Superheat spring Push rods Diaphragm Force from gas in bulb Body Temperature sensing bulb Capillary tube Needle valve Evaporator inlet pressure Outlet Inlet To evaporator discharge Figure 16-78.  An externally equalized expansion valve uses  evaporator discharge temperature and pressure to regulate the  amount of refrigerant passing through the valve and into the  evaporator. system and cockpit mounted switches for the fans, as well as  engaging and disengaging the system. When cabin air is cooled by flowing over the evaporator,  it can no longer retain the water that it could at higher  temperature. As a result, it condenses on the outside of the  evaporator and needs to be collected and drained overboard.  Pressurized aircraft may contain a valve in the evaporator  drain line that opens only periodically to discharge the water,  to maintain pressurization. Fins on the evaporator must be  kept from being damaged, which could inhibit airflow. The  continuous movement of warm cabin air around the fins  keeps condensed water from freezing. Ice on the evaporator  reduces the efficiency of the heat exchange to the refrigerant. The compressor is the heart of the vapor cycle air conditioning  system. It circulates the refrigerant around the vapor cycle  system. It receives low-pressure, low-temperature refrigerant  vapor from the outlet of the evaporator and compresses it. As  the pressure is increased, the temperature also increases. The  refrigerant temperature is raised above that of the outside air  temperature. The refrigerant then flows out of the compressor  to the condenser where it gives off the heat to the outside air. The compressor is the dividing point between the low side  and the high side of the vapor cycle system. Often it is  incorporated with fittings or has fittings in the connecting  lines to it that are designed to service the system with  refrigerant. Access to the low and high sides of the system  are required for servicing, which can be accomplished with  fitting upstream and downstream of the compressor. from the cabin to the evaporator and from the evaporator  back into the cabin. Sometimes the cool air produced may  be introduced into an air distribution system where it can  blow directly on the occupants through individual delivery  vents. In this manner, the entire vapor cycle air conditioning  system may be located fore or aft of the cabin. A multiposition  fan switch controlled by the pilot is usually available.  Figure 16-80  diagrams the vapor cycle air conditioning system  in a Cessna Mustang very light jet. It has two evaporators that  share in the cooling, with outlets integrated into a distribution  Modern compressors are either engine driven or driven  by an electric motor. Occasionally, a hydraulically driven  compressor is used. A typical engine-driven compressor,  similar to that found in an automobile, is located in the  engine nacelle and operated by a drive belt off of the engine  crankshaft. An electromagnetic clutch engages when cooling  is required, which causes the compressor to operate. When  cooling is sufficient, power to the clutch is cut, and the drive  pulley rotates but the compressor does not. [ Figure 16-81] Dedicated electric motor-driven compressors are also used  on aircraft. Use of an electric motor allows the compressor  to be located nearly anywhere on the aircraft, since wires  can be run from the appropriate bus to the control panel and  to the compressor. [ Figure 16-82]  Hydraulically-driven  compressors are also able to be remotely located. Hydraulic  lines from the hydraulic manifold are run through a switch- activated solenoid to the compressor. The solenoid allows  fluid to the compressor or bypasses it. This controls the  operation of the hydraulically driven compressor. Compressor Figure 16-79.  The evaporator of this aircraft’s vapor cycle air  conditioning system is visible in the forward cabin sidewall behind  the right rudder pedal.  16-49