This paper presents a comparative analysis of backward curved impeller and backward inclined airfoil in centrifugal fans, focusing on their efficiency across different operational speeds. Results indicate that the backward inclined airfoil offers higher maximum efficiency (up to 81%) compared to the backward curved impeller (76%), particularly in clean, dry air applications. The study highlights the power characteristics and optimal performance conditions for both impeller types, establishing guidelines for their respective uses in various industrial applications.

1. *
Corresponding author e-mail: swatidayal@rediffmail.com Journal access: www.ijesft.com
Tel.: +91 9998670720 © 2015 A D Publication. All rights reserved
ID:IJESFT20150101001 January 2015
International Journal of
Engineering Science and Futuristic Technology
A Peer-reviewed journal
ISSN : 2454-1125
IJESFT 01 (2015) 001-009
Comparison of backward curved impeller with backward inclined
Airofoil of Centrifugal fan
S P Dayal *
Department of Mechanical Engineering, Government Polytechnic, Vadnagar, Gujarat, India
A B S T R A C T : The main objective of this paper is to show Comparison of backward curved impeller with backward
inclined Airofoil of Centrifugal fan. The best efficiency points at different speeds were obtained. The
produced static pressure and air volume at best efficiency points were recorded. Results were
plotted as fan characteristic curve and conclusion has been made.
© 2015 A D Publication. All rights reserved
Keywords: Centrifugal fan ,backward curved impeller , backward inclined airofoil
1. Introduction
Now a day in every textile spinning application waste evacuation is being done by continuous waste removal
process. The waste which is generated in preceding machines is being collected and guided through the duct
system to the filtering unit. The filtering unit comprises of different types of fan. i.e. Waste handling fan, Dust
handling fan, Clean air/Main fan etc.
Filter unit is heart of the textile mill as the breakdown in Filter unit leads to shutdown of the complete spinning
line results into huge losses in production hours. The Main Fan is the heart of the Filtering unit. The function of the
Main fan is to create required suction pressure at given waste volume to each machine. The losses because of
the ducting lengths, bends, surface roughness etc needs to be considered while project planning.
So the input for this project is to design the complete main centrifugal fan which can handle -2000 Pa static
pressure at 30000 m³/Hr air volume with optimum power consumption
According to their blade shapes, centrifugal fans can be subdivided into the following six categories: AF
(airfoil), BC(backward-curved). BI (backward inclined), RT(radial tip),FC(forward curved) and RB(radial blade).
These six commonly used blade shapes. Each of them has its own advantages and disadvantages. Accordingly
each is well suited for certain applications. Many fans, however, are built for low cost and have maximum
efficiencies below those shown, and occasionally – as we will see- even higher efficiencies are obtained with BI
and RT blades. The highest efficiencies can be obtained with airfoil blades, the lowest with radial blades. There
are six types of centrifugal fan wheel in common use according to the types of blades.
Centrifugal fan with AF blades has the best mechanical efficiency and the lowest noise level (For comparable
tip speed) of all centrifugal fans. Cast aluminium AF blades are often used in small sizes and for testing
development work.
Centrifugal fan with BC blades are having single thickness otherwise are similar to AF blades with respect to
construction and performances. They have slightly lower efficiencies but can handle contaminated air streams
because the single-thickness steel blades can be made of heavier material than can be used for hollow airfoil
blades.
Centrifugal fans with BI blades are economical in production, but they are somewhat lower in structural
strength and efficiency. Centrifugal fan wheels also can be used without scroll housing, in such applications as
unhoused plug fans, multistage units and roof ventilators.
Centrifugal fans with RT blades are curved, with good flow conditions at the leading edge. These RT wheels

