The Air Cycle Machine (ACM)
After the bleed air passes the pressure regulating and shutoff valves it then enters the ACM where it is conditioned and temperature regulated before entering the cabin.
The ACM conditions the air by cooling, compressing, then cooling it again before expanding the air as it leaves the ACM.
The basic principle of operation of an ACM is that by compressing the hot bleed air, more heat energy can be extracted from it as it passes through a heat exchanger (radiator).
Before the bleed air enters the compressor section of the ACM it passes through the primary heat exchanger to remove most of the heat before it is compressed.
The bleed air is then compressed in the compressor section of the ACM and is then fed through to the secondary heat exchanger to remove as much heat as possible before being expanded and introduced to the turbine section of the ACM which is used to drive the ACM’s compressor.
After passing through the turbine section of the ACM, the air is then ducted through a water separator to remove excess moisture from the conditioned air before it enters the cabin and cockpit via a one way check valve.
The excess water that is filtered out of the air is injected into the air that is flowing over the heat exchangers to increase cooling efficiency.
To regulate the temperature of the air entering the cabin, a bypass valve (mixing valve) allows some of the bleed air to bypass the ACM and be introduced into the conditioned air to increase its temperature.
The bypass valve can be automatically or manually controlled by the air conditioning controls mounted under the co-pilots control column.
The air conditioner controls, drive the bypass valve in the ACM to regulate the temperature of the bleed air entering the cabin.
The valve position is varied by an electrically powered motor that requires DC power to operate.
This valve can be opened (hotter) or closed (cooler) by either the automatic or manual functions of the system.
The temperature is automatically controlled when the AUTO TEMP SELECT switch is in any position other than MAN.
This selector is a rheostat that which sends a variable temperature setting to a cabin temperature controller.
In automatic mode this controller will sense the actual temperature compared to the desired temperature and vary the bypass valve in the ACM accordingly.
With the AUTO TEMP SELECT rheostat in the MAN position the mixing valve is controlled manually by the pilot using the MAN TEMP CONT switch.
This is a three position switch that is spring loaded to the centre position. When the switch is moved up towards the HEAT position, the bypass valve is opened allowing more mixing of hot bleed air with cool conditioned air in order to raise the temperature of the bleed air entering the cabin.
When the switch is moved down towards the COOL position, the bypass valve is closed to reduce the amount of bleed air bypassing the ACM allowing a greater percentage of cool conditioned air from the ACM to enter the cabin.
When the switch is released the bypass valve will remain in the current position allowing the current rate of bleed air to bypass the ACM.
The bypass valve will take approximately 21 seconds to travel from full open to full closed when operated manually.