Duct Calculator

8/3/2019 Duct Calculator

1/9

,-

STATIC REGAIN METHODEQ'UAL FRICT ON MET_ 00

Carr ier Corporation 11968


2/9

' . .."'

Th e Duct Calculator is offered as a significant 'COD-t ibuti to the id II t t ",ibution to tne mnustry, Ia n ms rumen, W rose purpose ISto expand the knowledge of and increase the quality of airconditioning systems ingeneral.

DUCT CALCULATORCONTENTS

PAG,E1 .13

, 3 - 67 ,8

8

IntroductionD uc t D es ig n MethodsInstructions for U sin g the Carrier Duct CalculatorExamples 1 ..6Example Answers 1 , .6C om p ariso n o :f D esign M eth od s

DUCT IDESIGN MIETIHODS

e. '

The fundamental requirement for the design of' an y ductsystem , is a sim ple" direct an d sym metrical layout. Supplya ir t erm ina l s must be selected and located to provide proper di t ib t" D !I h .. aloom air' istnr U .Ion. ,'uet connections to tte tenninr smust be integrated with structural members and the

h leal dl tical ...' , - . . , I ' . - I . ' II , - . . . . . - -1 ' . . . I - - ~ . _necessary mec arne: an ". e ,ectrics -equipment,The 3 methods of duct design (static regain, 'velocity'd ' tir -' _ rd .. I a ll f ' II' , ti n) .. ult s d!CC .. rt le el - ofIeruction , an u equt mcuon) r es ui . . ill,' rrrerent eveis '01,

accuracy, economy, and , use, Each method is brieflydescribed with examples. For full, d eta ile d .'reatrnent referto the Carrier System Design M an ual an d AS;HRAE Guide.

INTRODUCT'IONCarrier Air Conditioning Company is pleased to present

a new Duct Calculator. This simple tool permits ductsystem design by static regains as w ell as,velocity reductionand equal, friction methods, With the new Calculatorvstaticregain need no longer be consldered a luxury: its dollar-'saving advantages can n ow be enjoyed 'with ease; ; ~. . . i ..

S yste m b alan cin g problems are eliminated! Less I an p ow er is, n eed ed !. Quieter systems result!I . Fewer n o is e p r ob lem s! Fewer call-backs! Greater accuracy!

S'T,A,TIC REGA. I IN METHOIDThe basic principle underlying the static regain method

is that e ach s ec tio n o f' duct is sized so that 'the increase instatic pressure, due to the v el oc it y r ed uc ti on " offse 'S or

1


3/9

lances the friction loss in tile succeeding section of duct.the sam e static 'pressure is m ain tain ed at each bran ch

d terminal throughout the system. With a system.operly designed for no gain or loss, the total system~ . ~ l ~ 1 m , ~ d t L l , h ' . . . . , . l ' . , 1 ".(}SS IS turute to the ,Q'SS In t .. tmtiat section ,o~and branch ducts, plus takeofflosses.The initial duct size' is determined from tile initial air

antity und from 3. suitable initial duct velocity. Theduct size ismaintained U'p to the first takeoff. The

cceeding section is then sized. 'by allowing a reduction inct velocity to obtain a static regain equal to, the frictionof the duct to the second branch or terminal,

ucceeding duct sections are similarily sized to the end ofe run.In, some instances, the reduction in duct size may be tooll to , warrant a. change after th e outlet, resulting in a

gligible static' pressure gain. Conversely, when the ductis reduced to ' a, s tan dard, size som ew hat below that

d"~ ted all gl ~ ible etati 1. reate r, an equa '. y n~e.':19l.. e s ta ti c p r es su re .08S mayThese slight gains and losses tend to balance out over

e entire 'System ..It is also possible to design th e entire system for anstant: gain 'or a constant loss. Neither method .] S

co mm en d ed sin ce they 'both, d efe at th e basic advantages.the static regain method,The, static regain method .is equally applicable to , a l l

pes of supply duct distribution systems. It is particularlyp ortan t in , the design of' m edium and high, 'velocity's tem s an d system's with varyin g len gths of duct runs suchencountered on l ar ge I n st al la tio n s ...B ec au se e qu al pressures ar e mainta tned at each branchterminal, air balancing and corresponding noise problems

e red uced to a. minimum. S in ce fric tio n losses. a re limitedthe duct ru n to the branch or takeoff, lower fan static

essures are required with corresponding reductions in fa nr se p owe r ..

