The air pump is the simplest example of the operating principle of the aircompressor. A piston is moved inside a cylindrical housing. The forward stroke compresses the air. The return stroke motion sucks new air into the pumping space. Each compressing stroke movement thus increases the pressure of the enclosed air.
Figure 2.2.1
An air pump is, of course, an unsuitable means for producing large quantities of compressed air for industrial purposes. However, if the piston is driven not manually but electro-mechanically, designed for high pressures and fitted with suitable control units, this principle can be utilised for industrial application. However fundamentally simple or complicated particular types of compressor are designed, all compressors compress the ambient air to reach a specified final pressure.
Figure 2.2.2
Static pressure is the force effect per unit area of.a standard force FS.
Formula 2.2.1
Static pressure p or built up pressure results from the pressure exerted by a fluid at rest upon a wall lying parallel to the direction of flow.
Figure 2.2.3
On the other hand, dynamic pressure q or flow pressure, describes the force effect of a flowing mass of air onto a wall situated at a 90° angle to the direction of flow.
Formula 2.2.2
The sum of static and dynamic pressure is the total pressure of flowing compressed air exerting the required effect.
Formula 2.2.3
Referred to as free flow, differential pressure is the difference between flowing pressure and static pressure. The difference between two pressures, p1 and p2 is described as differential pressure delta p = p1,2. For purposes of calculation, the static pressure should, as a rule, be given as absolute pressure (differential pressure compared with vacuum pressure). In Fig. 2.2.4 pressure indications relative to barometric pressure pb are shown graphically.
Figure 2.2.4
For many practical applications, the term pressure is always understood as gauge pressure. Compared with atmospheric pressure, the numerical value of gauge or static pressure is less by 1 (bar). Static pressure is measured by means of a pressure gauge or with a manometer. In accordance with ISO 8573, gauge pressure is given the formula symbol pe. Here, e means exceeding.
