42 
BULLETIN 98, TJ. S. DEPARTMENT OP AGRICULTURE. 
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 
to*/ ««ss«rc- High PRESZune 
Fig. 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 lj-inch direct-expansion piping required for the room, is 
100,000X2.3 
96, or 10.4 cubic feet of room space for each linear 
24X10X10 
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. 
