Capacity Control on the Trane models M & R are electrically controlled hot gas by-pass unloaders. Both models consist of the same parts though they have a slightly different design. The main parts are the solenoid valve, unloader piston, unloader valve and the check valve.
There are two unloader head available, one that unloads both cylinders on the bank and one that unloads only one of the cylinders. Various combinations of the two heads and non-unloading heads make possible a wider variety of capacity control steps.
The solenoid valve controls operation, hi pressure gas through this valve to the unloader piston opens the unloader valve. The position of the unloader valve determines the route of the cylinder discharge. When closed all gas is sent thru a check valve to the discharge manifold, when open its sent back to the suction side of the compressor.
The check valve is necessary to prevent a direct connection between the discharge and suction sides of the compressor when the unloader valve is open. Since all cylinder banks connect to the discharge manifold, a direct connection would result in the compressor trying to unload all cylinders when the unloader valve opens, not just the cylinders within the unloader head.