How To Calculate AHU Cooling Coil Capacity?

To calculate an AHU cooling coil capacity, you need the airflow rate and the enthalpy difference across the coil; the core formula is:

Q = ṁ × (h₁ − h₂) (kW, SI units)

or in common HVAC field units:

Q = 4.5 × CFM × (h₁ − h₂) (Btu/h)

Below is a step-by-step guide with formulas and an example.

1. Basic Parameters Needed

• Airflow rate (ṁ): mass flow (kg/s) or volume flow (m³/s, CFM)

• Inlet air state (AHU mixed air): DB₁, WB₁ / RH₁ → get h₁ (kJ/kg or Btu/lb)

• Outlet air state (coil leaving): DB₂, WB₂ / RH₂ → get h₂

• (Water side, for verification): chilled water flow, Tchw_in, Tchw_out

2. Calculate Total Cooling Capacity (Air Side)

SI Units (kW)

1. Find air density ρ ≈ 1.2 kg/m³ (standard air)

2. Mass flow:

   ṁ = Qv × ρ (kg/s), Qv in m³/s

3. Total heat (sensible + latent):

   Q_total = ṁ × (h₁ − h₂) (kW)

IP Units (Btu/h, common in US)

• Q_total = 4.5 × CFM × (h₁ − h₂) (Btu/h)

• 4.5 = 60 min/h × 0.075 lb/ft³ (standard air density)

Sensible vs Latent Split

• Sensible: Q_sensible = ṁ × cp × (DB₁ − DB₂)

  cp ≈ 1.005 kJ/(kg·K)

• Latent: Q_latent = ṁ × hfg × (ω₁ − ω₂)

  hfg ≈ 2501 kJ/kg; ω = humidity ratio (kg/kg)

• Q_total = Q_sensible + Q_latent

3. Water Side Verification (kW)

If chilled water data is known:

Q = ṁ_w × 4.187 × (Tchw_out − Tchw_in)

• ṁ_w = water mass flow (kg/s)

• 4.187 = specific heat of water (kJ/kg·K)

4. Coil Sizing (Heat Transfer Area)

For coil selection, use LMTD (Log Mean Temperature Difference) and U-value:

Q = U × A × LMTD

• LMTD = (ΔT₁ − ΔT₂) / ln(ΔT₁/ΔT₂)

  ΔT₁ = Tchw_in − DB₂

  ΔT₂ = Tchw_out − DB₁

• U = overall heat transfer coefficient (W/m²·K, coil-specific)

• A = coil surface area (m²)

5. Example (SI Units)

Given:

• Airflow: 2.0 m³/s

• Inlet: 32°C DB, 26°C WB → h₁ = 80.5 kJ/kg

• Outlet: 14°C DB, 90% RH → h₂ = 37.8 kJ/kg

1. ṁ = 2.0 × 1.2 = 2.4 kg/s

2. Q_total = 2.4 × (80.5 − 37.8) = 102.5 kW

6. Key Notes

• Use a psychrometric chart or software to get accurate h₁, h₂.

• Add 10–15% safety factor for fouling, filter loading, and duct gains.

• Coil manufacturers use your calculated Q + inlet/outlet states to select rows, fins, and tube circuiting.