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Cooling coil power calculation
Calculation of enthalpy and dew point of moist air
Cooling coil sizing, enthalpy and dew point of moist air calculator
Professional psychrometric calculator for ventilation and air-conditioning designers. It combines two basic analyses in one tool: sizing the cooling coil power of an air handling unit (based on the enthalpy difference between outdoor air and air leaving the coil) and computing the full set of psychrometric parameters of any moist-air state — enthalpy h, dew point Td and absolute humidity w. It also lets you compare two air states side by side, which is useful when analysing air-handling processes (cooling, dehumidification, mixing, heating). The tool supports the calculation of summer cooling loads of supply-and-extract units for the Polish climate.
How to use the calculator
Cooling coil sizing — enter the outdoor air temperature, its relative humidity and the ventilation airflow. Then enter the desired temperature behind the coil and the coil wall (heat-exchange surface) temperature. The calculator returns the coil power in kW together with the parameters of the air leaving the coil.
Moist-air analysis — enter the temperature and relative humidity of any air state, and the calculator returns enthalpy h [kJ/kg], dew point Td [°C] and absolute humidity (moisture content) w [g per kg of dry air and g/m³].
Compare two states — the "Compare two air states?" toggle shows two independent enthalpy calculators side by side. This lets you compare e.g. outdoor air with supply air, the state before and after a coil, or the state before and after a heater.
Base equations — psychrometrics and enthalpy balance
Cooling coil power
The cooling coil power is computed as the product of the inlet/outlet enthalpy difference and the air mass flow rate:
Q = ṁ · Δh = ṁ · (h₁ − h₂)
where: Q — coil power [kW], ṁ — air mass flow rate [kg/s] (converted from the volumetric flow V using density ρ ≈ 1.2 kg/m³), h₁ — enthalpy of air entering the coil [kJ/kg], h₂ — enthalpy of air leaving the coil [kJ/kg]. The enthalpy difference Δh covers both the cooling of the air (sensible heat) and the condensation of water vapour (latent heat) when the coil wall temperature is below the dew point of the entering air.
Enthalpy of moist air
h = cp · T + w · (2501 + 1.86 · T)
where: h — enthalpy [kJ/kg of dry air], cp — specific heat of dry air ≈ 1.005 kJ/(kg·K), T — air temperature [°C], w — absolute humidity [kg of vapour/kg of dry air], 2501 kJ/kg — latent heat of vaporisation of water at 0 °C, 1.86 kJ/(kg·K) — specific heat of water vapour.
Dew point — Magnus formula
Td = (b · α) / (a − α), α = (a · T) / (b + T) + ln(RH)
where: Td — dew point [°C], T — current air temperature [°C], RH — relative humidity as a fraction (e.g. 0.5 for 50%), a = 17.27, b = 237.7. The Magnus formula provides sufficient accuracy (±0.4 °C) for typical meteorological and HVAC ranges from −40 °C to +60 °C.
Typical design values for ventilation cooling coils in Polish climate
The values below are starting points for quick calculations and reflect Polish design practice for comfort air conditioning:
- Summer design conditions for Poland (zone III, most of the country): outdoor temperature 30–35 °C (typically 32 °C), relative humidity 45–55%, outdoor air enthalpy around 65–75 kJ/kg.
- Comfort summer supply temperature: 16–18 °C for ceiling-level diffusers (Tsup − Troom around −6 to −10 K). The supply dew point is usually 10–14 °C, ensuring air dehumidification on the way through the coil.
- Chilled-water coils (fed by a water chiller) — wall temperature depends on the system operating parameters. Typical chilled water at 7/12 °C gives a mean wall temperature around 9–10 °C.
- Direct-expansion (DX, refrigerant) coils — wall temperature is close to the refrigerant evaporation temperature, typically 4–8 °C. They allow deeper air dehumidification.
- Sensible vs latent heat — when the coil wall temperature is above the dew point of the entering air, cooling is purely sensible (Δw = 0, no condensation). When the wall is below the dew point, part of the coil power is consumed by water vapour condensation — the latent share can reach 30–50% of the total power under summer conditions.
- Cooling coil bypass factor (BF) — the fraction of air that passes the coil without contacting its surface. Typical values: 0.1–0.2 for 4–6-row coils, 0.2–0.3 for 2–3-row coils. It affects dehumidification efficiency and the required chilled-water flow.
- Cooling loads of typical rooms (at 32 °C outside and 24 °C inside): office 60–100 W/m², conference room 100–200 W/m², server room 500–2000 W/m². These are rough figures — a detailed design requires a full building heat balance.
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