570 



U. S. BUREAU OF FISHERIES 



OTHER METHODS OF FREEZING 



The method already described in detail has come into widespread 

 ii:-e because it is simple, practical, and is equally applicable to all 

 varieties of fish. The chief and only serious objection to it is the 

 slow rate of freezing. In connection with the discussion of the 

 operation of sharp freezers numerous suggestions were made, in- 

 tended mainly to increase the rate of freezing, and if these were all 

 applied general improvement would result. But after everything 

 possible is done, sharp freezers are still far too slow to produce all 

 that could be desired because of the insurmountable difficulty of an 

 exceedingly poor heat conductivity and the low specific heat of air. 



In a sharp freezer most of the heat from the fish is absorbed by 

 air surrounding the fish, and the air in turn gives up its heat to 

 the pipes. The heat must pass from fish to air and from air to 

 pipes — it must be conducted. Of all substances, air is one of the 

 poorest conductors of heat known. Expressed in the metric system, 

 its conductivity, as compared with that of some other substances, is 

 shown in Table 16. 



Table 16. — Heat conductivity of various substances 



The specific heat of air likewise is exceedingly small; that is, a 

 relatively small quantity of heat causes a relatively large rise in the 

 temperature of air. The specific heat of a substance is defined as the 

 number of heat units required to raise a unit weight of that sub- 

 stance 1°. The specific heat of water is the standard, its specific heat 

 being taken as unity. The specific heat of air, as compared with that 

 of some other substances, is shown in Table 17. 



Table IT. — Specific heat of various substances 



The above figures are on the basis of weight and heat units, say, 

 pounds and British thermal units. That is, 0.24 B. t. u. would be 

 required to raise the temperature of a pound of air 1° F., while 1 B. 

 t. u., or four times as much heat, would be required to raise the tem- 

 perature of a pound of water 1°. But a pound of air occupies 11 

 cubic feet while a pound of water is approximately a pint. Only 

 a very small weight of air is actually in contact with the fish at one 



