Larger systems are commonly operated with the compressor running continuously and the capacity regulated by unloading. One mechanical method used with reciprocating compressors is shown in Figure 5.
Pressure from the receiver is transmitted to the underside of the piston of the device, which acts against a spring set at a predetermined pressure. When the pressure in the receiver exceeds this pressure, the spring compresses and allows air to flow to a piston on top of the suction valves which in turn depresses a claw which holds the suction valves off their seat, so that compression cannot take place in the cylinder. This is termed suction valve unloading.
When double-acting compressors are used, by having two air relays set at different pressures, it is possible to unload each side of the piston in turn, giving full load, half load and zero load. This is three step unloading.
A variation of this method is to allow the inlet valves to open normally to fill the cylinders but to keep them open for a timed interval of the compression stroke, after which compression can begin. This can provide stepless regulation, but is normally used only with process compressors.
In clearance pocket control, one or more pockets containing added clearance volume are connected to the first stage cylinder, thereby lowering the volumetric efficiency and the amount of air delivered. These pockets may be manually or automatically controlled. The pockets usually have a fixed volume, but may have continuously variable pockets. A normal method on double acting compressors is to have a combination of clearance pocket and suction valve unloaders to give 100%-75%-50%-0% unloading.