Don’t overlook critical backpressure

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.