CALIFORNIA AGRICULTURAL EXTENSION SERVICE CIRCULAR 111 April, 1939 PROTECTION OF ORCHARDS AGAINST FROST WARREN R. SCHOONOVER, E A. BROOKS, and H. B. WALKER Cooperative Extension work in Agriculture and Home Economics, College of Agriculture, University of California, and United States Department of Agriculture cooperating. Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914. B. H. Crocheron, Director, California Agricultural Extension Service. THE COLLEGE OF AGRICULTURE UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA CONTENTS PAGE Introduction 3 Economic conditions under which orchard heating may be justified 4 Probable additional income from heating . . 5 Probable costs of orchard heating 5 Atmospheric conditions on frost nights 8 Radiation of heat from the ground to the cold sky 8 Flow of cold air 9 Classification of atmospheric conditions on frost nights 10 Air temperature inversion and "ceiling" . . . 10 Effect of atmospheric moisture and smoke. . 12 Temperature in relation to frost damage 13 Safe temperatures for citrus fruits 17 Air temperatures at which heaters should be lighted in citrus orchards 17 Safe temperatures for deciduous fruits .... 18 Methods of adding heat to orchards 19 Use of orchard heaters 19 Border heating 20 Mass heating 20 Wind machines or blowers 21 Use of water 22 Use of steam 23 Use of central heating 23 Nocturnal heat from the ground 23 Heating equipment 24 Heaters 24 Open pails for heating deciduous fruits 25 Early smudge pots for citrus 25 Early orchard heaters 26 Choice of heaters 27 Heat and smoke output of oil-burning orchard heaters 33 Number of heaters required 36 Filling equipment 37 Hydrant-hose filling systems 39 Piped-heater systems 41 Lighting 42 Thermometers 42 Thermographs 45 PAGE Frost alarms 45 Fuels and fuel storage for orchard heating ... 46 Fuels for bowl- or distilling- type heaters. . . 46 Fuels for drip-type heaters 49 Fuels for generator-type heaters 49 Pour-back oil 53 Storage of fuel oil 53 Location of oil-storage tanks 54 Capacity 54 Foundations 54 Materials for tanks 54 Concrete tanks 54 Metal tanks and protection from rust 55 Tank covers, vents, and manholes 55 Float gauge 55 Pipe-line outlets 55 Filling inlet 55 By-pass inlet 55 Provision for sludge removal 55 Protection with embankments 55 Heating values of fuels 55 Coke and briquettes 66 Wood 56 Rubber tires 56 Butane-propane mixtures 56 Natural gas 57 Electricity 57 Routine practices of orchard heating 58 Lighting procedure 60 Lighting methods 64 Regulating the burning rate of heaters 65 Extinguishing heaters 65 Refilling 66 Cleaning 66 Labor for operations 66 Care of orchard heaters 66 Appendix: Open-crucible self-burning oil residue test 69 Scope 69 Apparatus 69 Procedure 69 Tolerances 70 PROTECTION OF ORCHARDS AGAINST FROST' WAEEEN R. SCH00N0VER,2 F. A. BROOKS,^ and H. B. WALKER* INTKODUCTION The protection of orchards against frost damage is a major problem in many California orchard districts. Almost one third of the entire Cali- fornia citrus acreage now has heater equipment, a second third does not suffer from frost damage often enough to warrant heating systems, and in the other third the need for heaters is marginal. At present 92,000 acres of citrus orchards are equipped with heaters to prevent frost dam- age." Four million orchard heaters are in service. Each one must be indi- vidually lighted and regulated and all except 160,000 pipe-line heaters must be individually filled. During the severe freeze of 1937 about two million barrels of fuel oil and 17,000 tons of solid fuel (coal, coke, wood, and rubber tires) were burned during approximately 15 nights from January 5 to 27. The gas oil consumed in these three weeks drained all southwestern storage, emptied outbound tankers, and required large emergency shipments from distant refineries. The total quantity burned in three weeks was about 3 per cent of the whole United States annual refinery output of this grade of fuel. The available supply of solid fuel was adequate for only a small acreage. Although, in 1937, there was a local shortage in fuel oil of the desired 27 + ° A.P.I. (American Petroleum Institute) gravity Diesel grade, oil is the only fuel which can be considered for the general practice of or- chard heating. Because the refiners cannot carry adequate stocks, many growers have installed ranch storage, ranging in capacity from 350 to 3,000 gallons per heated acre. Mr. Floyd Young, in the statistical study just cited, reports a total ranch storage of about 61,000,000 gallons which, ^ This publication is a revision of and supersedes Extension Circular 40, Frost Pro- tection in California Orchards, by Warren R. Schoonover, Robert W. Hodgson, and Floyd D. Young; it is not a report of the research project in orchard heaters and heating now being carried on by the Agricultural Engineering Division of the Uni- versity of California in the laboratories at Davis and in the field at the Citrus Experiment Station at Riverside. This project is not complete but some of the new observations will be cited at their appropriate places in the text to clarify the general questions of orchard heating. 2 Extension Specialist in Subtropical Horticulture. 3 Associate Prof essor of Agricultural Engineering and Agricultural Engineer in the Experiment Station. * Professor of Agricultural Engineering and Agricultural Engineer in the Experi- ment Station. ^ Young, Eloyd D. California orchard heater statistics. California Citrograph 23(9) :371, 386, 388. July, 1938. [3] 4 California AGRicuLTURAii Extension Service [Cir. hi with 25,000,000 gallons stored at packing-houses, is equal to the 1937 usage. Orchard heating is used for producing a rather small modification of outdoor temperature, namely, counteracting local cooling to maintain a safe minimum temperature, and raising the temperature of inflowing air near the ground a maximum of about 10° Fahrenheit. Whether orchard heating should be used more extensively than at present is primarily an economic problem. In general, heating should be avoided if there is any doubt as to its being worth while. It seems likely that present heating equipment will still be in use when fuels will have changed somewhat in quality and when they will probably be higher in cost. ECONOMIC CONDITIONS UNDER WHICH ORCHARD HEATING MAY BE JUSTIFIED Frost damage has occurred with sufficient frequency in California or- chards so that frost protection is now recognized as an essential orchard- management practice in many of the orchard districts. The losses vary from minor crop losses or impaired quality of part of the fruit to severe tree damage which may result in decrease of crops for several seasons in the case of the more tender subtropical fruits. Past experience in the citrus industry has led to the installation of frost-protection equipment on about one-third of the acreage in the state. Deciduous-fruit orchards and vineyards have, on the whole, less loss ex- pectancy and are not as fully protected. It is probable that most of the citrus acreage on which protection is justified is already equipped, al- though often inadequately so, against severe frosts. There is evidence that heating is practiced in some orchards where it is not economically sound. The question of when or under what conditions it becomes advisable for the grower to install orchard-heating equipment in all or part of his orchard is difficult if not impossible to answer positively, since there are so many variable factors such as differences in frost hazard, production costs, possible net returns with and without heating, and other considera- tions. In the last analysis this question must be decided by each grower. Certain factors entering into the decision may be determined with some degree of accuracy. There is every reason to believe that over wide areas in the fruit-pro- ducing sections of the state the occurrence of temperatures occasioning severe damage is so infrequent that the savings effected by the use of heaters would not in the long run equal the costs of installing and oper- ating the heating equipment. It is also probable that there are certain Protection of Orchards Against Frost 5 localities planted to fruits where frost damage is so extensive and so fre- quent in occurrence that over a period of years the cost of heating would exceed the value of the crops saved. Orchards so situated should be top- worked to varieties that are more resistant to frost damage, that bloom later, or that mature earlier. Unless this is possible, it is doubtful whether such orchards should be maintained, and in most cases they must even- tually be abandoned or replaced by other crops. It is unsafe to make a decision concerning the advisability of installing orchard heaters from the experience of only one season. The question should be given careful study and a decision reached only when it is apparent from local experience over a number of years that orchard heating is not only necessary to obtain satisfactory crops but that it will probably pay returns on the investment in capital and labor involved in its installation and operation. The primary factors which should determine the advisability of or- chard heating are the overhead and operating costs involved, and the probable additional income which may result. The overhead costs of or- chard heating can be determined with some degree of accuracy from the extensive experience at hand, as can also the operating costs provided the average number of hours of heating per year required to save the crop is known. In many districts frost-hazard data can be supplied by the local representative of the Fruit Frost Service of the United States Weather Bureau or by the county farm advisor. Many of the farm advisors, in addition, can supply local data on overhead and operating costs as re- ported to them by cooperators in their enterprise-efficiency studies. Cost estimates should be based on available data and in all cases should be liberal. Probable Additional Income from Heating. — The income from saved fruit is difficult to compute since this is determined by the production per acre which may be expected under the methods of management in use and the average price which may be expected for the fruit saved. It is necessary, therefore, to estimate the volume of fruit per acre that is likely to be saved from frost damage over a twenty-year period and the average price for this production. A fair allowance should also be made for protection against tree damage. The factors of production per acre and average price received for the fruit are of great importance in determining the probable profits from orchard heating. It is clear that where both average yields and prices are low, heating is likely to result in a loss. Probable Costs of Orchard Heating. — Orchard-heating costs should be divided into overhead costs and operating costs. The overhead costs 6 California Agricultural Extension Service [Cir. ill will depend upon the type of equipment chosen and the care given to it. Operating costs depend mainly upon the fuel consumption which in turn is proportional to the degree-hours^ of temperatures below the danger point and to the general atmospheric conditions prevailing during the period of heating. The cost of satisfactory equipment for locations of average frost haz- ard is approximately $175 per acre for oranges, $280 per acre for lemons and avocados, and $80 per acre for deciduous fruits. These costs are based on new equipment which is adequate but which is not the most ex- pensive nor the cheapest available. Costs will be higher for areas affected by the more rigid smoke abatement ordinances or places having extreme frost hazards. Where heating is practiced generally over an extensive acreage there is a marked effect on temperatures over the whole area so that the amount of equipment required can be reduced 10 to 20 per cent with a corresponding reduction in the above-estimated costs. An adequate supply of fuel in storage will cost, at present prices, about $60 per acre for oranges, $100 for lemons and avocados, and $30 for de- ciduous fruits. The total investment will be about $235 per acre for or- anges, $380 for lemons and avocados, and $110 for deciduous fruits. An annual charge of 10 per cent should be ample to cover normal deprecia- tion, interest, and ordinary fuel losses, making annual overhead costs $23.50, $38.00, and $11.00 per acre for the respective classes of fruits. Operating costs fall into two classes : first, those costs which are in- curred every year whether heaters are lighted or not ; and second, cost of fuel and labor for firing and refilling. The first group includes placing heaters in the field, the initial filling, removing from the field, ordinary repairs, painting, etc. Minimum costs for good practices will run about $6 per acre for oranges and deciduous fruits and $10 for lemons and avo- cados. The costs in the second group vary widely according to conditions. The orchards of the Citrus Experiment Station at Riverside are typical of a relatively large area of moderate frost hazard. In January, 1937, there was general heating for 11 nights. Several of the nights were se- verely cold but all of the equipment was used on only 2 nights. Only half of the heaters were lighted on the remaining 9 nights. Oil consumption on the entire 150 acres heated averaged a little over 12 gallons per acre per hour, taking the elapsed time from the beginning of lighting to the shutting-down of the last heater. This amounted to 0.42 gallon per heater per hour burned. The cost of fuel was 4.5 cents per gallon and the total cost for fuel and labor averaged 81 cents per acre per hour. ^ Further discussion of degree-hours is to be found in the section, "Temperature in Eelation to Frost Damage." Protection of Orchards Against Frost The Division of Agricultural Engineering and the Agricultural Ex- tension Service cooperated in conducting an orchard-heating survey dur- ing the summer of 1937. The results of this survey indicate that costs vary from the above as an approximate minimum to three or four times as much. In extreme cases oil consumption exceeded 100 gallons per acre per hour for short periods. The survey data indicate that, on the average, with distilling types of oil-burning heaters of 9 gallons capacity, the cost of lighting is about one-sixth of the total operating cost, the cost of refilling another sixth, and the cost of fuel two-thirds of the total operating cost. In addition to normal depreciation on containers there is faster depreciation on stacks and covers, and extra work for cleaning that should be covered by a TABLE 1 Typical Costs of Orchard Heating Per Acre Items Interest and normal depreciation Care and handling of heaters (no heating) Heating, 20 hours Heating, 30 hours Heating, 10 hours Extra depreciation and maintenance Total Oranges dollars 23.50 6.00 16.00 1.60 47.10 Lemons and avocados dollars 38.00 10.00 36.00 87.60 Deciduous fruits dollars 11.00 6.00 10.00 1.00 28.00 charge of 10 per cent of the combined fuel and labor cost. Assuming three typical cases with an average number of hours of heating per acre per year and average fuel consumption rates, the costs may be summarized as shown in table 1. These costs are presented merely as a guide for the grower who must decide whether to heat or not. The method of making these estimates is indicated in the text. Each grower who can secure data for his imme- diate locality can estimate his probable costs using the probable number of hours of heating required for his location and adjusting the interest and depreciation item in accordance with the cost and type of the equip- ment he contemplates purchasing. These probable costs should be weighed against probable additional income. The additional income de- pends, as stated above, upon the extent of probable frost losses which are to be prevented by heating, the yield of the orchard, and the price of fruit. Orchard-heating costs are likely to go up in the future rather than down. Heating should be undertaken only where a carefully estimated balance sheet indicates the probability of a profit. California Agricultural Extension Service [Cir. ill ATMOSPHERIC CONDITIONS ON FROST NIGHTS In the citrus districts located in areas not subject to subfreezing temperatures in the daytime, the frost damage is due primarily to the nocturnal loss of heat by radiation to the cold sky. The air itself does not lose much heat by radiation, but becomes chilled because of contact with the cold ground or other exposed surfaces, principally plant leaves, which radiate energy to the sky. Even in the cases of so-called "freezes" when a cold air mass of polar origin invades the citrus districts, it is the Fig. 1. — The effect of topography and the drift of cold air. The numbered lines are essentially isotherms expressed as the average number of nights per season during which heating was required. (From unpublished data compiled in 1930 by the Fruit Frost Service of the U. S. Weather Bureau, and supplied through the courtesy of Floyd D. Young, Senior Meteorologist.) regional chilling of this air at night which is most damaging. The cooled air is more dense than the warmer air above and remains close to the ground, getting colder as long as the surface temperature of the earth continues to fall. Topographical air drainage produces "cold spots" and there is large variation in heating requirements of different localities. The map of the Santa Paula district, California (fig. 1), shows broadly the topographical effect in terms of number of nights of heating. This is not a frost-hazard map because it does not indicate degree-hours of low temperature nor does it show local variations found in individual or- chards. Radiation of Heat from the Ground to the Cold Sky. — Since the pri- mary factor in the formation of a frost hazard is the loss of heat from the ground and other surfaces by radiation to the cold sky, it is of interest to Protection of Orchards Against Frost 9 note the net rate at which this radiation proceeds. The rate of radiation of energy from exposed surfaces to space varies from about 120 B.t.u/ per square foot per hour, when the ground temperature is 60° F, to about 90 B.t.u. after the ground has cooled to 20° F. Incoming radiation from the water vapor and carbon dioxide in the atmosphere will vary greatly with the dryness and temperature of the air overhead, but during radia- tion frosts 80 B.t.u. per square foot per hour can be assumed for a rough estimate. The net loss of heat from the ground to the sky would under these assumptions vary from 40 to 10 B.t.u. per square foot per hour be- tween sunset and sunrise. When the ground temperature approaches the danger point the net radiation loss is about 20 B.t.u. per square foot per hour, or about 900,000 B.t.u. per acre per hour. This net radiation loss is equivalent to the perfect utilization of the heat of combustion of 6% gallons of oil per acre per hour. Because the lower horizons of the ground are warmer than the surface during frost, heat flows out of the ground and supplies part of the net radiation demand. The thermal efficiency of artificial heating varies with many factors so no definite figure can be cited as to the fuel requirements to counteract the net radiation loss in the tract. Furthermore, additional heat sources exist in the cooling process of the air in contact with the ground surface or foliage and also in the condensation of water vapor (dew) and sometimes in the latent heat of freezing (white frost) . Flow of Cold Air. — Air movement is one of the major factors in deter- mining the effectiveness of artificial heat in raising orchard tempera- tures. The air chilled by ground contact is heavier than warmer air and hence will slowly flow downhill, underrunning the warmer air. The fill- ing of ground depressions by cold air from neighboring slopes increases the frost hazard in low spots, and conversely the frost hazard is less on hill slopes where air drainage prevents the accumulation of chilled air. If an inflow of cold air (sometimes 2 to 3 miles per hour) is to be warmed 10° F or an average of 5° for the full depth from ground up to say 40 feet, the total border-heating requirement is about 5,000,000 B.t.u. per hour for each 100 feet of windward frontage, needing roughly one row of heaters spaced every 10 feet plus 8 rows with heaters every 20 feet. This shows why it is difficult to warm small isolated tracts when there is any air drift. In the so-called "mass heating" of large areas the air-drift effect diminishes and the main requirement is to add to the heat given up by the ground enough artificial heat to counteract the normal local cool- ing, so that the temperature will not fall below the danger point. ^ B.t.u., a British thermal unit, is the quantity of heat required to raise the tem- perature of one pound of water one degree Fahronheit. 10 California AgricultTural Extension Service [Cir- ill Classification of Atmospheric Conditions on Frost Nights. — Because of the two factors contributing to frost hazard, namely, radiation cooling and inflow of cold air, it seems proper to consider atmospheric conditions on frost nights under four separate headings, as follows : A. Clear winter day and calm, clear night, atmosphere drier than average. Strong local nocturnal radiation cooling to frost temperatures occurs with slow air drainage (typical radiation frost). B. Cold wind by day, calm clear night. The daytime wind prevents ground temperatures from rising normally ; then the ground heat lost in even moderate radiation cooling at night drops the surface tempera- ture enough to produce a frost. C. Light rain in the daytime or early evening, calm clear night. Evap- oration cooling lowers the temperature of the ground surface. The effect is similar to that of type B. D. Cold night wind and clear sky. Local and regional nocturnal radia- tion cooling lowers further the already cold air temperatures at night. (Usually known as a "freeze.") In all cases except during a rare freeze occurring under clouds the air is cooled by contact with the colder ground or leaf surface. When there is no wind, the chilled air will accumulate near the ground and the air at higher elevations will be warmer than near the ground. There will not be much mixing of the colder ground air with the warmer air overhead, as long as the wind velocity at 8 to 10 feet above the tree tops is less than 6 miles per hour. If the air above is sufficiently warm, this inversion is an important factor in orchard-heating practice in that it restricts the rise of air warmed by the heaters. Air Temperature Inversion and ^^Ceiling.^' — Figure 2 indicates the change of temperatures throughout the night at four different levels on a hill slope. This shows the gradual nocturnal cooling and also that it is more pronounced on the low ground than at higher elevations. Figure 3 illustrates another example of temperature variation with altitude ob- served by a captive balloon at the Citrus Experiment Station in River- side, January 27, 1938, and by airplane sounding at San Diego, 90 miles south. The balloon soundings cover a period of 3 hours during which there was progressive cooling as indicated in the shaded band. This record is for much higher ground temperatures than that of figure 2, but it is of interest in showing a temperature inversion point at 125 feet. Above this point there was on that night no appreciable decrease in air temperature up to an elevation of about 2,000 feet, and in the next 2,000 feet there was a decrease of only about 6° F. Under steady weather conditions there is little fluctuation of air tern- Protection of Orchards Against Frost 11 perature above the inversion level, hence the overhead air temperature at night can be estimated from daytime soundings. The elevation at which the maximum temperature occurs, or in other words, the point of inver- sion, is the upper limit of the "ceiling." From an orchard-heating stand- point the height of the "ceiling" is thought of as the distance above ground to a natural temperature warm enough to restrict the rise of di- 8P.M. Fig. 2. — Temperature records taken at four different levels on a hill slope dur- ing a frosty night. (From: Young, Floyd D. Frost and the prevention of damage by it. U. S. Dept. Agr. Farmers' Bui. 1096:1-48. 1922. luted hot products of combustion. For instance, with cold dry air, under frost conditions (type B) there might be a low-level inversion point and yet the absence of warm air overhead would make heating difficult. Other balloon observations showed that the inversion point might be at 250 feet as on February 13, 1938, where the air was only 5° to 8° F warmer than at the surface ; or it might be as low as 60 feet, as observed February 26, 1938, when there was a temperature difference of 25° in this height. These figures of course do not agree with the hillside observa- tions, but represent the maximum and minimum observations by balloon during the winter of 1937-38 which was relatively warm. In conclusion, it can be seen that the frost damage is due primarily to the loss of heat from the ground and trees, and therefore the most effec- tive protection is to supply heat artificially to counteract the natural loss. In terms of heat, a common firing rate of 25 gallons gives about 3,500,000 12 California Agricultural Extension Service [Gib. m B.t.u. per acre per hour, which is nearly four times the radiation loss discussed previously. Effect of Atmospheric Moisture and Smoke. — The rate of nocturnal loss of heat by radiation varies with the moisture content of the atmos- phere and its temperature, particularly in the lower layers. This is be- 4000 J750 S500 3BS0 \3OOO L2730 \,^soo ^2250 \ ^2000 /500 2S0 \/2S0 ^ /ooo I 200 175 \ \ \ i/.S aero/o^/'co/ sou/?c///7gr ,— \ I 1 1 ■ '^^i /C^J a \ \ \ ^ \ \ \ ^ \ \ \ \ /SO 725 700 7S SO 2S . ^^, 1 \ /J fi/\/ers/c7s, Ji?^. £7, /S3S m /// y/f ^(' U 1 ,'/ 4 f r ■■/// n f / II/7 4'B ¥ / ■'Ik f///. 7777 M^ ai:£. =^ 'CC££- :fi^ laaaJ iU^ S6 S3 ■ 'F 60 62 64 66 6S 36 33 40 42 44 46 43 SO S2 34 Fig. 3. — Balloon and airplane observations of air temperatures at Eiverside and San Diego, January 27, 1938. Note change of scale in the upper half of the figure. cause the radiation from the water vapor of the atmosphere coming- toward the earth is dependent upon the amount of moisture in the air and its temperature. There is some transparency through the water vapor, but the net rate of radiation from the ground to the sky depends on the rate of radiation back to the ground from the moisture in the air. Protection of Orchards Against Frost 13 The smoke output from orchard heaters has no advantage in an eco- nomic sense even to the grower. There is an opinion that a smoke screen shielding the fruit from the early-morning sun decreases frost damage, and there is some laboratory evidence of the advantage of slow thawing. Thermally there is a disadvantage in smoke intercepting solar energy during the day. The lighter frost damage which is sometimes observed in sections of orchards shaded by windbreaks or buildings might be ex- plained by restricted radiation cooling at night. However, with adequate heating there will be no frost damage. The smoke is " serious nuisance to the general public, so orchard heating should be made virtually smoke- less as soon as possible. TEMPERATURE IN RELATION TO FROST DAMAGE The literature dealing with the effect of low temperatures on plants, blos- soms, and fruits is extensive, and no attempt will be made to give a gen- eral review of this literature here. Temperatures which will be endured without damage depend upon many variable factors, such as : 1. Duration and degree of the low temperature. 2. The amount of air movement. 3. The amount of moisture in the air. 4. The degree of exposure of the plant parts to direct radiation. 5. The cell-sap concentration of the plant part under consideration. 6. The heat capacity of the fruit or plant. 7. The degree of insulation. 8. The extent to which the fruit cools below the freezing point without the formation of ice crystals — ^namely, "undercooling." The present concepts of these factors, influencing the freezing of fruit, are as follows : 1. The duration of temperatures below some critical point is the most important factor in determining the degree of frost damage. A conven- ient way of expressing duration of damaging temperatures is degree- hours below the freezing point of the juice, or below some critical temperature which field experience has indicated results in practically no damage. By degree-hours is meant the total exposure to cold expressed as an integration of the product of degrees below the danger point and hours, for each hour below that point. A rough estimate of the damage to ripe oranges may be secured from a record of the degree-hours below 26° F to which the fruit has been exposed. Past experience indicates that 4 degree-hours will cause very little damage, 6 to 8 degree-hours serious reduction in grade, and 10 or more degree-hours almost complete loss of 14 California Agricultural Extension Service [Cir. ill crop.* This should not be taken as an exact or sole criterion because the degree-hours of low temperature represent only one of eight important factors determining injury. 2. Other factors being equal, fruit or plant parts will freeze more quickly in moving cold air than in still air. There are two reasons for this effect: first, the movement of the air substitutes, forced convection for free convection and thus greatly increases the rate of loss of the heat stored in the fruit ; and second, undercooling of the fruit does not occur to the same extent when the air is moving as it does in calm air. 3. For equal temperatures the amount of damage to fruit or plants will be greater on nights of high dew point than on nights of low dew point. The reasons for this are not fully understood but it is known that under- cooling occurs to a greater extent on dry nights. The greatest amount of undercooling occurs when the temperature of the dew point is so low that no frost forms on the fruit. Nights of high dew point are not usually as dangerous as nights of low dew point because the temperature does not fall as rapidly nor as far, but if the temperature does happen to go down very low on nights of high dew point, the damage will be excessive. Arti- ficial humidification of small areas retards the fall in temperature very little and may contribute seriously to frost injury. 4. Fruit and plant parts become cooled through contact with cold air and also through radiation of heat to the sky. Fruit on the inside of a tree will often have a temperature of 2° or 3° F higher than the fruit which is fully exposed to sky radiation. Trees near buildings or dense windbreaks are sheltered from part of the sky exposure. Often there is very little damage to fruits on these trees when nearly all of the fruit may be lost on trees further out in the orchard. 5. The concentration of the cell sap or the freezing point of the fruit juice is a most important factor in determining the temperatures which will be endured. Dissolved substances lower the freezing point of water. If atmospheric and soil conditions have been favorable for rapid growth immediately preceding a frost, the sap concentration is likely to be low and frost damage may occur at relatively high temperatures. If the plants have been exposed to low temperatures unfavorable for growth for a period of several days or several weeks, plants and fruits will en- dure very low temperatures for relatively long periods of time. The actual damage which occurred during the freeze of January 1937 « This formula should not be confused with the so-called "Dezell" formula used by the California Fruit Growers Exchange in estimating losses. The Dezell formula is based on a simple product of the number of hours below 26° F and the number of degrees that the minimum temperature is below 26°. The figures obtained in this manner are about twice those obtained by summation of the degree-hours. Protection of Orchards Against Frost 15 was very much less than would have been expected from previous experi- ence with similar temperatures. For example, one orchard of eight-year- old Fuerte avocados survived a minimum of 14° F with 6 hours below 20°. The trees were severely damaged but the trunks and main frame- work branches remained alive so that it has been possible to rehabilitate the orchard. In previous freezes similar trees have been killed outright by temperatures of 20°. Citrus trees and fruits in general suffered much less damage in 1937 than occurred with higher temperatures in 1922 and with similar temperatures in 1913. The experience of 1937 cannot be con- sidered a dependable guide regarding temperatures which will be en- dured. 