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The main aim of this interim report is to highlight the work done during semester A with the project on “Study on the Performance Analysis of Centrifugal Blower” as of 20th February 2009.
This report contains the important information of querying on the web and books. A few important technical information (on the web) have been given, which could be improved on further research. The main concept of impellers has been compiled in this report.
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Centrifugal blower is a radial flow machine which is used for deliver air. It has an impeller inside and which consists of blades mounted around a hub. The input power for centrifugal blower is mechanical energy and such as electrical motor of the drive shaft which is driven by the prime mover. As the impeller rotates, the centrifugal force is created and moves the air outward along the blade channels, the air which enters axially from opposite side, and exits radially to other side through fan housing.
The impeller is turned either by the direct drive or by an electric motor employing pulleys and belts.
Parts of the centrifugal blower are:
Impeller: rotates and transfers energy to the air.
Blade: the working surface. (vane, paddle)
Shroud: supports the blades ( cover disc, inlet plate,
backplate, rim, flange).
Hub: attaches to the fan/motor shaft but may support the blades directly (axial) or indirectly through a shroud (centrifugal). (boss, disc)
Housing: encloses the impeller and guides the air to
and from the impeller. (casing, scroll, panel, ring, volute)
Inlet: the opening to the impeller. (eye, suction, suction eye, inlet cone, inlet bell, inlet nozzle).
Outlet: the opening leaving the fan. (discharge, discharge cone, evase, diffuser).
Guide Vanes: when installed before the impeller are called pre-rotation vanes or inlet guide vanes. If they are adjustable they are called variable inlet vanes or simply VIV’s. When installed after the impeller they are termed straightening vanes or discharge guide vanes.
CONCEPT OF IMPELLERS
There are three different type impellers, which are used in the centrifugal blower. They are:
Forward Curved Impeller
Backward Curved Impeller
Radial Curved Impeller
Forward Curved Impeller: In this type of impeller, air curve in the direction of the fan wheel’s rotation. Normally this impeller is used, where there is high flow, low pressure applications and typically 3 – 5in static pressure. The advantage of the forward curve fan is its low speed and quiet operation. Forward curved wheels use higher horsepower at low resistance. It is highly dependent on the housing for performance, and is not suitable for plug or plenum fan applications. Without housing, the forward curve impeller becomes unstable and exhibits a relatively poor performance.
FC fans are made of riveted or welded stamped sheet metal, the common examples are a furnace blower and an air handler. Fans are mounted with large number of blades and short chord length. The forward velocity vector of the air coming off from the blades is high, this results in low fan speed. Static efficiencies are quiet low, because controlling and directing the velocity vector is difficult. An FC fan is used only for clean air applications. Compare to other impellers, efficiency is less, wheel is smallest and fabricated very light in weight.
Backward Curved Impeller: Air curve against the direction of the fan wheel’s rotation. The impeller blades are larger and heavier than forward curved blades. Blades are in three different shapes such as flat single thickness, curved single thickness, curved airfoil. Backward inclined fans are used to deliver medium to high airflow at static pressures up to 20″ water gauge. Pressures up to 40″ water gauge are attainable with special construction. Efficient performance range from 40% to 85% of free delivery. The horsepower increases to a maximum as airflow increases, and then drops off again. Typically the flat bladed design has efficiencies of about 82%, while the curved blade designs approach 86 to 90%, respectively.
These fans are used primarily in the industrial market for ventilation, clean side of commercial air cleaning devices, furnace draft and large commercial heating and cooling units. The air leaving the backward inclined impeller has less of its total energy in the form of velocity pressure.
Radial Curved Impeller: In this type of impeller, air is extended straight out from the hub. The impeller blades are generally narrower, deeper and heavier than forward curve and backward inclined blades. A radial blade impeller usually comprises six to twelve equally spaced flat blades extending radially from the center of the hub. These impellers are generally of simple design that lends itself to rugged construction and offers a minimum of ledges, etc., for the accumulation of dust or sticky materials. These fans are generally selected to operate from 35% to 80% of free delivery.
The disadvantages are, they generate more noise than forward curve and backward inclined fans, primarily because of the impeller design and high operating velocities and they exhibit the same rising horsepower characteristic as the forward curve fans. Because they are low volume fans, larger sizes are generally required, taking up a larger installation space. Fan efficiencies are lower than both the forward curve and backward inclined type.
Forward Curved Blade ( 3000 RPM)
Blower Speed in RPM ‘N’
Torque in Kgm. ‘T’
Static Head rise in mm of water ‘hsta’
Flow differential head of pitot tube in mm of water ‘hf’
Energy meter reading time in secs for 5 revolutions
Electrical Input ‘HP I/P Watts
Output of AC Motor ‘HP O/P’ Watts
Static Pressure rise meter of air column ‘Hsta’,m
Volume flow rate ‘Q’ m^3/sec
Blower O/P horse power ‘HP blower’ Watts
Blower Efficiency N blower, %
Electrical input to AC motor as read on energy meter:
HP i/p = 2*100*60*60
= 1.17*10^3 Watts
Output Horse power from the AC motor as read on torque meter:
HP o/p = 2*pi*Nmotor*T
= 7.33 Watts
Static head developed due to air in the duct:
Hsta = hsta
——– * [ pw/pa – 1 ]
= 0.5/1000 * [ 1000/1.29 – 1 ]
= Hsta = 0.774 * hsta
= 0.774 * 0.5
Volume flow rate:
Q = A (âˆš2 gHf )
A = ( Ï€/4 ) * d^2
= Ï€/4 * ( 0.162 )^2
= 0.021 m^2
Q = 0.39 * âˆšhf
= 0.39 * 0.3
= 0.214 m^3/sec
Air horse power calculated from the output of blower:
HPblower = Pa * Q * Hsta
= 1.29 * 0.774 * 0.39
= 5.2 * 10^-3
HP blower = 5.2*10^-3 * hsta * âˆš hf
= 5.2 * 10^-3 * 0.5 * âˆš 0.3
= 1.42 * 10^-3
Efficiency: N blower = Output / Input *100
= 7.33 / 1170
= 63 %
Discussions with the supervisor
of the project
Selection of project proposal and project planning preparation
Study of Theoretical methods
Experimental setup &experimental Procedure
Analysing the result
Interim report preparation
Conclusion and Recommendation
Preparing the poster presentation
Final project report
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