12.2 Evaporator TD and Superheat Diagnostics
Key Takeaways
- Evaporator TD is space or process temperature minus saturation temperature at the evaporator; using compressor suction overstates TD by suction-line drop.
- High TD means underfeed, iced coil, oil logging, high load, or low airflow — UA is down or Q is up.
- Low TD is overfeed/flooded wetting, light load, extra UA, or suction held high by a control; undersized coils run high TD, not low.
- DX superheat too high means a starved coil; superheat too low or zero on DX feeding a screw is floodback risk.
- Flooded and liquid-overfeed returns are saturated by design — do not force DX superheat targets on a wet suction.
12.2 Evaporator TD and Superheat Diagnostics
Evaporator temperature difference (TD) tells you whether the coil or chiller is actually moving the load. Superheat tells you whether a DX coil is fed, starved, or dumping liquid at the compressor. CIRO screens mix those two ideas; the supervisor's job is to keep them apart — and to stop applying DX superheat rules to a flooded overfeed return.
TD: space or process minus suction saturation
Evaporator TD = space or process temperature − saturation temperature at the evaporator.
The sat T must be taken at coil or chiller pressure, not blindly at the compressor. Suction-line drop, wet returns, and vessel drop all make compressor suction lower than evaporator pressure. Using compressor suction overstates TD (you subtract a colder sat T).
On a process heat exchanger, process temperature is the leaving or entering fluid the design uses — the stem will say which. On an air unit, use room or blast-cell air, not the fan-discharge thermometer, unless the question asks for air-side range.
Simplified heat transfer still rules the diagnosis: Q = UA × TD. For a given load Q:
- UA falls (ice, oil, underfeed, dead fans) → TD rises
- Q rises (hot product, extra infiltration) → TD rises
- UA is high (wet overfeed surface, extra circuiting, lots of air) → TD falls for the same Q
Worked freezer example — account for suction-line drop
Blast cell air is −10°F. Evaporator pressure corresponds to −22°F on the P/T chart. Coil TD = −10 − (−22) = 12°F — a normal industrial freezer band (many ammonia air units live in 8–15°F).
The compressor suction is 0 psig. Ammonia at 0 psig is about −28°F. If you compute TD from the compressor, you get −10 − (−28) = 18°F and you start chasing an 18°F coil that does not exist. The extra 6°F is suction-line and vessel drop, not ice.
Always ask which pressure the screen used for suction: coil, packaged vessel, or compressor flange. The same caution applies when you compute superheat: the vapor thermometer and the pressure for sat T must be the same plane.
High TD — the coil is not keeping up
High TD means the refrigerated space or process is warmer relative to evaporating temperature than design. Causes CIRO expects you to name:
- Underfeed. DX TXV/EEV too tight, plugged distributor, flash gas in the liquid line, low high-side pressure, failed overfeed solenoid, or a starved recirculator. Part of the surface is dry. UA drops. Superheat on a DX coil will be high at the same time.
- Iced or frosted coil. Frost is insulation. Defrost that is too short, too rare, or terminated on time instead of coil temperature leaves a blanket on the tube. UA drops; operators pull suction down; TD climbs further.
- Oil-logged evaporator. Oil films the inside of the tubes or the flooded vessel. Same UA penalty as a mild ice coat. Look for oil in the surge drum, a recent compressor dump, or a missing oil still or pot drain.
- High load. More product, more infiltration, more lights, a warm pull-down. Q is up; TD must rise until capacity catches. This is not a defective coil — it is a load story. Check production schedule and door time before you rebuild the coil.
- Low airflow. Belt off, iced fan ring, plugged return, closed damper. Air-side UA collapses. The room sensor may climb; the operator lowers suction; TD on the screen looks terrible.
High TD is not a 2 a.m. surprise from an undersized coil if that coil used to hold design TD. Undersized equipment does run a high design TD — that is a nameplate and selection problem, visible from day one. Overnight changes are feed, frost, oil, load, and air.
Low TD — do not call it undersized
Low TD means sat T is close to space or process temperature. Heat is transferring easily, the load is light, or you are holding suction too high.
- Overfeed / flooded coils run wet and typically show lower TD than an equivalent DX coil because UA is higher. A 3:1 or 4:1 overfeed that suddenly looks too good may be overfeeding or the room may be at light load.
- Control problem: a back-pressure regulator or zone valve holding evaporating pressure up (warmer sat T) shrinks TD by definition. The room may then miss setpoint even though TD looks small.
- Light load / extra surface: extra coil, extra air, empty freezer. TD collapses. That can be fine — or on DX it can mean the valve is hunting toward floodback because load vanished.
Exam trap: undersized evaporators produce high TD, not low. If the stem offers an undersized coil as the explanation for a 4°F freezer TD, reject it. Low TD is overfeed, extra UA, light load, or suction held high. The question mark in many operators' notes — low TD, or undersized? — is resolved by Q = UA × TD. Small UA (undersized or iced) needs a larger TD to move the same Q.
Superheat — DX only unless the stem says otherwise
Superheat = vapor temperature − sat T at the pressure where you measured that vapor.
- Too high (often 15–25°F+ when design is about 8–12°F): starved DX. The coil is being used as a superheater. Capacity is down, discharge may be hot, and evaporator TD is usually high. Causes: TXV/EEV underfeed, lost bulb charge, clogged strainer, flash gas, low receiver pressure, wrong orifice.
- Too low or zero on a DX coil feeding a screw: floodback. Liquid and cold rich oil wash into the suction. Screws do not like liquid. Oil dilutes, viscosity collapses, bearings and rotors suffer. Zero superheat at the package on DX is a stop-and-fix, not a tune-it-later.
Flooded and liquid-overfeed returns are saturated by design. A wet suction into a separator is not floodback; it is the machine. Superheat at the compressor on a two-stage or high-stage suction may still be small. Do not open a hand expansion to get 10°F superheat on a recirculator coil — you will starve it and raise TD.
Pair the two KPIs
| Screen | DX meaning | Overfeed meaning |
|---|---|---|
| High TD + high superheat | Starved coil | Underfeed or ice/oil; check solenoids and frost |
| High TD + low superheat | Unusual on DX (huge load with valve wide) | Ice, oil, or low air on a still-wet coil — UA problem |
| Low TD + low superheat | Floodback risk | Normal wet coil or light load |
| Low TD + high superheat | Check sensors and tap locations | Sensor or location error |
On the CIRO screen, compute TD from room versus evaporator sat, compute DX superheat from suction temperature versus the same pressure plane, and write down which plane you used. That habit scores items that wreck operators who subtract wet-bulb from suction or who chase 10°F superheat on a flooded freezer.
A blast freezer holds −10°F air. Coil pressure corresponds to −22°F saturation. Compressor suction is 0 psig (about −28°F saturation). What is the coil TD, and what happens if you use compressor suction instead?
A DX ammonia coil feeding a screw shows 24°F superheat at the coil outlet and a room-to-coil TD several degrees above design. What does the high superheat indicate?
A DX package feeding a screw compressor shows zero superheat at the suction temperature sensor and liquid-line feed is wide open. What is the primary risk?
An overfeed freezer coil shows a 5°F TD at light production load. A candidate claims the evaporator must be undersized. Why is that claim wrong?