7. Operation
A. The operation of the equipment cooling system is automatic once electrical power is applied to the airplane. With the blower selector switch positioned to NORMAL the main blower will operate unless the GEN BUS No. 1 is de-energized. If for this reason or any other reason the main blower fails to operate, the blower selector switch may be positioned to ALTERNATE. The alternate blower will then operate.
(1)
AIRPLANES WITH FLOW CONTROL VALVES P/N 10-60704-1 THRU -6; The selected blower operates continuously and the flow control valve is in the open position when the differential pressure is less than 2.0 to 2.8 psi.
(2)
AIRPLANES WITH FLOW CONTROL VALVES P/N 10-60704-7; The selected blower operates continuously and the flow control valve is in the open position when the differential pressure is less than 0.7 to 1.1 psi.
B. When electrical power is connected to the airplane, circuit breaker panel P18-3 is energized and the blower and the airflow detector are energized. While the cabin pressure differential is low the blower creates most of the airflow necessary for cooling. In flight, the cabin pressure differential increases causing a greater flow through the blower duct. The increased differential is sensed by the flow control valve that begins to close.
(1)
AIRPLANES WITH FLOW CONTROL VALVES P/N 10-60704-1 THRU -6; When the differential pressure reaches 2.0 to 2.8 psi, the valve is closed. While the pressure differential is greater than 2.0 to 2.8 psi, the equipment cooling air will exhaust mainly to the forward cargo compartment.
(2)
AIRPLANES WITH FLOW CONTROL VALVES P/N 10-60704-7; When the differential pressure reaches 0.7 to 1.1 psi, the valve is closed. While the pressure differential is greater than 0.7 to 1.1 psi, the equipment cooling air will exhaust mainly to the forward cargo compartment.
C. During normal operation the airflow through the equipment cooling system is sufficient to remove heat from the detector heater, preventing the temperature sensitive switch from activating. When the airflow through the system is not sufficient to remove heat from the heater, the temperature around the heat sensitive switch rises, actuating the switch. This illuminates the OFF warning light. The OFF warning light on the overhead panel will remain on until airflow through the system is sufficient to remove heat from the heater.
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21-58-0 Page 6 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. Feb 01/95
EQUIPMENT COOLING SYSTEM - TROUBLE SHOOTING
1. General
A. In trouble shooting the equipment cooling system two cooling problems may exist. One concerns the equipment in the racks and sizing of the orifice openings. The other will result from faulty components in the air-moving portion of the system.
B. Fixed orifice openings are provided to allow proper airflow for all equipment installed in the airplane. If orifices are not set accordingly, or if additional orifices are added to provide cooling for equipment installed after delivery, it may upset the cooling balance and allow overheating of some electrical/electronic equipment. For a more detailed description of orifices, refer to 21-58-0, Adjustment/Test.
CAUTION: IF ORIFICE OPENINGS ARE IMPROPERLY ADJUSTED, EQUIPMENT ON THE RACKS MAY OVERHEAT. DO NOT ADD EQUIPMENT OR CHANGE ADJUSTMENTS WITHOUT YOUR ENGINEERING DEPARTMENT APPROVAL.
(1)
Furthermore, it should be noted that cooling of equipment utilizes control cabin exhaust air. This air, drawn through the equipment, may contain impurities such as tars and nicotine that may settle on the equipment. If equipment is not cleaned for a considerable period of time, these deposits could adversely affect their cooling.
(2)
A performance test is provided in Equipment Cooling System - Adjustment/Test to check that airflow through the system is within limits and to isolate any fault which might cause system performance to stray from those limits.
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