High discharge pressure on a dairy walk-in cooler after a fan motor replacement
A 12 m³ walk-in cooler for dairy storage is set to 4 °C and uses R-404A with a 3 HP semi-hermetic condensing unit and a thermostatic expansion valve. On a 40 °C afternoon the compressor cuts out on the high-pressure switch after about 8 minutes and restarts only after manual reset. The condenser coil was washed yesterday and looks clean. The condenser fan motor was replaced last week; the technician reports strong air at the fan grille. A thermometer in the air leaving the condenser reads 46 °C. Gauges show suction 4.2 bar and discharge 23 bar; the liquid line is hot to touch. The sight glass is clear. Measured suction-line temperature is 1 °C, and the liquid-line temperature is 45 °C; the PT chart gives evaporating saturation -5 °C and condensing saturation 55 °C. Compressor current is 9.8 A against a rated 8.6 A. The store will not accept a shutdown during the day and asks for a quick diagnosis.
1. Calculate the superheat, subcooling and condensing approach. What fault do these readings point to, and what would you check first?
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Superheat = suction-line temperature minus evaporating saturation = 1 - (-5) = 6 K. Subcooling = condensing saturation minus liquid-line temperature = 55 - 45 = 10 K. Condensing approach = condensing saturation minus ambient = 55 - 40 = 15 K.
A normal approach for a clean air-cooled condenser is roughly 5-8 K; 15 K is high. The condenser is rejecting heat poorly even though the coil looks clean and the fan is moving air. The air temperature rise across the condenser is only 46 - 40 = 6 K, which is low for a heavily loaded condenser, so the problem is likely low or recirculated airflow rather than a dirty coil. A fan blade fitted backwards, a motor of the wrong speed or rotation, a missing fan guard, or hot air recirculating back into the condenser can all produce this pattern.
Overcharge would normally show higher subcooling and a high head, but it would not explain the weak air-side temperature rise. Non-condensables would also raise discharge pressure, but the recent fan-motor work is the stronger clue. First check fan rotation direction, blade pitch and orientation, motor RPM and whether air is actually being discharged away from the coil. Then check for recirculation and nearby obstructions. Do not adjust the refrigerant charge until airflow is proven correct.
2. The store asks you to bypass the high-pressure switch until tomorrow. Explain why you must refuse and what temporary measures are safe.
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Bypassing the high-pressure switch removes the last protection against an unsafe discharge pressure. With the condenser already at a 15 K approach, the next likely events are compressor valve damage, motor overload, blown gaskets, oil breakdown, or a refrigerant release. The switch is not nuisance tripping; it is reporting a real heat-rejection fault. Defeating it would also endanger the technician and the stored food.
Safe temporary measures are limited to correcting the cause: restore correct fan rotation and blade orientation, remove any recirculation path, clean the coil if needed, and confirm that the condenser fan motor is the correct speed and is running. If the fan cannot be corrected immediately, keep the unit off and move product to a working room or arrange a temporary condensing unit. Never install a larger fuse, jumper the HP switch, or simply add refrigerant. Record the pressures and approach before and after the correction.
3. After the fan problem is corrected, what acceptance readings would convince you the condenser is healthy, and what should be recorded for preventive maintenance?
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Acceptance should be based on more than 'it is cooling'. With the box near set point and the compressor loaded, expect the condensing approach to fall to roughly 5-8 K, subcooling to settle near the manufacturer's range, superheat to remain within the TXV setting, and compressor current to fall below the 8.6 A rating. The HP switch should remain stable with no reset. The air temperature rise across the condenser should increase compared with the faulty 6 K reading because more heat is being rejected.
Record ambient temperature, discharge and suction pressures, condensing and evaporating saturations, liquid-line and suction-line temperatures, calculated superheat and subcooling, compressor current, fan motor current, and the condition of the coil and fan blades. Also note the HP switch setting and confirm that it has not been altered. This baseline lets the next technician detect a gradual airflow or charge problem instead of waiting for another trip.