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Fan Selection

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Published in: Mechanical
5,820 Views

This PPT Is On Selection Of Fan And Its Calculation

Juned K / Navi Mumbai

10 years of teaching experience

Qualification: M.Tech (DR BATU LONERE MANGAON - 2015), Ph.D (Amity Mumbai Campus - 2023), B.Tech/B.E. (FINOLEX ACADEMY OF MANAGEMENT AND TECHNOLOGY RATNAGIRI - 2011)

Teaches: All Subjects, Hindi, Mathematics, Science, Accountancy, Arts Group, Physics, Biology, Chemistry, B.Sc Tuition, B.Tech Tuition, Drawing, Engineering Graphics, Mechanical, Mechanics, Production

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  1. Fan Selection Basics
  2. Fan Selection Basics • Centrifugal Fan Selection • Fan Laws ' Specific Speed Multiple Fan Arrangements ' Fan Features and Considerations
  3. Fan Selection Basics Howden Centrifugal Fan Selection Centrifugal Fan Centrifugal force Air Enters the Impeller Axially, Then Passes Through the Impeller Radially The Blades Drag the Air in a Circular Motion and Centrifugal o Forces Accelerate the Airflow Radially Outward The Airflow is Then Collected in The Volute Casing and Discharged at Right Angles to the Inlet Flow
  4. Fan Selection Basics Centrifugal Fan Selection Centrifugal force Increasing on particle Dotted line showing the track Of the particle, Here it can be seen entering the blade passage and starting to rotate Howden Motion of Air The Increase of Centrifugal Force With Radius Produces a Pressure Gradient, Whereby the Static Pressure in the Blade Increases With Radius The Blades Drag the Air in a Circular Motion and Centrifugal Forces Accelerate the Airflow Radially Outward
  5. Fan Selection Basics Centrifugal Fan Selection Centrifugal force increasing on particle The particle leaves the impeller following an almost tangential track. The particle is now almost spinning at the same speed as the impeller, generating the maximum centrifugal force Howden Motion of Impeller The Motion of the Rotating Blades Adds Rotational Momentum to the Flow, Increasing the Velocity of Flow in the Tangential Direction As the Air Moves Radially Through the Impeller, the Absolute Velocity of the Flow Will Increase, With the Absolute Maximum Velocity at the Impeller Outlet The Increase in the Absolute Velocity of the Air Leads to an Increase in the Kinetic Energy and Dynamic Pressure Across the Impeller
  6. Fan Selection Basics Centrifugal Fan Selection Howden Air Flow Through The Casing A Proportion of the Dynamic Pressure From the Air Leaving the Impeller is Transferred Into More Useful Static Pressure The Fan Casing is Usually Volute Shaped, Allowing Diffusion of the Air Leaving the Impeller to Take Place
  7. Fan Selection Basics Fan Laws "The Fan Laws" Howden It is not practical to test every fan at every variable which may be encountered. Fortunately, by use of a series of equations, commonly referred to as the "Fan Laws" it is possible to predict with good accuracy the performance of a fan at other variables. The variables include : Fan Size - D Fan Speed- N Fan Air Density - Pän Flow Rate - Fan Total Pressure - PFT PFV Fan Velocity Pressure — Fan Static Pressure — PFS Fan Input Power— Ps Fan Total Efficiency— 11T IIS Fan Static Efficiency— Compressibility Coefficient— K Sound Power Level - L
  8. Fan Selection Basics Fan Laws - Volume Volume = D *W *C Volume 1 Blade Blade Inlet Detail Howden 1 W 1 c ml • Volume D * tan 1 1 ml 1
  9. Fan Selection Basics Fan Laws - Pressure Pressure : Howden *kpl Pressure = [(1J2 * cu2) -(UI * CUI)] * p but CUI = O when there is no pre-swirl. Pressure = (IJ2 * cu2) * p Pa cu = - (CM2/tan 132) Pressure D2 * p pa N22 N12 2 Blade Tail Detail
  10. Fan Selection Basics Fan Laws - Power Volume * Pr essure Power pu2 = pul * Howden Power . N13 *DI
  11. Fan Selection Basics Fan Laws - Summary Howden Fan Law Summary Volume : Pr essure : Power : Pu2 N22 *1022 PI * p2 N23 *D25 = Pul * 11
  12. Fan Selection Basics Fan Laws - Example Howden Given a 36.5-in. dimneter fan delivering 10000 cfm at 1.85 in. wg, 0.075 lbm/ft3, 600 rpm, and 3.4 hp, find the corresponding perfonnance of an homologous fan of 73.0-in. dianleter at 0.070 Ibnx/ft, and 1200 Use Fan Law I because the known quantities (independent variables) are D N , and p . Assume incompressible flow. Q PFTa P FTb Pia ib 3 73.0 1 1200 1 1 x 1 1 = 16.00 36.5 600 2 73.0 365 5 73.0 2 1200 600 3 1200 1 0.070 1 X 0.075 1 1 0.070 1 X 0.075 1 = 14.93, and = 238.93. 3 6.5 600 Qa = = 160 000 cfm = 1.85 x 14.93 = 27.62 in. wg, and PFTa Pia = 3.4 x 238.93 = 812.4 hi).
