REFRIGERATION OF FISH 543 



will now remain approximately constant for some time; that is, 

 until the fish are so frozen that they give up heat at a diminishing rate. 

 Then a series of changes in the opposite direction occurs. The brine 

 pipes absorb heat from the air faster than the fish give up heat; the 

 air becomes colder. The ammonia absorbs heat faster than the brine 

 gets it from the air, so the brine grows colder, likewise the ammonia, 

 and the pressure on the suction gauge drops. This continues until the 

 fish are frozen through and until there are only small differences in 

 temperature all around. The temperature at the finish may be 10 or 

 20° below zero, but it is not proper, for practical purposes, to call this 

 the temperature of the room. It would do the fish no good to cool 

 them to 50 or 100° below zero after they are frozen; in fact, there 

 is little doubt but that it would do them harm. The important 

 thing is not how cold they are eventually, but how fast they freeze. 

 As the rate of freezing, as we have seen, is determined by the dif- 

 ference between the temperature of the air and the temperature of 

 the fish while the freezing is going on, we may say that the speed 

 with which they freeze is determined by the coldness of the air 

 around them while they are freezing. The temperature of a sharp 

 freezer thus may be defined as the maximum, approximately con- 

 stant, temperature of the room after it has been loaded and the doors 

 have been closed. 



Failure to understand this principle often leads to difficulties and 

 poor operation. A case that recently came to the writer's attention 

 may well illustrate the point. A new freezer, approaching its max- 

 imum fish production of the season, contains a room with piping 

 designed for a storage temperature of about 0°. When closed and 

 empty it had a temperature of about 5° below zero. The management, 

 anticipating a shortage of freezer space, reasoned that as a tempera- 

 ture of 5° below zero was as good as that of a sharp freezer (when 

 it is loaded) the room might be used as a sharp freezer during the 

 rush of fish production by the aid of wooden frames for the pans 

 of fish. The management was surprised, of course, to find that 

 the temperature of the loaded room rose many degrees above zero, 

 and that the fish froze very slowly and were greatly damaged thereby. 

 There was not enough pipe in the room to absorb the heat from the 

 air as fast as the air absorbed it from the fish. The air grew warmer, 

 the difference between the temperature of the air and that of the 

 fish diminished, and the rate of freezing was retarded. This brings 

 out another important rule ; namely, each square foot of pipe surface 

 absorbs a definite number of thermal units per hour per degree of 

 difference in temperature. Under the conditions pevailing in a sharp 

 freezer this figure is about 5 B. t. u. From 25,000 pounds of fish 

 approximately 3,480,000 B. t. u. must be removed in order to freeze 

 it, and the required number of square feet of pipe surface must be 

 available to carry this amount of heat away in the given time. 



On the basis of this definition we have few exact data regarding 

 the degrees of temperature maintained in sharp freezers. Final 

 temperature in freezers ranges as low as 20° or 25° below zero, and 

 during the heavy load it runs as high as 20° above zero. The temper- 

 ature actually prevailing at any time in the sharp freezer depends 

 on the piping, temperature and rate of flow of brine, quantity and 

 initial temperature of fish, insulation, opening of doors, air circula- 



