Although there are compressors that use rotating impellers to generate air pressure, positive displacement compressors are more common and include models used by homeowners, woodworkers, mechanics, and contractors. Here, the air pressure is increased by reducing the size of the space containing the air. Most compressors you will use use reciprocating pistons to accomplish this.
Like small internal combustion engines, traditional piston compressors have a crankshaft, connecting rods and pistons, cylinders and valve heads. The crankshaft is driven by an electric motor or a gas engine. Although some models consist only of a pump and an electric motor, most compressors have an air tank that can keep a certain amount of air within a preset pressure range. The compressed air in the air tank drives the pneumatic tool, and the motor will cycle on and off to automatically maintain the pressure in the air tank.
On the top of the cylinder, you will find a valve head with intake and exhaust valves. Both are simple thin metal discs-one on the bottom and one on the top of the valve plate. When the piston moves down, a vacuum is created above it. This allows outside air to push open the intake valve at atmospheric pressure and fill the area above the piston. When the piston moves upward, the air above it compresses, keeping the intake valve closed and pushing the exhaust valve open. The air moves from the outlet to the water tank. With each stroke, more air enters the tank and the pressure rises.
Typical compressors are available in 1-cylinder or 2-cylinder versions to suit the requirements of the tools they provide power. At the homeowner/contractor level, most 2-cylinder models operate in the same way as single-cylinder models, except that there are two strokes per revolution instead of one. Some commercial 2-cylinder compressors are 2-stage compressors, with one piston pumping air into the second cylinder, thereby further increasing the pressure.
When the tank pressure reaches a preset limit (approximately 125 psi for many single-stage models), the compressor uses a pressure switch to stop the motor. However, in most cases, you do not need that much pressure. Therefore, the air line will include a regulator that you can set to match the pressure requirements of the tool used. The pressure gauge before the pressure regulator monitors the tank pressure, and the pressure gauge after the pressure regulator monitors the air pressure. In addition, there is a safety valve on the water tank. If the pressure switch fails, the safety valve will open. The pressure switch can also be equipped with an unloading valve, which reduces the tank pressure when the compressor is turned off.
Many articulated piston compressors use oil lubrication. That is, they have an oil bath that splashes to lubricate the bearings and cylinder walls as the crank rotates. Pistons have rings that help keep compressed air at the top of the piston and keep lubricating oil away from the air. However, the ring is not completely effective, so some oil will enter the compressed air as aerosols.
Oil in the air is not necessarily a problem. Many pneumatic tools require oiling, and in-line oilers are often added to increase the uniform supply of tools. The downside is that these models require regular oil checks, regular oil changes, and must be operated on level ground. Most importantly, some tools and situations require oil-free air. Spraying oil in the air stream can cause paint problems. And many new woodworking pneumatic tools (such as nails and sanders) are designed to be oil-free, so there is no chance of getting oil on the wood surface. Solutions to the problem of oil in the air include the use of oil separators or filters in the air line, but a better idea is to use an oil-free compressor that uses permanently lubricated bearings instead of an oil bath.
A variation of the automobile type piston compressor is to use an integrated piston/connecting rod model. Because there is no wrist pin, the piston will tilt left and right when the eccentric journal on the shaft moves up and down. The seal around the piston maintains contact with the cylinder wall and prevents air leakage.
In situations where the air demand is not high, a diaphragm compressor may be effective. In this design, the diaphragm between the piston and the compression chamber seals the air and prevents leakage.
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