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Table of Contents

Background Preparation
Units & Dimensions
Psychrometric Chart

Psychrometric Process

Plotting On The Chart
Psychrometric Analysis

residential heating design


Introduction to the Psychrometric Chart

Before commencing with the design phase, it is important to look at the climate you are going to be designing for and assess the thermal comfort conditions throughout the year. The psychrometric chart is a good analysis tool to use for this purpose. As we have seen in the first part of this tutorial, atmospheric factors, namely the air temperature and the moisture in the air pretty much define thermal comfort in the presence of air movement and radiation. The psychrometric chart represents the state of a given atmosphere by a point which gives the dry-bulb, wet-bulb, relative humidity, specific volume and saturation temperature of the atmosphere.

The figure below represents a typical psychrometric chart. The psychrometric chart differs only by altitude or barometric pressure.

Psychrometric Terms
The chart might seem a little overwhelming in the number of factors it represents and their relationship to one another but we will look at its components individually in the following order:

  1. Saturation humidity line:
    Absolute humidity (AH) is the vapor content of air, given in grams of water vapour per kg of air, i.e. g/kg. Air at a given temperature and pressure can support only a certain amount of moisture and no more. This is referred to as the saturation humidity (SH). If this is plotted on a graph against the dry bulb (air) temperature (DBT), we have the basis of the psychromeytric chart and we get what is called the saturation humidity line.

  2. Relative humidity line:
    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:

    RH = 100 x AH/SH (%)

    Relative humidity lines are plotted on the psychrometric chart by halving each SH ordinate to obtain the 50% curve, and further proportionate subdivision gives any intermediated RH curve.


  3. Wet bulb temperature lines:
    Wet bulb temperature (WBT) is measured by a hygrometer (or psychrometer). This consists of two thermometers, one measuring the DBT, the other having its bulb enclosed in a wet wick. 'Web bulb depression' is noted as the difference in the temperatures between the wet wick thermometer and the DBT. The difference happens as the wet wick thermometer is cooled down by the evaporation on the wick. The amount of evaporation is a direct indication of the moisture carrying capacity of the atmospheric air at that temperature and that lowers the WBT.
    When the air is saturated, there is no evaporation, thus the DBT and WBT readings are identical, the depression is zero. In this way, the 'status point' is determined at the intersection of the vertical DBT line and the sloping WBT line of the psychrometric chart.


  4. Specific volume lines:*
    Specific volume (Spv) (reciprocal of density, in m
    3/kg) of the air-vapour mixture is indicated by another set of slightly more sloping lines on the psychrometric chart. This will be useful for the conversion of volumetric air flow quantities into mass-flow rates, eg. in air conditioning calculations.

    NOTE : The above is given for information sake. The use of specific volume may not be necessary for comfort analysis.


  5. Enthalpy lines:*
    Enthalpy (E) is the heat content of unit mass of the atmosphere, in kJ/kg, relative to the heat content of 0°C dry air. It was omitted from the psychromteric chart shown above to avoid confusion which a third set of lines will cause. The enthalpy lines would almost, but not quite, coincide with the WBT lines. To avoid the confusion in representing it, the enthalpy scale is given at the perimeter of the chart and can be read by using a straight edge.
    For air condition P the enthalpy is read at point A. The sensible heat component can be read at point B, corresponding to the enthalpy of dry air at the same temperature. The remainder, ie, A - B, is the latent heat content.

    NOTE : Enthalpy is rarely used in architectural context. It is more useful for making air conditioning assessments and calculations.

In the next SECTION, we will discuss the psychrometric processes that can be represented on the the psychrometric chart.


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