RE'DUCT.ION METHODThe velocity reduction design procedure is based o:n the

of a suitable initial duct velocity, with. arbitraryreductions in velocity" m ade after each takeoffin air quantity) along the entire duct run. The

stem friction loss is , then calculated as the sum , of theiction losses of the various duct sections, and the required

operating pressure, based on the corresponding airantities and velocities..Frequently, the static pressure required at th e fan

isch arge is considered tlo be equal to the system friction,of the longest e qu iv alen t (a ctu al len g th plus allowances

r fittings) duct run . In more s op h is tic ate d d es ig n s, aredit is allow ed for the static, p ressu re regain dn e to the

redulction, fro 'm th e first to th,e, l a's t du .c t se,ctio'n.fot static pressure regain i S I us;ually' ,calculate:,d. as

qual to 75'% ol.f the v elo city .p tessU Ie difference. O.ptimums .u lt s ,a re olbtained ' w h , e n the s tatile p r es su re .r eg ain o:f the

m a t c . h e : s " lor i s e'qual to the static pressure loss of fh.e

2

Return an d exhaust air system s m ay be sized in a s im i la rr n a n n er s t . i a ting with t h e 10 v e s t velo..ity at th e Ia t h e s t_ ___ _r . , . , . r ] _ _ . ' " J I . _ , . . _ w .. . 1... . ,~~I..Intake an d p r o gr es si ve ly i n cr ea si ng the velocity u n til a rr iv -ing at the fan, intake, Dampers arc required for finalba lanc ing of th e system in all cases,

The velocity' reduction. method requires broad know ...-led ge a n d e xp erien c e in duct design fo .r reasonable a,CC'UTBCYwithout numerous repetitive trial and errol! ' se lec tions . .Forthis reason, its U8e is generally' limited to short, simple,symmetrical duct layouts.EQUAL FRIC'TION METHOD

As its name implies, the design p ro ced ure u sin g theequal friction method is based on maintaining the samefriction loss per 1 . 00 ft of length : f O T the e n ti re d uc t system.and on the selection of a suitable initial duct velocity ..Initial a ir q ua n tit y and velocity are then used to establishthe friction rate. The system is sized, to maintain the sam e.friction, loss rate throughout. This loss is t he n c al cu la te d asthe product of the friction rate and, the largest totalequivalent length of duct in the s ys tem , in c lu din g necessaryallowances for fittings.

This method is reasonably' satisfactory for symmetricallayouts on 'which all runs have equal resistance. However, ifthe system i s ,composed of a mixture oflong and short runs"it will be quite difficult to balance since this method makesn o provision f or equa li zi ng pressure drops in ,branches or atS ucceed in s term' .In " " " " ' S J: ' . " : ' : ' - 1 . , , ' i . . : ' . ](U-'- .. . '-"-0 - - 'II

Occasionally, this method is modified by sizing tilev ar io us le n gth s ofduct at different friction rates to obtainequal friction losses for each ru n ..This requires selection ofthe maximum initial velocity and friction rate fo r thes ho rte st ru n with corresponding 'velocity' and friction ratereductions for t he l on g er runs.

As in . the case of the velocity reduction m ethod, thetotal system friction loss is frequently considered to' beeq U,....l to the s tat ic pressure required at th e fan discharge I - I -~ I 'at , :1 '., ' .. ... I . , ..... :~-"':'!'u' , . e ' . . . . . " , : , , . ' ' : . . ": ' . ' . , . 1 _ , > _ I : . _ . nmore sophisticated d es ig n s , p a r ti cu la rl y those for mediumor high velocity systems, c re dit s ho uld . be taken fo r thes ta ti c, p r es su re r egai n d ue ta, the , ve loci ty reduction from thef ir st t o the last duct sect ion . .

