On slight difference, however, is in the need for keeping the inside of the pump perfectly clean. Other than the fact that they do not require an oil separator, systems using dry air pumps are quite similar to those using a wet pump. These pumps use carbon vanes and do not require any lubrication, as the vanes provide their own lubrication as they wear away at a carefully predetermined rate.The more modern vacuum pumps are of the dry type.This valve has a spring-loaded poppet that offsets to allow cabin air to enter the pump and maintain the correct negative pressure inside the instrument case.The airflow through the instruments is controlled by maintaining the suction in the instrument case at the desired level with a suction relief valve mounted between the pump and the instruments.But before it could be used, this air was passed through a second stage of oil separation and then to the distributor valve and finally to the boots (See figure 12-2.).Aircraft equipped with rubber deicer boots used this discharge air to inflate the boots.Engine oil was metered into the pump to provide sealing, lubrication, and cooling, and then this oil, along with the air, was blown through an oil separator where the oil collected on baffles and was returned to the engine crankcase.The early vacuum pumps were vane-type pumps of what is called the wet type-one with a cast iron housing and steel vanes.Most of the early instruments used only paper filters in each of the instrument cases, but in some installations a central air filter was used to remove contaminants from the cabin air before it entered the instrument case.A suction relief valve maintained the desired pressure (usually about four inches of mercury) on the attitude gyro instruments, and a needle valve between one of the attitude indicators and the turn and slip indicator restricted the airflow to maintain the desired 2 inches of suction in its case. In order to overcome the major drawback of the venturi tube, that is, its susceptibility to ice, aircraft were equipped with engine driven vacuum pumps and the gyro instruments were driven by air pulled through the instrument by the suction produced by these pumps.
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