Tag: expansion valve

There has been much written and many jokes made about the misdiagnosis of TXV (Thermostatic expansion valves) and rightly so. This article will cut straight to the point to help those of you who may still need a bit of clarification and hopefully, we will save the lives of a few TXVs and the pocketbooks of some customers.

Q: What is a TXV?

A: A TXV (TEV) is a type of metering device. The metering device’s job is to create a pressure drop from the liquid line into the evaporator which will result in refrigerant boiling (changing from liquid to vapor) through the majority of the evaporator coil. This low temperature “boiling” absorbs heat from the space or product being cooled.


Q: How does a TXV Function?

A: A TXV “measures” the temperature and (usually) the pressure at the end of the evaporator coil with a bulb and a tube called an external equalizer. The bulb measures temperature and provides an opening force, the equalizer measures pressure and provides a closing force. There is also a spring that may have an adjustable tension that provides additional closing force. When working properly these forces achieve a balance and maintain the evaporator superheat to the designed of set levels at the end of the evaporator.  The TXV’s job is to maintain superheat within certain operational ranges and conditions. 


Q: How do they fail?

A: A TXV may fail either too far open or too far closed. Too far open is also called “overfeeding” and it means that boiling refrigerant is being fed too far through the evaporator coil, this would show up in low superheat. If the TXV fails closed it can be said to be “underfeeding” which means not enough boiling refrigerant is fed through the evaporator coil and superheat will be too high at the evaporator outlet. 

These failures can and do occur, but they are usually caused by contaminants or moisture in the system that have worked their way to the valve and caused it to stick or become restricted. Another cause of valve failure is a rub out on bulb tube and an external equalizer without a core depressor installed on a port that has a Schrader core in place.  

When a valve is overfeeding the first thing to check is bulb insulation, placement and strapping. If the numbing isn’t properly sensing the suction line it can lead to the valve remaining too far open.


Q: Why are they misdiagnosed so often? 

A: TXV’s are often incorrectly condemned in cases of low evaporator airflow or load. This happens because techs will find a system with low suction pressure and assume that means it is low on refrigerant. They will then start to add refrigerant and the TXV will respond by closing further the more refrigerant is added. The tech will see that the suction isn’t increasing and they will conclude that the TXV is failed. 

This occurs because the tech is paying too much attention to suction pressure without considering the other readings.


Q: What is the correct way to diagnose a TXV? 

A: First take all of your refrigerant readings as well as your liquid line and suction temperature at both ends (on a split system). This means superheat, subcooling, suction saturation (evaporator coil temp) and liquid saturation (condensing temp). For a TXV to do what it is supposed to you need a full line of liquid before the TXV, this means you need at least 1° of subcooling in theory but in reality, you will want to make sure that you have the factory specified subcooling which is usually around 10°. In refrigeration, we do this same thing by looking for a clear sight glass. On a split system checking the subcool outside and then confirming there is no big temperature difference inside to out is a great way to ensure that kinked lines or plugged line driers aren’t an issue. 

The next thing that a TXV needs is enough liquid pressure to have the required pressure differential. This amount of required pressure differential will vary a bit based on the valve but usually, we want to see a 100 PSI minimum difference between the liquid line pressure and the desired evaporator pressure. If the head pressure drops too low due to low ambient conditions this can come into play and impact the ability of the valve to do its job. 

Once this is all confirmed then it is simply a matter of checking the superheat at the end of the evaporator. Most A/C systems will be maintaining 6-14° of superheat at the evaporator outlet. If it is in that range then the valve isn’t bad, it’s doing its job. 

If it is lower than 6° of superheat at the evap outlet then it could be overfeeding (double check your thermometer and gauges) and if the superheat is well above 14° at the evaporator outlet, with the proper subcool and liquid pressure entering… then you have a failed closed (underfeeding valve). Keep in mind that some valves will have a screen right before the valve and this can be the cause of the restriction rather than the valve. You can intentionally freeze the coil and try to see the freezing point or use thermal imaging to help spot if it’s the valve or the screen. When you find the point of temperature you find the point of pressure drop, just remember that the TXV is DESIGNED to provide pressure to maintain a fairly fixed superheat. 


