86 Auto Heating and Air Conditioning Copyright by Goodheart-Willcox Co., Inc. Many late-model systems have a valve in the compres- sor that allows it to vary the amount of refrigerant pumped. Th ese systems always have an accumulator between the evaporator and compressor. A variation of the CCOT air conditioning system is shown in Figure 5-8. Th is type of system is used on many minivans and full-size vans. In addition to the normal cycling clutch system used to cool the front of the vehicle, it has an extra cooling circuit with an evaporator installed in the rear of the passenger compartment. Th e other refrigeration system uses a thermostatic expansion valve, Figure 5-9. Th e compressor on these systems operates at all times while the system is on. Older versions of these systems use an evaporator pressure con- trol device, located at the evaporator outlet. Some very old expansion valve systems did not have a pressure control device, and cycled the compressor clutch to control evapora- tor pressure. A temperature switch resembling an expansion valve controlled the clutch on these early systems. Evaporator An evaporator is a heat exchanger made from one or more lengths of tubing, folded into a series of loops, or coils. See Figure 5-10. Th is makes the evaporator as small as pos- sible so it does not take up a signifi cant amount of interior room. Fins are attached to the coils to aid heat transfer into the tubing. Th e fi ns are thin and fl at to allow maximum airfl ow. Th e refrigerant fl ows through every coil in the evaporator, absorbing heat as it travels. As moisture collects on the surface of the evaporator, it runs to the bottom of the evaporator case and drips out of the drain. Other evapora- tors are of diff erent designs. Accumulator Most modern refrigeration systems have an accumulator installed between the evaporator and the compressor. Th e accumulator is a small tank, usually made of aluminum, Figure 5-11. It holds refrigerant and contains a desiccant. Th e desiccant absorbs any moisture left in the refrigeration system during service, and any small amounts of moisture that enter through the rubber hoses. Th e design of the internal tubes of the accumulator separates the liquid and vaporized refrigerant. Th e accumulator may have a return line which allows any oil in the bottom of the accumulator to return to the compressor. Compressor Th e compressor is a pump used to compress the vaporized refrigerant and move it through the system, Figure 5-12. Most modern compressors are piston types, which draw in refrigerant through one-way valves called reed valves. Th e intake reed valve is pulled open when the piston is moving downward. Th is allows refrigerant to enter Figure 5-8. This system is a variation of the fixed orifice system. It has an extra low side circuit to cool the rear of the vehicle. (General Motors) Low-pressure liquid Low-pressure vapor High-pressure liquid High-pressure vapor Compressor Front condenser Rear liquid line Front evaporator Accumulator Rear vapor line Thermal expansion valve Rear evaporator Expansion tube (orifice) Front liquid line