AMCA International Technical Seminar 2009
AIRFLOW IN A SYSTEM
Presented by:
Bill Howarth, Illinois Blower, Inc.
The Air Movement and Control Association
International (AMCA), has met the standards
and requirements of the Registered Continuing
Education Providers Program. Credit earned on
completion of this program will be reported to
the RCEPP. A certificate of completion will be
issued to each participant. As such, it does not
include content that may be deemed or
construed to be an approval or endorsement by
NCEES or RCEPP.
Learning Objectives
•
•
•
•
•
•
Describe the elements of an air system
Know the physical properties of air
Describe the effects of system components on
airflow
Understand the concept of pressure
Understand how the conservation of energy
relates to airflow
Understand an air systems operating point
MOVING AIR
Air
Air at “A”
Air
Air at “B”
AIR SYSTEM
System
Air
Air
Air at “A”
Air at
“B”
AIR SYSTEM DESIGN
PARAMETERS
• Properties of Air
• Conservation of Energy
• Friction And Friction Losses
• Fan Characteristics
PROPERTIES OF AIR
• Standard Air
• Density
• Pressure
• Temperature
STANDARD AIR
The Reference Gas
for
Air System Design
RECIPE FOR
STANDARD AIR
Ingredients:
•
•
•
•
•
1.105 X 1025 Molecules of Nitrogen (N2)
1.480 X 1023 Molecules of Oxygen (O2)
6.558 X 1021 Molecules of Argon (A)
2.190 X 1020 Molecules of Carbon Dioxide (CO2)
Pinch of other trace gases
RECIPE FOR
STANDARD AIR
Mix Well in a sealed box one foot
on a side and one foot deep. Heat
to 68F. Warning! if you are in a
vacuum, it will take 2117 pounds of
force to hold the lid on the box.
2117 lbs
lbs
 14.7 in2
12 in. x 12 in.
RECIPE FOR
STANDARD AIR
If you followed the instructions
properly, the container will have
gained in weight by 0.075 lb. The
density of standard air is:
Weight
Density 
 0.075
Volume
lb
ft3
STANDARD AIR
DEVIATIONS
Due To:
• Change In Pressure
• Change in Temperature
• Addition of other
Component(s), such as Water
STANDARD AIR
Pressure
PRESSURE
1 Cubic Foot
at 68F.
Air molecules are in continuous random
motion.
The average impact of the
molecules against the sides of a container
result in the phenomenon known as
pressure.
PRESSURE
1 Cubic Foot
at 68F.
Forcing the same number of molecules to
occupy a smaller volume (compressing the
air) will increase the frequency of the
molecular impacts, which is an increase in
pressure.
PRESSURE
1 Cubic Foot
at 600F.
Increasing energy raises the random
motion and the temperature. Pressure also
increases. But; a cubic foot of air at 600F
and 14.7 lb/in2 has fewer molecules - It is
less dense.
BAROMETRIC PRESSURE
AIR
The weight of our
atmosphere
compresses air to a
pressure of 14.7
lb/in2 or 29.92 in. Hg
(average at sea level
with 50% relative
humidity).
PRESSURE
• Absolute Pressure
• Any Pressure referenced to
absolute zero pressure.
• Barometric Pressure is an
absolute pressure.
AIR DENSITY
