1. SNS COLLEGE OF ENGINEERING Coimbatore-107 Subject: Thermal Engineering
Psychrometric Processes BY
SIVAKUMAR P
AP/MECH

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 Psychrometry is the study of MOIST AIR or mixture of dry air and water vapour.

4. PROPERTIES OF PSYCHROMETRY
DEW 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.
To remove moisture from the air, an air conditioner evaporator coil must operate below the dew point temperature.
Moisture will collect on the cold surface of an evaporator coil and then drain through the condensate line.

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
(percentage)
The fourth property, relative humidity, is a comparison of the amount of moisture that a given amount of air is holding, to the amount of moisture that the same amount of air can hold, at the same dry-bulb temperature.
100% RH - Saturated
50% RH

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
Finally, humidity ratio describes the actual weight of water in an air – water vapor mixture. In other words, if one pound of air were wrung completely dry, comparing the weight of the water to the weight of the dry air would yield its humidity ratio.
Humidity ratio can be expressed as pounds of moisture per pound of dry air, or as grains of moisture per pound of dry air. There are 7000 grains of water in a pound. To appreciate the magnitude of these units of measurement, at sea level one pound of 70°F air occupies approximately 13.5 cubic feet, and one grain of water in that air weighs about two-thousandths (0.002) of an ounce.

9. HUMIDITY Specific 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 Humidity:
The mass of water vapor per unit volume of air containing the water vapor. This quantity is also known as the water vapour density.

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

11. Air conditioning Processes
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 1 2 t1 h1 t2 h2 Q

12. Sensible heating

13. 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 1 2 t1 h1 t2 h2 Q

14. 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.
KChatterjea, 2008

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

16. 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.
KChatterjea, 2008

17. Dehumidification by Cooling

18. Heating and Humidifying: It is the addition of heat and
moisture to moist air, it also involves sensible and latent
heat transfer. m 1 2 t1 h1 t2 h2 Q mw hw

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

20. Removing Sensible Heat and Moisture
Put all of these changes together on one chart and they show the direction the air condition will move when the dry-bulb temperature or moisture content is altered.

21. 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.

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

24. Adiabatic mixing
Governing equation

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

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

27. 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

28. Any Questions???

29. -THANK YOU FOR YOUR
ATTENTION….