Service & Maintenance / Extra Edition

Servicing Thermostatic Expansion Valves

April 3, 2006
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The thermostatic expansion valve (TEV or TXV) is erroneously considered by some to be a mysterious and complex device. As a result, many valves are needlessly replaced when the cause of the system malfunction is not immediately recognized.

Actually the TEV performs only one very simple function - it keeps the evaporator supplied with enough refrigerant to satisfy all load conditions. It is not a temperature control, suction pressure control, a control to vary the compressor's running time, or a humidity control.

How effective the valve performs is easily determined by measuring the superheat. Observing the frost on the suction line or considering only the suction pressure may be misleading. Checking the superheat is the first step in a simple and systematic analysis of TEV performance.

  • If not enough refrigerant is being fed to the evaporator - the superheat will be high.

  • If too much refrigerant is being fed to the evaporator - the superheat will be low.

    Although these symptoms may be attributed to improper TEV control, more frequently the origin of the trouble lies elsewhere.

    Note: TEVs with permanent bleed ports (BP) or Rapid Pressure Balancer (RPB) construction are applied on many air conditioning and refrigeration systems by original equipment manufacturers. Each application is tested and approved by the manufacturer. The primary function of these devices is to equalize high-to-low side pressures during the off cycle on systems equipped with low starting torque compressors.

    However, some BP type valves are applied to allow small amounts of liquid refrigerant to pass for compressor motor cooling. The specific function of the feature on a given unit must be determined from the system manufacturer. Once that is determined, it is easier to troubleshoot the system.

    The primary cause of difficulty with either the BP or RPB feature is dirt and other foreign materials that restrict or plug them. And if the system purpose intended for either feature is not being satisfied, the valve probably needs cleaning or replacing.

    The RPB type valve is not to be applied on systems using high starting torque compressors or "hard-start" electrical components, on outdoor coils of heat pumps, or on any refrigeration system, and it should not be used to replace BP type valves that are applied on those types of systems. On systems other than those described above, the RPB type valve can replace the BP type valve when necessary. Usually it is advisable to replace a valve with one of the same specification unless advised differently. Consult with the system manufacturer for assistance.

    The following are typical valve complaints, causes, and remedies.

    Moisture freezing in the valve port can prevent proper operation — a common source of trouble.

    Complaint "A"

    Valve does not feed enough refrigerant.

    SYMPTOMS:

  • Load temperature (air or water leaving evaporator) too high.

  • Superheat too high.

  • Suction pressure lower than normal with compressor unloaders locked out or hot gas bypass shut off. (Note: When the system has some form of capacity reduction - cylinder unloaders or hot gas bypass, a low suction pressure will not exist. Therefore, when checking TEV performance, a better analysis is possible when these devices are locked out or shut off so the suction pressure will respond to variations in load or valve feed.)

    THE CAUSE MAY BE:
    1. Moisture - Water or a mixture of water and oil frozen in the valve port or working parts of the valve will prevent proper operation. This is a common source of trouble on expansion valves. Since the valve is the first cold spot in the system, moisture will freeze and block the valve open, closed, or any position in between. If the valve is frozen in the intermediate position so that flow is restricted, the superheat will be high.

    Remedy - Install a filter-drier in the liquid line for removal of moisture from the refrigerant and oil.

    To determine a safe level of moisture in the system, install a moisture and liquid indicator.

    Excessive moisture has a damaging effect on all system components regardless of the evaporating temperature. Moisture must be removed for trouble-free performance.

    2. Dirt or foreign material - Contaminants such as copper oxide scale, metal chips, oil breakdown sludge, etc. will restrict the flow of refrigerant when it collects in strainers or other liquid line accessories. This produces a shortage of refrigerant at the TEV port. Conventional strainers frequently allow the material to pass through the screen and obstruct the flow at the valve port. If a moisture and liquid indicator is installed downstream of the restriction, bubbles will be visible. This should not be confused, however, with a refrigerant shortage or excessive liquid line pressure loss which are also indicated by bubbles in the moisture and liquid indicator.

    Remedy - Locate and remove the foreign material creating the restriction. Install a filter-drier to provide effective filtration of the refrigerant.

    3. Wax - Certain systems are contaminated with small amounts of wax which will precipitate at low temperatures in systems with refrigerants R-22 or R-502. Since the TEV represents the first cold point in the refrigeration cycle, wax is most likely to form at the valve port.

