21-58-03
 ú ú 01 Page 2 ú Nov 15/87
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL

Equipment Cooling System Circuit
Figure 2

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 ú ú 01 Page 3 ú Jul 15/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
 3.
4.

5.

A
737-300/400/500MAINTENANCE MANUAL
 C.  Equipment cooling exhaust blower operation is controlled by the blower selector switch on the overhead panel (P5). The switch positions are NORMAL and ALTERNATE. An EQUIP COOLING EXHAUST OFF light illuminates when the blower selected fails to operate.
Automatic Flow Control Valve
____________________________
 A.  The flow control valve is in the exhaust plenum under the floor in the aft section of the electronic compartment. The electronic compartment floor must be removed to gain access to the valve.
 B.  The automatic flow control valve is operated by airflow through the valve and sensed across its eccentrically mounted butterfly plate which acts as an airfoil. The butterfly valve, as an airfoil, provides initial movement toward the closed position at a given dynamic pressure. As the butterfly moves closed, an increased pressure differential is developed which acts on the off-centered butterfly to produce a closing torque.
 (1)  
AIRPLANES WITH FLOW CONTROL VALVES P/N 10-60704-1 THRU -6; At 2.0 to 2.8 psi cabin-to-ambient differential pressure the flow control valve will be completely closed and the overboard flow is directed under the floor to the forward cargo compartment.

 (2)  
AIRPLANES WITH FLOW CONTROL VALVES P/N 10-60704-7; At .7 to 1.1 psi cabin-to-ambient differential pressure the flow control valve will be completely closed and the overboard flow is directed under the floor to the forward cargo compartment.

Equipment Mounting Shelves and Ducts
____________________________________
 A.  The electronic racks are formed by shelves, header plenums, and supporting structure. Holes for airflow are suitably located above each shelf. The holes lead to the header plenum, which leads into the ducts, and these into the blowers. The holes are sized to provide the right amount of cooling airflow when a given differential exists between cabin pressure and plenum pressure.
 B.  Shelves E1-2, E2-1, E2-3, E3-1, and E3-3 are hollow and have individual holes under the equipment for "draw through" cooling. There are sealing strips along the edges of these trays. The holes lead to the plenum, which leads into the ducts, and these into the blower.
 (1)  When changing orifice sizes because of component changes, total orifice inlet area for each shelf must be maintained at the same value.
Airflow Detection System
________________________
 A.  The airflow detection system provides warning when the equipment cooling exhaust system is not supplying enough cooling air for safe electrical or electronic equipment operation. The airflow detection system consists of an equipment cooling exhaust OFF warning light and a low airflow sensor (Fig. 2). Electrical power for operation of the airflow sensor and OFF light press-to-test circuit is taken from the 28-volt dc bus on circuit breaker panel P18. The OFF light is also connected to the master test circuit.
 B.  The low flow sensor is a heated probe with associated electronic circuitry to measure the current required to heat the probe when the proper amount of cooling air flows past. Loss of airflow will change the current required to heat the probe, triggering the transitorized monitoring circuit which supplies a ground to the EQUIP COOLING EXHAUST OFF light and master CAUTION warning light on pilots glareshield.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 C.  The OFF amber warning light is located on the forward overhead panel (P5).
 6.  Check Valves____________
 A.  Two check valves are installed in the equipment cooling system. These check valves are installed in the blower discharge duct for each blower and prevent cooling air being discharged back through the nonoperating blower.
　
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