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PRINCIPLES OF MECHANICAL
REFRIGERATION
 
A. GENERAL EXPLANATION
 
4A1. Brief statement of principles. The mechanical refrigeration system used on board a submarine is a vapor refrigerating system. In this refrigerating process, the refrigerant passes alternately through its liquid and vapor states. Such a refrigerant, therefore, must have special qualities. It must boil at a low temperature and it must be able to change its state readily from liquid to vapor, and vice versa. Above all, it must be a safe refrigerant.   Safety is more important in submarines than in other types of vessels.

In the liquid state, the refrigerant picks up heat from the substances or from the air in a space, and in so doing, vaporizes. The vapor carrying the excess heat is then transferred to another location where it discharges that heat, and, in so doing, reverts back to the liquid state.

The mechanical system in which the

Figure 4-1. Schematic refrigeration cycle.
Figure 4-1. Schematic refrigeration cycle.
 
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refrigerant is contained is a single airtight circuit of pipes and mechanisms through which the refrigerant is pumped continuously. The same quantity of the refrigerant is used over and over. This requires an input of energy which is supplied by an electric motor. The complete operation is called the refrigeration cycle, a schematic diagram of which is shown in Figure 4-1.

Figure 4-1 shows the disposition of the essential elements of the system for a complete refrigeration cycle. These elements include evaporator, compressor, condenser, receiver,

  and thermostatic expansion valve. The liquid refrigerant picks up heat and vaporizes in the evaporator. The vapor then goes to the compressor, where it is compressed to a pressure at which its temperature is above that of the water flowing through the condenser. The compressed vapor then goes to the condenser where sufficient heat is transferred to the water to cause the refrigerant vapor to condense. The condensed refrigerant, now a liquid, flows next to the receiver, and then through the thermostatic expansion valve to the evaporator. A detailed description of the cycle is given in Chapter 6.
 
B. TYPES OF REFRIGERANTS
 
4B1. Definition of a refrigerant. A refrigerant is a substance capable of carrying heat which is picked up at a low temperature level and is compressed to a high temperature, whereupon the heat can be removed by the condensing medium, which is either cold water or cold air.

4B2. Primary refrigerants. In the main circulating system described in Section 4A1, the refrigerant changes its state from a liquid to a vapor and back again, and is called a primary refrigerant. There are several refrigerants of this type, most of which boil at temperatures below the freezing point of water. They vary greatly in their properties and their cost. The refrigerant selected for any given installation depends upon the conditions therein. Among the primary refrigerants are: Freon 12 (dichlorodifiuoromethane), carbon dioxide, ammonia, sulfur dioxide, and others.

Naval vessels use only one primary refrigerant; dichlorodifluoromethane, the common name of which is Freon 12, or F-12 for short.

4B3. Secondary refrigerants. A secondary refrigerant usually consists of a salt solution, or brine, that is used to carry heat from the space to be cooled to the coils that contain the primary refrigerant. This type of refrigerant is generally used in large ice-manufacturing equipment, or where the space to be cooled is remote from the ice machine. Secondary refrigerants

  are not used aboard a modern submarine.

4B4. Refrigeration ton, RT. Some unit of measurement by which to measure heat elimination and to specify the capacities of different refrigeration machines is necessary. It has been found that a fairly large unit is required. This unit is called, variously, the refrigeration ton, ton of refrigeration, and ton refrigeration. Inasmuch as the expression ton of refrigeration leads one to think of the making of a ton of ice, and has caused much con fusion, the term refrigeration ton is gradually becoming the most generally used.

The refrigeration ton is based on the cooling effect of one ton of ice at 32 degrees F melting in 24 hours. The latent heat of fusion of ice (that is, the number of Btu required to melt 1 pound) is approximately 144 Btu. The number required to melt one ton is 2000 x 144 = 288,000 Btu. Hence, one standard commercial refrigeration ton is defined as the transfer in a cooling operation of 288,000 Btu in 24 hours. In smaller time periods, this rate of heat transfer for cooling would be the same as 12,000 Btu per hour, or 200 Btu per minute. It must be emphasized again that a ton of refrigeration has no reference to the manufacture of a ton of ice; a refrigeration ton is a rate of heat transfer, and not a weight.

 
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Figure 4-1a. Graphic diagram of mechanical refrigeration cycle.
NavPers 17022, Amphib 39

Figure 4-1a. Graphic diagram of mechanical refrigeration cycle.
 
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