    It is sometimes difficult to observe the wax in a valve because it may exist in solid form only at very low temperatures. By the time the valve has been taken apart, the temperature has increased enough to cause the wax to melt and thus become difficult to detect. When wax is suspected, it can usually be detected on the pin and seat by packing the valve in dry ice while disassembling.

    Remedy - Clean the valve with solvent before reassembling the valve. Some filter-driers have a special activated charcoal desiccant that is designed to remove wax in the liquid line before it causes trouble. Therefore, to prevent wax problems, use these style driers on all low temperature systems using R-22 or R-502.

    4. Refrigerant shortage - The moisture and liquid indicator or sight glass in the liquid line will show bubbles when the system is short of refrigerant charge. Before adding more refrigerant however, be sure the bubbles are not produced by other causes (see No. 2 above and No.5 below).

    A lack of refrigerant charge may also be detected by a hissing sound at the TEV. Some systems not equipped with a liquid line sight glass will have test cocks or other devices for checking the refrigerant level in the receiver.

    Remedy - Add enough refrigerant to obtain desired result.

    5. Gas in the liquid line - As explained in No. 2 and No. 4, liquid line vapor can be produced by a partially plugged strainer or drier and by a shortage of refrigerant charge. In addition, gas in the liquid line can be caused by air or other noncondensable gases in the system or by excessive pressure losses in the liquid line as a result of:

  • Long or undersized line.

  • Liquid line vertical lift.

    Remedy - Verify the correct liquid line size for the equivalent length and system tonnage. Consult liquid line sizing data published in many manufacturers' catalogs and in textbooks. If undersized, repipe with the correct size.

    Determine amount of vertical lift, and obtain the resulting pressure loss. Calculate the subcooling necessary to prevent gasification with the existing pressure losses. Provide the necessary subcooling.

    6. Misapplication of internally equalized valve or incorrect location of external equalizer - If the pressure drop through the evaporator exceeds predetermined values, an externally equalized valve must be used. When an externally equalized valve is used, the equalizer connection should be made at a point in the suction line that will reflect the pressure existing in the line at the bulb location.

    Remedy - Replace internally equalized valve with one having an external equalizer.

    If external equalizer is installed incorrectly, change to correct location.

    7. Insufficient pressure drop across valve - One of the factors that influence expansion valve capacity is the pressure drop that exists between the inlet and outlet. Anything contributing to a reduction in this pressure drop will reduce valve capacity. Abnormally low condensing pressures, excessive liquid line pressure losses (even with adequate subcooling), undersized distributor nozzle or distributor tubes may also be responsible for a very low net pressure drop across the valve port.

    Remedy - Remove source of pressure loss, or install valve with adequate capacity at the reduced pressure drop. If inlet pressure to valve is low due to low condensing pressure, raise pressure.

    If the refrigerant distributor nozzle is undersized, replace with correct size.

    8. Dead thermostatic element or wrong thermostatic charge - If the element has partially or completely lost its thermostatic charge, the valve will be unable to feed sufficient refrigerant or will remain closed. A wrong charge may cause insufficient feed also.

    Remedy - Replace the element if it is dead. If charge is incorrect, replace with proper selective charge.

    9. Charge migration - For certain series valves, in order to maintain control at the bulb, the bulb must be kept at a lower temperature than the element (diaphragm case). If the thermostatic charge does migrate to the element because of a lower element temperature, the valve will throttle.

    Detection - Warm the element with a cloth saturated with hot water. If this produces more refrigerant feed and reduces the superheat to normal, charge migration is responsible for the starved evaporator.

    Causes:

  • Insufficient pressure drop between the valve outlet and bulb location, possibly due to an oversized distributor nozzle or no nozzle at all.

  • Excessive pushrod leakage, which allows the leaking refrigerant to cool the diaphragm case before passing into the equalizer line. This is a rare occurrence and should be carefully checked before arriving at this conclusion.

  • Cold location of TEV, or condensate drippage on the diaphragm case.

    Remedies:

  • Install distributor nozzle correctly sized in accordance with nozzle sizing procedure.

  • On valves with packed pushrod construction, remove element and tighten the pushrod packing nuts.

  • Relocate the TEV away from cold outlet air, or condensate drippage.

    10. Undersized valve

    Remedy - Install valve sized in accordance with proper procedures.

