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.
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
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
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