B737-800/900ER ATA 50 (CARGO) "Cargo Compartments Air"

Cargo Compartments Air

There is some confusion about the cargo compartments related to air inside the holds. The current Boeing CBT’s explain that air from the Equipment Cooling System and from the Recirculation Fans enter the (Fwd) Cargo Compartment what appears to refresh air inside and warm the compartment. 
This is NOT true as the Cargo Compartments are "Class C" fire compartments, meaning they confine a fire. So when the fire agent is relieved in the compartment, it takes away the O² without fresh air circulation to feed the fire.

The air from the Recirculation Fans are diffused in a “double skin” around the Cargo Compartments, thereby warming the compartments up without any regulation. The liner that creates that double skin is designed as a fire barrier to isolate the compartments from the rest of the aircraft.

The Forward Cargo Compartment (liner) also receives Equipment Cooling air when the aircraft is above ± 3.000 feet (> 1 PSID), when the Overboard Exhaust Valve is closed and reliefs into the liner.

The Aft Cargo Compartment (liner) only receives air from the Recirculation Fans and from the Cabin where the Outflow Valve creates an increased airflow through the liner depending on differential pressure at that moment warming up the Aft Compartment.

All together this normally results in a higher temperature in the Forward Compartment compared to the Aft Compartment.

The compartments are pressurized through an Pressure Equalization Valve, so as the aircraft climbs or descents there is a flow out and into the compartments for pressurization. At level altitude the compartments are “closed” and there is no airflow in or out creating those fire confining areas.

Last, the compartments are equipped with Blowout Panels to backup the Equalization Valves in case of a fast rate of change in pressure around the compartments as in a rapid decompression.