Specific Volume of Air on Psychrometric Chart

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.

1. What is specific volume?
2. Constant specific volume lines on psychrometric chart
3. Calculate unknown specific volume from two known air properties.

Volume of dry air

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

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

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.

Calculate specific volume

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.

Problem

A psychrometer shows its dry bulb temperature as 25⁰C and wet bulb temperature as 20⁰C. Find the specific volume of air.

Steps:

• Find the point of 25⁰C in the dry bulb axis. Then draw the constant dry bulb temperature line (vertical, yellow) from this point.
• Then, find the point of 20⁰C in the wet bulb axis. Then draw the constant wet bulb temperature line (inclined, yellow) from this point.
• Mark the interception point of above two lines.
• Identify the constant specific volume lines nearest to the interception point (blue colour).
• Draw a parallel line to constant specific volume lines through interception point (green colour). Extend this line until it intersect the dry bulb temperature axis. Mark that interception point.
• Measure distances (x and y) between each constant specific volume lines when they intersect the dry bulb temperature axis (see the figure).
• Then you can calculate the specific volume for drawn constant specific volume line.

Calculation using distance ratio

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