Density at a given temperature
and barometric pressure:
lbm Abs. press.
460 F  70 F
0.075 3 x
x
ft
29.92 in. Hg 460 F  Temp.
EFFECT OF HUMIDITY
The addition of
water vapor to
air will
decrease the
density of the
air.
PSYCHROMETRIC CHART
Approx.
Average
Increase in
Density Per
°F Wet-Bulb
Depression
Td-Tw
.000017
.000017
.000017
.000018
.000018
30
31
32
33
34
Density of Saturated Air for Various Barometric and Hygrometric
Conditions—Ibm/ft3
Increase in
Density Per
Barometric Pressure in. Hg
0.1 in. Hg
Rise In
28.5
29.0
29.5
30.0
30.5
31.0
Barometer
.07703
.07839
.07974
.08110
.08245
.08380
.00027
.07687
.07822
.07957
.08093
.08228
.08363
.00027
.07671
.07806
.07940
.08075
.08210
.08345
.00027
.07654
.07789
.07924
.08053
.08193
.08327
.00027
.07638
.07772
.07907
.08041
.08175
.08310
.00027
35
36
37
38
39
.07621
.07605
.07589
.07573
.07557
.07756
.07739
.07723
.07706
.07690
.07890
.07873
.07856
.07840
.07823
.08024
.08007
.07990
.07973
.07956
.08153
.08141
.08123
.08106
.08089
.08292
.08274
.08257
.08239
.08222
.00027
.00027
.00027
.00027
.00027
.000018
.000018
.000019
.000019
.000019
40
41
42
43
44
.07541
.07525
.07509
.07493
.07477
.07674
.07657
.07641
.07625
.07609
.07806
.07790
.07773
.07757
.07740
.07939
.07922
.07905
.07889
.07872
.08072
.08055
.08038
.08021
.08004
.08205
.08187
.08170
.08153
.08135
.00027
.00026
.00026
.00026
.00026
.000019
.000020
.000020
.000020
.000020
45
46
47
48
49
.07461
.07445
.07429
.07413
.07397
.07592
.07576
.07560
.07544
.07528
.07724
.07707
.07691
.07674
.07658
.07855
.07838
.07822
.07805
.07788
.07986
.07970
.07953
.07936
.07919
.08118
.08101
.08084
.08066
.08049
.00026
.00026
.00026
.00026
.00026
.000020
.000021
.000021
.000021
.000022
50
51
52
53
54
.07381
.07366
.07350
.07334
.07318
.07512
.07496
.07479
.07464
.07447
.07642
.07625
.07609
.07593
.07576
.07772
.07755
.07739
.07722
.07706
.07902
.07885
.07868
.07852
.07835
.08032
.08015
.07998
.07981
.07964
.00026
.00026
.00026
.00026
.00026
.000022
.000022
.000023
.000023
.000023
55
56
57
58
59
.07302
.07287
.07271
.07255
.07240
.07431
.07415
.07399
.07383
.07367
.07560
.07544
.07528
.07512
.07495
.07689
.07673
.07656
.07640
.07623
.07818
.07801
.07784
.07768
.07751
.07947
.07930
.07913
.07896
.07879
.00026
.00026
.00026
.00026
.00026
.000024
.000024
.000025
.000025
.000025
Dry-Bulb
Temp.
°F
60
.07224
.07352
.07479
.07607
.07734
.07862
.00026
.000026
61
.07208
.07336
.07463
.07590
.07718
.07845
.00026
.000026
62
.07193
.07320
.07447
.07574
.07701
.07828
.00026
.000027
63
.07177
.07304
.07430
.07557
.07684
.07811
.00026
.000027
64
.07161
.07288
.07414
.07541
.07668
.07794
.00026
.000028
Note: Approx. average decrease in density per 0.10F rise in dry-bulb temperature equals .000017 lbm/ft3.
Approx.
Average
Increase in
Density Per
°F Wet-Bulb
Depression
Td-Tw
.000028
.000029
.000029
.000030
.000030
65
66
67
68
69
