The TXV or Thermostatic Expansion Valve is preferred over other refrigeration metering devices because it only allows a specified amount of refrigerant to flow based on demand. Any type of refrigeration metering device creates a pressure drop in the refrigerant and with a pressure drop of the refrigerant a temperature drop also occurs.
This temperature is necessary to drop the refrigerant down to temperature lower than ambient temperatures inside the structure being cooled. This allows the refrigerant to absorb the heat contained in the air passing over the evaporator coil.
How the TXV or Thermostatic Expansion Valve Works
The TXV has a sensing bulb that is connected to the suction line leaving the evaporator coil. The sensing bulb is filled with a refrigerant that expands and contracts. This is based on the temperature of the refrigerant inside the TXV or thermostatic expansion valve sensing bulb.
The TXV sensing bulb is attached to the valve itself with a small copper capillary tube. Inside the valve is a bellows that moves according to how the refrigerant inside the sensing bulb reacts to the temperature. This creates a throttling effect from the expansion and contractions of the refrigerant through the capillary tube and to the bellows which moves a needle inside the valve to throttle the refrigerant.
As the superheat inside the suction line changes the TXV can react to the demand. If there is less demand then the TXV will meter less refrigerant to the evaporator coil. More demand the TXV will meter more refrigerant to the evaporator coil. Finally, now you know how a TXV works.
TXV or Thermostatic Expansion Valve - Common TXV Problems
Probably the most common problem with the TXV is improper installation. This is a good reason why it is important to find or select a good HVAC contractor. A good HVAC contractor will have knowledgeable certified technicians that know how to properly install and service HVAC equipment. This includes the proper position of a TXV in a new or retrofit installation.
The key is to always follow the manufacturers instructions when installing a TXV. The same for retrofits of air conditioner and heat pump installations. Instances of finding TXV sensing bulbs not even attached to the suction line is a common problem.
Another common problem with a TXV is the bulb in insufficiently insulated or not insulated at all. It is important to properly wrap the bulb and suction line where the TXV sensing bulb is attached. A good insulator is recommended. One typically provided by the manufacturer. If any of these steps are not followed the TXV will not function properly. Additionally, the air conditioner or heat pump will not realize the rated efficiency levels of the equipment. Despite what is written or illustrated here it is always recommended to follow the manufacturers installation instructions.
TXV or Thermostatic Expansion Valve - How a TXV Works
There are three specific functions of a TXV in order for it to function properly.
- The pressure created by the remote sensing bulb
- The evaporator equalization pressure (on the opposite side of the diaphragm from the pressure provided by the remote sensing bulb)
- The spring action inside the TXV (some are adjustable but many are not adjustable and engineered for specific application and capacity)
As mention above, the most common reason TXV’s do not work correctly is improper installation. It is always recommended to follow the manufacturers instructions on installation of any type of control including refrigeration parts. For it to work as engineered it has to be installed according to the instructions by the engineer. These instructions are found in the manufacturers literature. The spring creates an opposing force against the pressure of the remote sensing bulb. The reason proper installation is necessary is because TXV’s operates off of superheat coming from the evaporator. The action of the needle position moving open and closed maintains a precise amount of refrigerant in the evaporator. The desired amount of refrigerant for the load on the evaporator. Too much refrigerant and the evaporator gets flooded and the superheat drops making the system inefficient. Not enough refrigerant and the superheat spikes, again making the system inefficient.
TXV or Thermostatic Expansion Valve | Malfunction Causes for the TXV
- Moisture,dirt or debris in the refrigerant lines - this typically occurs because of bad installation practices. I always recommend using the triple evacuation method when working with refrigeration piping.
- Improper piping installation for refrigeration piping (similar to the above issue)
- Sensing bulb discharged - usually occurs when the cap tube rubs up against something
- Improper sizing in the system ie compressor or TXV improperly sized
- Bad bellows inside the TXV - I’ve never had this occur to me (if it did I was never aware this was the problem) but I’ve heard it happening to others.
- Incorrect bulb position (as noted above)
TXV or Thermostatic Expansion Valve - Conclusion
Additionally, operational problems can occur when improper practices are used when installing or retrofitting a new HVAC system. One of the biggest is foreign debris inside the refrigeration circuit. Debris can migrate to the TXV and plug it or cause the TXV to work improperly.
Furthermore, using proper brazing techniques (always use nitrogen to prevent oxidation) are necessary. Avoid any type of work that would causes metal shavings to get into the refrigeration circuit. Improper techniques such as cutting the copper piping with a saw instead of tubing cutters. Drilling a hole in the copper pipe rather than using an awl to punch a hole in the pipe.
Finally, using proper refrigeration system evacuation techniques is also recommended. This keeps the system clean and free of moisture. It is important to prevent the moisture from freezing at the TXV.
TXV or Thermostatic Expansion Valve