Capacity Control in Refrigerated Systems

Powerful refrigeration methods are being deployed to cool electronic devices in test and in end-use application. Cooling capacity control is required to prevent over- or under-cooling. Various levels of control precision are also required. Test applications demand precise temperature control while many end-use applications will accommodate a less sophisticated approach. Important determinants of the method employed to control refrigeration capacity include target operating temperature, absolute and dynamic power dissipation of the device being cooled, control precision required, refrigeration system design and construction, as well as application-related details of the electronics assembly. A variety of capacity control methods are needed to cover the breadth of electronics cooling applications. Control free, or open loop systems meet the needs of some applications. Other systems employ thermal expansion (TX) and hot-gas by-pass valves to provide controlled cooling of very high power electronics cooling vapor compression systems. Modulation of condenser efficiency by varying fan speed provides a very simple but limited range of temperature control. A broad range of precise temperature control requires a combination of approaches to precisely meter refrigerant flow to the cold plate and, if needed, to apply parasitic heat. This paper overviews various vapor compression refrigeration control architectures as they apply to electronics cooling. Comparative cost and performance data are presented.