The thermostatic expansion valve (TEV/ TXV) regulates refrigerant flow so that superheat at the evaporator outlet is maintained. Internal equalising TEVs sense the evaporator pressure at the valve outlet, which is acceptable only when the pressure drop through the evaporator is small (a few tenths bar) - as in a single-circuit small evaporator. Where the evaporator is large, or a distributor feeds several circuits (as in air-cooling rooms) so the pressure drop through the evaporator/distributor is significant, an internal equaliser would read a pressure lower than the true evaporator pressure at the sensing point, causing the valve to underfeed (higher superheat). An external equalising TEV takes a capillary from the evaporator outlet (downstream of the distributor/last pass) to the underside of the diaphragm, so the pressure at the diaphragm is the true evaporator outlet pressure. This corrects the balance so the valve maintains the correct superheat and full evaporator loading despite the pressure drop, preventing the starved (gassed) evaporator and poor performance that internal equalising would give in such a system.
The back pressure valve (constant-pressure, holdback or EPR - evaporator pressure regulator) is fitted in the suction line at the evaporator outlet to hold a minimum evaporating (back) pressure, i.e. to prevent the evaporator pressure/temperature falling below a set value even if the compressor or the load drops. Its purpose is to prevent the evaporator temperature falling too low - for example to stop water freezing in a cold store (maintaining temperature above 0 C), to prevent excessive drying of chilled cargo, or to protect the chilled-water circuit from frost - while allowing the compressor to run to its own suction set point lower down. It throttles the suction vapour to maintain the required minimum pressure in the space/evaporator.
If the back pressure valve leaks: it fails to hold the minimum pressure, allowing the evaporator/suction pressure to fall below the set point. The result is the evaporator temperature drops too low - water/air may freeze in the space or chilled-water cooler, frost may form and the product may be over-cooled or damaged, the compressor may run with abnormally low suction, and control of the room temperature is lost. Leakage can also cause hunting/false operation and higher power.
An electronic expansion valve (EEV) replaces the mechanical diaphragm, sensing element and spring with a valve driven by an electric actuator (stepper motor or pulse-width-modulated solenoid). Temperature and/or pressure sensors (thermistors) at the evaporator outlet (and inlet) feed a microprocessor/controller. The controller continuously calculates the actual superheat (temperature minus saturation temperature at suction pressure, or direct differential sensing) and compares it with the set-point superheat. If superheat becomes too high the controller opens the valve, admitting more refrigerant; if superheat falls too low (risk of liquid returning to compressor) it closes the valve, reducing flow. Being a proportional/integral (PI) controller it responds quickly and precisely to load changes, holds a small stable superheat over a wide range (which maximises evaporator efficiency and guards the compressor against liquid slugging), and can be programmed (e.g. for pull-down/defrost modes). Its advantages are accuracy, flexibility, fast response and better evaporator utilisation compared with a mechanical TEV.