ℹ️ About This Calculator
The Air Handling Unit (AHU) is the central element of a ducted HVAC system. It conditions the supply air (cools, heats, humidifies or dehumidifies) and distributes it through the duct network. Correct AHU sizing determines the supply air temperature differential, airflow rate, coil duty, and fan power - all of which must be coordinated to meet the room cooling load without overcooling or undercooling.
ASHRAE 62.1 specifies minimum outdoor air ventilation rates for acceptable indoor air quality. NBC Part 8 provides the Indian equivalent. For offices: minimum 10 L/s per person plus 0.3 L/s per m² of floor area. AHUs must be designed to handle the minimum fresh air requirement in addition to the recirculated air for cooling. The mixing of fresh and return air happens in the AHU mixing box before the filters and coil.
📐 AHU Sizing Formula (ASHRAE)
ASHRAE 62.1 / NBC Part 8
Supply Airflow: Q_air (m³/h) = Cooling Load (kW) × 3600 / (ρ × Cp × ΔT) ΔT = Room DB − Supply Air Temp (typically 24−13 = 11°C) ρ = 1.2 kg/m³, Cp = 1.006 kJ/kg·K CHW Flow Rate: Q_chw (L/s) = Cooling Load (kW) / (4.187 × ΔT_chw) ΔT_chw = Return − Supply (typically 12−7 = 5°C) Fan Power (estimate): P_fan (kW) = Q_air × ESP / (3600 × η_fan) ESP = External Static Pressure (Pa), η_fan = 0.65–0.75 AHU Coil Face Velocity: 2.0–2.5 m/s (typical)
🧮 Worked Example
If the AHU fan must overcome an external static pressure of 400 Pa at 70% fan efficiency, fan power P_fan = (27,100/3600) × 400 / 0.7 ≈ 4.3 kW. This confirms the selection needs roughly 27,000 m³/h airflow, 5 L/s CHW flow, and a fan motor of about 5.5 kW with margin.
📊 AHU Sizing Reference (ASHRAE — Airflow, Coil & CFM/Ton)
Two numbers size an AHU: the supply airflow that carries the room sensible load away at your chosen supply-air ΔT, and the coil duty that removes the total (sensible + latent) load from that air. Fresh-air ventilation is added on top, in the mixing box, ahead of the coil.
Typical AHU Design Parameters
| Parameter | Typical value (comfort cooling) |
|---|---|
| Supply-air ΔT (room − supply) | 10 – 12 °C (18 – 22 °F) |
| Airflow per ton (TR) | ≈ 340 – 400 CFM/TR |
| Coil face velocity | 2.0 – 2.5 m/s (400 – 500 fpm) |
| Chilled-water ΔT | 5 – 6 °C (7 °C flow / 12 °C return) |
| Cooling-coil rows | 4 – 8 rows (more rows for higher latent load) |
| Fresh air (offices) | ASHRAE 62.1 / NBC Part 8 — ≈ 10 L/s per person |
| Sizing margin | +10 – 15 % on airflow and coil duty |
Airflow & Coil-Load Formulas
| Quantity | SI | IP (US units) |
|---|---|---|
| Supply airflow | Q(m³/h) = Load(kW) × 3600 / (1.2 × 1.006 × ΔT°C) | CFM = Sensible(BTU/h) / (1.08 × ΔT°F) |
| Cooling-coil load | = ṁair × Δh (enthalpy drop) | BTU/h = 4.5 × CFM × Δh(BTU/lb) |
| Mixed-air temp | Tmix = (%OA × Toutdoor) + (%RA × Treturn) | |
The classic rule of thumb — ≈ 400 CFM per ton of refrigeration — only holds at a normal comfort ΔT. High-latent spaces (kitchens, coastal offices, gyms) use colder supply air, so they need less airflow per ton but a deeper coil. Where outdoor air is significant, size the coil on the mixed-air on-coil condition, not the room condition, or the coil will be undersized.
Indicative design values — confirm against the ASHRAE Handbook, the coil manufacturer's selection software, and NBC 2016 Part 8 / ASHRAE 62.1 ventilation rates for the final design.
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