82 Auto Heating and Air Conditioning Copyright by Goodheart-Willcox Co., Inc. You cannot repair a modern refrigeration system unless you know how it works. Knowledge of what is actually happening in a working refrigeration system is essential to determining what is going on when the system is not work- ing. In this chapter, you will learn the underlying principles and components of the refrigeration system. The Purpose of Refrigeration Before studying the operation of the refrigeration system, we must review its purpose. Remember from Chapter 1, the main job of the air conditioner is to cool the incoming air and remove moisture. Th e other purposes of the system are to remove dust and airborne gases, and to circulate air through the vehicle. Removing heat is an obvious benefi t of the air con- ditioner. Less obvious is the need to remove humidity. Humidity is the amount of water vapor in the surrounding air. Th e basic reason for humidity control is humans are warm-blooded animals. We produce heat as a byproduct of muscle movement. In warm weather, we produce too much heat, and the body tries to remove this heat by perspiring, or sweating. As perspiration forms on our bodies, it removes heat as it evaporates. However, if the humidity is high, the air will not immediately absorb additional moisture, and perspiration will remain on the body for a much longer time. Th is is why humid air feels more uncomfortable than non-humid air at the same temperature. When the air con- ditioner removes humidity, your perspiration mechanism becomes more eff ective. Air movement also aff ects heat removal. Th e more air moving over your body, the more heat is removed by convection. Airfl ow also removes moisture from the skin. In cold weather, this eff ect is called wind chill. Air move- ment through the passenger compartment when the air conditioner is operating removes more heat than would be possible if the air was still. Basic Principles of Refrigeration To understand how the air conditioner works, you must fi rst learn about some of the basic principles of refrigeration. Th e principles the refrigeration system uses are: • Heat transfer, especially by convection. • Change of state from liquid to gas and back again. • Sensible and latent heat of vaporization and condensation. • Th e eff ects of pressure and temperature. Th e underlying principles of refrigeration deal with some things you may have thought about. For instance, you may have wondered what causes things to be hot or cold. You may have also wondered why pressurizing the cooling system keeps the coolant from boiling out of the radiator. Th is part of the chapter addresses these questions. Heat and Cold When something feels cool to the touch, we say it is cold. When something feels warm to the touch, we say it is hot. What we refer to as heat and cold are actually varying levels of heat. In science, there is no such thing as cold. An object we call cold merely contains less heat than a warmer object. Th e only temperature point in which an object is considered to have no heat is referred to as absolute zero, which is -459°F (-273°C). Heat Transfer All substances are made up of atoms. Atoms combine to create molecules. Th e bonds between the atoms in a molecule cause it to move, or vibrate. Th e vibrations of an individual molecule are too small to be felt as movement. However, the vibration of billions of molecules can be felt as heat. Th e more molecular vibration, the more heat. Heat transfer is actually the movement of these vibra- tions from an object with more heat to an object with less heat. Heat always travels from a warmer object to a cooler one. Th ere are three methods of heat transfer, convection, conduction, and radiation. If all objects in the vicinity are at the same temperature, no heat transfer can occur. In actual practice, this seldom occurs and some movement of heat is always taking place. Convection Convection is the transfer of heat by air. Th e hot object heats the air, which then heats any surrounding objects. You have probably noticed the air several feet above the surface of an operating stove or barbecue grill feels warm. Th is is caused by convection of the heat from the cooking surface. Th e most common example of convection is the transfer of heat from a hot water heater to air passing over the heater’s fi ns, Figure 5-1. Figure 5-1. Hot water radiators, such as this baseboard heater, transfer heat to the surrounding air, which then heats the room. The fins increase the air contact surface to allow quicker heat transfer.