Moist Air Specific Heat Calculator

Formula Used

1) Saturation pressure from dry-bulb temperature

P_ws = 0.61094 × exp[(17.625 × T) / (T + 243.04)]

2) Vapor partial pressure

P_v = RH × P_ws / 100

3) Humidity ratio

w = 0.62198 × P_v / (P − P_v)

4) Moist air specific heat on dry-air basis

c_p,da-basis = c_p,da + w × c_p,v

5) Moist air specific heat on moist-air basis

c_{p,moist-basis} = (c_p,da + w × c_p,v) / (1 + w)

6) Enthalpy reference relation

h = c_p,daT + w(2501 + c_p,vT)

This implementation uses common HVAC and psychrometric engineering approximations. It is excellent for design studies, comparisons, and educational work. For extreme pressure or non-ideal mixtures, a higher-order model may be needed.

How to Use This Calculator

Select whether you want to calculate from relative humidity or humidity ratio.
Enter dry-bulb temperature and choose its unit.
Enter total pressure and choose its pressure unit.
Enter RH if using humidity percent, or enter humidity ratio directly.
Keep the specific heat constants at default values unless your standard requires different constants.
Choose graph minimum and maximum temperatures for the trend plot.
Press Calculate Specific Heat to show results below the header and above the form.
Use the CSV and PDF buttons to export the current result summary.

Dry-bulb Temp (°C)	Pressure (kPa)	RH (%)	Humidity Ratio (kg/kg dry air)	Specific Heat Dry-air Basis (kJ/kg dry air·K)	Specific Heat Moist-air Basis (kJ/kg moist air·K)	Enthalpy (kJ/kg dry air)
20	101.325	40	0.0058	1.0168	1.0109	34.7978
25	101.325	50	0.0099	1.0243	1.0143	50.2630
30	101.325	60	0.0160	1.0358	1.0195	71.1027
35	101.325	70	0.0251	1.0527	1.0269	99.6522

FAQs

1) What does moist air specific heat mean?

It is the heat needed to raise the temperature of humid air by one degree. Because water vapor stores energy, moist air usually has a higher specific heat than dry air.

2) Why are there two specific heat results?

One result is expressed per kilogram of dry air, which is common in psychrometrics. The second is expressed per kilogram of the total moist mixture, which is useful for whole-mixture energy discussions.

3) How does relative humidity affect specific heat?

Higher relative humidity generally raises the humidity ratio at the same temperature and pressure. That adds water vapor to the mixture, increasing the moist air specific heat.

4) Why does pressure matter in this calculator?

Pressure changes the relationship between vapor partial pressure and humidity ratio. At lower total pressure, the same moisture condition can produce a different humidity ratio and slightly different specific heat.

5) Can I use humidity ratio directly?

Yes. Direct humidity-ratio mode is useful when your psychrometric chart, process data, or simulation output already reports moisture content as kilograms of water per kilogram of dry air.

6) What constants are used here?

The calculator defaults to 1.006 kJ/kg·K for dry air and 1.860 kJ/kg·K for water vapor. These are standard engineering approximations for ordinary HVAC and atmospheric calculations.

7) Is the Magnus saturation-pressure relation accurate enough?

For common engineering temperatures, it is usually accurate enough for educational use, HVAC sizing, and quick design checks. Extremely precise work may require a more rigorous property model.

8) When should I avoid this simplified calculator?

Avoid it for very high pressures, unusual gas mixtures, near-condensation edge cases requiring rigorous accuracy, or applications where your governing code specifies another psychrometric property formulation.