A substance changes state when the inherent amount of heat is varied. Ice is water in a solid state and steam is a vapor state of water. A solid is changed to a liquid and a liquid to a vapor by applying heat. Heat must be added to vaporize or boil a substance. It must be taken away to liquefy or solidify a substance. The amount of heat necessary will depend on the substance and the pressure changes in the substance.
Consider, for example, an open pan of boiling water heated by a gas flame. The boiling temperature of water at sea level is 212ºF (100°C). Increase the temperature of the flame and the water will boil away more rapidly, although the temperature of the water will not change. To heat or boil a substance, heat must be removed from another substance. In this case, heat is removed from the gas flame. Increasing the temperature of the flame merely speeds the transfer of heat. It does not increase the temperature of the water.
A change in pressure will affect the boiling point of a substance. As the altitude increases above sea level, the atmospheric pressure and the boiling temperature drop. For example, water will boil at 193°F (89.4°C) at an altitude of 10,000 ft. At pressures below 100 psi, water has a boiling point of 338°F (170°C).
The relationship of pressure to refrigeration is shown in the following example. A tank contains a substance that is vaporized at atmospheric pressure. However, it condenses to a liquid when 100 lb of pressure is applied. The liquid is discharged from the tank through a hose and nozzle into a long coil of tubing to the atmosphere (see Fig. 4-10).
As the liquid enters the nozzle, its pressure is reduced to that of the atmosphere. This lowers its vaporization or boiling point. Part of the liquid vaporizes or boils using its own heat. The unevaporated liquid is immediately cooled as its heat is taken away. The remaining liquid takes heat from the metal coil or tank and vaporizes, cooling the coil. The coil takes heat from the space around it, cooling the space. This unit would continue to provide cooling or refrigeration for as long as the substance remains under pressure in the tank.
All of the other components of a refrigeration system are merely for reclaiming the refrigeration medium after it has done its job of cooling. The other parts of a refrigeration system, in order of assembly, are tank or liquid receiver, expansion valve, evaporator coil, compressor, and condenser.
Figure 4-11 illustrates a typical refrigeration system cycle. The refrigerant is in a tank or liquid receiver under high pressure and in a liquid state. When the refrigerant enters the expansion valve, the pressure is lowered, and the liquid begins to vaporize. Complete evaporation takes place when the refrigerant moves into the evaporator coil. With evaporation, heat must be added to the refrigerant. In this case, the heat comes from the evaporator coil. As heat is removed from the coil, the coil is cooled. The refrigerant is now a vapor under low pressure. The evaporator section of the system is often called the low pressure, back pressure, or suction side. The warmer the coil, the more rapidly evaporation takes place and the higher the suction pressure becomes.
The compressor then takes the low-pressure vapor and builds up the pressure sufficiently to condense the refrigerant. This starts the high side of the system. To return the refrigerant to a liquid state (to condense it), heat picked up in the evaporator coil and the compressor must be removed. This is the function of the condenser used with an air- or watercooled coil. Being cooler than the refrigerant, the air or water absorbs its heat. As it cools, the refrigerant condenses into a liquid and flows into the liquid receiver or tank. Since the pressure of the refrigerant has been increased, it will condense at a lower temperature.
In some systems, the liquid receiver may be part of another unit such as the evaporator or condenser.