6. Full-grown oranges, grapefruit, and lemons will endure more de- gree-hours below the actual freezing point of the juice than will small fruits and blossoms. This is because the rate of cooling by free convection is less for larger size, and the mature fruit has a greater heat capacity. Fruits become warmed during the daytime and then cool more slowly than the surrounding air when the temperature falls at night. When the temperature is falling rapidly on a frosty night following a warm day, the interior of the fruit may be as much as 6° to 7° F warmer than the sur- rounding air, and the temperature inside the fruit may lag from an hour to an hour and a half behind the air temperature. For these reasons many growers use fruit thermometers as a means of determining when to start lighting orchard heaters. The use of fruit thermometers will be discussed in another section. 7. Another reason why mature citrus fruits freeze slowly is because of the insulation value of the thick pithy rind. This principle is applied artificially to protect young trees against frost damage. A layer of corn stalks, tules, or other porous insulating material 2 or 3 inches in thickness tied tightly around the trunks of young trees will retard heat loss from the trees and protect them from injury even though temperatures go to exceedingly low points. 8. Undercooling, sometimes called supercooling, is the name used for the physical phenomenon of cooling a liquid below its freezing point without the occurrence of actual solidification or change of state.^ When a liquid is undercooled, it is in an unstable condition and will freeze rapidly if anything causes a rearrangement of the molecules to start, so that freezing or crystallization may occur. A sudden jar or the addition of a small crystal as a source of inoculation for crystal formation will often cause instantaneous freezing of the whole mass of liquid. Such » Dorsey, N. Ernest. Supercooling and freezing of water, Jour. Research National Bureau of Standards (Research Paper 1105) 20(6) -.799-808. June, 1938. 16 California Agricultural Extension Service [Cm. ill freezing is accompanied by a rise in temperature to the freezing point. Fruits such as oranges frequently cool as much as 3 degrees below the freezing point of the juices and are in an unstable undercooled condition. No damage will occur if fruit in this condition becomes warmed above the freezing point without actual freezing having occurred. Undercool- ing explains the lack of damage on many severely cold nights. It is most likely to occur on nights which are dry, and particularly on nights which are both dry and relatively calm. In view of the many variables which influence the extent of frost in- jury it is difficult to specify temperatures which will be safe for various kinds of fruit and below which damage will occur. However, the fruit grower needs some specification as a guide to his orchard-heating opera- tions. The suggestions in this circular are believed to be conservative and safe. They are based upon extensive experimental work and field obser- vations by the Fruit Frost Service of the United States Weather Bureau under the direction of Floyd D. Young, Senior Meteorologist, Pomona, California.''" The table for deciduous fruits and suggestions in the text are based on a standard reference point, which is the temperature indi- cated by a tested thermometer exposed in a satisfactory shelter at a height of 5 feet from the ground. If a grower follows the recommenda- tions, he will inevitably light his heaters many times when no damage would have occurred without heat. On the other hand, unusual conditions may occur when fruits might be injured at higher temperatures than those indicated in tables which are based upon past experience. If heating costs go up and fruit prices go down, growers will wish to supplement the tables with their own experience, and not maintain higher temperatures than are actually necessary to prevent damage. No grower should be guided by the experience of only one season. For example, during 1937 green Valencia oranges withstood lower temperatures without damage than mature navel oranges. This experience is contrary to that of some previous years so one should not conclude that it is safe to allow Valencias to reach lower temperatures than navels. In order to save expense, growers may decide to take certain chances, and not try to save all of the fruit. More small lemons will be saved by maintaining orchard temperatures above 30 °F than by maintaining them above 28°. For some locations, however, it is better from an eco- nomic standpoint to permit the freezing of some of the small lemons and to save expense by allowing temperatures to go as low as 28°, before starting to light heaters. Some growers allow the most exposed oranges ^" Young, Floyd D. Frost and the prevention of frost damage. U. S. Dept. Agr. Farmers' Bui. 1588:1-62. 1929. Protection of Orchards Against Frost 17 on trees to start freezing before lighting their orchard heaters. This will result in a slight amount of damage but may materially reduce the cost of heating. Other growers will look upon heating as insurance and en- deavor to maintain safe temperatures at all times, being guided by the slogan, "It is better to be safe than sorry." Safe Temperatures for Citrus Fruits. — Safe temperatures for citrus fruits depend upon the variety, the degree of maturity, and the atmos- pheric conditions. The freezing point of the juice of green oranges varies from 28.5° to 29.5° F ; for half -ripe oranges and grapefruit from 28° to 29° ; and for ripe oranges and grapefruit from 27° to 28°. The time at which lighting should occur may be determined by using air tempera- tures indicated by properly sheltered thermometers or fruit tempera- tures or a combination of the two as a guide. When air tempertures are falling, fruit temperatures will usually lag. Therefore, the temperature of exposed fruits obtained with fruit thermometers will be a valuable aid in determining the time of lighting. Fruit temperatures should be taken only in oranges of more than average sky exposure. If the air tempera- ture is below the freezing point of the juice, it is not safe to delay heating operations after the fruit temperature has dropped to the freezing point of the juice, or after the fruit has reached a temperature near this point and remained at a constant temperature for about half an hour. When a fruit is cooled its temperature falls to the freezing point of the juice and then if the fruit starts to freeze, its temperature will remain constant at the freezing point until the fruit is frozen, even though the air temperature continues to fall. The leveling-off of temperature in an individual fruit surrounded by air which is falling in temperature indi- cates that the freezing point of that particular fruit has been reached even though the temperature is above the supposed freezing point for the variety and stage of development. If the fruit undercools, the tempera- ture will drop below the freezing point. Air Temperatures at Which Heaters Should he Lighted in Citrus Or- chards. — For those growers who do not wish to use fruit temperatures in conjunction with air temperatures, the following suggestions issued by the Fruit Frost Service of the United States Weather Bureau'' will be useful. They are based on the use of sheltered thermometers and antici- pate completing first lighting of heaters within 30 minutes. On cold nights following warm days (highest temperature 60° F or over) with steady temperature fall to the danger point: Fire ripe oranges or grapefruit when the air temperature reaches 26°. Fire green or half -ripe oranges and grapefruit at 27°. ^1 Placards printed by the California Fi'uit Growers Exchange. 18 California Agricultural Extension Service [Cm. ill On cold nights following cool days (highest temperature 59*^ or lower) with very slow temperature fall near danger point : Fire ripe oranges or grapefruit at 27°. Fire green or half -ripe oranges or grapefruit at 27.5°. Keep the shelter thermometer up to 28° or higher on both types of night after firing is begun. Damp nights are more dangerous than dry nights, with similar temperatures. Citrus fruits begin to freeze at a higher temperature when they are covered with ice than when they are dry. The temperature fall is usually slow and steady on damp nights. On dry nights look out for sudden and rapid drops in temperature. If the air temperature fluctuates rapidly, owing to mnd, take the average of the high and low points as the effective temperature. During November the navels in most districts are in the half -ripe stage and will freeze at a temperature about 1° higher than they will later in the winter. Similar suggestions apply to lemons except that higher temperatures need to be maintained. The temperature at which heating is begun for the protection of lemons will depend on whether it is desired to save the blossoms and small "button" lemons, or only the mature fruit. If blos- soms and small fruits or "button" lemons are to be protected, the temper- ature must be held at 30° F or higher, while the larger lemons will not be injured with an air temperature of 28° for several hours. Even though lighting is delayed until the temperature reaches 28° it is usually desira- ble afterward to maintain the temperature of lemon orchards around 30°. The small green fruits are more susceptible to damage by frost than the blossoms. In the protection of avocados the general recommendations for "but- ton" lemons should be followed. Safe Temperatures for Deciduous Fruits. — The data in respect to de- ciduous fruits are not adequate. The recommendations do not cover all of the stages of growth and should be considered merely as a guide which will be safe but which may result in burning unnecessary amounts of fuel. It is very difficult to forecast damage to a deciduous fruit crop from a severe frost at any stage prior to the young green-fruit stage. This is because blossoms will withstand different temperatures at different stages of development, and not all of the blossoms on the trees are in the same stage of development. In California, owing to warmer winters than in many other fruit-growing regions, the rest period of deciduous trees is not completely broken. The prolongment of the rest period results in the lengthening of the blossoming season. Severe frosts have often been observed to kill a large percentage of the blossoms without subsequent reduction in crop. A second, lighter frost may cause a complete loss of crop if it occurs when fruits which missed the first frost are in a tender stage. It is therefore necessary for the deciduous fruit grower to become Protection of Orchards Against Frost 19 a careful student of temperature conditions as related to frost injury in order to secure adequate protection without unnecessary expense. Table 2 indicates the temperatures in degrees Fahrenheit which will be endured for 30 minutes or less by deciduous fruits in various stages of development. Orchard heating is often of considerable benefit during severe freezes even though it has been impossible to prevent air temperatures from dropping far below those ordinarily considered safe. The thermometer in TABLE 2 Temperatures Endured for Thirty Minutes or Less BY Deciduous Fruits* stage of development Kind of fruit Buds closed but showing color Full bloom Small, green fruits Apples op 25 25 28 25 25 25 23 26 30 30 op 28 27 28 28 28 28 27 27 31 30 op 29 Peaches 30 Cherries 30 Pears 30 Plums 30 Apricots 31 Prunes 30 30 Grapes 31 Walnuts, English 30 * Temperatures determined from sheltered thermometers. Source of data: Young, Floyd D. Frost and the prevention of frost damage. U. S. Dept. Agr. Farmers' Bui. 1588:1-62. 1929. the standard shelter may indicate complete failure when some fruit may have been protected by radiation from heaters which did not register upon the thermometer. Often the hot products of combustion blow di- rectly through trees and do not affect thermometers out away from the trees. There may be other reasons also for the benefits which result from heating even though thermometers have not indicated that fruit would be saved. A grower who is unable to maintain satisfactory temperatures by using all of his equipment at reasonable burning rates should not become discouraged and stop his heating operations as a result of finding that his thermometer records dangerously low temperatures. METHODS OF ADDING HEAT TO ORCHARDS Use of Orchard Heaters. — As previously described, orchard heaters are successful in counteracting nocturnal radiation cooling and in warming the inflowing cold air mainly because of the usual temperature inversion 20 California Agricultural Extension Service [Cir. m which confines the artificial heat to a relatively shallow depth of air near the ground. The nocturnal atmospheric condition is very different from that of the daytime in that a cold ground surface stabilizes the air, whereas a warm ground surface promotes circulation. In the daytime air need be warmed only a few degrees above normal to become less dense than the air over- head and rise almost without limit. At night when the ground is colder than the air overhead, the ground air uniformly warmed a few degrees will still be heavier than the even warmer air overhead and hence will rise only a short distance before it reaches a balance. With this in mind it is evident that the ideal method of orchard heating is the use of a very large number of small heating units distributed over the area to be protected. Experience in 1937 showed that the minimum number of heaters of any kind is about 35 per acre to afford reasonable protection ; and of course more units would have been more efficient in frost protection. Larger combustion units might be more efficient as heat producers, but in an orchard the advantage in small units seems to lie in the more uniform radiant and convective energy distribution. Border Heating. — When there is an inflow of cold air on an exposed frontage it is necessary to bank the heaters on the upwind side to protect the first rows of trees. If the heaters are located one or two per tree on the exposed side, the radiant energy from the heaters and possibly also the products of combustion will warm the near trees directly and thus afford protection although the heat output of this first bank of heaters is not adequate to warm the entire depth of incoming air stream to the height where the air temperature due to inversion naturally is the same as that of the dispersed products of combustion. Hence, several rows of heaters inside the border also serve to warm the incoming air in addition to coun- teracting radiation cooling, and the incoming air will not all be warmed to the height of natural safe temperature until it has passed many tree- rows in from the exposed border. Under these conditions one would ex- pect the need for heat per unit area to gradually taper off down-wind from close border-row spacing until only radiation cooling need be coun- teracted. Mass Heating. — The "mass-heating" requirement to counteract only radiation cooling is usually low, and at most is equivalent to the heat of combustion from 35 heaters per acre burned at the rate of % gallon per hour. Not all the heat output of heaters is available but some of the heat needed comes naturally from the ground, so even lower burning rates are often used successfully on calm nights when radiation cooling is moderate. Protection of Orchards Against Frost 21 This low burning rate to counteract radiation cooling in the center of large heated areas no doubt explains why certain orchards with inade- quate protection have suffered little frost damage. Furthermore, the general practice of orchard heating in some districts has now altered the local conditions so that the frosts at control stations do not appear as severe as in the past during similar general weather conditions. Where most of the frost damage occurs in localized "cold spots" much fuel can be saved by noting temperature differences in each acre or two so that firing will be regulated to suit the need of the areas threatened. Wind Machines or Blowers. — Because under ordinary nocturnal radia- tion cooling there is g, large quantity of warm air overhead, a great many installations of blowers have been made to artificially mix the ground air with the higher warm air. Large blowers or fans mounted on towers as high as 50 feet above the ground have been used in California for twenty years. Some of these are driven by electric motors and others by gas engines, varying in size from a Ford V-8 engine to a 425-hp. airplane engine. The greatest use of this equipment has been in the central California citrus district, but the prac- tice has been extended to other areas, and a few installations have been made in walnut, apricot, and pear orchards. Observations and the experiences of owners of typical wind machines indicate that under certain conditions blowers have reduced frost dam- age. Temperature observations in some orchards when there was a tem- perature inversion of 10° F in 50 feet have shown the air to be about 3° warmer in the area near the blowers than that outside. The effectiveness of blowers is not apparent beyond approximately 300 feet for small ma- chines and about 500 feet for large ones. In the winter of 1934-35 when the oranges in two orchards were graded for frost injury, it was evident that the fruit nearest the blower was damaged least, and that the damage increased with the distance from the blower." Similar results were later obtained in a walnut orchard. Growers cannot expect protection from such equipment if there is no warm air available overhead. In a typical freeze there is no warm layer above if there are no orchard heaters in the neighborhood. However, there is some evidence that a few blowers operated near heated areas decreased the frost damage during the 1937 freeze. The combination of a large furnace with a revolving blower has not yet been proved success- ful. The blown furnace air, being much hotter than the orchard air, does not carry well into the trees before rising. 