The Grumman Aerospace Corporation chose a Freon 12 vapor cycle ACS to provide avionics equipment cooling in the E-2 "Hawkeye" aircraft. This system, the VEA6-1, is described in this section. The basic difference between the basic vapor cycle system and the VEA6-1 system is the method of compensating for the variations in ram air temperature and the variation in the flow of ram air, which is dependent on aircraft speed. Figure 3-12 is a schematic diagram of the VEA6-1 vapor cycle ACS. In the E-2 configuration, the vapor cycle system cools, filters, and distributes avionics compartment air at a temperature of 38 5 F. The system consists of a vapor cycle cooling scoop assembly, an evaporator group assembly, and air distribution ducting interconnected by refrigerant lines and electrical wiring. The evaporator assembly (fig. 3-13) is a compact, quick-change package that can be easily installed, removed, and serviced as a unit. The assembly is composed of five quick-dis-connect couplings; two shock mounts; tem-perature controls; a hydraulic, motor-driven, self-lubricating Freon compressor; a receiver; a subcooler; a thermostatic expansion valve; an evaporator; hydraulic motor-driven fan; and an oil separator.

The vapor cycle cooling scoop assembly is mounted on the top of the fuselage and consists of a condenser assembly, ejector nozzles, an actuator and flap, and a refrigerant pressure actuator control switch. The Freon 12 in the closed system is the primary coolant. The forced air that is drawn through the evaporator in a continuous cycle is the secondary coolant. The electronic equipment is cooled by the secondary coolant, which removes heat by direct contact with the equipment to be

Figure 3-12.- Vapor cycle air-conditioning system.

Figure 3-13.- Evaporator assembly.

cooled and the transfer of this heat to the primary coolant through the evaporator assembly. A header assembly attached to the discharge side of the evaporator assembly directs the secondary coolant air to distribution ducts throughout the electronic equipment compart-ment. A filter between the header assembly and the evaporator assembly removes dirt and dust particles and traps moisture from the air. The closed system consists of the evaporator group assembly and the condenser group assembly. The coolant circulates between the evaporator group assembly (where it absorbs heat) and the condenser group assembly where it discharges or dissipates the heat to the atmosphere through the vapor cycle scoop. During flight, ram air flowing through the scoop cools the condenser group assembly. The airflow through the scoop is controlled by a condenser pressure control system. The actuator in the scoop modulates the airflow through the scoop to provide sufficient cooling for con-densation of the refrigerant. When the aircraft is on the ground with engines running and ram airflow is insufficient for cooling, a condenser ejector air shutoff valve opens to permit engine bleed air to discharge through the ejector assembly. The ejector consists of a set of tubes that permit bleed air to escape into the ram air duct behind the condenser. The escaping bleed air creates a negative pressure (suction) area behind the condenser and causes ambient air to be drawn into the scoop and across the condenser. If the heat load applied to the evaporator and the ram air temperature and flow were constant, a simple opening would be all that was required to control the boiling point of the refrigerant entering the evaporator. Since these three factors are not constant, they must be compensated for. In the model VEA6-1 system, if the heat load is changed, the flow of refrigerant is changed by using a thermostatic expansion valve in place of a fixed opening. The pressure of the refrigerant in the evaporator is maintained constant, regardless of the refrigerant flow, by varying the speed of the compressor. When the EQUIPMENT COOLING switch is set to ON, solenoid-operated shutoff valves are energized and hydraulic pressure is directed to the evaporator fan motor and the compressor motor. The compressor motor will be automatically shut off when either aircraft engine is in autofeather and the landing gear is down. The evaporator fan motor will continue to operate.

With the evaporator fan and compressor motors operating, low-pressure, low-temperature refrigerant Freon 12 vapor enters the compressor assembly through the low-pressure line leading from the evaporator assembly outlet. The vapor entering the compressor inlet combines with lubricating oil that is fed to the compressor. The oil-refrigerant mixture is compressed to raise its condensing temperature. From the compressor, the high-temperature, high-pressure mixture flows to the oil separator, where the oil is removed from the refrigerant vapor, filtered, and fed back to the compressor. If the refrigerant vapor pressure exceeds 250 5 psi in the line downstream from the oil separator, the high-pressure cutoff switch will cause the cockpit EQUIP COOLING caution light to illuminate and the compressor motor solenoid valve to shut off hydraulic pressure to the compressor motor, thus shutting down the compressor. If the cutout switch failed to operate properly, the relief valve in the compressor discharge line would relieve the system of pressure in excess of 325 psi. Refrigerant vapor from the oil separator next enters the condenser assembly where ram air lowers its temperature and changes the vapor to a liquid. Refrigerant pressure on the high side of the system is controlled by regulating the amount of cooling air flowing across the condenser. A pressure transducer in the high side refrigerant line provides a signal to a control amplifier, which, in turn, causes the control actuator and flap to open and close as necessary to regulate pressure. The system is calibrated so that the condenser flap is fully closed when high side pressure is 107 3 psi; fully opened at 151 3 psi condensing pressure; and modulates the flap travel for intermediate pressures within that range. If the cooling air is inadequate to maintain the pressure at 151 3 psi with the flap fully open, the system pressure will exceed the control range. When the pressure reaches 250 5 psi, the high--pressure cutout switch will shut down the vapor cycle system. From the condenser assembly, liquid Freon flows to the receiver in the evaporator group assembly. The receiver stores surplus refrigerant and thereby prevents surges in the refrigerant flow rate. Liquid refrigerant flowing from the receiver passes through a subcooler and then through a filter drier, where foreign matter and water are removed. Before entering the thermostatic expansion valve, the liquid refrigerant passes through a sight glass, which provides a visual indication of flow and proper refrigerant charge. The refrigerant is metered by the thermostatic expansion valve, and then enters the evaporator assembly. The hydraulic motor-driven evaporator fan forces warm electronic equipment compart-ment air through the evaporator assembly, where it is cooled by transfer of heat to the refrigerant. The refrigerant leaves the evaporator as a superheated vapor. The temperature of evaporator discharge air to the equipment compartment is controlled by controlling the speed of the compressor motor. The evaporator pressure control system maintains the refrigerant pressure within a specified range so that the average temperature range of the refrigerant is between 29.8 and 32.9 0.6 F. This temperature range consequently controls air temperature to approximately 38. The difference between the air and refrigerant temperatures is due to the efficiency of the heat exchanger. If the equipment compartment temperature increases, refrigerant pressure on the low side will also increase. The increase in pressure is sensed by a pressure transducer located in the compressor inlet line, and a signal is sent to the evaporator pressure control system amplifier. The amplifier, in turn, sends an appropriate signal to the servo portion of the compressor hydraulic motor calling for a speed increase to prevent pressure increase and thus maintain a constant refrigerant pressure. If the temperature increase calls for a compressor motor speed above a maximum of 12,000 rpm, the temperature rise cannot be compensated for and the refrigerant pressure will rise. At 250 5 psi compressor discharge pressure, the high side cutout switch will shut the vapor cycle system down. If the equipment compartment temperature drops, a reverse situation exists. Compressor motor speed will decrease to a minimum of 4,000 rpm. If the temperature at the fan inlet continues to drop beyond the range that can be compensated (30 F), the low-temperature cutoff switch de-energizes the compressor power relay and shuts down the compressor motor. The refrigerant stops flowing while the evaporator fan motor continues to circulate compartment air. When the fan inlet temperature rises to 40 2 F, the compressor is cut in and refrigerant flows through the evaporator and the subcooler and returns to the compressor to repeat the cycle.

The purpose of each major component in the vapor cycle system is discussed in the following paragraphs.