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Understanding Fan Operation and Performance

Walter HilbishWalter Hilbish

Perry Nuclear Power PlantPerry Nuclear Power Plant

FirstEnergyFirstEnergy

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Fan Definition

What is a “fan”? Defined by ASHRAE “A fan is an air pump that creates a pressure difference and causes airflow. The impeller

does work on the air, imparting to it both static and kinetic energy, which vary in proportion, depending on the fan type.”

2000 ASHRAE Systems and Equipment Handbook Chapter 18

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FansA fan is a constant volume machine. For a fixed speed, the fan will move the same volume of gas irregardless of gas density.

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Fans

• Most manufacturer’s rate the fan based upon tests conducted using AMCA 210.

• Testing uses ideal configurations (i.e straight duct runs, 10 duct diameters, no obstructions, etc.).

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AMCA Installation Types•Four basic test installations are defined.

•AMCA Certified Ratings require that the test installation be identified.

Example:“Performance certified is for installation Type A: Free inlet, Free outlet. Performance ratings do not include the effects of appurtenances (accessories). Performance ratings do not include the effects of cross winds.”

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Fan Performance Tables

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Fan CurvesGraphical depiction of pressure-volume performance of given fan size and speed operating at various points from shutoff to free delivery. Graph may also include power, efficiency, system, and stall curves.

STP - standard temperature (70° F) and pressure (29.92 in Hg) with gas density of 0.075 lb/ft3.

Unless otherwise specified the curve is at STP .

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Fan Testing

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Fan Laws

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Fan Laws

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Fan Laws

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Types of Fan

Two basic types of fan are:

Centrifugal - rotating impeller to increase velocity of an airstream.

Axial - air is pressurized by the aerodynamic lift of the fan blade.

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Centrifugal Fan Components

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Axial Fan Components

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Axial Fan Components

Vaneaxial fan shown

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Centrifugal Fans - Blade Designs

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Centrifugal - Airfoil• Blade curves away from direction of

rotation• Highest efficiency• Relatively low noise• High structural strength• Power reaches max near peak

efficiency, then reduces twoards free delivery (self-limiting).

• For given duty, has highest speed of centrifugal design

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Centrifugal - Backward Inclined• Efficiency slightly less than

airfoil design• Pronounced region of instability

where low efficiency and pulsations (stall, surge, bi-stable flow)

• Power reaches max near peak efficiency, then reduces t0wards free delivery (self-limiting).

• Consideration for use in applications where airfoil blade erosion concern

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Centrifugal - Backward Curved• Power reduces towards free

delivery (self-limiting).• Promote smoother flow than BI

fan • Operate at higher speeds and

therefore require sturdier construction than FC

• Less susceptible to flow/fan instabilities due to less pronounced instability region

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Centrifugal - RadialRugged blade design, self-cleaning• Lowest efficiency centrifugal• Used for material handling due to

blade strength• Moderate to high pressure industrial

applications• Power rises continually to free

delivery (overloading).

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Centrifugal - Radial Tip

• Blade is radial at outer edge of wheel, curved in direction of flow as it moves inward

• More efficient than Radial blade• Used for mildly erosive environments• Higher pressure than Airfoil

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Centrifugal - Forward Curved

Also called “Squirrel Cage”• Blade curved forward in direction of

rotation • Low pressure HVAC applications

(residential, etc.)• Lower speed than other centrifugals • Single thickness blade, lightweight

construction

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Axial Fans - TypesPropeller - generally operate at

< 1/2 inch water gauge, inefficient, steeply rising power curve (overloading).

Tubeaxial - propeller mounted in cylindrical tube, ranges up to ~ 4 inches water gauge, overloading.

Vaneaxial - utilizes guide vanes to recover rotational energy, up to 10 inches (single stage), overloading.

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Axial Fans - Curves

Propeller Tubeaxial

Vaneaxial

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Axial Fan - Guide VanesFunction - reduce the helical flow pattern of the fan.•Inlet - vanes guide airflow into a rotational pattern opposite to fan rotation to provide neutralize deflection by blades and result in axial flow

•Outlet - vanes take rotational pattern from blades and redirects it to an axial flow, converting some rotational velocity into static pressure.

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AMCA ClassDefinition AMCA 99 - An established minimum performance level for fan aerodynamic performance in terms of pressure and airflow rate.

Simply put in general terms - the higher the Class, the greater mechanical strength (and price).

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Gaps/Clearances•Ensure gaps and clearances set per Vendor Manual

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Gaps/ Clearances continued

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Centrifugal Fan Volume Control

Generally 3 methods:• Variable speed – changing fan speed results in a

change in fan flow.• Outlet damper – functions by changing system

resistance, forcing the fan to operate against higher backpressure and reducing the flow.

• Inlet damper – combination of resistance change and potential flow characterisitic change (spin, reduced turbulence) at fan inlet.

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Centrifugal Fan VortexThe inlet box damper (positioned properly) and both types of inlet vane dampers pre-spin the air in the direction of fan rotation. By creating spin in the direction of fan rotation, the airflow, pressure and horsepower of the fan are reduced, resulting in a new fan performance curve.

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RecognitionFollowing references were used in the generation of this presentation for technical or pictorial support:

CML Northern Blower Fanfacts (1991)Improving Fan System Performance: A

Sourcebook for Industry - DOE/AMCA (2003)

Engineering Papers - Moore Fans

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Recognition (continued)CML Northern Blower Fanfacts (1991)Fan Handbook: Selection, Application, and

Design - Frank P. Bleier (1997)ASHRAE Systems and Equipment

Handbook Chapter 18, Fans (2000)AMCA Fan Application Manual,

Publications 200-98 & 201-2002 AMCA Standards Handbook Publication 99

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Recognition (continued)

Fan Engineering, 9th Edition - Howden Buffalo

Value in the Air: Why Direct Drive Backward Curved Plenum Fans? - AAON

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Additional Information may be found at the following websites:

•http://www.moorefans.com/•http://www.aaon.com/literature.aspx?id=1•http://www.chicagoblower.com/service/default.asp•http://www.cincinnatifan.com/catalogs/EngData-203-internet.pdf•http://www.northernblower.com/download/fanfacts.pdf•http://www.nyb.com/eng_letters.asp•http://www.greenheck.com/library/articles?c=2•http://www.hartzellfan.com/lib_pdf/Bulletin%20A-108-N.pdf•http://www.tcf.com/TCFBlower/literature.asp•http://www.howden.com/en/Library/HowThingsWork/•http://www.lorencook.com/PDFs/cookbook.pdf

For some history of fans, see the following:Benjamin Franklin Sturtevant•http://www.sturtevantfan.com/index.htmlHistory of the Vaneaxial Fan•http://www.jba-1.com/HistoryOfVaneaxialFan.htm