Capacity control
Also known as: capacity modulation, part-load control, staged operation
Short definitionThe ways of raising and lowering the output of the refrigeration plant to match the need of the moment. The method chosen affects electricity consumption, how much the room temperature swings and how often the compressor starts.
Equipment is selected for the hottest day and the heaviest loading, yet at night and in winter the load falls to 30–50% of that. In a small room, stopping and starting the compressor is enough. Larger plants need steps: in a multi-compressor pack the compressors cut in one after another, in a semi-hermetic reciprocating compressor some cylinders are unloaded, in a screw compressor the slide valve throttles, and a digital scroll works almost steplessly between 10% and 100%.
An inverter gives the smoothest control by changing the speed and can cut annual consumption by 15–30%. With hot gas bypass the room temperature does not swing, but the compressor goes on drawing electricity for nothing; it is used only in sensitive processes. When choosing steps in a quotation, look at the pipes as well: at a low step the gas slows down, and the suction riser must be sized to carry the oil. An electronic expansion valve and speed-controlled condenser fans keep the system stable at low load.