Density of Saturated Air for Various Barometric and Hygrometric
Conditions—Ibm/ft3
Increase in
Density Per
Barometric Pressure in. Hg
0.1 in. Hg
Rise In
28.5
29.0
29.5
30.0
30.5
31.0
Barometer
.07145
.07272
.07398
.07525
.07651
.07777
.00026
.07130
.07256
.07382
.07508
.07634
.07760
.00026
.07114
.07240
.07366
.07492
.07618
.07744
.00026
.07098
.07224
.07350
.07475
.07601
.07727
.00026
.07083
.07208
.07333
.07459
.07584
.07710
.00026
70
71
72
73
74
.07067
.07051
.07035
.07020
.07004
.07192
.07176
.07160
.07144
.07128
.07317
.07301
.07285
.07268
.07252
.07442
.07426
.07410
.07393
.07377
.07568
.07551
.07534
.07517
.07501
.07693
.07676
.07659
.07642
.07625
.00026
.00025
.00025
.00025
.00025
.000031
.000031
.000032
.000033
.000033
75
76
77
78
79
.06988
.06972
.06956
.06940
.06925
.07112
.07096
.07080
.07064
.07048
.07236
.07220
.07203
.07187
.07171
.07360
.07343
.07327
.07310
.07294
.07484
.07467
.07451
.07434
.07417
.07603
.07591
.07574
.07557
.07540
.00025
.00025
.00025
.00025
.00025
.000034
.000034
.000035
.000036
.000036
80
81
82
83
84
.06909
.06893
.06877
.06861
.06845
.07032
.07015
.07000
.06983
.06967
.07155
.07138
.07122
.07105
.07089
.07277
.07261
.07244
.07227
.07211
.07400
.07383
.07366
.07349
.07333
.07523
.07506
.07489
.07472
.07454
.00025
.00025
.00024
.00024
.00024
.000037
.000033
.000039
.000039
.000040
85
86
87
88
89
.06829
.06812
.06796
.06780
.06764
.06950
.06934
.06917
.06901
.06885
.07072
.07056
.07039
.07022
.07005
.07194
.07177
.07160
.07143
.07126
.07316
.07299
.07281
.07264
.07247
.07437
.07420
.07403
.07385
.07368
.00024
.00024
.00024
.00024
.00024
.000041
.000042
.000043
.000043
.000044
90
91
92
93
94
.06748
.06731
.06715
.06698
.06682
.06868
.06852
.06835
.06818
.06801
.06989
.06972
.06955
.06938
.06921
.07109
.07092
.07075
.07058
.07041
.07230
.07213
.07195
.07178
.07161
.07351
.07333
.07316
.07298
.07280
.00024
.00024
.00024
.00024
.00024
.000045
.000046
.000047
.000048
.000049
95
96
97
98
99
.06665
.06648
.06632
.06615
.06598
.06785
.06768
.06751
.06734
.06717
.06904
.06887
.06870
.06853
.06835
.07024
.07006
.06989
.06972
.06954
.07143
.07126
.07108
.07091
.07073
.07263
.07245
.07227
.07209
.07191
.00024
.00024
.00024
.00024
.00024
.000050
.000051
.000052
.000053
.000054
Dry-Bulb
Temp.
°F
100
.06581
.06700
.06818
.06937
.07055
.07174
.00024 .000055
Note: Approx. average decrease in density per 0.10F rise in dry-bulb temperature equals .000017 lbm/ft3.
GAGE PRESSURE
Barometric
Pressure
Water
Gauge Pressure is a
Differential Pressure.
1 in. wg
STATIC PRESSURE
Ai
r
Water
Barometric
Pressure
1 in. wg
Static Pressure
Fan Static Pressure is a gage
pressure, indicating
compression of the air.
VELOCITY PRESSURE
Barometric
Pressure
Ai
r
1 in. wg
Water
Velocity
Pressure
Velocity pressure is a measurement of the energy
needed to accelerate air to a given velocity.
VelocityPressure  VP 
2
Kenetic Energy
Volume
 Velocity
VP  
x Density

