Tuin RtAITogOther More A TEV is a proportional control devic capacity changes are direc subcooling an re drop across th 二:二=2 -Mos affirm WhatIs supOreatunungP lps for preventing superheat Superheat hunting is the fesult of the Iing InTa'S luctuntonas h s setio temperature. anda utimate customer dissatisfaction. WITCHhunt IiSCtHEtMtetemt service tech should consider when = Laudun Undercharged system: Intermittent loss af subcooling is causing loss af Loss or delay of temperature signal to load on expansion valve bub m时出出如 ed n e field, a doser examination and a4 Putting It All Together A TEV is a proportional control device. That means capacity changes are directly proportional to changes in superheat. Figure 9 shows the characteristics of three TEVs. To simplify the diagram, each characteristic is shown as a single curve. The slope of the characteristic is called its gain. The greater the gain, the steeper the valve characteristic, and the bigger the capacity change for a given change in superheat. Each of the curves in Figure 9 shows the capacity increase for a 1?change in superheat. In an actual operating system, a TEV's gain will change with variations in subcooling and pressure drop across the valve. Figure 9. Valve gain for three different TEV designs. I n an operating system, gain will vary with changes in subcooling and pressure drop. More… In an actual operating system, a TEV's gain will change with variations in subcooling and pressure drop across the valve. Figure 10 shows how three valve characteristics with different slopes "fit" an evaporator characteristic. Remember that the evaporator characteristic we want the valve to fit is to the right of the unstable zone, but as close to it as possible. It's the valve closing curve that's closest to the evaporator characteristic. The range shown in green is the system's most-efficient operating range. It is important to take the hysteresis band into consideration because the valve opens in the stable area where there is only gas at the sensing bulb, and closes just before the system becomes unstable. In the first two examples in Figure 10, the valve closing curves are completely outside the unstable zone. Due to the slope of the characteristics in those examples, there is a comparatively large area of high efficiency (green). In the third example, due to a steeper characteristic slope, the upper portion of the closing curve falls inside the unstable zone, and the high-efficiency range is much smaller. Knowing how hysteresis affects valve operation, how refrigerant quality changes in an evaporator, and how valve and evaporator characteristics need to match will give a service technician a base of understanding that will make it easier to tune refrigeration systems for optimum performance at minimum stable superheat (MSS). Where the valve characteristi c's closing curve fits most closely to the evaporator characteristi c (the MSS line), system efficiency is greatest. Tips for preventing superheat hunting in TXVs what is superheat hunting? Superheat hunting is a cyclical fluctuation in suction superheat due to varying refrigerant flow rate in the system. Superheat hunting is the result of the expansion valve (see Figure 1) excessively opening and closing in an attempt to maintain a constant operating condition. Hunting can be seen indirectly by regular fluctuations in suction temperature, and in extremes, suction pressure. Excessive hunting can reduce the capacity and efficiency of the system, resulting in uncomfortable conditions, loss of product, wasted energy, and ultimately, customer dissatisfaction. Figure 1. A conventional balanced port thermostatic expansion valve and the three forces it responds to. F1: thermal bulb pressure times the diaphragm effective area; this force acts on the top of the diaphragm, which tends to open the valve. F2: evaporator pressure times the diaphragm effective area; this force acts on the underside of the diaphragm and tends to close the valve. This force is transmitted to the diaphragm through the valve body with internal equalized valves and through the external connection in external equalized valves. F3: superheat spring force which assists in closing the valve. Why TXVs hunt? There are several common reasons the service tech should consider when determining why a TXV hunts. Oversized valve: The expansion valve may be oversized for the application or operating condition of the system. Valve capacity significantly exceeds the requirements of the system and when the valve attempts to adjust to system load, it overcompensates because it is oversized. Incorrect charge selection: The charge selected does not have the necessary control characteristics and/or dampening ability to stabilize operation. Undercharged system: Intermittent loss of subcooling is causing loss of expansion valve capacity and resulting intermittent high superheat. Poor bulb contact: Loss or delay of temperature signal to the bulb, causing erratic and unpredictable operation. an imbalanced heat exchanger (multi-circuit coil): An imbalance in the heat load on each circuit creates a false temperature signal to the expansion valve bulb and results in erratic operation. Since this problem is commonly overlooked in the field, a closer examination and a possible solution are the focus of this article. TXVs on multi-circuit heat exchangers TXVs respond to the temperature of the suction line. (They respond to pressure too, but this is not the concern of this article.) At the expansion valve outlet, flow is divided into two or more paths (circuits) at the inlet of the evaporator by the distributor; these paths then recombine as they exit the evaporator into the suction manifold. (See Figure 2.) Ideally, each circuit is equally loaded and absorbs an equivalent amount of heat. If one assumes the refrigerant flow rate and heat load through each circuit is equal, then the superheat condition exiting each circuit will be equal and when all of the flow streams recombine, the result is a “true” average condition of the evaporator suction gas. When one or more circuits has a lighter heat load, some refrigerant from that circuit remains unevaporated when it exits the coil. When this unevaporated liquid refrigerant combines with the other superheated flow streams, the recombined suction flow no longer represents an average condition. Excessive hunting can reduce the capacity and efficiency of the system, resulting in uncomfortable conditions, loss of product, wasted energy, and ultimately, customer dissatisfaction. The suction temperature where the bulb is mounted will be lower than the “true” average of the circuits if they were all properly superheated. Sensing a “cold” suction condition will cause the valve to close down because it is sensing a condition that is not superheated enough; when the valve closes down, it restricts flow to all circuits and eventually “dries out” the circuits which are flooding. By this time, the remaining circuits have become highly superheated due to the reduced flow rate. At the point the “flooding” circuit(s) begin to be superheated, the suction temperature rises rapidly because there is no more liquid present to falsely reduce the suction temperature. Sensing a now “warm” suction condition, the valve opens to decrease superheat and the lightly loaded circuit begins to flood into the suction manifold again. Suction temperature drops rapidly again, the valve closes down again, and the whole sequence repeats in a cyclical fashion. Figure 2. At the expansion valve outlet, flow is divided into two or more paths (circuits) at the inlet of the evaporator by the distributor. These paths recombine as they exit the evaporator into the suction manifold