ℹ About This Calculator
The cooling coil is the heart of an air handler, where warm air gives up heat to chilled water. This calculator estimates the coil face area, face velocity, number of rows and chilled-water flow from the total cooling load and design conditions, following ASHRAE and AHRI 410 practice.
Face velocity is kept around 2.0-2.5 m/s - higher velocities carry condensate off the coil and increase pressure drop. The number of rows and fin spacing set how much the air is cooled and dehumidified; deeper coils give a lower off-coil temperature but higher air-side resistance. Chilled-water flow follows from the load and the water temperature rise (usually 5-6 °C).
Cooling Coil Formulas
ASHRAE / AHRI 410
Face area = Q_air / V_face Water flow = load / (4.186 × ΔT_water) Q_air = supply airflow, m³/s V_face = coil face velocity, m/s (2.0-2.5) load = coil cooling load, kW 4.186 = water specific heat, kJ/kg·K ΔT_water = chilled-water temperature rise, °C (5-6)
Worked Example
Example: An AHU handling 2 m³/s of air at a 2.25 m/s face velocity needs a face area of 2 / 2.25 ≈ 0.89 m². For a 35 kW coil load with a 5.5 °C water rise, chilled-water flow = 35 / (4.186 × 5.5) ≈ 1.52 L/s. A 4-6 row coil typically achieves the required off-coil condition for comfort cooling.
Coil Face Velocity & Rows Reference (AHRI 410)
Face velocity and row count are the key selection levers. Guidance:
| Parameter | Typical range | Effect |
|---|---|---|
| Face velocity | 2.0 – 2.5 m/s | Above ~2.5 m/s risks condensate carry-over |
| Rows (comfort cooling) | 4 – 6 | More rows = lower off-coil temp, higher ΔP |
| Fin spacing | 1.8 – 3.2 mm | Tighter fins = more capacity, easier fouling |
| Water temp rise | 5 – 6 °C | Larger rise = lower flow, smaller pipes |
Sensible vs Total Load
The coil handles both sensible (temperature) and latent (moisture) load. High-latent applications (kitchens, dense occupancy) need a colder, deeper coil to reach a low enough off-coil dew point. Always select on the total load and the required off-coil condition, not the sensible load alone.
Indicative values - final selection uses the manufacturer's coil-rating software to AHRI 410.
Frequently Asked Questions
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