VAV-107 - page 5

5
Total Pressure Concept
Chicago Blower Design 47 perform-
ance is published in terms of Total
Pressure.Total Pressure=StaticPres-
sure+VelocityPressure
“Static Pressure” is unrelated to air
motion. It is a pressure that exerts a
forceequal inall directions, like theair
pressure inaballoonor pressureves-
sel, and canbePositiveor Negative.
“Velocity Pressure” is the kinetic en-
ergyapplied tomotionor speedof the
air through a duct system. It is always
Positive in character.
Afan“sees”onlyTotal Pressure. It can-
not recognize Static or Velocity Pres-
sureasseparatequantities.Therefore,
the “Total PressureConcept”must be
considered toassure that anyvariation
in the actual installation from the in-
stallation in which the fan was tested
is accounted for.
A fan is normally tested blowing into a
duct of the same cross-sectional area
as the fan outlet.When it is rated on
StaticPressure, the rating isbasedon
TotalPressureminusVelocityPressure
in the test duct.The rating is only valid
if the fan is installed similar to the test
setup. If it is not, the true Static Pres-
sure capability will be greater or less
than the ratedStaticPressure.Thedif-
ferencewill be the difference between
actualVelocityPressureand theVeloc-
ityPressurebasedon the fanarea.
Conesmay be used to transition from
the fan to ducts larger or smaller than
the fan.Outlet conesmayalsobeused
tominimizeVelocityPressure lossand
regainStaticpressure.Air leavesadi-
verging discharge cone (Point B) at a
lower velocity than at the cone inlet
(Point A), therefore, at a lowerVelocity
Pressure. Consequently, more of the
fan’sTotal Pressure capability is avail-
able for StaticPressure thanwouldbe
For the Standard VAV cone effective
Pv is:Pv
E
=Pv
A
- (Pv
A
- Pv
B
) .8
Whereacone isused, the ratedStatic
Pressure is increased by the amount
equal toPv
E
- Pv
B .
Use of Outlet or Inlet Cone
AIR
FLOW
FAN
P
V A
P
V B
CONE
A B
Where the fan has an open discharge
(does not blow into a cone, duct or
transition), theVelocityPressuremust
be based on the annular area; that is,
the fan casing area minus the hub
area. Since annulus velocity is gener-
ally very high, the benefit of using a
conewhendischarging toatmosphere
is apparent.
Note:AMCAcertified ratings donot apply
when factors areused.
available with the fan blowing into a
duct of diameter equal to the fan.
Fan
Areas
(
Ft
2
)
1650
1825
2000
2225
2450
2700
3000
3300
3650
4025
4450
4900
5425
6000
6650
7300
8112
1.48
1.82
2.18
2.70
3.27
3.98
4.91
5.94
7.27
8.84
10.80
13.10
16.05
19.63
24.12
29.07
35.89
2.18
2.70
3.27
3.98
4.91
5.94
7.27
8.84
10.80
13.10
16.05
19.63
24.12
29.07
35.89
44.17
54.54
0.66
1.00
1.36
1.88
2.45
3.16
4.09
5.12
6.45
1.45
2.16
3.09
4.12
5.45
7.02
8.98
11.28
14.23
5.50
7.46
9.76
12.71
3.22
4.79
6.75
9.05
12.00
15.58
20.07
25.02
31.84
FANSIZE
CASING
AREA
CONE
AREA
HUBA
HUBB
HUBLB
HUBC
ANNULARAREAS
VelocityPressure= –––––––––––––
FanArea x 4005
whereFanArea= cross sectional area
at the flowpoint considered
CFM
(
)
2
1,2,3,4 6,7,8,9,10,11,12,13,14,15,...16
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