The problems faced by pneumatic controls are the blockage of dust particles and moisture in the fine capillary passages of the various nozzle and flapper arrangements and their combinations which give the desired control action.
To prevent this blockage , the air system is fitted with micro filters and silica gel driers at every stage of control so that blockage is prevented. Moisture is gathered from the atmospheric air because of its humidity and although being regularly drained it is still carried over with the control air. Dust is carried in the air and enters the compressor even through the filters .. the filter mesh size is limited to an extent which allows easy operation of the compressor. If the mesh size is made very small the volumetric efficiency of the compressor is reduced considerably. Hence the need for on line filters and driers
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