    11. High superheat adjustment

    Remedy - Turn the adjusting stem counterclockwise until the correct superheat is indicated.

    12. Feedback from another valve - On multiple evaporator installations, the piping should be arranged so that the flow from any valve cannot affect the bulb of another.

    Remedy - Check the bulb temperature and calculate the superheat. If superheat is normal but too little refrigerant is flowing through the evaporator, check the piping for possible refrigerant flow from another evaporator affecting the bulb. Repipe if necessary. Approved piping practices including the proper use of traps ensures individual control for each valve without the influence of refrigerant and oil flow from other evaporators.

    13. High pressure drop through evaporator

    Remedy - Check the pressure at the evaporator inlet and outlet with gauges. If pressure difference is greater than normal values, use an externally equalized valve.

    14. Restricted, plugged, or capped external equalizer - If the pressure under the diaphragm builds up due to pushrod leakage and cannot escape through the external equalizer line, the valve will remain closed.

    Remedy - Check the external equalizer line to be sure it is open or not capped.

    Complaint "B"

    Valve feeds too much refrigerant.

    SYMPTOMS:

  • Liquid returns to compressor.

  • Superheat is low.

  • Suction pressure is normal or higher than normal.

    THE CAUSE MAY BE:
    1. Moisture - Water or a mixture of water and oil frozen in the valve port or working parts of the valve will prevent proper operation. This is the most common source of trouble on TEVs. Since the valve is the first cold spot in the system, moisture will freeze and block the valve open, closed, or any position in between. If the valve is held in the open position by ice, liquid floodback will occur.

    Remedy - Install a filter-drier in the liquid line for removal of moisture from the refrigerant and oil.

    For additional protection, install a moisture and liquid indicator for a positive indication of when a safe moisture level is reached.

    2. Dirt or foreign material - Contaminants such as copper oxide scale, metal chips, oil breakdown sludge, etc. may pass through ordinary strainers and lodge at the TEV port and prevent the valve from closing.

    Remedy - Disassemble the valve and remove all foreign material from the internal parts. Install a filter-drier in the liquid line. This will filter out the smallest particles of foreign material that might interfere with the operation of any system component.

    3. Expansion valve seat leak - When the valve port does not seat tightly, refrigerant will pass through during the off-cycle and fill the evaporator with refrigerant. If the seat leak is severe, the valve will feed too much refrigerant during the operating cycle as well. (Not applicable to valves with permanent bleed ports or RPB feature.)

    Remedy - If the valve seat is leaking, a gurgling or hissing sound can usually be heard during the off-cycle. Also, a sight glass or moisture and liquid indicator in the liquid line may indicate continued refrigerant flow for a long period after the compressor has stopped. Make certain however, that the bubbles are not the result of back-flow through a vertical liquid line.

    Disassemble the valve to be certain that dirt or foreign material is not responsible (see No. 2 above). If the pin and seat are worn or damaged and an internal parts kit is available, replace the parts. When parts are not available, the valve must be replaced.

    4. Oversized valve - Check valve ratings considering all the factors which affect its capacity.

    Remedy - Install correctly sized valve.

    5. Incorrect bulb installation - The bulb should be securely fastened to a straight, clean, section of the suction line using two bulb straps for good thermal contact. Also, the temperature of the bulb should not be influenced by ambient temperature - an external heat source such as a steam pipe or heating coil.

    Remedy - Install bulb correctly.

    6. Low superheat adjustment

    Remedy -Turn the adjusting stem clockwise until the correct superheat is indicated.

    7. Incorrect thermostatic charge

    Remedy - Select and install the correct selective charge.

    8. Incorrectly located external equalizer

    Remedy - Relocate external equalizer or the connection between evaporator and any other temperature or pressure sensitive evaporator control valve near bulb location.

    9. Inefficient compressor - If the compressor is inefficient or for some other reason lacks capacity, the suction pressure will operate higher than normal. This may or may not be accompanied by low superheats.

    Remedy - Consult with compressor manufacturer.

    Complaint "C"

    Valve feeds too much refrigerant at startup only.

    SYMPTOMS:

  • Liquid returns to compressor.

  • No superheat.

  • Suction pressure higher than normal.

    THE CAUSE MAY BE:
    1. Refrigerant drainage - Drainage of refrigerant from the evaporator (during the off-cycle) when installed at a higher level than the compressor.

    Remedy - Install a trap-riser to top of evaporator or use pump-down control.

