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Engineering Info


Industrial Air Flow Design Principals for Fan / Blower Applications.


As each industrial air moving system has it's own particular design and system effects, it would be impossible to predict the infinite amount of all possible system effects or phenomenon that may occur in a particular application. This information will center on heavy duty industrial process air moving applications.  For Air Conditioning design principals, light duty commercial (Schools - Restaurants Etc.) please search the web for keywords, "commercial air moving" or look in the consumer yellow pages under "air conditioning".


Table of Contents


Introduction.

The number 1 consumption of power use in the world is that of a Fan or a Blower.  For the average household has an average of 11 fans all fractional horsepower, consider the fans in the kitchen, the restroom, the blower in the dishwasher, the condenser fan in the refrigerator, the cooling fans in the computer, 1 fan and 1 blower in each car .  The average general industrial plant may have as few as 10- 3 phase industrial fans or blowers and often up to 50 or more. Fans and Blowers have been around since the mid 1800's with few changes.  Only since adopted test standards in the 1920's have improvements and advancements been made.  Fans and Blowers can be very efficient up to a maximum of 86%, however many of the systems they are integrated into are inefficient, due to space constraints, and design considerations such as cost.  In most applications, the use and application of a Fan or Blower in a system is more important than the consequences associated with environmental or health issues brought about by industrial process air pollution which generates the need for a clean environment, safe work place and uncontaminated air to breath. 


Process air moving applications.

  • Dryers.
  • Heaters.
  • Chillers.
  • Humidifiers and Dehumidifiers.
  • Air Supply and Exhaust.
  • Boilers.
  • Combustion.
  • Dust Collection.
  • Pneumatic conveying.
  • Incineration.
  • Solvent recovery.
  • Paint and coating lines.
  • Ovens / Curing systems.
  • Aeration.
  • Scrubbers .
  • Odor control.
  • Air filtration.
  • Blow off.
  • Gas extraction.

Industrial Fan / Blower Types - three main categories.

Centrifugal
  • Backwardly inclined -Airfoil, Single thickness, Backward curved.
  • Forward curved - general light duty multiplied slow speed.
  • Radial blade type - Open, Backplated, Rimmed, Reinforced, for rugged applications.
  • Inline centrifugal - using Airfoil or Single thickness backwardly inclined wheel.
  • High Pressure. - Narrow width open and radial tipped design.
  • Radial Tipped - High rim or tip speed capability, resulting in high flow and pressure..
Axial
  • Propeller.
  • Vaneaxial.
  • Tubeaxial.
  • Ductaxial.
  • Inline Centrifugal.
  • Ring / panel Venturi.
Plug and Plenum Fans Unhoused impeller using one of the centrifugal impellers listed above.
Others Many other sub categories, however most are all based on above main impeller types

Fan / Blower Performance.

As a fan wheel or impeller rotates, the Impeller through centrifugal force creates vacuum or low pressure at its inlet suction side.  In turn, the impeller creates a positive pressure, inducing a force of air on the discharge side. These basic principals are similar to a pump or water wheel, in which the air is conveyed from the suction to discharge.

The affinity laws or centrifugal laws apply to fans similarly as in centrifugal pumps.

Flow varies in direct proportion to change in RPM.

CFM (new) = [RPM(new) / RPM(old)] x CFM(old)

SP varies in proportion to change in RPM�

SP(new) = [RPM(new) / RPM(old)] x SP(old)

Fan brake horsepower varies proportionally as the cube of the change in RPM.

BHP(new) = [RPM(new) / RPM(old)] x BHP(old)

These factors only apply with changes to speed.   Should the system be altered in anyway through addition of "resistance" devices, such as dampers or changes in existing system design, a new system design must be calculated.

All fans are tested, Rated and cataloged based on 70F and Sea Level conditions.

Should a Fan need to be installed at an elevation above sea level, the following

correction should be applied to Mass Flow and Pressure to attain a proper quantity of air

Corrections for Altitude

Altitude -
Ft. Above Sea Level
Factor   Altitude -
Ft. Above Sea Level
Factor
0 1.00   5000 1.20
500 1.02   5500 1.22
1000 1.04   6000 1.25
1500 1.06   6500 1.27
2000 1.08   7000 1.30
2500 1.10   7500 1.32
3000 1.12   8000 1.35
3500 1.14   8500 1.37
4000 1.16   9000 1.40
4500 1.18   10000 1.45

Note: A fan will move the same number of Cubic feet per minute (CFM) regardless of Altitude

or Temperature, however Mass flow will vary in lbs/min and Pressure / resistance will vary, due to changes

in resistance.


