Psychrometrics

Psychrometrics is the thermodynamics of moist air — the branch of physics and engineering concerned with calculating and visualizing the state variables of a mixture of dry air and water vapor.

Key psychrometric variables

Variable	Symbol	Unit
Dry-bulb temperature	$t$	°C
Wet-bulb temperature	$t_{wb}$	°C
Dew-point temperature	$t_d$	°C
Relative humidity	φ	%
Humidity ratio	$x$ ($W$)	g/kg dry air
Enthalpy	$h$	kJ/kg dry air
Density of moist air	ρ	kg/m³
Barometric pressure	$p$	Pa

Physical basis

Moist air is modeled as an ideal mixture of ideal gases (Dalton’s law of partial pressures):

$$p = p_a + p_w$$

where $p_a$ is the partial pressure of dry air and $p_w$ that of water vapor.

The saturation vapor pressure $p_{ws}$($t$) is computed from the Hyland–Wexler equations (ASHRAE) — semi-empirical equations giving a very accurate dependence on temperature over the range −100 °C to +200 °C.

The Mollier diagram as a tool of psychrometrics

Historically the h–x diagram (named after Richard Mollier, 1923) was a paper tool for solving psychrometric problems graphically. In the Mollier convention the humidity-ratio axis is horizontal and the enthalpy axis oblique (in the ASHRAE/Carrier psychrometric chart the two are transposed). Today it is superseded by computer programs — such as PsychroView — that combine accurate numerical calculation with an interactive diagram.

Reference standards

• ASHRAE Handbook — Fundamentals (2021), Chapter 1 — Psychrometrics (SI and I-P) — the main international reference
• EN 16798-1 — Energy performance of buildings; indoor environmental input parameters
• ISO 7730 — Ergonomics of the thermal environment (PMV/PPD index)
• EN ISO 52016-1 — Energy performance of buildings — calculation of energy needs for heating and cooling

Use in engineering practice

Psychrometrics underpins:

• design of air-handling units (air flows, coil and humidifier duties)
• calculation of condensation and dew point in building fabric
• assessment of thermal comfort (PMV, PPD)
• building energy-balance calculations
• sizing of industrial dryers, chambers and process equipment

Covered in depth, with formulas and worked examples, in the article What is psychrometrics?.