T he eq ual 'fric tio n method, is com mon ly used, for sizingreturn and exhaust air systems, So that allowable duct'velocities a re - n o t e xc ee de d, friction rates ar e based on the,air quantity and velocity at th e fa n inlet,

The equal friction method 'has been the most Commonly 'used because of' its, appa ren t simplicity, Duct v el oc it ie s arcreduced in . the direction of' air flow , thereby somewha treducing noise problems ..However , it does not differentiateb etw ee n 'v ar io us lengths of duct, nor does it provide fo requal static' pressure at ,succeeding terminals . Thus, bal. . .Q,ncing , d Q , m p e , r s must , b , e u se d th .r olu g ho ut the system andsp'ecial p r o v ' i s . i o . n s fo r sound , a t t e . n u a t o r s may ,h'e require 'dwhe' re h (gh p l r e s . s u r e ' l O . s s e s occur~ F 'o r th es e r e a s o l n s , ;, ' u s e ofthe equal 'friction :meth.old sho'uld b'e , r e : s . t r i c t e d to fairly,simple" symmetr ic ,al t lo w v'elo'city duc t sy'ste.ms.


4/9

W Find duct diameter for section B-13~Fig. 1.a. Set brown. scale at 1800 cfrn and 43 ~ 5 ' ft (air

quantity and equivalen t length in.section B~13),~'b ,~ Set reid curve at 3600 cfm an d 22 in, (c fro a nd duct

size'before takeoff, section A.~B)iljC~ I Follow red. curve to 180"0 cfrn (air quantity in

section :B-13)~d. Read 1 8 . in. duct diam eter on yellow seale.W Find duct diameter for section 13-14, Fig. 1.a. Set brown sea cs at 300 cfm and 20 ft (air quantity

a n d equ iv al en t length :-'nsection 1,3'-l'4),~b. Set red curve at 18.0 '0cfm an d 18 in. (cfm an d duct

size before takeoff, section B...S),c. Follow reid curve to 1.50 10 cfm (air quantity in

section 1:3-14),~d,~ Read 1.8 in, duct diameter on yellow scale.W Find duct diameter for sections 14 fum 18, Fig. 1.

Since these sections arc symmetrical to section13,-,14.- (same cfm at takeoffs an d same distancebetween takeoffs) 'use same brow n scale an d . re dcurve setting,

b. Follow red curve an d : r e a , d . duct sizes on yellowsc ale at in te rv als o f 3'00 cfm:

NOTES,::1 @ Had ld,uct layout in . Step S ' been nonsymmetrical to

previous section, brown scales and re d curve wouldrequire resetting as in previous steps.

2.~ For summary of Examp le I~ . refer to an swersection,W Find duct diameter for branch A-6 and 8-12.

a, Repeat procedure . .fo llowed in Steps ,4 , an d ,5Static Pr95SUr\B Loss

Whe n the in tersection of the duct diameter ~ airquantity, and velo city' lines falls to the right of point " " 0 "Io n th e red curve, read th e H slHv' value Gin. the re d CUIV'e~,

I:NSRUCTIONS 'F 'OR USING T'HE CAR,RI'ERDUCT CALCULAT'OR, 'b . Stet re d curve at intersec-tion of ai r quantity b e fo ['e first takeoff (5 4 '0 '0 cfm )and initial duct diameter(2~'4,In.).

The: following instructions explain the operation of th eCarrier Duct Calculator j For convenience, color has 'beenused in . the examples to identify corresponding scales on .the calculator,

Either side of the calculatormay be used, T he fron t sideis for static regain, The reverse side is , f : O I I velocity reductionor equal friction duct design. It also provides equivalentlengths for rectangular ,and round duct componen t s , as 'wellas 'pressure drop thru branch takeoffs .. Each side containsthe basic operating instructions,

C, ~ Follow red curve to ai rquantity in. duct sectionA ,-B ( 3 1600 cfm),

d. Read duct diameter (22in.] ,o'n yellow scale.

'EX.A:MPLE 1 - S'T'ATIC REGIAIN - ROUNID ,DUCT S~ZINGGIVEN:: : D-ct sy stern fo r g en e ra l o ffic e (Fig 1 - ).:I, " ~" ": '.~ " ," ;; . ' . .. , :.._ ,I~.,U_ .. " : < . J lL l i O ~ l . '.

