92 Auto Heating and Air Conditioning Copyright by Goodheart-Willcox Co., Inc. Cooling Phase—Evaporator We will begin the basic refrigeration cycle at the evapo- rator. Th e evaporator is the place where heat is transferred from the air to the refrigerant. Th e refrigerant enters the evaporator as a low pressure, low temperature liquid. Warm air passes through the evap- orator. Heat from the air is transferred to the evaporator metal by convection. Th e heat passes from the evaporator metal to the refrigerant by conduction. Since the pressure is low, the heat causes the refrigerant to turn to a gas, or vaporize. As the refrigerant vaporizes, it absorbs heat from the air. Th is heat becomes latent heat in the refrigerant vapor. Th e cooled air enters the passenger compartment. Th e refrigerant leaves the evaporator as a low pressure, low temperature vapor. Th e lines and accumulator will be cold to the touch. Th e vaporized refrigerant travels to the accumulator, on its way to the compressor. Refrigerant Superheat in the Evaporator Earlier in this chapter, superheat was discussed as it applies to steam. Th is section applies the principle of superheat to refrigerant. You know from previous sections that refrigerant evaporates (turns to vapor) when it absorbs enough heat. Th e refrigerant vapor can also absorb addi- tional heat, which is called superheat. For example, assume that in a particular evaporator, the refrigerant evaporates at 21°F (11.6°C). Th e temperature of the refrigerant as it leaves the evaporator is 31°F (17°C). Th is means that the refrigerant has 10°F (5.5°C) of superheat. Th e refrigerant has absorbed extra heat from the air passing over the evaporator. A small amount of superheat increases evaporator effi ciency. Superheat also ensures that no liquid refriger- ant reaches the compressor, where it could damage the compressor valves. An evaporator operating at maximum effi ciency will have about 8–12°F (4.5–6.6°C) of superheat at the evaporator outlet. Too much superheat, however, can damage the compressor. Excessive superheat can cause oil to collect in the evaporator. Th is occurs because the heated refriger ant molecules expand and cannot hold the heavier oil molecules in suspension. Th e oil molecules fall to the bottom of the evaporator and do not return to the compressor. If the oil cannot return to the compressor, the compressor will be damaged from lack of lubrication. Excessive superheat can occur because of a refrigerant undercharge. For this reason, it is very important to make sure the refrigeration system has the right amount of both refrigerant and oil. Pressure Increase Phase—Compressor Th e refrigerant vapor is drawn from the accumula- tor into the compressor. In the compressor, the refrigerant is packed, or compressed, into a smaller space. Forcing the refrigerant vapor to occupy a smaller space does two things: • It increases the pressure of the refrigerant. • It forces the heat in the vapor into a smaller area. Th e refrigerant enters the compressor as a low tempera- ture, low pressure vapor, and leaves as a high temper ature, high pressure vapor. Except on mild days, the vapor will be superheated. Th is means the vapor is hotter than the temperature at which it will condense. Th e lines containing the superheated refrigerant will be very hot to the touch. From the compressor, the high pressure vapor travels to the condenser. Heat Transfer Phase—Condenser From the compressor, the refrigerant enters the condenser as a high temperature, high pressure vapor. Air is constantly passing through the condenser, either through vehicle movement or pulled by the engine fan. Although this air is the same temperature as the air passing through the evaporator, it is not as hot as the compressed refrigerant. Heat in the refrigerant is transferred to the condenser metal by conduction. Th e heat passes from the condenser metal to the outside air by convection. At this time, any superheat is removed. Although the refrigerant is under high pressure in the condenser, the loss of heat causes the refrigerant to condense, or turn to a liquid. As the refriger- ant condenses, it gives up the latent heat it absorbed in the evaporator. Th is heat migrates out through the condenser metal to the atmosphere. Th e heated air passes through the vehicle radiator and exits out of the bottom of the engine compartment. Th e refrigerant leaves the condenser as a high pressure, high temperature liquid. Even though the liquid refrigerant is still hot, it has changed to a lower state and is now ready to be reused in the evaporator. On some systems, the refriger- ant will go through a receiver-drier, on its way to the fl ow restrictor. Refrigerant Subcooling in the Condenser Th e main purpose of the condenser is to remove heat from the refrigerant vapor until it condenses back to a liq- uid. An effi cient condenser and fan will lower the refriger- ant temperature at least 8°F (4.4°C) below its condensation point. Th is is called subcooling. Subcooling is important because it ensures that all refrigerant vapor is condensed back to a liquid and that only liquid refrigerant is present at the expansion valve or orifi ce tube. Since liquid refrigerant removes heat by changing from liquid to vapor in the evaporator, any refrigerant vapor enter- ing the evaporator reduces system effi ciency. Extra subcool- ing does not present a problem, since the system becomes more effi cient as more heat is removed from the refrigerant. Proper subcooling is especially important when the vehicle uses an orifi ce tube system. Systems with expan- sion valves have a receiver-drier between the condenser and expansion valve. Th e receiver is designed to separate vapor from liquid refrigerant. Orifi ce tube systems have no receiver-drier, and vapor could reach the evaporator if there is no subcooling. Some large vehicles with orifi ce tube systems have a tank between the condenser and expansion