Pocket HVAC
What is enthalpy?
Enthalpy is the heat the air holds, counting both the sensible heat you read on a thermometer and the latent heat stored in its moisture. For air it's given per pound of dry air (Btu/lb) or per kilogram of dry air (kJ/kg).
It's how coil capacity is worked out. The drop in enthalpy from the air entering a coil to the air leaving it, times the mass of air going through, is the total heat the coil removed, sensible and latent together. For standard air in US units that's Btu/h = 4.5 × CFM × the enthalpy difference in Btu/lb.
How it's calculated
Saturation pressure: ASHRAE eqns 5 (over ice) and 6 (over water) Humidity ratio W = 0.621945 × pw ÷ (p − pw) Wet bulb: ASHRAE eqns 33 and 35, solved by bisection Dew point: eqns 5 and 6 solved for the temperature where pws = pw Enthalpy h = 0.240 t + W (1061 + 0.444 t) Btu/lb, or 1.006 t + W (2501 + 1.86 t) kJ/kg Pressure at elevation z: p = 14.696 (1 − 6.8754×10⁻⁶ z)^5.2559 psia
Sources
- ASHRAE Handbook—Fundamentals (2017), chapter 1, Psychrometrics: equations 3, 5, 6, 20, 26, 30, 33 and 35
- Meyer & Thevenard (2019), PsychroLib: a library of psychrometric functions to calculate thermodynamic properties of air
If a number doesn't match your chart, email me what you entered and what you expected, and I'll check it against the source. You can also read how the numbers are checked.
Checked by Paul Williams · Updated
Questions
What is the enthalpy of air at 75°F and 50% humidity?
The enthalpy of air at 75°F and 50% relative humidity is 28.1 Btu per pound of dry air at sea level. In metric, 24°C and 50% is 47.8 kJ/kg.
Why is enthalpy in Btu per pound of dry air?
The amount of dry air passing through a coil doesn't change, but its moisture does. Measuring per pound of dry air lets you subtract the leaving enthalpy from the entering enthalpy to get the total heat a coil removed, both the sensible and the latent part.
Why do Btu/lb and kJ/kg values not convert directly?
The two systems measure from different zero points: ASHRAE's I-P tables count enthalpy from dry air at 0°F, and SI tables from dry air at 0°C. Differences in enthalpy convert directly (1 Btu/lb = 2.326 kJ/kg); single values don't.
