Mollier Diagram Calculator (Humid Air h-x)
Enter dry-bulb temperature, relative humidity, and pressure to estimate the state point used on a Mollier diagram (enthalpy vs humidity ratio).
What Is a Mollier Diagram?
A Mollier diagram is a thermodynamic chart that lets engineers visualize air or steam properties at a glance. In HVAC work, the most common form is the h-x chart (specific enthalpy vs humidity ratio) for moist air. Instead of solving many equations manually every time, you can locate a state point and read properties such as enthalpy, moisture content, and saturation behavior directly from the chart.
This calculator gives you the same practical benefit in digital form. You provide basic measured values, and it computes the key psychrometric outputs needed to place and interpret your point on the diagram.
How to Use This Mollier Diagram Calculator
Step-by-step
- Enter the dry-bulb temperature in °C.
- Enter relative humidity in %.
- Confirm or adjust local atmospheric pressure in kPa (use a lower value for high-altitude locations).
- Click Calculate to generate thermodynamic properties and the plotted point.
The chart point is displayed using humidity ratio (g/kg dry air) on the horizontal axis and enthalpy (kJ/kg dry air) on the vertical axis, consistent with a Mollier h-x representation.
Calculated Outputs Explained
1) Humidity Ratio (w)
Humidity ratio is the mass of water vapor per unit mass of dry air. It is usually expressed as kg/kg dry air, but many charts use g/kg for readability.
2) Specific Enthalpy (h)
Enthalpy combines sensible and latent heat content in moist air. It is one of the most useful values for cooling load, heating load, and coil process analysis.
3) Dew Point Temperature
The dew point is the temperature where condensation begins if air is cooled at constant pressure and moisture content.
4) Wet-Bulb Temperature (Approx.)
Wet-bulb temperature is estimated using a common engineering approximation. It helps when evaluating evaporative cooling and adiabatic processes.
5) Vapor Pressure and Specific Volume
Vapor pressure indicates the partial pressure of water vapor in air, while specific volume gives volume per kilogram of dry air—useful for fan and duct sizing checks.
Equations Used in This Tool
The calculator uses standard psychrometric relationships:
- Saturation pressure (kPa) from a Magnus/Tetens-type approximation.
- Vapor partial pressure from RH and saturation pressure.
- Humidity ratio: w = 0.62198 × Pw / (P − Pw).
- Enthalpy: h = 1.006T + w(2501 + 1.86T).
- Specific volume based on ideal-gas humid-air approximation.
- Dew point from inverse vapor pressure relation.
These formulas are widely used for design-level calculations and quick checks in building systems, process ventilation, and energy analysis.
Practical Engineering Use Cases
- HVAC coil calculations: compare entering and leaving air enthalpy to estimate cooling or heating energy.
- Indoor comfort control: verify humidity ratio and dew point against target comfort range.
- Dehumidification studies: quantify moisture removal potential and latent loads.
- Ventilation analysis: compare outdoor vs indoor air states across seasons.
- Educational thermodynamics: quickly validate classroom problems against chart behavior.
Example
If you enter 25 °C, 50% RH, and 101.325 kPa, you should see a humidity ratio around the comfort-zone range and an enthalpy near typical occupied-building conditions. By changing only RH while holding temperature fixed, you can immediately see how latent load increases on the Mollier chart.
Limitations and Good Practice
- This is an engineering approximation tool, not a substitute for high-precision property libraries.
- Extreme temperatures and pressures can increase model error.
- For compliance, research, or critical design, validate with ASHRAE-grade psychrometric software or standards tables.
FAQ
Is this for steam Mollier charts too?
No. This page is focused on the humid-air Mollier concept (h-x psychrometric use). Steam Mollier diagrams (h-s) require different fluid property models.
Why does pressure matter?
At higher altitude, atmospheric pressure is lower, which changes humidity ratio and related properties for the same temperature and RH.
Can I use this for SI-only workflows?
Yes. All inputs and outputs are presented in SI-compatible units (°C, kPa, kJ/kg, m³/kg, kg/kg).