Psychrometric Properties, Processes and Chart

1. PSYCHROMETRY
Properties, Processes andCharts
AppleNathaleeC. Villegas
Bachelorof Sciencein AgriculturalEngineering
Universityof SouthernMindanao
Kabacan,Cotabato

2. Psychrometry
Psychrometry is the Study of the properties of mixtures of
air and water vapour.

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

5. PROPERTIES OF
PSYCHROMETRY
 Dewpointtemperature
 Relativehumidity
 Humidityratio
 Dry-bulbtemperature
 Wetbulbtemperature
 Humidity
-Specific humidity
-Absolutehumidity
 Psychrometric ratio

6. 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.

7. Relative Humidity
Relative
Humidity
Amountofmoisturethata given amountofairis holding
=
Amountofmoisturethata given amountofaircan hold
(percentage)

8. Humidity Ratio/Specific
Humidity
Massofwaterassociatedwitheach kilogramofdryair.
W=kilogramofwatervapor=0.622Pv
kilogramofdryair Pt- Pv

9. DRY -BULB
TEMPERATURE
Thedry-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 addedto temperature to distinguish it from wet-bulb and
dewpoint 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

12. HUMIDITY
Specific Humidity:
Specific humidity is defined as the proportion of the mass of water vapour perunit
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 Humidity:
Themass of water vaporper unit volume of air containing the water vapor. This
quantity is also known as the water vapour density.

13. PSYCHROMETRIC RATIO
Thepsychrometricratio is the ratio of the heat transfer coefficient tothe
product of mass transfer coefficient and humid heat at a wetted surface. It
may beevaluated with the following equation:
r = hc/ ky cs
where:
 r =Psychrometricratio,dimensionless
 hc =convective heattransfercoefficient,W m-2 K-1
 ky=convective mass transfercoefficient, kg m-2 s-1
 cs =humidheat,J kg-1 K-1

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

18. Psychrometric Processes or
Air conditioning Processes
1. Sensible Heating : It is addition of heat tomoist air
without the addition of moisture. It follows a constant
humidity ratio line onthe psychrometric chart.
m m
1 2
t1
h1
t2
h2
Q

19. Sensible heating
tcmQ p  

20. 2. Sensible Cooling: It is the removal ofheat from moist
air
without the removal ofmoisture. It also follows a constant
W on the psychrometric chart.
m m
1 2
t1
h1
t2
h2
Q

21. Sensible Heat & Cooling
Processes
– Heating & Cooling process –addition or
removal of sensible heat without change in
absolute moisture content.
– NOTE:RH changesas temperature changes.
21

22. 3. Cooling and Dehumidifying: It is the removal
moisture from moist air, it involves sensible and latent
heat transfer.
m m
1 2
t1
h1
t2
h2
Q
mw
hw

23. Psychrometry - Dehumidification
Dehumidification by cooling –in
cooling coils temp of air reduces and the
saturation point (dewpoint) is reached.
23

24. Dehumidification by
Cooling

25. 4. 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
t1
h1
t2
h2
Q
mw
hw

26. 5. HUMIDIFYING : It is the addition of moisture to moist
air without the addition of heat.
m m
1 2
t1
h1
t2
h2
mw
hw

27. Removing Sensible Heat
and Moisture

28. ADIABATIC OR
EVAPORATIVE COOLING
A psychrometric process which invovles the cooling without
heat loss or gain. Sensible heat lost by air isconverted to latent
heat in the added water vapor.

30. ADIABATIC MIXING OF MOIST
AIR & STREAM
A psychrometric process thatinvovles no netheatloss or
gain during themixingof two air streams.

31. Adiabatic mixing
• Governing equation
hmQhm
outin
  

32. Psychrometry – Climate
Classification
Climate Classification shows the
various approximate zones of
temperature and humidity on the
psychrometric chart.
32

34. 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…..

35. -THANK YOU FOR YOUR
ATTENTION….