One Iced Evaporator in a Four-Circuit Produce Cooler
A 20 ft x 30 ft x 10 ft produce cooler is served by four matched unit coolers, each with a 1-ton TEV and a 24 V solenoid. The controller runs four 20-minute off-cycle defrosts per day. The customer reports water on the floor and warm product near the back wall. A technician finds the far evaporator heavily frosted while the other three are clear. Its fan is running, the solenoid is energized, and the suction line at that evaporator is frosted back to the header. Suction pressure is 32 psig, box temperature is 39°F, the liquid-line sight glass is clear, and the other three evaporators show about 8°F superheat. The iced coil's TEV remote bulb is clamped to the top of the horizontal suction line about 6 in. from the coil and is not insulated.
1. The suction line is frosted back to the header on only one circuit. What does that pattern suggest about superheat and refrigerant feed on that evaporator?
Read the worked answer
Frost back to the header on one circuit means that circuit is running at very low superheat, likely returning liquid. It is not a whole-system undercharge: the other three circuits have normal superheat and the sight glass is clear. One flooding circuit can depress overall suction pressure and cause the other circuits to look for load. The likely causes are a TEV that is overfeeding, a bulb that has lost charge or is poorly located, a missing or plugged equalizer, or a coil with low airflow that cannot boil off the refrigerant. The frost pattern alone does not prove the TEV is bad; it tells you to check feed and load on that circuit.
2. Evaluate the TEV bulb location. How would you correct it, and what else would you check before adjusting the TEV?
Read the worked answer
The bulb is incorrectly located. On a horizontal suction line it should be clamped to the side (about 4 or 8 o'clock), clean and tight, and insulated from ambient. Mounting on top and leaving it uninsulated can make the bulb sense a temperature that is not representative, especially with oil or liquid stratification, and it can cause the TEV to overfeed. Correct the location and insulation first, then check the bulb charge, external equalizer, inlet screen, and coil airflow before turning the adjusting stem. Do not adjust the TEV to compensate for a bad bulb installation.
3. Outline the diagnostic sequence to separate an airflow problem from a TEV or defrost problem, and state the final repair verification.
Read the worked answer
Diagnostic sequence: measure air temperature drop across the iced coil and compare with the other three; check fan blade, motor speed, and coil blockage. If airflow is normal, measure superheat at that evaporator and inspect TEV inlet for flash gas. Verify the defrost schedule and termination; off-cycle defrost may not clear a heavily iced coil, but only one coil icing points to a local problem. After relocating and insulating the bulb, set superheat to the manufacturer's target and observe a full defrost and cool cycle. Verify no frost returns to the header, box temperature recovers, and condensate drains properly.