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.

**Two independent intensive properties**are required to completely specify the state of air in the psychrometric chart.

- Psychrometric chart illustrates enthalpy scale (line with a slope), dry-bulb temperature scale (horizontal line), and specific humidity scale (vertical line). Wet-bulb temperature scale is indicated on the saturated curve (relative humidity 100%).
- Furthermore, it also illustrates the constant relative humidity curves, constant wet-bulb temperature lines, constant enthalpy lines and constant specific volume lines.
- Psychrometric charts are extensively used in
**air-conditioning applications**.

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 * 10^{5} Pa, we can only read values related to that pressure.

Total pressure is also called as **barometric pressure**.

Design pressure ( total pressure of air , P_{T}) is the sum of **dry air pressure** (P_{da}) and **partial pressure of
water vapor** (P_{wv}) .

P_{T} = P_{da} + P_{wv}

Example

You are provided a psychrometric chart which is designed for 1.01325 * 10^{5} Pa. And you are said to find relative humidity of a
room using given chart. The total pressure in the room is 0.89214 * 10^{5} 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.

- Bottom line of the graph indicate temperature values of the dry bulb. Here temperatures are numbered by 5 by 5 and
started from -10
^{0}C. - Dry bulb temperature axis is highlighted(red).
- Examples: 10
^{0}C and 20^{0}C temperature lines of the dry bulb are highlighted in green and blue respectively.

Numbering is same as the dry bulb line, but it's one part is curved and highlighted(red) on the following figure.

Examples: 15^{0}C and 25^{0}C 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 30^{0}C and wet bulb temperature is 25^{0}C.
Calculate the moisture content.

Moisture content measures by ** g/1kg dry air **

Solution

- Draw a vertical line at 30
^{0}C from dry bulb temperature axis (1). - Draw the line of 25
^{0}C of wet bulb temperature axis (2). - Those two lines intercept at one point (3).
- Draw a horizontal line from interception point (4).
- Read the moisture content.

Some of humidity curves parts are highlighted(red) in the figure.

Dry bulb temperature is 30^{0}C and wet bulb temperature is 25^{0}C.
Calculate the relative humidity.

Solution

- Draw a vertical line from dry bulb temperature axis at 30
^{0}C (line 1). - Draw the line of 25
^{0}C of wet bulb temperature axis (line 2). - Those two lines intercept at one point (point 3). Interception point lies between humidity curves of 60% and 70%.
- Calculate relative humidity by ruler.

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 30^{0}C and wet bulb temperature is 25^{0}C.
Calculate the enthalpy of that air.

Enthalpy is measured by kJ/kg Dry Air

Solution

- Draw a vertical line from dry bulb temperature axis at 30
^{0}C (line 1). - Draw the line of 25
^{0}C of wet bulb temperature axis (line 2). Interception point of line 1 and line 2 is point 3. - Extend line 2 until enthalpy line meets (4).
- interception of line 3 and enthalpy line is the enthalpy of air(Point 5).

Answer: 77 kJ/kg Dry air

Question 1

In an experiment, dry bulb temperature and wet bulb temperatures are found as 28^{0}C and 20^{0}C
respectively. Researches need to find following parameters to continue the experiment using psychrometric chart.

- Absolute humidity or specific humidity
- Relative humidity
- Enthalpy of air

Solution

First, we should identify which values we know. In this example we know values of dry bulb
temperature (28^{0}C ) and wet bulb temperature (20^{0}C ). mark these two points in related axis
and draw lines through that points. See the figure. Mark intersection point of dry bulb
temperature (28^{0}C ) line and wet bulb temperature (20^{0}C ) line as 1.

Draw a horizontal line towards right through intersection point of dry bulb
temperature (28^{0}C ) line and wet bulb temperature (20^{0}C ) 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 (28^{0}C ) line and wet bulb temperature (20^{0}C ) 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 (28^{0}C ) line and wet bulb temperature (20^{0}C ) 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 30^{0}C.

Absolute/Specific humidity 20g/1kg of dry air

Find values of following properties.

- Wet bulb temperature
- Enthalpy of air
- Relative humidity

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 = 27.2^{0}C

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.

- between intersection point and 50% relative humidity curve (b)
- between intersection point and 60% relative humidity curve (a)

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)