Pressurization Outflow
Cabin air outflow is controlled by the outflow valve and the overboard exhaust valve. A
small amount is also exhausted through toilet and galley vents, miscellaneous fixed vents,
and by seal leakage.
Outflow Valve
The outflow valve is the overboard exhaust exit for the majority of the air circulated through
the passenger cabin. Passenger cabin air is drawn through foot level grills, down around
the aft cargo compartment, where it provides heating, and is discharged overboard through
the outflow valve.
Overboard Exhaust Valve
On the ground and in flight with low differential pressure, the overboard exhaust valve is
open and warm air from the E & E bay is discharged overboard. In flight, at higher cabin
differential pressures, the overboard exhaust valve is normally closed and exhaust air is
diffused to the lining of the forward cargo compartment.
737-700
The overboard exhaust valve is driven open if either pack switch is in high and the
recirculation fan is off. This allows for increased ventilation in the smoke removal
configuration.
737-800 and 737-900
The overboard exhaust valve is driven open if either pack switch is in high and the right
recirculation fan is off. This allows for increased ventilation in the smoke removal
configuration.