12 Moses, Ben D. Blowers for frost protection. Agricultural Engineering. 19(7) : 307-08. 1938. 22 California Agricultural Extension Service [Cir. m The explanation of the occasional apparent success of this type of frost protection in spite of observed low temperatures is obscured by highly complicated physiological factors of the trees and fruit, such as dormancy, so no specific limits can be stated. If blowers are used, experi- ence indicates that they should be started before the temperature drops to 32° F. If an increase in air temperature cannot be obtained, the wind machine should be stopped at the freezing temperature of the fruit to avoid forced cooling. If the grower is satisfied with partial frost protection, a blower may be considered because of its convenience and smokelessness. However, partial protection by heaters would be normally more economical. A favorable place for blowers is in narrow canyons where insufficient nat- ural air drainage can be artificially increased so that the formation of cold air pockets is prevented. Again, this use does not afford protection against freezes. Use of Water. — If the orchard is irrigated at night, just before an ex- pected frost hazard, the water will provide extra heat, first in the cooling from its original temperature to the freezing point, and second in the giving up of latent heat when forming ice. The amount of heat delivered by a cubic foot of water in cooling from 56° F to the freezing point is approximately 1,500 B.t.u., and the amount of heat liberated when a cubic foot of water changes to ice is about 9,000 B.t.u. Hence, the total available heat from a cubic foot of water at 56° is about 10,500 B.t.u. when transformed to ice. Maximum nocturnal radiation alone for 6 hours would cool a 2-inch layer of water to the freezing point or cool and freeze a %-inch layer. Ice does not form rapidly except at temperatures below 26° so the latent heat of freezing does not afford satisfactory frost pro- tection. If water and ice remain throughout the next day, the orchard will start cold the following night ; but if there is another radiation frost, a second flooding could be resorted to. The limitation of this method of frost pro- tection is the availability of enough water, and its effect on the root sys- tem. If ice is allowed to form, it must be flooded frequently to prevent its temperature from dropping much below 32° F. Of course, flooding will not protect against low-temperature winds as in freezes, but water on the ground can be used as a partial substitute for artificial heat. The water should never be sprayed on the trees because the ice load is likely to break the trees. Furthermore, the introduction of water by spray promotes evaporation which absorbs heat from the air rather than supplying heat. The belief that an ice coating will prevent the fruit from going below 32° F is not correct, because ice does not remain at 32° if Protection of Orchards Against Frost 23 the loss of heat continues. A layer of ice has some thermal insulation value, but there is evidence that the presence of ice on the outside of the fruit affects the physiological condition of the fruit so that there might actually be more damage than if the surface were dry. Use of Steam. — If steam could be liberated throughout an orchard so that it would warm the air before rising above the trees, a large quantity of heat could be carried by a small quantity of water. The steam in con- densing (forming a fog or drizzle) gives up about 65,000 B.t.u. for each cubic foot of condensed water. The cooling and freezing of condensed steam would add another 20,000 B.t.u. per cubic foot of water. However, steam heating has not been considered practical for orchards because of the high cost of the steam plant and distribution piping. Mr. W. H. Hutchinson, of Sonora, proposed in 1933 that the steam be generated without a boiler by injecting water directly into a furnace. All the products of combustion plus the extra water vapor were to be dis- tributed throughout the orchard by a blower and large ducts. The heat output of such a central heating system would include the steam values mentioned previously and in addition those obtained from the cooling of the air and products of combustion. The pertinent question of such a sys- tem is whether the hot fog could be confined to the orchard displacing the heavier colder air, and whether the central heating and distributing sys- tem would be too expensive. If the protection is not adequate, there would be greater frost damage with wet fruit. Previous attempts at steam heat- ing have been abandoned. Use of Central Heating. — A few installations have been made in small orchards of central heating with warm air piped to each tree. In these cases it is intended to warm the trees but not the spaces between trees. Single-tree experiments at Riverside show this to be difficult of accom- plishment unless the air is so quiet that smoke from the usual orchard heater rises straight up. The apparent economy of this type of installa- tion may be partly due to adjacent mass heating. Installation costs seem to be high. Nocturnal Heat from the Ground. — There is an advantage in having a moist soil during frost periods in that with the ground damp the rate of heat transfer into and out of the ground will be greatly facilitated, and hence the diurnal temperature range of the wet ground surface will be less than if the surface were dry. In other words, a dry surface soil will obstruct the flow of heat into the ground when the sun is shining and will also obstruct the flow of heat from the ground at night. Hence, when the energy discharged by radiation at night is drawn from the ground, the temperature gradient in dry ground must be much greater than in 24 California Agricultural Extension Service [Cm. ill wet ground, and the surface temperature will fall much lower with dry soil than moist. This is extremely important in typical radiation frosts because the air is largely cooled by the ground. Late spring frosts seldom occur when the spring rains have been so spaced that the soil over the countryside is always somewhat moist. The heat discharge from moist soil at night is complicated by evapora- tion which absorbs energy day and night so the above general statement cannot be made specific for a given orchard without detailed studies of the local soil and air conditions. HEATING EQUIPMENT The necessary items of equipment for orchard heating, which will be dis- cussed in this section, are heaters, filling equipment and torches, ther- mometers, thermographs, and frost alarms. Heaters must use fuel that is available. The commonest usage at pres- ent is of 27+° Diesel oil which is the ''heart cut" of the crude oil and for which there is growing demand in the production of gasoline. Heaters. — Many types of heaters have been put on the market during the last twenty-five years. New and improved types are brought out nearly every year but no present heater meets all of the requirements of a good orchard heater. According to the generally accepted ideas as to the more important specifications of an ideal oil-burning heater, it should : 1. Hold sufficient fuel to burn all night without refueling, or else be piped for continuous fuel supply. 2. Be capable of sufficient regulation to give its greatest heat just be- fore sunrise even though the fuel in the bowl is low by this time (ordi- nary burning rates are from % to % gallon an hour) . 3. Be able to burn any of the ordinary grades of heating fuels on the market without objectionable smoking and without leaving troublesome residues. 4. Be rain-proof so that water will not enter the oil container. 5. Deliver the heat and products of combustion near the ground, but without excessive heating of the ground. 6. Be easy to light and regulate by inexperienced labor under all weather conditions. 7. Be capable of being lighted with the control set as required for nor- mal burning. 8. Burn at a uniform rate without frequent regulation. 9. Be readily extinguished by merely closing the regulator and cap- ping the stack or by closing a valve. 10. Burn several nights if possible without the necessity of cleaning. Protection of Orchards Against Frost 25 11. Be so designed that the oil can be burned to the bottom of the heater bowl without damage. 12. Avoid excessive oil condensation on the stack or cover. (A little condensation may be beneficial in preserving the stack and cover.) 13. Be easily filled without removing stack or cover. 14. Be of reasonable cost. (See p. 6 for average installed cost per acre.) 15. Be made of good material and show small annual depreciation. 16. Be easy to take apart, clean, and store. New heaters being installed for use near populated districts should be as smokeless as possible and old heaters should be modernized so as to reduce their smokiness. Open Fails for Heating Deciduous Fruits. — The simplest type of or- chard heater is an ordinary pail with a close-fitting cover. So-called "lard pails" of 5-, 8-, and 10-quart capacity are manufactured especially for deciduous orchard heating. Growers also use secondhand paint pails, powder cans, no. 10 tin cans, and garbage pails of from 5- to 12-gallon capacity. Garbage pails need to be of especially good construction or they will leak after soldered seams have been exposed to heat. Other simple types consist of square or rectangular pans with sliding covers. These simple heaters or "smudge pots" are effective from a frost-protec- tion standpoint provided burning rates are controlled properly. Burning rates depend upon the surface area of oil exposed in relation to depth to the oil level in the container. A 5-quart lard pail with full surface ex- posed will burn about 2^/2 hours and a 10-quart pail the same length of time, giving average burning rates of one-half and 1 gallon per hour respectively. The burning rate is far from uniform, however, being very high the first hour and low the last. The burning rate may be partially controlled by the spider, which is a device for partially covering the top of the pail (fig. 4, A) . The spider should be left in place until the pail has burned half empty and then be removed. The larger pails and the pans have the burning rates controlled by sliding metal covers. These simple heaters are not recommended for citrus orchards because the excessive smoke soots up the fruit and reduces its value. In thickly settled areas they create a smoke nuisance which is intolerable. Under these conditions all such heaters should be discarded. For deciduous-fruit growers who are away from the cities and in dis- tricts where the hours of operation are so few that no general smoke nuisance is created, such heaters are satisfactory and economical. The smoke does no damage to the blossoms of deciduous fruits and does not interfere with pollination. Early Smudge Fots for Citrus. — The next development in heaters was 26 California Agricultural Extension Service [Cir. m the smudge pot of large capacity, usually 7 to 9 gallons. Such heaters in- clude the Dunn, the Riverside or Citrus, with stub stack, or with 15-inch stack (fig. 4, B, C and D respectively) , and several models of the Hamil- ton. These heaters are effective but are ordinarily too smoky for use in the citrus districts. The smoke output often exceeds that of the lard-pail C D Fig. 4. — "Smudge pot" orchard heaters. A, Lard-pail heaters of the Bolton and Caneo types; B, Dunn heater with the umbrella cover removed; C, Citrus Regular, stub stack ; and D, Citrus, 15-inch stack. (A and B from Bui. 398 ; C and D from Bui. 536.) types. These heaters can be modernized by fitting collars onto the covers and using "lazy-flame" stacks of recent design. In the lazy-flame type, the stack serves as a mixing chamber for the petroleum gases and air, and the combustion is mostly above the stack. In many cases it will not be worth while to modernize old Riverside or Hamilton heaters with loose- fitting covers or old Dunn heaters which have been in use for twenty years or more. Such heaters should be discarded. Early Orchard Heaters. — Other equipment which needs modernizing includes the Baby Cone, the 5V2-inch Exchange stack, the National and Protection of Orchards Against Frost 27 ■-'^^^j^- '> £:;^ Wliiimrrms^ Fig. 5.— Early types of orchard heaters. A, National Baby Cone • B National Exchange model, 5 1/2 -inch stack; C, National, double stack; and D, Hy-Lo double stack. These heaters are all of 9-gallon capacity; the illustrations are not of the same scale. (From Bui. 536.) 