Menu
Process Engineer's Tools Logo

Psychrometric Chart (Mollier Diagram)

Follow us on Twitter  Twitter
Question, remark ? Contact us at contact@myengineeringtools.com

Section Summary
1. What is a Psychrometric Chart?
2. Interactive Online Calculator
3. Psychrometric Chart Picture
4. The Chart Explained
5. Governing Equations

1. Introduction

What is a psychrometric chart or Mollier Diagram ?

The psychrometric chart is used for all operations handling humid air. It relates dry bulb temperature, absolute humidity, relative humidity, and enthalpy. It is essential for HVAC design, drying operations, and cooling tower sizing.

Note: The Mollier diagram is functionally identical to the psychrometric chart but is mirrored and rotated by 90 degrees. Both provide the same thermodynamic state of the air-water mixture.

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided for preliminary estimation and educational purposes. Always verify results with certified psychrometric software or tables for critical design.

Air Property Calculator

kPa
°C
%
Humidity Ratio (\(\omega\)): 0.0098 kg/kg
Specific Enthalpy (\(h\)): 50.22 kJ/kg
Wet Bulb Temp (\(T_{wb}\)): 17.9 °C
Dew Point Temp (\(T_{dp}\)): 13.8 °C
Specific Volume (\(v\)): 0.858 m³/kg
Relative Humidity (\(RH\)): 50.0 %

2. Psychrometric Chart

The diagram below is based on SI units at sea level (101.325 kPa).

Psychrometric chart

3. The Psychrometric Chart Explained

Each point on the graph defines a unique thermodynamic state of air:

Dry Bulb Temperature: The actual air temperature measured by a standard thermometer. Dry Bulb Lines
Wet Bulb Temperature: The lowest temperature air can reach via evaporative cooling. Wet Bulb Lines
Dew Point: The temperature at which water vapor begins to condense into liquid. Dew Point Lines
Enthalpy: The total energy content of the air (Sensible + Latent). Enthalpy Lines
Relative Humidity: The ratio of current water vapor pressure to saturation pressure. RH Curves

Industrial Rules of Thumb

  • Standard Comfort: Target \(T_{db} \approx 22-24^\circ\text{C}\) and \(RH \approx 40-60\%\).
  • Cooling Coil Design: Typical air leaves a cooling coil at \(90-95\% RH\).
  • Pressure Sensitivity: Properties change significantly with altitude (lower pressure increases humidity ratio for the same RH).
  • Process Drying: Raising air temperature lowers RH dramatically, increasing the "driving force" for moisture removal.

4. Governing Equations

The following equations are used to calculate the properties of humid air:

\[ P_{ws} = \exp\left( \frac{C_1}{T} + C_2 + C_3 T + C_4 T^2 + C_5 T^3 + C_6 \ln(T) \right) \]

Where \(P_{ws}\) is the saturation pressure and \(T\) is absolute temperature (K).

\[ \omega = 0.62198 \cdot \frac{P_w}{P - P_w} \] \[ h = 1.006 \cdot t + \omega(2501 + 1.86 \cdot t) \]

Note: The Specific Heat Capacity of dry air is assumed to be \(1.006 \text{ kJ/kg}\cdot^\circ\text{C}\).