The thermostatic expansion valve (TEV/TXV) starts by throttling liquid refrigerant into the evaporator and maintaining the correct superheat at the outlet. The valve balances the diaphragm pressure (from the sensing bulb) against the spring and the evaporator pressure. In an internally equalised valve the evaporator pressure is sensed at the valve outlet, which is acceptable only if the evaporator has a small pressure drop. Where the evaporator is large or a distributor feeds several circuits, the pressure drop through the distributor and evaporator is considerable, and the pressure at the valve is higher than at the evaporator outlet. The internal equaliser would then give a false (higher) pressure under the diaphragm, causing the valve to close and underfeed (starve) the evaporator. The external equalising connection takes a capillary from the evaporator outlet (downstream of the distributor) to the underside of the diaphragm, so that the diaphragm senses the true evaporator outlet pressure. This gives correct superheat control, full evaporator loading and stable operation despite the pressure drop; hence external equalisation is fitted on distributor-fed and large evaporators.

The back pressure (holdback/EPR) valve is placed in the suction line at the evaporator outlet and is set to hold a minimum evaporating pressure in that circuit/space. It throttles the suction gas so that the evaporator pressure (and hence temperature) does not fall below the set value even when the compressor pulls the suction down further. Its purpose is to prevent the evaporator temperature dropping too low - for example to prevent water freezing in a chilled-water cooler or cold store, to prevent overshooting of temperature, or to balance several rooms on one compressor at different temperatures. If it leaks, it cannot hold the minimum pressure; the result is that the evaporator/suction pressure falls below the set point, the evaporator temperature drops too low (frost/ice formation and over-cooling), the compressor may run with abnormally low suction and the room/temperature control is lost. Leakage also wastes capacity and can cause excessive frost.

An electronic expansion valve is driven by an electric actuator (stepper motor or pulse-width-controlled solenoid) instead of a mechanical diaphragm and spring. Sensors (thermistors/thermocouples) measure the temperature at the evaporator outlet (and, in some, the chilled/refrigerant pressure or superheat differentially). A microprocessor control unit computes the actual superheat (outlet temperature minus the saturation temperature corresponding to the suction pressure) and compares it with the controller's set-point superheat. When superheat is too high the controller opens the valve to admit more refrigerant; when superheat is too low (risk of liquid returning to the compressor/liquid slugging) it closes the valve to reduce flow. Being a fast, proportional-integral controller, an EEV holds a small, stable superheat over a wide load range, responds quickly to changes, gives better evaporator utilisation and efficiency, and may be programmed for pull-down, defrost or start-up sequences, giving more precise control than a mechanical TEV.