90 Auto Heating and Air Conditioning Copyright by Goodheart-Willcox Co., Inc. to a diaphragm. On the other side of the diaphragm is a chamber. Th e chamber is attached to a tube, which extends into a cavity at the outlet end of the evaporator. Th e cham- ber and tube are sealed and contain a gas. Figure 5-18A shows the placement of an expansion valve in relation to the evaporator. A variation of the expansion valve, called a block valve, is shown in Figure 5-18B. If the evaporator temperature becomes too low, the gas contracts. Th is causes the diaphragm to move and close the valve. Closing the valve cuts off refrigerant fl ow into the evaporator. Once the evaporator temperature begins to rise, the gas expands and causes the diaphragm to open the needle valve. Refrigerant can once again enter the evapora- tor. Th is cycle repeats as necessary. Note: Both types of flow restrictors will have a mesh screen just ahead of the restrictor in the high pressure (liquid) line. This screen catches any debris that might clog the restrictor. Evaporator Pressure Control Some older vehicles have an evaporator pressure control installed between the evaporator and the compressor. Th e pur- pose of any evaporator pressure control is to keep condensed moisture from freezing on the evaporator. Th ese devices are all variations of a pressure regulating valve. As long as the evaporator pressure is above 28-30 psi (193-207 kPa), the valve is open. When the evaporator pressure drops below this fi gure, the valve closes, keeping the compressor from drawing any more refrigerant from the evaporator. When more refrigerant enters through the fl ow restric- tor, evaporator pressure goes up and the valve opens. Keeping the evaporator pressure at 28-30 psi (193-207 kPa) or above keeps the evaporator temperature above 32°F (0°C) and the condensed moisture cannot freeze. Figure 5-19 shows the location of a typical evaporator pressure control. Some names for these pressure controls are suction throttling valve (STV), pilot operated absolute (POA) valve, valves- in-receiver (VIR), and evaporator pressure regulator (EPR). Connecting Lines Metal tubing and fl exible hoses connect the components of the refrigeration system. Metal tubing is used between parts that do not move. Hoses connect parts that move in relation to each other, for example, the engine-mounted compressor and the body-mounted condenser. Tubing and hoses must stand up under high pressures and not leak. Sometimes the lines used on a refrigeration system are called vapor or liquid lines. Vapor lines carry refrigerant vapor when the system is operating normally. Examples would be the lines from the evaporator to the compressor, and from the compressor to the condenser. Liquid lines carry liquid refrigerant during normal system operation. Th e line from the condenser to the fl ow restrictor is an example of a liquid line. Service Fittings Service fi ttings, sometimes called service valves or service ports, allow the technician to gain access to the refrigeration system. Service fi ttings allow pressure testing, Figure 5-18. A—This type of expansion valve is located at the entrance to the evaporator. The sensing bulb is attached to the evaporator outlet line. B—All of the parts are contained inside of the block expansion valve. Note the attachment of inlet and outlet tubes of the evaporator. Figure 5-19. The clutch cycling switch can be installed any- where on the low pressure side of the refrigeration system. This switch is installed on the line between the evaporator and accumulator. Other clutch cycling switches are installed on the accumulator body. A B