PSYCHROMETRY

A detailed descripction on the topic of psychrometry.. its charts and its properties....

1. PSYCHROMETRY PROPERTIES, processes AND charts - Presented by- PRABHAV JOSHI (ENROLL NO- 120070125031 ) -Guided by- PURVI MA’M

2. CONTENTS What is PSYCHROMETRY? Properties of PSYCHROMETRY PSYCHROMETRIC Processes PSYCHROMETRIC CHART

3. Psychrometry Psychrometry is the science dealing with the physical laws of air – water vapour mixtures. When designing an air conditioning system, the temperature and moisture content of the air to be conditioned, and the same properties of the air needed to produce the desired air conditioning effect . In other words, we can say that P sychrometry is the study of MOIST AIR or mixture of dry air and water vapour .

4. PROPERTIES OF PSYCHROMETRY D EW POINT TEMPERATURE RELATIVE HUMIDITY HUMIDITY RATIO DRY-BULB TEMPERATURE WET BULB TEMPERATURE HUMIDITY Specific humidity Absolute humidity PSYCHROMETRIC RATIO

5. Dew Point The temperature at which moisture starts to condense out of the air is known as DEW POINT . Dew point is also known as saturation temperature. Dew point temperature is determined by moving from a state point horizontally to the left along lines of constant humidity ratio until the upper, curved, saturation temperature boundary is reached.

6. Dew Point Condensation occurs at the Dew Point Temperature

7. Relative Humidity Relative Humidity Amount of moisture that a given amount of air is holding = Amount of moisture that a given amount of air can hold 50% RH 100% RH - Saturated (percentage)

8. Humidity Ratio Actual weight of water in an air – water vapour mixture Amount of moisture per unit of dry air Can be defined as, W=m/G

9. DRY -BULB TEMPERATURE The dry-bulb temperature is the temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation. The term dry-bulb is customarily added to temperature to distinguish it from wet-bulb and dew point temperature .

10. WET BULB TEMPERATURE Wet bulb temperature is the temperature recorded by thermometer when the bulb is enveloped by cotton wick saturated with water. The accuracy of a simple wet-bulb thermometer depends on how fast air passes over the bulb and how well the thermometer is shielded from the radiant temperature of its surroundings.

11. THERMOMETERS Dry Bulb Wet Bulb Measured in degrees Fahrenheit ( o F)

12. HUMIDITY   Specific Humidity: Specific humidity is defined as the proportion of the mass of water vapour per unit mass of the moist air sample (dry air plus the water vapour ); it is closely related to humidity ratio and always lower in value .   Absolute H umidity : The mass of water vapor per unit volume of air containing the water vapor. This quantity is also known as the water vapour density.

13. PSYCHROMETRIC RATIO The  psychrometric ratio  is the ratio of the heat transfer coefficient to the product of mass transfer coefficient and humid heat at a wetted surface. It may be evaluated with the following equation : r = h c / k y c s where :   r= Psychrometric ratio, dimensionless   h c = convective heat transfer coefficient, W m -2  K -1   k y = convective mass transfer coefficient, kg m -2  s -1   c s = humid heat, J kg -1  K -1

14. PSYCHROMETRIC PROCESSES SENSIBLE HEATING SENSIBLE COOLING DEHUMIDIFICATION HUMIDFIYING ADIABATIC COOLING ADIABATIC MIXING (MOIST AIR & WATER VAPOR)

15. Psychrometric Processes or Air conditioning Processes 1. Sensible Heating : it is addition of heat to moist air without the addition of moisture. It follows a constant humidity ratio line on the psychrometric chart. m m 1 2 t 1 h 1 t 2 h 2 Q

16. Sensible heating

17. 2. Sensible Cooling: It is the removal of heat from moist air without the removal of moisture. It also follows a constant W on the psychrometric chart. m m 1 2 t 1 h 1 t 2 h 2 Q

18. KChatterjea, 2008 18 Sensible Heat & Cooling Processes Heating & Cooling process – addition or removal of sensible heat without change in absolute moisture content. NOTE: RH changes as temperature changes.

19. 3. Cooling and Dehumidifying: It is the removal of heat and moisture from moist air, it involves sensible and latent heat transfer. m m 1 2 t 1 h 1 t 2 h 2 Q m w h w

20. KChatterjea, 2008 20 Psychrometry - Dehumidification Dehumidification by cooling – in cooling coils temp of air reduces and the saturation point (dew point) is reached. Further cooling results in reduction of absolute humidity.

21. Dehumidification by Cooling

22. Heating and Humidifying: It is the addition of heat and moisture to moist air, it also involves sensible and latent heat transfer. m m 1 2 t 1 h 1 t 2 h 2 Q m w h w

23. 5. HUMIDIFYING : It is the addition of moisture to moist air without the addition of heat. m m 1 2 t 1 h 1 t 2 h 2 m w h w

24. Removing Sensible Heat and Moisture

25. Adiabatic or evaporative cooling A psychrometric process which invovles the cooling without heat loss or gain. Sensible heat lost by air is converted to latent heat in the added water vapor.

26. ADIABATIC MIXING OF MOIST AIR & STREAM A psychrometric process that invovles no net heat loss or gain during the mixing of two air streams.

27. Adiabatic mixing Governing equation

28. KChatterjea, 2008 29 Psychrometry – Climate Classification Climate Classification shows the various approximate zones of temperature and humidity on the psychrometric chart.

29. Dry Blub Relative Humidity Dew Point 100% Saturation Enthalpy Wet Blub Grains of Moisture Specific Volume Breakdown of the Lines

30. Psychrometric chart description Vertical lines are dry-bulb temperature Horizontal lines are humidity ratio (right axis) or dew point temp (left axis) Slanted lines are wet-bulb temp and enthalpy Specific volume are the “other” slanted lines

31. REFERENCES I have collected information regarding PSYCHROMETRY topic from the book “ENGINEERING THERMODYNAMICS” by Mr. P. K. NAG... Overview of Psychometrics: http://www.handsdownsoftware.com/Overview_of _ Psychrometrics.pdf For some topics I have taken a help of GOOGLE and WIKIPEDIA…..

32. Any Questions???

33. - THANK YOU FOR YOUR ATTENTION ….