FROST 



FROST 



1289 



actual cost of fuel-oil to the acre in the following way: 

 The quantity of fuel-oil necessary to raise the tempera- 

 ture 5 F. above the surrounding air in an orchard in 

 full bearing with wide-spreading trees will be approxi- 

 mately twelve and one-half gallons an hour to the 

 acre. This will be true if the air remains calm or does 

 not move more than 1 or 2 miles an hour. In young 

 orchards with the fruiting area low, practically double 

 the quantity of fuel-oil as given will be necessary. The 

 simple lard-pail type of heater, or, for that matter, any 

 type of orchard-heater providing a burning area of 

 about 44 square inches, will burn about one quart of fuel- 

 oil an hour. Fifty one-gallon heaters will burn approxi- 

 mately twelve and one-half gallons of fuel-oil an hour, 

 and if this oil costs 4 cents a gallon, the cost of an acre- 

 hour will be about 50 cents when all the pots are burn- 

 ing. In other words, under the above conditions, it 

 will cost approximately 1 cent an hour for each heater 

 used, providing this heater does not have a burning 

 surface greater than 44 square inches. It will require 

 less fuel to heat an orchard in the square form than 

 one of any other shape. The more the orchard-heaters 

 are massed in the form of a square, the better the 

 results. In fact, it is almost impossible to protect an 

 orchard of only a few rows because there is no mass- 

 ing of the heat generated on account of the lack of 

 braking effect on air-movement. 



The physics of orchard-heating. 



By common consent, the one-gallon or ten-pound 

 lard-pail type of orchard-heater has been made the 

 standard. This heater has a top diameter of 7J^ inches, 

 a bottom diameter of 6% inches, and a depth of 8 

 inches. It has a top burning surface of about 44 square 

 inches, an average burning surface of 40 square inches, 

 and under actual field conditions will hold five quarts. 

 As will readily be seen, the rate of burning fuel-oil 

 will not remain con- 

 stant owing to the 

 form of the pot. Since 

 the rate of burning 

 does not remain con- 

 stant, it is natural to 

 suppose that a cor- 

 responding change in 

 the orchard tempera- 

 ture-characteristic 

 takes place. In other 

 words, the difference 

 in temperature be- 

 tween the air outside 

 the heated area and 

 that in the heated 

 area does not remain 

 constant. The char- 

 acteristic for any 

 orchard-heater having 

 sloping sides will be 

 approximately the 

 same as for the standard lard-pail type. 



By repeated experiment it has been shown that the 

 standard lard-pail type heater will burn about two 

 pounds (two pints) of fuel-oil an hour, providing the 

 oil is neither too heavy nor too light gravity. A very 

 simple rule which will determine the burning time of 

 any fuel-oil in the standard pot is to divide the weight 

 of a gallon (in pounds) by two. This will give the num- 

 ber of hours a gallon will burn under actual conditions 

 in the field. 



For any other size heater of the lard-pail type, the 

 burning time will vary in accordance with the following 

 law: K=T (DH) 2 , in which T=the time of burning, 

 D=the mean of the top and bottom diameters, and 

 H=the depth, K=a constant. In other words, it has 

 been found that if the mean diameter times the depth 

 be squared and multiplied by the burning time (in 



1589. Smudging used to prevent frost-injury. 



hours) of a unit quantity (one gallon) of fuel-oil, the 

 product will be the constant for the same fuel-oil 

 burned under the same conditions, no matter what the 

 size of the pot may be, providing it is not too much out 

 of proportion. 



The calorific power of fuels. Analyses of various 

 fuel-oils on the market have shown that the heating 

 power averages about 18,000 British thermal units 

 to the pound. A pound of dry pine wood, or, for that 

 matter, the best oak wood, if perfectly dry, will generate 

 about 6,000 B.T.U.'s under perfect combustion; bitu- 

 minous coal under the same conditions will generate 

 12,000 B.T.U.'s. It will be seen, therefore, that the 

 ratio of wood, coal and fuel-oil is about as 1:2:3. A 

 cord of well-seasoned pine will weigh about 2,000 

 pounds and that of oak about 4,000 pounds. These 

 figures are, of course, only approximate, but will serve 

 as a basis for calculation in case anyone should desire 

 to use wood or coal for orchard-heating purposes. 

 Since the calorific power of fuel-oil is about 18,000 

 B.T.U.'s to the pound, the standard fuel-pot burning 

 two pounds of fuel-oil an hour will generate 36,000 

 B.T.U.'s. This is largely theoretical as, owing to the 

 character of the heater, combustion is by no means 

 perfect. Nevertheless, the amount of oil, as indicated, 

 is consumed, and will, therefore, be accounted for on 

 the basis of complete combustion. By repeated experi- 

 ment, it has been shown that the quantity of fuel-oil 

 necessary to raise the temperature 5 F. above the 

 surrounding air in an orchard in full bearing with the 

 trees forming a protective covering and the air calm 

 or moving not more than 1 or 2 miles an hour, is approxi- 

 mately twelve and one-half gallons or 100 pounds an 

 hour to the acre. This quantity of fuel-oil is approxi- 

 mately the amount which would be burned in one 

 hour by fifty standard orchard -heaters. In very 

 young orchards, or with a wind of 10 to 15 miles 



an hour, fully two or 

 three times as much 

 fuel-oil will be neces- 

 sary to maintain a 

 temperature 5 F. 

 above that of the sur- 

 rounding air. Under 

 average orchard con- 

 ditions, with the trees 

 in good bearing, the 

 maximum height at 

 which the bulk of the 

 fruit crop is borne is 

 usually not more than 

 12 to 15 feet above the 

 surface of the ground. 

 In many cases frost- 

 injury does not occur 

 above this height, the 

 cold-air stratum being 

 relatively thin. Dur- 

 ing periods of injuri- 



ous low temperatures, the atmosphere is very dry and, 

 therefore, calculations may be based upon the heating 

 of the air only, since the small amount of water-vapor 

 present at such times is of little importance. Taking an 

 acre as the unit area, and assuming that the height to 

 which the air must be heated to protect the crop is 12 

 feet, the total weight of the air inclosed within this space 

 would be 42,000 pounds. As stated above, a pound of 

 fuel-oil has a thermal capacity of 18,000 B. T. U.'s, 

 and since it will require 100 pounds of fuel-oil an hour 

 to the acre to maintain the temperature of the air 5 F. 

 above the surrounding atmosphere, 1,800,000 B. T. 

 U.'s or about 700 mechanical horsepower will be 

 expended an hour to the acre on 42,000 pounds of air. 

 But 1,800,000 B. T. U.'s would raise the temperature 

 of 360,000 pounds of water 5 F., and since the specific 

 heat of air is 0.24, it would raise the temperature of 



