As is known to those skilled in the art, air compressors are normally driven by such means as an electric motor or an internal combustion engine, depending upon which is more convenient to use. Normally, when the internal combustion engine is used as the driving means, an unloader valve device is actuated to a vent position by the governor to allow the compressor to pump compressed air to atmosphere, usually at an idling speed, when storage reservoir pressure has attained the high setting value of the governor, said unloader valve being restored to a closed position when the governor responds to reduction of reservoir pressure to the low setting value thereof for cutting in compressing action again.
If an electric motor is used as the driving means, the governor simply controls a pressure switch for deenergizing or energizing the motor as necessitated by the pressure condition of the storage reservoir.
In either of the cases above discussed, a drain valve device is normally installed in the system at a location where moisture condensate is most likely to accumulate, said drain valve being operable to a momentarily open position for allowing the accumulated moisture condensate to drain to atmosphere in response to either a pneumatic pressure control impulse or an electrical control impulse, depending upon the system, coinciding with the cutting-out phase of operation of the compressor. Normally, presently known drain valve devices are so constructed as to be operable either exclusively pneumatically or exclusively electrically. In the event, therefore, that the means for driving a compressor were changed from an internal combustion engine to an electrical motor, or vice versa, it may also be necessary to change the drain valve device to the appropriate type suited to the set-up.