Chapter 5 Principles of Refrigeration 85 Copyright by Goodheart-Willcox Co., Inc. The Pressure-Temperature Relationship Pressure has a great eff ect on the temperature at which a substance changes state. Th e pressure-temperature relationship is diff erent for every substance. An increase in pressure always results in an increase in the temperature at which a change of state will take place. A decrease in pres- sure always causes a decrease in the temperature at which a change of state takes place. For instance, water boils at 212°F (100°C). However, if the pressure of water is raised, such as in a pressurized engine cooling system, the boiling point is raised. For every 1 psi (6.9 kPa) rise in temperature, the boil- ing point of the water goes up by 3°F (1.6°C). Th erefore, in a cooling system containing only water, the boiling point will be raised to 257°F (124°C) when a 15 psi (104 kPa) pressure cap is used. Figure 5-6 shows the relationship of pressure to change of state for one particular refrigerant. As you study the chart, note that something will turn to a gas at a higher temperature when it is pressurized, it will also condense at a higher temperature. To make the pressure-temperature relationship work in a refrigeration system, we must have a substance that can change from a liquid to a gas, and from a gas to a liquid, at the pressures developed by the refrigeration system. Various types of refrigerants have been developed, but not all of them will work in an automotive air conditioning system. Later in this chapter, we will discuss the action of refriger- ants in more detail. Refrigeration Components Th e following sections contain a brief overview of the components of the refrigeration system. Th ese sections con- centrate on the eff ect each component has on heat transfer, change of state, and pressure development. Detailed discus- sions of each component will be given in later chapters. The Two Main Types of Refrigeration Systems Th ere are two major types of automotive refrigeration systems. Notice both systems use an evaporator, compres- sor, and condenser. Th e type of evaporator pressure control device used identifi es the type of refrigeration system. Th e most common modern refrigeration system is the cycling clutch orifi ce tube (CCOT) system shown in Figure 5-7. It uses a fi xed fl ow restrictor. Th is system always has a way to control the amount of refrigerant drawn into the compres- sor. Many orifi ce tube systems use a cycling compressor clutch. Th e compressor control may be a pressure switch installed on the low side of the system, or a temperature switch mounted on the evaporator outlet. Figure 5-7. The system shown here is a modern refrigeration system using a fixed orifice tube. (General Motors) Compressor Condenser Evaporator Accumulator Switch, pressure cycling Bag, desiccant Hole, oil bleed Orifice tube Low-pressure liquid Low-pressure vapor High-pressure liquid High-pressure vapor Figure 5-6. Pressure affects the temperature at which a change of state occurs. Notice how the boiling points of water and a common refrigerant increase when the pressure is increased. Pressure-Temperature Relationship Pressure Water Refrigerant 0 212°F (100°C) –15°F (–26°C) 5 227°F (108°C) –6°F (–21°C) 10 242°F (116°C) +5°F (–15°C) 15 257°F (124°C) +15°F (–9.4°C) 20 272°F (133°C) +20°F (–6.6°C)