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1 Thermal Expansion Valve 陈江平 上海交通大学制冷研究所 《制冷原理与技术》讲义 Installation, Operation, and Troubleshooting of TEVs In a nutshell, the role of the TEV is to control liquid injection into an evaporator as a function of the load. The controlling parameter is superheat at the evaporator. As the load on the evaporator increases, the valve responds to an increase in superheat and opens to allow more liquid refrigerant to flow into the evaporator. In so doing, the TEV maximizes the usable evaporator heat transfer surface and protects the compressor by making sure that only vapor returns to it. Parts Of a TEV · The power head assembly, enclosing a diaphragm; · The capillary tube and bulb connected to the power head containing a charge which, as it expands and contracts, exerts a varying pressure on the diaphragm; and · The valve body, with one or more pushpins operated by the diaphragm; the pushpins regulate the opening of an orifice through which the refrigerant is metered into the evaporator. The movement of the pushpins depends on the pressure on the diaphragm, which is opposed by the force of a spring. Spring force, which determines static superheat, can be fixed or adjustable. How TEVs Work The function of a TEV depends on the relationship between three fundamental pressures. Bulb charge pressure acts on the upper surface of the diaphragm, moving it in the valve-opening direction. Two pressures oppose bulb pressure. Evaporating pressure is introduced by either internal or external equalization. This equalization pressure acts on the underside of the diaphragm in the valve closing direction. Note: Evaporat-ing and equalization pressures should always be the same. Spring pressure also acts on the underside of the diaphragm in the closing direction. In a valve with adjustable superheat, the spring pressure can be adjusted manually. As the expansion valve regulates, there is balance between bulb pressure on one side of the diaphragm and equalization pressure plus spring pressure on the other side. This balance can be upset in either of two ways: 1. When spring force is adjusted manually, there is a proportional change in the TEV’s static superheat. 2. A change in the cooling load will change the evaporating pressure of the refrigerant and hence the equalization pressure under the diaphragm. This change occurs in proportion to the change in temperature at the evaporator outlet tube where the bulb is strapped. Any change in pressure is transmitted from the bulb through the capillary tube to the diaphragm. The balance of forces is disturbed until a new equilibrium is reached as more refrigerant is injected into the evaporator and the cooling load demand is met. Static superheat + opening superheat = operating superheat. What is Superheat? Physically, superheat is the temperature difference between the external pipe wall temperature and the evaporating pressure converted to temperature (saturation temperature) measured in °C. The level of superheat equals the temperature increase above the saturation temperature at the existing pressure. A vapor is superheated when its temperature is higher than the saturation temperature corresponding to its pressure. For example, R-22 at 70 psig has a saturation temperature of 41? and if its temperature actually is 51? it is said to be superheated by 10? With respect to valve operation, superheat has two distinct components: 1. Static superheat is the superheat at which spring force is met and the valve is ready to open. 2. Opening superheat is the amount of superheat above static superheat that opens the valve to allow refrigerant flow. The superheat measured at the outlet of the evaporator is the sum of the two and is called operating superheat. On valves with adjustable superheat, we are only changing spring force, and therefore only the static superheat. By adjusting the static superheat, however, we are effectively adjusting operating superheat. The opening superheat cannot be adjusted and is dependent on the system load or operating pressures as transmitted from the sensing bulb. Taking the measurements for superheat calculation; for the most accurate readings, place your gauge and thermometer in the positions shown. How to Measure Superheat 1. Measure the suction pressure at the evaporator outlet (or, if there is no fitting there, at the compressor inlet service valve). 2. Clean an area of the suction line near the bulb. 3. Tape your thermocouple to the cleaned area and insulate it; connect the thermocouple to a calibrated electronic thermometer and read the temperature. 4. Convert the suction pressure to a temperature using a refrigerant slide rule or chart, and subtract the temperature measured near the bulb. The difference is the superheat. A common but inaccurate method for determining superheat in the field uses evaporator inlet temperature instead of the saturated suction temperature equivalent to the evaporator outlet pressure. The problem with this method is its inaccuracy, which is most often due to misplacement of the inlet thermocouple or the inability to access the inlet at all. Make sure the sensing bulb is mounted in the corre t position, according to the tubing size
