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MASTERING
FAN SYSTEM
EFFICIENCY
Rosy Wang,
Global Solutions Director for
Cement, Schneider Electric,
explains how to boost the overall
efficiency of the process fan
system – and therefore reduce
energy consumption in the
cement plant – and improve the
decision making process behind
fan drive selection.
Fan basics
In cement plants, gases flow through
a fan, which is located in the gas flow
system. Part of the energy it receives
from its drive motor is converted into
gas movement. There are two types of
fans in cement plants: axial fans and
centrifugal fans. Axial fans are used to
cool down hot surfaces (e.g. the kiln)
and for ventilation. Centrifugal fans
are used to blow air into the systems or
take the gases or even solid material out
of the systems (Figure 1). This article will
take centrifugal fans as its focus.
Table 1. Key process fans in typical cement plants
Sub-process
Designation
of process
fans
Plant D (3500 tpd)
Flow rate
(m3/h)
Pressure
(Pa)
Power
(kW)
Flow rate
(m3/h)
Pressure
(Pa)
Power
(kW)
Raw grinding
Raw mill fan
490 000
11 500
2500
430 000
10 500
1800 x 2
Burning
Kiln exhaust
fan
688 700
4000
1120
900 000
3800
1400
ID fan
679 500
8000
2000
850 000
7500
2500
Cooler
exhaust fan
447 652
4000
800
620 000
3500
900
Cooler fans
(e.g. fan 1)
F1: 18 585
F1: 12 360
1565 (12 fans)
F1: 18 478
F1: 12 000
2011 (17 fans)
Roller press
circulation
fans
-
-
-
250 000
3500
355 x 2
Separator fans
165 000
5650
400 x 3
231 000
5800
560 x 2
Cement mills
fans
57 800
5650
160 x 3
72 000
4500
132 x 2
Fan before
coal mill
160 000
2500
185
-
-
-
Exhaust fan
144 000
10 300
710
90 000
8000
315
Finish grinding
Coal grinding
Plant G (5000 tpd)
Sum of key process fans
10 200 kW
12 820 kW
Installed power of whole plant
31 800 kW
41 140 kW
Importance of process fans on total power
32.1%
31.2%
Fan performance curve
The fan curve is a graph of the fan pressure as a
function of flow rate, as shown in Figure 2. A gas
balance is established when the fan pressure equals the
head loss (pressure drop) of the circuit.
The point of operation of the fan and system
together is at the intersection of the fan curve and the
system curve. The head loss of a circuit is the curve of
the circuit resistance to flow. It should not exceed the
critical point.
Fan laws
Figure 1. A typical fan structure.
The rotational speed (N) of the fan blades has a major
influence on energy cost, due to the following:
ll The volumetric flow rate is directly proportional to N.
ll The change in gas total pressure is proportional to N2.
ll The absorbed power of the fan is proportional to N3.
Note: energy consumption is strongly dependent on the
changes of fan speed.
Evolutions of fan diagrams
Figure 2. Fan performance curve.
Reprinted from worldcement.com [Sep 12]
In real applications, the fan diagram will be changed
along with the following factors: fan speed, opening
degree of damper before fan, gas temperature and mass
density. At the point of operation, the provided power
from the fan is equal to the head loss of the circuit.
If the fan speed changes (Figure 3a), or if the
inlet louver opening changes (Figure 3b), the point of
operation will move along the system head loss curve.
Drives: For a fixed speed fan, when the louver opening
changes from 100% to 60%, the gas flow through the fan
will be reduced, but the pressure generated by the fan
Figure 3a. Speed changes.
Figure 3b. Opening.
Figure 3c. Temperature and mass
density.
Why talk about fan efficiency?
The functions of process fans
Process fans are critical equipment in cement plants.
Figure 4 shows the process flow in cement clinker
manufacturing with gas handling by key process fans
that have heavy duties. The fans are numbered following
the size of their installed power.
ll Fan no. 1 is the raw mill fan, which carries the gas and
the raw meal. The pressure drop across the system is
high. This fan is usually the largest in terms of power if a
VRM is used for raw grinding.
Figure 4. Key process fans in cement clinkering process.
