Modulation (capacity control) valve

Pneumatic regulator that meters control air to the inlet valve so the machine throttles between 100 % and about 40 % capacity.

Also called: modulator · throttling valve · capacity regulator

How it works

Line pressure acts on a diaphragm against an adjustable spring. As line pressure rises above the modulation setpoint, the valve passes progressively more control air to the inlet valve actuator, closing the inlet and reducing capacity. Efficiency is poor below about 60 % — that's why VSD and load/unload exist.

Specifications

Typical modulation band

10 PSI (0.7 bar) from full load to minimum

Minimum capacity

Roughly 40 – 60 % of full flow

Adjustment

Set with the machine at rated pressure and a slowly closing demand valve

Field test procedure

1. Slowly throttle plant demand and watch amps: they should fall smoothly, not step.
2. Hunting means a sticking inlet linkage or a torn diaphragm.

Symptoms & probable causes

• Machine will not unload, only throttles
  Set for modulation-only control; check the mode selection on the controller.
• High kW at low demand
  Modulation control on a lightly loaded system — switch to load/unload or VSD.