One of the most confusing aspects of industrial pneumatic technology concerns the relationship between air pressure and air flow. We know that pressure measures the force per unit area — typically in psi; while flow measures the volume of air that moves in a unit of time — typically in scfm. If we need more force to move a load, we crank up the pressure. Because F = pA, and area A is constant, more pressure (p) generates more force (F). When we want to cycle faster, the tendency is to crank up the pressure again — perhaps on the assumption that it will have the same effect as pushing the gas pedal to make a car go faster.

The confusion begins because of the direct relationship between compressed- air flow and pressure. The only reason that air will flow from point X to point Y is that there is a difference in pressure. Point Y must be at a lower pressure for air to flow toward it, similar to the flow of water from a higher elevation to a lower elevation. The greater the difference in pressure, the greater the flow of air — up to a point.

Just as an object falling from an airplane reaches a maximum speed called terminal velocity, flowing air reaches a speed called critical velocity. This is the speed of sound — roughly 1100 fps or 750 mph. Air can flow no faster.

Critical velocity is achieved when downstream pressure is 53% or less of upstream pressure. In pneumatics, the 53% figure is known as critical backpressure. For example: if supply pressure is set at 75 psi, and air is exhausting from a cylinder at 50 psi (67% of supply), you can increase the stroke speed by increasing the supply pressure. But at 95-psi supply, the 50-psi exhaust pressure is 53%. Any increase in supply pressure cannot increase cylinder speed. It will only waste energy and strain components unnecessarily.

Charles Post
VP/general manager
TSI Solutions
Stone Mountain, Ga.