|Figure A Typical Refrigeration Plant|
Various types of refrigerating systems are used for naval shipboard refrigeration and air conditioning. The system that is used most often for refrigeration purposes is the vapor compression cycle with reciprocating compressors.
Figure B is a simple drawing of the vapor compression refrigeration cycle. As you study this system, try to understand what happens to the refrigerant as it passes through each part of the cycle. In particular, be sure you understand why the refrigerant changes from liquid to vapor and from vapor to liquid and what happens in terms of heat because of these changes of state. We will trace the refrigerant through its entire cycle, beginning with the thermostatic expansion valve (TXV).
Liquid refrigerant enters the expansion valve, which separates the high-pressure side of the system and the low-pressure side of the system. This valve regulates the amount of refrigerant that enters the cooling coil. Be-cause of the pressure differential, as the refrigerant passes through the TXV, some of the refrigerant “flashes” to a vapor (changes state from a liquid to a gas).
From the TXV, the refrigerant passes into the cooling coils or evaporator. The boiling point of the refrigerant under the low pressure in the evaporator is usually maintained at about 20°F lower than the temperature of the space in which the cooling coil is installed. As the liquid boils and vaporizes, it absorbs latent heat of vaporization from the space being cooled. The refrigerant continues to absorb latent heat of vaporization until all the liquid has been vaporized. By the time the refrigerant leaves the cooling coils, it has not only absorbed its latent heat of vaporization but has also picked up some additional (sensible) heat. In other words, the vapor has become SUPER-HEATED. As a rule, the amount of superheat is 8° to 12°F.
The refrigerant leaves the evaporator as low-pressure superheated vapor. The remainder of the vapor compression cycle serves to carry this heat away and convert the refrigerant back into a liquid state. In this way, the refrigerant can again vaporize in the evaporator and absorb the heat. The low-pressure superheated vapor flows out of the evaporator to the compressor, which provides the mechanical force to keep the refrigerant circulating through the system. In the compressor cylinders, the refrigerant is compressed from a low-pressure, low-tem-perature vapor to a high-pressure vapor, and its temperature rises accordingly. The heated high-pressure R-12 vapor is discharged from the compressor into the condenser, which is simply a heat exchanger that uses water or air as a coolant. Here the refrigerant condenses, giving up its superheat (sensible heat) and latent heat of condensation. The cooled refrigerant, still at high pressure, is now a liquid again.
From the condenser, the refrigerant flows into a receiver, which serves as a storage place for the liquid refrigerant in the system. From the receiver, the refrigerant goes to the TXV and the cycle begins again.
This type of refrigeration system has two pressure sides. The LOW-PRESSURE SIDE extends from the TXV up to and including the intake side of the compressor cylinders. The HIGH-PRESSURE SIDE extends from the discharge valve of the compressor to the TXV.