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7/24/2019 L34 - The Psychrometric Chart
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Moist Air Properties
2
Using a combination of the ideal gas and real-fluid models
along ith ideal mi!ing e ha"e disco"ered#
( ) ( )# ,a w wa
Hh h T h T P
m= = +
$ote% The te!tboo& puts a '() sign on the enthalpy to remindus that its mass basis is per lbm of dry air in the mi!ture*
Applying this notation to the heating problem from +ecture ,#
( ) ( )2 2 2 1 1 1a a w a wQ m h h h h = + + & &
( )# #2 1aQ m h h= & &
Q
1
1
1
14 psia65 F0.50
300 cfm
P
T
V
=
=
=
=&
2
2
14 psia110 F
P
T
=
=
7/24/2019 L34 - The Psychrometric Chart
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Moist Air Properties
,
Question% .s there an easier ay to determine h
(
/Answer% 0ES1
As ith pure fluids# a real-fluid mi!ture formulation has been
de"eloped for the dry air-ater "apor mi!ture* The
formulation uses the real-fluid beha"ior of the components
along ith real mi!ing* As such# the resulting formulation is
comple!* ortunately# it can be displayed graphically in a
series of charts &non as the psychrometric charts* The
psychrometric charts are published by AS34AE* They eredran at the Center for Applied Thermodynamic Studies
5CATS6 at the University o !daho* There are 7 .P 5English
unit6 charts# and S. charts*
7/24/2019 L34 - The Psychrometric Chart
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Department ofMechanical Engineering
ME 322 Mechanical Engineering
Thermodynamics
"sychrometric "roperties
The Psychrometric Chart
7/24/2019 L34 - The Psychrometric Chart
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Chart $o* 8 is a"ailable on the 9b+earn ebsite15
7/24/2019 L34 - The Psychrometric Chart
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:et 9ulb and Dry 9ulb Temperatures
:et bulb and dry bulb temperatures are thermodynamic
properties of the moist air mi!ture* They are measured ith
a de"ice &non as a psychrometer*
;
7/24/2019 L34 - The Psychrometric Chart
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Psychrometric Properties using the Chart
#iven% Moist air mi!ture in our classroom< T# Tbmeasured
ith a sling psychrometer*
$ind%The folloing psychrometric properties of the mi!ture
using the Psychrometric Chart
5a6 relati"e humidity
5b6 humidity ratio
5c6 specific enthalpy
5d6 specific "olume
5e6 de point temperature
T
#h
dpT
wbT
v
dpT
7/24/2019 L34 - The Psychrometric Chart
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Psychrometric Properties in EES
=
#iven% Moist air at 8 atm#
=7># ?@ relati"e
humidity
$ind%The folloing
psychrometric propertiesof the mi!ture using EES
5a6 humidity ratio
5b6 specific enthalpy
5c6 specific "olume5d6 et bulb temperature
5e6 de point temperature
7/24/2019 L34 - The Psychrometric Chart
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Department ofMechanical Engineering
ME 322 Mechanical Engineering
Thermodynamics
Air &onditioning "rocesses
7/24/2019 L34 - The Psychrometric Chart
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The i"e 3BAC Processes
3eating :ith and ithout humidification
Adiabatic humidification
Cooling :ith and ithout dehumidification
Adiabatic mi!ing of moist air streams
E"aporati"e cooling
8@
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3eating :ithout 3umidification
88
HQ&
1 2
1T 2T
1
2#
1h
#
2h
1
2
1 2 =
( )# #2 1H aQ m h h= & &
Constant humidity ratio 4elati"e humidity
decreases utlet relati"e humidity
may be too dry to becomfortable
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3eating ith 3umidification
82
HQ&
1 2
water
1T 1'T
1
1'
#
1h
#
1'h
1 1'1
1'
2
#
2h
22
( ) ( )# #2 1 2 1H
water
a
Qh h h
m =
&
&
# #
1 2H a water water aQ m h m h m h+ + =&
& & &
1 2
2 1
2 1
w water w
water w w
a a a
m m m
m m m
m m m
+ =
= =
& & &
& & &
& & &
2T
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Adiabatic 3umidification
8,
1 2
water
1T
1
1 1
2b
( ) ( )# #2 1 2 1 0waterh h h =
# #
1 2a water water am h m h m h+ =& & &
1 2
2 1
2 1
w water w
water w w
a a a
m m m
m m m
m m m
+ =
= =
& & &
& & &
& & &
2c2a
The location of state 2 52a#2b# or 2c6 depends on the
state of the ater being
inFected for humidification
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Cooling :ithout Dehumidification
8?
CQ&
1 2
1T2T
1
2
#
1h
#
2h
1
2 1 2 =
( )# #1 2C aQ m h h= & &
This process is not "ery
common in 3BAC systems*
ften# the surface of the
heat e!changer is belo
the de point hich causesater to condense
1'T
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Cooling :ith Dehumidification
87
CQ&
1 2
1T2T
1
2
#1h
#
2h
12
1
.f Tateris not gi"en# it iscommon to assume
that it is eGual to T2
water
dpT
2
# #
2 1c a water water aQ m h m h m h+ + =&
& & &
2 1
1 2
1 2
w water w
water w w
a a a
m m m
m m m
m m m
+ =
= =
& & &
& & &
& & &
( ) ( )# #1 2 1 2C
water
a
Qh h h
m =
&
&
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Adiabatic Mi!ing
8;
1 (cold)
2 (hot)
#
1h
#
2
h
1
2
1
2
# # #
1 1 2 2 3 3a a am h m h m h+ =& & &
3 (warm)
3
#
3h
3
1 2 3a a am m m+ =& & &
( )# # #1 1 2 2 1 2 3# #
1 2 3
# #
2 3 1
a a a a
a
a
m h m h m m h
m h h
m h h
+ = +
=
& & & &
&
&
( )1 1 2 2 1 2 31 2 3
2 3 1
a a a a
a
a
m m m m
m
m
+ = +
=
& & & &
&
&
1 1 2 2 3 3a a am m m + =& & &
# #
2 3 2 3
# #
3 1 3 1
h h
h h
=
8-2-, are on a straight line1
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E"aporati"e Cooling
8
1 2
water
1T
1
1
1
( ) ( )# #2 1 2 1 0waterh h h =
# #
1 2a water water am h m h m h+ =& & &
1 2
2 1
2 1
w water w
water w w
a a a
m m m
m m m
m m m
+ =
= =
& & &
& & &
& & &
2
2
2
2T
This is the adiabatic
humidificationprocess hen the
ater used for
humidification is
colder than T8
This system or&s "ery ell in hot#
dry climates* $otice that there canbe a significant increase in the
humidity le"els 5both relati"e
humidity and humidity ratio6*