Valves scapegoated for a multitude of problems

By Mike Squires

Hashtag “Bad TXV” is a common phrase in the online refrigeration community where it has morphed into quite the joke. The TXV, also known as a thermostatic expansion valve, is sometimes accused of being the root of all evil in terms of refrigeration problems. I find it funny when someone posts a photo of a refrigeration problem and people blame the valve as a joke, even when the issue obviously has nothing to with a TXV.

What it does

Parts of a TXV, a thermostatic expansion valve.

The main purpose of this valve is to meter the refrigerant into the evaporator of the refrigeration system where the work is done to transfer heat from the substance being cooled. The valve rated capacity is directly related to the size of the port or pathway in which the refrigerant will flow through the valve from the high side high pressure liquid refrigerant line to the low-pressure evaporator section of the refrigeration system. A basic needle and seat type of valve is what is in many TXVs.

The flow of the refrigerant through the valve is maintained by opposing forces – closing forces and opening forces.  The closing forces are the evaporator pressure via the internal or external equalizer (depending on the valve type) and the spring pressure.  The spring is typically connected to an adjustment screw, which allows a technician to increase or decrease the spring pressure. If spring pressure is higher, the valve will close and take more force to open.

It also has one opening force: the bulb/powerhead pressure. The thermostatic bulb, which is connected to the top diaphragm of the power element via a capillary line, contains a special type of refrigerant engineered to match the type of system in which it is installed. This is why you would have different TXV power elements for R-22, R-410A, R-134A, R-448A, and so on. The refrigerant mixture in the bulb would have similar acting properties as the refrigerant in the system.

The power element bulb senses the temperature of the suction line pipe on the outlet of the evaporator. As the temperature rises in the suction line, the pipe warms up the sensing bulb and the refrigerant in the sensing bulb expands and creates a higher pressure in the element. This increases internal pressure and pushes down the powerhead diaphragm for the opening force on the pushrod. The expansion valve then opens.

As the suction line cools and the power element bulb cools, the valve closes. When the evaporator suction line outlet temperature drops the power element pressure drops and the evaporator pressure, along with the spring valve, close the valve. The spring pressure is used to maintain a specific closing force. This is measured by the technician to maintain a proper evaporator superheat, which is required for proper operation and protection of other system components − namely the refrigeration compressor.

System Problems

Why is the TXV blamed for system problems? It’s tough to know why but unfortunately the little fella is often falsely accused. Other system problems would have quite an influence on the operation of the TXV. If you see a TXV and the powerhead of the valve is very rusty or in poor condition it is a good sign you really have a TXV problem because the power element charge could have leaked out rendering the power head nonfunctional. If you have a nonfunctional valve and you replace it, take apart that valve and look at the inside components. Do all the parts move freely? If not have a look for contaminants. If particulates are moving along with the refrigerant many of those contaminants could separate from the refrigerant and build up in the TXV.

When it comes to working with a TXV be careful on adjusting the screw as a first means of service. It is a good idea to look at the full system operation. If possible, it is also good to know a bit of the valve’s history. Is this an old system that has been operating for years? Is it a newer system with a history of problems? Has the system been recently serviced? If there is any doubt check all the pressures and temperatures at each part of the system.

Pressure Difference

The refrigerant pressure difference across the valve, meaning the different condenser and evaporator pressure, can really influence the operation of the valve. Assuming temperatures are not changing too much, if the pressure differential increases the volume of refrigerant flow will increase and if the pressure differential is lowered the flow volume is reduced. If you have a 10-ton valve, when the pressure drop across the valve is 100 psi, the valve is at its design pressure drop. The valve is operating at its rated capacity. If you increase the pressure drop across the valve it is now capable of a capacity higher than 10 ton. Conversely, if the pressure drop is reduced the valve capacity would no longer be 10 tons.

Variable load systems can struggle with maintaining proper superheat flow due to the changes in pressure drop. A wide range of operating pressures is reason for the application of a balanced port TXV. When a balanced port valve is used pressure drop differences have little to no influence on the valve capacity.

With pressure drop on the valve the temperature of the refrigerant entering the TXV can have influence on the capacity of the valve. As the temperature or sub cooling of the liquid refrigerant increases the capacity of the valve increases. This is because as the liquid changes from high to low pressure through the valve there is less refrigerant that immediately evaporates. This rapid expansion of the liquid refrigerant is called “flash gas.” The more subcooled the refrigerant the less flash gas you have and the more low-pressure liquid refrigerant remains in the evaporator to perform more beneficial work.

What is Flash Gas?

Flash gas is refrigerant in gas form produced spontaneously when the condensed liquid is subjected to boiling. The presence of flash gas in the liquid lines reduces the efficiency of the refrigeration cycle. It can also lead to several expansion systems working improperly and increase superheating at the evaporator.

New technologies simplify the maintenance of refrigeration parts

Innovative technologies are assisting in system diagnostics and preventive maintenance. Smart sensors monitor the performance and health of refrigeration components, including the TXV. Real-time data ensures the system is running smoothly.

The Internet of Things (IoT) connects devices, including refrigeration systems, to a single network where you can remotely manage and monitor devices. If the TXV isn’t performing as it should you’ll know about it.
Artificial Intelligence (AI) has also made its way to refrigeration maintenance. AI-based predictive analytics can forecast potential issues before they become serious. It could be thought of as a crystal ball for your expansion valve.

Finally, there is Augmented Reality (AR), which can overlay information onto real-world objects – a great boost  for maintenance. A technician can point their device at an expansion valve, and instantly see specs, history, and even step-by-step repair instructions.

These technologies are transforming refrigeration maintenance, making it more efficient, less prone to costly mistakes and less likely to malign the TXV.

Get to know your TXV

If you struggle with understanding the operations of the TXV you would greatly benefit from teaching yourself and familiarizing yourself with the components and the theory of operations. Take an old valve apart and have a look at the internal components to visualize its operation. You can always find manufacturers’ educational material online. One that personally helped me is the Parker/Sporlan Bulletin #10-9.

 

Mike Squires, RSE, is manager service accounts and technical training at Neelands Group Limited in Burlington, ON, where he works with customers and in-house staff on all matters relating to refrigeration. He can be reached at [email protected].

 

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