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2 TEV Installation The TEV must be installed in the liquid line, ahead of the evaporator and as close to it as possible. The bulb is tightly strapped to the suction line, as close to the evaporator outlet as possible. The bulb will give false signals to the diaphragm if it is installed after a desuperheater or close to components with large mass, such as large valves or flange connections. Any evaporator with a distributor or with a significant pressure drop requires an externally equalized TEV. If the valve is externally equalized, the equalizing line must be connected, otherwise the valve will not operate. The equalization connection is made at a point in the suction line immediately after the bulb, and in a 12 o’clock position on the tube to avoid oil logging the equalizer line. Mount the TEV’s sensing bulb on a horizontal suction line tube at the outlet of the evaporator in a position between 12 and 4 o’clock. The location depends on the suction line diameter. Tubes smaller than 3/4 in. should have the bulb located at the 12 or 1 o’clock position; ¾- and 7/8- in. tubes require a bulb position at 2 o’clock; for tubes 1 in. and larger, the correct position is from 3 to 4 o’clock. Never locate the bulb at the bottom of the suction line because of the possibility of a false signal caused by oil lying there. For the same reason, the bulb must not be mounted in areas where the suction line is bent and may act as an oil trap, as on a riser. Remember that the optimum location is on a horizontal part of the suction line immediately after the evaporator outlet. The bulb must have good thermal contact with the suction line. (Danfoss valves with double-contact bulbs improve thermal conduction.) The bulb mounting strap transfers heat to the area of the bulb that is not in contact with the copper tubing. Never use plastic straps such as cable ties for bulb mounting. Time and temperature will loosen the plastic material; contact as well as heat transfer will be lost. The bulb mounting strap supplied by TEV manufacturers is made of heat-conductive material, and should always be tight, but not so tight as to deform the piping or bulb. Although not a requirement, if heat-conductive paste is available, you can use it on the contact surfaces to enhance heat transfer. Because it needs to be able to sense the temperature of superheated suction vapor, the bulb must not be located in a position where external heat or cold will affect it. Insulating the bulb will help, but in cases where the lines operate below 32? the insulating material must be chosen to seal against moisture that might freeze around the bulb. Insulation of the bulb is also recommended if the bulb is exposed to a warm air current. On systems where a liquid distributor is used, remember that the TEV must be externally equalized, and the distributor should be mounted vertically, head outlets downward. It is extremely important that the feeder tubes from the distributor be of the same diameter and length. It is important to keep pressure drops across the distributor tubes as equal as possible for good liquid distribution. Avoid liquid traps when routing the distributor tubing. Piping must be carefully designed and executed to prevent any unwanted effects. For instance, where a circuit has multiple evaporators at different elevations, a higher evaporator can affect the TEV sensing bulb on a lower one. Also, in multi-fixture circuits, you may find situations where another technician has mislocated a sensing bulb so that it is actually reading the temperature of the common suction line rather than the evaporator it is meant to serve. Setting, Adjusting Superheat All expansion valves are supplied with a factory superheat setting appropriate for most applications. TEVs with fixed superheat do not allow readjustment in the field. Other valves, though, are designed to allow field setting by adjusting the spring force. To adjust the static superheat, turn the valve’s setting stem. Turning clockwise increases static superheat and effectively reduces refrigerant flow through the valve. Turning counterclockwise reduces static superheat and increases refrigerant flow. In addition to TEV sizing, correct superheat setting and proper sensing bulb positioning are two more of the many important determining factors for proper operation of an evaporator, and for compressor protection. The bulb will give false signals to the diaphragm if it is installed after a desuperheater or close to components with large mass, such as large valves or flange connections. Superheat Problems Expansion valves are often suspected of causing system problems. But generally speaking, a TEV is operating properly if it maintains superheat of 5?to 15? If superheat is low (lower than 5?, there is a potential for flooding refrigerant back to the compressor. If superheat is higher than 15°C, the evaporator is probably operating inefficiently. Stop Fiddling and Find the Problem There are countless possible causes for problems in a system. Superheat is one of the last things we adjust. Expansion valves are designed and set by their manufacturers to serve as “plug-and-play” devices which, right out of the box, can operate effectively in a wide range of applications. The temptation to adjust them is there because it is very easy to get to them before taking time to properly diagnose the refrigeration system. Unless there is absolute certainty of incorrect superheat, leave the TEV alone. Here are some problem areas that can cause low and high superheat. These areas should be investigated before adjusting superheat. Causes of low superheat include: · An improperly adjusted valve; · A significantly oversized valve; · Poor