During June and July every year, humidity control becomes the most unstable factor in cleanrooms. Air conditioning systems that run steadily normally may suffer insufficient dehumidification capacity under high-humidity conditions, triggering a chain of problems including increased particle counts, pressure differential fluctuations and abnormal microbial data. Combining common field cases, this article sorts out technical points for cleanroom operation and maintenance in plum rain season from impact analysis, AHU countermeasures, filter management, pressure differential control and operation monitoring.

Impacts of Plum Rain Season on Cleanrooms

Rising particle concentration High humidity raises water molecule content in the air, making particles easier to agglomerate and settle. Actual tests show 0.5μm particle counts are generally higher in plum rain season. This is not caused by reduced filter efficiency, but changes in air physical properties.

Accelerated microbial growth When relative humidity exceeds 65%, microbial reproduction speeds up significantly. For medical device and pharmaceutical cleanrooms, settling bacteria and airborne bacteria levels tend to approach or exceed control limits in plum rain season. Some workshops maintain single-digit settling bacteria normally, which may nearly hit limits in humid weather.

Corrosion of equipment and enclosure structures High humidity accelerates rusting of stainless steel panels, galvanized air ducts and equipment casings. In some projects, color steel plates near return air outlets peel after just one plum rain season.

AHU Dehumidification Capacity & Adjustment Methods

The combined air handling unit (AHU) used in purification workshops relies mainly on cooling coils for dehumidification. Fresh outdoor air is cooled by cooling coils; water vapor condenses and drains away, then air is reheated to the required supply temperature. This process works stably under normal conditions. However, in plum rain season, air moisture content outdoors can exceed 20g/kg dry air, nearly double the usual level. The cooling coil bears much higher moisture load. With unchanged chilled water parameters (typically 7℃ supply /12℃ return) and heat exchange area, condensation dehumidification efficiency drops, supply air moisture content rises, and workshop humidity gets hard to control.

Available adjustments:

1. Lower chilled water supply temperature Some sites can reduce supply water temperature from 7℃ to 5℃ or lower to drop the coil dew point and boost dehumidification. Check unit design margin carefully. Excessively low water temperature may freeze cooling coils and reduce air volume.
2. Adjust fresh air volume Fresh air contributes most of the moisture load in plum rain season. Properly cut fresh air rate and increase return air ratio on the premise of guaranteed pressure differential and personnel health to ease coil burden. Cleanroom fresh air usually accounts for 10%~30% of total supply air. Adjustments must meet minimum requirements for breathing and positive pressure maintenance.
3. Add rotary dehumidifier section For strict humidity control scenarios such as lithium battery and solid dosage workshops, install rotary dehumidifier after AHU. Based on adsorption principle, rotary dehumidifiers work stably regardless of inlet air moisture, yet with higher capital and operating cost, normally not equipped for general Class 10,000 cleanrooms.

FFU & Filter Moisture Absorption Effects

When humidity rises in plum rain season, HEPA filter media absorbs moisture and increases resistance. Normal FFU operating resistance sits at above 100 Pa and may exceed 200 Pa under high humidity. Higher resistance reduces air volume and air change rate. Constant-air-volume FFUs automatically speed up motors to compensate resistance and keep stable airflow; fixed-speed FFUs suffer obvious airflow attenuation in humid weather. It is recommended to test FFU air volume before and after plum rain season for comparison. If airflow drops over 15%, replace filters in advance or add frequency conversion control.

Pressure Differential Fluctuations & Solutions

Air density changes under high humidity, breaking supply and exhaust balance and causing more dramatic pressure fluctuations. Coupled with frequent door opening by staff, pressure gradient between clean corridors and core operating rooms fluctuates sharply. One IVD project recorded stable +12 Pa normally, swinging between +5 Pa ~ +15 Pa during plum rains. Root causes were air density variation and exhaust airflow fluctuation. The solution is adding humidity compensation logic into variable-frequency supply/exhaust control. Older systems without this function require manual adjustment or on-site program upgrade during maintenance.

Condensate Drain & Fresh Air Inlet Maintenance

Condensate drainage Large dehumidification load generates massive condensate in plum rain season. Insufficient slope of condensate pipes or dried water seal in traps due to negative suction will lead to poor drainage or backflow. Stagnant water in drain pans easily creates odor and breeds bacteria. Inspect condensate pipe smoothness and trap water seal height before plum rain arrives.

Pre-filter at fresh air inlet Frequent rainfall clogs pre-filters at fresh air intake with rainwater and dust, increasing resistance and cutting fresh air volume. The AHU fan runs under heavy load with insufficient inlet air, triggering a chain reaction: insufficient fresh air, falling positive pressure and out-of-control humidity. Weekly visual inspection and timely replacement are required in plum rain season.

Key Monitoring Points in Plum Rain Season

Focus on these parameters:

1. Supply & return air temperature and humidity: hourly logging, set alarm thresholds on BMS if available
2. Pressure differential of each zone: track pressure gradient of clean corridors and core rooms
3. FFU / AHU operating current and filter resistance: rapid resistance rise indicates heavy filter moisture absorption
4. Chilled water supply & return temperature: monitor coil icing risk when lowering supply water temperature
5. Fresh air inlet pre-filter: weekly visual inspection, replace once clogged

Plum rain season brings systematic impacts on cleanrooms. Humidity management requires coordinated control of AHU dehumidification, filter status, pressure balance, condensate drainage and routine inspection. Troubleshoot weak points in advance rather than handling emergencies after problems occur.