Formula for determining capacity (typical supply and exhaust systems)

CFM Required =  Volume of space (WxHxL)

                           No. of Min. per air change

Space to be ventilated No. air changes per hour No. of Min. per air change
Auditorium 12 5
Bakeries 20 3
Banquet Halls 20 3
Boiler Rooms 60 1
Bowling Alleys 12 5
Cafeterias 12 5
Class Rooms 10 6
Engine Rooms 30 2
General Factories 10 6
Foundries 12 5
Garages 10 6
Kitchens 30 2
Laboratories 12 5
Laundries 20 3
Machine Shop 10 6
Mills 12 5
Packing Houses 20 3
Plating Rooms 20 3
Printing Plants 15 4
Ship Holds 6 10
Transfer Rooms 20 3
Tunnels 6 10

Pressure drop Calculations

Determining pressure drop through a system would be taking in to account all losses through pipe, elbows, hoods, filters, coils, duct expansions and contractions and branches as applicable. To thoroughly describe all infinite possibilities would be difficult. The best solution would be to employ the services of a design consultant or registered professional engineer.  In some cases a fan manufacturer will assist in the system calculation to select a fan, or review the system layout to ensure that there are no improper fittings or elbows before or after the fan.  The following is an example of a homograph to determine pressure loss through a given diameter of duct.

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The above is one example of a fan discharging into a stack, however the air exiting the blower is turbulent, due to the abrupt change in direction of airflow. 

 

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Improper discharge connection. Note the direction of rotation of fan is CCW (counter clockwise), however the discharge transition from fan outlet to stack is Clockwise.   A better solution would have been to discharge straight UP from the fan and vertically support the discharge stack or to have ordered the fan in a CCW Bottom Angular Up Blast Discharge position and transition into the stack shown.(This photo was actually snapped from a company's web site that sells duct collectors - nice looking equipment, nice colors, improper installation)  Horsepower and energy savings could be attained by rearranging this fan and discharge connection.  Air is not discharged from these blowers with a uniform velocity profile. The main reason for this is the fact that air has mass and it is thrown to the outside of the scroll.


Fan / Blower Accessories.

  • Inlet Silencer.
  • Inlet Damper.
  • Inlet Box.
  • Inlet Box Damper.
  • Inlet filter.
  • Inlet vanes.
  • Discharge Damper.
  • Access doors.
  • Drain.
  • Shaft seals.
  • Skid bases with Isolation.
  • AC Variable speed drives.
  • Osha belt, shaft and bearing guards.
  • Alloy construction, Aluminum, Stainless, Nickel.
  • Abrasion resistant construction.
  • High temperature construction.
  • Insulated housings for sound and thermal reduction.
  • Special Coatings.
  • FRP - Fiberglass / Resin / Vinylester / Polyester.
  • Special Motors.
  • Split housings.
  • Vibration switches.
  • Bearing temp switches.

COMMON FAN PROBLEMS

Vibration - Causes remedies

  1. Material build-up on the wheel.
  2. Loose mounting setscrews, bearings, bolts,  or couplings.
  3. Misalignment or excessive wear of belts coupling or bearings.
  4. Bent shaft.
  5. Material build-up on the wheel.
  6. Excessive system pressure or restriction of airflow due to closed dampers.
  7. Inadequate structural support or mounting.
  8. Externally transmitted vibration.

Lack of Performance

  1. Incorrect system design calculation or testing procedures.
  2. Incorrect blower RPM.
  3. Blower wheel rotating in wrong direction - check motor leads.
  4. Improper wheel to inlet cone clearance.
  5. Inlet or discharge air leaks, clogged filters, coils or damper settings.
  6. System effect due to improper inlet or discharge connection as illustrated above.

Excessive Noise

  1. Fan operating near “stall” due to incorrect system design or installation.
  2. Vibration originating elsewhere in the system.
  3. System resonance or pulsation.
  4. Improper location or orientation of fan intake and discharge.
  5. Inadequate or faulty design of supporting structures.
  6. Nearby sound reflecting surfaces.
  7. Loose accessories or components.
  8. Loose drive belts.
  9. Worn bearings.

Premature Component Failure

  1. Abrasion or corrosion of internal fan components.
  2. Vibration due to Impeller out of balance.
  3. Lack of lubrication of bearings.
  4. Misalignment or power transmission components or bearings.
  5. Bearing failure from incorrect or contaminated lubricant or grounding through the bearings while arc welding.
  6. Extreme ambient or airstream temperatures.

 

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