ASHRAE Journal Supplement - AMCA InMotion - October 2020 - 13

however, inlet and outlet conditions rarely mimic
those of laboratory-test setups, as the installaRequired operating-point
flow and pressure

tion and ducting are inﬂuenced by the existing
infrastructure and space limitations.

Static pressure

Fan curve adjusted
for system effect

Common Causes of System Effect

System effect

The most common causes of system effect include
Fan curve

uneven or spinning airf low at a fan's inlet,
obstructions to airflow at the inlet or outlet,
improperly conﬁgured ductwork at the inlet or
outlet, and/or failure to correct for losses caused
by fan accessories.
System effect can be avoided by accounting

System curve

for all factors, including the shape of the tran-

Airﬂow

sition points between the fan and existing

FIGURE 1. Impact of system effect on system performance.

ducts, ductwork conﬁguration close to the fan,

Source: AMCA "System Effect" online educational module

and accessories. For optimum air performance,
airﬂow at the fan's inlet needs to be uniform,

associated performance testing, engineering analysis,

symmetrical, and free of swirl. Similarly, airﬂow must be

ﬁeld service, long-term maintenance, and lost production

able to diffuse and fully develop across the fan's outlet.

from unplanned downtime and extended startup can

Even minor improvements to airﬂow stability can reduce

be costly.

system effect and, in turn, increase fan performance and

With any installation, system effect and all other

operating efﬁciency.

system-associated losses must be considered to under-

An inlet vane damper is a modulating device that

stand how a fan will perform relative to its laboratory

affects fan performance. As a damper is closed, air begins

tests once installed. During the fan-selection process,

to pre-spin into the fan; the fan wheel no longer can move

the combined impact of those losses should be taken

as much air, and ﬂow, pressure, and brake horsepower all

into account. For optimal system performance and cost-

decrease. Even when a damper is fully open, the vanes

effective operation for years to come, best practices for

interfere with normal ﬂow and reduce fan performance.

fan ducting and installation need to be followed.

If the losses are not accounted for, the fan will have to
run at a speed higher than the one speciﬁed during fan

Laboratory vs. Real-World Conditions

selection. Fans should be tested with accessories that

To recognize how system effect impacts air-system perfor-

determine the losses and the fan speed required to over-

mance, it is important to understand the conditions under

come them.

which fans are tested in a laboratory. Housed fans typically are tested with an open inlet that includes a bell

Reducing System Effect at a Fan's Inlet

mouth to eliminate entry losses and achieve uniform

A lack of uniform airﬂow entering a fan's inlet is one of

airﬂow across the inlet. The discharge includes a straight

the greatest and most common causes of system effect.

run of ductwork that produces fully developed airﬂow

Often, these losses are the result of elbows and isolation

(free of swirl or turbulence) prior to the air entering the

dampers being installed too close to a fan's inlet.

test chamber. Uniform, fully developed airﬂow enables

Depending on the application, a variety of strate-

a fan to move air efﬁciently and quietly through a duct

gies can be employed to improve airﬂow at a fan's inlet.

system without causing excessive vibration. In the ﬁeld,

Take, for example, the 90-degree round elbow located

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

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

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