2. S P Dayal / International Journal of Engineering Science and Futuristic Technology 2
Nomenclature
AF airfoil
BC backward-curved
BI backward inclined
RT radial tip
FC forward curved
RB radial blade
are used mainly in large sizes, with wheel diameters from 30 to 60 in., for industrial applications, often with severe
conditions of high temperature and light concentrations of solids.
Centrifugal fans with FC blades as the name suggests, are curved forward i.e. in the direction of the rotation.
This results in very large blade angles and in flow rates that are much larger than those of any other centrifugal
fan of the same size and speed. These fans are used in small furnaces, air conditioners and electronic
equipments, wherever compactness is more important than efficiency.
Centrifugal fans with Radial blades have comparatively low efficiencies because of the non tangential flow
conditions at the leading edge. These fans can handle not only corrosive fumes but even abrasive materials from
grinding operations.
Centrifugal fans can be used for exhaust or for supply. With exception of the models with reduced blade
width, these fans are for large air volumes and for moderately high static pressures. They have high efficiencies
and nonover loading brake horsepower curves. They are for general ventilation, forced or induced draft, boilers
and bag houses and clean or slightly dirty air. This means that they are for industrial applications. Most fan
manufacturers offer these units with a choice of the following four different wheel types in each size: Air handling
wheels(AH), Material handling wheels(MH), Long shaving wheels(LS) and Long shaving open wheels(LSO).The
AH unit is used for supply as well as for exhaust. It is used for handling air, gas or fumes that are clean or only
slightly dusty. The MH wheel can handle air or gases containing small particle dust and granular materials from
wood or metal working operations without plugging up the blade passages. In the case of LS fans, the shroud has
been omitted so the risk of plugging up has been further reduced and even long shavings and abrasive materials
can be handled. Temperatures up 1600˚F can be tolerated. The LSO fan consists of a rugged wheel that can
handle not only long shavings but extremely abrasive and corrosive materials and can tolerate high temperatures.
2. Flow Analysis
In this paper comparison the complete parameters of two different kinds of Impellers have been done.
Both Impellers are having the same diameter, no of blades, blade height, Inlet diameter etc. The only difference
being kind of blade and geometry of blade. This paper presents the comparison of backward curved impeller with
backward inclined Airofoil.
Series of experiment has been carried out at various RPM and result recorded.
Table 1 Motor speed 1000 RPM
Type of Blade
Air Volume
(m³/Hr)
Static Pressure
(Pa)
Motor Power
(Kw)
Efficiency
(%)
Backward Airfoil
26181 982 9.1 78.48
Backward Airfoil
23687 1105 9 80.78
Backward Airfoil
21195 1230 9 80.46
Backward curved
27428 983 10.03 74.67
Backward curved
24934 1105 10.03 76.30
Backward curved
22440 1228 10.3 74.32

3. 3 S P Dayal / International Journal of Engineering Science and Futuristic Technology
Table 2 Motor speed 1100 RPM
Type of Blade
Air Volume
(m³/Hr)
Static Pressure
(Pa)
Motor Power
(Kw)
Efficiency
(%)
Backward Airfoil
32415 980 12 73.53
Backward Airfoil
29920 1105 11.9 77.17
Backward Airfoil
28674 1230 12.39 79.07
Backward Airfoil
26180 1351 12.15 80.86
Backward Airfoil
21195 1474 11.13 77.97
Backward curved
33662.5 983 13.13 70.01
Backward curved
31168 1106 13.05 73.38
Backward curved
28674 1229 12.95 75.59
Backward curved
26181 1351 12.92 76.05
Backward curved
22450 1474 12.62 72.84
Table 3 Motor speed 1200 RPM
Type of Blade
Air Volume
(m³/Hr)
Static Pressure
(Pa)
Motor Power
(Kw)
Efficiency
(%)
Backward Airfoil
37402 982 15.27 66.81
Backward Airfoil
36154 1105 15.7 70.68
Backward Airfoil
33662 1228 15.3 75.05
Backward Airfoil
29921 1475 15.3 80.13
Backward Airfoil
22440 1720 13.8 77.69
Backward curved
38649 983 16.16 65.31
Backward curved
37402 1105 16.76 68.50
Backward curved
34909 1228 16.55 71.95
Backward curved
33662 1351 17.12 73.79
Backward curved
31168 1474 16.87 75.65
Backward curved
28674 1597 16.71 76.12
Backward curved
23687 1720 15.56 72.73

4. S P Dayal / International Journal of Engineering Science and Futuristic Technology 4
Table 4 Motor speed 1300 RPM
Type of Blade
Air Volume
(m³/Hr)
Static Pressure
(Pa)
Motor Power
(Kw)
Efficiency
(%)
Backward Airfoil
42390 983 19.1 60.60
Backward Airfoil
39896 1228.5 19.77 68.86
Backward Airfoil
37402 1474 20.53 74.59
Backward Airfoil
33662.5 1720 20.32 79.15
Backward Airfoil
29921.2 1965 20.14 81.09
Backward curved
43635 983 20.05 59.43
Backward curved
42390 1105 20.35 63.94
Backward curved
41143 1228 20.93 67.05
Backward curved
38649 1351 20.55 70.58
Backward curved
37402 1474 21.12 72.51
Backward curved
36154 1597 21.66 74.05
Backward curved
33662 1720 21.25 75.68
Backward curved
31168 1842 20.91 76.27
Backward curved
27428 1965 20.33 73.64
Table 5 Motor speed 1400 RPM
Type of Blade
Air Volume
(m³/Hr)
Static Pressure
(Pa)
Motor Power
(Kw)
Efficiency
(%)
Backward Airfoil
44882.7 1228 24.17 63.34
Backward Airfoil
42390.3 1474 24.75 70.13
Backward Airfoil
39896.1 1720 25.36 75.16
Backward Airfoil
36154.9 1965.7 24.97 79.06
Backward Airfoil
32415 2211 24.56 81.06
Backward curved
46129 1228 25.2 62.44
Backward curved
44882 1351 25.66 65.64

