Frozen Display Case EEV Superheat Hunting
A frozen food display case with an electronic expansion valve (EEV) and a case controller. The case setpoint is -5°F, but product temperature is 8°F. The EEV superheat is oscillating between 5°F and 25°F every 2 minutes. Suction pressure at the rack is 14 psig (about -18°F saturated). The case has a 10-foot line run and a 3/8 in. liquid line. The EEV uses a pressure transducer and a temperature sensor on the suction line. The sensor is clamped 10 in. from the evaporator outlet, and the insulation is missing. The rack has a liquid subcooler and 10°F subcooling. The case has a defrost cycle every 8 hours, 20 minutes. Store staff report the case never recovers after defrost, and the product is soft.
1. Why is the EEV hunting, and how does that explain the high product temperature?
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The EEV is hunting because its superheat sensor is giving a false, fast-changing signal. The sensor is only 10 in. from the evaporator outlet and the insulation is missing, so it is influenced by ambient air and by the coil rather than the true suction-line temperature. The controller responds to the false temperature swing by driving the valve open and closed, creating the 5°F to 25°F oscillation. The 10°F subcooling and 14 psig suction are otherwise reasonable, so the rack is probably not the cause. Check the pressure transducer calibration as well. The EEV PID settings may also be too aggressive, but correct the sensor location and insulation first. A hunting EEV reduces average coil capacity and can cause the case temperature to rise to 8°F.
2. What repair and verification sequence would stabilize the case?
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Repair the sensor installation: move the sensor to the manufacturer's specified distance from the evaporator outlet, clean the suction line, and insulate it fully. Verify the pressure transducer reading against a gauge. Then check the controller's superheat setpoint and PID tuning; reduce the proportional gain if the valve still oscillates. Run the case and record superheat and case temperature for at least 30 minutes. Superheat should stabilize near the setpoint (typically 6–10°F for a frozen case) and case temperature should pull down toward -5°F. If hunting persists after the sensor repair, inspect the EEV stepper motor and valve orifice for debris, and confirm the liquid line is not flashing.