How to use this calculator

Enter the total pressure and the dry-bulb temperature, then pick the second property you know: relative humidity, wet-bulb temperature, dew point or humidity ratio. Add the dry-air mass (or mass flow) to get the water mass and total enthalpy.

Formulas

Humidity ratioω = 0.622 P_v/(P − P_v)

Relative humidityφ = P_v/P_g(T)

Enthalpyh = 1.005T + ω(2501 + 1.82T) kJ/kg dry air

Adiabatic saturationω₁ = [c_p(T₂ − T₁) + ω₂h_fg2]/(h_g1 − h_f2)

Worked example

Çengel Example 14-4: air at 1 atm, 25 °C dry bulb and 15 °C wet bulb

At the wet-bulb temperature, saturated air has ω₂ = 0.622(1.7057)/(101.325 − 1.7057) = 0.01065.
The adiabatic saturation balance gives ω₁ = [1.005(15 − 25) + 0.01065(2465.4)]/(2546.5 − 62.98) = 0.00653 kg water/kg dry air.
P_v = ωP/(0.622 + ω) = 1.05 kPa, and P_g(25 °C) = 3.17 kPa, so φ = 0.332.
h = 1.005(25) + 0.00653(2501 + 1.82 × 25) = 41.7 kJ/kg dry air.

Frequently asked questions

Why is the wet-bulb temperature lower than the dry-bulb?

Water evaporating from the wet wick takes its latent heat from the air flowing past, cooling it. The drier the air, the more evaporates and the bigger the difference. At 100% humidity they are equal.

What is the dew point?

The temperature at which the water vapor already in the air starts to condense when the air is cooled at constant pressure: the saturation temperature at the vapor partial pressure.

Is the enthalpy per kilogram of moist air?

No. Psychrometric enthalpy and volume are per kilogram of dry air, because the dry-air mass stays constant through heating, cooling and humidifying processes.