Figure 5. The chart for fan efficiency diagnosis.
is the same. Part of it is used to overcome the pressure
drop of the damper (ΔPd) and part of it is to overcome
the system loss. That is why a variable speed drive is
commonly used to adjust the fans’ flow without damper:
to avoid the energy loss on the damper.
If the temperature of the gas changes, or if the
mass density of the gas changes (Figure 3c), then the
characteristic fan curve and the system head loss curve
will be modified.
ll Fan no. 2 is the ID fan. This fan is usually the largest in
terms of power if a ball mill is used for raw grinding. It
is located after the preheater and is considered the ‘lung’
of the clinker burning system. Without it there is no gas
flow, the fuel combustion in the kiln and calciner cannot
happen, the gas from the combustion and decomposition
cannot be taken out of the system, and the heat exchange
between cold raw meal and hot gas cannot occur. If
it is not properly designed and operated, it will affect
throughput and power consumption.
ll Fans marked 3 in the diagram are cooling fans;
usually there are 5 – 18 cooling fans in one clinker line
depending on kiln capacity. They blow cold air to the
cooler, to cool the clinker from 1300 ˚C to about 100 ˚C.
This process protects the equipment and facilities after
the cooler, quenching the clinker to ensure the proper
forms of main minerals (C3S, C2S, C3A, C4AF) remain,
thereby ensuring adequate clinker quality. The air
becomes hot and is then partly used for fuel combustion
in the kiln (secondary air, via kiln) and calciner (via
TAD- tertiary air duct). If it is not properly designed
and operated it will affect the clinker quality, power
consumption and even damage the conveying system
and clinker silo.
ll Fan no. 4 is the kiln system dedusting fan (known as the
raw mill exhaust fan, or stack fan). To produce 1 t of
clinker, this fan handles about 2.2 t of gases (mainly CO2,
N2, O2, NOx and SOx) with a certain dust content.
ll Fan no. 5 is the cooler exhaust fan. It moves about 1.8 t
of exhaust air per t of clinker production, which is part
of the air blown into the cooler by the cooling fans and
not useful for fuel combustion.
ll Fan no. 6 is the coal mill fan.
[Sep 12] Reprinted from worldcement.com
For one large clinker line, usually there are 2 – 3
cement mills equipped for different types of products
and depending on each cement mill size limitation. In
each cement mill system, there is a separator fan, mill
fan and roller press fan if roller press is used.
In addition to the key process fans mentioned above,
there are more than 100 non-process fans in a cement
plant. These fans all work together, with bag filters, for
the dedusting of various conveying systems, with air
slides for material transportation purposes or to cool the
hot surfaces, etc.
How much energy are fans
consuming in a cement plant?
Table 1 shows the key process fans, their specifications
and installed power in two typical plants.
For example, in a 5000 tpd cement plant, the total
installed power is approximately 41 MW, with the total
power of only the key process fans being around 13 MW.
The installed power for key process fans is about a third
of the plant’s total installed power, coming in second
behind the mill in terms of power consumption. If the
actual load is 80% then the power consumption on these
key process fans will be about 10 MW.
Table 2a. Data input
Fan operating efficiency
(input)
Symbol
Values
Gas volumetric flow rate
Qf
80 m3/sec
Static pressure at fan suction
Pfss
-8440 Pa
Static pressure at fan
discharge
Pfds
120 Pa
Cross-section area of exit fan
(or duct)
Ae
2 m2
Motor input power/VFD input
power
PWm
1000 kW
Combined efficiency of power
conversion/transmission
equipment
ŋc
97%
Dust concentration in the air
or gas stream
Cdc
93 g/m3
Gas temperature at fan exit
te
101 ˚C
Density of air/gas at fan inlet
condition
ρi
0.6146 kg/m3
Density of air/gas at fan outlet
condition
ρe
0.6711 kg/m3
Isentropic exponent (air = 1.4/
flue gas 1.3)
K
1.3
Efficiency calculation
(output)
Symbol
Values
Total static head developed
by fan
Pfst
8560 Pa
Velocity of air/gas at fan exit
Vae
40 m/s
Barometric pressure at local
condition
Po
79 495 Pa
Velocity pressure at fan exit
Pfve
536.88 Pa
Total pressure developed by
fan
Pft
9096 Pa
Mean density of air/gas
across the fan
ρm
0.64285
kg/m3
Motor output power or fan
shaft power
PWfsh
970 kW
Fan efficiency
Since process fans are the second biggest power
consumers, cement plants should try to master fan
efficiency with the help of experts. The suggested
workflow is ‘measure-calculate-diagnosis-action’.
