Question

Humid air is enclosed in a 5.00-liter flask at 40.0°C. The flask is slowly cooled. When the temperature reaches 20.0°C, drops of moisture become visible on the flask wall. Although the pressure in the flask changes when the temperature drops, it remains close enough to 1 atm for the psychrometric chart to provide a close representation of the behavior of the system throughout the process. Use a psychometric chart to determine the relative humidity, absolute humidity, and wet-bulb temperature of the air at 40.0°C. And please explain how you found it!

Answer 1

To understand this let us furst see what all useful information we can get from the chart

The temeprature of the humid air is 40 ^OC. This is nothing but the dry bullb temperature.

Dry bulb temperature = 40 ^OC

now it is said than moisture becomes visible on flask at 20 ^oC. This means that water vapour in air has condensed and this occurs at dew point . At dew point water is completely saturated that implies relative humidty at dew point is 100 %.

Dew point temperature =20 ^oC

Now let us used this dew point and dry bulb temperature data to determine Relative and absolute humidity and also the wet bulb temperature

We have used the follwing psychometric chart

Now let us mark 20^oC anf 40 ^oC on x axis. the curves lines (arrowed with red line) represents relative humidty lines. Since we know that relative humidty is 100% at dew point hence we will draw a vertical line from 20 ^oC to 100% relative humidty line (Yellow line)

Now we will also draw a vertical line from 40 ^oC ( pink line). Now we we will move horizontally from the point of intersection of Dew point line (yellow line) and 100% relative humidty curve (yellow curve) and cut the vertical 40 ^oC ( pink line) line. We can move horizontally because the absolute humidity doesn't change. that means amount of water remains unchanged.

This point of intersection (Green circle) is the actualy state of the system. We can all information we rewuire with the help of this point.

Now if we just extend horizontal line through this point( Green circle ). on y-axis, we will get absolute humidity.

From Figure : Absolute humidity = 0.015 Kg water vapour/ Kg of air

Relative humidty line are curves line (indicated by Red arrow)

Now we will draw a curve of relative humidity passing through this point (green curve)

Since the point doesnt exactly passes through drawn lines. The green line which we have drawn will give us a approximate ans

Relative Humidty = 25 %

Now Let us evaluate Wet bulb temperature .

Wet bulb temperature lines are negative lines ( a dark blue color arrow is show to indicate this line)

Now let us draw this line at our point to get the answer.

(Since here temperature values are now written along the curve we will simply project the value on X-axis to just know the value of temperature.

From graph, Wet bulb Temperature = 26.67 ^Oc