Compressor

How to read it

The DOE’s Mechanical Science handbook says an air compressor provides a continuous supply of pressurized air and names the reciprocating compressor as the most common design; a centrifugal compressor uses the same centrifugal force as the centrifugal pump. In the example process in Kuphaldt’s Lessons In Industrial Instrumentation, a compressor pulls a vacuum on an evaporator and carries the vapour on to a knockout drum.

Its symbol varies more than most. Kuphaldt’s chart draws an axial compressor as a trapezoid narrowing in the direction of flow, driven by a motor box marked M, and gives separate pictures for reciprocating and rotary screw compressors. The DOE’s chart draws compressors as outlines of the machine. A diagram’s legend sheet says which form it uses.

Also searched as: compressor p&id symbol, compressor symbol.

What the P&ID shows around it

Kuphaldt uses a compressor to show the difference between diagrams. The process flow diagram shows only a flow transmitter on its suction line. The P&ID adds a differential pressure transmitter, a flow indicating controller and a recycle control valve that lets some gas from the discharge go back round to the suction.

Look-alikes

In fluid power diagrams the DOE’s handbook uses a different set of symbols: a pneumatic compressor is a circle with hollow arrowheads, where a hydraulic pump has solid ones.