Reciprocating Air Compressors
A simple reciprocating compressor uses a piston moving back and forth in a cylinder with one closed end containing one intake valve and one discharge valve. The intake valve allows ambient air to flow into the cylinder, and the discharge valve allows air to flow out of the cylinder. As the piston moves away from the closed end of the cylinder it creates a vacuum which draws ambient air through the intake valve. When the piston reverses direction and moves towards the closed end of the cylinder it compresses the air and forces it out of the cylinder through the discharge valve. A crankshaft and connecting rod create the reciprocating motion of the piston, and piston rings seal the clearance between the piston and the cylinder. The compressor described above is a single stage unit, meaning that ambient air is compressed to the desired discharge pressure in one step. Since compressing air releases more and more heat as pressure increases, single stage compression becomes impractical for discharge pressures over 100 psi. Efficiency, longevity and pressure capability can be improved through multistage compression, where two or more cylinders are involved. In a 2 stage design, the first stage (low pressure) cylinder compresses ambient air part of the way and sends it through a heat exchanger called an intercooler to the second stage (high pressure) cylinder, where it is compressed to the final discharge pressure. In a 3 stage compressor another intercooler and cylinder are added.