Mollier diagram

A Mollier diagram is the name given to a chart that describes the properties of a gas and has enthalpy as one of its scales. Mollier diagrams exist for steam, refrigerants and air for example. Here the focus is on air. Technically, the Mollier diagram for humid air is a psychrometric chart, which is the term generally used for it in English speaking countries.

The diagram is used to illustrate heating, cooling and humidification processes. We can see the effect of the process and we can see how it relates to requirements for the comfort conditions in the room, for example.

History

Richard Mollier was a German professor of Applied Physics and Mechanics and a pioneer of research in thermodynamics, particularly for water, steam and moist air. He created a diagram in 1904, when he plotted the total heat H against entropy S. At the 1923 Thermodynamics Conference it was decided to name any thermodynamic diagram using enthalpy as one of its axes "Mollier diagram", in his honor.

Mollier diagrams are used by engineers in the design work associated with power plants, compressors, steam turbines, refrigeration systems and air conditioning equipment to visualize the working cycles of thermodynamic systems.

Source: Wikipedia

Properties of air

It is important to understand the properties of air and what they are used for.

Dry air
Dry air is made up of a mixture of gases. 78% is Nitrogen, 21% is Oxygen and the remaining 1% is made up of gases such as carbon dioxide, argon, neon, helium etc. Dry air is mixed with an amount of water vapour up to about 4%.

Humid air
The main physical properties of humid air that needs to be consider are:

Temperature
Pressure
Humidity
Density
Enthalpy

The Mollier diagram describes the enthalpy, humidity and temperature at a fixed density and pressure. Read more about the properties of humid air below.

Composition of dry air

Properties of humid air

Temperature

Pressure

Humidity

Humidity is the amount of water vapour in the air. Air can hold up to about 4% water at the normal range of outdoor air temperature. The maximum amount of water that the air can carry depends on the temperature and increases rapidly with temperature. Air at minus 20 degrees centigrade can hold less than 1 gram of water per kilogram of air while at plus 20 degrees the air can hold 15 grams.

Air holding the maximum possible amount of water is called saturated. Saturation depends on temperature and the water content but is also affected by pressure.

Saturation line (x-axis shows moisture content, y-axis shows temperature)

There are two important measures of humidity: Relative humidity and specific humidity.

Relative humidity tells us how much water there is in the air compared with the amount of water at saturation at the same temperature. It is expressed as a ratio in percent, e.g. 50% RH.
Specific humidity tells us how much water there is in the air as a ratio of the mass of water to the mass of the air-water mixture in kg/kg or g/kg.

Two other measures of humidity:

Wet bulb temperature is measured using a thermometer that has a bulb soaked in water. The water evaporates and cools the thermometer. The lower the humidity, the greater the evaporation and the reduction in temperature. The difference between wet bulb and dry bulb temperature is related to humidity.
Dew Point temperature is the dry bulb temperature at which air becomes saturated when cooled.

Density

Enthalpy

Enthalpy is a measure of the amount of energy in the air. The enthalpy of moist air is the sum of the enthalpy of the dry air and the enthalpy of the water moisture. Enthalpy increases with increasing temperature and with increasing moisture content. Enthalpy is given in kJ/kg.

When air is heated or cooled, the enthalpy changes and the change in enthalpy multiplied by the mass flow of air gives us the power, making it important in air conditioning processes.

The Mollier diagram layout

Relative humidity and enthalpy are plotted in a diagram against temperature and specific humidity to form the Mollier diagram. The curved lines in the diagram are relative humidity while the diagonal lines are enthalpy.

The left hand diagram is the Mollier diagram used in many countries. The right hand diagram is normally called a Psychrometric chart and is used in most English speaking countries. The diagrams are really the same – they give the same results but the temperature and specific humidity scales are reversed.

Mollier diagrams for typical HVAC system processes

The diagram is used to illustrate heating, cooling and humidification processes. The effect of the process is visualised as well as how it relates to requirements for the comfort conditions in the room.

The equations used to plot the Mollier chart are used in selection programs to calculate the results such as the change in temperature and humidity, as well as power consumption etc.

Learn more and see diagrams of typical processes used in HVAC systems

The Swegon Mollier diagram

The Swegon Mollier tool is a web based program.

Learn more about the tool and how it works

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Additional information

Relative humidity is an important aspect of indoor climate.

The video to the left explains how humidity affects both buildings and human health. To learn more about the importance of correct humidity levels for an optimal indoor climate, please explore below resources: