48 BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 
lj-inch pipe, this is equivalent to a uniform coating of ice fj inch 
thick. 
In raising the temperature of 1 pound of a 15 per cent solution of 
common salt from the freezing temperature of 12.2° to 27.6°, 13 
B. T. U. are required. For the same range in temperature of 1 pound 
of brine ice, approximately 135 B. T. U. are required. In other 
words, under the above conditions there is about 10 times more re- 
frigeration available in the 1 pound of brine ice than there is in the 
1 pound of brine. Consequently, by freezing a portion of the brine 
on the coils the hold-over period can be greatly increased. 
AIR-CIRCULATING SYSTEM. 
There is a further modification of both the direct-expansion and 
brine-circulathig systems, known as the indirect air-circulating sys- 
tem. This system is seldom used, except in connection with ice- 
cream hardening, where low temperatures must be obtained in a short 
time. It requires, in addition to the necessary members of the di- 
rect-expansion or brine systems, a fan, located in the cooling room 
over the coil bunkers and driven from without the room. The air 
is simply the circulating medium for producing the desired refrigera- 
tion and is forced by the fan over the cooling coils and down into the 
room below, where it comes in contact with and absorbs heat from 
the goods in storage. This results in a pure dry atmosphere, as the 
moisture content of the air is deposited on the surface of the coils in 
the form of ice and the greater portion of the microorganisms which 
were originally contained in the air are inclosed and rendered innoc- 
uous. 
If the initial and final temperatures of the air, together with the 
corresponding moisture contents, are known, the refrigeration re- 
quired for the simultaneous cooling and drying of the air can be ascer- 
tained. To cool the air alone necessitates the withdrawal, for every 
pound of air, of a number of B. T. U. equal to the product of the 
difference of temperature and the specific heat of ah at constant 
pressure. 
As it is common practice to measure air by volume, it is most con- 
venient to express its specific heat in a unit of volume instead of 
weight. Since, however, the density of ah varies with its tempera- 
ture, it is impossible to obtain a value that will apply other than ap- 
proximately to more than one condition. Taking, for example, air 
under standard conditions, density at 70° F., and a relative humidity 
of 70 per cent, a cubic foot weighs 0.07465 pound, and its specific heat 
is 0.0177. 
If uie initial temperature of the air in the room is 70° F. and it is 
cooled to, say, 20° F., the refrigeration to cool 1,000 cubic feet of air, 
neglecting the moisture, is 1,000x0.0177 (70 -20) =885 B. T. U. 
