In a nutshell, the role of the TEV is to control liquid injection into an evaporator as a function of the load. The controlling parameter is superheat at the evaporator. As the load on the evaporator increases, the valve responds to an increase in superheat and opens to allow more liquid refrigerant to flow into the evaporator.
In so doing, the TEV maximizes the usable evaporator heat transfer surface and protects the compressor by making sure that only vapor returns to it.
The movement of the pushpins depends on the pressure on the diaphragm, which is opposed by the force of a spring. Spring force, which determines static superheat, can be fixed or adjustable.
Bulb charge pressure acts on the upper surface of the diaphragm, moving it in the valve-opening direction.
Two pressures oppose bulb pressure. Evaporating pressure is introduced by either internal or external equalization. This equalization pressure acts on the underside of the diaphragm in the valve closing direction. Note: Evaporat-ing and equalization pressures should always be the same.
Spring pressure also acts on the underside of the diaphragm in the closing direction. In a valve with adjustable superheat, the spring pressure can be adjusted manually.
As the expansion valve regulates, there is balance between bulb pressure on one side of the diaphragm and equalization pressure plus spring pressure on the other side. This balance can be upset in either of two ways:
1. When spring force is adjusted manually, there is a proportional change in the TEV’s static superheat.
2. A change in the cooling load will change the evaporating pressure of the refrigerant and hence the equalization pressure under the diaphragm. This change occurs in proportion to the change in temperature at the evaporator outlet tube where the bulb is strapped. Any change in pressure is transmitted from the bulb through the capillary tube to the diaphragm.
The balance of forces is disturbed until a new equilibrium is reached as more refrigerant is injected into the evaporator and the cooling load demand is met.
A vapor is superheated when its temperature is higher than the saturation temperature corresponding to its pressure. For example, R-22 at 70 psig has a saturation temperature of 41Â°, and if its temperature actually is 51Â°, it is said to be superheated by 10Â°.
With respect to valve operation, superheat has two distinct components:
1. Static superheat is the superheat at which spring force is met and the valve is ready to open.
2. Opening superheat is the amount of superheat above static superheat that opens the valve to allow refrigerant flow.
The superheat measured at the outlet of the evaporator is the sum of the two and is called operating superheat.
On valves with adjustable superheat, we are only changing spring force, and therefore only the static superheat. By adjusting the static superheat, however, we are effectively adjusting operating superheat. The opening superheat cannot be adjusted and is dependent on the system load or operating pressures as transmitted from the sensing bulb.
1. Measure the suction pressure at the evaporator outlet (or, if there is no fitting there, at the compressor inlet service valve).
2. Clean an area of the suction line near the bulb.
3. Tape your thermocouple to the cleaned area and insulate it; connect the thermocouple to a calibrated electronic thermometer and read the temperature.
4. Convert the suction pressure to a temperature using a refrigerant slide rule or chart, and subtract the temperature measured near the bulb. The difference is the superheat.
A common but inaccurate method for determining superheat in the field uses evaporator inlet temperature instead of the saturated suction temperature equivalent to the evaporator outlet pressure. The problem with this method is its inaccuracy, which is most often due to misplacement of the inlet thermocouple or the inability to access the inlet at all.
To adjust the static superheat, turn the valve’s setting stem. Turning clockwise increases static superheat and effectively reduces refrigerant flow through the valve. Turning counterclockwise reduces static superheat and increases refrigerant flow.
In addition to TEV sizing, correct superheat setting and proper sensing bulb positioning are two more of the many important determining factors for proper operation of an evaporator, and for compressor protection.
If superheat is low (lower than 5Â°), there is a potential for flooding refrigerant back to the compressor. If superheat is higher than 15Â°F, the evaporator is probably operating inefficiently.
Expansion valves are designed and set by their manufacturers to serve as “plug-and-play” devices which, right out of the box, can operate effectively in a wide range of applications.
The temptation to adjust them is there because it is very easy to get to them before taking time to properly diagnose the refrigeration system. Unless there is absolute certainty of incorrect superheat, leave the TEV alone.
Here are some problem areas that can cause low and high superheat. These areas should be investigated before adjusting superheat.
Causes of low superheat include:
Causes of high superheat include:
Next, ensure that there is a nominal (or design) load on the evaporator; use a dummy load if necessary. Now remove the stem cap to expose the adjustment stem.
Setting superheat is a trial-and-error procedure that will require several changes.
1. Adjust the valve to a point where you get unstable superheat readings, unless you have confirmed that the system has unstable superheat to begin with. (When superheat is unstable, the system is out of control and temperature and pressure are randomly fluctuating.)
2. Proceed to adjust the valve by turning the valve stem clockwise to increase superheat until the system is just stable. Then a further one-quarter to one-half turn clockwise will compensate for system variables during operation.
3. The valve manufacturer’s instructions give the number of stem turns per degree of superheat. You need to measure superheat after each adjustment, until the new value results in correct evaporator temperature under the nominal (or design) load.
4. Recheck the superheat under low-load conditions, too. Now you can be sure that the valve is set correctly.
When there’s a refrigeration problem, don’t start working on a remedy before making a careful diagnosis. Adjusting superheat without careful observation and measurement is asking for trouble.
The same goes for pumping down a system to replace the expansion valve, only to find out that the system is still not working. It wastes time and can be rather embarrassing.
But some systematic troubleshooting, examining system pressures and temperatures, will likely lead to a solid diagnosis, a timely solution, and a satisfied customer.
Strouboulis is application engineering manager and Robinson is market communication manager, Air-conditioning and Refrigeration Division, Danfoss Inc., Baltimore, MD. For further information, e-mail maxrobinson@ danfoss.com. Visit the company’s website at www.danfoss.com.
Publication date: 12/06/2000