Chapter 5 Principles of Refrigeration 91 Copyright by Goodheart-Willcox Co., Inc. refrigerant removal, recharging, and other service opera- tions. Every type of refrigerant requires the use of a diff erent type of service fi tting. Service fi ttings will be discussed in more detail in later chapters. Seals and Gaskets To hold in pressurized refrigerant, seals and gaskets are used through the refrigeration system. Th ere are many variations of seals and gaskets. Seals and gaskets will be dis- cussed in more detail in Chapter 7. Which Parts Are Used on Which Systems? Not all of the parts described in this chapter are used on every refrigeration system. Th ere is considerable variation as to what parts are used on which system. A few general rules do apply to refrigeration systems made since the 1960s. For instance: • Most refrigeration systems will never have both an accumulator and a receiver-drier. A few medium-duty trucks have both an accumulator and receiver-drier. • An evaporator pressure regulator valve is never used with a fi xed orifi ce fl ow restrictor. • Usually a system with an expansion valve type of fl ow restrictor will not have a cycling clutch compressor. • Systems with a cycling clutch will not have an evapora- tor pressure control valve. • Most systems with a cycling clutch will not have a sight glass. • Only a few systems have a fi lter. Th ese rules apply in most cases. Always consult the service literature and make a careful search of the system to determine just what is being used. Refrigerants Th e substance circulating inside of the refrigeration system is called refrigerant. Using the right refrigerant is critical to the proper operation of the refrigeration system. Most importantly, the refrigerant must be able to evaporate and condense at the pressures and temperatures developed by the refrigeration system. Th e two refrigerants in common use, R-134a and R-12, will evaporate at –14.7°F (-26°C) and –21.7°F (-30°C) at sea level. To allow refrigerants to evaporate at a temperature that will not make the air too cold, they must be pressurized. Evaporator pressures are usually held at about 21-30 psi (145-207 kPa), depending on the type of refrigerant and other system design factors. Controlling the pressure raises the refrigerant boiling point to slightly over 32°F (0°C). To allow the refrigerant to condense to a liquid, pressures in the condenser are raised to about 200 psi, (1380 kPa), the temperature at which the refrigerant will condense is raised to about 135°F (57°C), which is much higher than outside air temperature. Condenser pressures may go much higher on very hot days. Note these pressures can easily be produced by the refrigeration system. The Refrigeration Cycle In this section, we will discuss how refrigeration prin- ciples are used with physical components to make the air conditioning system work. As you read, concentrate on the overall process rather than on individual components. Refer to Figure 5-20 as you read these sections. Figure 5-20. A typical cycling clutch orifice tube air conditioning system. (Four Seasons) Suction line Discharge line Liquid line 110° ACCUMULATOR EVAPORATOR Cold air Pressure cycling switch COMPRESSOR Hot air CONDENSER Ram air IN-LINE FILTER Warm air EXPANSION TUBE Low-pressure liquid Low-pressure gas High-pressure liquid High-pressure gas