Inlet valve (modulating vs load/unload)

Inlet valves regulate the air entering a compressor under part-load conditions. Two main strategies exist: load/unload, in which the valve is either fully open (compressor making air) or fully closed (compressor unloaded and bleeding to atmosphere); and modulating, in which the valve can throttle to any partial opening, reducing SCFM proportionally.

What it is

In a load/unload rotary screw, the inlet valve slams shut and a blow-down valve dumps separator pressure when the system reaches setpoint. The motor keeps running at idle power until the unload timer expires or demand resumes. In a modulating compressor, the valve gradually closes against rising line pressure, gradually reducing intake volume and intake pressure to slow the compression process. Modulating maintains stable pressure but at relatively poor part-load efficiency.

Why it matters

Load/unload is the right control for plants with infrequent partial demands; modulating is appropriate when steady pressure matters more than energy efficiency. Both are inferior to VSD on variable demand. Picking the wrong control mode wastes 10-20 percent of annual compressor energy on a typical mid-sized plant.

How it is used in our catalog

Each rotary screw compressor in our catalog publishes available control modes. Filter to compare load/unload, modulating, and VSD options against your duty profile.

Common questions

Can a compressor have both modes?

Yes. Many modern controllers support load/unload, modulating, and dual control (modulate then unload) selectable at the panel. The choice is configuration, not hardware.

Why is modulating less efficient than load/unload?

A modulating compressor compresses every cubic foot of inlet air, even when the inlet is throttled to lower pressure. The work is still substantial. An unloaded compressor draws roughly 25 percent of full power; a modulated compressor at 50 percent flow can draw 70-80 percent.