After collecting and measuring data as input, the fan
efficiency can be calculated as output, shown in Table 2.
Cement plants should know where they are in terms of
fan efficiency level compared to benchmarks. Table 3
provides benchmark values for fan efficiency.
The key factors affecting fan efficiency
At design/manufacturing stage:
ll The design of fan casing.
ll The design of fan inlet cone.
ll The design of fan blades.
ll The electric drive.
Table 2b. Data output
Compressability factor
Kp
0.9634
At operation stage:
Fan output power (static)
PWfst
659 kW
ll Fan operating speed.
Fan output power (total)
PWft
701 kW
ll Dust deposit on impeller.
Fan input power (shaft power)
PWfsh
970 kW
ll Damaged fan casing.
Fan static efficiency
ŋfst
68%
ll Age of fan (i.e. impeller).
Fan total efficiency
ŋft
72.3%
ll Operating point in the performance
curve.
ll Level of impeller trimming or
extension from original size.
Fan operating efficiency
diagnosis
Cement plants are advised to organise
an audit on key process fan efficiency.
Figure 5 provides a chart for fan efficiency
diagnosis.
Reprinted from worldcement.com [Sep 12]
Figure 6. Overall efficiency of the fan system.
Table 3. Fan efficiency benchmark values
In Figure 7, the
gas law indicates how
the gas characters
(temperature, pressure
Preheater fan
>80%
and volume) will change
Raw mill fan
>80%
from condition 1 to
Coal mill fan, ball mill fan, ID fan, cement mill recirculation fan
>75%
condition 2 for the same
Cooler vent gas fan
>50%
amount of gas. If the
temperature increases,
Primary air fan
>60%
the volume will increase.
Bag filter fan (if motor rating is >50 kW)
>70%
The equation of fluid
Bag filter fan (if motor rating is 25 – 50 kW)
>65%
dynamics shows: if the
Cooler fans
>80%
flowrate (Q v) increases,
the pressure drop (or
route resistance) will
Table 4. Examples of how process system efficiency affects power consumption
increase (proportional
Plant
Kiln feed
Clinker
SPC on IDF T at
P at
Fan power
to Q v2).
(tph)
production
(kWh/t of
preheater
preheater
consumption
It has been observed
(tpd)
clinker)
outlet (˚C)
outlet (Pa)
(kW)
that the specific power
consumption (SPC) on
Plant C
82
1230
12.6
330
7000
644
IDF largely changes
from plant to plant
Plant D
245
3675
10.4
338
5400
1590
because the systems
Plant G
400
6000
8.1
316
5270
2017
they serve (kiln/
preheater system) are
different. The two key performance factors are gas flow
rate (which is not easy to measure so temperature can
be looked at instead) and pressure at the top of the
preheater:
Fan application
Benchmark value for fan
design efficiency
ll The flow rate (m 3/s or m 3/kg of clinker) depends on
combustion efficiency, heat consumption, fuel type,
operation choice on extra air level, gas temperature,
air leakage level, etc. It is not easy to achieve
online measurement of the flow rate to present it to
operators. It is a hidden parameter.
Figure 7. Gas law and fluid dynamics equation.
Overall efficiency
The overall efficiency is a combination of the efficiency
of the fan and drive system and the process system it
serves (Figure 6).
Fan efficiency and drive efficiency (95 – 98%) tend to
remain consistent after installation. This installation is
usually undertaken by the project team or by an EPC.
The process system efficiency (eg. kiln and preheater
system) is partly determined by the initial design (the
sizing of cyclones, dip tubes, connecting ducts, etc).
