Thermal Expansion Valve (TXV) Operation and Diagnostics
Many TXV's do not actually need to be replaced – A bold statement..
So, here it is…
Approximately 30% of the warranty claims from field service technicians to replace thermal expansion valves did not need to be replaced.
First, a little bit of grace for those who I have surprised with this statement.
Thermal expansion valves (TXV’s) are indeed mechanical devices and mechanical devices do indeed fail from time to time.
For this reason, it’s easy to see when a technician notices a thermal expansion valve malfunctioning to quickly come to the conclusion that the valve is defective and requires replacement.
What I will be covering in this blog post is a sequence of diagnostic activities that if followed can dramatically reduce misdiagnosis of thermal expansion valves.
In doing so service providers will potentially save time and create even more happy customers AND THATS just good for business.
SO HERE WE GO.
First of all, I will assume that my reader’s refrigeration skills are to the point that they understand how to take a refrigeration system to superheat readings on a running system.
Pro-tip, If you need to brush up on your refrigeration diagnostic skills, check out our:
FREE Basic Refrigeration Mini-Course.
With that out of the way, here are a few steps to take when faced with evaporator feed issues, followed by a suspicion that you have a TXV issue.
If you follow these recommendations, I am 99.9% sure you will avoid the 30% of the refrigeration service practitioners that performance a TVX replacement only to find out it did not fix the problem.
Step#1: Get the big picture: Before performing any diagnostic activities on a refrigeration system, make sure to look over the entire system. You must first rule out any maintenance issues that could impact refrigeration performance.
Your pre-diagnostic workup should include:
– Have a conversation with your customer: This could be an essential diagnostic activity in the technician’s toolbox.
The fact that you have been requested to come out and look into a problem indicates that someone has noticed that the refrigeration equipment is not working correctly. Taking a few minutes to get a history from the customer regarding what they witnessed can be very helpful.
Suppose you happen to be lucky (or maybe not 🙂 fortunate enough to come in behind another service technician, sometimes it is helpful to ask the customer to provide you with service records from prior visits. There may be some trending information that you can gain from looking at the preceding technician’s paperwork, so don’t be afraid to ask.
– Inspection of both the condenser and the evaporator: If you happen to be working on a process chiller, make sure you look around for any plugged filters or strainers that could be impacting fluid process flow through your evaporator. The bottom line, you want to rule out any heat transfer issues that can be causing the refrigeration cycle performance issues.
– Look around for refrigerant leaks: Make sure to inspect the equipment cabinet base pan for any signs of refrigerant oil. If your refrigeration system is not functioning correctly due to a leak, performing diagnostics will be a waste of time until the leak is repaired. The system is returned to its factory refrigerant charge.
After performing the above checks, you have now ruled out potential maintenance issues.
YOU MUST perform these tasks before digging further into this system.
Checking your refrigeration pressures: If the particular machine you’re working on is equipped with a PLC that reads refrigeration pressures, it’s always a good idea to use the PLC data to calculate you’re superheat and subcooling readings first.
Pro-Tip: Even if you have low-loss fittings on your refrigeration gauge set, each time the gauges are connected and disconnected from the refrigeration system, you will lose a small amount of refrigerant. Over time, small amounts of refrigerant losses add up.
– Checking your superheat and subcooling readings: Before starting your diagnostics, you will have to create conditions to keep the refrigeration system running for an extended period of time. You can do this by manipulating the refrigeration system set point in most cases.
With the refrigeration system running a minimum of five minutes using your compressor suction and head pressures, you can calculate both your evaporator superheat(SH) in your condenser sub-cooling(SC) values.
Pro-Tip: What you are looking for before taking your SH and SC readings in stabilized compressor suction and discharge pressures. Suppose your system can stabilize in less than five minutes, great. If not, you should wait until they do.
With SH and SC readings in hand, you will have the ability to determine these two major components’ performance.
A good rule of thumb on most refrigeration systems is your superheat and subcooling, on a healthy refrigeration system, will generally run between 8 and 12°F.
Additionally, you’re superheat and subcooling readings should eventually balance where these two readings will be very close to each other.
For example, if you have a superheat reading at 10°F, your subcooling reading will generally be within 2°F of your superheat reading.
