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IMPROVEMENTS TO FAN EFFICIENCY OFFER ATTRACTIVE ROIs
Examining the Benefits
of Retrofitting Centrifugal
Fan Rotors
By Eranga Devasurendra
THIS ARTICLE EXPLAINS HOW DIFFERENT FLOW CONtrol methods, inlet dampers versus variable frequency
drives (VFDs), affect a fan’s efficiency and plant operating costs. Case studies provide an understanding of the
data required to assess eligibility for fan efficiency retrofits. Opportunities for retrofit are practical when fans are
oversized, processes are changed, or equipment is being
added to the system. Benefits of energy-efficiency retrofits
can be predicted from field studies and a return on investment (ROI) determined. Whether fan casing, ductwork,
Digital Object Identifier 10.1109/MIAS.2018.2875211
Date of current version: 4 September 2019
and existing motors and motor components can be reused
is considered in the cost analysis. Rebates and incentives
from local utilities that can contribute funding to help provide the necessary returns are also explored. This article
compares predicted ROI to actual field results. Nomenclature used throughout the article is given in Table 1.
In an increasingly competitive marketplace, industrial
facilities are constantly looking for an edge regarding production costs and reliability. Over the last few decades,
this has manifested itself in various forms, with one of the
primary buzzwords being energy-efficiency. The term itself
conjures up images of wholesale changes to equipment,
systems, and processes. However, there are steps that can
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Table 1. Nomenclature
ACFM
Actual cubic feet per minute; the volume of
air moved per minute.
AMCA
Air Movement and Control Association
International, Inc.
Brake
horsepower
(BHP)
A measure of the rate of energy expended.
One BHP is equivalent to mechanical energy
consumed at a rate of 33,000 ft lb/min.
Fan damper
An accessory installed at the fan inlet or
outlet to modulate air volume.
Impeller
Another term for fan wheel. This is the rotating
portion (less the shaft) of the fan designed to
increase the energy level of the gas stream.
Inches of
water column
(in-wc)
A unit of measure of static pressure.
ROI
A measure of the amount of profit on an
investment relative to the investment’s
cost.
Static
efficiency
Ratio of the fan power output to the power
supplied to the fan. Static efficiency uses
static pressure, which does not include the
kinetic energy, to calculate the efficiency. It
can be found by multiplying the mechanical
efficiency by the ratio of fan static pressure
to fan total pressure.
Static pressure
The measure of the potential energy of the
airstream. In simpler terms, this is the resistance
to airflow caused by air moving through a duct.
relatively small investments that are paired with readily
measurable metrics and highly desirable ROIs.
Table 2 shows consolidated data from a survey by
the U.S. Department of Energy and U.S. Census Bureau
that highlight the potential annual savings at individual facilities in a wide cross section of industries [1].
This estimate examines only fan retrofits and does not
include savings associated with improving the efficiency
of the motors driving these systems or using VFDs
instead of fan dampers for volume control. Factoring
motor replacement and VFDs into the equation may
increase project cost but could provide a significant
boost to overall savings.
Fans and System Resistance
Fans are designed to deliver a specific amount of airflow
at a designated static pressure. This required static pressure (or resistance to flow) is tied in to process ductwork
and other equipment upstream and downstream of the
fan. For a fan system, a value for airflow delivery cannot
be discussed in a bubble without correlating it to a given
static pressure.
Figure 1 shows a typical fan performance curve including (a) airflow versus BHP and (b) airflow versus static
pressure. Point A 1 simulates a duct system where there
is no resistance to flow, which is referred to as wide
49-in Single-Width Backward-Inclined 700 r/min
ρ = 0.075 lb/ft3
30
BHP
be taken to address inefficiencies on the equipment side
that do not require complete overhauls or disturbing the
surrounding manufacturing ecosystem. Older centrifugal
fans have proven to be one piece of the puzzle, requiring
Table 2. The estimated annual power savings
available per industry based on fan retrofits [1]
Annual Savings per
Facility for Fan Upgrades
Paper mills
US$695,000
Petroleum refining
US$946,000
Industrial inorganic chemicals
US$283,000
Paperboard mills
US$492,000
Blast furnaces and steel mills
US$358,000
Industrial organic chemicals
US$91,000
Industrial gases
US$116,000
Plastic material and resins
US$121,000
Cement
US$219,000
Pulp mills
US$483,000
D2
C2
B2
A2
20
10
F2
(a)
E1
5
F1
4
Static Pressure
Industry
E2
D1
3
C1
2
B1
1
0
A1
0
1
2
3
4
10,000 CFM
(b)
5
6
FIGURE 1. Two graphs showing (a) airflow versus BHP and
(b) airflow versus static pressure. Points A1 to F1 lie along the fan
performance curve. Each point has a unique system resistance
curve associated with it.
