2 Psychrometric Processes
Transcript of 2 Psychrometric Processes
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Psychrometric processes
Introduction:
In the design and analysis of air conditioning plants, the
processes being performed on air. Once identified, the
processes can be analyzed by applying the laws of
conservation of mass and energy. All these processescan be plotted easily on a psychrometric chart. This is
the changes taking place in important properties such as
processes that air undergoes in a typical air conditioning
plant are discussed below.
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Fig. 1: Sensible cooling process O-A on psychrometric chart
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Fig.2.:Sensibleheatingprocessonapsychrometric chart
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Example:-
heats 0.1 m3/s of air from 15oC and 80% RH to 55oC? The
barometric pressure is 101.325 kPa.
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C):-c) Cooling and dehumidification (Process O
-
it in contact with a cold surface as shown in Fig. 3, some
of the water vapor in the air condenses and leaves the airstream as liquid, as a result both the temperature and
humidity ratio of air decreases as shown.
The heat and mass transfer rates can be expressed in
conservation of mass and conservation of energy
equations as given below:
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Fig.3 : Cooling and dehumidification (Process O-C)
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By applying mass balance for the water:
By applying energy balance:
from the above two equations, the load on the cooling coil,Q is given by:
the 2 term on the RHS of the above e uation is normall small
compared to the other terms, so it can be neglected.
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It can be observed that the coolin and de-humidification
process involves both latent and sensible heat transfer
processes, hence, the total, latent and sensible heattransfer rates (Qt, Ql and Qs) can be written as
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A lower value of SHF, say 0.6, implies a high latent heat load
such as that occurs in a humid climate.From Fig. 3, it can be seen that the slope of the process line
Fromthe
definition
of
SHF,
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Fromtheaboveequations,wecanwritetheslopeas:
Thuswecanseethattheslopeofthecoolinganddehumidification
lineis
purely
afunction
of
the
sensible
heat
factor,
SHF.
Hence,
we
candrawthecoolinganddehumidificationlineonpsychrometric
c ar e n a s a ean e are nown. nsomes an ar
psychrometric charts,aprotractorwithdifferentvaluesofSHFis
andinparalleltothegivenSHFlinefromtheprotractorasshownin
Fig.4.
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Fi . 4: A s chrometric chart with rotractor for SHF lines
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By pass factor:
In Fig.3, the temperature Ts is the effective surface
tem erature of the coolin coil, and is known as a aratus
dew-point (ADP) temperature. In an ideal situation, when
all the air comes in perfect contact with the cooling coilsur ace, en e ex empera ure o a r w e same as
ADP of the coil. However, in actual case the exit
apparatus dew-point temperature due to boundary layerdevelopment as air f lows over the cooling coil surface and
also due to temperature variation along the fins etc.
Hence, we can define a by-pass factor (BPF) as:
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It can be easily seen that, higher the by-pass factor
and the cooling coil temperature. When BPF is 1.0, all the
air b - asses the coil and there will not be an coolin or
de-humidification. In practice, the by-pass factor can be
increased by increasing the number of rows in a coolingco or y ecreas ng e a r ve oc y or y re uc ng e n
pitch.
,
is given by:
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Example:
coolingcoil
at
25oC
and
50%
RH
and
leaves
the
cooling
coil
at
11oC
and90%RH.Theapparatusdewpointofthecoolingcoilis7oC.
Find:
a)The
required
cooling
capacity
of
the
coil.
ens e ea ac or or eprocess.
c)Bypassfactorofthecoolingcoil.
.
condensatewater
to
leave
the
coil
at
ADP
(hw=
29.26
kJ/kg)
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Heating and Humidification (Process O-D):
During winter it is essential to heat and humidify the room
.
heating the air and then adding water vapour to the air
stream through steam nozzles as shown in the fig.6.
Mass balance of water vapor for the control volume
, . .
Where m a is the mass flow rate of dry air.
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Fig. 6:- Heating and Humidification Process .
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From ener balance:
WhereQ h is the heat supplied through the heating coil .
Hw is the enthalpy of steam.
mass transfer, we can define a sensible heat factor for
the rocess in a wa similar to that of a coolant anddehumidification process.
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e) Cooling & humidification (Process O-E):
, ,
temperature drops and its humidity increases. This
rocess is shown in Fi . 7.
This can be achieved by spraying cool water in the air
stream. The temperature of water should be lower thanthe dry-bulb temperature of air but higher than its dew-
DPT w O .
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Fig. 7: Cooling and humidification process
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Itcanbeseenthatduringthisprocessthereissensible
ea
rans er
rom
a r
o
wa er
an
a en
ea
rans er
fromwatertoair.Hence,thetotalheattransferdepends
.
Ifthetemperatureofthewatersprayedisequaltothe
,
willbezeroasthesensibleheattransferfromairtowater
.
Ifthe
water
temperature
is
greater
than
WBT,
then
there
.
temperatureislessthanWBT,thenthenetheattransfer
.
Theprocessofcoolingandhumidificationisencountered
,
coolingtowers
etc.
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F):-humidification (Process O-Heating and de
This process can be achieved by using a hygroscopic
,
the moisture. If this process is thermally isolated, then
the enthalpy of air remains constant, as a result thetemperature of air increases as its moisture content
decreases as shown in Fig. 8.
In general, the absorption of water by the hygroscopic
released during this process, which is transferred to air
and the enthalpy of air increases.
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Mixingofairstreams:
Mixing of air streams at different states is commonly
encountered in many processes, including in air
conditioning. Depending upon the state of the individual
streams, the mixing process can take place with orw ou con ensa on o mo s ure.
.
mixing of two moist air streams during which nocondensation of moisture takes place.
As shown in the figure, when two air streams at state
points 1 and 2 mix, the resulting mixture condition 3 can
e o a ne rom mass an energy a ance.
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g. . x ng o wo a r s reams w ou con ensa on
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From the mass balance of dry air and water vapor:
From the above equations, it can be observed that
the final enthalpy and humidity ratio of mixture are
weighted averages of inlet enthalpies and humidity
ratios
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ii) Mixing with condensation:
As shown in Fi . 10, when ver cold and dr air mixes
with warm air at high relative humidity, the resulting
mixture condition may lie in the two-phase region, as aresu ere w e con ensa on o wa er vapor an some
amount of water will leave the system as liquid water. Due
,
wil l be less than that at point 4. Corresponding to this willbe an increase in temperature of air due to the release of
latent heat of condensation.
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Fig. 10. Mixing of two air streams with condensation
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