AHU Design Specification for High Humidity / Tropical Humid Climate

(Monsoon region, Southeast Asia, coastal tropical area, continuous high ambient RH, frequent rainfall, ambient RH often >80%)

1. Main Challenges in High Humidity Environment

• Massive latent cooling load; heavy condensate generation

• Risk of condensate dripping, cabinet water leakage

• Mold, fungus, biofilm growth inside AHU, coils & drain pan

• Surface condensation on cabinet outer walls

• Filter rapid dampness, breeding microorganisms

• Risk of supply air overcooling without proper reheat → space over-dry or surface condensation indoors

2. Core AHU Section Design & Upgrade

2.1 Air Intake & Filtration

• Louver: water droplet eliminator (mist eliminator) to trap rain splash

• Filtration sequence:

  G4 Pre-filter → F7 / F8 medium filter

• Optional: anti-microbial coated filter media

• Quick access door for frequent inspection; avoid trapped damp air inside filter compartment

2.2 Cooling Coil (Most critical section)

• Oversized evaporator coil for sufficient dehumidification capacity

• Lower coil surface temperature to achieve deep dew-point dehumidification

• Hydrophilic, anti-mold coated aluminum fins

• Limit coil face velocity ≤2.0 m/s to prevent water carryover (water droplets blown into duct)

• Optional: coil condensate separator after cooling coil

2.3 Reheat Section (High humidity climate highly recommended)

Two common options:

1. Hot water reheat coil

2. Electric reheat

   Purpose:

   After air is cooled & dehumidified, air temperature becomes too low. Reheat adjusts supply air temperature, stabilizes indoor RH, avoids cold air condensation on indoor walls/windows.

For projects requiring strict indoor humidity control (office, factory, warehouse): reheat is not optional

2.4 Drainage System (Failure here = water leakage complaint)

• Extra deep, widened stainless steel 304 drain pan, continuous slope ≥2%

• Extended water collection area under cooling coil

• Deep P-trap matched to unit internal static pressure

• Anti-siphon design, trap must keep water seal all the time

• Cleanout port for regular sludge & algae removal

• Multiple drain outlets for large-size AHU; avoid local water pooling

2.5 Cabinet Construction Anti-Condensation

• Double skin insulated panel, thick PU insulation (minimum 50mm)

• Vapor barrier inside cabinet to stop water vapor penetration

• All panel seams fully sealed with neutral anti-mold silicone

• Prevent external sweating (cabinet outer surface condensation)

• Internal rounded corners; no dead air zones where moisture accumulates

2.6 Fan Section

• Centrifugal EC / belt driven fan

• Anti-corrosion fan wheel (epoxy coating / 304 SS optional)

• Flexible canvas connector (waterproof, anti-mold material)

3. Control Logic Special for High Humidity

• Priority control: humidity first, temperature second

• DDC monitors return air RH; automatically adjust chilled water valve to maintain dehumidification

• Interlock: Drain pan high water level alarm (prevent overflow)

• Anti-mold periodic operation: Periodic fan running to ventilate unit during shutdown

• Humidity limit protection to avoid over-dehumidification

4. Optional Upgrade Packages

1. UV-C germicidal lamp: Installed after cooling coil, inhibit mold and bacteria on coil surface

2. Titanium / epoxy coated coil: For coastal humid + salt spray combination

3. Stainless steel internal frame for long service life

4. Condensate drain pan heating trace for occasional low-temperature conditions

5. Air to air heat recovery (ERV) to reduce fresh air latent load

5. Typical Design Target Reference

Indoor common requirements (tropical commercial / factory):

• Temp: 24 ~ 26°C

• RH: 45% ~ 60%

Fresh air carries huge moisture load; always calculate latent load first during sizing.

6. Trouble Shooting Common Issues in Humid Climate AHU

1. Water blow through from supply air duct → Reduce coil face velocity + install droplet eliminator

2. Cabinet outer sweating → Improve insulation + vapor sealing

3. Drain pan algae & odor → Regular flush, anti-mold coating, periodic cleaning plan

4. Mold inside AHU → Add UV lamp, periodic dry ventilation cycle