Answer EKM Question 9

With respect to control air supply system for Main Engine Control:
A. Define the essential conditions, which must be satisfied by the air supply for a pneumatic control system;
B. Sketch a control air supply arrangement and give a reasoned explanation for positioning of dryers and filters.
Answer: A. The air supplied to a pneumatic control system should be clean, essentially free of dirt or dust particles and moisture droplets. The air sucked from the atmosphere by the air compressor contains dust particles as well as moisture content due to the relative humidity in the engine room. Although air filters are used in air compressors at the air suction , the filtration is not 100 percent perfect and hence some dust particles find its way into the compressed air. Similarly moisture enters the compressed air although drains at intercoolers are being operated regularly. It is therefore essential that control air lines are fitted with microfine filters and driers in the control lines. The pneumatic controllers have fine capillary passages in their control elements and these passages can be blocked by dust particles and moisture drops. Hence the need for microfine filters and driers.
B. The line sketch shows the filters and driers incorporated in the control liner has to be incorporated immediately after the reducing valve, since the air gets cooler after throttling causing condensation of water particles from the air.