Everything You Need to Know About TXVs

Before the minimum efficiency standard of 13 SEER, it was very rare to find thermostatic expansion valves (TXVs) in residential air conditioning systems. These were used in some systems with a higher SEER and in heat pumps (usually in the outer coil), although they represented a relatively small part of the overall market.

However, this has changed drastically as now virtually all new SEER 13 equipment will be manufactured with TXV. The result: there is a growing need, for many technicians, to reacquaint themselves with TXVs in terms of their operation, fault detection and fix, and replacements.

With higher SEER air conditioners, technicians need to reacquaint themselves with thermostatic expansion valves.

Refrigerant flow

P1 = P2 + P3

Measuring the flow of refrigerant to the evaporator is the unique function of a TXV. It must measure this flow at precisely the same rate at which the refrigerant is evaporated by the heat charge. The TXV does this by keeping the coil with enough coolant to maintain the correct overheating of the suction gas coming out of the evaporator coil.

There are three forces that govern the operation of the TXV. Refer to Figure 1, which indicates the equilibrium diagram of the basic forces of a TXV. In the table:

P1: Pressure of the power element and the remote bulb.

P2: Evaporator pressure.

P3: Equivalent pressure of the overheating spring.

In order for the valve to be stabilized, the forces need to be balanced; that is: P1 = P2 + P3.

As the temperature of the evaporator outlet increases, the pressure (P1) increases, causing the diaphragm to flex in a downward direction. This forces the valve needle into the open position, resulting in an increased flow of coolant.

The lower part of the diaphragm always senses the pressure of the evaporator (P2). As this pressure increases, it forces the diaphragm into an upward or closing position, decreasing the flow of coolant.

Spring pressure (P3) also acts on the bottom of the diaphragm. This spring is adjusted to provide static overheating for the valve. Static overheating is the amount of overheating needed to initiate the movement of the valve needle to start moving. This is defined as a 0.002-inch stroke.

Gradient. It is the amount of overheating required to move the chaveta from the static set point to the assigned stroke. Figure 2 shows how a TXV regulates flow in response to changing overheating.

Starting from the origin, no change occurs in the valve stroke as overheating slowly increases. The valve begins to open only when the static set point is reached. From this point, the following increases in overheating result in a proportional increase in valve stroke, until the maximum stroke position is obtained.

Gradient is an important aspect of the performance of a TXV in a system. With a very low gradient, the valve will be unstable and will have a tendency to "oscillation" (we will talk more about this later). If the gradient is too high, more overheating will be needed for the valve to open, resulting in high operational overheating and poor evaporator efficiency.

Measurement of overheating.

Since good overheating control is the criterion for the performance of a TXV, accurate measurement of overheating is vital. This involves four steps, as shown in Figure 3. These are:

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3. Measure the temperature of the suction line in the bulb.

4. Subtract the saturated temperature from the temperature measured in the suction line to find the superheat.

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1. Measure the suction pressure at the evaporator outlet with a precision pressure gauge. If a connection for a pressure gauge is not available, a T can be installed on the equalizer line.

2. Taking as a reference a P/T table for the refrigerant used in the system, find the saturation temperature that corresponds to the pressure observed in step 1.

3. Measure the suction line temperature on the remote sensor bulb. This can be done with an adjustable type thermometer or with an electronic device.

4. Subtract the saturation temperature determined in step 2 from the suction gas temperature measured in step 3. The difference is operational overheating.

In most air conditioning systems, evaporators are quite large and therefore have significant pressure drops.

TXV equalized internally and externally.

In a system with a relatively small evaporator, the pressure drop in that evaporator is so small that it can be assumed to be zero. Therefore, the TXV outlet pressure and the evaporator outlet pressure are the same.

By drilling a small bleeding passage between the bottom of the diaphragm and the valve outlet, you can feel the pressure internally and eliminate the need for an external connection. Valves produced in this way are called internally equalized valves.

In most air conditioning systems, evaporators are quite large and therefore have significant pressure drops. In order for the TXV to sense the pressure of the evaporator outlet, a separate line is necessary from the suction line (near the position of the TXV bulb) to the connection of the external equalizer in the valve body).

If a distributor is used to supply refrigerant to the various evaporator circuits, an externally equalized valve should be used. Distributors typically have a pressure drop of 15 psi to 30 psi. Therefore, use them only with externally equalized valves.

Balanced port valves. With a conventional TXV, the pressure differential throughout the valve results in a force that tends to "open" the valve. As operating conditions vary, this pressure differential changes and results in a variation of the original overheating. Engineers have designed balanced port TXVs to compensate for this effect (Figure 4).

In this design, inlet pressure is applied to the entire valve needle as well as an undercut on the push rod. As these forces are in opposite directions, they cancel or balance each other, resulting in no changes in overheating, regardless of operating conditions.

Balanced port valves are ideal for use in systems that operate with major changes in operating conditions. An example of this case is a commercial air conditioning system that must operate in both summer and winter, resulting in the operation of the system under highly varying head pressures.

Fault Detection and Fix in TXVs

There are actually only three types of failure modes that a TXV may experience:

1. Deprivation. This is defined as an insufficient refrigerant flow that causes high overheating at the evaporator outlet. Symptoms include high overheating at the compressor inlet, high discharge temperature, and possible overheating of the compressor (the protector fires).

2. Flooding. This occurs when the flow of refrigerant to the evaporator is so high that it cannot evaporate inside the coil. The result is liquid refrigerant returning to the compressor. Symptoms include low overheating of the evaporator, diluted oil, and noisy compressors. If not corrected, it can cause permanent damage to the compressor.

3. Oscillation. Oscillation is said to exist when overheating in an operating system constantly changes from very little or no overheating to very high overheating. You can easily recognize this effect by noticing extreme cyclical changes in evaporator or suction pressure.

Oscillation is caused by many factors, although it usually occurs when the valve is too large for the load. Before ruling out a valve for this symptom, make sure the evaporator is frost-free and has adequate airflow, as these conditions will result in very low loads that can potentially produce "good" valve oscillation.

Control of the operation of the TXV

If a TXV is suspected of not working properly, overheating control is the only way to make sure. Do this with precision instrumentation for meaningful results.

Review the air conditioning equipment manufacturer's service and installation manual to verify what is the acceptable overheating for that particular model. As an approximate rule, overheating is operational from 8°F to 12°F are considered normal. Below are some "tips" to help detect and fix flaws in the performance of a TXV:

• Check the bulb to make sure it is properly connected to the suction line. If you can move the bulb manually, it means that it is not properly secured.

• Some manufacturers isolate the bulb to protect it against the effects of an air current. If the original manufacturer had done this, make sure the insulation is still intact.

• Check the equalizer line for constraints (bends) or frost signals. A frosted equalizer line indicates internal leaks and will require Valve Replacement. A bent equalizer will need to be repaired or replaced for the valve to operate properly.

TXVs are designed to measure the flow of liquid coolant. If the refrigerant at the valve inlet contains sudden gas, the capacity of the valve will be reduced. Make sure the system is properly charged and that there is some subcooling at the valve inlet before discarding the TXV. With the use of R-410A and POE oils, there is an increased risk of dirt and contaminants circulating within the system. Some manufacturers use filters or strainers to prevent debris from clogging the valve. If this condition is found, clean and replace the filter. It would also be good to install a large filter/dryer at the entrance of the TXV to avoid service calls.

"When replacing a valve in a heat pump, verify that the original valve has internal check"

Replacement of TXVs

If it is determined that the valve needs to be replaced after the overheating has been checked, here are some tips to ensure a correct replacement:

1. Whenever possible, use the valve recommended by the equipment manufacturer. If this is not possible, make sure the replacement has the same:

• Assigned capacity.

• Refrigerant designation.

• Type of load.

• Internal/external equalizer style.

• Internal check valve (if supplied in the original valve).

• Measurement and type of internal/external connection.

2. To maintain the cleanliness of the system, replace the filter every time you open the sealed system. While this has always been a recommended service procedure, it takes on even greater importance with HFC/SOE systems due to the hygroscopic nature of POE oils and their increased solvency.

3. Do not overheat the valve during the welding process. Overheating can cause deterioration in the internal seals, and this could produce frosted equalizer lines. To avoid this:

• Wrap the valve with wet rags, as shown in Figure 5.

• Keep the blowtorch flame away from the valve body.

• Never allow the blowtorch to come into contact with the bulb.

TXVs in heat pumps

In heat pump applications, liquid coolant must flow through or around the TXV when operating in the reverse direction. Historically, this was achieved by installing a check valve around the valve.

However, in recent years, TXV manufacturers have modified their products with internal check valves. Many OEMs have adopted them as they eliminate gaskets and potential losses.

Figure 6 is a cross-sectional plane of one of these valves. In the forward flow direction, the pressure of the inlet pushes the ball against the seat, forcing it to close. All flow must then pass through the main valve port and the valve operates like a normal valve.

When the flow is reversed, the inlet pressure pushes the ball upwards, allowing flow through the holding port. In this mode, the flow bypasses the main port and the liquid flows through the valve with only a small pressure drop.

When replacing a valve in a heat pump, verify that the original valve has internal check. If you have it, make sure the replacement has it too. If you don't have any available, use a standard valve. You will need to install a check valve to bridge the TXV when the flow is in reverse.

Systems using TXVs are quickly becoming the norm in this post-SEER-13 world. Understanding their function and operation will allow you to give a correct service to the systems that use such devices. Follow the basic replacement and fault detection and fix guides that have been discussed here, to ensure optimal system performance and to avoid permanent compressor damage.

The HFES series (or other balanced port valve) will control the flow of refrigerant in any direction. The sensor bulb in such a case would have to be placed in a common suction line such as the central tube of a 4-way valve (see Figure 1).

Also, for such a system to operate properly, the system would have to be "closely connected" this means that the evaporator and condenser would have to be physically located in close proximity to each other such as in a package-type system.

For split systems, the long stretch of pipe between the VTE and the coils does not make them practical for using such a model.

For such systems, two expansion valves should be used; one for the inner coil and one for the outer coil. Check valves should be installed around each of the VTEs to allow flow around the valve when it is operating in the reverse direction (see Figure 2).

Location of the Sensor Bulb

Q: What is the correct position for the sensor bulb of a VTE?

The placement of the VTE remote sensing bulb on the suction line is critical for proper VTE operation. The important points to follow are:

• Clean the suction line near the evaporator outlet

• The total length of the sensor bulb must be in contact with the cleaned portion of the suction line

• The sensor bulb should be placed a few centimeters before the connection of the external equalizer

• The sensor bulb must be fastened at the 12 o'clock position on any suction line of 7/8 diameter or less. In lines larger than 7/8 in diameter the bulb should be placed either on the 4 or 8 of the clock. The bulb should never be placed on the 6 of the watch

• Always isolate the sensor bulb after installing it

• A sensor bulb can be installed on a vertical suction line if necessary, but never place the bulb after a trap. It is recommended to place the bulb before a trap.

• On suction lines 7/8 in diameter or smaller, place the bulb on the 12th of the watch.

• On suction lines larger than 7/8 in diameter, place the bulb on the 4 or 8 of the watch.

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