pressure generated in the bulb is transmitted to the diaphragm in the power section of the valve, causing the valve pinto open. A decrease in the temperature of the gas leaving the evaporator will cause the pressure in the remote bulb to decrease, and the valve pin will move toward the closed position. The superheat spring is designed to control the amount of superheat in the gas leaving the evaporator. A vapor is superheated when its temperature is higher than that necessary to change it from a liquid to a gas at a certain pressure. This ensures that the Freon returning to the compressor is in the gaseous state. The superheat spring is adjustable and is factory set to provide approximately 9° of superheat in this particular vapor cycle system. Superheat setting is calculated in relation to evaporator size and heat loads applied; therefore, it should never be tampered with in the field as serious inefficiencies will result. The equalizer port is provided to compensate for the effect the inherent evaporator pressure drop has on the superheat setting. The equalizer senses evaporator discharge pressure and reflects it back to the power head diaphragm, adjusting the expansion valve pin position to hold the desired superheat value. The purpose of the multioutlet configuration of the valve is to ensure an even distribution of the refrigerant in the evaporator. EVAPORATOR.— The evaporator (fig. 3-16) is a plate fin heat exchanger forming passages for cooling airflow and for Freon 12 refrigerant. The evaporator assembly houses a hydraulically driven fan and a low-temperature cutout switch. When the vapor cycle system is operating, refrigerant from the expansion valve flows into the Freon passages of the evaporator. At the same time, the hydraulically driven fan is forcing air from the electronic equipment compartment across the coils of the evaporator. The temperature of the air is rather high since it is affected by being circulated through the electronic boxes. This air, in passing through the evaporator, readily gives up its heat to the liquid Freon 12. The Freon is receptive to the heat exchange and, in absorbing the heat, a change of state comes about, changing the Freon from a liquid to a gas at approximately the same temperature that it was changed from a gas to a liquid. Since the Freon compressor is maintaining a constant pressure in the evaporator, the Freon vaporizes at a ^- temperature that causes the air discharging from the evaporator to the electronic compartment to be at approximately 40°F. Vapor leaving the evaporator is also at a temperature of about 40°F. Attached to the discharge side of the evaporator is a header duct assembly, bolted to the perimeter of the evaporator. This header is used to direct the discharged cooling air to the various distribution ducts. A set of movable louvers in the header is designed to act as a shutoff valve during ground cooling cart operations. During this time an external cart is attached to a receptacle on the right-hand side of the fuselage and feeds to the distribution system for ground operations, if desired. This air, however, would also escape in reverse direction through the evaporator and discharge into the forward compartment, thereby reducing the airflow to the electronic equipment. The louvers are actuated by a single control knob located at the top of the header duct. The knob is a two-position control (open and close) and is placarded to explain operation. To prevent the louvers from being inadvertently left in the closed position with the possibility of starving the avionics gear of cooling air after ground cart operation has been terminated, an overcenter device is incorporated. This device will automatically open the louvers as soon as a pressure is felt on them from the evaporator fan. The header duct also contains a discharge air filter, which filters the recirculated air and also removes the majority of the moisture (if present) in the cooling air on its way to the electronic equipment. COMPRESSOR ASSEMBLY.— The purpose of the compressor assembly (fig. 3-17) is to evacuate the evaporator, keeping it at a constant pressure, and also to superheat the Freon vapor and feed it to the condenser where it is condensed back into a liquid for reuse. The compressor houses two intermeshing helical rotors that rotate in counterrotating directions. This action causes cool Freon gas to be taken from the evaporator and compressed. This increases its temperature and pressure to a value where it may be fed to the condenser for ambient air to change it back into a liquid. The compressor controls the pressure in the evaporator by varying its speed in response to signals from a suction line pressure switch. The two intermeshing helical screw-type rotors are enclosed in a close tolerance housing, containing an inlet and an outlet port. Since the rotors mesh, they may be distinguished one from the other by calling one the male and the other the female. The male rotor is directly coupled to, and driven by, a variable speed hydraulic motor. The 3-24