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open volume or free delivery. A fan will operate at
the point where the system line intersects the fan curve.
To generate the different test points shown (B 1 through
F1), the system resistance had to be modulated by simulating resistance (i.e., back pressure in the test duct).
Connecting the test points generates the fan performance curve. Point F1 is the point where the fan delivers
no airflow and is purely generating pressure. This point
is sometimes referred to as shut off or no delivery. As
a result, each test point has a unique system resistance
curve associated with it, and there is an infinite number
of system lines between points A 1 and F1 .
Oversized Fans and How to Identify Them
performance (airflow, static pressure, and BHP) and
comparing these results to the original fan curve. Finding the operating point and then comparing it to the
as-designed fan curve will provide all the indicators
necessary for determining the variations from design
and the path to right-sizing the fan.
How Oversized Fans Affect Power Consumption
Once the conditions leading to oversized fans and how to
identify them in operation are understood, it is important
to consider the effect they have on power consumption.
Table 3 summarizes the performance conditions of the
installed fan shown in Figure 2.
Although the increase in BHP may be considered
minimal, the delivered flow is too high for the system,
so an inlet damper is used to bring the flow back to
Static Pressure (in-wc)
Oversized fans are one of the biggest culprits in high
operating costs and considered energy vampires in the
industry. Although there are various reasons for fans being oversized
(including process and equipment
50
changes over time), the biggest
45
System Resistance Curve
contributor is conservative-design
(as Designed)
40
engineering practices. These pracFan Curve
System Resistance Curve
35
tices often lead to the specifica(as Installed)
tion, purchase, and installation of
30
fans that exceed system require25
ments. Engineers include a margin
20
of safety in sizing fans to compen15
sate for uncertainties in the design
process. Built-in contingencies for
10
Expected
future system capacity increases
Performance
Actual Performance
5
that never actually materialize are
0
present as well. Figure 2 highlights
0
100,000 200,000 300,000 400,000 500,000 600,000 700,000
the effects of oversizing on a fan
Airflow (ACFM)
curve by showing how less-thananticipated static pressure require- FIGURE 2. A graph showing as-designed and as-installed system resistance curves for an
ments shift the system resistance oversized fan installation.
50
45
Static Pressure (in-wc)
to the right of the calculated values, producing more airflow during
wide-open damper operation.
A way to identify an oversized
fan is to observe the operation of
its inlet flow control devices. If inlet
vanes and dampers remain partially
closed through the lifetime of the
fan, it is a glaring indication that the
fan is producing more airflow than
required. As a result, the system continually operates against an excessive load, and the fan’s operational
cost is unnecessarily high [2]. Figure 3 shows the same fan from Figure 2 but dampered back to hit the
flow required in the original design.
Other ways to identify oversized fans involve air performance
testing to get a snapshot of fan
40
Fan Curve
35
30
25
20
15
10
5
0
Damper Curve
0
100,000 200,000 300,000 400,000 500,000 600,000 700,000
Airflow (ACFM)
FIGURE 3. A graph of the fan from Figure 2 with the addition of the performance curve
resulting when an inlet damper is used to achieve original fan performance.
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desirable system levels. As shown in Table 4, the use of
an inlet damper to achieve this condition reduces the
static efficiency from 84 to 66%. A common misconception here may cause one to say that the horsepower is
only 1,560 BHP, so it’s actually lower than the estimated
design horsepower of 1,800 BHP, and we’re saving
money! While the point about horsepower is technically
true, it neglects the underlying problem, which is that 8
in-wc of static pressure is absorbed by the damper operation to move the flow back from 360,000 to 310,000
ACFM. It’s this misuse of the static pressure that causes
the large drop in static efficiency of the fan.
Table 3. The performance comparison of fan
as-designed versus installed with miscalculation
of system resistance
As-Designed Conditions
As-Installed Conditions
Flow (ACFM)
310,000
Flow (ACFM)
360,000
Static pressure
(in-wc)
31
Static pressure
(in-wc)
28
BHP
1,800
BHP
1,900
Static efficiency (%)
84
Static efficiency (%)
83
50
Static Pressure (in-wc)
45
40
Methodology for Improving Fan Efficiency
To reduce the required flow, a new fan must be selected
to match the new operating conditions. Once an efficiency
opportunity has been identified, a full air performance test
is conducted to benchmark the maximum requirements for
the fan. At this point in the timeline, the estimates shown
in Table 4 can be confirmed. The AMCA has helpful standards to assist in field-testing centrifugal fans, and most test
contractors follow these guidelines to ensure consistency
and repeatability of results. With this real-world data in
hand, a fan manufacturer can start the process of identifying the required component changes.