bulb or equalizer location; · Overcharged system; and · Excessive oil blocking the evaporator and acting as an insulator. Causes of high superheat include: · Low refrigerant charge, resulting in flash gas in the liquid line; · Dirt in the system trapped in the valve; · A restriction such as a plugged filter-drier in the liquid line, again causing flash gas; · A saturated or undersized drier in the system; unremoved moisture is likely to result in the formation of ice, restricting the TEV’s orifice; and · Improper system design, resulting in little or no liquid subcooling. If You Need to Adjust Earlier we talked about the proper way to take a superheat reading. If the valve is adjustable, and if you determine that the superheat needs to be set (for example, because the system is hunting), prepare for the adjustment by ensuring that you have operational head pressure and a proper flow of refrigerant to the TEV, without flash gas. Next, ensure that there is a nominal (or design) load on the evaporator; use a dummy load if necessary. Now remove the stem cap to expose the adjustment stem. Setting superheat is a trial-and-error procedure that will require several changes. 1. Adjust the valve to a point where you get unstable superheat readings, unless you have confirmed that the system has unstable superheat to begin with. (When superheat is unstable, the system is out of control and temperature and pressure are randomly fluctuating.) 2. Proceed to adjust the valve by turning the valve stem clockwise to increase superheat until the system is just stable. Then a further one-quarter to one-half turn clockwise will compensate for system variables during operation. 3. The valve manufacturer’s instructions give the number of stem turns per degree of superheat. You need to measure superheat after each adjustment, until the new value results in correct evaporator temperature under the nominal (or design) load. 4. Recheck the superheat under low-load conditions, too. Now you can be sure that the valve is set correctly. When there’s a refrigeration problem, don’t start working on a remedy before making a careful diagnosis. Adjusting superheat without careful observation and measurement is asking for trouble. The same goes for pumping down a system to replace the expansion valve, only to find out that the system is still not working. It wastes time and can be rather embarrassing. But some systematic troubleshooting, examining system pressures and temperatures, will likely lead to a solid diagnosis, a timely solution, and a satisfied customer. TEV Hysteresis and Evaporator Characteristics
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3 Superheat Affects Valve Actions1 Friction, which results in resistance to the movement of the pushpins, causes hysteresis. All TEVs are affected by hysteresis. A certain amount is needed, as we will see, but more than that is definitely detrimental to the refrigeration system's efficiency. Figure models the action of a typical TEV. Although valve response is often represented by a single curve, there are actually two, one for valve opening and the other for closing. The area between the two curves is called the hysteresis band. The hysteresis band Superheat Affects Valve Actions2 A load increase starting from the valve opening curve. Let's say we're on the valve opening curve and the load on the evaporator increases, in turn requiring an increase in cooling capacity (Figure 3). The superheat increases, raising the bulb pressure, which opens the valve. The capacity delivered by the valve changes almost instantly. Superheat Affects Valve Actions3 If we're on the opening curve and the load decreases, the story is different (Figure 4). Before the valve can start closing, superheat must decrease by the distance between the opening curve and the valve closing curve. With any smaller decrease in superheat, we will still be in the hysteresis band, and the valve will not begin to close. Only after reaching the closing curve is the valve ready to begin closing with any further decrease in superheat. The relationships between superheat changes and valve actions are similar if we start on the valve closing curve. There, a decrease in flow through the valve occurs without hysteresis (Figure 5), but since an increase in refrigerant flow requires crossing the hysteresis band, superheat must increase by the distance between the two curves, overcoming the valve's hysteresis (Figure 6). Capacity can only increase on the opening curve, and can only decrease on the closing curve. Figure 4. A load decrease starting from the valve opening curve. Figure 5. A load decrease starting from the valve closing curve. Figure 6. A load increase starting from the valve closing curve. Hysteresis can't be computed. Valve manufacturers make laboratory measurements over the valve's capacity range to determine the curves for a given design. At Danfoss, this is done using precision automatic measuring and recording instruments. The process takes place under standardized, controlled conditions. The design of a valve determines its internal friction, and therefore its hysteresis. Valves with low internal friction have correspondingly low hysteresis. But if there were no hysteresis at all, the opening and closing curves would become one, and the valve would react instantaneously to any change in load, even extremely small changes. While that might sound great at first, it would cause the valve to be too sensitive. The system could then