Air compressors, air treatment and pressure regulation
83
causes the main diaphragm to rise, opening the central vent allowing air to escape from the load and pressure to be reduced. Action of pilot diaphragm and inlet air bleed approximates to integral action, giving a form of P + I (proportional plus integral) control. In the steady state, the outlet pressure equals the set pressure and there is no pressure droop with increasing flow.
Service units In pneumatic systems a moisture separator, a pressure regulator, a pressure indicator, a lubricator and a filter are all frequently required, local to a load or system. This need is so common that combined devices called service units are available. Individual components comprising a service unit are shown in Figure 3.30a, while the composite symbol of a service unit is shown in Figure 3.30b. r . . . .
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Pressure
Separator
P ressu re regulator
indicator
Lubricator
(a) Symbols for individual components
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I I I
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iJ
(b) Composite symbol
Figure 3.30
The service unit