Air Systems -Air Conditioning System Description
737 Operations Manual
Overheat detection is provided by temperature sensors located downstream of the packs. An overheat condition causes the appropriate mix valves to drive full cold and the DUCT OVERHEAT light to illuminate. A temperature higher than the duct overheat causes the appropriate pack valve to close and the PACK TRIP OFF light to illuminate.
Flight Deck
Since the flight deck requires only a fraction of the air supply provided by the left pack, most of the left pack air output is mixed with the right pack supply and routed to the passenger cabin.
Conditioned air for the flight deck branches into several risers which end at the floor, ceiling, and foot level outlets. Air diffusers on the floor under each seat deliver continuous air flow as long as the manifold is pressurized.
Overhead diffusers are located on the flight deck ceiling, above and aft of the No. 3 windows. Each of these outlets can be opened or closed as desired by turning a slotted adjusting screw.
There is also a dual purpose valve behind the rudder pedals of each pilot. These valves provide air for warming the pilots’ feet and for defogging the inside of the No. 1 windshields. Each valve is controlled by knobs located on the Captain’s and First Officer’s panel, respectively.
Passenger Cabin
The passenger cabin air supply distribution system consists of the mix manifold,
sidewall risers, and an overhead distribution duct. Sidewall risers go up the right and left wall of the passenger cabin to supply air to the overhead distribution duct. The overhead distribution duct routes conditioned air to the passenger cabin. It extends from the forward to the aft end of the ceiling along the airplane centerline and also supplies the sidewall diffusers.
Recirculation Fan
The recirculation fan system reduces the air conditioning system pack load and the engine bleed air demand. Air from the passenger cabin and electrical equipment bay is drawn to the forward cargo bay where it is filtered and recirculated to the mix manifold. The fan is driven by an AC motor. The fan operates with the recirculation fan switch in AUTO except with both packs on and one or both in HIGH.
Equipment Cooling
The equipment cooling system cools electronic equipment in the flight deck and the E & E bay.
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Air Systems -Air Conditioning System Description 737 Operations Manual

The equipment cooling system consists of a supply duct and an exhaust duct. Each duct has a normal fan and an alternate fan. The supply duct supplies cool air to the flight deck displays and electronic equipment in the E & E bay. The exhaust duct collects and discards warm air from the flight deck displays, the overhead and aft electronic panels, circuit breaker panels in the flight deck, and electronic equipment in the E & E bay.
Loss of airflow due to failure of an equipment cooling fan results in illumination of the related equipment cooling OFF light. Selecting the alternate fan should restore airflow and extinguish the OFF light within approximately 5 seconds.
If an overtemperature occurs on the ground, alerting is provided through the crew call horn in the nose wheel well.
Forward Cargo Compartment
The recirculation fan system circulates air from the passenger cabin around the lining of the forward cargo compartment. On the ground, or with the cabin differential pressure less than 2.5 psi, the exhaust fan air is blown through a flow control valve and exhausted out the bottom of the airplane. With increasing airflow at greater cabin differential pressures, the flow control valve closes and exhaust air from the equipment cooling system is now diffused to the lining of the forward cargo compartment for in-flight heating.
Conditioned Air Source Connection
A ground air conditioning source may be connected to the mix manifold to distribute preconditioned air throughout the airplane.
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Air Systems -Air Conditioning System Description
Air Conditioning Distribution Schematic

GROUND PRECONDITIONED CONDITION: IN FLIGHT CONDITIONED AIR AIR SOURCE

PACKS ON
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Air Systems -Air Conditioning System Description 737 Operations Manual

Intentionally
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737 Operations Manual

Introduction Airplanes with CPCS
Cabin pressurization is controlled during all phases of airplane operation by the cabin pressure control system (CPCS). The CPCS includes one automatic controller and one standby controller available by selecting AUTO or STBY, and two manual (MAN) pilot-controlled modes.
　
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