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The purpose of this HVACR training project is to visually show your students the change in the amount of liquid flowing through a TXV or Piston Metering Device during operation. Using water as the liquid during this experiment helps them visually see this in action!
Why does the TXV Matter?
A TXV (Thermostatic Expansion Valve) can change the amount of liquid refrigerant traveling into an evaporator coil to absorb heat. This regulation device allows the system to act in an efficient manner to allow the proper amount of refrigerant into the coil during changing heat loads. It does this based on measuring and maintaining the superheat across the coil. The superheat has direct correlations to pressures and these pressures are applied to the TXV, which allow it to function.
If you want to learn about superheat, make sure to read this article: https://www.acservicetech.com/post/the-hvac-total-superheat-charging-method-explained
If you want to learn about subcooling, make sure to read this article: https://www.acservicetech.com/post/the-hvac-subcooling-charging-method-explained
We will be looking at and using a balanced port TXV which does not allow the inlet pressure to change its internal formula. These types of TXVs operate using three pressures.
P1=P2+P3.
P1 is the bulb pressure which is an opening force.
P2 is the external equalizer pressure which is a closing force.
P3 is the spring pressure which is a closing force initially set by the factory.
By comparison, a piston metering device cannot change the amount of refrigerant flowing through it because it has a fixed hole size. Also, the amount of refrigerant flowing through a piston is subject to change due to any change in the liquid pressure at its inlet. Liquid pressure may change due to a high outdoor temperature, low outdoor unit airflow, or outdoor coil fin degradation.
We can use the following project to compare a piston to a TXV. We can also show the TXV operation as we change either the P1 or P2 pressure.
Building the Display
The TXV training project can be built in a number of ways, using a variety of different materials. For the following example, we will show you how to build this setup using an old evaporator coil with a piston chamber and using an old TXV that fits onto a piston chamber. A large sized piston and a small sized piston size are needed too.
Tools Required
Standard drill
Small tubing cutter
Grinder with a cutting wheel, or roto-zip tool
Linesman's pliers
Deburring wheel
Step drill bit
Propane torch, solder, flux, sand paper
Access to water supplied by a hose
Nitrogen bottle, regulator, refrigerant hose
1/4" male by 1/4" male brass flare coupling
Parts Required
A scrap evaporator coil with a piston chamber and TXV mounted
Piston size #59 or smaller
Piston size #84 or larger
One Brass Garden Hose End Cap https://amzn.to/3SxK9ea
One Garden Hose Adapter with Shutoff Valve https://amzn.to/464sZYy
Step #1
To begin, remove the TXV bulb from the suction line. On the empty evaporator coil, loosen and remove the external equalizer flare nut from the suction line's 1/4" port. Disconnect the TXV body from the two sides of the piston chamber using two adjustable wrenches. Make sure to save the two white/blue Teflon rings located at the nuts for later. Cut the distributor tubes at the sections where they connect to the coil.
Step #2
Straighten the distributor tubes and cut them to a length of 6" past the piston chamber using a cutting wheel on a grinder or roto-zip tool. You could also use a pointed file to cut through the distributor tubes without them collapsing. Once the tubes are shortened to the appropriate length, deburr the end of the tubing with a deburring tool.
Step #3
Once the ends are deburred, use linesman's pliers to pinch the ends of the tubes. Make sure it is not fully pinched closed all the way. This reduced size opening will produce a more powerful stream once water flows through the TXV instead of simply water droplets.
Step #4
At this point, set aside the piston chamber assembly and focus on the liquid line section which includes the piston chamber nut. Also get the hose end cap and step drill bit ready.
Remove the rubber grommet from the inside of the hose end cap. Use the step drill bit to drill a hole into the brass hose end cap that is the same outside diameter as the liquid line.
We wanted to let you know we have our 2nd Ed. Refrigerant Charging Book available!!
Step #5
Cut and deburr the liquid line tubing at a length of 2" from the nut using a tubing cutter and deburring tool. Typically, there is a strainer screen mounted in the tube which starts from the nut and extends into the tube roughly 1.5 inches. Leave this strainer screen in place as it will help protect the metering device from any debris entering with the water.
After the tube is cut, sand the tubing and brass end cap section where the hole is. Add flux to both the brass section where the hole is and the end of the tubing. Insert the tube 3/8" tube partially into the brass hose end cap.
Step #6
Add flux to the joint and solder the connection with a low temperature torch and standard lead-free solder.
Step #7
After the newly soldered fitting has cooled and any remaining flux has been wiped off, add the rubber grommet back into the garden hose end cap. Connect the garden hose end cap to the hose adapter with shut off valve. This adapter will allow you to turn on or off the water flowing from the hose to the device.
Step #8
At this point you can add the small #59 sized piston into the piston chamber. Add one Teflon ring to the end of the chamber and tighten the fitting.
Step #9
Secure the piston chamber to a vertically level frame. In this instance, we used the piston chamber flange holes to screw the chamber to the side of a secured 2" by 4". After tightening the assembly, run your first test with water running through the piston chamber assembly.
Step #10
After testing the length of the water stream with the smaller #59 piston size, turn the water off and switch the piston with the larger #84 piston size. Turn the water on again and note the change in the length of the water stream. It should now be noticeably longer.
Step #11
After testing the flow of water using both piston sizes, turn the water off and remove the piston from the chamber. Attach the TXV (thermostatic expansion valve) into the middle of the piston chamber assembly.
Make sure to add a Teflon ring on both sides of the TXV at the chamber nuts.
Step #12A
Next, pressure must be added to the external equalizer line. To do this, a 1/4" by 1/4" male flare adapter is needed. Attached the equalizer nut to one end of the 1/4" by 1/4" male flare adapter and attach the valved end of a nitrogen hose to the other side.
Step #12B
12B show a different setup using the body of a valve core removal tool to lock the pressure into the external equalizer line. With this setup, pressure can be measured using either a digital or compound pressure gauge stub. Add a specific amount of nitrogen pressure through the hose and into the external equalizer and lock this pressure in there so it does not change during the experiment.
For an R-22 TXV, an average pressure of 70 PSI can be added to the equalizer line.
For an R-410A TXV, an average pressure of 120 PSI can be added to the equalizer line.
Step #13
Once the setup is built, turn the water on and apply temperature changes to the TXV bulb to demonstrate how this affects the refrigerant flow through the TXV. Essentially, by changing the bulb temperature, you are changing the measured superheat at the TXV. When an ice pack or cold water is applied to the bulb, the water stream is small and shallow. This is because when the bulb temperature is lower, less pressure is exerted from the bulb's refrigerant charge to the head of the TXV as an opening force.
Step #14
On the flip side, increasing the temperature on the TXV bulb will increase the opening pressure within the TXV body, allowing more liquid through. By placing a firm grip over the bulb, the heat transfers from your hand to the bulb's refrigerant charge, which increases the pressure and increases the length of the water stream!
Using the Display
After the display is complete, you can now easily demonstrate a TXV and a piston metering device's role in the refrigeration cycle using water and without a complicated setup involving refrigerant. Use an old kiddy pool to catch the water if showing this in your classroom. Practice this ahead of time before you show off the display and perform your magic show!!
If you want to know more about Thermostatic Expansion Valves, Watch This Video!
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Published: 1.25.25 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 17 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel
