The document discusses the science of psychrometry, which deals with air-water vapor mixtures. It defines key psychrometric properties like dry bulb temperature, wet bulb temperature, dew point, relative humidity, and specific humidity. It also explains various psychrometric processes like sensible heating and cooling, humidification, dehumidification, and adiabatic mixing. The psychrometric chart is introduced as a tool for analyzing these processes. Finally, the document discusses factors that affect human thermal comfort like air temperature, humidity, and heat transfer methods in buildings.

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2. CONTENTS
 What is PSYCHROMETRY?
 Properties of PSYCHROMETRY
 PSYCHROMETRIC Processes
 PSYCHROMETRIC CHART
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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.
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4. PROPERTIESOF PSYCHROMETRY
 DEW POINT TEMPERATURE
 RELATIVE HUMIDITY
 HUMIDITY RATIO
 DRY-BULB TEMPERATURE
 WET BULB TEMPERATURE
 HUMIDITY
• Specific humidity
• Absolute humidity
 PSYCHROMETRIC RATIO
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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.
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6. 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.
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7. 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.
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8. 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.
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9. Psychometric 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 = hc / ky cs
 where:
 r= Psychrometric ratio, dimensionless
 hc = convective heat transfer coefficient, W m-2 K-1
 ky= convective mass transfer coefficient, kg m-2 s-1
 cs= humid heat, J kg-1 K-1
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10. 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
t1
h1
t2
h2
Q
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11. Sensible heating
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tcmQ p 

12. 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
t1
h1
t2
h2
Q
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13. Sensible Heat & Cooling Processes
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 Heating & Cooling process –
addition or removal of sensible
heat without change in absolute
moisture content.
 NOTE: RH changes as
temperature changes.

14. 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
t1
h1
t2
h2
Q
mw
hw
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15. Psychrometry - Dehumidification
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 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.

16. Dehumidification by Cooling
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17. 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
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18. 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
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19. Removing Sensible Heat and Moisture
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20. 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.
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22. ADIABATIC MIXING OF MOIST
AIR & STREAM
 A psychrometric process that invovles no net heat loss
or gain during the mixing of two air streams.
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23. Adiabatic mixing
 Governing equation
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hmQhm
outin
  

24. Psychrometry – Climate Classification
 Climate Classification
shows the various
approximate zones of
temperature and humidity
on the psychrometric chart.
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Basic Factors That Affect Human
Comfort In The Internal Environment

26. Thermal and air quality
 What affects the surroundings you live in?
 Air quality is affected by how hot it is outside or inside your environment
 What is humidity and what affects humidity?
 The amount of moisture that is present within the air will have an effect on
humidity, which is linked to the amount of ventilation entering
 What is the normal temperature of a human being?
 Human temperature maintain an average core temperature of 37º depending
on the metabolic rate
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27. Nature of heat
• What is the measure of temperature
• Temperature is measured in degrees celsius
• The lower is 0 fixed at a melting point of ice at a stand at atmospheric
pressure of 101.32kN/m2
• The upper point is 100 degrees – temperature of steam above the boiling
point
• What is the acceptable value of temperature taken at normal design?
• Normal design temperature are taken at 21 degrees inside and -1 degrees
outside on average
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28. Thermodynamic temperature scale
• This is another measure of temperature in degrees Kelvin
• 0 degree celsius= 273.16 Kelvin (K)
• 100 degree celsius = 317.16 Kelvin
• The unit of thermodynamic temperature is the fraction of the
thermodynamic temperature at the triple point water
• (equilibrium point of the temperature and pressure at which three known
phases of substance can exist i.e. liquid, water vapour and pure ice)
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29.  How do we measure the quantity of heat?
 Heat is measured in joules (J) which is a measure of work done
 The rate of expenditure of energy or doing work or of heat loss is
measured in watts (W)
 1 watt is = 1 Joule per second
 1 W =1 J/s
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30. Heat transfer
 Name three ways heat is transferred from one mass to another, for instance a
person sitting next to a radiator.
 Conduction
 Convection
 Radiation
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31. Thermal comfort
 In high activity the temperature rises and the more heat you will give off.
Several factors influences the level heat is generated (metabolic rate)
including:
 Your surface area
 Age
 Gender
 Level of activity
 e.g.
 Sleeping heat output 70W. Lifting 440W.
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32. Typical heat output of an adult male
Activity Example Heat output
Immobile Sleeping 70W
Seated Watching TV 115W
Light work Office 140W
Medium work Factory Work 265W
Heavy work Lifting 440W
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33. Heat losses from buildings
 Comfortable temperature for humans is provided by balancing the heat lost
through conduction and ventilation through the fabric with similar heat
 Optimum temperature will depend on material used , type of construction,
orientation of the building and degree of exposure to the rain and wind
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34. Room temperatures
 What would you consider in design to maintain temperature in buildings?
 The resistance of a material to the passage of heat and the thermal
conductivity of the material in passing the heat along are the basics of
understanding of maintaining a steady temperature and a comfortable
thermal indoor environment
 In order to maintain a comfortable room temperature the building must be
provided with as much heat as is lost through ventilation
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35. What will the loss of heat in buildings
depend on?
 Materials used
 Type of construction
 Orientation of the building in relation to the sun
 Degree of exposure to rain and wind
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36. RELATIVE HUMIDITY
 Humans are used to a relative humidity of between 40 and 60%. Greater
than this we start to describe air as being ‘Humid’.
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37. HEAT LOSS DUE TO VENTILATION
 Natural ventilation leads to the complete volume of air in a room changing a
certain number of times in one hour
 Type of room Air changes in hr
 Halls 1.0
 Bedrooms /lounges 1.5
 WCs and bathrooms 2.0
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