PRESERVATION OF FISHERY PRODUCTS FOR FOOD. 
37o 
of the pressure exerted upon the gas or to the relative volume to which it is reduced, 
and this heat is withdrawn from the compressed gas by forcing it through coils of 
pipe in contact with cold water, the heat being transferred to the water. The gas is 
now ready to assume a liquid state, and in so doing transfers additional heat to the 
water surrounding the pipes. The liquid gas thus obtained is allowed to enter coils 
of circulating pipe at a pressure much lower than that required for retaining the gas 
in a licpiid state, whereupon it reexpands and extracts from the pipes and the sub- 
stances surrounding them a (piantity of lieatecpial to that which was previously given 
up by the gas during the period of condensation and liquefaction. The gas is then 
drawn from the expansion coils by the pniiqis at a pressure of 10 to 15 pounds above 
that of the atmosphere, and is again comiiressed iu the condensing coils at a pressure 
of 125 to 175 pounds to the scuuire inch, and the same cycle of operations is repeated. 
V arious modifications of the above, as well as auxiliary processes, have been introduced, 
but the principles are the same in all compression machines, the differences being in 
their application. 
The absorption system, which is comparatively little used at present, is based on 
the fact that many vapors of low boiling point are readily absorbed by water, but can 
be separated again by the application of heat to the mixed liquid; and the machinery 
in an absorption system differs from that in a compression plant principally in the 
substitution of an absorber for the condenser and iu applying heat to the ammonia 
water to drive off the anhydrous ammonia at a high pressure. 
Formerly, in order to avoid danger fi’om leakage of gas through the circulating 
l)ipes carrying the cold ammonia, those pipes were not passed through the freezing 
and storage chambers, but were stored in a large tank surrounded by some liquid 
w^'ose freezing point is very low, such as salt briue, or, when lower temperature is 
desired, a solution of chloride of calcium, and this cooled liquid is pumped through 
pipes circulating in the freezing and storage rooms. The improvements in the manu. 
facture of freezing machinery have resulted in the making of much tighter pipes, so 
that at present iu many freezers the ammonia coils pass directly through the freezing 
rooms, and in some instances they also pass through the storage rooms, but briue or 
chloride of calcium circulation is yet preferred for the storage rooms. 
In the mechanical freezing-houses there is a machinery room containing the 
boilers, mmpression pumps or absorption tank, according to the system employed, 
brine pump, etc. Apart from these and within well-insulated walls are the cold rooms, 
of which there are two kinds — one for the freezing of fish and the other for their 
storage after being frozen, the capacity of the latter being usually much greater than 
that of the former. In the freezing-room the circulating pipes containing the cooling 
material are i inch to 2 inches iu diameter and arranged in shelves or nests with hori- 
zontal layers 4 or 5 inches, and sometimes 10 inches, apart, ranging from the floor to 
the ceiling, the entire room being occupied with these nests, except sufficient space 
for moving about. Tliese pipes are sometimes made in separate coils, so that if desired 
the brine may be circulated through only a xiortion of the pipes, and there is generally 
a vertical row of pipes on each side of the freezing-room. The temperature depends, 
of course, on the quantity of green fish and the progress of the freezing xirocess, but 
with direct circulation, or using briue made of chloride of calcium as the circulatory 
medium, a temperature of — 10° F., or even less, is obtainable. In this room the green 
fish are frozen, and then removed to the storage rooms. 
