psychrometry
DESCRIPTION
A detailed descripction on the topic of psychrometry.. its charts and its properties....TRANSCRIPT
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PSYCHROMETRY
PROPERTIES, processes AND charts
-Presented by-PRABHAV JOSHI (ENROLL NO-120070125031)
-Guided by-PURVI MA’M
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CONTENTS What is PSYCHROMETRY? Properties of
PSYCHROMETRY PSYCHROMETRIC
Processes PSYCHROMETRIC CHART
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Psychrometry
O Psychrometry is the science dealing with the physical laws of air – water vapourmixtures.
O 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.
O 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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PROPERTIES OF 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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Dew Point
O The temperature at which moisture starts to condense out of the air is known as DEW POINT.
O Dew point is also known as saturation temperature.
O 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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Dew PointOCondensati
on occurs at the Dew Point Temperature
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Relative Humidity
RelativeHumidity
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)
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Humidity RatioOActual weight of
water in an air – water vapour mixture
OAmount of moisture per unit of dry air
OCan be defined as, W=m/G
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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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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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THERMOMETERS
Dry Bulb
Wet Bulb
Measured in degrees Fahrenheit (oF)
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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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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 = 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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PSYCHROMETRIC PROCESSES
SENSIBLE HEATING SENSIBLE COOLING DEHUMIDIFICATION HUMIDFIYING ADIABATIC COOLING ADIABATIC MIXING (MOIST AIR & WATER VAPOR)
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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 m1 2
t1
h1
t2
h2
Q
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Sensible heatingtcmQ p
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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 m1 2
t1
h1
t2
h2
Q
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18 KChatterjea, 2008
Sensible Heat & Cooling Processes Sensible Heat & Cooling Processes
O Heating & Cooling process – addition or removal of sensible heat without change in absolute moisture content.
O NOTE: RH changes as temperature changes.
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3. Cooling and Dehumidifying: It is the removal of heat and moisture from moist air, it involves sensible and latent heat transfer.
m m1 2
t1
h1
t2
h2
Q
mw
hw
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20 KChatterjea, 2008
Psychrometry - DehumidificationPsychrometry - Dehumidification
O Dehumidification by cooling – in cooling coils temp of air reduces and the saturation point (dew point) is reached.
O Further cooling results in reduction of absolute humidity.
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Dehumidification by Cooling
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4. Heating and Humidifying: It is the addition of heat and moisture to moist air, it also involves sensible and latent heat transfer.
m m1 2
t1
h1
t2
h2
Q
mw
hw
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5. HUMIDIFYING : It is the addition of moisture to moist air without the addition of heat.
m m1 2
t1
h1
t2
h2
mw
hw
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Removing Sensible Heat and Moisture
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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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ADIABATIC MIXING OF MOIST AIR & STREAM
OA psychrometric process that invovles no net heat loss or gain during the mixing of two air streams.
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Adiabatic mixingO Governing equation
hmQhmoutin
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29 KChatterjea, 2008
Psychrometry – Climate Classification
Psychrometry – Climate Classification
O Climate Classification shows the various approximate zones of temperature and humidity on the psychrometric chart.
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DryBlub
Relative
Humidity
DewPoint
100%Saturatio
n
Enthalpy
WetBlub
Grains of
Moisture
SpecificVolume
Breakdown of the Lines
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Psychrometric chart description
O Vertical lines are dry-bulb temperatureO Horizontal lines are humidity ratio (right
axis) or dew point temp (left axis)O Slanted lines are wet-bulb temp and
enthalpyO Specific volume are the “other” slanted lines
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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…..
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Any Questions???
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-THANK YOU FOR YOUR ATTENTION….