    2. Compressor or suction line in cold location - During the period when the system is not in operation, liquid refrigerant will condense at the coldest point in the system. Liquid will condense in the compressor or suction line, if they are located in an ambient temperature below that of the evaporator during the off-cycle. Upon re-starting, this liquid will slug the compressor.

    Remedy - Keep compressor or suction line warm during the off-cycle. Some compressors are equipped with crankcase heaters for this purpose. Another corrective measure is to install a suction line solenoid valve that is de-energized during the off-cycle.

    3. Restricted or plugged external equalizer - A momentary flood can occur when the load increases suddenly, such as at start-up because the higher suction pressure cannot reach the underside of the diaphragm and help close the valve. If the pressure under the diaphragm increases due to any pressure leakage around the pushrods, the valve will eventually throttle.

    Remedy - Remove the restriction or plugged portion of the external equalizer.

    4. Liquid line solenoid valve seat leak or interrupted pumpdown - Liquid refrigerant can continue to feed the TEV and/or remain in evaporator upon shutdown causing floodback to the compressor upon startup.

    Remedy - Disassemble and clean solenoid valve and/or replace damaged internal parts if seat leakage is the problem. If the pumpdown cycle isn't completed before the compressor cycles off, or the thermostat calls for cooling and reopens the liquid line solenoid before the evaporator has been properly evacuated, check the low-pressure cut-off setting or the electrical controls for possible causes.

    Exploded view of a thermostatic expansion valve.

    Complaint "D"

    Valve doesn't feed properly.

    SYMPTOMS:

  • Poor system performance.

  • Superheat normal or lower than normal.

  • Suction pressure lower than normal with compressor unloaders locked out or hot gas bypass shut off. (Note: When the system has some form of capacity reduction - cylinder unloaders or hot gas bypass, a low suction pressure will not exist. Therefore, when checking TEV performance, a better analysis is possible when these devices are locked out or shut off so the suction pressure will respond to variations in load or valve feed.)

    THE CAUSE MAY BE:
    1. Unequal circuit loading (multi-circuit evaporators and parallel evaporators connected to a single refrigerant distributor) - When each circuit is not subjected to the same heat load, the lightly loaded circuits will allow unevaporated refrigerant or low temperature vapor to enter the suction line and throttle the valve. This will cause normally loaded circuits to be deprived of their share of refrigerant. The net result is a loss of refrigerated evaporator surface.

    Remedy - Make necessary modifications which will allow each evaporator circuit to receive the same percentage of the total load.

    2. Poor refrigerant distribution (multi-circuit evaporators and parallel evaporators connected to a single refrigerant distributor) - If the refrigerant distribution is faulty, the circuits receiving the largest portion of refrigerant will have the controlling influence on the TEV. The result is the same as in No. 1 above.

    Remedy - Correct refrigerant distribution.

    3. Low load - Low evaporator load may be caused by insufficient air over the coil as a result of an undersized blower, dirty air filters, or an obstruction in the air stream. In addition, frost formation on the coil or low entering air temperatures will reduce the evaporator load.

    Remedy - Correct the condition responsible.

    4. Flow from one coil affecting TEV bulb of another (multiple evaporator systems only) - The temperature of the bulb may be falsely influenced by flow from another evaporator usually because of incorrect piping.

    Remedy - Correct the piping.

    5. Improper compressor-evaporator balance - If the compressor is too large for the load and evaporator capacity, the low suction pressure which results will cause poor system performance.

    Remedy - Consult with the manufacturer or consulting engineer, or the ASHRAE Handbook on component balancing. If necessary, change or correct the improperly sized component. Hot gas bypass may be used to balance properly.

    6. Evaporator oil-logged - Poor heat transfer occurs and unpredictable performance takes place. If erratic performance is observed over a period of time, and other causes are omitted from consideration, review the amount of oil in the system. Turbulent compressor oil level with little or no return to the compressor sump indicates oil problems.

    Remedy - Remove excessive oil from evaporator and connecting piping. Many times the evaporator temperature will be too low for the oil to be removed. Therefore, the system must be allowed to warm sufficiently to get cold oil to drain. Analyze system components for possible causes of oil problem before restarting the system. Consult with the compressor manufacturer for specific details on their compressor.

    Complaint "E"

    System hunts or cycles.

