metered to obtain optimal cooling. This is accomplished by  monitoring the temperature of the gaseous refrigerant at the  outlet of the next component in the cycle, the evaporator.  Ideally, the expansion valve should only let the amount of  refrigerant spray into the evaporator that can be completely  converted to a vapor.  The temperature of the cabin air to be cooled determines  the amount of refrigerant the expansion valve should spray  into the evaporator. Only so much is needed to completely  change the state of the refrigerant from a liquid to a vapor.  Too little causes the gaseous refrigerant to be superheated by  the time it exits the evaporator. This is inefficient. Changing  the state of the refrigerant from liquid to vapor absorbs  much more heat than adding heat to already converted vapor  (superheat). The cabin air blowing over the evaporator will  not be cooled sufficiently if superheated vapor is flowing  through the evaporator. If too much refrigerant is released by  the expansion valve into the evaporator, some of it remains  liquid when it exits the evaporator. Since it next flows to  the compressor, this could be dangerous. The compressor is  designed to compress only vapor. If liquid is drawn in and  attempts are made to compress it, the compressor could break,  since liquids are essentially incompressible. The temperature of superheated vapor is higher than  liquid refrigerant that has not totally vaporized. A coiled  capillary tube with a volatile substance inside is located at  the evaporator outlet to sense this difference. Its internal  pressure increases and decreases as temperature changes.  The coiled end of the tube is closed and attached to the  evaporator outlet. The other end terminates in the area  above a pressure diaphragm in the expansion valve. When  superheated refrigerant vapor reaches the coiled end of the  tube, its elevated temperature increases the pressure inside  the tube and in the space above the diaphragm. This increase  in pressure causes the diaphragm to overcome spring tension  in the valve. It positions a needle valve that increases the  amount of refrigerant released by the valve. The quantity  of refrigerant is increased so that the refrigerant only just  evaporates, and the refrigerant vapor does not superheat.  When too much liquid refrigerant is released by the expansion  valve, low-temperature liquid refrigerant arrives at the outlet  of the evaporator. The result is low pressure inside the  temperature bulb and above the expansion valve diaphragm.  The superheat spring in the valve moves the needle valve  toward the closed position, reducing the flow of refrigerant  into the evaporator as the spring overcomes the lower  pressure above the diaphragm. [ Figure 16-77] Vapor cycle air conditioning systems that have large  evaporators experience significant pressure drops while  refrigerant is flowing through them. Externally equalized  expansion valves use a pressure tap from the outlet  of the evaporator to help the superheat spring balance  the diaphragm. This type of expansion valve is easily  recognizable by the additional small-diameter line that comes  from the evaporator into the valve (2 total). Better control of  the proper amount of refrigerant allowed through the valve  is attained by considering both the temperature and pressure  of the evaporator refrigerant.  [Figure 16-78] Most evaporators are constructed of copper or aluminum  tubing coiled into a compact unit. Fins are attached to increase  surface area, facilitating rapid heat transfer between the cabin  air blown over the outside of the evaporator with a fan and  the refrigerant inside. The expansion valve located at the  evaporator inlet releases high-pressure, high-temperature  liquid refrigerant into the evaporator. As the refrigerant  absorbs heat from the cabin air, it changes into a low-pressure  vapor. This is discharged from the evaporator outlet to the  next component in the vapor cycle system, the compressor.  The temperature and pressure pickups that regulate the  expansion valve are located at the evaporator outlet.  The evaporator is situated in such a way that cabin air is pulled  to it by a fan. The fan blows the air over the evaporator and  discharges the cooled air back into the cabin. [ Figure 16-79] This discharge can be direct when the evaporator is located in a  cabin wall. A remotely located evaporator may require ducting  Figure 16-77.  An internally equalized expansion valve. Evaporator Superheat spring Push rods Diaphragm Force from gas in bulb Body Temperature sensing bulb Capillary tube Needle valve Expansion valve outlet pressure Outlet Inlet 16-48