Venturi Restrictor Valve

AVIONICS COMPARTMENT VENTILATION – GENERAL DESCRIPTION

General

During usual flight operation, the primary cooling fan pulls the air through the forward accessory and electrical/ electronics compartments for ventilation and equipment cooling. If the primary fan does not operate, the standby cooling fan comes on automatically. On the ground, the two fans operate at the same time to give satisfactory ventilation and cooling. When the fans do not operate in- flight, the flight crew can open the venturi restrictor valve. The open valve pulls the air through the compartments and sends it overboard to keep satisfactory ventilation and cooling.

Ventilation

Conditioned air goes to the flight compartment from the cooling and distribution systems. The cooling fans, in the ducts in the forward cargo compartment, pull the air out of the flight compartment and into the forward accessory compartment. Most of the air then goes under the flight compartment floor and into the electrical/ electronics compartment.

The air in the electrical/ electronics compartment goes through the electronic equipment and into the equipment racks. The warm air in the racks goes out of the compartment through the

cargo compartment heating ducts. Some of the air in the forward accessory compartment gives

cooling to the installed equipment. The warm air from the forward accessory compartment goes through a duct that connects to the cargo heating duct, in the electrical/ electronics compartment.

Cooling Fans

The cooling fans operate to make a negative pressure that pulls air from the flight compartment and through the forward accessory and electrical/ electronics compartments. The primary cooling fan operates on the ground and in- flight. The standby cooling fan operates on the ground or automatically comes on in- flight, if the primary cooling fan stops. The primary cooling fan has a differential pressure switch that controls the automatic operation of the standby cooling fan.

Venturi Restrictor Valve

During unusual flight conditions, the flight crew can operate a switch to open the venturi restrictor valve and stop power to the cooling fans. The valve opens and makes a negative

pressure in the cargo compartment heating ducts. This negative pressure pulls the air from the ducts and gives ventilation for the forward accessory and electrical/ electronics compartments. Check valves, in the ducts, keep cargo compartment air from the venturi restrictor valve.

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