AK-PC 772 is complete regulating units for capacity control of compressors and gas cooler in a small CO2 booster systems. The controller is with heat recovery function and CO2 gas pressure control. In addition to capacity control the controllers can give signals to other controllers about the operating condition, e.g. forced closing of expansion valves, alarm signals and alarm messages. The controller’s main function is to control compressors and gas cooler so that operation all the time takes place at the energyoptimum pressure conditions. Both suction pressure and gas pressure are controlled by signals from pressure transmitters and temperature sensors.