Humid air psychrometric
Relative humidity, wet bulb, dew point, humidity ratio, enthalpy and density of moist air — from the dry bulb and any one other property, at any altitude.
Inputs
Results
Need the full version for your plant?
The Cooling Tower Thermal Rating and Upgrade Excel tool does everything here and more: predict cold water for any weather, flow, load or fan condition — and compare fill, fan and extra-cell upgrades with payback. Print-ready report, any units, visible formulas, no macros.
Join PlantCalcKit — free
Be first to hear about new calculators, Excel tools and field guides — one short email when something new is ready. The calculators stay free with or without an account.
By joining you agree to our privacy policy. Unsubscribe or delete your details any time.
You are a PlantCalcKit member
Signed in as . We will email you when new tools and guides are ready.
How this calculator works
Moist-air properties by the equations of the ASHRAE Handbook — Fundamentals (2017), chapter 1: saturation pressure over liquid water and ice by Hyland and Wexler, humidity ratio W = 0.621945·pw/(p − pw), the wet-bulb relation (eqs 33 and 35, solved by iteration), enthalpy h = 1.006·t + W·(2501 + 1.86·t) and specific volume v = 0.287042·(t + 273.15)·(1 + 1.607858·W)/p. Give the dry bulb and any one of relative humidity, wet bulb, dew point or humidity ratio. The atmospheric pressure comes from the site altitude (ASHRAE eq. 3) or the barometer.
Example: air at 30 °C and 50 % relative humidity at sea level has a humidity ratio of 13.31 g/kg dry air, a wet bulb of 22.0 °C, a dew point of 18.4 °C, an enthalpy of 64.2 kJ/kg dry air and a specific volume of 0.877 m³/kg. The values were checked against PsychroLib, the reference implementation of the ASHRAE equations, at 50 states from −10 to 50 °C, at sea level and at 80 kPa: humidity ratio, enthalpy and volume agree to 0.01 %, wet bulb and dew point to 0.01 °C.
Uses: cooling tower design wet bulb, air-cooler and compressor inlet conditions, instrument-air and dryer dew points, HVAC and drying loads. In US units the enthalpy uses the ASHRAE 0 °F datum, so it is not a direct conversion of the SI value. References: ASHRAE Handbook — Fundamentals (2017) ch. 1; Hyland and Wexler, ASHRAE Transactions 89 (1983); Meyer and Thevenard, PsychroLib (2019).