Flow v Pressure

Controlling Flow and Pressure

Regulating pressure in a pneumatic system does not control flow. This article explains the difference between flow and pressure and how they should be controlled individually. The regulation of pressure in a compressed air system does not precisely control flow and can lead to increased energy costs.


Definitions of Pressure and Flow

Pressure is a force applied across a given area. Its units are N/m2 which is the same as Pascal (Pa); you will also see bar and psi (pounds per square inch).

Turning this around, the force generated will be the pressure multiplied by the area over which the pressure is applied. Flow is the volume of fluid (in this case compressed air) moved in a given time; it is often measured in dm3/s or litres/minute. 1 dm3 is the same as a litre.


How does pressure control work?

The force generated is equal to pressure multiplied by area. Thus a large input pressure over a small area can be the same as a lower output pressure over a larger area. This is the principle of the pressure regulator, with both input and output forces acting against that generated by a compressed spring. As the spring is compressed further the spring force increases; the input force remains constant (within tolerances), so the output force increases to balance the load.


How does flow control work?

A flow regulator works by essentially restricting the orifice through which air can flow; so, as the orifice is closed less air can flow, at a given pressure, in a given time. Restriction is often achieved using a needle closing or opening against a matching seat.


Wouldn't you expect pressure and flow to be directly connected?

In an ideal system. But, as we know, systems are never ideal; the pressure/flow graph of a regulator shows large area's on nonlinearity, so to use pressure regulation as a way of controlling flow will not give the precision required. It most likely will lead to increased costs due to more air flow than required (refer to our article on Energy Saving to see how much this could cost). There is also a risk of overpressure in the system which, clearly, can damage components and/or products. Our recommendation is to use separate flow control downstream.

More information

Find out more by viewing our pressure regulator and flow controller ranges.