REFRIGERATION AND FREEZING OF FISH 337 



to indicate the true temperature of the rooms and where they may be easily read. 

 The rooms are electrically lighted, and some have pilot lights outside to indicate 

 when the lights are on inside. Concrete walls are usually not covered, but wooden 

 walls are whitewashed or painted. 



Mechanical Refrigeration. In nearly all large commercial plants for freezing 

 fish ammonia refrigeration machines are used, and the great majority of these are 

 of the compression type. 



A number of relatively small fish freezers and cold storages now have machines 

 which use "Freon-12" as the refrigerant. This refrigerant, or some other refrigerant 

 of the "Freon" group, should be used on freezers placed on board ship since 

 ammonia and other poisonous refrigerants are dangerous when used at sea. 



Absorption machines are used to some extent with excellent results; they 

 easily maintain very low temperatures, though are usually not as economical 

 to operate as the compression machines. If absorption machines are used, brine 

 is circulated; if compression machines are used, either brine may be circulated 

 or liquefied ammonia delivered directly to the coils in the rooms for evaporation. 

 This is known as the "direct expansion" system. In some cases where steam is 

 used to drive compression machines there is an amount of exhaust steam coming 

 off at low pressure which is used to operate an absorption machine, brine being 

 circulated through the rooms. Some very large fish freezers are refrigerated by 

 this combination. 



Electricity is being used more and more as a prime mover in fish-freezing plants; 

 it is less complicated, requires little attention, and, where nonpeak-load contracts 

 are made with the power companies, compares favorably in expense with the 

 steam-driven plants. 



When "direct expansion" is used for chilling the rooms, the liquid ammonia is 

 sometimes fed in through expansion valves at the top of the coils, the tempera- 

 tvire being regulated by the expansion valve. The so-called "flooded system" makes 

 use of much larger quantities of ammonia to fill the banks of coils to a high level 

 by expansion valves at the bottom. In this way advantage is taken of the much 

 better heat conductivity of liquid than gaseous ammonia in contact with the pipes. 

 Many plants which operate without flooding the coils often entirely lose the 

 benefit of much of the pipe surface installed. 



While it is impossible in the scope of this chapter to discuss the design, con- 

 struction, installation, and operation of the machinery, it is nevertheless of the 

 very greatest importance. Effort is frequently made to operate an expensive plant 

 with low-paid mechanics, who do not understand the machinery, either in theory 

 or practice. Naturally the results are highly unsatisfactory. In the smaller plants 

 automatic machines are often used (i.e., plants controlled by thermostats in the 

 cold rooms which stop and start the machinery as necessary to maintain the de- 

 sired temperature). However, few machines other than those of the ammonia 

 compression or absorption type are used for fish freezing. 



For the technology of refrigeration machinery reference should be made to 

 the numerous books on the subject. 



Preliminary Handling Aboard Boats 



On small inshore boats the catch of fish usually receives no preliminary chill- 

 ing, but is covered by tarpaulins or other covering and brought ashore. Many 