I n it ia l v elo ci ty is to be equal ta l o r less th an . 1 '7 '0 1 0fprn,. ,, iT- t , . . . .1 ,-:...- .' .. rtit- - - 5 4 1 0 . ' : - 0 . . .frro .at ,alI qu.an I..- ...-"'. C.m.E l - b ' . - . . ' 9 " 0 : 0 . " . . - th R /' D " : " . _ , I i 5 '.ows... smo,o, i . / , _ ~ ;

W '. '~'..~",3600 CFM,i 18,00 CFMA - B ~ . w ..25~1

~20

- ~1 0 ,

.. _ . . . . . . -.: . . . . . . . . .

I F A , ' N ;: 5 , q. ,O O C F " M 'i2.0' '1'200\57'V' \ 0 / ':6 '90 : 0 ' ' "-~....,0"9 ' 0 ' 0 ', .

6 0 0 . I ' . . .'2 ' 0 : ' : -. 600 il20"'. .

iii20 3 0 . 0 . .. 20' 1 1 3 0 0a,_'I

'FI\:'D: Round duct sizes .. Use Fig, 1 for duct systemconditions. Refer to Example 1 in answer sectionto verify results.

PROCE.D'URE.:W sing Calculator. find d uct diameter from fa n to firstoutlet or takeoff (point A,. Fig..1 ) as follows:a,~ In yellow window, findpoint of intersection of'

, 5 [ 4 , [ ) 0 cfm and 1700 fpmand read '2 4 in. 'all yellowscale,

1200 cfm - 18 in.90'0 cfm .~I, in",6 100 CfI11 - 14 in.,3 0 10 cfm ~ 11 in .

W Find duct diameter for sectionA-H" 'F ig : ~1 .a I' Set " br own . s ca le a t . airquantity' of ' first branch

takeoff (18010 cfrn) an..dequivalent length of' ducttiC n . e : x t takeoff (20 ft)~

. '.-. -::;... I.. ","r; ---::.-:. .r . . . . . . . . , . i 'r -: ,r . . . " " '" " 1 : .- . .. . I - - - x--...:..... _. .(... ); ..., I. :_ . f - : : . . ' I )~ ~_ . _ : . .- -- - -_ _ '- oJ ' .~ ' . .. .. . . ' ; ~ ~ : : : . : . . > . " .. -- ~_ I ~

. _ _ . ' . ~'_- . . . . - . .;... '. .. . . . . . : - : . .~~'- . . . . . . . . . . .~ .. . . . . ; . . . '.'~' \. '\.

. '1 V '-,/ ' , ) - ' - - ";-..~~r : ! o . . - . ; ' . . . , ~ '.,.. ~---..:.__:. . _.-' . .. . 1.--c-: ~... ::.. ...... :-:'. (.':'r .~ ". , ," ,. " . :-. . . . . _. ..(. " = ' - ' ~ . . . . . '~ . ' . . . .. . . I"" ,". .. _. ";.~. ~ .. . .._ _. .. .. . I~ .. .. .. . -~ ' - ; a -" . . -" - .. ~ r: .( - . ' \

. __ _______ \ . . - : . ~1 . -1

3

- r. :~ . ~ :- . . . .JI .. _.. ~..._ " 1 _ .- " 1 .. - .. : : : : " ~ : . - .. "" . . . . . . . . . ::... . .- . , . . : : . .. _., . 1 . _ , ," : . -- - ._ ; - . .

./:. . . . . . . . . : :_ _ . . . . ~ ~ ~ ' . . . . . . . - ~ . - _ - . . . . . . . . . . _ " ' l ; I F . 'I ~ J - . .

. . : . : . . . .. ." : .-:

.. .-". s - c -..::" - . _ ~.:" , .-

.-----_L.~----r--,__---------,.. ..------..;.. - ...~ .i -

... .~ . ~ 1

~. . . . ., .~. . . .~. .. ~.__ -" _ ..1 ... _ _ .~~ 22~.... .. . . . .. . . . . . . . . .. . ., .'" . . . . .- ." ; - - -..~ . . I I .: _ . _

- _ .r "" - ._ : 1 10 1 .- .. -" . . - -: :: : . . .- . . . . - - ..


