ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 14

D

Impeller
rotation

Length
of duct

R

FIGURE 3. Non-uniform airﬂow into
a fan inlet induced by a rectangular
inlet duct.

Counter-rotating swirl

FIGURE 4. Example of a forced inlet
vortex.

FIGURE 2. Non-uniform airﬂow into a
fan inlet induced by a 90-degree, threepiece section elbow-no turning vanes.

Source (ﬁgures 2-4): AMCA Publication 201-02 (R2011), Fans and Systems

at the inlet of a fan shown in Figure 2. Air entering the

The effective length of ductwork at a fan's outlet is one of

fan wheel is not uniform, loading on various parts of the

the most important factors in fan and system efﬁciency.

fan wheel instead of at the center as designed for optimal

In most cases, the proﬁle of the air coming out of a fan

performance. Some air is circulating back into the elbow,

is asymmetrical, causing turbulence and a lack of static-

creating additional losses. In this case, the addition of

pressure regain.

turning vanes in the elbow will help direct air toward
the center of the fan wheel.

For symmetrical and uniform ﬂow to be achieved,
outlet ducting must be long enough to allow airﬂow to

Figure 3 shows the effect of a rectangular inlet

diffuse and fully develop. This is called 100-percent

box mounted directly to the inlet of a fan. Again, the

effective duct length. As a rule of thumb, the effective

fan wheel is not being uniformly loaded, resulting

length of outlet ducting should be no less than 2.5 duct

in performance loss. With an inlet box, the cavity,

diameters when duct velocity is 2,500 fpm (13 m/s) or less.

or dead area, below the outlet is where air will get hung

For every additional 1,000 fpm (5 m/s), one duct diameter

up, creating additional losses. Improving the shape of

should be added.

the inlet box or adding straightening vanes can help to
redirect the ﬂow of air into the wheel.

Figure 5 shows the velocity proﬁ le of air as it exits
a fan. Air is forced against the outside of the scroll,

Figure 4 shows air entering a fan from the side as

resulting in uneven ﬂow at the outlet. An effective run

opposed to straight through the inlet.
Blast area

The air is spinning in the opposite direction of the fan wheel. Consequently, the

Cutoff

Outlet area

Discharge duct

fan is having to work harder, resulting in
greater energy consumption and stress on
fan components.

Reducing System Effect
at a Fan's Outlet

100% effective duct length

Sim ila r to inlet f low, out let f low is
impacted signiﬁcantly by the placement
and distancing of ductwork and dampers.

14

2020 A M CA i n m o t i o n

Centrifugal fan

FIGURE 5. System-effect curves for outlet
ducts-centrifugal fans. Source: AMCA Publication
201-02 (R2011), Fans and Systems

w w w. a m c a .o r g

http://www.amca.org

ASHRAE Journal Supplement - AMCA InMotion - October 2020

Table of Contents for the Digital Edition of ASHRAE Journal Supplement - AMCA InMotion - October 2020

Contents
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - BB1
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - BB2
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - Cover1
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - Cover2
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - Contents
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 2
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 3
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 4
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 5
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 6
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 7
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 8
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 9
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 10
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 11
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 12
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 13
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 14
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 15
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 16
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 17
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 18
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 19
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 20
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 21
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 22
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 23
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 24
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 25
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 26
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 27
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 28
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 29
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 30
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 31
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 32
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 33
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 34
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 35
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 36
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 37
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 38
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 39
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 40
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 41
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 42
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 43
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 44
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - Cover3
ASHRAE Journal Supplement - AMCA InMotion - October 2020 - Cover4
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