Air compressor types
While there are small models that are comprised of just the pump and motor, most compressors have an air tank to hold a quantity of air within a preset pressure range. The compressed air in the tank drives the air tools, and the motor cycles on and off to automatically maintain pressure in the tank.
Compressors use a pressure switch to stop the motor when tank pressure reaches a preset limit-- about 125 psi for many single-stage versions. A lot of the time, however, you do not need that much pressure. The air line will include a regulator that you set to match the pressure requirements of the tool you're using. A gauge before the regulator keeps track of tank pressure and a gauge right after the regulator monitors air-line pressure. In addition, the tank has a security valve that opens up if the pressure switch malfunctions. The pressure switch might also incorporate an unloader valve that reduces tank pressure when the compressor is turned off.
At the top of the cylinder, you'll find a valve head that keeps the inlet and discharge valves. Both are simply thin steel flaps-- one installed beneath and one mounted on top of the valve plate. As the engine goes down, a vacuum is created above it. This allows outside air at air pressure to push open the inlet valve and fill the area above the piston. As the piston goes up, the air above it compresses, holds the inlet valve closed and pushes the discharge valve open. The air moves from the discharge port to the tank. With each movement, more air enters into the tank and the pressure rises.
And while there are compressors that use rotating impellers to generate air pressure, positive-displacement compressors are more common and include the models used by homeowners, woodworkers, mechanics and contractors. Here, air pressure is increased by reducing the size of the space that contains the air.
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Typical compressors can be found in 1- or 2-cylinder types to match the requirements of the tools they power. On the homeowner/contractor level, most of the 2-cylinder models operate just like single-cylinder versions, apart from that there are 2 strokes per revolution rather than one. A few commercial 2-cylinder compressors are 2-stage compressors-- one piston pumps air in to a second cylinder that even more enhances pressure.
Here is a good video showing how they work: