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FANS

VIEWS: 150 PAGES: 20

									Fans
Fans
Purpose:
 To supply the necessary static pressure to move air
  in a         ventilation or exhaust system.
Basic classifications of air moving devices:
 Ejectors
 Fans




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Ejectors
  Low operating efficiencies.
  Used only for special material handling applications (corrosive
   material, flammable material, hot or sticky material).
  Used when not desirable to have contaminated air passing
   directly through the air moving device.
  Used for air streams containing materials that might degrade
   fan performance.
  Used in pneumatic conveying systems.




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Fans
Primary air moving devices used in industrial applications

Basic groups of fans are:
 Axial fans
 Centrifugal fans (home furnaces, hair dryers, vacuum cleaners)
 Special type fans




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Axial Fans
Propeller fans:
 Moves air against low static pressures (less than 1” wg)
 Commonly used for general ventilation
 Very sensitive to added resistance
 Blade types:
       Disc blade
       Propeller blade
2. Tube axial fans:
 Moves air against moderate pressures (less than 2” wg)
 Blade type: propeller type with no straightening vanes
3.Vane axial fans:
 Moves air against high static pressures (up to 8” wg)
 Limited to clean air applications
 Blade types: air foil blades with straightening vanes



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Centrifugal Fans
1. Forward curved impellers:
 Blades curve towards the direction of rotation.
 Fans have low space requirements and low tip speeds.
 Used against low to moderate static pressures.
 Not recommended for dust or particulate that would adhere to blades.
2. Radial impellers:
 Blades are in a radial direction from the hub.
 Fan have medium tip speeds.
 Radial blade shape resist material build up.
 Can handle either clean or dirty air.
3. Backward inclined/curved impellers:
 Blades are inclined opposite to the direction of fan rotation.
 High fan efficiency and relatively low noise levels.
 Blade shape is conducive to material buildup.



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Special Types of Fans
1. Inline centrifugal fans:
 Backward inclined blades are used.
 Pressure versus flow rate performance curves are similar to
    centrifugal fans.
 Space requirements are similar to axial fans.
2. Power roof ventilators:
 Packaged units that can be either axial flow or centrifugal type.
 Centrifugal type discharges around the periphery of the ventilator.
3. Fan and dust collector combination:
 Fans and dust collectors are packaged in a unit.

Power exhausters and air foil are other special type fans.

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Fan Selection
Considerations for fan selection are :
1. Capacity:
       Flow rate based on system requirements. Expressed as actual cubic feet
        per minute (acfm).
       Pressure requirement based on system pressure requirements. Expressed
        as FSP or FTP in inches of water gauge.
2. Air stream:
 Material handled through fan.
      Small amount of smoke or dust - backward inclined centrifugal or

        axial fan is selected.
      Light dust fume or moisture - backward inclined or radial fan is
        preferred.
      Heavy particulate loading - radial fan is selected.

 Explosive or flammable material.
      Spark resistant construction is used.
      Explosion proof motor is used.




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Fan Selection
3. Physical Limitations:
   Fan size is determined by
     Performance requirements
     Inlet size and location
     Fan weight
The most efficient fan size may not fit the physical space available.

4. Drive arrangements:
   Electric motor is the power source of fans.
    Unlike packaged fans, for larger units the motor is coupled
      directly to the fan or indirectly by a belt drive.




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Fan Selection
Standard drive arrangements are:
 Direct drive:
     Offers more compact assembly and assures constant fan speed.
     Fan speeds are limited to available motor speeds.
 Belt drive:
     Offers flexibility in changing the fan speed.
     Important in applications where changes in system capacity or
       pressure requirements are needed.

5. Noise:
 Generated by turbulence within he fan housing.
 “White” noise which is a mixture of all frequencies is mostly produced.
 Radial blade fans produce a pure tone at a frequency BPF.
              BPF = rpm * n * CF.
    Where:
    BPF - blade passage frequency.
    RPM - rotational rate.
    N - number of blades.
    CF - conversion factor, 1/60.

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Fan Selection
6. Safety and accessories:
 Safety guards are required at inlet, outlet, shaft, drive and
   cleanout doors.
 Accessories help in future maintenance requirements.
 Flow control can be done using dampers.
Outlet dampers:
 Mounted on the fan outlet.
 Adds resistance to the system when partially closed.
Inlet dampers:
 Mounted on the fan inlet.
 Pre-spins air into the impeller.
 Lowers operating horsepower.




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Fan Selection
Various factors effecting fan selection are:
 Volume required (cfm)
 Fan static pressure
 Type of material handled
 Explosive or inflammable material
 Direct driven vs belt driven
 Space limitations
 Noise
 Operating temperature
 Efficiency
 Corrosive applications

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Rating Tables
 Fan size and operating RPM and BHP can be obtained from these
  tables
 Tables are based on the following pressure relationships
  FanTP = TP out - TP in = (SP out + VP out) - (SP in + VP in)
  FanSP = FanTP - VP out = SP out - SP in - VP in
Refer to IV manual, table 6-1 (page 6-14)




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Point of Operation
 Fan and system have variable performance characteristics which can be
  represented graphically.
 “Point of operation” is the single point at the intersection of fan curve and
  system curve.

Fan Performance Curves:
 Curve represents fan performance variables plotted against flow rate.
 Curve is specific to a fan of given size operating at a single rotation rate (
   RPM ).
 Even with size and rotation rate fixed, power and pressure requirements
   vary over a range of flow rates.

System Requirement Curves:
 Duct system pressure varies with volumetric flow rate.
 Curve represents the variation of pressure plotted against volumetric flow
   rate.



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Fan Laws
 Useful when changes in fan performance are required.
 Principles relate the performance variables for any homologous
  series of fans.
 Predict the effect of varying size, speed, capacity, pressure and
  power requirement as follows:

   Q2 = Q1 (size2/size1)3 (rpm2/rpm1).

   P2 = P1 (size2/size1)2 (rpm2/rpm1)2 ρ2/ ρ1.

   PWR2 = PWR1 (size2/size1)5 (rpm2/rpm1)3 ρ2/ ρ1.




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Simplified Fan Laws
For changes of rotation rate:
     Flow varies directly with rotation rate
     Pressure varies as square of rotation rate
     Power varies as cube of the rotation rate
        Q2 = Q1 (rpm2/rpm1)
        P2 = P1 (rpm2/rpm1)2
        PWR2 = PWR1 (rpm/rpm)

For changes of gas density:
     Flow is not affected by a change in density
     Pressure and power vary directly with density
        Q2 = q 1
        P2 = P1 (ρ2/ρ1)
        PWR2 = PWR1 (ρ2/ρ1)


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Limitations of Fan Laws
Fan laws rely on the fact that the performance curves are
  homologous and the ratios are for the same relative
  points of rating on each curve.

When applying fan laws to the following cases special care
 must be exercised.

Case 1: when pressure does not vary as the square of the
 flow rate.
Case 2: when the system has been physically altered or
 for some reason operates on a different system line.


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Fan Selection at a Density Other Than
Standard
Fan performance is affected by changes in gas density.
 Corrections must be employed if density varies by more than 5% from
   the standard 0.075lbm/ft3
 Corrected Pressure is given by:
               Pe = Pa (0.075/ρa)
   Pe = Equivalent or corrected pressure
   Pa = Actual pressure
   ρa = Actual density, lbm/ft3

 Actual power requirement is given by
               PWRa = PWRt (ρa/0.075)
   PWRa = Actual power requirement
   PWRt = Power requirement in rating table.
   ρa = Actual density, lbm/ft3

 Fan selection at non-standard density requires knowledge of actual
   volumetric flowrate, actual pressure requirement and the density of gas
   at the fan inlet .
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Fan Installation and Maintenance
 Fan rating tests are conducted under ideal conditions i.e.,uniform straight
  air flow .
 In practice duct connections cause non-uniform air flow.
 Location and installation of fan must consider the location of duct
  components to minimize losses.

System Effect:
 This is the estimated loss in fan performance due to non-uniform air flow.
 System effect factor is obtained from resulting fan performance curve and
   actual system curve.
A vortex or spin may be created by non-uniform flow conditions.
 This may be caused due to poor inlet box, multiple elbows or ducts near the
   inlet.
 If vortex or spin cannot be avoided, the use of turning vanes, splitter sheets
   will reduce the effect.


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Fan Installation and Maintenance
Inspection and Maintenance:
 Wear or accumulation on an impeller will cause weakening of the
   impeller structure .
 Severe vibrations may cause damage or failure at the bearings or fan
   structure.


Scheduled inspection of following items of fans is recommended:
 Bearings for proper operating temperature
 Excessive vibration of bearings or housing
 Belt drives for proper tension and minimum wear
 Correct coupling alignment
 Fan impeller for proper alignment and rotation
 Impeller free from excess wear or material accumulation

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