Lesson System components · Evaporator
Evaporator: superheat and feed
Superheat is how much the suction vapor warmed after the last drop of liquid boiled off. It tells you whether the evaporator is starved, correctly fed, or flooding the compressor. Combined with how the coil is fed (TXV, capillary, flooded), superheat is the main field language for evaporator performance—not “add gas until the box feels cold.”
What superheat actually is
At a given evaporating pressure, refrigerant has a saturation temperature (from the P–T chart). Measure suction-line temperature at the coil outlet (or manufacturer point).
Superheat = suction-line temperature − saturation temperature at that pressure.
Example: pressure corresponding to 20°F saturation, suction line 30°F → 10°F superheat.
No liquid left at that point: the vapor has been heated another 10°F by the coil and air.
Why the compressor cares
- Too little superheat → liquid risk → slugging, broken rods, damaged scrolls.
- Too much superheat → coil mostly empty of liquid → low capacity, hot compressor, inefficient operation.
- Target depends on application and control (often roughly 8–12°F on many TXV A/C coils; follow OEM for freezers and specialty).
Superheat is a safety and capacity tool, not a vanity number.
Dry expansion vs flooded feed
| Feed style | Liquid in coil | Typical control | Superheat idea |
|---|---|---|---|
| Dry / DX | Mostly evaporates before outlet | TXV, EEV, capillary, fixed orifice | Maintain positive superheat at outlet |
| Flooded | Liquid level maintained in shell / vessel | Float, level, pump recirculation | Different rules; do not chase DX targets |
Most walk-ins and comfort A/C you service are dry DX. Flooded chillers and some industrial evaporators need level thinking, not “10°F SH everywhere.”
Where and how to measure
- Pressure at the evaporator outlet / suction service valve (account for pressure drop on long lines if OEM says so).
- Convert to saturation T.
- Temperature on the suction line at the OEM location—usually after the coil, on clean metal, insulated probe.
- On TXV systems, bulb location and contact matter as much as the reading.
Bad habits: probe on paint or insulation; reading at the compressor only on a long line; ignoring a starved distributor.
High superheat: starved coil
Causes you will see:
- Undercharge or leak
- Restricted drier, TXV screen, distributor feeder
- TXV stuck closed / lost charge in power head
- Low load with fixed feed (capillary) in some conditions
Box warm, suction low, superheat high: think not enough liquid in the coil, not “compressor weak” first.
Low / zero superheat: flooding risk
Causes:
- Overcharge (especially fixed orifice / capillary)
- TXV stuck open or bulb loose / poorly insulated
- EEV control error or sensor fault
- Extremely low airflow so coil cannot boil liquid fast enough
Treat low superheat as an emergency for the compressor until proven safe.
Feed devices in one sentence each
- TXV: meters liquid to hold roughly constant superheat.
- Capillary / fixed orifice: feed depends on pressure difference and charge; superheat drifts with load.
- EEV: electronic metering; needs correct sensors and controller setup.
- Hand expansion / float: specialty and flooded systems.
Next lessons cover valves in depth; here remember: feed method decides how you interpret SH.
Airflow still owns capacity
Even perfect superheat cannot fix a plugged filter. Low CFM drops load on the refrigerant side: pressures and SH move in ways that tempt wrong charging. Always confirm air (or water) flow before using SH to decide charge.
Field routine
- Confirm product / space temperature complaint and runtime.
- Verify fans, filters, doors, and defrost schedule.
- Measure SH (and later subcooling) under stable conditions.
- Compare to OEM target for that feed device.
- Only then adjust charge, valve, or replace restrictions.
Field case
Situation. Reach-in cooler warm; tech last week “topped off” refrigerant. Gauges: low suction, high superheat (~35°F). Liquid line warm near drier.
How to think it.
- High SH + warm liquid-line feel near drier suggests restriction / starve, not “add more.”
- Temperature drop across the filter-drier confirms restriction.
- Replace drier, evacuate, weigh in correct charge; TXV behaves again with ~10°F SH.
Takeaway: superheat diagnosed feed starvation; charge was already wrong as a band-aid.
In the field
Symptom
Warm box, ice at inlet only, compressor noisy / tripped
Where to look
SH at coil outlet, airflow, TXV bulb, drier ΔT, charge history
Likely causes
- Restriction, wrong charge, valve fault, low air
What to measure
- P/T → SH
- entering/leaving air
- drier temps
What not to do
- Charge by suction pressure alone without SH
Checklist
- I define superheat as T_suction − T_sat at evaporating P
- I measure at the correct OEM location with good probe contact
- I know dry DX vs flooded feed changes the rules
- High SH → starved; low SH → flood risk
- I fix airflow before chasing charge with SH