Coil capacity calculator (two measured points)
Find the actual cooling or heating capacity from temperature, humidity and airflow measured at the coil inlet and outlet. The temperature formula (ρ·Q·Cp·Δt) and the enthalpy method (ṁ·Δh) are shown side by side with the difference and its reason.
Basis
The enthalpy method uses the same definitions as the process card in the psychrometric calculator. Volume airflow is converted to dry-air mass flow with the specific volume at the inlet.
| Item | Equation | Notes · basis |
|---|---|---|
| Dry-air mass flow | ṁ = V / v₁ | V is the volume airflow measured at the inlet, v₁ the inlet specific volume (m³/kg dry air). ASHRAE Fundamentals 2017 ch. 1 Eq. 28 |
| Total heat (enthalpy method) | Q_t = ṁ · (h₁ − h₂) | h is moist-air enthalpy (Eq. 32). For cooling, the heat removed by the coil |
| Sensible heat | Q_s = ṁ · (1.006 + 1.86 W₁) · (t₁ − t₂) | Moist-air specific heat at the inlet humidity ratio, as in the calculator's process card |
| Latent heat and condensate | Q_l = Q_t − Q_s, m_w = ṁ · (W₁ − W₂) | Latent heat is total minus sensible. Condensate is the drop in humidity ratio |
| Temperature formula (field shortcut) | Q = ρ · V · Cp · Δt, ρ = 1.2 kg/m³, Cp = 1.006 kJ/(kg·K) | Approximate dry-air density and specific heat near 20 °C. It ignores the moisture change (latent heat), so on a dehumidifying coil it gives only the sensible part |
Worked example
Inlet 27 °C dry bulb, 19.5 °C wet bulb (RH 49.8 %, humidity ratio 11.10 g/kg, enthalpy 55.48 kJ/kg, specific volume 0.8655 m³/kg); outlet 14 °C dry bulb, 13 °C wet bulb (RH 89.6 %, 8.92 g/kg, 36.62 kJ/kg); airflow 10,000 m³/h at the inlet.
ṁ = 10,000 / 3,600 / 0.8655 = 3.210 kg/s. Total = 3.210 × (55.48 − 36.62) = 60.54 kW (17.2 RT). Sensible = 3.210 × (1.006 + 1.86 × 0.01110) × 13 = 42.84 kW, latent = 17.70 kW, condensate 25.2 kg/h.
The temperature formula gives 1.2 × 2.778 × 1.006 × 13 = 43.59 kW, 28.0 % below the enthalpy method. Most of the gap is latent heat (29 % of total); the rest is the ρ·Cp shortcut.
Limits and measurement error
Measurement error changes the result a lot. In humid air such as a coil outlet, a 0.5 K wet-bulb error is about 1.3 to 1.7 kJ/kg in enthalpy (outlet and inlet of this example), roughly 7 to 9 % of total capacity. Measure wet bulb with a wetted wick and enough air speed (about 3 m/s or more); RH sensors lose accuracy near saturation at the outlet.
Airflow error (pitot traverse, anemometer) carries straight into the result. At the outlet, average several points to avoid stratification right after the coil and carried-over droplets.
The calculation uses standard pressure (101.325 kPa). Fan heat and duct heat gain are not separated.
Sources
- ASHRAE Handbook, Fundamentals 2017 (SI), ch. 1 Psychrometrics: enthalpy (Eq. 32), specific volume (Eq. 28), wet bulb (Eq. 33, 35)
- Field shortcut ρ 1.2 kg/m³, Cp 1.006 kJ/(kg·K): standard air at 20 °C, sea level
Last updated 2026-10-07.