1097


TOTAL PRESSURE
Total Pressure=
Static Pressure + Velocity Pressure
or
TP  SP  VP
ACFM vs. SCFM
1 Cubic Foot
at 600F.
1 Cubic Foot
at 600F.
1 Cubic Foot
at 68F.
Actual Cubic Feet Per Minute (ACFM)
Standard Cubic Feet Per Minute (SCFM)
ACFM  SCFM
CONSERVATION
OF
ENERGY
BERNOULLI'S LAW
For ducted airflow which is:
• Constant with time
• Incompressible
• Without friction
TP  SP  VP  Constant
(If we neither add nor subtract
energy, energy is constant.)
FLOW THROUGH A NOZZLE
Area 2
Airflow
TP1
Pitot Tube
TP1  TP2  SP2  VP2
VP2  TP1  SP2
SP1
 VP2 
Velocity2  

Density


CubicFeet
CFM 
 Velocity2 x Area 2
Minute
0.5
x 1097
BERNOULLI'S LAW
• May be used in system calculations
wherever friction can be ignored.
• Do NOT use for:
• Abrupt
Expansion
• Abrupt
Contraction
FRICTION
AND
FRICTION LOSSES
TOTAL PRESSURE
IN AN AIR SYSTEM
Duct
Loss
Total
Pressure
Duct Length
Total Pressure declines
as duct length increases.
FRICTION LOSS
• Caused by non-uniform velocities
across the ductwork, coupled with
the viscosity of air.
• Always results in the conversion of
Total Pressure to Heat
• Turbulence (irregular or chaotic air
flow) will amplify the friction loss.
LOSS FACTORS FOR
ROUND ELBOWS
Coefficient
R
D
Loss C x VP
R/D
0.50
0.75
1.00
1.50
2.00
2.50
C
0.71
0.33
0.22
0.15
0.13
0.12
LOSS FACTORS FOR
STRAIGHT DUCTS
D
100
Coefficient
Loss C x VP
D
0.25
C per 100 Ft.
0.33
6.53
0.50
3.95
0.67
2.78
0.75
2.40
1.00
1.69
1.33
1.19
1.67
0.91
2.00
0.73
2.50
0.56
3.00
0.45
3.50
0.38
4.00
0.32
5.00
0.25
9.35
SYSTEM LOSSES
Duct Friction Chart
• Based on
standard air, 0.075
lbm/ft3 .
• This chart based
on galvanized
ducts with Beaded
slip joints every
48” (=0.0003).
• Other charts
available.
LOSSES IN A REAL
AIR SYSTEM
• Add losses for each component.
• Add a safety factor to all for the impact
of one component connected directly to
the next.
Example:

AIR SYSTEMS
Basis for development
of an Air System
• Ventilation Rate
• Air Changes/Hour
• Face Velocity
• Exhaust
Requirements
• References:
• Fan Application Manual
• ASHRAE Handbooks
• Industrial Ventilation
Guide
AIR SYSTEM
Convert Ventilation Rate in to Flow Rate (CFM)
Develop a detailed duct system layout.
AIR SYSTEMS
Don’t:
Do:
• Calculate:
• Actual Cubic Feet Per
Minute
• Static Pressure
Requirement
• Air Density
• Include all entrance
and discharge points
• Pay careful attention
to fan entry and exit
conditions
• Simplify component
losses
• Abruptly change
velocity through
the air system
• Neglect System
Effects on the fan
• Inlet and Outlet
• Density
SYSTEM CURVE
2
CFM


2
 C x VP  C 
x
0.075

Constant
x
CFM
 Area x 1097 
System Resistance Curve
System Losses Plotted
Pressure
System
Loss
CFM
THE FAN’S JOB
The purpose of a fan is to
supply an air system with
energy (in the form of
pressure) necessary to
maintain airflow.
FAN
CHARACTERISTICS
FANS
• There are many types of fans.
• For each type, there may be many
sizes.
• All fans have one thing in common:
Accurate prediction of aerodynamic
performance requires a test.
THE AERODYNAMIC
PERFORMANCE TEST
Power
At:
Pressure
Constant
Speed,
Known
Density
Airflow
THE FAN LAWS
• Are used to calculate fan
performance at:
• Other Speeds
• Other Densities
• Other Fan Sizes
THE FAN LAWS
First Law:
3
 DIA2   RPM 2 
CFM 2  CFM1  
 

 DIA1   RPM 1 
THE FAN LAWS
Second Law:
2
2
 DIA2   RPM 2    2 
SP2  SP1  
 
  
 DIA1   RPM 1   1 
THE FAN LAWS
Third Law:
5
3
 DIA2   RPM 2    2 
H 2  H1  
 
  
 DIA1   RPM 1   1 
THE FAN LAWS
P
r
e
s
s
u
r
e
P
o
w
e
r
Airflow
Changes in Speed
FAN SELECTION
Airflow
Desired CFM
Select a fan which will generate the
required pressure at the desired airflow.
FAN SELECTION
• There is only one intersection
between the fan curve and
system curve.
• Fans are load matching
devices.
• Fans handle ACFM only.
OPERATING POINT
Fan - Air System Interaction
FAN SELECTION
COROLLARIES
• Any air system fan which
generates the required system
pressure will also deliver the
required airflow.
• If an air system fan generates the
specified static pressure but not
the desired airflow, the system
resistance has been miscalculated.
FAN PERFORMANCE
TOLERANCES
• Account for:
• Test Uncertainty
• Manufacturing
Imperfections
AMCA TOLERANCES
• The fan must perform within 2.5% of its air
performance rating and within 5% of its
power rating.
• To meet rated performance, the fan RPM
might have to be increased up to 2.5%, and
the power increased up to 5% of the rated
power.
• The AMCA Tolerances DO NOT account for
System Effect or for errors in the system
calculations.
Questions?