CONTROL SYSTEM
TEMPERATURE
CONTROL
The two air conditioning packs reduce the temperature of the bleed air to that required in the cabins. The air is cooled using air to air heat exchangers and an air cycle machine (cold air unit).Hot air from the pneumatic system, passing through the pack valve is divided by the mix valve. Some of the air is passed on, still hot to the mixing chamber. The remainder is sent through the air conditioning packs. The temperature control system positions the mix valve to control the amount of air being by-passed or cooled.The mix valve is of the dual type; both valves being positioned by a common 115 volt AC actuator motor. The valves are positioned by signals from the temperature control system such that as one valve moves towards its closed position, the other valve moves towards its open position.The temperature control system is comprised principally of a selector switch, regulator and temperature sensors, located at selected points in the system. The whole system operates automatically and continuously monitors the mix valve position. In the event of failure of the temperature regulator, the mix valve can be positioned manually using the temperature selector switch.When the selector switch is in the ‘auto’ position and at a desired cabin temperature, a potentiometer within the switch establishes a reference resistance value in an arm of a control bridge circuit of the temperature regulator. A cabin temperature sensor is in the other arm of the control bridge circuit so that if the cabin temperature is at a level other than that selected, then the bridge will be unbalanced. As a result, a signal is developed in the circuit of the mix valve motor so that it will drive the valves to either a hot or cold position, as required, to attain the selected cabin temperature. At the same time, conditioned air is sensed by an anticipator sensor, and limit sensors both of which are located in the ducting to the cabin, and are connected in an electrical bridge configuration. The purpose of the sensors is to modulate any rapid changes demanded by an unbalanced control bridge, so that when the actuator control moves the mix valve it will produce cabin temperature changes without sudden blasts of hot or cold air, and without rising duct temperatures above limits.
OVERHEAT
PROTECTION
A 90°C duct overheat thermal switch protects the system by preventing the mix valve staying in a too hot position. Should the duct temperature reach 90°C, the mix valve moves to the fully cold position and the duct overheat light illuminates. After correcting the overheat condition, the system can be returned to normal by pushing the reset button.Another thermal switch protects from duct overheat, should control electrical power be lost. At approximately 120°C this switch closes and the pack valve closes, illuminating the pack trip off light. Pushing the reset switch after the condition has been corrected will return the air conditioning pack to normal.If the ACM (Air Cycle Machine) compressor discharge overheat switch, or the ACM turbine inlet overheat switch are actuated, the pack valve will close. The air cycle machine will then stop.
MANUAL
TEMPERATURE CONTROL
During manual operation, the mix valve is directly controlled by the temperature selector. The indicating system allows monitoring of cabin temperature, cabin supply duct temperature and mix valve position.
CONTROLS /
INDICATIONS
The pack valves are controlled by switches on the engine bleed air control panel. Left and right pneumatic system duct pressure is also shown on this panel.
AIR
CONDITIONING TRIM AIR SYSTEM
The trim air system maintains the temperature in the flight control and passenger compartments at preselected temperatures.Trim air is hot engine bleed air which is directed into the flight control and passenger cabin zones to provide zone heating. The desired temperature for each cabin zone is set on the temperature selectors on the temperature control panel in the flight control compartment. During normal operation of the air conditioning system, the air conditioning pack temperature (zone) controller will control the pack output temperature to the lowest zone temperature selected. The zone controller will then control the trim air modulating valves to direct warm bleed air into the supply ducts of the zones which require a warmer temperature. Trim air is selected by placing the trim air switch OPEN. This is located on the zone temperature control panel. Selecting OPEN allows the trim air pressure regulating valve to operate and activates the zone temperature control circuits. Selecting CLOSE removes electrical power from these circuits, stopping the operation of the trim air system.
TRIM AIR
CONDITIONING SYSTEM
RAM AIR
SYSTEM
Figure below shows examples of National Advisory Committee for Aeronautics (NACA) ram air inlets. Atmospheric air is used to cool the heat exchangers. The amount of air permitted to flow across the heat exchangers is determined by the altitude of the aircraft. During climbs and descents, and when the aircraft is on the ground, insufficient ram air flows across the heat exchangers. During these periods, a turbo fan induces air to flow across the heat exchangers. The turbo-fan is driven by engine bleed air. During flight, the ram air inlet doors and exit louvres open and close to control the airflow across the heat exchangers. The actuator which adjusts the size of the inlet and exit is controlled by the ram air controller sensor. This is located downstream of the ACM compressor. By controlling the size of the ram air inlet and exit, a temperature of 110°C is maintained at the ACM compressor outlet.
NACA RAM AIR INLETS
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