Even if the noise-reducing enclosure is sealed too tightly, it will not overwrite the evaporation temperature settings programmed into the PLC. The setpoint values in the program will not increase on their own; however, since heat cannot be dissipated from around the unit, the actual evaporation temperature of the equipment will continue to drift upward. Consequently, the operating temperature reading displayed on the HMI will rise accordingly, which can easily lead to a misinterpretation on-site that the program settings have increased on their own.

In low-temperature heat pump evaporator units, the compressor and condenser are the primary heat-generating components and are typically housed inside the noise-reducing enclosure. If the enclosure is sealed too tightly, the ventilation and heat dissipation channels are blocked, preventing hot air inside the enclosure from escaping, causing the internal temperature to gradually rise. With the condenser located in a high-temperature, sealed chamber, the heat exchange temperature difference is reduced, condensation cooling capacity decreases, and the refrigerant’s condensing pressure is forced to rise. As the condensing pressure rises, the output temperature on the heating side of the heat pump system increases accordingly. The controller continues to output heat based on the original evaporator temperature setpoint, but the heating capacity on the heat source side is passively strengthened, resulting in the amount of heat supplied to the evaporator chamber exceeding the heat normally consumed by the system during evaporation.

Under fixed vacuum conditions, the evaporator chamber inherently has a corresponding saturated evaporation temperature. When the input heat exceeds the heat required for moisture vaporization, the excess heat directly raises the actual temperature of the feed liquid. The actual operating temperature then exceeds the theoretical saturation temperature, causing the temperature reading displayed on the screen to rise continuously. To suppress this temperature rise, the vacuum control system increases the vacuum pump’s pumping capacity in an attempt to lower the chamber pressure. However, due to limitations in the vacuum pump’s capacity, the vacuum cannot be reduced indefinitely, and the condition of elevated temperature persists.

Another contributing factor is environmental interference affecting the sensors. The wiring for the temperature probes runs through the noise-reducing enclosure, where the internal environment remains consistently high-temperature. The probe junction boxes and transmitters are exposed to this high-temperature environment, introducing additional thermal interference on the measurement side. This causes a positive drift in the temperature signal, resulting in even higher readings fed back to the control system and creating a dual upward drift phenomenon. It is important to distinguish between two scenarios: one where the actual temperature of the material has indeed risen, and another where the sensor is affected by high ambient temperatures, resulting in falsely elevated readings. In most field situations, both conditions occur simultaneously.

Operating conditions can amplify this phenomenon. During summer when ambient temperatures are high and equipment is running at full load, the heat generated inside the enclosure is significant, and the temperature drift caused by limited heat dissipation becomes particularly pronounced; at night or during periods of low processing load, when heat generation is minimal, the phenomenon is somewhat alleviated. If the side panels of the noise-reduction enclosure are opened to allow ventilation, heat dissipation conditions are restored, the condensing pressure drops, and the evaporation temperature can return to the design range. This characteristic can be used for rapid on-site troubleshooting.

Consequential chain reactions include: high condensing pressure, increased compressor power consumption, and higher energy consumption per metric ton of water for the entire unit; prolonged high-pressure operation of the refrigerant system increases the risk of triggering compressor overload protection; and once the actual chamber temperature deviates from the designed low-temperature evaporation range, the process fluid is more prone to salt and colloidal deposits on the walls, accelerating the rate of scaling.

During many on-site commissioning phases, in an effort to reduce equipment operating noise, all gaps in the noise-reduction enclosure are sealed, all gaskets are compressed to the limit, and all reserved ventilation openings are closed off. This approach prioritizes noise reduction metrics while neglecting the heat pump system’s requirement for forced heat dissipation. After a failure occurs, repeatedly adjusting the PLC temperature settings or calibrating the temperature probes only provides temporary relief; as long as the poor heat dissipation conditions within the enclosure persist, temperature deviations will continue to recur.

It is important to dispel this misconception: it is not a change in the numerical settings within the program that causes the issue, but rather the deterioration of heat dissipation that alters the heat pump’s operating conditions, resulting in actual operating temperatures deviating from the target values. The solution should not be to blindly pursue a completely sealed enclosure for noise reduction; instead, reasonable air intake and exhaust pathways must be maintained to ensure adequate ventilation and heat exchange for the condenser. Ventilation openings can be equipped with sound-absorbing louvers to balance noise reduction and heat dissipation; a fully enclosed enclosure is not the answer.