28 California Agricultural Extension Service [Cir. m Hy-Lo double-stack heaters (fig. 5) . The last two heaters can be modern- ized by placing adapters under the present stacks, but the Baby Cone and 5%-inch Exchange stack should be replaced by new stacks of better de- sign. The Baby Cone is an example of a heater which burns reasonably well at one burning rate, which rate cannot be maintained without regu- lating the heater at frequent inter- vals. This difficulty can be overcome in part by pounding down (closing) the second, third, and fourth rows of louvres, starting from the top. There are many old models of stacks with combustion chambers which can be operated with reasonable freedom from smoke if great care is exercised to keep them within the range of burning rates for which they are designed. Heater manufac- turers should be consulted regard- ing modernization of old models. Heaters with combustion chambers in the stacks are satisfactory only if there is a favorable air-fuel ratio in the combustion chamber, if good mixing takes place, and if the capacity of the combustion chamber is not exceeded. If the burning rate is too high for the size of the combustion chamber, a portion of the fuel vapor becomes overheated before it can Fig. 6. — Eomid- and square-bowl orchard heaters of the "lazy-flame" type. rig. 7. — Covers equipped either with down draft tubes (right) or internal chimneys (left). burn ; and cracking takes place, causing excessive smoke production. All combustion-chamber heaters are sensitive to regulation. Other factors such as air-fuel ratio and mixing being equal, the larger the combustion chamber the wider the range of burning rate and the easier the regula- tion. The grower should make burning tests to determine the range of burning rates over which his heaters will burn without appreciable smoke and should regulate them accordingly. Other heaters especially sensitive to regulation are the Apollo, the Beacon, and the 6-inch Exchange stack. Protection of Orchards Against Frost 29 Choice of Heaters. — The grower purchasing heaters today has four general types from which to choose : 1. Distilling type or bowl heater : These may be obtained with round or square bowls (fig. 6). Observed differences in carbon-residue formation due to bowl shape or interior parts have not been sufficiently significant to justify recommendations for any particular type of bowl if the covers fit tightly. Field experience indicates that square-bowl heaters must be rig. 8. — Combustion-chamber heaters. A, Jumbo Cone, B, Lemoraj and C, Hy-Lo, No. 148 Special handled more carefully than round-bowl heaters to maintain the cover fit sufficiently airtight to permit proper control of the burning rate. Stack performance is almost independent of bowl designs now in use. Heaters are usually equipped with downdraft tubes or internal chim- neys (fig. 7) . Each serves to distribute primary air, makes starting easier, and makes it possible to burn the oil to the bottom. Field experience indicates that with downdraft tubes starting is somewhat easier than with internal chimneys. Both devices tend to clog with soot. This soot is usually washed out of the downdraft tubes at each refilling, but slots in the internal chimney must be cleaned (usually with a stick) to keep the holes open and to maintain the full burning rate. Wicks facilitate the starting of new or cleaned heaters, but if left in place apparently increase carbon residue. Until a few years ago, the cover of the 9-gallon round bowl was fur- nished with a lighting cup underneath the draft opening. A V-type de- 30 California Agricultural Extension Service [Cir. ill flector, fluted deflector, or downdraft tube was fltted into the lighting cup. Some of the cups had two holes pointing toward the base of the stack, some had one hole, and some were without holes. These holes cause a great increase in smokiness for all stacks placed on such covers and the holes should be plugged with small sheet-metal disks which can be obtained for this purpose. /.o 0.8 I I 0-6 O.^ 0.2 / / )> i— ■"' i i^^*^ .....A n ' ^ 3 ^ S 6 7 Fig. 9. — Smokiness of the Kittle drip-type heater. Two general types of stacks are available for use on these bowls. These are the lazy-flame or short stacks (fig. 6), and the combustion-chamber stacks (fig. 8). Field experience indicates a preference for short stacks of the lazy- flame type. The main reasons advanced are that the heat is liberated close to the ground ; the stacks are easily cleaned ; the stacks last longer because the metal is not overheated ; and they are easier to light and regulate. The lazy-flame stacks have a narrower range of burning rate than some of the more expensive stacks of the combustion-chamber type. However, the maximum burning rate for good operation is adequate if enough heaters are provided per acre, and it is preferable to burn more heaters at rates for which operation is satisfactory than to burn fewer heaters at excessive rates. 2. Drip-type heater : The Kittle is the only drip-type heater in general use at the present time. It differs from the distilling type heater in hav- Protection of Orchards Against Frost 31 ing fresh oil fed constantly to the burner, either from a reservoir at one side or from an oil line connected with a small reservoir in which the oil level is controlled by a float (fig. 9) . The operation of this heater is satis- factory at burning rates of Vs to % gallon per hour, with a possible maxi- mum of % gallon. The smokiness when burning a good grade of the Marine Diesel fuel is also shown in figure 9. It is essential that the fuel trough in the burner be kept level and that the residue be scraped out from time to time. The usual difficulty is that the trough fills with coke Fig. 10. — Generator-type heaters. The Fugit is shown at the left; the California at the right. (1937 models.) and then the oil overflows into the base pan where it ignites and causes a great deal of smoke. High burning rates aggravate this difficulty. Poor- quality fuels leave more residue and cause the trough to fill up in a short time. Some samples of Marine Diesel fuel, bunker grade (27+° A.P.I.) can be burned successfully if the trough is frequently cleaned. If one wants to be sure of avoiding difficulty with samples which may be deliv- ered in any one main grade of fuel, it will be best to specify fuels of the "kero-distillate" type. 3. Generator-type heaters : In generator-type heaters, commonly known as "pipe-line" heaters (fig. 10), the fuel is passed through a small evapo- ration chamber in which the fuel is volatilized. The fuel vapor then issues from an orifice, as a vapor jet, where it burns in a self-induced draft. Special fuel is essential for the heaters now in use and the range of burn- ing rate is very limited. The minimum burning rate is too high for the best utilization of the heat in an orchard. The cost of installation is so high that there is a tendency to install too few heaters per acre and burn them at relatively high burning rates. They are likely to be wasteful of fuel, especially on nights which are only moderately cold. These heaters are popular because they avoid the necessity of hauling oil through the orchard. They have many advantages, but have not yet been developed to 32 California Agricultural Extension Service [Gib. m the point where they can be operated without systematic regulation and attention during the night. 4. Solid-fuel heaters : Less than 10 per cent of the orchard heating is done with heaters burning petroleum coke, or briquettes made from coke, carbon black, or other materials. Briquette heaters are of two types : a small-sized heater (fig. 11) in which the burning rate is difficult if not Fig. 11. — Briquette heater with funnel to facilitate filling, and tub for fuel storage. (From Bui. 398.) impossible to control, and which varies between 1^^ and 3 pounds of fuel per hour ; and a larger type with some control of burning rate and a maximum burning rate of about 5 pounds per hour. The amount of heat available from 5 pounds of briquettes or coke is approximately equal to that developed from burning % gallon of oil. Therefore, a large-sized bri- quette heater at its maximum burning rate will produce heat equivalent to that produced by an oil heater at average burning rates, and some of the small briquette heaters may produce heat at a rate as low as one-third of this. The number of heaters required will be inversely proportional to the average burning rate of the kind selected, and will vary between 50 and 100 per acre. The first cost of satisfactory equipment for frost pro- tection is less if solid-fuel heaters are used than in the case of oil heaters. Protection of Orchards Against Frost 33 The former deteriorate much more rapidly under use than the latter. They may, however, be a good choice where the frost hazard is not great and where general fixed charges are a more important item of total cost than operating expense. There is no practicable method of extinguishing most of the briquette heaters and they must be kept refueled until sunrise, after which time the remaining fuel burns out without being useful. Therefore, some waste //i/-Lo ^^SO A/(7i!/'o/7a/ Jr Lot/t^&r 2 3 4 S 6 Fig. 12. — Smokiness of cleaned lazy-flame stacks ; the band combines the performance characteristics of all stacks of this type. of fuel is inevitable and operating expense is higher than with oil heaters. Solid fuel can be more easily handled than oil by growers who do not have horses, trucks, or tractors available. Real and Smoke Output of Oil-hurning Orchard Heaters. — The total heat output of all orchard heaters is practically the same for a given weight of oil burned, but there is a large difference in the ratio of total to sensible heat — manifested by the feeling of warmth — because the pro- portion of radiant heat is greatest in a horizontal plane from large, tall, hot stacks. The smoke output from different heaters varies greatly and the operating characteristics differ. The choice of heaters and stacks, therefore, should be based mainly on minimum smokiness and on reliable operation. 34 California Agricultural Extension Service [Cir. m Figure 12 shows a band covering the normal smoke production of rea- sonably clean stacks of the lazy-flame type when operated in calm air. The figure combines the performance characteristics of all stacks of the lazy- flame type now available on the market. The Riverside Junior Louver 18-inch stack exhibits cycles of smokiness especially if burned at rates above or below the optimum, which is about % gallon per hour. It will therefore show a smoke output considerably above the indicated band for /.o as 0.6 \ ^ 0.4 \ i Coi//?ii/ Ord/^a/7ce //>/i'-x y| J ^ ^ B TTTTT^ ^ ^ p WWWV WWWW \\\ W^j^" #^ .Jc//7?6o Cone as 4 S 6 7 3i/r/?/'/?y ra^e^ />o^//?c/s per /^o^r Fig. 13. — The normal band of smoke output for reasonably clean stacks of the combustion-chamber type of heater. brief periods when in a smoky cycle. All lazy-flame stacks are smoky whenever low burning rates or soot accumulations cause the flame to drop to the lowest row of holes or louvres in the stack. Stacks with combustion chambers are usually designed for a wider range of burning rate than is possible with lazy-flame stacks. As com- bustion-chamber size is increased, the mixing of the oil vapors with air becomes more of a problem, so stacks have been built with sharp conical sections and with internal gadgets such as flame spreaders to aid in the mixing. Such changes have not been wholly beneficial since they have increased the difficulty of cleaning the stacks. All combustion-chamber stacks must be kept clean and the burning rate must not be allowed to exceed the capacity of the combustion chamber. These stacks show a very Protection of Orchards Against Frost 35 rapid increase in smokiness when the capacity of the combustion cham- ber is exceeded, at which time the performance may be as bad as that of ordinary smudge pots. ^ s /.6 /.4 /.a /.o a4 as 1 1 1 1 1 1 1 1 / Coc//?6j/ Orc//'r?^r?ce //^/<^-^ / / / / / / f / / / / / ^ /^ / y" 3i/rr?/r?y r^^ff^ />oi//?a^s per /^06/r Fig. 14. — Smokiness of 6- and 7-inch Exchange Stacks. Figure 13 indicates the normal band of smoke output for reasonably clean stacks of the combustion-chamber type now available on the market operated in calm air. The Hy-Lo 148 Special operates as a combustion- chamber stack at low and moderate burning rates, and at high burning rates assumes the characteristics of the lazy-flame stack. At the higher 36 California Agricultural Extension Service [Cir. m burning rates its smoke output is normally near the lower edge of the band and it is responsible for the bump in the curve of figure 13 at 3% pounds per hour. The 6-inch and 7-inch straight stacks (Exchange type) are more erratic in their performance than the stacks with conical cham- bers such as the Jumbo Cone, the Lemora, and the Hy-Lo 148 Special. The 6-inch straight stack is easily cleaned and if clean is satisfactory up to about % gallon per hour. The diagram and smoke output with a clean stack is shown in figure 14. The same stack after being operated 20 hours without cleaning showed a smoke output too high to be plotted on figure 14. The actual performance was as follows : Burning rate, pounds per Smoke, grams hour per minute 3.6 2.1 4 2.4 5 3.4 6 4.0 (or more) This stack overheats and deteriorates rapidly unless kept at burning rates below 4 pounds per hour. The 7-inch straight stack, Exchange type, will operate satisfactorily over a wider range of burning rates than the smaller ones, as shown in figure 14. It also requires frequent cleaning and is subject to rapid de- terioration at high burning rates. Number of Heaters Required. — The number of heaters per acre, and the best way of using the different types will vary somewhat with their operating characteristics. The grower should not get away from the basic principle of a large number of small fires per acre. The number of heaters usually installed is 50 bowl-type heaters for oranges and 60 to 80 for lemons and avocados. The number may be reduced about 20 per cent if large areas are heated, and must be increased, especially around the borders of orchards, in cold spots, and in colder areas. For deciduous fruits the grower should be prepared to maintain fires at from 40 to 50 locations per acre under severe conditions. This would require a minimum installation of 40 heaters per acre having a capacity of 5 or more gallons per heater or about 100 to 150 of the small containers which burn empty in so short a time that some must be held in reserve for burning later in the night. Solid-fuel heaters are not well adapted to the heating of deciduous orchards because blossoms and small fruits freeze quickly. When a drop in temperature is sudden, it is impossible to develop heat rapidly enough with solid-fuel heaters unless excessive amounts of kindling are used. Usually older trees are easier to protect than young. It is important Protection of Orchards Against Frost 37 from the standpoint of effective heating to have enough heaters so that it is unnecessary to exceed the range of burning rates over which they oper- ate best. The most efficient burning rate from the standpoint of heat con- servation in the orchard is probably from % to % gallon of oil (2% to 3% pounds) per heater per hour. A number of heaters, including all of the present generator heaters and the Jumbo Cone heater, will not burn satisfactorily at rates as low as % gallon per hour. Such heaters are more effective for border protection and for severely cold nights. One large lemon orchard is equipped with heaters having stacks of large combus- tion chambers and lazy-flame stacks alternating. The lazy-flame heaters are used under ordinary conditions and the heaters with combustion chamber stacks are used when conditions become more severe and higher burning rates are desired. Some growers have found it practical to equip their orchards with generator heaters at the rate of about 20 per acre and to fill in between with 20 bowl heaters. The 20 generator-type heaters take care of ordinary nights, and since the bowl heaters are not used very often the hauling of oil will not be very difficult. This method of operat- ing has the disadvantage that it utilizes heaters of high burning rate on nights when lower burning rates would be more desirable and more eco- nomical of fuel. Filling Equipment. — Facilities must be provided for refilling the heat- ers after each night of burning. Oil is ordinarily distributed in tanks which may be horse-drawn if the capacity is 400 to 500 gallons ; 600- to 800-gallon sizes may be mounted on a light truck ; and those of even greater capacity may be on a trailer pulled by a tractor. In some in- stances these trailers have track treads instead of wheels. While the aver- age oil consumption will be less than 100 gallons per acre per night, the maximum oil consumption may be around 400 gallons per acre per night. It is therefore essential that everything be provided which will speed up the refilling operations. Oil is distributed to the heaters from the tanks either through 1%-inch to 2-inch hoses or carried in 5-gallon pails. Some operators have both hose and faucets connected to the tank, filling heaters near the drive from the hose and heaters in the second row by carrying the oil in pails. Data from a limited number of records taken in an or- chard-heating survey conducted in 1937 show that 45 per cent of the growers used pails and distributed an average of 131 gallons per man per hour ; 28 per cent used hose and distributed 180 gallons per man per hour ; and the remaining 27 per cent used the combination system and distributed 139 gallons per man per hour. Table 3 shows the details as reported from the various counties. The table indicates that the range of performance for any one method is much greater than the differences 38 California Agricultural Extension Service [Cir. m between the methods, so it is difficult to draw conclusions as to what method will be most effective for a given set of conditions. The table is valuable as an indication of field experience in filling. Inasmuch as equip- ment is more difficult to obtain than hand labor, the amount of oil dis- tributed per tank and crew, per hour, is a more important consideration than the amount distributed