Step#2: Looking into the TXV: Since this is an article about TXV’s, I will now create a hypothetical scenario to continue with the troubleshooting steps to diagnose a thermal expansion valve properly.
Again, if you follow these steps, there is an excellent chance you can keep out of the 30% covered above.
So here is our hypothetical scenario…
So in this scenario, you have performed ALL OF THE Step#1 items, right?
You now see that your superheat is running more than 25°F.
As far as you can tell, your condensers and compressor are operating fine. However, you did notice a slightly elevated sub cooling reading of about 18°F, and your compressor was getting a bit warmer than usual.
At this point, you have determined that there is a potential issue with the thermal expansion valve.
Pro tip: To ensure you are making diagnostic decisions based on accurate data, you have connected your calibrated refrigeration manifold set to the system to verify that the data you are reading from the PLC is correct.
You have verified that it indeed is correct.
Now, let’s get into diagnostics on the TXV:
– Remove the cork tape or other insulating material from the thermal expansion valve sensing bulb.
– While monitoring your superheat reading, add a small amount of supplemental heat to the expansion valves sensing bulb. Important note, this additional heat only needs to be 20 to 35°F above your ambient condition.
– With your supplemental he applied, you should see a superheat reaction within no more than 30 seconds.
– The reaction you’re looking for is an immediate reduction in your superheat calculation. If the expected decline does not happen, you have essentially confirmed that you have a problem with the power at an assembly or the valve itself.
Following our hypothetical situation, let’s assume that you did not see much of a response from the valve. Many technicians would move to condemn the thermal expansion valve at this point.
If you are getting bored, power through this ok… It will be worth it!
Pro-tip: Remove the TX of these superheat adjustment cover and thread the stem fully clockwise. Once fully clockwise until it stops.
Then, thread the stem counterclockwise until it reaches the stop once again. As you turn the stem counterclockwise, make sure to count the number of turns. From the fully counterclockwise position, thread the stem back down to half the total amount of turns.
For example, if the number of turns from fully clockwise to fully counterclockwise is 16 turns, you will set the stem from the full counterclockwise position to approximately eight clockwise.
This procedure will reset the TXV superheat setting to its original factory setting. This will help rule out the valve malfunctioning appearance related to someone adjusting the superheat setting once this procedure is complete. Repeat the application of supplemental heat to the valve’s power had to see if the valve will actuate.
Before moving to condemn the TXV, here are some additional considerations.
If applicable, you must next look for additional components in the refrigeration cycle that appear like a no open response from a TVX when you add supplemental heat to the powerhead.
Let’s take the extra steps to rule out these other components:
– liquid line solenoid: Many refrigeration systems are equipped with a liquid line solenoid upstream of the TXV inlet. If you have a restriction within your liquid line solenoid, It could indeed result in the remaining elevated superheat we discussed above.
Pro tips: On how to perform diagnostics on a liquid line solenoid:
If your refrigeration system’s liquid line is under 1 1/8 inches, you can expect your amp draw of the valve coil to be from .8 – 1.2 amps.
If your valve is experiencing a mechanical problem, preventing the valve from opening fully, your actual amp draw will be significantly elevated.
Another step you can take to verify if your liquid line solenoid is feeding correctly is to take accurate temperature measurements across the valve inlet and outlet. Suppose you have a temperature difference that exceeds 2°F. That would be an indicator that you have some internal restriction.
– Filter drier: As with the solenoid valve, you can use the same temperature differential check across the inlet and outlet of the dryer to determine if there is a restriction. Although these restrictions tend to be rare, they do indeed happen.
With all of your good diagnostic efforts out of the way, you have made the determination both your liquid line solenoid and dryer are indeed okay.
Based on that, you conclude that the thermal expansion valve is indeed malfunctioning in this hypothetical scenario.
Pro-tip: Surprisingly, many technicians will recommend changing out the entire thermal expansion valve. Such an effort requires a lot of work, including reclaiming the refrigerant and brazing.
If your thermal expansion valve is less than 10 years old, a good possibility of that specific valve is still available.
An alternative to replacing the entire valve would be to purchase a new valve and change out first, just the powerhead and, if that does not resolve the issue, you can change out just the guts leaving the brass body.
Depending on the accessibility of the particular valve you’re working on, this Pro tip could save you hours and a significant amount of your customer’s money.