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PSYCHROMETRIC CHART
DR HAMID YUSOFF
MEC 351
CHAPTER 2 PSYCHROMETRIC CHART
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CHAPTER 2 PSYCHROMETRIC CHART
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2.1 Atmosphere presure and moisture
2.2 What is a Psychrometric Chart?
2.3 Psychrometric Processes: Sensible Cooling and Sensible Heating of Air
2.4 Psychrometric Processes: Heating & Humidification
2.5 Psychrometric Processes: Cooling with dehumidification2.6 Psychrometric Processes: Cooling with Adiabatic humidification (Water injection)
2.7Psychrometric Processes: Heating with Adiabatic dehumidification
(sorbent/desiccants)
2.8 Psychrometric Processes: mixing of two streams of air
CHAPTER 2 CONTENTS
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Air is a mixture of N2, O2and small amount of other
gasses.
Atmospheric air:Air in the atmosphere normally
contains some water vapor (a moisture) and isreferred to as atmospheric air.
Dry air:Air that contains no water vapor is called dry
air. The temperature of air in air-conditioning
application ranges between -10OC to 50OC.
2.1 Atmosphere presure and moisture
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Taking 0OC as reference temperature, enthalpy
and enthalpy change of dry air can be
determined from
In air-conditioning processes we are
concerned with the changes in the enthalpy
TCkgkJTCh OPairdry ./005.1 kgkJ
TCkgkJTCh OPairdry ./005.1
h
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The atmospheric air can be treated as a n ideal-gas mixture whose pressure is the sum of thepartial pressure of dry air Paand that of watervapor Pv.
where
p = total pressure of air (Pa, N/m2) pa=partial pressure dry air(Pa, N/m
2)
pw=partial pressure water vapor(Pa, N/m2)
Vapor Pressure:The partial pressure of watervapor is usually referred to as the vapor pressure.It is the pressure water vapor would exert if itexisted alone at the temperature and volume of
atmospheric air.
va PPP kPa
Daltons Law of Partial Pressures in Moist Air
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Absolute or specific humidity:
The mass of water vapor present in a unit
mass of dry air. [Note: Also called humidityratio]
(kg water vapor / kg dry air)a
v
m
m
a
v
aa
vv
aaa
vvv
a
v
P
P
RP
RP
TRVP
TRVP
m
m622.0
v
v
PP
P
662.0
For dry air: 0
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Saturated Air:
There is a limit on the amount of vaporthe
air can hold at a given temperature.
Air that is holding as much moisture as it can
at a given temperature is called saturated air.
Any moisture introduced into saturated air will
condense.
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Relative Humidity ():The amount of moisture the
air holds relative to the maximum amount of
moisture the air can hold at the same temperature.
where
The relative humidity ranges from 0 for dr y air to 1 for
saturated air.
See Example 13.1 (Text Book)
g
v
vg
vv
g
v
P
P
TRVP
TRVP
m
m
Tsatg PP @
gPP
622.0g
g
PP
P
622.0
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Total enthalpy of atmospheric air is the sum ofthe enthalpies of dry air and water vapor:
(kJ/kg dry air)
vvaava hmhmHHH
ga hhh
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Dry bulb temperature (Tdb):The ordinary
temperatureof the atmospheric air is
refereed to as the dry bulb temperature.
Dewpoint temperature (Tdp):The
temperature at which condensation begins
when the air is cooled at constant pressure.
vPsatdp TT @
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When the temperature of a cold drink is
below the dew-point temperature of the
surrounding air, it sweats.
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Relative humidity and Sp. Humidity of air can
be determined by measuring the adiabatic
saturation temperature of air, which is thetemperature air attains after flowing over
water in a long adiabatic channel until it is
saturated
21
212
1
fg
fP
hh
hTTCg
2
2
2
622.02
g
g
PP
P
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Wet bulb temperature
(Twb
):A thermometer
whose bulb is covered
with a cotton wick
saturated with water
and to blow air over thewick as shown in figure.
The temperature
measure in this manneris called wetbulb
temperature.
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Sling Psychrometer:The wet bulb
temperature can alsobe measured byplacing the wetwicked thermometer in
a holder attached to ahandle and rotatingthe holder rapidly. A
device that works onthis principle is calledSling Psychrometer.
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2.1 What is a Psychrometric Chart? Psychrometric charts are graphic representations of the
psychrometric properties of moist air. By using psychrometric
charts HVAC engineers can graphically analyzedifferent types of
psychrometric processes and find solution to many practicalproblems without having to carry out long and tedious
mathematical calculations.
