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Air Conditioning Pack
The flow of bleed air from the main bleed air duct through each air conditioning pack is controlled by the respective pack valve. Normally the left pack uses bleed air from engine No. 1 and the right pack uses bleed air from engine No. 2. A single pack is capable of maintaining pressurization and acceptable temperatures throughout the airplane up to the maximum certified ceiling.
For airplanes equipped with the Garrett -129 or Sundstrand -2000 APU, two pack operation from a single bleed air source is not recommended due to excessive bleed air requirements.
For airplanes equipped with the Garrett -280 APU, two pack operation from a single engine bleed air source is not recommended due to excessive bleed air requirements. The APU is capable of supplying bleed air for both packs on the ground or in flight.
Airflow Control
With both air conditioning pack switches in AUTO and both packs operating, the packs provide “normal air flow.” However, with one pack not operating, the other pack automatically switches to “high air flow” in order to maintain the necessary ventilation rate. This automatic switching is inhibited when the airplane is on the ground, or in-flight with the flaps extended, to insure adequate engine power for single engine operation. Automatic switching to “high air flow” occurs if both engine bleed air switches are OFF and the APU bleed air switch is ON, regardless of flap position, air/ground status or number of packs operating.
Air Systems -Air Conditioning System Description 737 Operations Manual

With the air conditioning pack switch in HIGH, the pack provides “high air flow.” Additionally, an “APU high air flow” rate is available when the airplane is on the ground, the APU bleed air switch is ON and either or both pack switches are positioned to HIGH. This mode is designed to provide the maximum airflow when the APU is the only source of bleed air.
Ram Air System
The ram air system provides cooling air for the heat exchangers. Operation of the system is automatically controlled by the packs through operation of ram air inlet doors.
On the ground, or during slow flight with the flaps not fully retracted, the ram air inlet doors move to the full open position for maximum cooling. In normal cruise, the doors modulate between open and closed. The RAM DOOR FULL OPEN light illuminates whenever the ram door is fully open.
A turbofan is located in each ram air exit duct. It augments the ram airflow on the ground or during slow flight (flaps not retracted). The fan operates pneumatically using bleed air. It is activated electrically, when the pack is on, by the air-ground safety sensor or flap limit switch.
Deflector doors are installed forward of the ram air inlet doors to prevent slush ingestion prior to liftoff and after touchdown. The deflector doors extends when activated electrically by the air–ground safety sensor.
Cooling Cycle
The flow through the cooling cycle starts with bleed air passing through a heat exchanger for cooling. The air then flows to an air cycle machine for refrigeration and to a water separator which removes moisture. The processed cold air is then combined with hot air. The conditioned air flows into the mix manifold and distribution system.
Overheat protection is provided by temperature sensors located in the cooling cycle. An overheat condition causes the pack valve to close and the PACK TRIP OFF light to illuminate.
Air Mix Valves
The two air mix valves for each pack control hot and cold air according to the setting of the CONT CABIN or PASS CABIN temperature selector. Air that flows through the cold air mix valve is processed through a cooling cycle and then combined with hot air flowing from the hot air mix valve.
Copyright . The Boeing Company. See title page for details.
2.30.2  June 4, 2004

Air Systems -Air Conditioning System Description
737 Operations Manual
In the automatic temperature mode, the air mix valves are operated by the automatic temperature controller. The automatic temperature controller uses inputs from the respective temperature selector and cabin temperature sensor. The automatic temperature controller is bypassed when the temperature selector is positioned to MANUAL.
Anytime the pack valve closes, the air mix valves are driven to the full cold position automatically. This aids start-up of the cooling cycle and prevents nuisance hot air trips when the pack is turned on.
Air Systems -Air Conditioning System Description 737 Operations Manual

Air Conditioning Pack Schematic 
PASS  TO MIX 
CABIN  MANIFOLD 

RIGHT
BLEED AIR
BLEED AIR
Air Conditioning Distribution
Conditioned air is collected in the mix manifold. The temperature of the air is directly related to the setting of the CONT CABIN and PASS CABIN temperature selectors.
　
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