Ultrasonic Inspection: Compressors

Introduction

Air compressors are devices used to raise the pressure of air to above atmospheric pressure. The pressure of air is measured by the amount of pressure in pounds per square inch (PSI). Normal atmospheric is approximately 14.7 PSI, but compressors can deliver air up to 200 PSI and higher.

The rate at which a compressor can deliver a volume of air is called cubic feet per minute (cfm). CFM varies with atmospheric pressure because atmospheric pressure plays a role in how fast air moves. Other factors that vary CFM include temperature and humidity. This is one reason it is impossible to measure air flow rate. One way to standardize a CFM rating is to refer to it as SCFM, or standard cubic feet per minute, which is the CFM rate at sea level with 68 degrees Fahrenheit at 36% relative humidity. SCFM ratings are given at certain pressures. If the pressure goes up, the SCFM rating goes down and vice versa.

Air compressors are used in many different situations from powering tools to driving conveyors. It is becoming commonplace for people to have air compressors at home. The air compressor uses a single motor to convert electrical energy into kinetic energy. This saves from having to use multiple motors to power mechanical systems.

While there are many types of compressors, they each do the same job. They force air into a storage tank or small area such as a tube, causing pressure to build. Most compressors use a reciprocating piston. The motor operates the piston. As the piston moves down, a vacuum is created that allows outside air to push open an inlet valve. When the piston moves up, the inlet valve gets shut forcing the air to move into the storage tank. With each stroke of the piston, more air enters the tank, causing an increase in pressure.

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