per man per hour. In order to keep the tanks working to the best advantage, outlet pipes and fittings from elevated storage tanks and pumps from underground storage tanks should be of TABLE 3 Methods of Filling Heaters Reported in Orchard-Heating Survey of 1937 Pails Hose Pails and hose County Number of records Gals, per man-hr. Number of records Gals, per man-hr. Number of records Gals, per man-hr. Range Av. Range Av. Range Av. Los Angeles 20 2 19 24 9 8 52-266 75-125 60-183 60-207 100-280 78-150 130 100 115 136 184 105 7 3 11 11 21 109-350 100-185 44-194 89-216 110-400 225 150 127 162 205 I 20 10 4 73-350 90-165 52-186 28-200 125-205 171 114 Riverside San Bernardino Tulare Ventura 105 106 168 large capacity so that truck tanks can be filled rapidly. Discharge outlets from wagon or truck tanks should be sufficiently large so that all of the faucets on a discharge line can run simultaneously without diminishing the flow. Some growers even mount pumps driven by small gasoline en- gines on their truck tanks in order to speed up discharge rates. For slop- ing land it is desirable to have outlets from the front of the tank as well as the rear. This precaution is necessary in order to remove all of the oil when going down hill. The fuel for briquette heaters is usually carried loose in a cart, truck, or trailer ; and scoop shovels and large funnels are used to fill the heaters. In other cases briquettes are distributed during the summer time and stored in the field in apple boxes, old tubs, or other containers. Filling is then done by hand by pouring from the container into the heater through a large funnel, as is shown in figure 11. Refueling at night is done by taking fuel from boxes or other containers by hand. Kindling is carried in a separate container and is placed in the heater at the time of filling. Considering the field troubles with tank wagons and the heavy work of carrying pails of oil over muddy ground, the most promising improve- ment in filling technique seems to lie in piping the fuel oil from the ranch storage tank to well-placed hydrants from which all heaters can be reached by hose. Protection of Orchards Against Frost 39 Hydrant-Hose Filling Systems. — It appears that one man can distri- bute about twice as much oil per hour from a pipe-line system as he can as a member of a crew distributing from a tank. A brief investigation of hose-filling operations showed that filling was much faster with a short hose handled by one man than with two men ® 3) U^yj (i> :-.v®-.-.-c ® .^ ® .-.-® (^ (^\f(^'"(^.(m9\M"(^\((^ © ® © ^^ ® G QA© ^^A©: QaQ G^^© O Q#Q qV© G%G) QV© Q'®Q Fig. 15. — Suggested work pattern for filling heaters from hydrants. on a long hose. A %-inch hose 100 feet long was as much as any one man could handle effectively, and then only if the filling pattern followed was regular and with no danger of displacing heaters while dragging the hose from one location to another. Figure 15 shows a suggested work pattern for filling heaters spaced every other tree in every row. "With hydrants every two rows there will be no interference between heaters and hose if the route indicated is followed for either heaters in the tree spaces as shown in the upper part of figure 15 or for heaters in the tree rows as shown in the lower part of the diagram. In moving the hose from hydrant 40 California Agricultural Extension Service [Cm. ill //^(fr^/7^ P/'p/'/?y li7yo^/^ /br £6 roH^s o/' ^rees Tree s/><7c//7y ^^'xe2'. SO ^rees per (7cre 8S4 irees 1 ' ' (^r\r^r>. ~ " " ^ 1^4 } ^ ■^r; \ 4 z, y /^^/27/7/S ) ^ J .^ i^ III- ) t ^ 1- ; I M. A I, ^ ^ ^ f'/f' m/i 1 1 £ 1 ;^ , J! v^ .* £ ^ >» s £ "■\ 1 1 1 1 1 1 1 1 '"" 1 1 . ^ |- ^ ^ ^ \ f a ^1 ( U> r \ c '^ ('< >| g. r {, ^C, >c ^1 r '^(^ N # « t r^c ^f >■■<; w . [i_ _ _ __ _ _ _ _ _ _ _ _ — — \j^ J^ -f 3- Fig. 16. — Suggested layout for piping to hydrants in orchard heating. to hydrant both ends are hauled together and care must be taken always to pass between the heater nearest the hydrant and the last one to be filled. When filling is started the hose must be dragged around the first heater so as to lie in the space between heater rows as filling proceeds. A rate of oil flow of 10 gallons per minute is easily managed with a hose equipped with a hand valve at the nozzle. About half the time would be Protection of Orchards Against Frost 41 spent in moving from heater to heater. The fuel output per man in one such system installed late in 1937 was well over 400 gallons per hour."" A typical hydrant layout for 10 acres for 9-row hydrants is shown in figure 16. The recommended method of connecting the pump to the stor- age tank is shown in figure 17. The important difference from common (7a ^e ^a/i/e P(//7?p (j(7s e/^f/y?e {4ctfk) \ Ta orc/?arc/ 7bn/( Fig. 17.- -Diagram of connections and fittings of orchard pipe-line systems to pressure pumps and storage tanks. practice is that the by-pass is carried back into the tank to prevent over- heating and foaming of the oil when the pump is running but no oil is being used. The material cost for such a hydrant system is high, and the system should not be installed unless definite savings can be foreseen. Fiped-Heater Systems. — The engineering principles of piping oil to heaters have been studied and mimeographed recommendations are ^3 Daybell, Frank. A pipeline system for filling orchard heaters. California Citro- graph 23(6) ;251. April, 1938. 42 California Agricultural Extension Service [Cir. m available/* Numerous field installations have already been made in which fuel is piped to each heater for both drip and generator types."^^ Lighting. — Lighting is accomplished by the use of torches which drip burning torch fuel into the heaters. A torch consists of a container with a spout, a wick, and a wire gauze in the base of the spout. The wick is made of asbestos, usually wrapped in a piece of screen or a wire spiral. It is placed either directly in the spout, loosely enough so that the fuel will flow freely through it, or in a slot close to the end of the spout. In either case the wick must be so arranged that the fuel leaving the spout flows over or through it. The lighted wick ignites the torch fuel as it flows out. The most important feature of the torch is the protective wire gauze at the base of the spout and under no circumstances should a grower use a torch in which the wire gauze is lacking or defective. Fatal explosions have occurred with homemade torches. The usual gauze fire screen is of a fine mesh brass or copper screen and is generally soldered into the base of the spout. It works on the same principle as the miner's safety lamp. Slight explosions sometimes occur in the spout where there may be enough air to make an explosive mixture. The spout should screw tightly into the container against a metal gasket. A long, small tube for the air intake to the container helps to preserve a fuel- vapor atmosphere inside too rich to burn. Torches which are nearly empty should be refilled before further use. An electric flashlight is the safest light to use when filling torches at night. The most commonly used torch fuel is a fresh mixture of equal parts of gasoline and kerosene. This mixture will carry fire clear to the ground if poured from a burning torch held 3 feet from the ground. Oily clothes worn by the operators are dangerous during lighting operations or near open-flame heaters. Thermometers. — An adequate supply of accurate thermometers is an essential part of orchard-heating equipment. Temperatures vary several degrees in different parts of even small orchards. It would seem advisa- ble to have one thermometer for each two acres on the small properties, and one for each four or five acres on the large properties. The thermome- ters should be distributed through the orchard in some relatively uni- form pattern, with the locations marked in such a way that they can be easily found at night. One thermometer should always be placed outside of the orchard or on the upstream edge with respect to the air drift. " Division of Agricultural Engineering. Oil distribution systems for orchard heat- ing. Division of Agricultural Engineering, University of California. 33 p. 1937. (Mimeo.) Available at farm advisors' offices. ^^ McCracken, J. H., and K. B. Low. Centralized orchard heating system as installed at the Limoneira Eanch. California Citrograph 23(6): 246, 267. April, 1938. Protection of Orchards Against Frost 43 The horizontal alcohol- or toluene-filled minimum thermometer, which indicates the lowest temperature reached, is the most satisfactory type. The minimum temperature is marked by a glass indicator which is set at the top of the liquid column by turning the thermometer upside down. The thermometer is then placed with the bulb about 1 inch lower than the top of the stem and as the temperature falls the glass indicator is pulled down by surface tension. When the temperature goes up again the liquid flows past the glass indicator leaving it in such position that the top end indicates the lowest temperature reached since the last setting. Thermometers of this type, designed by the United States Weather Bu- reau especially for orchard-heating work, cost about $3.00 each. One or two of these in an orchard may be supplemented by somewhat cheaper vertical short-range instruments, designed especially for orchard use. Even carefully constructed thermometers are sometimes inaccurate, so it is advisable to have all thermometers tested when purchased, and be- fore each danger season. The Fruit Frost Service of the United States Weather Bureau performs this service free of charge in all districts where they have field men. If there is no local representative of the Serv- ice in the district, the county farm advisor sometimes arranges to test thermometers or have them sent to the nearest place where tests are made. If thermometers are so placed in the orchard that the bulbs are exposed to the sky they will lose heat by radiation, the amount depending on the type of instrument used, and the temperature indicated therefore will not be correct. This may vary as much as 3 or 4 degrees from the true air temperature. To provide accurate reading, thermometers must be placed in shelters which shade them from the sky. A very satisfactory shelter may be made from two 1-inch boards about 8 inches wide and 16 inches long. One board is placed at right angles to the other, one constituting the back of the shelter, the other furnishing a cover for the thermometer. The cover is hinged so the indicator can be set, which is done by elevating the bulb end of the instrument (fig. 18, B) or lowering the stem. Re- cently growers have been using a small double-roof shelter with venti- lated back and ends (^g. IS, A). This shelter provides for adequate air movement and protection against radiation. The standard exposure is at a height of 4% to 5 feet from the ground with the shelter facing north so as to prevent the sun from striking any part of the thermometer. All thermometer shelters should be painted white. The best thermometers for orchard use are graduated to a short range and may be injured by ex- treme heat. During the summer they should be removed from the shelters and stored with the bulb ends down, in a cool place. An open flame from match or torch should never be used to read a 44 California Agricultural Extension Service [Cm. ill thermometer ; the only safe light is that from an electric flashlight. The reading should be made quickly and the observer must take care not to breathe on the bulb of the thermometer. If the minimum thermometer is roughly handled or improperly ex- posed, the column of liquid may separate. The thermometer can some- times be put back into condition by attaching a 3-foot length of stout IT""""'""! i ^^H — „— — — ji^^^i Fig. 18. — A, Minimum thermometer in a simple shelter. B, Detail showing setting of the indicator. string to the top end and whirling it rapidly. Repairs should be made by a United States Weather Bureau representative if possible. The U-tube type of thermometer which reads both maximum and mini- mum is less reliable and is not recommended for use in connection with orchard heating. Fruit thermometers as shown in figure 19 are very valuable when used in conjunction with air-temperature thermometers in deciding the proper time to light orchard heaters. A satisfactory fruit thermometer should be filled with mercury. It should not have a metal shield to pro- tect the bulb because the metal will conduct heat away from the fruit. Instead, the fruit should be pierced with a nail before inserting the ther- mometer. Fruit thermometers should always be of the short-range type, preferably calibrated to half degrees and with the marks far enough apart so that accurate readings can be made. It is important that fruit thermometers be carefully tested and all which are inaccurate be dis- carded. Two or three fruits should have thermometers inserted and be used together to avoid misinterpretation. Protection of Orchards Against Frost 45 Thermographs. — Many growers have thermographs so that they have a continuous record of the temperatures they have maintained in the orchards. The temperature record charts shown in figure 2 are taken directly from an orchard thermograph record. The thermograph should be checked daily against a tested thermometer in the same shelter, with due allowance for time lag if the temperature is changing rapidly. Fig. 19. — A fruit thermometer inserted in an orange. Frost Alarms. — Many growers use frost alarms to sound a warning when the temperature reaches the danger point. Unfortunately, there are many frost alarms in use which are not dependable. The essential requirements of a good frost alarm are as follows : 1. It should operate on a closed circuit, attached either to a 110-volt lighting circuit or to a battery. (Different types of relays will be re- quired for the different sources of power.) 2. It should have a dependable relay which will close the bell circuit 46 California Agricultural Extension Service [Cm. ill when the current from the orchard unit is broken. The bell circuit should always be battery-operated, and if batteries are used on the orchard cir- cuit, a separate battery should be used for the bell circuit. 3. The orchard unit should consist of a carefully constructed large- bore mercury-filled thermometer with a fixed alarm contact point sealed in. Such alarm units are not adjustable. One thermometer unit must be purchased for each alarm temperature desired. Some first-class mercury thermoregulators are in use which can be reset at the factory. There is no dependable frost alarm at present on the market which can be ad- justed by the grower. All frost alarms should be tested each night during the danger season by breaking the orchard circuit with a switch provided for making the test. The thermometer unit should be placed in the coldest part of the orchard in an instrument shelter. More than one thermometer can be at- tached to the same alarm box. This makes it easy to have the alarm ring at different temperatures ; a switch in the house may be set for any se- lected unit to ring the alarm. Two wires run from the thermometer unit to the alarm box. Each additional thermometer requires an additional wire, the ground wire from the first unit serving the second unit also. FUELS AND FUEL STORAGE FOR ORCHARD HEATING Fuels for Bowl- or Distilling-Type Heaters. — More than 90 per cent of all orchard heating is done with fuel oil and most of the oil-burning heaters are of bowl or distilling type that use gas oil generally known as bunker grade marine Diesel fuel of 27+° A.P.I., selected for low pour point. The refiners list this grade of oil as Pacific Standard 200. Such a specification necessarily has rather broad limits so some selection should be made, first to get fuel with a pour point of 0° F or lower (as stated in Commercial Standard No. 3) ; second, to favor a low carbon residue; and third to favor a high degree of A.P.I, gravity. Table 4 lists the specification values for Pacific Standard 200 and the corresponding Commercial Standard No. 3'^ and also includes typical values as shown by regular stock samples of the former from seven Cali- fornia refineries, drawn in May 1938. A special carbon-residue test is included in table 4 which is practically a miniature lard-pail heater — the regular Conradson crucible — being left uncovered and the oil burned from a free surface without external application of heat. This test (described in the Appendix) is simpler than " United States Department of Commerce. Fuel oils, commercial standard CS 12-38, (4th ed.) IT. S. Dept. Commerce Natl. Bur. Standards. 