Enthalpy of Moist Air Calculator

Calculator Inputs

Formula Used

1) Saturation vapor pressure
P_ws = 0.61094 × exp[(17.625 × T) / (T + 243.04)]

2) Actual vapor pressure from relative humidity
P_w = φ × P_ws

3) Humidity ratio
W = 0.62198 × P_w / (P − P_w)

4) Enthalpy of moist air
h = 1.006T + W(2501 + 1.86T)

5) Specific volume
v = 0.287042(T + 273.15)(1 + 1.607858W) / P

Here, T is dry bulb temperature in °C, P is total pressure in kPa, φ is relative humidity as a fraction, W is humidity ratio in kg/kg dry air, h is enthalpy in kJ/kg dry air, and v is specific volume in m³/kg dry air.

How to Use This Calculator

Choose whether you know relative humidity or humidity ratio.

Enter dry bulb temperature in degrees Celsius.

Enter total air pressure in kPa.

Fill either relative humidity or humidity ratio, based on your selected mode.

Set the graph span and graph step for the temperature sweep.

Press Calculate Enthalpy to display the result above the form.

Review the detailed psychrometric outputs and the Plotly chart.

Use the CSV or PDF buttons to export the calculated report.

Example Data Table

Dry Bulb (°C)	Relative Humidity (%)	Pressure (kPa)	Humidity Ratio (g/kg)	Enthalpy (kJ/kg dry air)
18	40	101.325	5.110	30.970
25	50	101.325	9.880	50.320
30	60	101.325	16.030	71.010

These sample values are rounded examples for quick reference and comparison.

FAQs

1) What does moist air enthalpy represent?

Moist air enthalpy is the total heat content of an air-water vapor mixture. It combines sensible heat from air temperature and latent heat from the water vapor carried by the air.

2) Why is humidity ratio useful?

Humidity ratio directly measures the mass of water vapor per unit mass of dry air. It is a stable psychrometric property and is widely used in HVAC, drying, and atmospheric calculations.

3) Can I use relative humidity or humidity ratio?

Yes. This calculator supports both methods. Use relative humidity when that value is known from measurement, or use humidity ratio when you already have psychrometric or process data.

4) Why does pressure affect the result?

Pressure changes the relationship between vapor pressure and humidity ratio. At different elevations or controlled systems, the same relative humidity can produce a different humidity ratio and enthalpy.

5) What units does the calculator use?

It uses °C for temperature, kPa for pressure, g/kg dry air for displayed humidity ratio, and kJ/kg dry air for enthalpy. Internally, humidity ratio is converted to kg/kg dry air.

6) Is the formula suitable for engineering estimates?

Yes. The enthalpy equation used here is a standard engineering approximation for moist air. It is suitable for many educational, HVAC, and process calculations within common temperature ranges.

7) What does the graph show?

The Plotly graph shows how moist air enthalpy changes as dry bulb temperature changes across your selected range. It keeps the same humidity basis from your chosen input mode.

8) When should I export CSV or PDF?

Use CSV when you want spreadsheet-friendly output for further analysis. Use PDF when you need a compact report for printing, sharing, or attaching to project documentation.