Understanding relationship between air pressure and flow

“Pressure” and “flow” are common terms when discussing air compressors, but the relationship between the two is often misunderstood.

Pressure, typically measured in psi, determines an air compressor’s ability to perform a certain amount of work at any given point in time. Consider an application as simple as sliding a wooden block across a table with compressed air. In this application, 75 psi (5.2 bar) of air might not deliver enough force to move the block, but 100 psi (6.9 bar) would.

Figure 1 (left). Pressure is typically measured in psi and determines an air compressor’s ability to perform a certain amount of work at a given point in time. Figure 2 (right). Flow is typically measured in cubic feet per minute at a specific pressure and indicates an air compressor’s ability to continuously perform a task. Image courtesy of Atlas Copco.

At a minimum, the compressor must provide the pressure (force) needed to perform the job. If the minimum pressure required is 100 psi but the compressor can’t provide it, the job won’t get done. However, too much pressure can damage compressed air equipment or cause it to operate erratically. Each pneumatically powered device has a stated pressure range in which it works reliably, and these limits must not be exceeded.

Flow is the ability of an air compressor to continuously perform a task. In the U.S., flow is typically measured in cubic feet per minute (cfm) at a specific pressure. The amount of flow depends on the length of time needed to complete the task.