5/9

hi: is the static pres sure l_ ,S 'with respect to velocitypr.ssur ~~ " I tua stati 'pressu,' IO I.,'~ " -, the due sectionrn a I . ~ .. foun d on the I,{;a 0 : s .a '~ s ny set ing I ne velocit- attlF arr _w and re: din : - the static pressure loss I pposite leH , J . , ' - _ - I u ~

~e II c it )' , pl re~ '.l ur e . '- , f o r . a l c la i l l ] ] P' f a ' _ I I.. 0n ~ ' . r s i o n - '. i o 'Sor .lOS~:I._S. may b r~ad from -Ile-'"'an,~ scale for corre-s iondirtz i I, IOU' l t and duct V locities.

I..on' ersion 1 0 - - lin =OI~75 -.- ')ll , I I ' i .e .sure d r - - ' f -11C,-Conver ~.In lo : S := = 1 ..i x ver city p res ur I. i I fer n eeTille total static pr ssure r e q u i r e : a t the a l ] d i: c ha rg e . l l i s

t _I SU - 0 ~ the d ct rictiun l .s and the , ' .rminal oerating ~

IFrjctio-' Loss:Th -fri 'tion lass 1 1 L11ei n ; j. I due s e c tion frorn the , a n

tl " e fiJ" '1_ ot r ] 1 ' 1 ' -11-" ' 1 1 f md 1 1 1 the reverse I e _ alhe alculator ~Set uct dia ',. ter actually

I .se - I L_- inch S' I p p r . si te :' .r rov . 1 1 . b lu ._ale , On redscale pposite iniia ] air quantity I 5,400 cfrn r ad tileC rrescond il ,g f r r u .tion rat - O.1Sill," wg/l 00 I L d u _t .~

Thi oressure !I,SS, in til. duct section i the nrot UCI- "j isc uival snt lengtl1 to firs' takeoff. including allowance forcquivalerr len gt 10'[ fittir g s, time: the I riction rate justestab lished,EXA _ IL 2 -,S I "T-IC REG,A,IN'~ RIECT'A-~UIL,AR'UlleT SIIZIN I

luct system for gen era of ice ~]g,. :_.~itial _elo . i' l ii' ole e ua l or ~I. S 1 1 n 700

fr fi1..I>D :: R ectan gular duct sizes. '''., - F iz. ....1 I 0 I f due.. sys . .m

.n ditio ns. R efer to, - xa- . ole ~ in , II' :'er sectiont, veri y results ~

''17'V35m ."600 CFM ..A B_ II" " 1 5.'~ 1

1-180 10 C'F M 20 uj ,-1800lCF M '2 0" I~ O i- . .

2.0~

II _20 1'200 '20' 1200IFAIN5400C -I'2,0 ~200

20 . II20 - I'2000':',.

600 6 I 0 - . '__

4

PR 'CED UR - .'W Usin ca l .ulator, fin d d ct diameter from fa ' " fi rou 1:" or c" ke ffl ooin t A 1 - , ~ 2I "1 follo- .~a. In yellow \ i -d ,IQW ~ find poi t of in. I rsec ion . f

'.. _00 I. trn '~:d 1701 10fp m and read ~ , I~ .~ on ' ..1 1ca1~

b~ _"oll 1\ rr .nd duct procedi r _' a IJ COII_ ver roundduct iarn ter '[0 r _ _ran gular duct siz .'. 1 0 1 1 gre,.~.1

. , I E : 1 1 ' . xtren ' a curacy is desir ..d . use ...p ect ratiocorrect: . tor, s described fo r ree ~ngul ~ d u o t ncal: alat r ~'S_~ti,c Prassu 'e I '01S5

When the in ersection of th e duct d iam eter .. IiIquantityan d .elo cit ' o n e I fall tOI . h e right f p o i I. L ~!o t i l cu r I e ' . r e a d t e H s / H . . .~ va ue ) . 1 '1 1 1 ._ reid I . UTV'_


6/9

FIND,: Round duct sizes. Use Fig, 3 for duct 'system,conditions.Refer to Example 3 in answer section to verifyresults ..

, L I , I . 'I I -IBOIO C:FM '20 10 ' ~:800C,FM 20 I 'O~" J3 .""

'I,10

1500

FA, I N ".I ."5 '" 4 : " 0 '0 , . :' C i lL, M ." .. .. . " _ .' _. - . - , . .. ' f" "' ' . 2 'O~ 1'2.00''I2, 0 !1,200

2 -~o 900 2 , O ~ 1 900

20h 6 -..' i " . >'0 "'0.'..... " .... _.0'0" , ,- . ' _ 6 0 " " , 0 ', ."

