For a fan transporting a large volume of air at low static pressure the motor

For a fan transporting a large volume of air at low

This preview shows page 29 - 36 out of 229 pages.

For a fan transporting a large volume of air at low static pressure the motor power required during continuous operating may not be enough to accelerate the fan and a correct designed and adjusted motor protection system may stop the fan before windings overheats and insulation is damaged. Motor torque should be checked against fan wheel torque up to 90% of synchronized speed during design. 26 | P a g e Big Industrial Fan
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Faculty of Mechanical Engineering and Manufacturing MCD Project Report Motor Starting Torque Motor starting torque for a belt driven fan can be calculated as T = ℑ= 1.1 If ( Nf Nm ) where , I = moment of inertia that the motor must be capable of turning at the motor shaft (lbm ft2, kg m2) a=angular acceleration If = moment of inertia of the fan wheel (lbm ft2, kg m2) nf = fan speed (rpm, min-1 nm = motor speed (rpm, min-1) For direct drive fan Im always exceeds If. 2. Torque from drag and lift force to push air The efficiency of axial flow fans is greatly dependent on the profile of the blade, and the aerodynamic characteristics of the fan blades are strongly affected by the shape of the blade cross section. The blade performance characteristics may be predicted from the aero- dynamic characteristics such as lift and drag coefficients of the chosen aero foil section and given by the following equations: C 1 = 2 Fl ρV 2 A (1) C d = 2 Fd ρ V 2 A (2) where : Clis thecoefficient of lift, Cdisthe coefficient of drag Flis thelift force , Fdisthe drag force 27 | P a g e Big Industrial Fan
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Faculty of Mechanical Engineering and Manufacturing MCD Project Report The aerodynamic lifting force is a vital component and must be much greater than the drag component. Since lift contributes to the head generated by the fan and the drag causes loss due to skin friction in the wake behind the vane, the profile offering higher L/D ratio is considered more efficient. The lift and profile drag of the aero foil shaped blades, when move through air, vary with the structure of the aero foil and variations in the angle of attack (α). As the angle of attack increases, the coefficient of lift increases in a near-linear manner. However, at an angle of attack usually between 12 and 18°, breakaway of the boundary layer occurs on the upper surface. This causes a sudden loss of lift and an increase in drag, known as stall condition. 28 | P a g e Big Industrial Fan
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Faculty of Mechanical Engineering and Manufacturing MCD Project Report Since the fan blade its swiping the, the torque is calculated as, T = B ∗( F R L 2 ) When, B = number of blade F = Resultant force of lift drag L = lengthof blade T total = T Startingovercomeweight + T swiping 29 | P a g e Big Industrial Fan
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Faculty of Mechanical Engineering and Manufacturing MCD Project Report 1.4 PROJECT DESIGN PLANNING CHART 30 | P a g e Big Industrial Fan
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Faculty of Mechanical Engineering and Manufacturing MCD Project Report CHAPTER 2.0 DESIGN AND ANALYSIS 31 | P a g e Big Industrial Fan
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Faculty of Mechanical Engineering and Manufacturing MCD Project Report 2.1 Concept and Development (sketching) 32 | P a g e Big Industrial Fan
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