Enthalpy Moist Air⚠ unverified
Physics / Thermodynamics · Compute the specific enthalpy of moist air
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| T | T | — | 1.0 | Dry-bulb temperature, in degrees Celsius |
| w | w | — | 1.0 | Humidity ratio, in kilograms of water per kilogram of dry air |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| result | h | — | Specific enthalpy of the moist air, in kilojoules per kilogram of dry air (kJ/kg dry air) |
The science & history
Understanding the Parameters
-
$T$ — dry-bulb in Celsius for these coefficients. Using kelvin without shifting the fit is wrong. Live calculator labels $T$ as dimensionless — treat as degC.
-
$w$ — humidity ratio (not relative humidity). $w = 0$ is dry air: $h \approx 1.006\,T$.
- $h$ — enthalpy per kg dry air (standard ASHRAE basis), including sensible + latent terms.
Derivation (Approaching a Proof)
Model moist air as dry air + superheated vapour. Approximate:
$$h = c_{p,a} T + w\bigl(h_{fg} + c_{p,v} T\bigr)$$
with $c_{p,a}\approx 1.006\,\text{kJ/(kg}\cdot\text{K)}$, $h_{fg}\approx 2501\,\text{kJ/kg}$ at 0 degC, and $c_{p,v}\approx 1.86\,\text{kJ/(kg}\cdot\text{K)}$. This is an engineering correlation, not a first-principles EOS.
History
Psychrometric enthalpy formulas underpin HVAC load calculations and the ASHRAE psych chart.
Related Concepts: Psychrometric Wet Bulb, First Law DeltaU, Thermal Efficiency
Notes: Registry calculator enthalpy-moist-air (unverified). Unit bugs: $T$, $w$, and result
labelled dimensionless; physically degC, kg/kg, and kJ/kg_da. Empirical SI coefficients.