What is the formula for calculating humid air enthalpy?

The general formula is H' = Cp' * (T - T0) + w * Λ0, where Cp' is the humid heat capacity (1005 + 1884*w J/kg_dry_air·K), T is temperature, w is absolute humidity (kg water/kg dry air), and Λ0 is the latent heat of vaporization of water at 0 °C (2.5023 x 10^6 J/kg).

Why is liquid water enthalpy at 0 °C taken as the reference state?

In standard chemical and psychrometric engineering, 0 °C (273.15 K) liquid water is chosen as the zero enthalpy reference point. Heating dry air from 0 °C requires Cp_dry * T, while adding water vapor requires vaporizing water at 0 °C (latent heat Λ0) plus superheating the vapor to temperature T.

How does relative humidity affect the specific heat capacity of air?

Water vapor has a higher specific heat capacity (1.884 kJ/kg·K) than dry air (1.005 kJ/kg·K). As humidity increases, the effective humid heat capacity Cp' increases according to Cp' = 1.005 + 1.884 * w kJ/(kg_dry_air·K).

Humid Air : Enthalpy

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1. Definition of enthalpy of moist air
2. Calculation of enthalpy of humid air
3. Interactive Online Humid Air Enthalpy Calculator
4. Humid Air enthalpy calculator Excel
5. Practical Engineering Rules of Thumb & Safety Limits

1. Definition of enthalpy of moist air

The enthalpy of humid air is, at a given pressure, the energy necessary to heat 1 kg of dry air from the reference temperature T₀ to the temperature T, to vaporize at T₀ the quantity of water in the humid air having an absolute humidity of ω, and to heat up the water vapor from T₀ to T.

2. Calculation of enthalpy of humid air

Moist air enthalpy formula general form :

H' = C_p' \times (T - T_0) + \omega \times \Lambda_0

With :

H' = enthalpy of humid air (J/kg_dry_air)
Cp' = specific heat of humid air (J/K/kg_dry_air)
T = temperature of calculation (K)
T₀ = temperature of reference (K)
ω = Absolute humidity of air (-)
Λ₀ = vaporization enthalpy at T₀ (J/kg)

In practice, humid air calculation formula at 273 K :

H' = (1005 + 1884 \times \omega) \cdot (T - 273.15) + 2.5023 \times 10^6 \times \omega

3. Interactive Online Humid Air Enthalpy Calculator

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided exclusively for preliminary estimation and educational purposes. It is not intended for detailed design or equipment procurement without certified vendor rating. No warranty, expressed or implied, is provided, and no liability is assumed.

Unit System:

SI Metric (°C, bar, kJ/kg) US Customary (°F, psi, BTU/lb)

Calculation Mode:

Air Temperature (°C):

Atmospheric / Process Pressure (bar):

Relative Humidity (%):

Absolute Humidity ω (kg/kg dry air):

Psychrometric Calculation Results

Specific Enthalpy (H'): 50.05 kJ/kg dry air

Humid Heat Capacity (Cp'): 1023.42 J/(kg·K)

Absolute Humidity (ω): 0.009775 kg water / kg dry air

Relative Humidity (ε / RH): 50.0 %

Saturation Vapor Pressure (P_sat): 3167 Pa (0.0317 bar)

Water Vapor Partial Pressure (P_v): 1584 Pa (0.0158 bar)

Humid Volume (v'): 0.857 m³/kg dry air

4. Humid Air enthalpy calculator Excel

MyEngineeringTools.com has developed a free Excel calculator that allows you to calculate the enthalpy of humid air, knowing the absolute humidity and the temperature, it allows also many other calculations related to humid air : Humid air enthalpy calculator Excel

Warning : this calculator is provided to illustrate the concepts mentioned in this webpage, it is not intended for detail design. It is not a commercial product, no guarantee is given on the results. Please consult a reputable designer for all detail design you may need.

💡 Plant Engineering Rules of Thumb & Psychrometric Safety Margins

Reference Zero Enthalpy State: Standard psychrometric air charts define zero enthalpy at 0 °C (273.15 K) for dry air and liquid water. Heating dry air uses \(C_{p,\text{air}} = 1.005 \text{ kJ/kg}\cdot\text{K}\), whereas water vapor requires latent heat of vaporization \(\Lambda_0 \approx 2502.3 \text{ kJ/kg}\) plus superheating with \(C_{p,\text{vapor}} = 1.884 \text{ kJ/kg}\cdot\text{K}\).
Condensation & Duct Caking Risk: In industrial drying and pneumatic conveying system exhausts, maintain duct wall temperatures at least 5 °C to 10 °C above the dew point to prevent moisture condensation, corrosion, and severe powder caking.
Humid Heat Approximation: For general HVAC and drying calculations below 100 °C, the linear relation \(C_p' = 1.005 + 1.884 \cdot \omega \text{ [kJ/kg}_{\text{dry\_air}}\cdot\text{K]}\) provides an error margin of less than 0.5%.
Atmospheric Pressure Adjustments: For high-altitude plant installations (e.g. > 500 m above sea level), always adjust total pressure \(P\). Lower operating pressure significantly increases the absolute saturation humidity \(\omega_{\text{sat}}\) at a given temperature.