Pneumatics and Pneumatic Power

"Pneumatics" is the general term used to describe the mechanics of gases. And "pneumatic power" can be defined as: production or control of mechanical outputs for useful work by means of a pressurized gas in a closed circuit. In simpler language, fluid power provides a very reliable "muscle" function ranging from something as simple as pressure for blowing an air horn to vacuum for lifting a huge metal workpiece into a precise position on a worktable.

Although the definition allows for any gas, air is almost always used in practical industrial systems. Only air is considered in the remainder of the book.

Pneumatic power is one of the two branches of "fluid power." The other is "hydraulic power," in which the working fluid is a liquid. In pneumatic systems, unlike hydraulic systems, the pressures can be positive or negative.In pneumatic systems, the pressure differentials necessary to do work are produced by air compressors. They push more air into the system, increasing the pressure above that of atmospheric air.

In vacuum systems, the pressure differentials are produced by vacuum pumps. These pull air out of the system, decreasing the pressure below atmospheric.

Even though we sometimes refer to vacuum as negative pressure, this can be misleading. In an absolute sense, pressure is always positive. Pressure can be "negative" only in relation to some other, higher, pressure- But since we are constantly surrounded by the atmosphere, it's often natural to describe below atmospheric pressures as negative.

Compressed Air and Vacuum Systems—Fig. 1 compares the basic operation of compressed air and vacuum systems. In both systems a prime mover such as an electric motor or gasoline engine operates an air compressor or vacuum pump, converting electrical or chemical energy into pneumatic energy.

Note how atmospheric air enters and leaves each system, and the direction of the arrows indicating transmission of pneumatic energy. At the end of each system, an appropriate control valve and work device (air cylinder, air motor, etc.) converts the pneumatic energy into useful mechanical force or power. In either case, the air is a working fluid that is unchanged over a complete operating cycle.

Although the pressure differentials generated are exactly opposite in vacuum and pressure systems, there is considerable similarity in the equipment used. Air compressors and vacuum pumps use the same basic mechanisms.

In principle they can be considered the same machine but with the inlet and outlet ports reversed. That is, each takes air at a lower inlet pressure and transforms it to air at a higher outlet pressure. But in a compressor the inlet is usually at atmospheric pressure and the outlet is connected to the system; in a vacuum pump it is the outlet that is at atmospheric pressure.

Sometimes compressors and vacuum pumps are assembled in part from the same interchangeable stock of components. Valving, porting, and oilers usually differ, however.

Another major difference is in the drive power needed. Depending on its pressure rating, an air compressor may require from 150 to 400 percent more power than a vacuum pump of the same open-capacity rating.

System Categories—A clear understanding of the basic types of compressor and vacuum pump systems and their relationships should be helpful. These are summarized in Fig. 2. Keep in mind that, in all cases, a power-driven device transforms air at some initial intake pressure to air at a higher outlet pressure.