If your air compressor is constantly running without shutting off, this could be a sign of a problem—and it is probably costing you a bundle. It is very rare to see system compressors exactly match the facility demand, requiring the need for constant full-load operation. The most obvious thing you should check for is leaks or a system machine malfunction that could be consuming excess air, but failing that here are some things to consider.

There are two basic types of compressors commonly found in shops and manufacturing plants: reciprocating and screw type. These compressor types have different characteristics, so depending on what type you have, the answer varies.

If you have reciprocating compressors, then these definitely should not be running all the time without regularly shutting off. These units generate lots of heat, but usually have quite basic cooling systems, typically consisting of a simple fan on a pulley blowing on cooling fins. The system is simple in order to keep the manufacturing costs down. But this simple system is problematic in that the compressor will usually overheat if the it runs more than about 50% duty cycle.

Reciprocating units need time to stop and cool off, or the internals will become damaged. When damage occurs, the compressor can experience valve failures, causing the compressors to run constantly. If your reciprocating compressors are never turning off, you should investigate. If you have multiple compressors, your system should be set up so as not to allow constant operation.

For screw compressors, the cooling systems are much better, so the compressors can run constantly with no issues. But running constantly usually means the units are not very efficient. Screw compressors have a few different ways to control the output to match the actual system flow. The two most common are called “modulation” and “load/unload” control.

With modulation, a control valve chokes off the inlet of the compressor to control the flow and pressure. This is nice and simple—and gives good pressure control, but is very inefficient in terms of energy consumption. For example, for lightly loaded compressors such as in service shops, this would cause the compressor to run constantly while consuming about 80% of its rated power even though it might be only 30% loaded, consuming more than 2.5 times the required energy.

With load/unload control, the compressor alternates between loaded (producing air) and unloaded (no output) condition, all the while keeping the compressor motor running, still consuming power. A lightly loaded compressor would consume about 55% of rated power at about 30% load, much better efficiency.

A much more efficient way to run compressors is to have them start and stop, which is the way reciprocating compressors operate. Screw compressors can only start and stop so many times, so they usually have an internal timer limiting the number of stops. If your compressor has too little storage and a narrow pressure band, the timer will never allow the compressor to shut down, decreasing the efficiency and causing the compressor to run constantly.

If installed with large storage, and operated with a wide pressure band, small screw compressors under about 30 hp can operate in a more efficient start/stop mode. In this mode, since the compressor motor turns off between cycles, a 30% loaded compressor will consume only 30% rated power, almost perfect efficiency.