The simple thermostatic expansion valve relies on the pressure under the diaphragm being approximately the same as that at the coil outlet, and small coil pressure drops can be accommodated by adjustments to the spring setting.
Where an evaporator coil is divided into a number of parallel passes, a distribution device with a small pressure loss is used to ensure equal flow through each pass. Pressure drops of 1-2 bar are common. There will now be a much larger finite difference between the pressure under the diaphragm and that at the coil inlet. To correct for this, the body of the valve is modified to accommodate a middle chamber and an equalizing connection which is taken to the coil outlet, close to the phial position. Most thermostatic expansion valves have provision for an external equalizer connection (see Figure 8.4).
Figure 8.4 Thermostatic expansion valve with external equalizer
The thermostatic expansion valve is substantially an undamped proportional control and hunts continuously, although the amplitude of this swing can be limited by correct selection and installation, and if the valve always works within its design range of mass flow. Difficulties arise when compressors are run at reduced load and the refrigerant mass flow falls below the valve design range. It is helpful to keep the condensing pressure steady, although it does not have to be constant and can usually be allowed to fall in colder weather to save compressor power. Valves on small systems may be seen to fully close and fully open at times. Excessive hunting of the thermostatic expansion valve means that the evaporator surface has an irregular refrigerant feed with a resulting slight loss of heat transfer effectiveness. If the hunting is caused by a time lag between the change of valve position and the effect at the evaporator outlet, a solution can be to increase the mass of the sensor phial which will increase damping. Over-sized valves and incorrect phial position can also give rise to hunting. The phial should always he located on the horizontal outlet, as close to the evaporator as possible and not on the underside of the pipe. Electronic expansion valves are now becoming increasingly popular for many applications.
Posted in Refrigeration and Air Conditioning