CHAPTER 13 AIR-CONDITIONING SYSTEMS

Air conditioning is the treatment of air to ensure control of temperature, humidity, and dust (or foreign particles) at levels most suitable to personal comfort. A good example is the air-conditioning system used by astronauts; their air-conditioning units must supply all life-sustaining conditions to support their existence. In this chapter, we examine the basic principles of refrigeration, system components, troubleshooting, and the repair of these systems. Furthermore, in closing, the changes to automotive air systems and how they may affect you as a mechanic are also examined.

PRINCIPLES OF REFRIGERATION

Refrigeration is the process of producing low temperatures. It is usually associated with refrigerators or freezers rather than with vehicles. An understanding of heat transfer, basic refrigeration, pressure- temperature relationship, and the qualities of refrigerants is essential for a working knowledge of the air-conditioning system.

HEAT TRANSFER

It may seem a bit silly to cover heat transfer in connection with air conditioning. Keep in mind, however, that heat, like light, is a form of energy. As you remove light, a room grows darker. Likewise, when you remove heat, an area becomes colder. The process of transferring heat is the basis for air conditioning. Generally, when two objects of different temperatures are close to each other, heat energy will leave the warmer object and travel to the cooler. This is quite clearly illustrated in North America each fall and winter. As the rays of the sun become less direct and consequently give off less heat, we experience a drop in temperature. Cooler weather (refrigeration) results from this removal of heat.

Refrigeration applies a physical principle that is known to most of us through our everyday experiences. We have experienced the application of rubbing alcohol and its cooling effect. This example illustrates that an evaporating liquid absorbs heat. The evaporating moisture in the air on a hot day soaks up heat like a sponge. This removal of heat is exactly the same process used in automotive air conditioning. Heat is removed from the vehicle by an evaporating refrigerant and transferred into the atmosphere.

PRESSURE TEMPERATURE RELATIONSHIP

Different liquids have different boiling evaporating points; however, the boiling pint of any liquid increases when pressure is increased. When pressure is decreased, the boiling point is then decreased. This process of removing the pressure and allowing the coolant to boil is a vital part of any refrigeration system.

REFRIGERANT

With the exception of changes in state, gases used in refrigeration are recycled much like engine coolant. Different pressures and temperatures cause the gas to change state from liquid togas and back to a liquid again. The boiling point of the refrigerant changes with system pressure. High pressure raises the boiling point and low pressure reduces it. These gases also provide good heat transfer qualities and do not deteriorate system components. Two gases commonly used in the refrigeration process are Refrigerant-12 and Refrigerant-22. Use extreme caution when handling them. Refrigerant-12, otherwise known as R-12, Freon-12, or F-12, boils at -21.7°F (-29.8°C) when at sea level. Because of this low boiling point and its ability to pass through the system endlessly, R- 12 is almost the ideal refrigerant. (R-12 is currently being replaced by a refrigerant that is less harmful to our environment).

WARNING When you are working with R-22, keep in mind that this refrigerant contains methyl alcohol which can be a fire hazard. For this reason, automotive air-conditioning systems contain R-12.

HANDLING REFRIGERANT

R-12 is classified as a safe refrigerant because it is nonexplosive, nonflammable, and noncorrosive; however, you must observe certain precautions when