5. 5 S P Dayal / International Journal of Engineering Science and Futuristic Technology
Backward curved
43635 1474 26.2 68.19
Backward curved
42390 1597 26.74 70.32
Backward curved
39894 1720 26.19 72.78
Backward curved
38649 1842 26.68 74.12
Backward curved
35154 1966 26.11 73.53
Backward curved
32415 2088 26.34 71.38
Backward curved
34909 2211 28.08 76.35
Table 6 Motor speed 1500 RPM
Type of Blade
Air Volume
(m³/Hr)
Static Pressure
(Pa)
Motor Power
(Kw)
Efficiency
(%)
Backward Airfoil
44882 1720 30.25 70.89
Backward Airfoil
42390 1965 30.79 75.15
Backward Airfoil
39896 2211 31.32 78.23
Backward Airfoil
37402 2457 31.8 80.27
Backward curved
46129 1720 31.98 68.92
Backward curved
44882 1842 32.48 70.70
Backward curved
43635 1965 32.97 72.24
Backward curved
39896 2211 32.63 75.09
Backward curved
36154 2457 32.3 76.39
Backward curved
31168 2702 32.04 73.01
3. Result and Discussion
Fig.1 fan characteristic curve

6. S P Dayal / International Journal of Engineering Science and Futuristic Technology 6
High efficiency backwardly inclined airfoil bladed wheel are designed for clean, dry air applications.BCA
wheels exhibit non-overloading power characteristics and stable performance over the entire pressure curve.
Noise levels are lowest in the peak efficiency range of the performance curve.
Fig.2 fan characteristic curve
High efficiency backward curved wheel has blade shape similar to the convex shape of the BCA airfoil
wheel. This shape provides nearly identical performance characteristics at a given speed at a slightly lower
efficiency. BCS wheels also exhibit the same non-overloading power characteristics and stable performance over
the entire pressure curve. BCS wheels should be specified in moist or lightly contaminated air systems. Noise
levels are lowest in the peak efficiency range of the performance curve. They are less costly in production.
Fig.3 comparision of fan characteristic curve
The Graph is showing the comparison of fan characteristics of two fans.
From the graph we can conclude that for the same fan speed backward curved fan is developing more
static pressure and air volume up to certain range. (1000 RPM to 1200 RPM) after that at elevated speed the
static pressure and air volume is very much comparable. Fan characteristic conclude that each fan is made for a
specific range.
Fig.4 fan efficiency VS air volume

7. 7 S P Dayal / International Journal of Engineering Science and Futuristic Technology
The maximum efficiency (81%) of backward inclined airfoil impeller occurs at following parameters.
At 1000rpm,23000 m³/Hr@-1200 Pa static pressure
At 1100rpm,27000 m³/Hr@-1300 Pa static pressure
At 1300rpm,30000 m³/Hr@-2000 Pa static pressure
At 1400rpm,33000 m³/Hr@-2250 Pa static pressure
The maximum efficiency (76%) of backward curved impeller occurs at following parameters.
At 1000rpm,24000 m³/Hr@-1100 Pa static pressure
At 1100rpm,26000 m³/Hr@-1250 Pa static pressure
At 1200rpm,29000 m³/Hr@-1550 Pa static pressure
At 1300rpm,33000 m³/Hr@-1700 Pa static pressure
At 1500rpm,37000 m³/Hr@-2400 Pa static pressure
Fig.5 power characteristic curve
Power characteristic reveals that for the same air handling capacity, backward curved impeller requires
more power compared to airfoil backward inclined impeller
Conclusion
A Parametric analysis is also carried out to compare the performance of Backward curved Impeller with
Airfoil backward inclined impeller. Back to back analysis carried out that means the both Impellers are having the
same outer diameter, no of blades, blade height, Inlet diameter etc. The only difference is being kind of blade and
geometry of blade. We will present the comparison of backward curved impeller with backward inclined Airofoil
impeller.
High efficiency backwardly inclined airfoil bladed wheel are designed for clean, dry air applications.BCA
wheels exhibit non-overloading power characteristics and stable performance over the entire pressure curve.
Noise levels are lowest in the peak efficiency range of the performance curve.
High efficiency backward curved wheel has blade shape similar to the convex shape of the BCA airfoil
wheel. This shape provides nearly identical performance characteristics at a given speed at a slightly lower
efficiency.
BC wheels also exhibit the same non-overloading power characteristics and stable performance over the
entire pressure curve. BC wheels should be specified in moist or lightly contaminated air systems. Noise levels
are lowest in the peak efficiency range of the performance curve. They are less costly in production.

