ASHRAE Journal Supplement - AMCA InMotion - September 2019 - 4

1.5
1.25

in. wg

Static pressure

kPa

2.0

8

0.68

2.7

0.0

100%

system curve reaches zero ﬂow rate is the
nominal static-pressure set point. Often,

70%

2
1.2

0

IES 90.1 and the International Energy
Conservation Code (IECC). Where the

5.0

4

0.3

System curve

6

1.0
0.5

with recent versions of ANSI/ASHRAE/

this is about 0.75 in. wg (187 Pa). Both
ANSI/ASHRAE/IES 90.1 and the IECC

40%
0

0.0

3

1.5

6

7.2

3.0 3.4

9

12

12.6

4.5

6.0

15

7.5

18.0

21
24
cfm x 1,000
10.5

8.5

Airﬂow

12.0
m3/s

require it to reset based on VAV-box
position, so it effectively is much lower
in properly operating systems.
In a VAV air-conditioning system,
the peak ﬂow rate is needed on only

FIGURE 1. Operating points in a typical VAV system.

the hottest days ("100%" in Figure 1).
On average days, with the building

Selecting the Correct FEI for a VAV System

requiring less cooling and less flow, we can slow the

FEI is a solid indicator of the ability of a fan to serve a duty

rotational speed of the fan using a variable-frequency

point efﬁciently. However, it is based on a single point of

drive, allowing the fan speed to track the changing cooling

operation; in a VAV system, there are many points of opera-

needs of the building. For our discussion, let's say the

tion. Figure 1 shows the system curve of a VAV air handler

average ﬂow rate is 70 percent and the minimum ﬂow rate

that might serve one ﬂoor of a multistory high-rise. The

is 40 percent, as illustrated in Figure 1.

curve is offset upward from the origin because the controls

In this case, the FEI rating should be determined at

are set to provide a minimum static pressure in older systems

the peak ﬂow rate. Determining it at the average ﬂow rate

and a minimum pressure reset downward from an initial

would be problematic for two reasons: the ﬁrst related to

set point based on VAV-box position in systems complying

compliance, the second to structural integrity.

Impeller diameter,
in. (mm)

Fan-impeller type
(all double-width)

FEI at
40% ﬂow1

FEI at
70% ﬂow2

FEI at
100% ﬂow3

18 (464)

Airfoil

1.07

0.97

0.89

20 (508)

Airfoil

1.21

1.14

1.01

22 (565)

Airfoil

1.23

1.20

1.13

24 (622)

Airfoil

1.27

1.29

1.25

16 (406)

Backward-inclined

1.01

0.89

OVERSPEED

18 (464)

Backward-inclined

1.02

0.90

0.82

20 (508)

Backward-inclined

1.10

1.01

0.93

22 (565)

Backward-inclined

1.21

1.12

1.04

24 (622)

Backward-inclined

1.24

1.23

1.16

Backward-inclined

1.22

1.23

1.18

27 (686)
m3/s,

1. 7,200 cfm, 1.2 in. wg (3.40
304 Pa)
2. 12,600 cfm, 2.7 in. wg (5.95 m3/s, 674 Pa)
3. 18,000 cfm, 5.0 in. wg (8.49 m3/s, 1,245 Pa)

TABLE 1. FEI ratings at various duty points for a range of fan sizes.

4

20 19 A M CA i n m o t i o n

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ASHRAE Journal Supplement - AMCA InMotion - September 2019

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