1938. Protection of Orchards Against Frost 47 the Conradson on 10 per cent residuum, and appears to be as useful for judging orchard-heater fuel oils, although it is not an official test of the American Society for Testing Materials. In bowl- or distilling-type heaters the fuel is evaporated from a large free oil surface. The presence of a flame in the bowl atmosphere of fuel vapor causes the release of carbon regardless of the nature of the fuel. TABLE 4 Standard Limits of the Diesel-Fuel Grades Represented by Pacific Standard 200 and Commercial Standard No. 3. Characteristics Gravity at 60760° F, ° A. P. I. (American Petroleum Institute; Viscosity at 100° F, seconds Saybolt Uni- versal Viscosity at 35° F, seconds Saybolt Uni- versal Pour point, ° F Pour point on 25 per cent bottoms, ° F Open crucible self-burning residue, per cent by weight Conradson carbon, per cent by weight Conradson carbon on 10 per cent residuum, per cent by weight Flash point, Pensky-Martens, closed cup, ° F Distillation, [initial boiling point, ° F. American So- 10 per cent by volume, ° F ciety for Test- ing Materials D158-28 -• 50 per cent by volume, 90 per cent by volume. End-point, °F Distillation residue at 700° F, per cent Water and sediment, percent Sulfur, per cent Pacific Standard 200 and suggested limits Commercial standard No. 3 Maximum Minimum Maximum Minimum 27* 55 35 55 80* 0* ot 0.15 1.00* 150 230 110 425 600 675 600J 0.10* 0.10 0.75* 0.75§ 7 samples of Pacific Standard 200, May 1938 observed range 34.5to30.3 39.7to35.7 79.5 to 54.5 4-5 to -45 -f60to -10 1.21 too. 76 0.073 too. 016 0.60 to 0.08 214 to 172 438 to 370 485 to 440 542 to 498 668 to 603 732 to 661 6 to 2 * Additional limits suggested by the authors (not listed in the general standard) for orchard-heater fuel, pending further studies. t Specification limit when low pour point is required. t Minimum distillation of 600° may be waived if A.P.I, gravity is 26° or lower. § Specification limit when low sulfur content is needed. However, there are other types of residue formed in the bowl, the amount of which will vary with the nature of the fuel, probably being greater for the oils higher in naphthenes. The smoke output from free-surface burn- ing would also be greater for the oils higher in naphthenes, hence the oils with higher paraffin tendencies seem more desirable for bowl heaters. The paraffinic or naphthenic tendency in a fuel oil can be judged some- what by the relation between the 50 per cent distillation temperature and the open crucible self -burning residues. Figure 20 shows the test results 48 California Agricultural Extension Service [Cm. ill of all bowl-type heater fuel oils tested by the special self -burning open crucible method in relation to 50 per cent distillation temperatures. The 50 per cent temperature is chosen as being the most reliable indicator of the distillation characteristics. The temperatures are higher for the more paraffinic oils ; the burning residues are greater for the more naphthenic oils. A 50 per cent distillation temperature of 530° F appears from field reports to be the maximum acceptable for the more paraffinic fuel oils. t?2 4 06 OS /O /.e // /6 /S 20 22 £4 2 6 2S SO J.£ 34 3-6 J3 10 Open crucible residue, per cent Fig. 20. — Relation of open-crucible residue to 50 per cent distillation tempera- tures for samples of Diesel oil for boAvl-type orchard heaters. Those more naphthenic might be excluded by limiting the burning resi- due to 0.9 per cent maximum. The more naphthenic fuels with much higher burning residues up to about 1.7 per cent maximum have been used by growers without unusual trouble, but the 50 per cent distillation temperatures of these oils are low. A maximum temperature of about 470° appears to be a fair limit for oils with 1.72 per cent residue. Most of the fuels tested (which were not complained of in the field) show a burning residue of less than 1.3 per cent and a 50 per cent distillation temperature below 500°. Fuels with the lower residues and low distilla- tion temperatures are preferred. The present field data are not sufficient to indicate whether the saving in heater trouble would warrant the pay- ment of a higher price for fuel oil, but it seems evident that extra troubles in orchard heating should be expected when using oils with higher self- Protection of Orchards Against Frost 49 burning residues than shown below for the corresponding 50 per cent distillation temperatures : Open-crucible self-burning ' 50 per cent residue, per distillation cent temperature, "F 0.9 530 1.3 500 1.7 470 For intermediate values of burning residue, intermediate distillation temperatures would be expected. The characteristics of the samples of fuel oils for which trouble was reported in January, 1937, are given in table 5, arranged in the decreas- ing order of self -burning residue. Fuels for Drip-Type Heaters. — The best fuel for the Kittle heater comes under Pacific Standard 100, but satisfactory operation can be obtained from Pacific Standard 200 if specific lots are carefully chosen for higher quality than average. In this type of heater, burning is from a small free oil surface but under a strong draft and excess air. In the Kittle there is very little space for residue accumulation so the main service requirement is to clean out the burner trough frequently to prevent overflow. Although many operators mentioned fuel trouble with Kittle heaters, only one of the oil samples received in 1937 was identified with drip heaters. This was said to burn well (self -burning residue 0.8 per cent). The manufacturer specifies fuel of the 32° A.P.I, gravity or better, and judging by the samples included in this group a maximum open-crucible burning residue of 1.0 would exclude most of the undesirable oils. No recommendation for an upper limit for the 50 per cent distillation temperature can be made at present because of the lack of adequate data. Fuels for Generator-Type Heaters. — In these heaters the fuel is evapo- rated in a small closed chamber. Ideally, the temperature of the genera- tor, when the heater is in regular operation, should be high enough to volatilize the fuel, but below that at which cracking will occur. These heaters can operate with generator temperatures a little lower than the end boiling point because the vaporized portion can carry some liquid out through the orifice. Present experimental information is insufficient to specify fuel characteristics, but in general the fuel should be a straight- run distillation product. Generator fuels are generally marketed as kerosene distillate or stove oil, and come under Pacific Standard 100 or Commercial Standard No. 1. (See footnote 16, p. 46.) Table 6 gives the 50 California Agricultural Extension Service [Cir. ill -fj a> ga "-3 ^ V (M O ■«*< lO o (h"© eC to CO CO CD O 3 ft '^ 1. §^ ^ OJ fto c c 83 1^^ .«t< CO 05 C^ -H u'o C CO lO -^ lO lO S^ -fJ 0) •3 ga 5 o^ CD CO 05 >0 C^ i-'o ^ 00 CO CD t- to -* ■* ^ -*! -i'!- O "5 "? >0 >0 OO «0 CC iCi .^ f^aS|° (M (N (M (M (M • a ^l o ^-S'^ ^•Sh^^ m o »o o >o '-' O^ . Co S cq (M - CO lO c^ esi ^ i-i o .J-! (-. a lo o >o o lo g-SHSf=. CO C; ^1 ^ m'c ^ g S § 8 -kJ -tJ 03 °^ o '^^ p 3 12; M >> •Ic^ ^1 00 CO Oi 1-1 -*l §1 OS -rt^ CO . t-H 03 i-H lO ^ ^ T-i c^ (M CO CO CO CO o° § ■^ ^ ° .2.2; to ■* -f CO ."ll 2::. CO CO 71 c o r« ft OJ H s sS 2 C u Protection of Orchards Against Frost 51 standardized limits for these specifications and also includes typical values of regular stock from three California refineries in April, 1937. Because it is desirable to have as wide a difference as possible between evaporation temperatures and cracking temperatures (cracking inside the generator chamber produces coke in the passage) fuels of the more paraffinic type are to be preferred. Of the 20 field samples of fuel for TABLE 6 Standard Limits of the Stove Oil Grades Eepresented by Pacific Standard 100 and Commercial Standard No. 1 Characteristics Gravity at 60760° F, ° A. P. I. (American Petroleum Institute) Viscosity at 100° F, seconds, Saybolt Uni- versal Pour point, ° F Conradson carbon, per cent by weight Conradson carbon on 10 per cent residuum, per cent by weight Flash, Pensky-Martens, closed cup. ° F Distillation, American So- ciety for Testing Materials, [End-point, Distillation residue, per cent . Water and sediment, per cent. Sulfur, per cent Initial boiling point, ° F. . 10 per cent by volume, ° F 50 per cent by volume, ° F 1 90 per cent by volume, ° F F.. Pacific Standard 100 Commercial Standard No. 1 Maximum Minimum Maximum Minimum 0* 0.12t 165 110 165 100 (or legal) (or legal) 420 350 410 550 450 590t trace 0.5§ 3 samples of Pacific Standard 100, April 1937 observed range 38.3to37.0 34 to 31 to -20 0.02 to 0.01 180 to 145 381 to 359 405 to 383 451 to 429 520 to 495 600 to 558 2.0tol.5 trace * Limit when low pour point is needed. t Limit is 0.05 per cent for sleeve-type blue-flame household burners. t Limit is 560° F for sleeve-type blue-flame household burners. § Limit when oil of low sulfur content is needed. generator heaters analyzed in 1937, a maximum acceptable self -burning residue of 0.5 per cent would insure getting oils of paraffinic tendencies. This limit would exclude only 1 of the 20 samples received and that one was reported as having given trouble. To recommend an upper limit for the 50 per cent distillation tempera- ture is not feasible at present. Field troubles have been reported on sam- ples with the 50 per cent distillation temperatures throughout the range of all the samples (435° to 485° F). The design of most generators has been altered from year to year, so that no consistent behavior is evident. Field tests have shown that close attention is required to operate the generator-type heater at a proper burning rate, hence many of the field 52 California Agricultural Extension Service i^^^- m 11 "3 00 Oi CO •* CO r~-. i3> oo o CO •^ lO tK lO CO CO 00 05 00 — , ? a > •M (M ■*" c^ s § s s s CO o O 05 e<: o CO »o ": lO »o »o lo «3 O »o "* »o •o ■§ <=>^ ^f^ 05-0 Q-o c 2 c r « 2 S^ « 3 e*5 o t^ OJ a> »o CO CO t^ o CO CO O CO »o Oi c: I^ 05 o a1 c» t^ CO t— oo t^ t^ CO »o lO t-- eo m '^ •^ »n §H ■ -a" »o O I-- t— OO Oi ■* •* •«*< CO •»»<•>«< ""T ""ti CO CO ■* co ■<»< eo eo ■^ CO CO ^^ , -; a 5 3 t: o yn «c tn o o lO O lO kO lO U5 US »« o >o tn iO »o »o 2 2 (M t^ i-H rf< 00 l~- ^ a CO oo f^ao cs c^ o o § ":> »« «5 lO U5 irt) lO "5 O »c »o o tei >o o »o t;-| c< j« o 2 22 ^ ^ t- JO a eo CO u: 2 t- *c »o < o adson on on frcent uum, cent o O t^ t^ 00 t^ i^ r- 00 i-H o t- CO O -^ CO CD CO 05 00 3'ife M Oa Oi t^ CO lO CO CO CO CO CO eo -H O 00 oc 00 oo t— t^ o ■* •^ ■«*' ■<*< -* Tf< Tl< T*. Tt< -<1< •^ -^f •«t< CO eo CO eo CO CO d ^ o ^ S: ft Pour point A.S.T.M. maximum, OF* lO O »o lO o U5 O "5 O O o o o iC O O g i «5 o "3 1 CO 1 CO 1 1 1 CO Tt< CO CO CO 1 1 1 1 1 CO 1 T T ■>»< CO eo 1 1 1 7 CO 1 eo 1 (O — < '2^ P^ c (3 t- CO t~- Oi CO »« -"ti c< eo m O »0 r-H 8| s ■^ '* ■>«' ■* -* •^ "* ^ ^ ^ -* "* m'S ^= 12 p<4 o CO CO CC CO >o CO »0 uti lO lO m lO CO O CO ^ >o CD CC CO § CO CO CO CO CO CO CO CO CO CO CO CO eo CO CO CO CO CO eo CO >M '^ c ^ j^ 2 & ^ _o c 3 a c c rt ^ .« G &' &i fl ^' fel ^ s: i c c c c T T3 £ o 1=2 ^ ht yello y light ; low _o _c % )h k. i: u (H a> > d ' ^ S . >, ^ ^ ^ 4^ ^ Q m Xi X, M "5 bi « b D S M 13 M C3 t" 3 1 sn bl 5 1j h D ^ •^ c '^ P ;ji >^ h3 Q ;_] >H •-. K*^ Q l^hH p^ 2-< -wO CC •>* oc •^ O 00 00 O O CO o 00 eg 00 00 oc ■«* o CO Tf< cSP ir CO CC OC Co' CO t^ 00 00 t^ t--i t^ t^ CO t^ t- 05 oo o M ec eo CO CO CO eo CO CO CO CO eo eo eo eo eo eo eo CO CO o° § I 6 OJ .2; O" CC Ifl CO oo ^ >0 03 o 03 00 t-- 05 O ■* CO O t~- »o Ph 0 ■* CO ■* TP CO CO ^ iCi "3 Protection of Orchards Against Frost 53 complaints may be due to faulty operation or obsolete generators. Table 7 gives the entire 1937 list of field fuel samples for generator-type heaters, arranged in order of self -burning residue. Field complaints were made about fuels Nos. 29, 25, and 53. Pour-lack Oil. — In bowl-type heaters the character of the oil changes as burning from the surface proceeds because the lighter fuel fractions are burned more readily than the heavier and some polymerization oc- curs in the bowl. As an indication of the magnitude of this change figure 21 gives the distillation curves for the original poor oil and after burning __ — - " ST ^^ 1^ — "" ■— ■^ ^ ^ >•/ A fax .a P. 4 t ,rr ''/ ,^/ f^ ^ 5"?. 6 fo ^*' ^ .■sL. '> > ■^ Vt r.Z \^ "" ~ ~ ~ 1 b ^ •t^ 5. 90 ^, on >u b^ e L jr. i ?./ y u^ ^ L ^ -if 01 /V on e L Hu 1S.W /"/ J/Z e ^ •^ ^ 1 (U >(( ^h rOi oh m ff ^5; /// in ^* ** x* /\ y fi ^ ^ > X ^ R' F 1 ft -f r? to Y /r / lev t t' ■^ 1 ^ K f^ ifS o( t- 5f % to c r-- — 1 ^^ ft?. :7 / I.. u ^ ^ U-J s ' ) 5? ,4 ^ -- "^ l« r y ,♦• ** v^ p ^ ^4S0 / ^ ^ «c f1 '7 // ^ Oi 'it 'la *io / c t *- u P. s O'A -i ^2L )'/ ■ ^ n xh ai" d n& at »A '/ 6t 400 B //v ^ei 'if '6 ch eu ^ ra 'et *■ 'n\ 'ia u oif "Q P'' tin f-j 95 'F ^ Bi eit hei ia t -/4 ■3 } 350 300 1 2 3 4 s £> 6 7 8 9 /c /'ipr c<»/7^ v^ V. "n \ s^ ]6uiujf>g VffV~9uo 1 \ > V r ^) ^ / wB///c//jS ^^/^" ^^ \ J ■ — Suw. 91/4 OJ fJ f \ mq 9Jop-\ 94fi9(/ OS ^ ^N >^^^ / > J ; < {AJ3P04S7, SJ9JP9VJ0 euiujnq] 4fP(/-9uo^ ; / / ; ^ / > ,/ .^-jy Buiujnq f 'SMOJ J9fiJ» J94P9VJ0j /' / y Jj r 7 apimo f 1 .^ ^ y P94llSff 9J9A 'OJlfUf 9J ' SJ»4P»(/ 1 f6p94094l 1 \ ^-;^ r{' \ \ \ 1 \ i_ S S> J? •^ «M «M (j,) 9jn4PJ9tfu/»2 rr! CO )-t Cvj OS a. ^ iH P) ;h TtH o OO o TtH JSCtH C in 03 O % 03 1 rt ^ ^^3 o -«-' -M ixi 03 o O ^ 1=1 >^ ^73 :S fl fl ctf o ^^ -2 03 03* o rt £X O fl ;-i ctf o f ) x^ pTj rt tJO o CI) ,r) fl 13 a o o rt S bx o o ^ -aQ 2 'TJ fc(C ?>» o O s JH ,ra bX) tr i=l pi o coP^ (M .. bo s o f^ ;h P=H 62 California Agricultural Extension Service [Cm. in ords from instruments located in neighboring heated and unheated navel- orange orchards the same night. The solid line shows the temperature in the unprotected grove, and the dotted line the temperature record for a grove protected with fifty 7-gallon oil heaters per acre. The following facts should be noted: (1) The heat from the border rows of heaters merely delayed the fall in temperature of the heated orchard with the outside temperature steadily falling; (2) the burning of 25 heaters per 11 p. m. mdt 1 a. m. 2 a. m. 3 a. m. 4 a. m. 5 a. m. 6 a. 7 a.m. 8 a.m. rig. 24. — Temperature records in a heated orchard and at an outside check station on the night of April 13-14, 1919. (From: Young, Floyd D., and C. C. Gate. Damaging temperatures and orchard heating in the Eogue River Valley, Oregon. Monthly Weather Review 51:617-31. 1923.) acre from 10 :00 p.m. until 1 :00 a.m. did not maintain a safe temperature even though these heaters were burned at the maximum rate ; (3) fifty fires per acre did maintain a satisfactory temperature as long as they were kept burning; (4) failure to provide sufficient field fuel capacity by having heaters of larger size or some unlighted heaters in reserve, coupled with the waste of oil early in the night from improper regulation, was responsible for the severe drop in temperature starting at 4 : 15 a.m. when the first lighted heaters burned dry. The loss of fruit in this or- chard was not great but if fifty properly regulated heaters had been kept burning from 9 :30 p.m. until 8 :00 a.m., a much more satisfactory tem- perature control would have been obtained. Figure 24 shows superimposed thermograph records from neighboring pear orchards, one protected with small lard-pail-type heaters and the Protection of Orchards Against Frost 63 other unprotected. The records were taken during the night of April 13-14, 1919, when the temperature inversion was only 3° F in 35 feet and heating conditions were difficult. With no other regulation than an increase in the number of heaters burning per acre a satisfactory tem- perature was maintained all night with the exception of a few minutes SPM /O/^M ///M/ M^Z //?M P/7.M. 3/7M ^/7.M S/7.M 6/7M 7/7M <5/7At ■4P 40 J2 JO B8 26 \\ — — remperafure /n //rec/ ore/? arc/ TO /7eo fersj. \ (he ^orers r?or c(/rn/n^J heafers //n/7 J K n - — - - J fV of i/re a f /ei/e — J— : V. /f ^/c// /a/7 vH ^ V ■■ J \ •v. ■■■■-..J 1 ^ A i /! V \A / / S. ; ■ r p> ( 1 V 1/ D Fig. 25. — Continuous records of the temperature in a pear orchard on a calm, frosty night with considerable temperature inversion, showing the effect of orchard heating. Note that be- fore the heaters were lighted at 4 a.m. the temperature at the check station ran practically the same as that in the orchard equipped with heaters. On this night the stratum of air heated was only about 35 feet in depth. The increase in temperature of 7.5° F at the 5-foot level in the fired orchard with only thirty-six 5-quart lard-pail oil heaters to the acre burning was unusually large, owing to the strong temperature inversion and lack of air movement. (From: Young, Floyd D. Frost and the preven- tion of frost damage. U. S. Dept. Agr. Farmers' Bui. 1588:1-62. 