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

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


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AMCA InMotion - September 2019

Table of Contents for the Digital Edition of AMCA InMotion - September 2019

Contents
AMCA InMotion - September 2019 - BB1
AMCA InMotion - September 2019 - BB2
AMCA InMotion - September 2019 - Cover1
AMCA InMotion - September 2019 - Cover2
AMCA InMotion - September 2019 - Contents
AMCA InMotion - September 2019 - 2
AMCA InMotion - September 2019 - 3
AMCA InMotion - September 2019 - 4
AMCA InMotion - September 2019 - 5
AMCA InMotion - September 2019 - 6
AMCA InMotion - September 2019 - 7
AMCA InMotion - September 2019 - 8
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AMCA InMotion - September 2019 - Cover4
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