Climatic Design of Buildings - An Overview

5. PSYCHROMETRIC CHART

5.1 Psychrometrics

• Absolute humidity (AH) is the vapour content of air, given in grammes or kg of water vapour per kg of air, i.e. g/kg or kg/kg. It is also known as moisture content or humidity ratio. Air at a given temperature can support only a certain amount of moisture and no more. This is referred to as the saturation humidity.
• Relative humidity (RH) is an expression of the moisture content of a given atmosphere as a percentage of the saturation humidity at the same temperature.
• Wet-bulb temperature (WBT) is measured by a hygrometer or a sling psychrometer and is shown as sloping lines on the psychrometric chart. A status point on the psychrometric chart can be indicated by a pair of dry-bulb temperature (DBT) and WBT.
• Specific volume (Spv) , in m³/kg, is the reciprocal of density and is indicated by a set of slightly sloping lines on the psychrometric chart.
• Enthalpy (H) is the heat content of unit mass of the atmosphere, in kJ/kg, relative to the heat content of 0 deg ?C dry air. It is indicated on the psychrometric chart by a third set of sloping lines, near to, but not quite the same as the web-bulb lines. In order to avoid confusion, there are no lines shown, but external scales are given on two sides.
• Sensible heat (Qsen) is the heat content causing an increase in dry-bulb temperature. Latent heat (Qlat) is the heat content due to the presence of water vapour in the atmosphere. It is the heat which was required to evaporate the given amount of moisture.

Psychrometric processes, i.e. any changes in the condition of the atmosphere, can be represented by the movement of the state point on the psychrometric chart. Common processes include:

• Sensible cooling / sensible heating
• Cooling and dehumidification / heating and humidification
• Humidification / dehumidification
• Evaporative cooling / chemical dehydration

Figure 7 Psychrometric processes

5.2 Analysis Using PSYCHWIN

5.3 Bioclimatic Analysis for Hong Kong