Q: Do TXVs Ever Fail

A: They can fail internally but most often they fail because of a blocked inlet screen (if they have one), contaminants entering the valve, loss of charge from the power head, bulb location and positioning issues and overheating of the valve. In commercial and refrigeration applications you can often replace or clean the screen and replace the power head rather than replacing the entire valve. 


As I have said many times before diagnosis make sure your tools are well calibrated and working and that you are ACTUALLY reading the pressure correctly. I’ve seen many misdiagnoses just because a Schrader wasn’t pushing in or a multi-position valve cracked properly.

The piston (fixed orifice) and TXV (Thermostatic Expansion Valve) are the two most common metering devices in use today, with some modern systems utilizing an electronically controlled metering device called an EEV (Electronic Expansion Valve).  It should at least be noted that there are other types of fixed orifice metering devices like capillary tubes, but their use is not common on most modern A/C systems though you will see them in refrigeration.

While the compressor creates the pressure differential to get the refrigerant moving, by decreasing the pressure on the suction and increasing the pressure on the discharge side, the purpose of the metering device is to create a pressure drop between the liquid line and the evaporator coil or expansion line (the line between the metering device and the evaporator when there is one). When the high-pressure liquid refrigerant is fed into the metering device on the inlet the refrigerant flows out the other side and the immediate pressure drop results in an expansion of a percentage of the liquid directly to vapor known as “flashing”. The amount of refrigerant that “flashes” depends on the difference in temperature between the liquid entering the metering device and the boiling temperature of the refrigerant in the evaporator. If the difference is greater, more refrigerant will be “flashed” immediately and if the difference is less than less refrigerant will be flashed.

Piston

A piston is a replaceable metering device with a fixed “bore”. It is essentially a piece of brass with a hole in the center, the smaller the bore the less refrigerant flows through the piston and vice versa. The advantage of a piston is that it is simple and it can still be removed, the bore size changed and cleaned if required.

piston_flow

Some piston systems also allow the reverse flow of refrigerant as shown in the diagram to the above. In a heat pump system when the reversing valve is energized (cool mode), the unit will run in cool mode and the refrigerant will follow the path indicated on the bottom.  This seats the piston so refrigerant must pass through the orifice.  With the reversing valve de-energized the flow reverses.  This unseats the piston and allows the free flow of refrigerant.  In this case, there is a metering device in the condensing unit (outside unit) that meters the flow of refrigerant in heat mode and one inside that meters in cooling mode.

TXV

The TXV can vary the amount of refrigerant flow through the evaporator by opening and closing in response to evaporator heat load.  compared to a fixed orifice a TXV operates more efficiently in varying environmental conditions (theoretically at least).

To operate, the TXV has a needle and seat that restricts the flow of refrigerant and acts as the orifice.  This needle, when opened, allows more refrigerant to flow and, when closed, restricts refrigerant flow.  There are three factors that affect the flow of refrigerant flow through a TXV.  A sensing bulb filled with refrigerant exerts force to open the TXV.  Since gas pressure increases with a rise in temperature, the bulb, which is attached to the suction line after the evaporator coil, “senses” the temperature of the suction line.  If the suction line becomes too warm, the additional pressure created by the heated refrigerant opens the TXV more to allow additional refrigerant flow.  A spring inside the bottom of the TXV exerts pressure to close the valve.  An external equalizer senses pressure in the suction line after the evaporator, and also works to close the valve. In essence, the TXV is a constant superheat device, it sets a (relatively) constant superheat at the evaporator outlet by balancing bulb, spring and equalizer pressures.

The primary method of charging a system changes based on the type of metering device. A piston system uses the superheat method of charging and the TXV uses the subcooling method of charging.

No matter what primary method of charging you use it is still important to monitor suction pressure (Evap temperature) head (condensing temperature), Superheat, subcool and delta t (or some other method of air flow verification).

While a TXV and a piston function differently the end result is a pressure drop and boiling refrigerant in the evaporator.

— Bryan

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