become unstable, with the valve possibly going into a hunting mode. What Happens inside the Evaporator? Figure 7 shows the behavior in an evaporator at a given capacity. Given a negligible pressure drop across the system, if we place temperature probes (T1 and T2) at the inlet and outlet, we can determine the superheat across the entire evaporator (T2-T1). By moving T2 closer (to T2A), the temperature difference drops as we get closer to the liquid front. Moving T2 even closer (to T2B), we begin to see temperature fluctuations caused by T2B sensing both liquid droplets and vapor. The point just before the fluctuations can be seen is the minimum stable superheat (MSS) point. At MSS, the highest efficiency is achieved for the given load condition. The evaporator is most efficient at the MSS point because this is the point at which all of the refrigerant has finished evaporating and the evaporator is fully utilized. Figure 7. Refrigerant behavior in an evaporator, showing the MSS point and the changes in the quality of refrigerant. More… A curve can be graphed by determining the MSS point at different loads. This is shown by the red curve in Figure 8. For every evaporator, the MSS curve characteristic is unique. This curve can have many shapes, as it is primarily a function of evaporating temperature, airflow, and coil design. The gray area to the left of the MSS curve represents an unstable zone where liquid and gas coexist. When a system operates within this region, liquid refrigerant escapes at the evaporator outlet and overall system efficiency falls. The further we go into the unstable zone, the greater the potential for liquid slugging, which in turn leads to serious compressor damage. To the right of the MSS curve there is only superheated gas. Moving too far into this region also reduces system efficiency because the evaporator is not being fully utilized. In practice, achieving minimum stable superheat for optimum system performance may involve first decreasing the TEV's static superheat setting until the system begins to become unstable, then slightly increasing the static superheat until stability is just reached. Figure 8. A graphical representation of the action in an evaporator, showing the evaporator characteristic (the MSS curve)
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4 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
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5 Why circuits get loaded unevenly Again, the ideal situation is to assume each circuit is equally loaded and absorbs an equivalent amount of heat; however, this situation does not always occur. There are several reasons why circuits can become unevenly loaded. Poor heat exchanger design: In this case, each circuit is not of equal length and loading. Poor refrigerant distribution: This problem occurs due to the wrong choice of distributor or feeder tubes, partially blocked passageways of feeder tubes, unequal feeder tube lengths, and/or kinked feeder tubes. Uneven airflow: Airflow across the evaporator is reduced in some areas while increased in other areas. Dirty coils or damaged coil fins can have a similar effect on airflow. Diagnosing a hunting problem Diagnosing a hunting problem due to an imbalanced heat exchanger requires measuring the exit temperature of each circuit upstream of the suction manifold. To perform this process, average the temperatures of all of the circuits upstream of the suction manifold and compare this average temperature to the actual temperature of the suction manifold close to where the bulb is mounted. If the average value of the circuit exit temperatures exceeds the actual suction temperature value by more than 2°C, then there is probably one or more circuit(s) which are not completely superheated (flooding). A closer examination of the individual circuit temperatures and the associated suction pressure should reveal which circuit(s) are causing the problem. One simple rule to remember is that the valve’s response will favor the circuit that is flooding. Because of this favorable response, a heat exchanger can be operating at a reasonable exit superheat but still have a significant loss in capacity, because the expansion valve is responding to one or more flooding circuits while the other circuits remain highly superheated, and thus highly inefficient. Correcting the problem This can be a difficult task. First, the service tech must recognize the cause of the problem. If not, the problem can only be compensated for and this could mean a reduction in system performance. Here are some tips for correcting or compensating for an imbalanced heat exchanger: If possible, examine and correct any problems with airflow, coil circuitry, and distribution so that the circuits are more evenly fed and loaded. The goal is a more consistent circuit exit temperature on all circuits. One lightly loaded circuit may be tolerable if there are, for example, eight circuits. However, this is probably not the case if there are only three. Adjust the superheat of the valve to a slightly higher value. Attempting to control an evaporator near to or lower than 5癋 operating superheat can exceed the sensing capability of most expansion valves and result in hunting and subsequent intermittent flooding. If practical, move the bulb farther downstream on the suction line.Better mixing of the refrigerant prior to the bulb can smooth out the valve response, although capacity and efficiency may not improve significantly
