44 BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 



cost of installing the brine-circulating system is more than that of 

 the direct expansion, as it includes, in addition to the members of the 

 direct-expansion system, a brine tank, brine coils, and a brine pump. 

 The power necessary to operate the brine pump must also be con- 

 sidered. The excess power required for the operation of the brine- 

 circulating system over that of the direct expansion of the same 

 capacity is that necessary for pumping the brine from the tank 

 through the cooling coils back to the tank; that required to overcome 

 the friction of the brine in the pipes in traversing the above cycle ; 

 the additional refrigeration necessary to make up for the heating 

 effect produced mechanically by circulating the brine, and the heat 

 absorbed through the brine tank and through the brine piping; that 

 required to make up for the reduced efficiency by having to operate 

 the plant at a back pressure sufficiently low to obtain a correspond- 

 ingly low temperature in the evaporator coils to compensate for the 

 second heat transfer encountered between the air in the cold-storage 

 room and the refrigerant. 



The average difference in temperature between the circulating brine 

 inside the piping and the surrounding air is, of course, much less than 

 is the case when the refrigerant is expanded directly in the piping 

 located in the cooling room, assuming the back pressure on the 

 machine is the same in both cases. The difference in temperature 

 between the refrigerant and brine depends on the insulation of the 

 brine tank, piping outside of the rooms, etc., and as this varies 

 between wide limits it is impracticable to calculate the amount of 

 brine piping very closely on the basis of heat transfer, as was done in 

 the case of direct expansion ; consequently it is the common practice 

 to install from one-half to twice as much brine piping in the brine- 

 circulating system as would be used if direct expansion were employed. 

 This allowance, however, is very liberal and is made to cover unfavor- 

 able cases where the insulation is poor; and further, any surplus pip- 

 ing serves to increase the efficiency of the plant. 



Allowing 50 per cent more brine than direct expansion piping we 



have 96x1.50 = 144 linear feet of l^-inch piping required to do the 



same work as the direct expansion, or 7 cubic feet of storage space 



per linear foot of pipe. This amount of piping, when filled with 



brine, will contain 1.5 cubic feet. During the night the average 



difference in temperature between the inside and outside air will 



be about 43° F., therefore the heat that will come through the 600 



square feet of surface of the room during 16 hours' shut-down 



•j -iii. 600X2X43X16 _ „, TT 



period will be ^i = 34,400 B. T. U. 



If the initial temperature of the brine during the shut-down 

 period is 16° while the final is 32°, giving a rise of 16 degrees, the 

 heat that will be absorbed by the 1.5 cubic feet of brine contained 



