16 CIRCULAR 740, U. S. DEPARTMENT OF AGRICULTURE 



ity to pump out heat as fast as it enters the chamber, the desired low 

 temperature cannot be maintained. 



In extending the comparison, the factors determining the size of the 

 pumps are, in the case of the vacuum, (1) pressures, usually expressed 

 in pounds per square inch ; and (2) quantity of air, expressed as pounds 

 per minute. In the refrigerating system the factors are ( 1 ) tempera- 

 ture, expressed in degrees; and (2) heat, commonly expressed as Brit- 

 ish thermal units (B. t. u.). The term "B. t. u." (the heat required to 

 raise the temperature of 1 pound of water 1° F.) corresponds to the 

 term "pound" (in pumping air) , inasmuch as they both express definite 

 quantities of the thing to be handled. 



QUANTITY OF HEAT 



In dealing with refrigeration problems it is just as necessary to con- 

 sider the quantity of heat to be handled as to speak of pounds of air or 

 gallons of water when computing the necessary sizes of air or water 

 pumps for given jobs. Just as 1 pound represents a very definite and 

 measurable quantity of air, and it is still the same regardless of the 

 pressure under which it is placed, so 1 B. t. u. represents a definite 

 and measurable quantity of heat, and it too remains the same regardless 

 of existing temperatures. 



The refrigeration demand upon the machinery is frequently spoken 

 of in terms of "tons." This usage had its origin in a comparison of 

 refrigerating capacity, or demand, with the refrigeration obtained 

 from melting 1 ton of ice. As 144 B. t. u. of heat are required to change 

 1 pound of ice to water at the melting point, 288,000 B. t. u. are re- 

 quired to melt 1 ton of ice. Where it is necessary to remove 288,000 

 B. t. u. of heat in 24 hours, 1 ton of refrigeration is required. 



If, for example, a temperature of 32° F. is to be maintained in 

 a storage building, the refrigeration system will have to remove a 

 quantity of heat just equal to that which enters the building. The 

 heat entering may come from a number of sources. In the first place, 

 if the outside temperature is above 32°, some heat will come in 

 through the walls. This can be reduced by insulation, but not even 

 the best of insulation will exclude all heat leakage. If there are 

 cracks in the building, or if doors or windows are open and permit 

 warm air to enter, an increased quantity of heat will be introduced, 

 depending upon the outside temperature and the quantity of air. 

 Materials having temperatures above 32° placed in the cooled space, 

 will introduce still another quantity of heat, depending upon the 

 temperature, weight, and nature of the material. If the materials 

 are living, as for example, apples, they will produce heat continually ; 

 and this heat is in addition to that which they contained when first 

 put into storage. 



The heat from all these sources and from other incidental sources 

 combines into the total quantity of heat the refrigerating system 

 is expected to remove. If the system has sufficient capacity the 

 heat can all be pumped out. If the heat introduced into or produced 

 within the building exceeds the capacity of the refrigeration system, 

 some of it will remain in the fruit and cannot be taken out until 

 the rate of heat intake drops below the rate at which it can be removed. 



The quantity of heat that a refrigeration system can remove may 

 be increased or decreased by the conditions under which it operates, 



