1-When selecting a fan for ventilation system, why should you know about AMCA?2-What are the functions of fan?3-Use your own words to briefly describe the advantages and disadvantages of radial, forward- and backward-inclined fans, if you want to use them in LEV system.4-Please indicate the positive (+) or negative (-) pressure: Static pressure at the upstream of fan is _____, and at downstream of fan is _____. Pressure on the back side of blade is ____, and on the forward side of blade is _____. Static pressure on the inlet of fan (LEV) is ____, and on the inlet of fan (LEV) is _____.5-Why can FSP represent the system losses (p 8-4)?6-The inlet of fan has SP of -5 “w.g. and VP of 2 “w.g., while the outlet of fan has SP of +1 “w.g. and VP of 2 “w.g. Show your calculation for FTP and FSP.7-Why shouldn’t you have the fan running on the “backside” of fan curve (that is, not run on the left of the peak SP, but on the steep part of the curve)?8-1) Why should you avoid running the fan at a speed above 10% of rated rpm (that is at 110% rated rpm)? 2) Please calculate the % of changes in power (assume fan wheel diameter remains the same) if RPM increases 10%.9-According to the six in and three out rule, what are the lengths of straight duct that should be provided into and out of a fan which will be connected to 10″ round ductwork? You need to specify which duct into or out of the fan for full credit.10-You are designing a ventilation system with the following parameters. What is the required shaft horsepower (sph) for the ventilation system, and what rated power motor (rhp) would you like to choose? FTP = 15.0 inch w.g.; Q = 1,300 scfm; ME = fan efficiency = 0.65; Kdl = 1.05; STP (d = 1); c = 6356.
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Study Guide
Chapter 8: Fan Selection and Operation
©Miaozong Wu
Read textbook chapter 8, and make sure you understand the following questions and the body
of knowledge (BOK).
1. Fan functions and types:
? Ch5: LEV components – Fan function (SP & air movement)
? Fan basic components (p8-1): electric motor, fan blade, housing, air inlet and outlet, etc.
? Fan types:
? Axial fan: used for higher flow, lower SP systems
? Propeller-type blades
? Tube axial
? Vane axial
? Centrifugal fan: used for lower flows and higher SP system
? Radial impeller: LEV; rugged but low efficiency.
? Backward-inclined fans: LEV; less rugged but flat hp
? Forward-curved (squirrel cage): quiet but not rugged
2. Pressure:
ABIH: FTP = TPoutlet – TPinlet; FSP = SPout – SPin – Vpin
? Positive or negative pressure:
? at the upstream of fan is _____, and at the downstream of fan is _____.
? on the back side of blade is ____, and on the forward side of blade is _____.
? on the inlet of fan (LEV) is ____, and on the inlet of fan (LEV) is _____.
? FTP:
? Represents all energy requirements for moving air through the ventilation system.
? FTP = TPoutlet – TPinlet , different from TP = SP + VP (at a point)
? Fan total SP drop (FTP) = (SPout+VPout) – (SPin+VPin) = SPout – SPin (when VPin = VPout)
Practice: Exercise 8-1
? FSP:
? FSP = SPout – SPin – Vpin, because of FSP = FTP – VPin
? Represents the system losses (the amount of static pressure converted to useless
heat or noise): FSP = SPout – SPin – Vpin = Fan total SP drop – Vpin
Practice: Exercise 8-2
Updated on Apr 2020 (©Miaozong Wu)
1
3. Fan curve vs system curve:
? Fan RPM (revolutions per minute) can be adjusted
? Measure SP and Q on fan only (Fan curve) or on the duct system (system curve)
? Under AMCA rules, fans are tested under ideal conditions (air enters and leaves the fan
wheel in the smooth, uniform, balanced manner) and given the rated capacity.
? To prevent the system effect loss occurring when fan operated under non ideal
condition, the length of straight duct needs to be “six in and three out” of duct
diameters.
? Measure pressure (VP, SP and/or SP) and V at about _____ duct diameters
downstream, and ____ duct diameters upstream from any non-straight section point
(page 7-3).
? Measure SP at _____ duct diameters downstream from the hood (p 6-2).
4. Fan laws:
??
??
???? 1
ABIH equations: = ( )3 * (
) ;
??
??
????
?????????
?????????
??
= ( )2 * (
??
???? 2
) ;
????
????
????
??
= ( )5 * (
??
???? 3
)
????
? Q, pressure (SP) and power are related to fan impeller/ wheel diameter (d) and fan
speed.
? Avoid running the fan at a speed above 10% of rated rpm: change SP and overload
motor
Practice: If you run the fan at a speed above 10% of rated rpm, how will Q, SP and power
change. Assume the fan wheel diameter remain the same.
Solutions: RPM2/RPM1 = (100%+10%)/(100%) = 1.1
??
??
????
= ( )3 * (
) =1.1 (+10%)
??
??
????
Updated on Apr 2020 (©Miaozong Wu)
2
?????????
???? 2
) = 1.1^2 = 1.21 (+21%)
?????????
??
????
????
?? 5
???? 3
=( ) *(
) = 1.1^3 = 1.33 (+33%)
????
??
????
? If d2 =d1:
??
??
??
= ( )2 * (
=(
????
?????????
????
?????????
);
??
????
??
????
= ( ) 2;
??
=( )3
??
Practice: Exercise 8-8 to 8-9
5. Fan power:
? aph (Air horsepower): the minimum power to move Q against FTP
? bhp (Brake horsepower): also consider the fan loss (up to 75% power; radial fan
efficiency = ~60%), so it is the actual power fan needs. bph = aph / fan efficiency
? shp (Shaft horsepower): represent all powers needed. sph = bph * Kdl (1.05, 1.1 or 1.3)
? rhp (Rated horsepower): marked on fan nameplate
? Apply 33% power / 10% rpm when motor = 25 hp: order a fan with rhp > 1.33 shp
Note: this material is developed by Miaozong Wu, Ph.D. CSP, ARMTM, especially for 4160/5160
course. Under copyright, you must not distribute it to any other individual or parties (including
posting it online), without a permission from the developer.
Updated on Apr 2020 (©Miaozong Wu)
3

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