Download Humidity Ratio - SNS Courseware

SNS COLLEGE OF ENGINEERING
Coimbatore-107
Subject: Thermal Engineering
Psychrometric Processes
BY
SIVAKUMAR P
AP/MECH
CONTENTS
 What is PSYCHROMETRY?
 Properties of
PSYCHROMETRY
 PSYCHROMETRIC
Processes
 PSYCHROMETRIC CHART
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.
PROPERTIES OF
PSYCHROMETRY






•
•

DEW POINT TEMPERATURE
RELATIVE HUMIDITY
HUMIDITY RATIO
DRY-BULB TEMPERATURE
WET BULB TEMPERATURE
HUMIDITY
Specific humidity
Absolute humidity
PSYCHROMETRIC RATIO
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.
Dew Point
• Condensation
occurs at the
Dew Point
Temperature
Relative Humidity
Relative
Humidity
(percentage)
50% RH
Amount of moisture that a given amount
of air is holding
=
Amount of moisture that a given amount
of air can hold
100% RH - Saturated
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
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.
PSYCHROMETRIC PROCESSES






SENSIBLE HEATING
SENSIBLE COOLING
DEHUMIDIFICATION
HUMIDFIYING
ADIABATIC COOLING
ADIABATIC MIXING
(MOIST AIR & WATER VAPOR)
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.
Q
m
m
2
1
t1
t2
h1
h2
Sensible heating
 m
 c p t
Q
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.
Q
1
t1
h1
m
m
2
t2
h2
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.
14
KChatterjea, 2008
3. Cooling and Dehumidifying: It is the removal of heat and
moisture from moist air, it involves sensible and latent
heat transfer.
Q
1
t1
h1
m
m
2
t2
h2
mw
hw
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.
16
KChatterjea, 2008
Dehumidification by Cooling
4. Heating and Humidifying: It is the addition of heat and
moisture to moist air, it also involves sensible and latent
heat transfer.
Q
1
t1
h1
m
m
mw
hw
2
t2
h2
5. HUMIDIFYING : It is the addition of moisture to moist
air without the addition of heat.
m
m
2
1
t1
t2
h1
h2
mw
hw
Removing Sensible Heat and
Moisture
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.
ADIABATIC MIXING OF MOIST AIR
& STREAM
• A psychrometric process that
invovles no net heat loss or gain
during the mixing of two air
streams.
Adiabatic mixing
• Governing equation
 m h  Q   m h
in
out
Psychrometry – Climate Classification
• Climate Classification
shows the various
approximate zones of
temperature and
humidity on the
psychrometric chart.
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KChatterjea, 2008
Breakdown of the Lines
Enthalpy
Dew
Point
100%
Saturation
Specific
Volume
Dry
Blub
Relative
Humidity
Grains of
Moisture
Wet
Blub
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
Any Questions???
-THANK YOU FOR YOUR
ATTENTION….