In the majority of cases, this can be done without
replacing the entire fan assembly, which comes with its
own set of challenges, including foundation and ductwork
modifications. A new rotor with blades and dimensions
that are conducive to the most efficient operation can be
selected. Most of this modified operation is achieved by
changes to the geometry of impeller components such as
diameter, width, and blade angles. Figure 4 shows data
from such a selection operating inside the original host
fan casing.
In this example, selecting the correct fan for this
application revises the efficiency to the original intended
design, and the resultant power consumption is lower
than in the previous field operation that used an inlet
damper. Table 5 is a summation
of this new performance. The difference of 344 BHP (1,560−1,216)
results in real energy savings.
Assigning a Value to EnergyEfficiency Improvements
Original Fan Curve
35
30 Original Damper
25 Curve
New Retrofit
Fan Curve
20
15
10
5
0
0
100,000 200,000 300,000 400,000 500,000 600,000 700,000
Airflow (ACFM)
FIGURE 4. A graph of the fan from Figures 2 and 3 showing the performance curve after a rotor
retrofit to resize the fan to match the original design flow.
When touting energy-efficiency
numbers, it’s important to be able to
correlate them to real-world implications: namely, in megawatthours
(MWh) per year. In the case of the
fan under discussion, Table 6 illustrates this by demonstrating the
annual energy consumption of the
original fan system and the estimated
savings from the retrofit. Right-sizing
this fan would lead to a reduction of
2,135 MWh per year, which translates to a cost savings of US$143,045
Table 4. The performance condition using an inlet
damper to restrict flow
Table 5. The performance conditions after fan
rotor retrofit
Flow (ACFM)
310,000
Flow (ACFM)
310,000
Static pressure (in-wc)
21
Static pressure (in-wc)
21
BHP
1,560
BHP
1,216
Static efficiency (%)
66
Static efficiency (%)
84
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Table 6. The annual energy consumption and cost
savings based on US$0.067/kWh [3]
Table 7. A comparison of fan BHP to percentage
of motor full load
Existing
System
New System
After Retrofit
Case
BHP
Percentage of
Motor Full Load
Hours in operation per year
8,322
8,322
Original design
1,800
90
Fan energy usage (MWh/year)
9,681
7,546
As installed
1,900
95
Annual operating cost of fan
$648,627
$505,582
Postretrofit
1,216
61
Energy savings from retrofit
–
2,135 MWh/year
Energy cost savings from retrofit
–
$143,045
a­ nnually. This savings can be used to calculate the ROI for
such a project.
Role of Motors and Variable Frequency Drives in
Variable Load Operations
motors, this is not usually a concern since the efficiency
doesn’t drop off appreciably until the load percentage
drops below 25% [4]. This does become an issue, however, when the full-load condition represents 61% of motor
full load, and this percentage continues dropping for
lower-load conditions. In such cases, the ideal solution
with regard to efficiency and long-term operation is to
replace the existing motor with one that is appropriately
sized for the maximum operating condition.
Static Pressure Power (in-wc)
Power (hp)
The discussion so far has focused on fans operating at
peak load. However, for most facilities, there are variable load requirements dependent on factors such
1,400
100%
as production demand. In these
1,200
instances, fans can operate in a
80%
1,000
high-/low-load capacity, or they can
P1
60%
800
have a multitude of conditions in
40%
P2
20%
between. The go-to method for con600
0%
trolling these variable loads is the
400
use of inlet and outlet dampers. As
200
noted from the preceding examples,
0
static efficiency drops off drastically
30
once a damper moves away from
a completely open operation. As a
25
result, the annual energy cost sav20
ings will need to be recalculated
with the values of power consump15
tion tied to number of hours of operP1
10
ation at that given load. Depending
P2
on how often the fan runs at reduced
5
loads, the estimated annual savings
0%
20%
40%
60%
80% 100%
0
from Table 6 will be lower.
0
100,000
200,000 300,000 400,000
500,000
While the role of electrical effiVolume (ACFM)
ciency has been downplayed in
Power
System Resistance
Static Pressure
this analysis, it does come into play
once we start looking at low-load
conditions after a retrofit in which
Ratings:
Rating Point
P1
P2
the original motor was not downVolume (ACFM)
310,000
260,000
215,000
Static Pressure (in-wc):
21
15
10
sized. Table 7 compiles this informaDensity (lb/ft3):
0.046
0.046
0.046
tion using an assumed motor size of
Temperature (°F):
320
320
320
2,000 hp for the original fan.
