Expansion devices employed in refrigeration plants are based on two main designs: the capillary tube, or the orifice. The capillary tube is usually used in domestic appliances, where sophisticated control of the system is not required. However, in large refrigeration plants both the refrigerant mass flow rate and pressure levels have to be maintained to within acceptable working limits. Numerous valve configurations have been developed to accomplish these controls. The operation of these valves is based on a sharp-edged orifice whose throat area can be varied to control the operation of the plant