42 



BULLETIN 98, U. S. DEPARTMENT OP AGRICULTUPiE. 



machine is in operation. It is further assumed that the walls, floor, 

 and ceiling are insulated for a heat transmission of 2 B. T. U. in 24 

 hours per square foot per degree difference of outside and inside 

 temperature of room. The butter will come from the churn at about 



58° F. 



B. T. U. 

 The heat that will come through the insulation is 600X2(75— 32)=51,600 

 The heat to be removed from butter is 2,000X0.5494(58— 32)=28,574 



80,174 



Allowing 25 per cent to cover the additional work required for 

 opening doors, lights, etc., we have a total of, say, 100,000 B. T. U. in 

 24 hours. 



Assuming a back pressure of 15.67 pounds, which is equivalent to 

 zero temperature ammonia, and a difference of temperature between 



Pig. 19. — Elementary diagram of direct expansion system. 



the refrigerant inside the piping and the layer of air surrounding the 

 same of 10° F., and that 1 square foot of pipe surface will absorb 

 about 10 B. T. U. per hour for each degree of difference between the 

 inside and outside temperature of the pipe, the number of linear feet 

 of l|-inch direct-expansion piping required for the room, is 



100,000X2.3 .. in , ,.,',- , ur 



' . in =96, or 10.4 cubic leet ol room space lor each linear 

 —4. X 1 >J X 1U 



foot of piping. With the direct-expansion system the work of 

 refrigeration practically ceases with the shutting down of the 

 machine. The frost which has collected on the piping itself will tend 

 to keep the room temperature down for a short time, but this is so 

 small that it may be disregarded; consequently, it is necessary to 

 run the direct-expansion plant continuously in order to maintain low 

 temperatures. 



