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Aircraft Air Conditioning System: RAM VS BLEED AIR

The air for aircraft air conditioning system and pressurization is taken from the engine compressor or cabin compressor. Ram air and bleed air is both used for pressurized aircraft to maintain the right pressure inside the cabin and to give the right amount and temperature of the cabin air.


Aircraft Air Conditioning System

Similarities between ram and bleed air:

The ram air and bleed air are both used to heat the cockpit, cabin, and windscreen.

The Air source is from outside the aircraft: The ram air is from the inlet duct while the bleed air is from the engine compressor inside the cowling engine.

Ram and bleed air undergoes processes (i.e., cooling system) and are fitted with safety devices.


Differences between ram and bleed air:

Ram Air System

– Heated by exhaust or combustion heaters

– Used to heat gyroscopic instruments

– No possibility of leaks inside the chamber that contaminates the cabin air

– For light aircraft

– Used in unpressurized piston engine aircraft

– Used to cool the bleed air

Ram are used in unpressurized piston engine aircraft. The ambient atmospheric air is introduced to the cabin through forward facing air intakes.

Some ram air can beheated by exhaust or combustion heaters and then mixed with the cold air in varying proportions to give a comfortable cabin temperature.
The supply ram air must not come into contact with, or is contaminated by, the exhaust gases or the air used for combustion.

Ram Air Turbine

The ram air turbine or the RAT also gives emergency hydraulic power for the flight controls in the event of loss of engine-driven hydraulic power.

Ram Air Valves

The ram air valves (inlet and outlet doors) are opened and closed by the pack controller and regulate the amount of air entering the ram air duct. This is done automatically as part of the temperature control system and during landing and take-off in order to prevent ingestion of foreign matter.

location of ram air inlet and outlet valves in aircraft air conditioning system

Bleed Air System

– Small amounts of oil may leak in the stream and mix with the compressed air and contaminate the cabin air

– Came from the engine compressor

– For large aircraft

– Used in pressurized piston-engine aircraft

Hot pressurized air is supplied to the bleed air duct from the LP/HP compressor. A tapping is then taken from the duct to supply the air-conditioning system.

This air is passed through a mass flow controller or a modulated engine bleed air valve and since the bleed air supply is always at a higher temperature than that required for passenger comfort a means of cooling this air is accomplished by the air conditioning pack.

When a supply of air from the compressor of a gas turbine engine for air conditioning or pressurization is not available, cabin air supply may be provided by blowers driven through the accessory gearbox or by turbo compressors driven by bleed air.


What’s the difference between RAM and BLEED air?

The main difference between ram and bleed air is that ram air is heated by exhaust or combustion heaters while the bleed air came from the engine compressor.

Know more about the aircraft air conditioning system here.