A refrigeration system is sized for the hottest day of the year and the heaviest loading; for the rest of the year the demand is far below this. At night, in winter and in the hours when the doors are closed the load falls to 30–50% of the design value. Capacity control is how the system meets this changing load. The simplest method is on/off: the compressor runs at full power and stops when the room has cooled. It is adequate in small rooms; in large systems it leads to frequent starts and temperature fluctuation.
The stepped methods are these. In multi-compressor central systems the compressors come in one after another; four equal compressors give steps of 25%. In semi-hermetic reciprocating compressors cylinder unloading takes some of the cylinders out of action (for example 100, 66, 33%). In screw compressors the slide valve adjusts capacity between about 25% and 100%. The digital scroll, for its part, separates and brings together the scrolls within seconds and provides near-stepless adjustment between 10 and 100%. The lowest step of every method is checked separately for oil return.
An inverter (frequency drive) changes the speed of the motor; depending on the compressor model it provides stepless adjustment between 25–30 Hz and 60–75 Hz. Because the suction pressure rises and the discharge pressure falls at part load, efficiency increases; a saving of 15–30% in annual consumption is usual. Hot gas bypass, on the other hand, reduces capacity by feeding discharge gas back to the suction: it keeps the temperature constant but goes on using energy, and should be considered only as a last resort in sensitive processes.
When capacity is reduced the rest of the system has to follow. Because the gas velocity falls, oil return becomes harder; suction risers are sized for the lowest step. A thermostatic expansion valve can become unstable at very low load; an electronic expansion valve works better over a wide range. The condenser fans should also be stepped or speed controlled. The right starting point is knowing the load; the cooling load calculator gives the design load, and the part-load ratio is determined by the pattern of operation.