. . 1 120"". .l 3,00

PROC ,EDU RE ::1 'i Using th.e purple scale, set the initial air quantity (cfrn)

opposite the selected initial air velocity (fpm).2' i On the red, scale, opposite th e initial air quantity (cfm},

read, the duct friction loss (in . wg! lOO ft duct).3 t, On the blue scale, read the required round duct

diameter,,4,~ Calculate friction loss a,f the duct section by' multi-

plying the equivalent length, of the duct by the frictionloss found in Step '2~

5 , , ; Repeat the above procedure for a ll sections of ductsu si ng p rogres si ve ly reduced velocities fo r lea ch section,

,6,t Add the friction losses of the individual du,ct sections,found in Step 6, to find the total duct system frictionloss ..

7,~ (Optional) On the purple scale, read the velocitypressures corresponding to the initial and final ductve ocities, Apply 75% of the velocity pressuredifferen ce as a static pressure (regain) credit to . thetotal duct system friction ]OSSrj

8.. Velocity ' p res sures. fo t calculating fan conversion gainso r lo ss es " may also be read from corresponding fa noutlet and, duct velocities on the purple scale,

Conversion gain _ '0,75 x velocity pressurediff e : r e neeConversion 1 1 0 ss ,_ .1 ~ lOx velocity pressuredifference

9~, Find the total static pressure required at the fan dis..charge as the Slim of the duct fr ic tio n lo ss" the terminaloperating pressure and fan conversion less any allowablecredits: for "velocity regain, ..'

5 ,_. :'

EXAM:PLE ,4 _, V'ElOCITY' REDUCT'ION - R,ECTANGU,-L,A,R' oucr SIZINGGIV ,E 'N ': Duct system fo r general office (Fig, 4 )~

Initial 'velocity i s , to be equal to or less than 1700fpm.E lbow s '.' radius, no, vanes, R = 1 . ~25',DRectangular duct sizes. Use Fig, 4 'for duct systemconditions, Refer to Example 4 in aJ lSW I sectiontO I verify results.

F I ' N " D - ' " , ". ," I

35''t . : . . : . . " :3 6 '. '0 0" " 'C '' M ' - I , ' S " ' 0 ' " 0 " cr MA , " . . " " . . . . ' , . B - ' . t - , . " ' - - ~ - J - .,.I _~-1,8'00CFIM 20 lO ~ 180'0 C,F M 2 ,0 Id~ 11

.'.~'3.'1 , 5 : : 0 . 0 ' . . ..

. -"FAN540,0 CF'M I' 20- ~200 ! .20 1200

2,O'~ 900

2 "'1a 2 0 ' ~ 6 ' 0 ' '0 ". . .~, .' I ' . . ' .. '. ~. I I2 0 , "'" 600', ', . ,. I , . , , '

2 'O - ~ ' 3 : ' , ; 0 ". ' ' "! ' "

PROCE.DURE::1 ;0 Using purple scale" set initial air quantity (cfm)

opposite the' selected initial air velocity' (fprn),2~ On red scale, opposite initial air quantity (cfm), read

duct f ri ct io n , l os s (in. wg/IOO ft duct).3,~, On blue' s e a l e : " re ad ro un d duct diameter.4~, Conver t round duct size to rectangular duct size on,

Static Regain side of calculator. Set round duct size atblack arrow on green scale and read rectangular ductdepth and width.NOTE:: Do not use rec tan g ula r d uc t size s o ln blue scale.They are based on equal friction rather than equalvelocity.

5 Cal '1 t f ~ti ], _ "'CUla e :r]c Ion O S 'S :Establish aspect ratio correction factor (use Rec-tangular Ductwork. Correction Factor table onStatic Regain side),~

b. Determine effective length of duct by multiplyingequivalent length of duct by correction factor ~

c. Multip ly effective length of duct by' friction lossfound in Step 2~,

a.I

d~ Repeat this procedure for each duct section usingprogressively reduced velocities for each section ..

e' l Add th e resulting friction losses for each section todetermine total. system friction loss.

1 6 , ~ , (Optional) On purple scale, read velocity pressurescorresponding to the initial and final duct velocities.


7/9

App ly 7 ',5% of' th e velocity pressure difference as astatic pressure (regain) credit to the total duct systemfriction loss.

