11
System
Considerations
3. System effects for plenum discharges must also be
calculated and added to the design pressure. In a
properly designed plenum (see Duct System Effect
section), the system effect coefficient (C
o
) depends
on the orientation of the discharge relative to the fan
and duct connection (if any). For air plenums with
multiple discharges, calculate the system effect by
using the highest loss coefficient for all discharges.
In this example, the discharges are ducted so the
highest loss will be axial (C
o
"TTVNFUIBUUIF
two discharges will each handle 10,000 cfm.
Plenum Exit Velocity [ft/min]
7PMVNF<GU
3
NJO>0VUMFU"SFB<GU
2
>
<GU
3
NJO> JOYJO<JO
2
GU
2
>
<GUNJO>
System Effect
$
o
Y%FOTJUZ<MCGU
3
>Y
&YJU7FMPDJUZ<GUNJO>
2
YY
2
4. Now select a fan size from the catalog data based
on the corrected performance. The corrected static
QSFTTVSF PG JO XH QMVT UIF TZTUFN FGGFDU PG
JO XH NFBOT UIF GBO TIPVME CF TFMFDUFE GPS
20,000 cfm at 4.0 in. wg. A size 33 plenum will meet
this performance at 1290 frpm using 17.9 bhp.
(Note: The bhp does not include drive losses. Consult
AMCA Publication 203-90 for help in estimating drive
MPTTFT #BTFE PO UIF SFRVJSFE GSQN UIF VTFS IBT
the option of selecting either model QEM or QEP.
Airstream Temperature Variations
8IFO B GBO JT TFMFDUFE UXP UFNQFSBUVSFT JO UIF
airstream should be considered: Start-up and normal
PQFSBUJPO 8IJMF UIF IQ SFRVJSFE JT SFEVDFE BU IJHIFS
temperatures, the motor must be sized based the lowest
temperature that could be present in the airstream
(when air density is at its maximum value).
Reconsider the example assuming that the fan
was operating at 9,000 ft. elevation with a start-up
BJSTUSFBNUFNQFSBUVSFPG¡'BOEBOPSNBMPQFSBUJOH
UFNQFSBUVSFPG¡' /PUFUIFOFXOPSNBMPQFSBUJOH
conditions require the same density correction factor
as 13,000 ft. elevation at 70°F, so the fan selection is
still valid.)
'JSTUDBMDVMBUFIQBUUIFTUBSUVQDPOEJUJPO'PS¡'
at 9,000 ft. elevation, the air density correction factor
(Table 2A) is 1.14. Divide the cataloged bhp by the
correction factor.
CIQBUTUBSUVQ
Now calculate hp during normal operation using the
TBNFQSPDFEVSFCVUGPS¡'BUGUFMFWBUJPO
CIQEVSJOHOPSNBMPQFSBUJPO
The motor should be sized based on the larger of these
two values. So although the normal operation would
POMZSFRVJSFBIQNPUPSUIFGBOTIPVMECFTFMFDUFE
with a 20 hp motor based on the higher hp requirement
at start-up.
5"#-&" %FOTJUZ$PSSFDUJPO'BDUPST
*NQFSJBM6OJUT
ELEVATION (FEET ABOVE SEA LEVEL)
0
1000
2000
3000
4000
5000
6000
7000
8000
9000 10000 11000 12000 13000
AIRSTREAM TEMPERATURE (°F)
-50
0.77
0.80
0.83
0.86
0.89
0.93
0.96
1.00
1.03
1.07
1.11
1.15
1.20
1.24
-25
0.82
0.85
0.88
0.92
0.95
0.98
1.02
1.06
1.10
1.14
1.18
1.22
1.27
1.32
0
0.87
0.90
0.93
0.97
1.00
1.04
1.08
1.12
1.16
1.20
1.25
1.29
1.34
1.39
50
0.96
1.00
1.03
1.07
1.11
1.15
1.20
1.24
1.29
1.33
1.38
1.44
1.49
1.54
70
1.00
1.04
1.08
1.12
1.16
1.20
1.24
1.29
1.34
1.39
1.44
1.49
1.55
1.60
100
1.06
1.10
1.14
1.18
1.22
1.27
1.31
1.36
1.41
1.47
1.52
1.58
1.63
1.69
150
1.15
1.19
1.24
1.28
1.33
1.38
1.43
1.48
1.54
1.60
1.66
1.72
1.78
1.85
200
1.25
1.29
1.34
1.39
1.44
1.49
1.55
1.61
1.67
1.73
