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Electronically controlled electric valves have been accepted for years in larger applications, just as chillers have been, and have proven to add efficiency to the systems. The key to the increase in efficiency is due to their ability to control superheat to a low, stable setting. Since many chillers use almost infinitely variable screw compressors, an expansion valve that has the ability to follow these radical changes in capacity is required. Traditionally, mechanical thermostatic expansion valves (TXVs) are normally rated from about 50 to 110 percent of nominal rating. This rating is based on relatively constant head pressure and liquid quality conditions. These valves may not allow efficient control of the chiller during low load or head pressure conditions.
The electric expansion valve (EXV) has the ability to follow load, in most cases, from about 5 to 115 percent of nominal load.
In addition, flash gas in the liquid line is less damaging to the operation of the system because the relatively large port and large stroke of the EXV allows them to open wide, purge the flash gas, and then re-establish the desired superheat. The advantages of the EXV are clear, but since the valve is an electric component, it requires some form of electric or electronic control.
ELECTRONICS
Controllers for EXVs are becoming common and generally work in similar ways. Although electric, the EXV is still an expansion valve. As such, it should be used to control superheat. Controllers allow the EXV to do this by measuring the two components ot superheat: pressure and temperature.
Standard calculations of superheat are all the same, whether done by a technician in the field or an electronic controller. The pressure in the suction line, just after the evaporator, is measured. This pressure is converted to a saturation temperature by the use of a pressure-temperature chart for the refrigerant used. This saturated temperature is compared with the actual temperature measured on the suction line, also near the outlet of the evaporator. The difference between these two temperatures is the operating superheat.
SENSORS
The electronic controller (shown in a common...