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