1.79
1.86
1.93
2.00
250
1.34
1.39
1.44
1.49
1.55
1.61
1.67
1.73
1.79
1.86
1.93
2.00
2.07
2.15
300
1.43
1.49
1.54
1.60
1.66
1.72
1.78
1.85
1.92
1.99
2.06
2.14
2.22
2.30
350
1.53
1.58
1.64
1.70
1.77
1.83
1.90
1.97
2.04
2.12
2.20
2.28
2.36
2.45
400
1.62
1.68
1.74
1.81
1.88
1.95
2.02
2.09
2.17
2.25
2.33
2.42
2.51
2.60
450
1.72
1.78
1.85
1.91
1.99
2.06
2.14
2.21
2.30
2.38
2.47
2.56
2.66
2.75
500
1.81
1.88
1.95
2.02
2.09
2.17
2.25
2.34
2.42
2.51
2.60
2.70
2.80
2.90
550
1.91
1.98
2.05
2.13
2.20
2.29
2.37
2.46
2.55
2.64
2.74
2.84
2.95
3.06
600
2.00
2.07
2.15
2.23
2.31
2.40
2.49
2.58
2.67
2.77
2.88
2.98
3.09
3.21
650
2.09
2.17
2.25
2.34
2.42
2.51
2.60
2.70
2.80
2.90
3.01
3.12
3.24
3.36
700
2.19
2.27
2.35
2.44
2.53
2.62
2.72
2.82
2.93
3.04
3.15
3.26
3.38
3.51
750
2.28
2.37
2.46
2.55
2.64
2.74
2.84
2.94
3.05
3.17
3.28
3.40
3.53
3.66
800
2.38
2.47
2.56
2.65
2.75
2.85
2.96
3.07
3.18
3.30
3.42
3.55
3.68
3.81
850
2.47
2.56
2.66
2.76
2.86
2.96
3.07
3.19
3.31
3.43
3.55
3.69
3.82
3.96
900
2.57
2.66
2.76
2.86
2.97
3.08
3.19
3.31
3.43
3.56
3.69
3.83
3.97
4.12
950
2.66
2.76
2.86
2.97
3.08
3.19
3.31
3.43
3.56
3.69
3.83
3.97
4.11
4.27
1000
2.75
2.86
2.96
3.07
3.19
3.30
3.43
3.55
3.68
3.82
3.96
4.11
4.26
4.42
ELEVATION (METERS ABOVE SEA LEVEL)
0
250
500
750
1000
1250
1500
1750
2000
2250
2500
2750
3000
3500
AIRSTREAM TEMPERATURE (°C)
-50
0.76
0.78
0.81
0.83
0.85
0.88
0.91
0.93
0.96
0.99
1.02
1.05
1.08
1.15
-25
0.84
0.87
0.90
0.92
0.95
0.98
1.01
1.04
1.07
1.10
1.14
1.17
1.21
1.28
0
0.93
0.96
0.99
1.02
1.05
1.08
1.11
1.14
1.18
1.21
1.25
1.29
1.33
1.41
21
1.00
1.03
1.06
1.09
1.13
1.16
1.20
1.23
1.27
1.31
1.35
1.39
1.43
1.52
50
1.10
1.13
1.17
1.20
1.24
1.27
1.31
1.35
1.39
1.44
1.48
1.52
1.57
1.67
75
1.18
1.22
1.26
1.29
1.33
1.37
1.41
1.46
1.50
1.55
1.59
1.64
1.69
1.80
100
1.27
1.31
1.35
1.39
1.43
1.47
1.52
1.56
1.61
1.66
1.71
1.76
1.81
1.92
125
1.35
1.39
1.44
1.48
1.52
1.57
1.62
1.67
1.72
1.77
1.82
1.88
1.93
2.05
150
1.44
1.48
1.53
1.57
1.62
1.67
1.72
1.77
1.82
1.88
1.94
2.00
2.06
2.18
175
1.52
1.57
1.62
1.67
1.72
1.77
1.82
1.88
1.93
1.99
2.05
2.11
2.18
2.31
200
1.61
1.66
1.71
1.76
1.81
1.87
1.92
1.98
2.04
2.10
2.17
2.23
2.30
2.44
225
1.69
1.74
1.80
1.85
1.91
1.96
2.02
2.09
2.15
2.21
2.28
2.35
2.42
2.57
250
1.78
1.83
1.89
1.94
2.00
2.06
2.13
2.19
2.26
2.32
2.39
2.47
2.54
2.70
275
1.86
1.92
1.98
2.04
2.10
2.16
2.23
2.29
2.36
2.44
2.51
2.58
2.66
2.83
300
1.95
2.01
2.07
2.13
2.19
2.26
2.33
2.40
2.47
2.55
2.62
2.70
2.78
2.96
325
2.03
2.09
2.16
2.22
2.29
2.36
2.43
2.50
2.58
2.66
2.74
2.82
2.91
3.08
350
2.12
2.18
2.25
2.31
2.38
2.46
2.53
2.61
2.69
2.77
2.85
2.94
3.03
3.21
375
2.20
2.27
2.34
2.41
2.48
2.56
2.63
2.71
2.79
2.88
2.97
3.06
3.15
3.34
400
2.29
2.36
2.43
2.50
2.58
2.65
2.73
2.82
2.90
2.99
3.08
3.17
3.27
3.47
425
2.37
2.44
2.52
2.59
2.67
2.75
2.84
2.92
3.01
3.10
3.20
3.29
3.39
3.60
450
2.46
2.53
2.61
2.69
2.77
2.85
2.94
3.03
3.12
3.21
3.31
3.41
3.51
3.73
475
2.54
2.62
2.70
2.78
2.86
2.95
3.04
3.13
3.23
3.32
3.42
3.53
3.63
3.86
500
2.63
2.71
2.79
2.87
2.96
3.05
3.14
3.24
3.33
3.43
3.54
3.65
3.76
3.99
550
2.80
2.88
2.97
3.06
3.15
3.25
3.34
3.44
3.55
3.66
3.77
3.88
4.00
4.24
5"#-&# %FOTJUZ$PSSFDUJPO'BDUPST
.FUSJD6OJUT
