What is the purpose of unloaders and how do they work?

• A. They hold the suction valve open to prevent compression on one side of the piston, controlling capacity.
• B. They seal the discharge valve to prevent backflow.
• C. They lubricate the piston rings.
• D. They remove moisture from gas.

Answer: (A)

Unloading is about controlling how much gas the compressor actually compresses. Unloaders work by keeping the suction valve open so the gas can be drawn into the cylinder but isn’t sealed and compressed on that stroke. In effect, that cylinder stops contributing to pressure, which lowers the overall capacity of the compressor. This makes startup and light-load operation smoother, reduces the motor’s starting torque, and allows capacity to rise quickly when demand increases.

In a multi-cylinder setup, unloaders can be used on individual cylinders to dial down overall capacity as needed. They don’t seal the discharge valve, lubricate piston rings, or remove moisture from gas—their job is specifically to prevent compression on the suction side to modulate capacity.