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Psychrometrics

The Psychrometric Chart provides a graphic representation of the state or condition of the air at any particular time. The chart relates temperature along the horizontal scale to moisture content along the vertical scale. At first glance they can look quite complex, like something only an engineer could love. However, they play an important role in bio-climatic design and comfort analysis so it is worthwhile becoming reasonably familiar.

Figure 1 - The Psychrometric Chart. Display the different elements of the graph by selecting them from the pull-down list.
Figure 1 - The Psychrometric Chart. Display the different elements of the graph by selecting them from the pull-down list.

Select Display: :: Animated Description :: Dry Bulb Temperature :: Relative Humidity :: Absolute Humidity :: Wet Bulb Temperature :: Constant Air Volume :: Vapour Pressure :: Enthalpy

Air consists mainly of the gasses Nitrogen (N2 - 78%), Oxygen (O2 - 21%), Argon (A - 0.9%) and Carbon Dioxide (CO2 - 0.03%). Under normal atmospheric conditions there is also a certain amount of moisture vapour (H2O) present. This comes from evaporation over the sea, rivers and lakes as well as from the leaves of plants and our own skin when we perspire.

Figure 2 - Gasses that make up the air.
Figure 2 - Gasses that make up the air.

The amount of moisture vapour in the air varies quite significantly under different conditions. When the air is hot it can contain a a large amount of moisture vapour, sometimes as much as 5% by volume. When it is cold, its capacity to hold the moisture as a vapour is reduced. When the temperature of warm air begins to fall, the vapour also cools and, if cooling continues, will eventually condense into tiny moisture droplets. In the atmosphere this results in the formation of clouds and eventually rain, whilst it is more commonly manifest as condensation running down the outside of a glass of iced water.

When condensation is beginning to occur, the air is said to be saturated and cannot hold any more moisture, the point of 100% relative humidity. For a given amount of humidity, the temperature along the X-axis at which this occurs is called the dew point. For a given temperature, the humidity along the Y-axis at which this occurs is called the saturation point. The dew point for each temperature in the Psychrometric Chart above is therefore represented by the inner curved boundary. This is because the air simply cannot exist at a state above and to the left of this line. If the air is cooled beyond its dew point, excess vapour is lost as condensation.

Properties of Air

The following are the important properties of the air described by the Psychrometric Chart:

Psychrometric Processes

At any specific time, the condition of the air can be represented as a single point somewhere within the chart. If conditions change, then the point will move around the graph. The direction the point moves depends upon what properties of the air are changing.

Heating and Cooling

Heating of the air occurs when energy is absorbed from a heat source such as a warm fire, a rock face heated by the sun or an electric heating element. Cooling occurs when the air comes into contact with a cold object, losing its heat energy by conduction and convection, or when it radiates its heat out into space through the night sky.

Figure 3 - The effects of heating and cooling the air.
Figure 3 - The effects of heating and cooling the air.

Neither heating nor cooling change the absolute humidity, the amount of moisture in the air. However, the relative humidity does change in both cases. This is because the saturation point changes with dry bulb temperature. As relative humidity equals the absolute humidity divided by the saturation point, if the temperature and the saturation points change but the absolute humidity remains the same, then the relative humidity must change.

Condensation

If cooling occurs beyond the dew point, the air becomes saturated (RH = 100%). As the air continues to cool, the moisture vapour within the air will lose energy as well, some of which will condense back into moisture droplets and release its latent heat of vapourisation. Condensation in the air results in the formation of clouds and rain in the atmosphere or dew drops on icy cold glasses of water. You will notice from the graph below that, once 100% relative humidity at the dew point is reached, the absolute humidity begins to fall as the air continues to cool.

Figure 4 - Cooling air below the Dew Point results in the condensation of moisture vapour into water droplets.
Figure 4 - Cooling air below the Dew Point results in the condensation of moisture vapour into water droplets.

This process is also known as dehumidification by cooling. If you cool the air down a lot and then heat it back up again, it will have lost a lot of its water vapour content as condensation. If the condensed water is removed or the re-heating takes place faster than the condensation can re-evaporate, the absolute humidity will have been reduced. Many air conditioning systems utilise this phenomena to control the humidity of the air entering the interior.

Adiabatic Processes

The term 'Adiabatic' simply means without energy loss or gain. This refers mainly to the processes of evaporative cooling and desicant dehumidification.

Figure 5 - Adiabatic humidification and de-humidification.
Figure 5 - Adiabatic humidification and de-humidification.

Evaporative cooling occurs when warm air passes near water. Even at temperatures well below its boiling point, water molecules on the surface will absorb sufficient energy from the passing air to change phase into a gas and become moisture vapour. As the moisture vapour then becomes part of the air, the energy is transferred from sensible heat into latent heat of vapourisation. Thus the temperature of the air falls, but the absolute humidity rises, meaning that the overall energy content (or enthalpy) remains the same.

Desiccant dehumidification occurs when the air comes into contact with a substance such as Condies Crystals or silica gel. Such substances simply absorb moisture directly from the air. In the process, the latent heat of vapourisation is released back into the air, raising its temperature and reducing the absolute humidity. Once again, the net overall enthalpy remains the same.

Related Topics

  1. Human Comfort
  2. Comfort Prediction
  3. Bio-Climatic Design
Passive Design: Trombe Wall
Enthalpy

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