The psychrometric chart looks complicated with vast numbers of
lines and curves in it, but is very easy to understand if you know
the basic properties of air. You will also understand its worth
when you actually use it considering the fact that you wont have
to use any formulae to find the properties of air in different
conditions, all you will have to know is two parameters of air and
the rest are easily found on the chart.
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Various Lines and Curves in the Psychrometric Chart
Dry-bulb temperature
It is temperature of the air measured with on ordinary thermometer.
Dew point temperature: The remperature at which water by evaporating into moist air at
given dry-bulb temperature and humidity ratio (w) can bring the air to saturation
adiabatically at the same temperature Twb while the pressure is maintained constant.
Relative humidity: Relative humidity is defined as the ratio of the partial pressure of the
water vapour in moist air at given temperature T, to the partial pressure of the watervapour in saturated air, at the same temperature T.
Specific humidity/humidity ratio: As the mass of water vapour in kg which is associated
with one kilogram of dry air water vapour mixture.
Percentage saturation: The ratio of the moisture content of moist air at a given
temperature, T, to the moisture content of saturated air at the same temperature T.
Specific volume: Is the volume in cubic metres of one kilogram of dry air together with the
mass of water vapour associated with it.
Sensible heat: Heat which produces a change of temperature in a body is known as sensible
heat.
Latent heat: Heat which is necessary to produce a change of state of a material at a constant
temperature
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The psychrometric chart
A AbsoluteHumidity Axis
B Specific VolumeCDry Bulb Axis
D
RelativeHumidity
D
Dew PointE
EnthalpyF
Wet Bulb &
Saturation CurveG
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For a saturated air, the drybulb, wetbulb
and dewpoint temperature are identical.
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AirConditioning Processes
Air-conditioning processes include:
Simple heating (raising the temperature),
Simple cooling (lowering the temperature)
Humidifying (adding moisture), and
Dehumidifying (removing moisture).
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2.3 Psychrometric Processes: Sensible Cooling
and Sensible Heating of Air The basic function of the air-conditioners is to cool the air absorbed from
the room or the atmosphere, which is at higher temperatures.
In HVAC systems, air is typically cooled by passing it over a cooling coil.
Most cooling coils are finned-tube heat exchangers in which cool water or
refrigerant flows through tubes, and the tubes have external fins to
increase heat transfer area. If the air is cooled below the dew point
temperature, water will condense and must be drained from the bottom of
the cooling coil.
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2.3 Psychrometric Processes: Sensible Cooling
Observable features of a sensible
cooling process
the dry bulb temperature decreases
the relative humidity increases
the enthalpy decreases the wet bulb temperature decreases
the specific volume decreases
the humidity ratio, vapor pressure
and dew point remains constant = =
=
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2.3 Psychrometric Processes: Sensible heating
In HVAC systems, air is typically heated by passing it over a heating coil or useof electrical strip heaters. A schematic of a cooling coil is shown below.
Observable features of a sensible
heating process
The dry bulb temperature increases
the relative humidity decreases
the enthalpy increases
the wet bulb temperature increases
the specific volume increases
the humidity ratio, vapor pressureand dew point remains constant
= =
=
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2.3 Psychrometric Processes:
Governing Equation for Sensible Heating and Cooling
The heat transfer is given by; =
= ( )
= 1.005 + 1.888 ( )
= 1.005 + 1.88
= 1.005 + (2501 + 1.88)
=
is the ratio of volume flow rate to the specific volume
=
= ()
= 1 ,
= 2 ,
= (
)
= is the enthalpy of stream 1, (kJ/kg)
= is the enthalpy of stream 2, (kJ/kg)
= specific heat of moist air
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2.5Psychrometric Processes: Heating & Humidification
Observable features of a humidification
process are:
1. Humidification adds moisture to the air
which increase the absolute humidity
2. Water is added in vapor form
3. Water is converted from liquid to gas4. There is an increase in the energy level
Sensible heat added =
Latent heat added
= (3 )
= 3
h1
h3h2
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2.6Psychrometric Processes: Cooling with dehumidification
=
Observable characteristics of a cooling anddehumidifying process are:
1. dry bulb temperature decreases
2. humidity ratio decreases
3. vapor pressure decreases
4. dew point temperature decreases
5. wet bulb temperature decreases
6. enthalpy decreases (there is a decrease in
the energy level and with the loss of
energy, condensation occurs)
7. relative humidity increases
= + ( )
= ( )
= ( )
= +
=
+
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2.7 Psychrometric Processes: Cooling with Adiabatic
humidification(Water injection)
Evaporative cooling is an adiabatic process,
that is, there is no net heat loss or gain.
1. Constant wet bulb temperature
2. Decreasing dry bulb temperature
3. Increasing humidity ratio
4. Increasing dew point temperature
5. Increasing vapor pressure
6. Essentially constant enthalpy
7. Decreasing specific volume
8. There is no loss or gain of heat within thesystem because the amount of sensible
heat removed equals latent heat added to the
water.
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2.8Psychrometric Processes: Heating with Adiabatic
dehumidification(sorbent/desiccants)
Observable characteristics of a heating and
dehumidification process are:
1. dry bulb temperature increases2. humidity ratio decreases
3. dew point temperature decreases
4. vapor pressure decreases
5. relative humidity decreases
6. enthalpy remains essentially constant
7. wet bulb temperature remains essentially constant
When air is brought into contact with a desiccant
or sorption material the moisture will be
adsorbedand the air will become "drier". As this
occurs, the heat of condensation will enter the air
stream, thereby increasing its temperature. The
process is essentially adiabatic or isenthalpic. Aircan be dehumidified by passing it over
sorbents/desiccants that have affinity for
moisture. Examples; silica gel, activated alumina.
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2.9 Psychrometric Processes: mixing of two streams of air
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Psychrometric Processes: mixing of two streams of air
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Example
Solved Problem 1 -Assume
that the outside air
temperature is 32C with a
relative humidity = 60%.
Use the psychrometric chart
to determine the specifichumidity [18 gm-
moisture/kg-air], the
enthalpy h [78 kJ/kg-air], the
wet-bulb temperature Twb
[25.5C], the dew-point
temperature Tdp[23C], andthe specific volume of the dry
air v [0.89m3/kg]. Indicate all
the values determined on the
chart.
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Solved Problem 2:
Assume that the
outside air
temperature is 8C.
If the air in a room
is at 25C with a
relative humidity
= 40%, use the
psychrometric chart
to determine if the
windows of thatroom which are in
contact with the
outside will become
foggy.
The air in contact with the windows will become colder until the dew point is reached. Notice
that under the conditions of 25C and 40% relative humidity the dew point temperature is
slightly higher than 10C, At that point the water vapor condenses as the temperature
approaches 8C along the saturation line, and the windows will become foggy.
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One of the major applications
of the Psychrometric Chart is in
air conditioning, and we find
that most humans feel
comfortable when the
temperature is between 22Cand 27C, and the relative
humidity between 40% and
60%. This defines the "comfort
zone" which is portrayed on the
Psychrometric Chart as shown
below. Thus with the aid of the
chart we either heat or cool,
add moisture or dehumidify as
required in order to bring the
air into the comfort zone.
Solved Problem 3:
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Solved Problem 4:Outside air at 35C and 60%
relative humidity is to be
conditioned by cooling and
heating so as to bring the
air to within the "comfort
zone". Using the
Psychrometric Chart neatly
plot the required air
conditioning process and
estimate (a) the amount of
moisture removed [11.5g-
H20/kg-dry-air], (b) the heat
removed [(1)-(2), qcool=
48kJ/kg-dry-air], and (c) the
amount of heat added [(2)-
(3), qheat= 10kJ/kg-dry-air].
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Solved Problem 5:
Hot dry air at 40C and 10%
relative humidity passes
through an evaporative cooler.
Water is added as the air
passes through a series ofwicks and the mixture exits at
27C. Using the psychrometric
chart determine (a) the outlet
relative humidity [45%], (b) the
amount of water added [5.4g-
H20/kg-dry-air], and (c) thelowest temperature that could
be realized [18.5C].
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Solved Problem 6: Heating
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Solved Problem 7: Heating
Determine the load (kW) required to raise 0.5 m3/s of air at 10 degree C and 80% RH to 30 deg C.
What would be the final condition of air?
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Solved Problem 8: Cooling
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Solved Problem 9: Cooling with dehumidification
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Solved Problem 10: Cooling with humidification
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Solved Problem 11: Cooling with adiabatic humidifier
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CHAPTER 2 PSYCHROMETRIC CHART
/ /
Solved Problem 12: Heating with dehumidifier adiabatic process