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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
Figure 3-17.- Compressor assembly. female rotor is driven aerodynamically by the male. There is no physical contact between the two rotors or between the rotors and case. Inter-rotor contact is prevented by the rotors riding on a film of refrigeration oil. Both rotors are suspended by three pairs of ball bearings, one set on the discharge end and two pairs on the inlet end. Bearing lubrication is supplied by the refrigeration oil. Suitable carbon and labyrinth seals are incorporated to provide control of the flow of lubricating oil. Thin ridges are machined on the ends and flutes of the rotors to seal the mechanism against excessive rotor leakage. The compressor operates on the principle that if a given volume of gas is trapped and the area in which it is contained gradually decreases, the pressure and temperature of the gas will increase. The counterrotating rotors are fed a gas charge from the inlet port. This charge fills the void formed by the rotors. As they rotate, the charge is trapped and forced forward through the housing. The action of the rotors is to decrease the interlobe area in which the charge is contained as they revolve. This increases the pressure and tempera-ture of the refrigerant. As the outlet port is reached, the charge will be contained in the small-est area during its travel through the compressor. Therefore, it is at its highest temperature and pres-sure and is discharged into the system. The variable compressor speed is provided by the governor-controlled, hydraulically driven motor, which responds to electronic impulses from the Freon circuit to increase or decrease speed as demanded by the cooling load. The electrical wiring of the speed-sensing sys-tem is such that when the equipment cooling sys-tem is shut down, the servomotor will be driven to low speed. This relieves starting loads and also precludes the possibility of an overspeed during startup. The compressed Freon gas is discharged from the compressor and immediately passes through a check valve, which prevents the high-pressure discharge from motorizing the compressor in reverse at system shutdown. The compressor section requires lubrication; therefore, an oil is mixed with the Freon during system servicing. This oil is also discharged from the compressor outlet and is reclaimed by the oil separator.
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