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737-300/400/500MAINTENANCE MANUAL
 EQUIPMENT COOLING SYSTEM - DESCRIPTION AND OPERATION
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 1.  General_______
 A.  The equipment cooling system cools the electrical and electronic equipment on the racks in the electronic compartment, some circuit breaker panels in the control cabin and the main instrument panel.
 B.  Cabin air is the cooling medium. It is drawn through and around the equipment into a system of ducts and manifolds. Some of the ducts and/or manifolds form the equipment mounting racks. The system leads to the blower inlet plenum (Fig. 1). Two blowers are connected to the inlet plenum and exhaust into an exhaust plenum. The exhaust plenum contains an automatic flow control valve, a check valve and an exhaust port. The blower inlet plenum is outboard of Rack E3. The exhaust plenum is outboard of Rack E3 and under the electronic compartment floor. When the airplane is on the ground or during low-altitude flight, a blower draws cooling air through the ducts and discharges it overboard through the automatic flow control valve, exhaust port, and under the forward cargo compartment floor. During flight, when cabin pressure differential is adequate to close the flow control valve, equipment cooling air is discharged under the forward cargo compartment floor. The air circulates between the cargo compartment insulation and lining and heats the cargo compartment before it is discharged overboard through the forward outflow valve or recirculated into the main conditioned air distribution ducts.
 C.  Two blowers are provided. One blower operates continuously when power is supplied to the airplane. If the main blower fails, the alternate blower may be switched on so the cooling system can continue to operate.
 D.  The flow control valve controls airflow overboard. Normal operation of the cabin pressurization system will result in an increase of cabin pressure relative to ambient pressure. This increase in differential pressure will, in turn, tend to increase the airflow discharged through the nozzle and control valve. The increase in dynamic pressure is sensed by the flow control valve which begins to move toward the closed position to limit the airflow through the valve.
 2.  Equipment Cooling Blowers_________________________
 A.  Two identical blowers are used in the equipment cooling system to create the required cabin-to-ambient differential pressure for inducing airflow through the system. Each blower is an axial flow type with an electric motor mounted on the centerline of the blower cylindrical housing. The motor operates on 115 volt three-phase power taken from circuit breaker panel P18-3 (Fig. 2) and is designed for continuous operation.
 B.  The blowers are in the lower aft section of the electronic compartment. They are connected between the blower inlet plenum and exhaust plenum by short flexible ducts and duct clamps and are secured by the blower support.
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 21-58-00
 ú ú 03 Page 1 ú Feb 25/87
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
MAIN INSTRUMENT PANEL COOLING

Equipment Cooling System Component Location
Figure 1

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A 737-300/400/500MAINTENANCE MANUAL

Equipment Cooling System Circuit
Figure 2

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 3.
4.

5.

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737-300/400/500MAINTENANCE MANUAL
 C.  Equipment cooling blower operation is controlled by the blower selector switch on the overhead panel (P5). The switch positions are NORMAL and ALTERNATE. An EQUIP COOLING OFF Light illuminates when the blower selected fails to operate.
　
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