However, over time, its performance can change with
different operation and maintenance.
ll Pressure (simply expressed as P) at preheater outlet,
or route resistance, influenced by the shapes and
sizing of the cyclones, dip tubes, connecting ducts,
gas/solid ratio, gas temperature, etc.
ll Temperature (simply expressed as T) of exhaust,
influenced by efficiency of solid/gas heat changes
and combustion efficiency in calciner. According to
the equation of fluid dynamics in Figure 7, if the
temperature increases, the density reduces, the same
amount of gas will have higher volume flow rate, and
therefore more m 3/s need to be taken with the same
mass flow.
Here P and T are easy to see from any central control
room and they have an obvious impact on ID fan power
consumption/t of clinker. Table 4 provides an example.
Figure 8 shows the real operation conditions in
plant G and C. One can easily see the temperature and
pressure at the preheater top and see the ID fan power
consumption.
Case study 1: preheater ID fan
Energy saving
According to the previously explained fan laws:
the power needed on fan for the process system =
Qv x Pt (Qv is the flowrate, Pt is the total pressure of
the fan which has to suit the route resistance or pressure
drop).
Due to the good system design and operation in plant G,
the pressure and temperature of the exhaust at the
preheater top are lower. In addition, the VSD installation
enables the fan to run at a lower speed. Therefore,
among the three plants, plant G has the best specific
[Sep 12] Reprinted from worldcement.com
power consumption on ID fan, saving 2.3 kWh/t
of clinker vs. plant D and 4.5 kWh/t of clinker vs.
plant C. Plant G produces 5000 tpd of clinker and
1.6 million tpa; the power use price is US$100/MWh.
The energy saving just on the ID fan amounts to:
ll 1.6 million tpa x 2.3 kWh/t of clinker =
3.7 million kWh pa.
ll 3.7 million kWh pa x US$100/MWh =
US$370 000 pa.
Even for a given system, the energy saving
varies greatly because the process system
condition changes, along with shift changes. It is
worth digging about to see how to maintain the
IDF operation at its best level (Figure 9).
Operator/crew influence on fan
power consumption
Figure 8a. Preheater condition and ID fan power consumption at
plant C, 1200 tpd.
In cement plants, there are usually three shifts per
day, with 4 – 6 crews for each process section. There
is a big difference in performance from crew to
crew due to varying levels of responsibility and skill
(Figure 10). For the kiln operation, for example,
the KPIs are: output, specific power consumption,
specific heat consumption and quality. The power
consumption on the ID fan, cooling fans and cooler
exhaust fan is integral to their performance.
Case study 2: cooler exhaust fan
Figure 8b. Preheater condition and ID fan power consumption at
plant G (5000 tpd).
Figure 11 shows the cooler exhaust fan power
consumption seen from EOS (Energy Optimization
System) every half hour. The power consumption
varies substantially by 50% due to different process
conditions and shifts. This graph also demonstrates
why bigger fans are sized above its real need.
There are software and tools, such as EOS, that
can tell which condition consumes less kWh/t and by
which crew in real time. It provides tools and data
to energy specialists and analysts, allowing them to
identify how to keep the cooler operation at its best
level. Such data even lets the process experts, sitting
in the technical centre, comment and advise after
reviewing the data by remote access.
Decision making
Figure 9. Preheater ID fan specific power consumption at plant C
varies greatly.
Reprinted from worldcement.com [Sep 12]
When designing plants, fans have to be properly
sized. The design criteria for key process fans is
as follows: the fan capacity (its flow and pressure)
is designed to a larger scale than required at
nominal conditions, by around 20 – 30% or even
higher. For example, once there is a kiln push,
a huge amount of clinker rushes to the cooler
from the kiln. Enough air flow and high pressure
enables the clinker to cool. The reserved capacity
of the cooler exhaust fans protects equipment
from being burnt.
The fan with the best operation point should
be chosen to suit the normal condition of the
system (flow rate for cooling or combustion
purpose and pressure to overcome route
resistance).
The traditional way to control the fixed speed
fan is to use the (louver) damper by adjusting the
openings (usually 40 – 100%). In this case, the
damper creates additional resistance, part of the
pressure generated by the fan is used to overcome
the damper pressure (shown as ΔPd in Figure 3b),
which is a waste of energy and money.
