Read psychrometric chart,Dry,wet bulb temperatures,humidity axes

Introduction to psychrometric chart

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.

Axes in 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.

psychrometric chart axes

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.



Psychrometric chart is designed for a defined pressure (total pressure).

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.

barometric pressure psychrometric chart

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.


Read dry bulb temperature

Dry bub axis includes in the bottom of the chart.

  1. Bottom line of the graph indicate temperature values of the dry bulb. Here temperatures are numbered by 5 by 5 and started from -100C.
  2. Dry bulb temperature axis is highlighted(red).
  3. Examples: 100C and 200C temperature lines of the dry bulb are highlighted in green and blue respectively.
dry bulb temperature in psychrometric chart

Read wet bulb temperature

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.

wet bulb temperature

Read absolute humidity|specific humidity

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.

absolute humidity

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

  1. Draw a vertical line at 300C from dry bulb temperature axis (1).
  2. Draw the line of 250C of wet bulb temperature axis (2).
  3. Those two lines intercept at one point (3).
  4. Draw a horizontal line from interception point (4).
  5. Read the moisture content.
absolute humidity example


Relative humidity

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

relative humidity.jpg

Calculate relative humidity from psychrometric chart example

Dry bulb temperature is 300C and wet bulb temperature is 250C. Calculate the relative humidity.

Solution

  1. Draw a vertical line from dry bulb temperature axis at 300C (line 1).
  2. Draw the line of 250C of wet bulb temperature axis (line 2).
  3. Those two lines intercept at one point (point 3). Interception point lies between humidity curves of 60% and 70%.
  4. Calculate relative humidity by ruler.

Answer: approximately 66%

relative humidity example


Read enthalpy in psychrometric chart

Enthalpy line is divided into several parts. It's axis is highlighted(green) in the figure. Enthalpy is measured in kJ/kg dry air.

 read enthalpy in psychrometric chart

Read enthalpy values in psychrometric chart - Example

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

  1. Draw a vertical line from dry bulb temperature axis at 300C (line 1).
  2. Draw the line of 250C of wet bulb temperature axis (line 2). Interception point of line 1 and line 2 is point 3.
  3. Extend line 2 until enthalpy line meets (4).
  4. interception of line 3 and enthalpy line is the enthalpy of air(Point 5).

Answer: 77 kJ/kg Dry air

psychrometric chart enthalpy example


Examples and questions of psychrometric chart

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.

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

Solution

Identify points of psychrometric chart

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.

identify points of psychrometric chart

Read absolute/specific humidity value

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


Read the enthalpy value

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).


Read the relative humidity

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.

identify absolute relative enthalpy points psychrometric chart

Psychrometric chart example 2

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.

  1. Wet bulb temperature
  2. Enthalpy of air
  3. Relative humidity

Identify interception point in psychrometric chart

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.

psychrometric chart example problem

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

Reading relative humidity value

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.

relative humidity exercise problem
  1. between intersection point and 50% relative humidity curve (b)
  2. 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)

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