Superheat Adjustment on a thermostatic expansion valve system ( commercial fridge )

Superheat Adjustment Procedure

Superheat" is the term used to describe the difference between the vapor point (ie. temperature at which the refrigerant evaporates at a given pressure) and the actual temperature of the refrigerant exiting the evaporator coil.
To adjust the superheat you will need the following tools:
1. Refrigerant gauge set
2. 19mm socket or wrench
3. Flathead screw driver
4. Pressure-Temperature chart for HFC-134a
5. Accurate surface reading thermometer (remote probe type)

Warning - If you do not have these tools do not attempt to adjust the super heat setting. Do not attempt to adjust the superheat based on the "frost line" of the suction return.


Measuring Superheat
The adjustment process should be started when the holding plate is approximately 1/2 frozen. If the plate is fully thawed, allow the system to partially freeze down before taking your measurements.


Step 1
Ensure that the sensing bulb for the thermostat is firmly clamped to the suction line on a horizontal tubing run at the 10:00 or 2:00 o'clock position.

Step 2
Remove the protective cap covering the superheat adjustment screw.

Step 3
Connect the refrigerant gauge set and purge. Only the suction side need be connected. It is good to re-calibrate your gauge to "0" before connection.

Step 4
Securely attach the thermometer probe to the suction line immediately after the manifold elbow and fitting.

Step 5
Start compressor and allow the system to run at least 10 minutes to stabilize.

Step 6
Determine Suction line pressure drop. The suction side pressure you are reading at the compressor includes line losses as the refrigerant gas makes it's way back to the compressor. To accurately determine the superheat we need to know the suction pressure at the evaporator. Therefore, we need to determine, and then add back in, any additional pressure drop which occurs between the evaporator and the compressor. Fortunately, this loss can be easily seen on the pressure gauge as an immediate "jump" at the moment the compressor is turned off. Typically, line pressure drop will only amount to 1 or 2 psi. However, with long tubing runs it may be higher. Manually, turn the compressor off and on a couple of times to determine the pressure drop in your system. Once determined, leave the compressor running.

Step 7
Determine the evaporator temperature. Do this by reading your suction pressure displayed on the gauge and adding back in the line pressure drop. Use your temperature/pressure chart to determine the actual evaporator temperature. For example: gauge reading (10 psig) + line loss (2 psi)=12 psig. The temperature/pressure chart tells us that HFC-134a has a vapor point (evaporator temperature) of 100 F at 12 psig. Therefore, the system evaporator temperature is 100 F.

Step 8
Measure evaporator exit temperature. Use your surface reading thermometer to read the temperature of the suction line at the holding plate exit point. For example: 180 F.

Step 9
Calculate superheat. Evaporator exit temperature (180 F) - Evaporator temperature (100 F)=80 F superheat.


Adjusting Superheat
To change the superheat setting, turn the flathead adjustment screw clockwise to increase the superheat, counter-clockwise to decrease. Adjust the screw no more than 1/4 turn at a time and allow the at least 5 minutes running before re-measuring. Large adjustments and fine-tuning must sometimes be done over several "pull-down" cycles because of the amount of compressor run time required to stabilize the system after each adjustment.



Questions & Answers
Q - New laws in force world-wide mandate that only properly licensed service technicians (complete with recovery equipment "on-site" at all times) perform certain operations. Does the adjustment of superheat fall into this category?
A - In most cases. These laws apply anytime it is necessary to "break into the refrigerant stream". Unless you have permanently mounted gauges, you will need to connect them into place. By doing so, you then fall under the regulations. Fines up to $50,000 are now being regularly issued. Violations do not need to be documented in the usual fashion, as "reasonable likelihood" has been found sufficient cause for the issuance of fines.

Q - Why does my refrigeration serviceman always talk about the "frost-line" when referring to superheat settings.
A - Unfortunately, in spite of the fact that proper superheat adjustment in vital to the efficient operation of any refrigeration system, many technician have only a vague understanding of it. Many do know, however, that severely low superheat can (in some cases) cause physical damage to the compressor. An easy "rule-of-thumb" which ensures that the superheat is not set so low as to cause such damage is to adjust it until the "frost-line" is some distance from the compressor. While this practice does safeguard the compressor from damage it often, particularly in the case of freezer plates, gives in a superheat setting which is much too high. The result is inefficient operation and excessive compressor run time.

Q - Does under and/or overcharging effect superheat?
A - Yes and No. In the case of capillary tube systems, superheat adjustment in the field is accomplished entirely by the amount of the charge. In expansion valve systems (including all Glacier Bay equipment), the superheat would only change (increase) in the case of very severe under-charging. While cover-charging does cause other problems, it does not change superheat in an expansion valve system.

Q - Since the correct superheat setting have very little latitude for error, how do I know that the gauge and thermometer are accurate enough?
A - Most analog (dial type) refrigeration gauges are surprisingly accurate if they have not been abused and have been re-calibrated to "0" before pressure is applied. Thermometers, even very expensive ones, can be problematic. If possible, try to cross-check with another thermometer or two at the temperatures you expect to be reading.

Q - Where do I get a temperature/pressure chart?
A - From your local refrigerant dealer or your local commercial refrigeration supply house.

Q - If I don't have a temperature/pressure chart but have a gauge set with a temperature scale for HFC-134a, can I use that instead?
A - No. It isn't going to be accurate enough.