Chapter 5 Principles of Refrigeration 95 Copyright by Goodheart-Willcox Co., Inc. Vehicle Design Refrigeration system performance can vary from vehicle to vehicle, depending on two factors. Th e fi rst is passenger compartment space. Th e larger the passenger compartment, the harder the refrigeration system must work to cool the area. Obviously, it is much easier to cool the passenger com- partment of a pickup truck or compact car than a minivan. Th e second design factor that can aff ect system perfor- mance is the vehicle’s interior and exterior colors. Darker colors tend to attract and retain more heat than lighter colors. In some areas, you can see a notable diff erence in system performance between two identical model cars with the only diff erence being their color. Summary Heat and cold do not exist as such, but are actually varying levels of heat. Heat transfer is the movement of molecular vibrations from an object with more heat to an object with less heat. Th e three methods of heat transfer are radiation, conduction, and convection. Conduction and convection are the main heat transfer means used by the refrigeration system. Th e three states of matter are solids, liquids, and gases. Changing from a solid to a liquid or from a liquid to a gas is called changing to a higher state, and requires the addition of heat. Changing from a gas to a liquid or liquid to a solid is called changing to a lower state, and requires the removal of heat. Th e refrigeration system makes the most use of vapor- ization and condensation. All liquids and gases contain sensible heat and latent heat. Sensible heat is heat that can be felt. Latent heat is hidden in the liquid or gas. Only liquids and gases con- tain latent heat. Solids contain no latent heat, but must absorb heat to change to a liquid. To change from a liquid to a gas, more heat must be absorbed. This heat does not change the temperature, just the state. The types of latent heat are latent heat of freezing, latent heat of melting, latent heat of vaporization, and latent heat of conden- sation. Latent heat is important to the operation of the refrigeration system because change of state removes a large quantity of heat. Superheat is any heat added after the change of state. Pressure aff ects the change of state. Th e more the pres- sure is increased, the higher the temperature at which a change of state takes place. Th e most important principles in the operation of the refrigeration system are heat transfer, change of state, latent heat, and the eff ect of pressure on change of state. Removing heat is an obvious benefi t of the air conditioner. Reducing the humidity also helps us feel more comfortable. Air movement removes more heat than would occur when the air is still. Th e substance circulating inside the refrigeration system is called refrigerant. Using the right refrigerant is critical to the proper operation of the refrigeration system. Th e most important property of refrigerant is it is able to evaporate and condense at the pressures produced by the refrigeration system. Th ere are four major parts to the refrigeration cycle. Th e fi rst phase is cooling, which takes place at the evapo- rator. Th e next phase is pressure increase, which occurs inside of the compressor. Th e third phase is the heat transfer phase, which takes place in the condenser. Th e fourth and last phase is the pressure decrease phase that occurs at the fl ow restrictor Th e main refrigeration components are the evaporator, compressor, condenser, and fl ow restrictor. Other refrigeration system parts are the accumulator, the evaporator pressure con- trols, the receiver-drier, the sight glass, and fi lter, when used. Th ere are two kinds of fl ow restrictor. Th e fi xed orifi ce is used with a cycling clutch. Th e expansion valve is usu- ally found on older vehicles. Metal tubing and fl exible hoses connect the other components of the refrigeration system. Vapor lines carry refrigerant vapor when the system is oper- ating normally. Liquid lines carry liquid refrigerant during normal system operation. Th e actual use of parts depends on the design, the manufacturer, and the age of the system. Th e heat load is the total eff ect of heat and humidity of the outside, or ambient, air. Other factors aff ecting sys- tem performance are temperature, humidity, sunlight, and vehicle design. Review Questions—Chapter 5 Answer the following questions using the information provided in this chapter. 1. In science, there is no such thing as _____. 2. Heat always travels to an object _____ than its surroundings. 3. Match the heat transfer method with the descriptions. (A) Convection (B) Conduction (C) Radiation _____ How the sun heats the earth. _____ Why a hot exhaust manifold could burn you. _____ How refrigerant cools the evaporator metal. _____ How a chicken is cooked over a barbecue pit. _____ Makes use of infrared rays. _____ Heat transfer by direct contact. 4. Th e three states of matter are _____, _____, and _____. 5. To change water to its next highest state, it should be _____. 6. Sensible heat can be _____. 7. Defi ne superheat.