The thermostatic expansion valve (TEV) controls the flow of refrigerant to the evaporator so that it is completely vaporised at the evaporator outlet. It senses: (1) superheat at the outlet via the sensing bulb (bulb pressures), (2) the spring pressure, and (3) the evaporator pressure at the outlet. The valve holds the diaphragm in balance between these pressures. In a valve with an internal equalising connection, the evaporator pressure acting on the underside of the diaphragm is taken from inside the valve, i.e. at the valve outlet, which is the inlet pressure to the evaporator. In an evaporator with several parallel circuits or with a long distributing header, there is a pressure drop through the evaporator (between the valve outlet and the point where the bulb senses). This pressure drop means the pressure at the evaporator outlet (where the bulb is and where the gas leaves) is below the pressure at the valve outlet. If internal equalising is used, the valve sees the high inlet pressure, so it would under-feed: the effective superheat it senses is larger than the true superheat, and the evaporator would be starved, giving a large superheat and poor cooling.

The external equalising connection takes the evaporator pressure from the outlet (bulb) region via an external tube to the underside of the diaphragm. This cancels out the evaporator pressure drop, so the TEV controls on the true outlet superheat and feeds correctly. It is therefore fitted on evaporators with distributors, multiple circuits, or a significant internal pressure drop, to keep the control accurate. Without it, the evaporator would be under-fed (too little refrigerant), causing freezing of frost at the outlet but poor overall capacity, or even hunting.

A back pressure valve (an evaporator pressure regulator / suction-regulating valve or an outlet regulating valve) is fitted in the suction line to hold the evaporator pressure (and hence the evaporating temperature) at a set minimum value, regardless of the compressor suction pressure. Its purpose is to maintain a constant evaporator temperature in a multi-temperature or cold room system (e.g. to avoid freezing of a water/chilled-commodity chamber), or to protect a high-temperature evaporator from being pulled down by a compressor working to a lower pressure circuit. By throttling the suction gas it holds back pressure and stabilises the box temperature (e.g. to keep a vegetable room above freezing point).

Effect if the back pressure valve leaks: if it has an internal leak or fails to close properly, the evaporator pressure cannot be maintained; suction pressure falls, the evaporating temperature drops, and the box/space can overcool and freeze the product (e.g. damage to cargo or water). It also makes the compressor work harder at lower suction pressure, increasing energy consumption, and can cause ice formation on the evaporator. If it fails fully open, temperature control is lost and the room may go too cold. Leakage also unbalances the multi-temperature system, starving other higher-pressure circuits.

An electronic (electric) expansion valve replaces the thermal sensing bulb and diaphragm of a mechanical TEV with sensors and an electronic controller (e.g. a PLC/ECU) plus a motorised drive valve (the EEV itself). The EEV is a stepper/solenoid-driven needle valve in the liquid line. Function:

1. Temperature sensors (Pt100/thermistor) are fitted at the evaporator inlet and outlet; sometimes pressure sensors at the evaporator.
2. The controller computes the actual superheat at the evaporator outlet (outlet temperature minus the saturation temperature corresponding to the outlet pressure) continuously.
3. It compares this actual superheat with a set-point superheat, and adjusts the opening of the EEV (by stepping the motor) to maintain the set-point.
4. If superheat is too high (starved) it opens the valve more; if too low (flooded) it closes it.

Advantages: very accurate superheat control over a wide range, quick response, no hunting, better energy efficiency, allows lower stable set superheat (utilising the evaporator fully), remote adjustment, and protection functions (e.g. liquid slugging prevention). It is used for refrigeration/freezing and on systems where precise control and efficiency matter.