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TitleFan Performance
TagsMechanical Engineering Applied And Interdisciplinary Physics Building Engineering Gas Technologies Mechanical Fan
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Page 1

Fan Performance
By: Mark Stevens

AMCA International
Deputy Executive Director—Technical Affairs

Page 8

AMCA Publication 201 quantifies System Effect for a number of the more common causes,
and offers recommendations for avoiding System Effect.

On the fan’s inlet side, AMCA Publication 201 recommends that elbows near the fan’s inlet
be located at least three duct diameters upstream of the fan, while acknowledging that
elbows can cause System Effect when they are located up to five diameters upstream.

On the fan’s outlet side, AMCA Publication 201 introduces the term “Effective Duct Length.”
Effective Duct Length is 2.5 duct diameters when duct velocities are 2500 fpm or less, with
one duct diameter added for each additional 1000 fpm. A centrifugal fan needs 100% of an
Effective Duct Length on its outlet to avoid System Effect, while a vaneaxial fan needs 50%
Effective Duct Length.

When System Effect can’t be avoided, AMCA Publication 201 provides means of calculating
its magnitude. The following two systems are examples taken directly from the publication.

In the first system, shown in Figure 9, there is a short outlet duct on a centrifugal fan
followed by a plenum chamber with cross-sectional area more than 10 times larger than the
area of the duct.

Figure 9

If we consider the pressure losses in the opposite direction of flow, the velocity in the duct
from E to F is 14.4 m/s, equal to a velocity pressure of 124.5 Pa. At point F, the Pv is 124.5
Pa, the Ps is 0.0 Pa, and the Pt is 124.5 Pa. The friction of duct will cause a gradual increase
in Ps and Pt back to point E. If the duct has a uniform cross-sectional area the Pv will be
constant through this part of the system.

Since there is an energy loss of 49.8 Pa as a result of the abrupt contraction from the
plenum to the duct, the Pt requirement in the plenum is 921.3 Pa, or Pt = Ps (747 Pa) + Pv
(124 Pa) + Contraction (49.8 Pa).

Page 9

Air flowing across the plenum from D to E will have a relatively low velocity, Pv in the plenum
will be 0.0 Pa, and then Ps = Pt = 921.3 Pa.

At point D, there is an abrupt expansion energy loss equal to the entire Pv in the duct
discharging into the plenum. The outlet duct between the fan and the plenum is 2.5
equivalent diameters long, which is the same as used during the fan rating test. The Ps in
the outlet duct is the same as the Ps as measured during the rating test.


C-D Outlet duct on fan as tested 0 Pa

D Pv loss (also Pt loss) as a result of air velocity decrease.

Ps does not change from duct to plenum at D.

0 Pa

E Contraction loss – plenum to duct 49.8 Pa

E Ps energy required to create velocity at E 124.5 Pa

E-F Duct friction at Q = 1.42 m3/s 747 Pa

Required Fan Ps 921.3 Pa

The second system, shown in Figure 10, is similar to the first, except the duct at the fan
outlet has been omitted. The fan discharges directly into the plenum, which creates a
System Effect.

Figure 10

Page 15


AMCA 99-03, Standards Handbook, Air Movement and Control Association, Inc.

AMCA 200-95, Air Systems, Air Movement and Control Association, Inc.

AMCA 201-02, Fans and Systems, Air Movement and Control Association, Inc.

AMCA 211-05, Certified Ratings Program - Product Rating Manual for Air Performance, Air

Movement and Control Association International, Inc.

ANSI/AMCA 500-L-07, Laboratory Methods of Testing Louvers for Rating, Air Movement

and Control Association, Inc.

Cory, Bill. Fans and Ventilation. Elsevier, 2005.

Jorgensen, Robert, ed., Fan Engineering. 8th ed. Buffalo: Buffalo Forge Company, 1983.

Graham, Barrie. “The Importance of Fan Total Pressure.” Heating / Piping / Air Conditioning,

September 1994, 75-80.

Improving Fan System Performance. Washington DC, 1999.

Page 16

For more information about
Fan Performance, By: Mark Stevens

Please Contact
Deputy Executive Director—Technical Affairs: Mark Stevens (E) [email protected]

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