Bleed-off vs continuous duty

Bleed-off vs continuous duty: two often-confused terms. Bleed-off (or constant-run unloaded) is a control behavior, where a running compressor stays motorized but stops compressing by opening the inlet to atmosphere. Continuous duty is a mechanical/thermal rating, meaning the compressor can run loaded 100 percent of the time without exceeding its design envelope.

What it is

In bleed-off control, the compressor avoids start/stop cycling on small loads by simply running unloaded between demands. Power consumption while unloaded is roughly 20-30 percent of full load — wasted unless demand resumes within the unload window. Most fixed-speed industrial rotary screws use load/unload control with a programmable unload timer.

Why it matters

A 100 percent continuous-duty compressor can run loaded all day. A 50 percent duty cycle compressor cannot, regardless of control mode. Conversely, a continuous-duty compressor in bleed-off control is still wasting energy while unloaded; the right answer for variable demand is VSD, not just continuous-duty load/unload.

How it is used in our catalog

Catalog listings publish both: the mechanical duty rating (100 percent / continuous on industrial machines) and the available control modes (load/unload, modulating, VSD). Browse rotary screw compressors and check the control type alongside duty rating.

Common questions

Does a continuous-duty compressor always run loaded?

No. It can run loaded continuously, but plant demand rarely requires that. Control mode determines what happens at part load.

Is bleed-off the same as modulating control?

No. Modulating control closes the inlet valve partway to reduce SCFM, holding pressure within a narrow band. Bleed-off / load-unload bangs between full load and unload. Modulating saves less energy than VSD but more than aggressive load/unload at part load.