Dry Bulb, Wet Bulb Temperature Axis on Psychrometric Chart

Psychrometric chart includes so many axis which represent different parameters of air. Now we are going to learn two of those axis, dry bulb temperature and wet bulb temperature.

When we know dry bulb temperature and wet bulb temperature

When dry bulb temperature and wet bulb temperature are known, we can determine the relative humidity, absolute humidity, and enthalpy. Dry bulb temperature tells the actual room temperature.

Psychrometric chart - Full tutorial

Dry bulb temperature - T_db

It is defined as air temperature. When we measure the temperature of air, we refer the dry bulb temperature. Also, Dry bulb temperature can be measured by exposing a thermometer to the air.

Dry bulb temperature axis on psychrometric chart

Dry bulb temperature axis on psychrometric chart is located at bottom of the psychrometric chart and it is a horizontal line. Constant dry bulb temperatures appear as vertical lines in the psychrometric chart.

How to mark constant dry bulb temperatures on dry bulb axis?

Dry bulb temperature lines are vertical lines. We have marked three different dry bulb lines for 20⁰C, 30⁰C, 40⁰C in following figure.

Wet bulb temperature

The wet Bulb temperature is the adiabatic saturation temperature. Wet bulb temperature curve

Wet bulb temperature is lower than dry bulb temperature or equal to the dry bulb temperature. This equality occurs at only in air become saturated.

Wet bulb temperature axis on psychrometric chart

Wet bulb temperature axis is highlighted in following figure. Constant wet bulb temperatures appear diagonally from the upper left to the lower right in the Psychrometric chart.

How to mark constant wet bulb temperatures on wet bulb axis?

Wet bulb temperature lines are inclined lines. (not vertical or horizontal). We have marked three different wet bulb lines for 10⁰C, 20⁰C, 30⁰C in following figure.

Identify a point on psychrometric chart when wet bulb temperature and dry bulb temperature are known

When wet bulb temperature and dry bulb temperature are given, your first step is drawing correct lines of wet bulb temperature and dry bulb temperature. Then these two lines will meet at one point.

We discuss few examples to understand how this process is done.

Question

You are advised to find air properties from a psychrometric chart. You are given a psychrometer which can be used to measure dry bulb temperature and wet bulb temperature. You have observed that temperatures are 28⁰C and 24⁰C for dry bulb and wet bulb respectively. Identify the point on psychrometric chart where other air properties can be calculated?

• Find the dry bulb axis and 28⁰C point. Then draw a vertical line from that point.
• Find the wet bulb axis and 24⁰C point. Then draw a inclined line parallel to the wet bulb temperature lines from that point.
• Interception point of above two line will is used to calculate the other air properties.

Related topics to Chemical Engineering