8. S P Dayal / International Journal of Engineering Science and Futuristic Technology 8
Serial no Impeller
speed (RPM)
Air volume
(m ³/Hr)
Static Pressure
(Pa)
Best Efficiency
(%)
BC BCA BC BCA BC BCA
1 1000 25000 23000 -1100 -1200 76 81
2 1100 26000 27000 -1350 -1300 76 81
3 1200 28700 30000 -1600 -1475 76 81
4 1300 31100 30000 -1850 -1965 76 81
5 1400 35000 32400 -2200 -2200 76 81
6 1500 36100 37400 -2460 -2460 76 80
Reference
[1] Fan Handbook Selection,Application and Design- Frank P Bleier (Mc graw Hill)
[2] Inlet guide vane performance of centrifugal fan blowers. Trans ASME Ser.A 83,4,371-80 -Stepanoff A J
[3] Dissimilarity laws in centrifugal pumps and blowers Trans ASME Ser.A 83,4,381-9 -Stepanoff A J & Stahl H
[4] Blade design for turbomachinery (German) Konstruction1,3,77-82 -Pantell K
[5] Friction of discs rotating in Fluid (German) Forsch.Ing-Wes 16,4,97-108 -Pantell K
[6] Modern design recommendations for the construction of centrifugal fans (German) Heiz. Luft Haustech10,162-6 -
Gruber J
[7] Problem of optimum impeller width of radial flow fans (German) Heiz. Luft Haustech12,161-7,211-6 -Hoenmann W
[8] Improved design of fans with drum type rotors. Engineer’s Digest 20,1,71-3.-Laakso
[9] Investigations into flow separation in the suction space of radial fans (German) Heiz-Luft-Haustech 13,113-9,155-
7,176-81,260-6 -Hoenmann W
[10] Theory of the Impeller of Multi-blade Centrifugal Fans Rep Inst High Speed Mech Japan - Abe,S
[11] An experimental study of centrifugal pump impellers Trans. ASME 79,8,1821-39 -Acosta AJ and Bowerman R D
[12] The effect of the number of blades on the characteristic of backward curved centrifugal fan (German) Heiz Luft
Haustech14,5,159,166; 6,206-9,7-228-33 -Bommes L
[13] The influence of inlet form and bends on performance of centrifugal fan (German) Heiz Luft Haustech11,7,170-2-
Bouwman H B
[14] Investigation of the volumetric loss of a centrifugal fan Acta.Tech Acad Sci Hung18,255-61-Blaho M and Preszler L
[15] Three dimensional flow in centrifugal impellers.Bull JSME 1,1,42-59;3,275-82 -Fujie K
[16] Research on flow in a centrifugal pump impeller. Bull JSME 5,17,88-116;18,259-76-Murata S
[17] Fundamentals of Machine Design-Shigley
[18] The measurement of Air flow-Chapman & Hall
[19] A.S.H.R.A.E Guide and Data Book, Fundamentals and Equipment.
[20] Ventilatoren Springer-Verlag Berlin-Eck B
[21] Hagen H F (In discussion of paper by M S Kice) J I Mech E 154,115
[22] Noise Reduction Mc grawhill-Beranek LL

9. 9 S P Dayal / International Journal of Engineering Science and Futuristic Technology
[23] Dampf und Gasturbinen, 6 Aufl,Springer Berlin-Stodola.
[24] J I Mech E 124,481 –Kearton W J.
[25] Proc. I. Mech E 153,441 –Howell A R.
[26] Theory of Wing sections McGraw Hill–Abbot I H and Von Doenhoff
[27] The low speed performance of Related Aerofoils in Cascade–Carter A D S
[28] The elements of Aerofoil Theory Cambridge University Press–Glauert H
[29] Aerofoil catalogue and Bibliography Aero Res Council R & M 3311-Pankhurst R C, NPL
[30] Aerofoil Sections, Butterworth London –Riegels F W.
[31] Application of boundary layer theory in turbomachinery, Tran ASME Ser.D 81,543-51 –Schlicting.
[32] Kaup C.: Einsatz von freilaufenden Rädernals Ventilatorsystem in RLT-Geräten [Use of
plug-in fan impellers as a fan system in AHUequipment,
[33] Bommes L.: Problemlösungen bei der Gestaltung von Radialventilatoren [Solutions to problems encountered in
centrifugal fan design),HLH 25 (1974), No. 12, p. 420-425
[34] Leist H., Roth H.W., Schilling R., Zierep J:Neuere Entwicklungen auf dem Gebiet der Radialventilatoren hoher
Leistungsdichte [Recentdevelopments in centrifugal fans of high powerdensity], HLH 30 (1979), No. 11, p. 443-447
[35] DIN 45635, Measurement of noise emitted by machines, Part 9.
[36] DIN 24163, Fans; performance testing, standardized test airways, Part 2.