Electronic expansion valves have been increasing in popularity, and the technology has evolved to cope with increasingly sophisticated demands. However, the cost of electronic valves, which includes the sensors, regulator, actuator and the valve itself, is still much higher than for the simple and mechanical thermal expansion valve. Electronic expansion valves are therefore mostly found on very large systems and systems with a high demand for precise regulation. Two different models of electronic valves are discussed in this section: modulating electronic expansion valves, with a continuously adjusting orifice and electronically controlled ON/OFF valves, with a solenoid valve that is opened and closed periodically.
Modulating electronic expansion valves
Modulating electronic expansion valves are controlled by temperature or pressure sensors. The electronic regulating unit can be programmed to correct for differences in temperature and pressure at any point of the system. Because the electric actuator reacts only to signals from the regulator, there are good possibilities for achieving a lower level of superheating than with a thermal expansion valve. The signal follows the MSS signal continuously, because any hunting tendencies will be compensated immediately. Pressure differences between the valve and the sensor caused by refrigerant distribution systems are also corrected. Furthermore, the same valve can be used for different refrigerants after reprogramming. The electric actuator controls the shutter to adjust the orifice area continuously to allow a higher or lower mass flow of refrigerant to pass, depending on the signals from the regulator.
This type of valve can handle large variations in operating conditions, e.g. changes in pressure difference and cooling capacity. The biggest disadvantage of electronic valves is the relatively high cost and complexity of components. Programming the regulator box is not trivial, and system performance with a poorly adjusted electronic expansion valve may very well be lower than with a thermal expansion valve.
Electronically controlled ON/OFF valves
The electronic ON/OFF valve is actually an electronically controlled solenoid valve that functions both as an expansion valve and as a solenoid valve. When functioning as an expansion valve, ON/OFF control is used. During one cycle period, typically 6 seconds, the valve is opened and closed once. This type of valve is normally not recommended for use with brazed plate heat exchanger evaporators, because the sudden surge involved when the solenoid valve opens will create thermal and mechanical stresses in the evaporator that can lead to a reduced life expectancy. A system with an electronically controlled ON/OFF valve is shown in Figure 4.14.
The capacity of the expansion valve, i.e. the amount of refrigerant flowing through it, is determined by the relationship between the opening and closing times. A regulator controls the opening and closing of the valve in order to reach the correct level of superheating. The inputs to the regulator are the temperature and pressure at the evaporator outlet. The inputs could also be the inlet and outlet evaporator temperatures, as for an electronic valve with continuous control.
When the demand for refrigerant is high (high cooling capacities), the valve remains open for almost the entire 6 seconds. When the demand is very low (low cooling capacities), the valve opens only for a fraction of the 6 seconds. When the compressor is shut off, the valve closes and functions as a solenoid valve. The variations in opening and closing times over the cycle periods are shown in Figure 4.15.
In a system where a brazed plate heat exchanger condenser is used and no receiver is installed in the liquid line, the on/off operation of this type of valve can cause problems. This type of system is shown in Figure 4.14. Unless enough refrigerant is charged in the system, there is a risk of the condenser draining during the opening period and a gas/liquid mixture being fed to the valve. This will cause a decrease in the valve capacity, i.e. the refrigerant flow will decrease, and insufficient refrigerant will pass through. The result may be system hunting, or excessive gas superheating. To prevent this potential problem, the system must be charged with a sufficient amount of refrigerant.
The electronic ON/OFF valve can operate satisfactorily even with large variations in operating conditions, such as changes in pressure difference or cooling capacity. The valve capacity is adjusted simply by changing the relation between the opening and closing time. This allows the minimum stable superheating to be found for a wide range of operating conditions.