psychrometer 2.pptx

7/27/2019 psychrometer 2.pptx

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Psychrometry : It is the branch of science which deals with the study of

the mixture of dry air and water vapor. In air conditioning estimations air is

taken as a mixture of dry air and water vapor.

The principles ofpsychrometryapply to any physical system consisting of

gas-vapor mixtures.

The Pschrometry terms:

Dry airMoist air

Saturated air

Degree of Saturation

Humidity

Absolute HumidityRelative Humidity

Dry bulb temperature

Wet bulb temperature

Dew point Temperature

Dew point Depression


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Moist Air : The mixture of dry air and water vapor. The quantity of water vapor

present in air is known as moisture.

Saturated Air : When moist air contains maximum amount of water vapor that it

can hold, it is called saturated air.

Unsaturated Air : Moist air which is not saturated.

Humidity Ratio : The weight of water vapor per unit weight of dry air in avapor air mixture. It is also called Specific Humidity.

Degree of Saturation : The ratio of the prevailing humidity ratio of moist air to

the humidity ratio of saturated air at the same temperature and pressure.

Relative Humidity : The ratio of actual weight of water vapor in a given

volume to the weight of water vapor contained in the same volume at the same

temperature when the air is saturated.


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A Sling Psychrometer is

used to measure relative

humidity.

Following are the pschrometry relationship:

1. Specific Humidity, humidity ratio & moisture content.2. Degree of Saturation or percentage humidity

3. Relative Humidity

4. Pressure of Water vapour

5. Vapour density & absolute humidity


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Psychrometer Parts:

Dry Bulb Thermometer

Wet Bulb Thermometer

Reservoir and StockingWETDRY


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Dry-bulb Temperature : The temperature which is recorded by a

thermometer whose reading is not affected by the humidity ratio or

by thermal radiation.

Wet-bulb Temperature : The temperature which is recorded by a

thermometer when its bulb is covered with wet cloth and is exposed to a

current of moving air. The difference between dry-bulb temperature and

wet-bulb temperature is known as wet-bulb depression.

Dew Point Temperature : The saturation temperature which corresponds to the

existing humidity ratio and barometric pressure. Generally dew point

temperature is taken to be the temperature at which condensation will just begin

when the moist air mixture under consideration is cooled at constant pressure.

Enthalpy of Air: It is the Total heat of air.

It is considered that the enthalpy of moist air is a concern of Air conditioning

Engineer.


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On the side of the psychrometer is

a sliding scale which calculates

relative humidity for us.

Align the dry-bulb reading with thewet-bulb reading. The arrow will point

to the relative humidity reading on the

center scale. Its easy!


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Using our wet- and dry-

bulb temperatures, we can

consult tables made by

scientists

to determine humidityand saturation value.

Psychometric Chart : The graphical representation of various thermodynamic

properties of moist air. In this abscissa represents dry bulb temperature and ordinate

represents specific humidity.


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Psychometric Processes : The various processes which are involved in air

conditioning to change the psychometric properties of air according to requirement.


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But why do we sling it

around

and

around?


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What happens to your skin when you

sweat and then stand in a breeze?


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As the wet-bulb thermometer goes through the air

1. Water

evaporates

2. Energy is

used; water

changesfrom liquid to

vapor

3. Wet-bulb temperature goes down


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What influences how

much the wet-bulb

temperature goes

down?

Humidity

Temperature


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Now we can calculate relative humidity!


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Partial Relative Humidity Chart for 30 CDifference Between

Dry Bulb and Wet Bulb

Temperatures

Relative Humidity

None 100%

0.5 96%

1.0 93%

1.5 89%

9.0 44%

9.5 42%

14.5 19%

15.0 17%

18.0 5%

Fortunately, we dont have to do all this every time


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For Help:

AMSTI-GLOBE The GLOBE Programwww.amsti.org/globe www.globe.gov

Judy Reeves Lynn VaughanAMSTI-GLOBE AMSTI-GLOBEResource Specialist Resource Specialist

[email protected] [email protected]

Robin Nelson Jerry CobbsAMSTI-GLOBE AMSTI-GLOBEAdministrator Technology Specialist

[email protected] [email protected]
http://www.amsti.org/globehttp://www.globe.gov/mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]://www.globe.gov/http://www.amsti.org/globe

psychrometer 2.pptx

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