The Boeing 737 air-conditioning system consists of two independent air-cycle cooling systems, a cabin temperature control system, an air distribution system, recirculation system, and a pressurization system. The air-conditioning system is capable of supplying a total ventilation rate of 1,900 cubic feet per minute (CFM) on the 737-300/-500, and 2,100 CFM on the 737-400, up to the maximum certified operating altitude of the airplane. The system has sufficient capacity to maintain adequate cabin conditions to allow dispatch with any one subsystem inoperative.
The cabin air cooling portion of the air-conditioning system has two individual cooling packs located in an accessible unpressurized compartment under the wing centre section. Each cooling pack consists of a primary and secondary heat exchanger, modulated ram air duct system, air cooling turbofan, air cycle machine, water separator, anti-icing system and associated controls and ducts. Air from the pneumatic system first passes through the primary heat exchanger where the cooling process begins. The air then enters the compressor of the air cycle machine, is cooled again as it passes through the secondary heat exchanger. On the 737- 300/-500 temperature is greatly reduced as the air expands across the turbinestages, which also drives the compressor of the air cycle machine.
The excess moisture from the cooled air is removed by the water separator. The water separator is protected from freezing by a thermostatically-controlled warm air supply, which is obtained by by-passing the air cycle machine. On the 737-400 the air passes through the high pressure water separator (condenser, water extractors, and reheater) where the excess moisture is removed prior to entering the turbine, air expands across the turbine greatly reducing the temperature and driving the compressor stage of the air cycle machine. Freezing in the condenser is prevented by passing warm air around the air cycle machine through the stand-by pack temperature control valve. For in-flight operation the heat exchangers are cooled by ambient airflow into the ram-air inlet. For ground operation, or in-flight with
the flaps extended, a turbofan in the ram-air duct is driven by pressurized air, from the pneumatic supply, to provide cooling flow. In-flight flow of ram-cooling air is controlled automatically by a control system.