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Psychrometric chart (Mollier diagram)

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Section summary
1. Introduction
2. Diagram

1. Introduction

The psychrometric chart is used for all operations handling humid air. It allows to quickly relates the specific heat of air, the absolute humidity, relative humidity and temperature. It is used to calculate heat loads for air handling systems for example.

Note that the Mollier diagram is actually identical to the psychrometric chart but just mirrored and turned 90 degrees.

2. Diagram

The diagram is shared under creative common licence from Wikipedia, author ArthurOgawa : link. The file is svg, please download it for high definition.

Psychrometric chart
How to read / use a psychrometric diagram ?

Each point of the graph allows to determine :
- Dry bulb temperature
- Wet bulb temperature
- Dew point
- Enthalpy
- Relative humidity
- Absolute humidity
- Specific volume

Dry Bulb Temperature
Wet Bulb Temperature
Dew Point Temperature
Enthalpy
Relative humidity - red curves on the graph
Absolute humidity
Specific volume

The graph can also be used to determine the heat exchange required to go from one state to another. The heat exchange corresponds to the difference in between the enthalpy of points A and B. The example below shows the cooling of air from conditions A to B, reading the enthalpies for the 2 points allow to calculate a cooling requirement of 60-40 = 20 J/kg of dry air.