Understanding Fan Operation and Performance - W. Hilbish

7/30/2019 Understanding Fan Operation and Performance - W. Hilbish

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Understanding Fan Operation andPerformance

Walter Hilbish

Perry Nuclear Power Plant

FirstEnergy


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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 manufacturers

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 testinstallation be identified.

Example:Performance certified is forinstallation Type A: Free inlet, Freeoutlet. Performance ratings do not

include the effects of appurtenances(accessories). Performance ratingsdo not include the effects of crosswinds.


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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 variouspoints 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 increasevelocity of an airstream.

Axial - air is pressurized by the aerodynamiclift 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 fanshown


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Centrifugal Fans - Blade Designs


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Centrifugal - Airfoil

Blade curves away from direction ofrotation

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 andpulsations (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 lesspronounced instability region


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Centrifugal - Radial

Rugged blade design, self-cleaning

Lowest efficiency centrifugal

Used for material handling due to

blade strength

Moderate to high pressure industrialapplications

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 - Types

Propeller - generally operate at< 1/2 inch water gauge,

inefficient, steeply rising power

curve (overloading).

Tubeaxial - propeller mountedin cylindrical tube, ranges up to

~ 4 inches water gauge,

overloading.

Vaneaxial - utilizes guidevanes 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 Vanes

Function - reduce thehelical flow pattern ofthe fan.

Inlet - vanes guide airflowinto a rotational pattern oppositeto fan rotation to provideneutralize deflection by bladesand result in axial flow

Outlet - vanes takerotational pattern from bladesand redirects it to an axial flow,converting some rotationalvelocity into static pressure.


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AMCA Class

Definition AMCA 99 -An establishedminimumperformance level for

fan aerodynamicperformance in termsof pressure and

airflow rate.

Simply put in general terms - the higherthe Class, the greater mechanical

strength (and price).


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Gaps/Clearances

Ensure gaps andclearances setper VendorManual


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Gaps/ Clearances continued


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Centrifugal Fan Volume Control

Generally 3 methods:

Variable speedchanging fan speed results in achange in fan flow.

Outlet damperfunctions by changing systemresistance, forcing the fan to operate againsthigher backpressure and reducing the flow.

Inlet dampercombination of resistance changeand potential flow characterisitic change (spin,reduced turbulence) at fan inlet.


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Centrifugal Fan Vortex

The inlet box damper(positioned properly) and

both types of inlet vane

dampers pre-spin the air

in the direction of fanrotation. 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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Recognition

Following references were used in thegeneration 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 DriveBackward 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.asphttp://www.cincinnatifan.com/catalogs/EngData-203-internet.pdfhttp://www.northernblower.com/download/fanfacts.pdfhttp://www.nyb.com/eng_letters.asphttp://www.greenheck.com/library/articles?c=2

http://www.hartzellfan.com/lib_pdf/Bulletin%20A-108-N.pdfhttp://www.tcf.com/TCFBlower/literature.asphttp://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.html

History of the Vaneaxial Fan

http://www.jba-1.com/HistoryOfVaneaxialFan.htm

Documents

Understanding Fan Operation and Performance - W. Hilbish