What is Psychrometric Chart? Components of Psychrometric Chart?

In the series of articles on properties of air we saw the various components and important properties of air like relative humidity, dry bulb temperature, wet bulb temperature, dew point temperature, sensible heat, latent heat etc. We shall now see how the air behaves when it is subjected to changes in temperature and humidity to suit the various applications for which the air conditioning is meant. The behavior of the air can be studied very conveniently and accurately enough by the psychrometric chart.

The psychrometric charts are the graphic representations of the psychrometric properties of air. By using psychrometric charts the HVAC engineers can graphically analyze different types of psychrometric processes and find solution to many practical problems without having to carry out the long and tedious mathematical calculations.

The psychrometric chart looks complicated with number of lines and curves in it, but is very easy to understand if you know the basic properties of air. You will also understand its worth when you actually use it considering the fact that you won’t have to use any formulae to find the properties of air in different conditions, all you will have to know is two parameters of air and the rest ones are found readily from the chart.

All the properties of air indicated in the psychrometric chart are calculated at the standard atmospheric pressure and for other pressures relevant corrections have to be applied. The psychrometric chart looks like a shoe. The various lines shown in the chart are as follows (please refer the figs below):

The dry bulb temperature scale is shown along the base of the shoe shaped psychrometric chart forming the sole. The DB temperature increases from left towards right. The vertical lines shown in the chart are the constant DB temperature lines and all the points located along a particular vertical line have same DB temperature.

Moisture content is the water vapor present in the air and is measured in gram per kg of dry air (gm/kg of dry air). The moisture present within the air is indicated by the vertical scale located towards the extreme right. The horizontal lines starting from this vertical scale are the constant moisture lines.

The outermost curve along the left side indicates the WB temperature scale. The constant WB temperature lines are the diagonal lines extending from WB temperature curved scale downwards towards the right hand side of the chart. All the points located along the constant WB temperature line have the same temperature.

Since the dew point temperature of the air depends on the moisture content of the air, the constant moisture lines are also the constant DP temperature lines. The scale of the DP and WB temperature is same, however, while the constant WB temperature lines are diagonal lines extending downwards, the constant DP temperature lines are horizontal lines thus the constant DP and WB temperature lines are different.

1) Book: Principles of Refrigeration by Roy J. Dossat

2) Book: Basic Refrigeration and Air Conditioning by P N Ananthanarayan

1) http://www.acr-news.com/news/news.asp?id=633

2) http://www.sp.uconn.edu/~mdarre/NE-127/NewFiles/psychrometric_inset.html