Reducing the capacity is commonly called unloading, and devices that cause capacity reduction are called unloaders. Capacity control is often stated in terms of loading steps. Five-step unloading, without further clarification, means nominal capacity steps of 100, 75, 50, 25, and 0%; three-step means 100, 50, and 0%.
There are two general types of unloading devices. One type adds clearance to the cylinder to reduce the capacity. The other type fully deactivates one end of a cylinder. To avoid confusion between the two types, the former devices are often called clearance pockets (fixed or variable), clearance bottles, or clearance spacers; whereas, the latter devices are called unloaders. These are described further in Section 340.
There is some parasitic gas power loss in the unloaded end of a cylinder as a result of gas flowing back-and-forth through suction valves held open or through the port of a plug-type unloader. Moreover, when one end of a cylinder is unloaded, the capacity of the loaded end is reduced to some extent by heat build-up caused by the back-and-forth flow in the unloaded end. The heat built-up from the power loss causes a rise in suction temperatures, resulting in a lower weight flow. This effect is usually small but can be as much as 10–15% if the parasitic loss is proportionally high.
Bypassing, suction throttling, and varying speed are other means of capacity control that are external to the compressor. These methods are occasionally used for reciprocating compressors. Speed control is available only on units with variable-speed drivers, such as internal combustion engines.
Reciprocating compressor performance is usually presented in a tabulation of data showing Bhp’s and capacities for various pressure conditions and gas compositions. Also, if there are clearance pockets or many unloading steps to accommodate highly variable pressure conditions, curves may be furnished by the vendor (similar to Figures 300-13 and 300-14).