ASHRAE Journal - November 2020 - 60

COLUMN ENGINEER'S NOTEBOOK

FIGURE 3 Efficiency penalty for running two fans vs. one fan.

A. EPI Process Exhaust Fan Curve
7

N + 1 Bottom Lines

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N OVEM BER 2020

Static Pressure (in. w.c.)

100

Single-Fan
Operating Point

80%

Static Efﬁciency
Single-Fan Operation

6.0

70
66.9%

60
Static Efﬁciency
Two-Fan Operation 50
40
967 rpm

4
3
Design
System
Curve

2
1
0

10,000

903 rpm

14,000

20
10

28,000

20,000

30,000
Flow (cfm)

40,000

0
60,000

50,000

B. Typical 40 hp Motor Characteristics

100

93.50%
92.52%

Two-Fan
Operating Point

Single-Fan
Operating Point

70
60
Efﬁciency

50
40
30
20
10
0

19.78
0

33.01

20
25
30
Motor Load (hp)

C. 50 hp VFD Efﬁciency
96.00%
94.40%

100
90

Single-Fan
Operating Point

Two-Fan
Operating Point

80
70
Efﬁciency (%)

Efﬁciency (%)

If the system had been 2N, then we would have
saved energy. But, if the two-fan operating mode
avoided one fab crash per year, we would see a
net savings of about $8,000 a year, despite the
spinning reserve energy penalty. Even at current
utility rates, there would be signiﬁcant savings.
And, while modest, the service life of the motors,
bearings and drives were probably increased
due to the lower loads they experienced.
On the other hand, we would be wearing out
the fans at the same rate vs. having the option
of keeping one in reserve with less total runtime on it. But our operators were very rigorous
in doing preventive maintenance. And, they
could accomplish it by doing work on one fan
and allowing the other fan to handle the load.
Scheduling the work for periods of time when
the fab was out of production minimized ﬁnancial risk of a fab crash during maintenance.
Bottom line, I learned via experience that there
is more to the various N + relationships than
math; system dynamics and deﬁnitions were
critical, and nonenergy beneﬁts may prevail.
That may sound like heresy from a treehugging, salmon-loving Oregonian. But our
systems were there to provide a clean, safe,
comfortable, productive environment in the
facility. The energy consumption needed to
be considered in the context of that broader
mission. And the fans were not the only things
consuming resources. The $10,000 per crash
price tag ultimately represented an environmental hit in terms of wasted resources,
human energy and embedded energy.
So, I would like to think that
in addition to solving a ﬁnancial problem, our solution was
a holistic solution. If nothing
else, the experience taught me https://bit.ly/309ZpBE
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to think in broader terms about
how our systems use resources. Hopefully, this
column has given you similar food for thought.

Two-Fan Operating
Point

Do Not
Select
In This
Region

60
50
40
30
20
10
0

71.36%

42.31%
0

50
60
Load (%)

100

Static Efﬁciency (%)

Motor Speed (rpm)

Drive Efficiency. Belt efﬁciency will be the
same for either case. However, VSD efﬁciency
will tend to drop as the load drops (Figure 3c).

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ASHRAE Journal - November 2020

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