1929.) following 4 :40 a.m. when the first-lighted heaters burned dry. Reserve heaters were lighted until a total of 54 per acre were burning. Economy of fuel usage was obtained by keeping the temperature just above the danger point. This pair of records should be contrasted with another pair (fig. 25) obtained in two pear orchards on the night of May 4-5, 1919, when weather conditions were almost ideal for heating and the temperature inversion was about twice as great as on the night of April 13-14. The burning of 36 heaters per acre of the 5-quart lard-pail type rapidly raised the temperature several degrees above the danger point. Less oil 64 California Agricultural Extension Service [Cir. m would have been consumed if the grower had lighted only 9 heaters per acre at the first firing and then more if necessary, as was done the night of April 13-14. If the grower is to maintain a safe temperature with the minimum fuel consumption, it must be done by intelligent firing based on a careful check of actual temperatures at frequent intervals. Lighting Methods. — Simple smudge pots are most readily lighted by removing the cover, or sliding it back to the required position, and then pouring burning torch fuel around the inside edge of the exposed portion of the container. Bowl-type heaters vary in their lighting characteristics. Especially after the heaters have been burned once, most of the lazy- flame types can be lighted successfully with the draft open only a little, if any, more than is required for normal operation. Most of the heaters with combustion-chamber stacks cannot be lighted without throwing the draft cover wide open. In this case it is necessary for a second member of the crew to follow the first and regulate the heaters about 3 minutes after lighting. The only way of avoiding this procedure is to equip heaters with automatic regulators. Drip heaters are lighted by merely adjusting the flow of oil to the burner and pouring burning torch fuel into the stack. Each type of generator or pipe-line heater should be lighted according to instructions supplied by the manufacturer. Briquette heaters should be carefully prepared so that they may be lighted quickly. They are easily lighted if they are properly filled and provided with a sufficient quantity of good kindling. There should be one layer of briquettes or coke on top of the kindling. The best kindling is made by soaking, in crankcase drainings for several days, small blocks of wood, such as trimmings from 2 x 4's and smaller pieces of lumber. A few of these blocks per heater will provide sufficient kindling. Other types of kindling consist of oil-soaked shavings or wood briquettes, pieces of automobile tires about 2 or 3 inches long cut across the casing, or a large handful of peach pits. The heaters are lighted from the top with burning torch fuel. It may be advisable to replace part of the kerosene of the customary torch-fuel mixture with crankcase drainings when lighting briquette heaters. It is usually not worth while to attempt the regulation of burning rate of briquette heaters. They should usually be refueled at intervals of 1% to 2 hours. It is best to add only one layer of fuel each time, so as not to depress the fire too much by the addition of cold fuel. Solid fuel heaters which have been in the field more than one season without burning should have the fuel sifted to free it from ^'fines'' and dust. Lighting heaters is a more difficult task on a cold night than on a warm Protection of Orchards Against Frost 65 day. If the night is damp, the covers may be frozen on and the drafts frozen shut when lighting should begin. Pliers and draft tools should always be at hand. Some growers with a large acreage to heat remove the caps and open the drafts before night, if the forecasts and general weather conditions indicate the probability of a need for rapid lighting. Regulating the Burning Bate of Heaters. — Growers should be pre- pared to give careful and systematic regulation to their oil-burning heat- Fig. 26. — Smoke output of a heater capable of clear burning. A, Start- ing — the draft wide open ; B, burning clear — properly regulated ; C, burn- ing smoky — at too high a rate. ers throughout the night. This will save fuel and will avoid many difficul- ties which develop with heaters. It is usually necessary to open the drafts gradually as the oil level drops in the container. "Smudge pots" or old orchard heaters which are inherently smoky have been replaced in many districts and in such districts are no longer the major source of smoke. The smoke nuisance is therefore primarily due to careless operation of heaters w^hich can be burned without appre- ciable smoke if kept clean and regulated within the range of proper burn- ing rates. Figure 26 shows clearly how a satisfactory heater can become a nuisance when the starting period is longer than necessary or when the burning rate is excessive or cleaning is neglected. Extinguishing Heaters. — The time for extinguishing heaters should be determined from the temperature shown by the check thermometer situated outside the heated area, or if there is no check tliermometer, by extinguishing a few heaters near a thermometer on the upstream side of the air drift and watching the temperature on that thermometer. The temperature is frequently below the danger point for an hour or more 66 California Agricultural Extension Service [Cir. m after sunrise, and the fires should not be put out too soon. It is necessary to keep briquette heaters refueled up to sunrise even though they cannot be put out and some loss of fuel vrill be inevitable. Most types of heaters are best extinguished by closing the drafts tightly and capping the stacks. Refilling. — Refilling should begin as soon as the heaters are completely extinguished and should continue until all have been filled. Many losses have occurred through failure to refill after each night of burning, even though only a small part of the oil had been burned. If, for any reason, refilling is not completed, and lighting must start early, the full heaters from the reserve of the previous night should be lighted first the next night, so when the most fires are needed, usually about 4 :00 or 5 :00 a.m., there will be some fuel in all of the heaters. Cleaning. — The necessary cleaning of stacks, drafts, internal chim- neys, etc. should take place at the time of refilling. If stacks fit loosely enough, they should be removed from the covers for cleaning, so that the soot is not pushed down into the bowls. The easiest way to clean the stack is to pass a wire brush through it. The best type of brush is made by attaching a wire buffing wheel to a rod. It is important that the 6-inch Exchange stack be cleaned after each night of burning ; most of the lazy-flame stacks should be cleaned this often. It is absolutely essential to clean internal chimneys frequently. This is especially true of those types with narrow slots, like the ones found in the old-style Riverside heaters. If these slots become clogged with soot, the burning rate decreases until after a time such a heater will not burn at all. Labor for Operations. — One man can light 2% to 7 acres, according to the number and type of heaters and the amount of regulating required. Fruit pickers and high school and junior college students furnish the usual source of labor for orchard-heating operations. Heaters should be kept serviced and in good order throughout the en- tire season. They should not be removed from the field until danger of frost is over. The fact that the frost forecasts are discontinued on Febru- ary 15 does not mean that the danger season is over by then. Damaging frosts have occurred in the citrus districts as late as April. Care of Orchard Heaters. — No systematic studies have been under- taken to determine the best method of caring for orchard heaters. Some growers give them very little care and others spend too much money on this item. The care given to heaters depends upon the method of handling during the summer. It seems that the most economical and probably the most satisfactory method is to leave oil in the heaters. The heaters can be left either under the trees, in selected parking spots within the orchard. Protection of Orchards Against Frost 67 or hauled from the orchard on sleds and stored along the edge. Full heat- ers can be easily moved with tools like those shown in figure 27. If the heaters are put under the trees, care must be taken to see that they rest in a level position. They should be at least a foot from the trunk of the tree to avoid oil seepage which might kill the tree. A leaky heater would dam- age the tree even at this distance, so they should be carefully inspected. Growers have usually, however, been unduly concerned about spilling Fig. 27. — Tools for moving square- and round-bowl heaters while they are burning or while filled with oil. oil in the orchard. The only place oil, even in considerable quantities, can do serious damage is under the tree, so that the trunk and root crown become oil-soaked sufficiently to kill the bark. If the heaters are stored full of oil, the troublesome problem of pour- back oil is avoided although there may be new difficulties on account of water condensation in the heaters. Pour-back oil should in no case be mixed with fresh oil in the storage tanks. If growers do not wish to leave oil in the heaters, special storage should be provided for pour-back oil. With most of the grades of fuel a residue will form in the bowl at the rate of 1 inch or more for each 25 gallons of oil burned. The residue re- sults in part from impurities in the oil but mostly from soot which forms in the bowl and in the stack. This residue cuts down the capacity of the heater and should be disposed of from time to time to avoid this diffi- culty. One means of accomplishing this is to accumulate the residue in certain heaters and burn these at a rate higher than normal so that the residue is gradually consumed. If such heaters are burned dry or nearly dry, the remaining material can be dumped. It is unwise to burn these heaters dry if the draft must be opened wide in order to keep them burn- ing. A wide-open draft would develop a sufficiently hot fire to damage the heater, and the smoke nuisance even with a good stack would be in- 68 California Agricultural Extension Service [Cm. m tolerable. Other means of handling the residue are to clean the heaters by pouring from one to another through a fine screen, and hauling the material which remains on the screen to the dump. A recent development is a machine which removes the oil and residue from the heater, cleans it by means of a centrifugal process, and returns it to the heater or to the storage tank. In some areas it is possible to hire this work done on a con- tract basis. When the heaters are empty, either temporarily while pouring from one to the other for cleaning, or removed from the field for summer stor- age, they should be inspected for leaks, straightened out where bent, and repaired in any way that may be necessary. It was at one time customary to dip the bowls in hot asphaltum before storing. Practically all new equipment at present comes in galvanized iron so that dipping is less necessary, although the life even of the gal- vanized bowls may be prolonged by dipping the bottom. The most rapid deterioration occurs in covers and stacks, which are subjected to a great deal more heat than the bowls. It is not worth while to use expensive paints but cheaper coatings will be found useful, such as cheap asphal- tum-base roof paint, or that obtained by diluting melted asphalt with orchard-heater oil. No paint will remain on the heaters and be effective after they have been burned. A cheap paint applied at the end of the season will protect the metal against rust, and will be especially effective on those heaters which are not lighted again for one or more seasons. No blanket recommendations can be offered concerning methods of care of orchard heaters. These must of necessity be determined by each grower for himself in accordance with the type of heaters he has, the amount of burning required, the humidity and rainfall in the district, and the most economical and efficient method of handling them in his orchard. With good care the depreciation of heaters should not be excessive ; the bowls and covers last from fifteen to twenty years, and the stacks from three to ten years. Protection of Orchards Against Frost 69 APPENDIX : OPEN-CRUCIBLE SELF-BURNING OIL RESIDUE TEST The following method of residue test was found to give results more closely reproducible than the Hoffman California Residue Test" which was intended to indicate the general desirability of a fuel oil for the bowl-type orchard heaters. The determination may be carried out with parts of the usual standard Conradson^^ apparatus. Scope. — This method of test is a means of determining the amount of residue on burning an oil under specified free-surface conditions and is intended to indicate the soot and asphaltic residue-forming propensities of the oil. This test is intended for orchard-heater fuel oils, usually of 27° to 38° A.P.I. Residues of 0.3 to 4.0 per cent have been found in 60 samples. Apparatus. — The following pieces of the standard Conradson Carbon Residue Test apparatus (A. S. T. M. designation: D 189-36) are used: 1. Porcelain crucible — weight 12 to 14 grams. 2. Circular sheet-iron hood — 5 inches bottom diameter. 3. Iron tripod (9% inches tall), to support the hood. In addition, a wooden wind-screen (no holes in bottom) and crucible shelf are used (fig. 28) . The height of the crucible shelf should be 7 inches measured from the base upon which the tripod rests. The dimensions of the wind-screen as shown can be changed slightly without altering re- sults appreciably as long as all drafts are excluded. The whole apparatus is placed in a low- velocity laboratory draft hood. Procedure. — The test is conducted as follows : The oil to be tested should be free from moisture or other suspended matter. Measure 12 cubic centimeters and weigh in a tared porcelain crucible ; place on the asbestos-covered stand and center it to the hood tripod. Then add 1.5 cubic centimeters of high-test gasoline and ignite the mixture at the sur- face with a small flame. The oil is allowed to burn spontaneously. There should be no external application of heat to the crucible. The time for complete burning is usually from 30 to 40 minutes. When the fire dies out, remove the crucible and place in a desiccator to cool. When cool, ^' Hoffman, A. H. Laboratory tests of orchard heaters. California Agr. Exp. Sta. Bui. 442:29-30. 1927. ^^ Committee D-2 on Petroleum Products and Lubricants, American Society for Testing Materials. Standard method of test for carbon residue of petroleum products (Conradson carbon residue), A.S.T.M. designation D 189-36. (In: A.S.T.M. stand- ards on petroleum products and lubricants.) American Society for Testing Materials, Philadelphia, Pa. 70 California Agricultural Extension Service [Cir. ill L 26'' ^W o/p s/pe/f ■3/g'^ p/j/6oc?rc/ Fig. 28. — Diagram of open crucible self -burning residue .apparatus. weigh and express the residue as a weight percentage of the original sample. Tolerances. — Weights of oil sample should be accurate to within 10 milligrams. The residue of 12 cubic centimeters of the high-test gasoline should not exceed 2.5 milligrams. Tests should be run in quadruplicate, and repeated if necessary, until the average departure of individual per- centages of residue differ by not more than 5 per cent of the mean. 17m-6,'39(7266)