Properties of atmospheric air such as, the dry-bulb temperature, the wet-bulb temperature, the dew-point temperature, the specific humidity, the relative humidity, specific enthalpy and specific volume are discussed in relation to the psychrometric chart below.
Example: When dry bulb temperature and wet bulb temperature of air are known, we can calculate absolute humidity, relative humidity and enthalpy.
NOTE: Before understanding the psychrometric chart, you should have a good knoledge about air properties of air.
In this tutorial we first study axes of psychrometric chart, then questions and examples are discussed. Properly study axes of psychrometric chart with figures and get an idea, where different axes are located. After studying locations of axes, we look to do several examples and problems to understand psychrometric chart exactly.
Total pressure of air is changed in place to place. The defined pressure is displayed on the psychrometric chart. See the below figure (circled in blue). If design pressure (total pressure) of psychrometric chart is 1.01325 * 105 Pa, we can only read values related to that pressure.
Total pressure is also called as barometric pressure.
Design pressure ( total pressure of air , PT) is the sum of dry air pressure (Pda) and partial pressure of water vapor (Pwv) .
PT = Pda + Pwv
Example
You are provided a psychrometric chart which is designed for 1.01325 * 105 Pa. And you are said to find relative humidity of a room using given chart. The total pressure in the room is 0.89214 * 105 Pa.
The total pressure and psychrometric chart design pressure is not same. So we cannot use this psychrometric chart to find relative humidity.
Dry bub axis includes in the bottom of the chart.
Numbering is same as the dry bulb line, but it's one part is curved and highlighted(red) on the following figure.
Examples: 150C and 250C temperature lines of the dry bulb are highlighted in green and blue respectively.
Absolute humidity axis is in the right side of the chart. That axis is highlighted(black) in the figure.
Absolute humidity is measured by g/kg Dry Air
Green and blue lines are 10g and 20g per 1kg of dry air.
Example 1:
Dry bulb temperature is 300C and wet bulb temperature is 250C.
Calculate the moisture content.
Moisture content measures by g/1kg dry air
Solution
Some of humidity curves parts are highlighted(red) in the figure.
Dry bulb temperature is 300C and wet bulb temperature is 250C. Calculate the relative humidity.
Solution
Answer: approximately 66%
Enthalpy line is divided into several parts. It's axis is highlighted(green) in the figure. Enthalpy is measured in kJ/kg dry air.
Dry bulb temperature is 300C and wet bulb temperature is 250C. Calculate the enthalpy of that air.
Enthalpy is measured by kJ/kg Dry Air
Solution
Answer: 77 kJ/kg Dry air
Question 1
In an experiment, dry bulb temperature and wet bulb temperatures are found as 280C and 200C respectively. Researches need to find following parameters to continue the experiment using psychrometric chart.
Solution
First, we should identify which values we know. In this example we know values of dry bulb temperature (280C ) and wet bulb temperature (200C ). mark these two points in related axis and draw lines through that points. See the figure. Mark intersection point of dry bulb temperature (280C ) line and wet bulb temperature (200C ) line as 1.
Draw a horizontal line towards right through intersection point of dry bulb temperature (280C ) line and wet bulb temperature (200C ) line. This new line will meet absolute humidity(specific humidity) axis at some point. Mark that point and read the value. Answer is 11.7g moisture/kg dry air
Draw an inclined line parallel to the enthalpy lines trough intersection point of dry bulb temperature (280C ) line and wet bulb temperature (200C ) line. This new line will meet enthalpy axis at some point. Mark that point and read the value( approximately 57.5kJ/kg).
Check the two relative humidity curves which lies of intersection point of dry bulb temperature (280C ) line and wet bulb temperature (200C ) line. In this example we can see point 1 exactly lies on 50% relative humidity curve.
In an experiment, following air characteristics are known.
Dry bulb temperature is 300C.
Absolute/Specific humidity 20g/1kg of dry air
Find values of following properties.
We know two values( dry bulb temperature, absolute/specific humidity ). Therefore we can identify the related point(X) using that two values.
Green(1) and brown(2) lines are related to dry bulb temperature and absolute humidity values respectively. Study the figure.
Draw a line (red, line 3) parallel to the wet bulb temperature lines through interception point X. Extend this line until enthalpy line (blue, line 4) meets.
You can read values of wet bulb temperature and enthalpy easily.
Wet bulb temperature = 27.20C
Enthalpy = 86 kJ/kg of dry air
Intersection point(X) lie between two relative humidity curves. Identify those two curves 50% and 60% respectively.
Put a ruler square to the relative humidity curves. Then calculate following distances.
You can calculate relative humidity now using those distances.
R.H = less R.H + (b/(a+b))*(R.H.D)
R.H = relative humidity
R.H.D = relative humidity difference between adjacent R.H curves = 10
less R.H = 50% (in this example)
