Control of Electric Expansion Valves
Second of Two Parts
When using electric expansion valves (EEVs), you must first determine the manner in which superheat will be sensed. There are two basic schemes for sensing superheat: 1.True superheat is a pressure-temperature relationship specific to each refrigerant. When electronically derived, pressure-temperature superheat requires the use of a pressure transducer, a temperature sensor, and a pressure-temperature table or equation. 2. A simpler but less accurate measure of superheat is the two-temperature method. In the two-temperature method, the temperature is sensed at the inlet and at the outlet of the evaporator. The difference in temperatures is then assumed to be superheat. Refrigerants or blends with temperature glides may affect two-temperature superheat control. Ordinarily, superheat setpoint must be higher to overcome the effects of glide. An advantage to two-temperature superheat is cost; pressure transducers are far more expensive than thermistors. Additionally, two-temperature superheat works with any refrigerant without reprogramming. The temperature difference between the two sensors will indicate superheat no matter what the pressure-temperature relationship of the refrigerant. The main disadvantage of the two-temperature method is the uncertainty that the inlet sensor is located properly. For the two-temperature superheat method to be accurate, the inlet sensor must be located in a position that has saturated refrigerant present at all times. Often, only flow testing of individual evaporators will provide the information necessary to establish the correct location. Failure to find, or use, the proper location can lead to poor control or compressor damage.PRESSURE TRANSDUCERS
When choosing a pressure transducer, it is important that the device is refrigerant compatible and suitable for the pressures experienced in refrigeration systems.Typically, a pressure transducer is a three-wire device. Two wires supply power, and the third is an output signal. Generally, as the pressure rises, the voltage sent from the signal wire rises.
The controller uses this voltage to calculate the temperature of the refrigerant with the use of a pressure-temperature table encoded in the controller itself.
PRESSURE-TEMPERATURE TABLES
Pressure-temperature tables are familiar to the industry and are available in many forms. To be useful to an electronic device, they are encoded in a “lookup table.” This is nothing more than an area in the memory of the controller where the information is stored electronically.When a P-T (pressure-temperature superheat) controller is used, the lookup table for the specific refrigerant used in the system must be programmed into the controller.
As with mechanical thermostatic expansion valves (TEVs), when pressure and temperature are used for superheat control, the refrigerant must be known. The controller cannot be used with a different refrigerant without internal changes or reprogramming.
Since the lookup table for a refrigerant can be stored in a fairly small amount of electronic memory, some controllers have been programmed with a number of refrigerant tables. A switch on the controller selects the proper table for the application.
Another way the pressure-temperature relationships of one or more refrigerants are stored in the memory of a controller is by use of the “equation of state.” The equation of state is a mathematical description of a refrigerant’s properties.
Since EEV controllers are, in fact, small computers, they have the ability to process equations efficiently and quickly. It is the controller designer’s decision as to the best method of storing the P-T relationships.
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Once the pressure of the refrigerant is sensed and the lookup table is used to calculate the saturated temperature, only the real suction temperature must be sensed to determine the operating superheat. Temperature sensors detect suction temperatures.
TEMPERATURE SENSORS
Temperature sensors are available in different types. Most often, a thermistor is chosen because of availability, reasonable price, and good accuracy.A thermistor is a solid-state device that changes electrical resistance in response to a change in temperature. Other terms, such as PTC (positive-temperature coefficient) and NTC (negative-temperature coefficient) are sometimes used in thermistor literature, but that characteristic is not pertinent to this discussion.
The controller that calculates temperatures at the sensor location uses the change in resistance of the thermistor. The calculated temperatures are then used to generate superheat measurements, either by the pressure-temperature method or by the two-temperature method.
Temperature sensors are also used to allow electric valves to directly control temperature.
DISCHARGE AIR OR WATER CONTROL
A secondary routine in EEV algorithms may control the temperature of the discharge air or water directly. In this design, as long as superheat remains above some minimum value, the temperature of the medium being cooled is the control setpoint.If superheat falls, the controller resumes superheat control and attempts to raise superheat to the set value. Once superheat is re-established, discharge temperature control is resumed.
This type of algorithm may be suitable for some process applications, but has been found to be less desirable in supermarket display cases. In a refrigerated display case with direct air temperature control, the efficiency of the EEV allows less of the evaporator to be used, but at a higher TD (temperature difference). Higher TDs on the evaporator may lead to an increase in frost and require longer or more frequent defrost periods.
In general, EEVs using superheat control algorithms are not more likely to build frost. In systems with coils specifically designed for EEV control, or with provisions to float suction pressures, EEVs may increase control precision while saving energy.
CONTROL OF ELECTRIC EVAP PRESSURE REGULATORS
Traditional forms of temperature control involve thermostats. Thermostats generally stop cooling when the temperature setpoint is reached, and start cooling when the temperature rises a certain amount above setpoint.This difference is called “deadband,” and while modern thermostats may control to a 2° to 3° deadband, some temperature swing is inevitable.
Direct-acting or pilot-operated mechanical evaporator control valves are simple to apply and in many cases are very effective. However, both operate with a gradient; the valves must experience a pressure drop to open fully.
In addition, an adjusting screw sets mechanical evaporator pressure regulators (EPRs). If the desired pressure and temperature change, the valves must be mechanically reset. Electronic EPRs (EEPRs, Figure 1) are motor driven in response to a sensor. Because the sensors are very accurate and quick acting, EEPRs are not subject to gradient or large deadbands.
Case temperature setpoint changes can be made electronically, without needing physical access to the valve. EEPRs can be driven shut for defrost and driven fully open after defrost to allow extremely rapid pulldown.
TROUBLESHOOTING and FAILURE MODES
Electric valves respond only to the signal supplied by their controllers. Step motor valves will maintain their position whether voltage is removed intentionally or due to controller, power, or wiring failure.In those applications where system damage may result from valves failing in an open position, a liquid line solenoid should be placed before the valve. Testing has shown that under most operating conditions, EEVs modulate at about 30% of full open.
EEV or controller failures would most likely not lead to floodback, except immediately after defrost. Battery backup or uninterruptible power supplies (UPS) can be designed for step motor applications, but reliability testing indicates that this would be an unnecessary expense. Most modern controllers used with electric valves have built-in diagnostic capabilities.
When a failure is experienced in the valve/controller system, the first step is to define the failure as being controller related or valve related. The manufacturer should be consulted for specific troubleshooting information on the controller.
Dolan is with Sporlan Valve Co., 206 Lange Drive, Washington, MO 63090; 636-239
-1111.
Sidebar: Discharge Gas Bypass Valves
Mechanical, pressure-operated, discharge gas bypass valves (Figure 2) require a gradient, or change in pressure, to operate. This may result in temperature swings of up to 10°F in setpoint. Greater accuracy can be obtained electronically.Heat Reclaim
Heat reclaim involves diverting hot gas, rejected by a refrigeration or air conditioning system, into a secondary condenser for heating water or air.
Publication date: 09/18/2000