Uneven evaporator icing and high superheat: a TXV case
Heavy ice around the inlet and first section of a chiller-room evaporator, together with high superheat, pointed to poor refrigerant feeding. De-icing and reducing the TXV superheat setting restored sufficient cooling for the room to reach set temperature. The visit also identified a sensing-bulb and external-equaliser installation issue.
What was found
The evaporator had an uneven frost pattern, with particularly heavy ice around the inlet and the first part of the coil. High superheat was also identified. Taken together, these observations supported an underfeeding diagnosis: refrigerant was evaporating early, leaving less of the remaining coil effectively used for cooling. Accumulated ice can also restrict airflow and further reduce performance. High superheat itself does not create ice; the frost pattern and superheat must be assessed together.
The sensing bulb was after the equaliser connection
The concern was the order of the two connections, rather than simply the distance between them. Following suction-gas flow towards the compressor, the installed arrangement was:
Evaporator outlet → external equaliser tapping → sensing bulb → compressor.
The general arrangement is:
Evaporator outlet → sensing bulb → external equaliser tapping → compressor.
The bulb senses suction-line temperature, while the external equaliser supplies a pressure signal to the valve. These signals should represent the same evaporator-outlet conditions. Ramazan's field diagnosis attributed the excessive superheat and underfeeding to the bulb/equaliser installation arrangement.
As general installation guidance, Parker Sporlan recommends a pressure connection representative of the bulb location, usually immediately downstream of the bulb, while recognising equipment-specific arrangements. Position alone does not prove the size of the effect: a pressure difference between the tapping and bulb location can increase effective operating superheat. The actual equipment manufacturer's instructions and operating measurements should guide correction.
What was done during the visit
- De-iced the evaporator.
- Opened the TXV by reducing its superheat setting to improve refrigerant feeding.
- Checked the defrost probe.
- Set defrost to every 6 hours, with a 15°C termination temperature and 30-minute maximum duration.
- Monitored operation as the room temperature dropped.
The chiller room reached its set temperature and was left running. A check of the evaporator after a few days was recommended. No recurrence of icing has been reported in the supplied account. These defrost settings describe this visit; they are not universal settings for other equipment.
Successful adjustment versus permanent installation correction
Reducing the superheat setting improved operation, but it did not change the position of the bulb or external equaliser tapping. The installation correction remained a possible planned repair if further operating checks showed it was needed.
The proposed pipework work would involve refrigerant recovery, modification and brazing, pressure testing, evacuation and recommissioning. With the condensing unit on the roof, this job was assessed as requiring two engineers. That planned work was not carried out during this visit.
Why the freezer-room comparison matters
Ramazan's assessment was that, on a freezer-room application with the same installation problem, opening the valve would not be an adequate solution. In this chiller, adjustment provided enough cooling to reach setpoint. A freezer operates at a lower evaporating temperature, so this successful chiller result cannot establish that the same compensation would deliver adequate freezer capacity.
A freezer may still fail to pull down or maintain temperature if effective evaporator utilisation remains poor. The engineering lesson is to correct an inappropriate sensing arrangement and verify stable superheat and capacity under the actual operating conditions, rather than assuming a further valve adjustment will solve it. This was a chiller-room case; no freezer test was performed, so the comparison is an engineering assessment, not a measured freezer outcome.
What the visit confirmed
The recorded outcome was successful chiller pull-down after de-icing, TXV adjustment and defrost changes. Exact before-and-after superheat values, operating pressures, refrigerant type and room setpoint were not supplied, so none are invented here. Because the sensing arrangement was not relocated, this visit did not provide a before-and-after test of that permanent correction.
The practical lesson
An iced evaporator is not automatically a defrost fault. The distribution of frost, refrigerant feeding and superheat can reveal another part of the problem. Reaching set temperature confirms the observed cooling result; it does not, by itself, prove that every installation detail is correct.
Technical reference
Parker Sporlan Bulletin 10-11: Thermostatic Expansion Valves — Installation, Field Service and Assembly, external equaliser connection guidance, page 6. Consult the instructions for the valve and equipment actually installed.