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.
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.