    SYMPTOMS:

  • Suction pressure fluctuates. (Note: When the system has some form of capacity reduction - cylinder unloaders or hot gas bypass, a low suction pressure will not exist. Therefore, when checking TEV performance, a better analysis is possible when these devices are locked out or shut off so the suction pressure will respond to variations in load or valve feed.)

  • Superheat fluctuates.

  • Valve does not feed enough, and then too much refrigerant.

    THE CAUSE MAY BE:
    1. System characteristics - Certain design characteristics of the system may have an effect on the system's tendency to hunt or cycle. As an example, after the valve admits refrigerant to the evaporator inlet, there is a time delay before the bulb senses the effect at the evaporator outlet. This time delay is dependent on evaporator length, tube size, and load. Generally, there is more likelihood for hunting to occur when this time interval is long. Other influencing factors are circuit arrangement, load per circuit, and temperature difference.

    Remedy - When hunting is moderate particularly with no floodback, the effect on the system is insignificant and corrections are not necessary. If hunting is severe with floodback to the compressor, check the possible remedies shown in the paragraphs below.

    2. Valve size - An over-sized valve usually aggravates hunting. Carefully check the valve rating considering all the factors affecting its capacity.

    Remedy - Replace valve with one correctly sized. On multiple circuit evaporators using a refrigerant distributor, the capacity of the valve can be reduced, within certain limits, by installing a smaller distributor nozzle.

    3. Bulb location - If the bulb is located in a suction line trap, its temperature will be affected by liquid oil and refrigerant alternately collecting and evaporating at this point. This condition frequently results in severe hunting.

    Remedy - As a temporary measure relocate the bulb away from the trap, and any turbulent areas created by elbows, tees, etc. Also remove the bulb from the air stream or insulate. Repipe if necessary. Sometimes another position around the circumference of the suction line will minimize hunting.

    4. Refrigerant and load distribution - In addition to the effects of poor distribution explained in Complaint "D" No. 1 and No. 2 above, hunting also frequently results. This is caused by liquid refrigerant from the overfed circuits occasionally reaching the bulb of the valve.

    Remedy - Correct the faulty distribution.

    5. Superheat adjustment - TEVs are preset at the factory to give the best performance on the average system. A valve should not be adjusted unnecessarily, but occasionally another setting may prove to be better.

    Remedy - Turn the adjusting stem clockwise a turn at a time. If the hunting stops or is reduced, turn the adjusting stem counterclockwise a turn at a time to obtain the lowest superheat with stable operation.

    6. Moisture - As ice forms in a TEV from excessive moisture, a very erratic hunt may result.

    Remedy - Remove the moisture with the installation of a filter-drier. A safe moisture level can be determined by installing a moisture and liquid indicator.

    Complaint "F"

    System won't perform properly.

    SYMPTOM:

  • Cannot get valve to react or regulate at all.

    THE CAUSE MAY BE:
    1. No refrigerant being fed to evaporator. See Complaint A above.

    2. Too much refrigerant being fed to evaporator. See Complaint B above.

    3. Too much refrigerant being fed to evaporator at startup only. See Complaint C above.

    4. Refrigerant control is erratic. See Complaint D above.

    5. System is hunting or cycling. See Complaint E above.

    6. The TEV has been physically abused in an effort to make the valve work properly. This is usually the result of a mistaken analysis. It is frequently assumed that if a valve does not feed properly, it is stuck (either opened or closed). Beating the valve body with a hammer will only distort the body and make it impossible for the valve to work once the real cause is determined.

    If a valve "sticks," it is usually due to moisture freezing in the port, dirt and other foreign material restricting or plugging the internal parts, wax forming on the internal parts at low temperatures, or the valve has been physically abused so it cannot function.

    Remedy - Inspect the valve and its internal parts, including the inlet strainer. If plugged or restricted in any way, clean the parts thoroughly, oil the parts with a good grade of refrigerant oil, and reassemble the parts.

    If the valve is beyond normal cleaning processes, or if it is physically damaged in any way, replace the valve with its proper replacement model.

    Excerpted and reprinted from Sporlan Bulletin 10-11, "Thermostatic Expansion Valves: Installation, Field Service, and Assembly." For more information, contact Sporlan Division, Parker-Hannifin Corp., 206 Lange Drive, Washington, MO 63090; 636-239-1111; 636-239-9130 (fax); www.sporlan.com.

    Publication date: 04/03/2006

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