AHU Air Handling Unit - Tube-Fin Coil Design Considerations

AHU Coils: Cooling coils, heating coils (water-cooled/steam/hot water), finned structure, used in air handling units, responsible for cooling, dehumidification, reheating, and preheating functions.

Item Description:

Airflow (m³/h), face velocity (typically 1.8-2.5 m/s; 1.8-2.2 m/s preferred for dehumidification to avoid water carryover)

Inlet Air Conditions: Dry-bulb/wet-bulb temperature; return air + fresh air mixture

Outlet Air Conditions: Target supply air temperature and humidity; condensate volume needs to be calculated for dehumidification

Media Type: Chilled water (7/12℃), hot water, steam, ethylene glycol low-temperature antifreeze

Allowable Pressure Drop: Air-side pressure drop: 80-180 Pa; Water-side pressure drop ≤50-80 kPa

Frame: AHU enclosure dimensions, coil width × height × depth (number of rows)

Tube-Fin Structure Selection (AHU Industry Standard)

Copper Tube + Aluminum Fins (Most Popular)

Copper Tube: φ9.52 / φ12.7 mm Seamless Copper Tube

Fingers: Aluminum Foil, Fin Spacing 2.0-3.2mm

Dehumidification Cooling Coil: Fin Spacing 2.2-2.6mm (Anti-condensation and Frosting)

Dry Condition Heating Coil: Fin Spacing 2.8-3.2mm

Finger Type: Corrugated Fins / Open-Window Fins (Enhanced Heat Transfer); Hydrophilic Aluminum Foil is used for cooling and dehumidification coils to reduce dripping and lower air resistance.

Corrosion Resistance: Stainless Steel Tube + Stainless Steel Fins / Copper Tube + Epoxy Resin Coated Fins (Marine, Chemical Waste Gas AHU)

Number of Rows Selection (Coil Depth)

Cooling and Dehumidification: 2-4 Rows; 4-6 Rows for Deep Dehumidification

Hot Water Heating: 1-2 Rows

Steam Coil: 2-3 Rows

Pipe Layout: Staggered Layout (Enhanced Heat Transfer, AHU) (Almost all misaligned)

Key Logic of Thermal Design

Cooling Coil (Cooling + Dehumidification)

Air passes over the coil surface; the wall temperature is below the dew point, causing condensation.

It must be verified that the coil surface temperature must be less than the air dew point temperature for dehumidification to occur.

The condensate load needs to be calculated; the shell should be designed with a drip tray with a slope ≥1% for drainage.

Heating Coil (Dry Operation)

The wall temperature is above the dew point; there is no condensation, only temperature increase.

Water Flow Velocity: Water flow velocity inside copper tubes 0.8-1.6 m/s. Too low a velocity results in poor heat transfer; too high a velocity leads to high noise and pressure drop.

Structure and Installation Compatibility with AHU

Water Piping: Reversible left and right; inlet and outlet on opposite/same side; exhaust valve at high point, drain at low point

Drain Tray: Cooling coils must be equipped with a stainless steel drain tray with a drain outlet; this can be omitted for heating coils without condensation.

Water Baffle: For face wind speeds > 2.2m/s or high humidity conditions, install a water baffle downstream of the coil to prevent water droplets from being blown into the air duct.

Frame: Galvanized steel frame, flanges bolted to the AHU housing; sealing strips prevent air leakage.

Common Failure Issues & Design Mitigation

Water Splatter/Condensation: Excessive wind speed, insufficient fin spacing, lack of hydrophilic film, missing water baffle

Insufficient Heat Exchange and Inability to Achieve Temperature Drop: Insufficient number of rows, excessive face wind speed, insufficient water flow rate, clogged fins

Freezing Crack (Winter Low-Temperature Fresh Air AHU)

Fresh Air Pre-cooling/ Preheat the coils; add ethylene glycol antifreeze to the water system; design piping with venting or install antifreeze protection; Corrosion: In humid, salt spray environments, use a hydrophilic anti-corrosion coating and avoid using ordinary aluminum foil.