EKS-HUM-011 / Moist-air state and psychrometric properties

Moist-air Enthalpy

湿空气焓

Moist-air enthalpy is the combined specific enthalpy of the dry-air and water-vapour components, normally reported per unit mass of dry air in psychrometrics.

Aliases: moist air specific enthalpy

QUANTITY, SYMBOL AND UNIT

Quantity / objectSpecific enthalpy of moist air

Symbolh

SIkJ/kg_dakilojoule per kilogram of dry air

IPBtu/lb_daBritish thermal unit per pound of dry air

Definition

Moist-air enthalpy is the combined specific enthalpy of the dry-air and water-vapour components, normally reported per unit mass of dry air in psychrometrics.

Its numerical value includes sensible contributions and the energy assigned to the water-vapour component under a stated reference convention. Enthalpy differences are useful in energy balances; the absolute number is meaningful only with its basis and reference state.

Formula and variables

h = h_da + W h_g

Conditions: Moist-air enthalpy is reported on a dry-air mass basis. Numerical values require a stated reference state and property model.

Symbol	Meaning	Unit
`h`	specific enthalpy of moist air	kJ/kg_da
`h_da`	specific enthalpy of dry air	kJ/kg_da
`W`	humidity ratio	kg_w/kg_da
`h_g`	specific enthalpy of water vapour at mixture temperature	kJ/kg_w

Relations and distinctions

Enthalpy and temperature

Temperature is one state coordinate. Enthalpy also changes with humidity ratio and the chosen reference convention.

Enthalpy and latent load

Moist-air enthalpy is a state property per mass of dry air; latent load is an energy rate associated with moisture transfer.

Engineering meaning

Use inlet and outlet enthalpy differences with dry-air mass flow in steady air-stream energy balances.
Keep the same reference state, unit system and dry-air basis when comparing software or chart results.

Common misconceptions

MisconceptionA higher moist-air enthalpy always means a higher dry-bulb temperature.

Accurate explanationEnthalpy can also increase because humidity ratio increases, so different temperature-moisture combinations can have similar h values.

Application boundaries

Do not mix kJ/kg dry air with values reported per kg of moist air.
Enthalpy alone does not separate sensible and latent requirements or size a dehumidifier.

Supporting sources

ASHRAE / SRC-ASHRAE-F25-CH01

ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗

2025 SI edition / Chapter 1, Equations 29–32; lines 634–645 and notation lines 1375–1379
Supports: Moist-air enthalpy is the sum of dry-air and water-vapour partial enthalpies; The standard psychrometric basis is per unit mass of dry air; Numerical enthalpy depends on the reference-state convention

Maintenance record