  13. Fan Selection Basics Fan Laws - Example Howden Given a fan delivering 10000 cfm at 1.85 in. wg, 0.075 lbm/ft3, 600 113111, and 3.4 hp, find the con-espondmg performance of the sallie fan at 12000 cfin and 0.075 lblWft3. Use Fan Law 3 because the independent variables are D and p (which are to be held constant), and Q. Assume incompressible flow. 3 1 12000 1 1 1 1 1 1 1.85 x PFTa 1 1 Pia = X 1 1 10000 1 1 2 12000 1 1 1 x x x 10000 1 1 3 12000 1 1 2 1 = 720 1 = 2.66 in. wg, and = 5.9 hi). 10000 1 1
  14. Fan Selection Basics Specific Speed Low Specific Speed Specific Speed High Specific Speed Rotational Speed* Volume Equivalent Pressure Howden Fan Specific Speed The Ability of a Fan To Generate Pressure and Flow With Impeller Speed. It is the Speed at Which a Fan of Indeterminate Diameter Will Run at to Produce Unit Flow at Unit Pressure Low Specific Speed Fans Produce High Pressures at Low Flow Volume High Specific Speed Fans Produce Lower Pressures at High Flow Volumes Specific Speed is Proportional to Impeller Inlet Diameter and Impeller Width for a Given (Fixed) Wheel Diameter > 14 >
  15. Fan Selection Basics Impeller Styles and Specific Speed z' Howden Impeller Ranges Abridged Fan Range (with lines of constant specific speed plotted) 9 8 7 6 5 4 3 2 1 50 1 00 15 Ns=2000 I Metre dia. fans 1910 rpm 1 kg/m3 30 Ns = V 1/2 p 3/4 35 Ns=3000 Ns=5000 Ns=7500 40 5 Radial Impeller Curved plate blades 10 Radial Impeller Curved plate blades 20 Volume Radial Impeller Airfoil blades 25 m3/s 45 Axial Impellers, various hub sizes Airfoil blades
  16. Fan Selection Basics Multiple Fan Arrangements Multiple Fan Arrangements Fans in Series , Fans in Parallel Start Up in Series or Parrallel z' Howden
  17. Fan Selection Basics Multiple Fan Arrangements FAN FAN z' Howden Fans in Series 17
  18. Fan Selection Basics Multiple Fan Arrangements FAN FAN z' Howden Fans in Parallel 18
  19. Fan Selection Basis Multiple Fan Arrangements ONET-AN FAN Ap 2ND FAN NOT POWERED FAN z' Howden Start Up for Fans in Series 19
  20. Fan Selection Basics Multiple Fan Arrangements FAN FAN z' Howden Start Up for Fans in Parallel 20
  21. Fan Selection Basics Fan Features and Considerations z' Howden Fan Feature Rotational Speed Fan Drive Control of fan Volume / Pressure Type of fan, Single or Double Inlet Fan Arrangement Points to be Considered • Very important - the faster the speed the smaller the fan. • the smaller the fan, the lower the fan cost • 'clean' fans can usually be run at relatively fast speeds. • the operating process often mean that lower speeds are necessary, and is often the case. • Most common by electric motor, using a 50 Hz or 60 Hz asynchronous speed. • Variable speed electric motor • Steam turbine ( often dual drive with an electric motor) • Two speed electric motor • Radial Vane Control, gives better turn down efficiencies than a box vane control. • Inlet Box Vane Control lower cost than an RVC. Can be used on physically small fans. IJsed on many types of arduous duty fans • Variable speed electric motor (Or Turbine). Can be more expensive than the fan. Gives best turn down efficiencies but duties have to be near square law to benefit. e Damper in upstream or downstream duct (never used on high powered fans) • Centrifugal Flow (With plate blades) e Centrifugal Flow (with aerofoil blades ) e Mixed Flow • Single inlet fan with overhung impeller will be least cost. • Single inlet fan, between bearings, used when duties are unsuited to an overhung impeller. e Double inlet fan used when required duty not suitable for single inlet fan. Fan Features and Considerations
  22. Fan Selection Basics Fan Features and Considerations z' Howden Fan Feature Impeller Stress Rotor critical speed Inlet Boxes Bearings Points to be Considered • Dealing with centrifugal impellers. Get a knowledge of the approximate tip speed limitations of the various impeller designs. Generally the highest specific speed fans have the lowest tip speed capability, say around 130 m/sec. The low specific speed fans may be able to achieve tip speeds of around 260 m/sec. • If achievement of an acceptable critical speed ratio is a problem, bearing centres and chokage of the fan inlet by a large diameter shaft are items which will have a major influence on this value. e As most fans have an duct upstream (and downstream) inlet box('s) is the most common arrangement supplied. If an RVC is requested with a smaller fan, 2000mm diameter it may be possible with a 'separate' inlet box, (the box is not mounted on the casing front plate). e Bearing selection is extremely important, improperly selected bearings may mean the fan cannot be run even at fan first start up • Bearings supplied with Howden fans can be either rolling element or white metal sleeve (babbit metal designs. e Rolling element bearings often prove to be the most economic selection. Fan Features and Considerations

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