the psychrometric chart psychrometric properties in ees ... - the...the psychrometric chart...
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Department of Mechanical Engineering
ME 322 – Mechanical Engineering
Thermodynamics
Lecture 34
The Psychrometric Chart
Psychrometric Properties in EES
Air Conditioning Processes
Moist Air Properties
2
Using a combination of the ideal gas and real-fluid models
along with ideal mixing we have discovered,
# ,a w w
a
Hh h T h T P
m
Note: The textbook puts a ‘#’ sign on the enthalpy to remind
us that its mass basis is per lbm of dry air in the mixture.
Applying this notation to the heating problem from Lecture 37,
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
PT
V
2
2
14 psia110 F
PT
Moist Air Properties
3
Question: Is there an easier way to determine h# ?
Answer: YES!
As with pure fluids, a real-fluid mixture formulation has been
developed for the dry air-water vapor mixture. The
formulation uses the real-fluid behavior of the components
along with real mixing. As such, the resulting formulation is
complex. Fortunately, it can be displayed graphically in a
series of charts known as the psychrometric charts. The
psychrometric charts are published by ASHRAE. They were
drawn at the Center for Applied Thermodynamic Studies
(CATS) at the University of Idaho. There are 5 IP (English
unit) charts, and 7 SI charts.
Department of Mechanical Engineering
ME 322 – Mechanical Engineering
Thermodynamics
Psychrometric Properties
The Psychrometric Chart
Chart No. 1 is available on the BbLearn website! 5
Wet Bulb and Dry Bulb Temperatures
Wet bulb and dry bulb temperatures are thermodynamic
properties of the moist air mixture. They are measured with a
device known as a psychrometer.
6
Psychrometric Properties using the Chart
7
Given: Moist air mixture in our classroom; T, Twb measured
with a sling psychrometer.
Find: The following psychrometric properties of the mixture
using the Psychrometric Chart
(a) relative humidity
(b) humidity ratio
(c) specific enthalpy
(d) specific volume
(e) dew point temperature
T
#h
dpT
wbT
v
dpT
Psychrometric Properties in EES
8
Given: Moist air at 1 atm,
85°F, 40% relative
humidity
Find: The following
psychrometric properties
of the mixture using EES
(a) humidity ratio
(b) specific enthalpy
(c) specific volume
(d) wet bulb temperature
(e) dew point temperature
Department of Mechanical Engineering
ME 322 – Mechanical Engineering
Thermodynamics
Air Conditioning Processes
The Five HVAC Processes
• Heating
– With and without humidification
• Adiabatic humidification
• Cooling
– With and without dehumidification
• Adiabatic mixing of moist air streams
• Evaporative cooling
10
Heating Without Humidification
11
HQ
1 2
1T 2T
1
2#
1h
#
2h
12
1 2
# #
2 1H aQ m h h
• Constant humidity ratio
• Relative humidity
decreases
• Outlet relative humidity
may be too dry to be
comfortable
Heating with Humidification
12
HQ
1 2
water
1T 1'T
1
1'
#
1h
#
1'h
11'
1
1'
2
#
2h
22
# #
2 1 2 1
H
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
Adiabatic Humidification
13
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 (2a,
2b, or 2c) depends on the
state of the water being
injected for humidification
Cooling Without Dehumidification
14
CQ
1 2
1T2T
1
2
#
1h
#
2h
12 1 2
# #
1 2C aQ m h h
This process is not very
common in HVAC systems.
Often, the surface of the
heat exchanger is below
the dew point which causes
water to condense
1'T
Cooling With Dehumidification
15
CQ
1 2
1T2T
1
2
#
1h
#
2h
121
If Twater is not given, it is
common to assume
that it is equal 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 2
C
water
a
Qh h h
m
Adiabatic Mixing
16
1 (cold)
2 (hot)
#
1h
#
2h
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 3
1 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
1-2-3 are on a straight line!
Evaporative Cooling
17
1 2
water
1T
1
11
# #
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
humidification
process when the
water used for
humidification is
colder than T1
This system works very well in hot,
dry climates. Notice that there can
be a significant increase in the
humidity levels (both relative
humidity and humidity ratio).