Air contains gases and water vapor. There are inclined lines of constant specific volumes. Here we are going to calculate specific volume using psychrometric chart. When two air properties are known except specific volume, specific volume can be calculated.
In this tutorial, we will discuss followings.
Density of air is low than solids and liquids. So volume occupied for 1kg of mass is much higher than solids and liquids.
Specific volume of air in psychrometric chart is defined as the number of cubic meters occupied by one kilogram of a dry air. So units of specific volume is m3 kg-1.
Specific volume = volume / mass of dry air
Lets see an example to understand this.
If a room occupy 5 m3 of volume. There is 4.6 kg of dry air. Then what is the specific volume?
Specific volume = 5 m3 / 4.6 kg
Specific volume = 1.09 m3 kg-1
Constant specific volume lines on psychrometric chart are inclined and they have a negative slope. Constant specific volume lines on psychrometric chart have been marked on following psychrometric chart figure.
On one constant specific volume line, unit is written as specific volume (m3) / kg of dry air.
When we find a point where two air properties are known, we can calculate specific volume. If this point does not lie on a constant specific volume line, we have to draw a constant specific line parallel to the other constant specific volume lines through that point.
A psychrometer shows its dry bulb temperature as 250C and wet bulb temperature as 200C. Find the specific volume of air.
Specific volume of left side constant specific volume line is 0.86 m3 kg-1 and right side constant specific volume line is 0.88 m3 kg-1.
From distance measuring, ratio of x:y is approximately 2:3 .
So now we can find the specific volume from those data
Specific value = 0.86 + (0.88-0.86) * 2 / (2+3)
Specific value = 0.868 m3 kg-1
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