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Pressure regulation is accomplished by the use of free air unloading.The purpose of such control to unload theair compressor.When more air is being compressed than is being consumed.Thus reducing to a minimum the power required to drive the compressor and providing maximum economy during the unloading period.This control was done by manual operated type unloaders such as "M" type or "W" type unloaders.Actually this is old technology and replaced by "Electro-Pneumatic Controls" which is also replaced by "Program Logic Control".Still many companies are using old technology and therefore,I am giving details about "M" Type Unloader.
* The operation of the unloader is simple and positive.When the pressure in the airdischarge line from the compressor to the receiver reaches the pressure point at which the unloader is set to operate,as determined by the position of the adjusting screw,the plate valve is forced away from its seat on the unloader body.When this happens,air from the discharge line from the compressor to the receiver passes through the unloader and to the unloading suction valve covers,holding these valves open.Then the compressor operates against the atmosphere pressure only.
* When the air pressure in the discharge line from the compressor to the receiver is reduced to the predetermined pressure setting of the unloader as determined by the number of unloader shims,then the compression of the spring,over-comes the pressure of the air in the discharge line from the compressor acting against the plate valve,forcing the valve back against its seat on the unloader body.The air trapped in the unloader piping is then vented out to the atmosphere through the vent hole of unloader.
* The pressure at which the unit will unload is determined by the position of the adjusting screw.Turning this screw in,increase the compression pressure on the spring and the unloading pressure of compressor.Turning the screw out,lower the unloading pressure.To decrease the loading-unloading range,add the shims and to increase the range,remove the shims.
* The most important care that the unloader should receive is to be kept clean.If the operation of it becomes sluggish,the operator should first make sure that all unloader piping is free from leakages since leakage from these places will affect the operation of the unloader.Also the vent hole in the unloader valve seat is open.The gauze of the air cleaner in the unloader line should be removed and cleaned.If problem is not solved,remove the unloader valve seat and clean.If the cleaning and adjusting fail to make the unloader operate satisfactorily,the valve may require grinding.
* Use fine grinding compound for valve seat grinding.The valve is provided with two small holes on the top surface for the insertion of the tool to turn it in grinding.Avoid excess grinding since this may change the point of contact sufficiently to affect operation adversely.Clean out all the compound thoroughly after grinding.
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