Speed (r/min):
1,180
1,180
1,180
As evident in Table 7, the mechanBHP:
1,216
987
829
ical efficiency of the retrofit is 84%,
Static Efficiency (%):
84
62
41
but the motor is actually operating
at only 61% of its full-load capability. For larger, high-efficiency FIGURE 5. Two graphs showing performance curves for a fan operating with inlet box dampers.
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Conclusions
1,400
Power (hp)
1,200
1,180 r/min
1,000
800
1,010 r/min
P1
600
829 r/min
400
P2
Static Pressure Power (in-wc)
200
0
30
25
20
15
P1
10
P2
5
0
829
r/m
0
100,000
in
1,
10
18
r/m
in
200,000 300,000 400,000
Volume (ACFM)
Static Pressure
Ratings:
Volume (ACFM)
Static Pressure (in-wc):
Density (lb/ft3):
Temperature (°F):
Speed (r/min):
BHP:
Static Efficiency (%):
1,0
Power
Rating Point
310,000
21
0.046
320
1,180
1,216
84
0
r/m
in
500,000
System Resistance
P1
260,000
15
0.046
320
1,010
758
81
P2
215,000
10
0.046
320
829
418
81
Fan systems are critical components in industrial plant operations.
A significant portion of all energy
consumed by motor-driven equipment in manufacturing facilities is
for process fans and air distribution [1], [2]. Because of their critical nature in supporting production,
many of these industrial fans operate continuously, with their only
downtime being scheduled maintenance. Run times of this nature
mean that they contribute heavily to
energy consumption and the overall annual operating expenditure
for a plant. With electricity costs
being a quantity that’s relatively easily measured, improvements to fan
efficiency reduce plant operating
expenditures. Retrofitting an existing fan versus replacing the entire
unit yields a payback period that
is not just attractive but a repeat
source of savings for years to come.
Finally, variable speed operation
of the fan system offers additional
energy-efficiency gains by eliminating the dampers needed for reduced
flow operating points.
Author Information
Eranga Devasurendra (edevasuren
dra@clarage.com) is with Twin City
FIGURE 6. Two graphs showing performance curves for the fan from Figure 5 operating with a
Clarage, LLC, Pulaski, Tennessee. This
VFD instead of inlet box dampers.
article first appeared as “Consider the
Benefits of Retrofitting Centrifugal
To further fine-tune the efficiency calculation, it be­­
Fan Rotors” at the 2018 IEEE-IAS/PCA Cement Industry
comes necessary to address the inefficiency of continuing
Conference. It was reviewed by the IAS Cement Industo use inlet dampers after a retrofit to control flow. To
try Committee.
recap: we’ve been able to resize the fan at its high-load
point to 84% mechanical efficiency but forced to settle for
References
[1] U.S. Office of Energy Efficiency and Renewable Energy. (2002) “United
lower efficiencies at low-load conditions. This is where
States industrial electric motor systems market opportunities assessthe use of variable speed, specifically a VFD, enters the
ment: Executive summary.” U.S. Dept. of Energy Office of Industrial Tech.
discussion. A VFD can modulate the speed of a motor to
Olympia, WA. [Online]. Available: http://tinyurl.com/nacewxn
[2] U.S. Department of Energy’s Industrial Technologies Program and
achieve results identical to those of an inlet damper. The
Air Movement and Control Association International, Inc., “Improving
one glaring difference is that there is no wasted static presfan system performance: A sourcebook for the industry,” U.S. Dept. of
sure loss across the operation of the damper. Figures 5
Energy, Washington, DC, Rep. DOE/GO-102003-1294, 2003. [Online].
Available: https://www.energy.gov/sites/prod/files/2014/05/f16/fan
and 6 show curves that contrast a retrofit option operating
_sourcebook.pdf
with inlet box dampers to one operating with a VFD.
[3] T. Persful and E. Rogers, “Opportunities and financial incentives
Although there is no variation in the high-load efficienabound for industrial fan retrofits,” ASHRAE J. Suppl. AMCA Intl. Inmotion, pp. 8–11, July 2014.
cy, it’s clear that speed control yields superior efficiency
[4] U.S. Department of Energy, “Determining electric motor load and
across the operating range for low loads. Coupled with
efficiency,” 1997. Accessed on: Dec. 12, 2017. [Online]. Available: https://
the appropriately sized motor, the gains in both mechaniwww.energy.gov/sites/prod/files/2014/04/f15/10097517.pdf
cal and electrical efficiency are vastly greater when using
speed control with a rotor retrofit.
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