Vapor cycle systems make use of the scientific fact that a liquid can be vaporized at any temperature by changing the pressure above it. Water at sea level barometric pressure of 14.7 psi will boil at 212 F. The same water in a closed tank under a pressure of 90 psi will not boil at less than 320 F. If the pressure is reduced to 0.95 psi by a vacuum pump, the water would boil at 100 F. If the pressure is reduced further, the water would boil at a still lower temperature; for instance, at 0.12 psi, water will boil at 40 F. Water can be made to boil at any temperature if the pressure corresponding to the desired boiling temperature can be maintained. Liquids that boil at low temperatures are the most desirable for use as refrigerants. Com-paratively large quantities of heat are absorbed when liquids are evaporated; that is, changed to a vapor. For this reason, liquid Freon 12 or 22 is used in most vapor cycle refrigeration units whether used in aircraft or in home air conditioners and refrigerators. If liquid Freon 12 were poured into an open container surrounded by standard sea level pressure, it would immediately begin to boil at temperatures above -22 F (-30 C). There would be a continuous flow of heat from the warm surrounding air through the walls of the container to the boiling Freon. Moisture from the air would condense and freeze on the exterior of the container. This open container system would work satisfactorily insofar as cooling alone is con-cerned. A drum of Freon could be connected to a coil and the vaporized Freon piped outdoors. A system such as this would provide satisfactory refrigeration, but the cost of continuously replacing the refrigerant would be prohibitive. Because of the cost involved, it is desirable to use the refrigerant over and over. To accomplish this, additional equipment, over and above that already mentioned, is required.

Vapor Cycle Theory

Refrigerant used in the vapor cycle re-frigeration system occurs as both a liquid and as a vapor. Conversion from a liquid to a vapor will occur at temperatures above - 21 F ( - 34 C) at sea level. If the refrigerant pressure is increased, conversion to a vapor will occur at higher temperatures. Maximum heat transfer efficiency occurs when the refrigerant is at the boiling point (the point at which the liquid will vaporize). The refrigerant must be delivered to the evaporator as a liquid if it is to absorb large quantities of heat. Since it leaves the evaporator in the form of a vapor, some way of condensing the vapor is necessary. To condense the refrigerant vapor, the heat surrendered by the vapor during condensation must be transferred to some other medium. For this purpose, water or air is ordinarily used. The water or air must be at a temperature lower than the condensing temperature of the refrigerant. At any given pressure, the condensing and vaporizing temperature of a fluid are the same. If a refrigerant that vaporizes at 40 F (5 C) is to be condensed at the same temperature, water or air at a lower temperature is needed. Obviously, if water or air at this lower temperature were available, mechanical refrigeration would not be required. As the temperature of available water or air is usually always higher than the temperature of the boiling refrigerant in the evaporator, the refrigerant must be condensed after it leaves the evaporator. To condense the vapor, its pressure must be increased to a point that its condensing temperature will be above the temperature of the water or air available for condensing purposes. For this purpose a compressor is needed. After the pressure of the refrigerant vapor has been increased sufficiently, it may be liquefied in the condenser with comparatively warm water or air. In a practical refrigeration circuit, liquid flows from the receiver to the expansion valve, which is essentially nothing more than a needle valve. The compressor maintains a difference in pressure between the evaporator and the condenser. Without the expansion valve, this difference in pressure could not be maintained. The expansion valve separates the high-pressure part of the system from the low-pressure part. It acts as a pressure reducing valve because the pressure of the liquid flowing through it is lowered. Only a small trickle of refrigerant fluid flows through the valve into the evaporator. The valve is always adjusted so that only the amount of liquid that can be vaporized in the evaporator passes through it. The liquid that flows through the evaporator is entirely vaporized by the heat flowing through the walls of the evaporator. This heat has been removed from the air being cooled. After leaving the evaporator, the vaporized refrigerant flows to the compressor where its pressure is raised to a point where it can be condensed by the condenser airflow available. After being compressed, the vapor flows to the condenser. Here, the walls of the condenser are cooled by the water or air; and as a result, the vapor is liquefied. Heat is transferred from the condensing vapor to the water or air through the walls of the condenser. From the condenser the liquid refrigerant flows back to the receiver, and the cycle is then repeated.