VSD is a solution to precisely control fan
speed, hence the flow rate and pressure. VSD can
greatly reduce power consumption since power is
proportional to N 3 (N is fan speed) if it is properly
used, but it is an expensive solution. Today, many
cement plants are investing in VSDs and there are
common issues in the decision making process.
Case study 3: decision making on fan
drives
When installing VSDs on existing fans many may
assume, for example, that if a cooler fan opening
is only 50% then installing a VSD will save power.
This is usually done by an electrical department
within the plant. Some large companies make
corporate procurement for VSDs for all their plants
without considering actual process conditions.
An example with cooler fans will now be
provided. The process department finds that,
with the low speed of fan by VSD, the pressure
provided by the fans is too low and believes a
certain pressure must be maintained to cool certain
thickness of clinker on the cooler grate. The kiln
operators then have to run the previous damper
with a 50% opening and keep the original speed
with VSD at 50 Hz. Figure 12 shows the real life
example of plant B, in which operators run the
VSD at full speed and the opening of the damper is
40 – 70%.
However, if one always uses VSD at full speed,
even more energy is being consumed as the drive
system itself also consumes 2 – 5% of the energy.
In some instances, operators are not bothered
about adjusting the VSD speed according to need.
Table 5 indicates the typical issues on the system
design of the fan and drive. In plant B, even when
the operators run the VSD at full speed at 50 Hz,
the pressure provided by the fan is still lower than
other plants (for example in Chamber 3).
This case suggests that the decision making process
should involve not only electrical engineers but also
process engineers and operators. It is a management
issue at the operational level of a company.
Figure 10. SPC on cooler exhaust fan (by different shift).
Figure 11. Plant D cooler exhaust fan power consumption seen
from EOS remote access.
Possible solutions
ll Option 1: to dismantle the VSD to avoid 2 – 5% of
energy consumption compared to the current case.
ll Option 2: to choose a new fan with different
performance curve (usually higher pressure)
while considering if it will work with a VSD and
at a proper operation point, to save more energy,
compared to option 1. This represents a 20 – 40%
energy saving opportunity but with a new initial
investment.
Figure 12. The real life example of plant B.
[Sep 12] Reprinted from worldcement.com
Table 5. Cooler fan operation data in typical cement plants
Plant
name
Cooler
chamber
C1
B
P from fan
9812
P under grate
5552
P from fan
10 000
P under grate
N/A
P from fan
12 676
P under grate
8250
P from fan
12 000
P under grate
5500
C
D
G
C2
C3
C4
C5
C6
C7
Kiln
capacity
3500
3462
3223
1334
1481
1267
1200
6193
4730
3148
2564
3500
6403
4510
1811
1083
3800
3600
3100
2500
Conclusion
ll Process fans are critical equipment in cement plants;
they play important roles and consume one third of
the plant’s energy. They are the second biggest energy
consumers behind mills.
system efficiency (fan, drive,
process system design and
operation) can be improved.
llCrew/operators make a big
difference to system efficiency
and power consumption.
An energy management
or optimisation system
can help make continuous
improvements.
ll When making decisions (e.g.
sizing a fan or installing VSD
for a fan), the process needs
5000
to involve not only electrical
1400 1300
engineers but also process
engineers and operators. If
the VSD is installed without proper design and planning,
it will not save energy and may even increase power
consumption. This is a common management issue at the
operational and even the business level of companies.
516
ll It is recommended that experts carry out a fan efficiency
audit by measurement, calculation, benchmarking and
diagnosis.
Bibliography
•
RANGARAJUN Ej, EnergySTEP for Cement, Schneider Electric,
Energy Step Training (2012)
ll The overall efficiency is a combination of the fan
efficiency, drive system and process system it serves. In
a preheater ID fan system, the gas flowrate, pressure
drop and temperature are all important to impact the
fan power consumption per t of clinker. The whole
•
VIJAYARENGAMANI R., and Fredric DUMAS., Schneider
Electric, Energy Audit Report in South East Asia, 12 (2009).
•
WANG, R., ‘Discover Cement Manufacturing Process’,
E-Learning, Schneider Electric University, (2010).
•
LANG, Q. Schneider Electric, Analysis Report on Remote Energy
Service, 06 (2012).
Reprinted from worldcement.com [Sep 12]