7.. Velocity pressures 'for calculating fan conversion gainsor losses may also 'be read from corresponding fanoutlet and duct velocities on the purple scale j

Conversion gain = 10.75 x velocity p ressu redifferenceConve rsi on loss = 1..1 0 x 'velocity' pressuredifference

8. Find the total static pressure required. at the fan dis-charge as the sum of the duct friction loss the terminaloperating pressure, and fall conversion less any allow-able credits for velocity regain,

EQUAL FRICTION METHODDue sizing by means of th e equal friction method is

performed entirely on the equal friction side o,f the DuctCalculator as described in the following examples:EXAMP'LE 5 - EQUAL FR1C'TION - ROUNID 'DUCTSIZINGGIVEN: Duct system for general offic,e., (Fig. 5).

Initial velocity is to 'b e equal to or less than 1700fpm.Total ai r quantity = 54,00 cfmElbows ,_9010 smooth, RID '= 1~S

'FIND,~ Round duct sizes, Use Fig. 5 for duct systemditio'n _ I lons.

Refer to Example 5 in answer section to v erifyresults, ..

F . A N~'4110:' 0 C -" 'F " ~M '

J ,"' .: "

. i20 12,00 2,0' 1'200 2 0 ' ~ ~ , ' ' 0 " ' 0 ' :. I t _ . . . ' "

201 900 _ iii20, 900

2 0 . . : . . ~ ' 60,0 2 0 t iii 6 ; O ,0'. .1 1 . - . . ,.'. . . . .

~20 30,0

' P ~ R O : C E D I . :U R E , : ' : . . .". ..' . I.. _. . I . ..1l Using ' th ,e purp ' le scale, set the initial air quantity (cfm)

opposite the, selected initial ail' velocity (fpm),2;0 IOn red scale, opposite initial ai r quantity (cfm), read

duct friction loss (in. w'g/lIOO ft duct),3, . On the blne 'Scale, read thie, round duct diameter.

6

4 , F 'aT 'succeed in g duct sections, se t the air quantity (efm)on reid scale fo r the respective sections opposite th epressure loss found fo r the initial duct section an d readtllie required round, duct diameter on blue s ca le . .

5. The total friction loss in . the duct system is the productof the longest run of duct, including allowances for th,eequivalen t len gths of fittings, and the design frictionloss per 100 ft of duct I

6,~ (Optional) On. the purple scale, read the velocitypressures corresponding to the initial and final ductvelocities. Apply 7,5% of the velocity pressure dif-ference as a static pressure (regain) credit to the totalduct system friction loss.

7,~Velocity pressures for calculating fan conversion gainsor losses may also be read from corresponding fanoutlet and duct velocities on the purple scale.

Conversion g ain = ' O ' i . 7S ' x 'velocity pressuredifferenceConver si on loss = 1.10 x velocity pressured if fe r enee

8 ~ Find the total static pressure required at the fandischarge as the sum of the duct friction loss" theterminal operating pressure, and fan conversion less anyallowable credits for velocity regain,

EXAMPLE 6 - EQUAL FR,ICTION - RECTA,NGULARDUC'T S ZINGGIVEN': Duct system for general office (Fig, !6).

Initial velocity is to be equal to or less than 1700fpm,Elbows ...radius, no vanes, R, = 1 .2 5 : D

FIND: Rectangular duct sizes. Use Fig, 6 for duct systemconditions, Refer to Example 6 in answer sectionto 'verify results ..

A 3600 CFM B 18001 'CFM

FAN540'0 CF'M

.i ~0020 12,.' 20~ ~200

. I20 9'0,0~20 900

20 1.( .. :1600 60,0,

I,'2 0 300

,PR.OCEDURE:Follow same procedu 'c as m""Example 5 but in-st ead of'. " , . ', . . . . . ~ ' " . ; ' ; : - . ! . ~ : . . I . . r : ,I. '1,_ . < ' . . I, .... " .... . 1 1, . " . ' .. . 1.: : .,:\ . ',1.,round duct diameter, read rectangular duct depth andwidth 'on. blue scale,


8/9

E,XA:MPLE 1 1 ~ A' , ISWERISSTATile R ,EG 'A I, --- R IQIUN ID O IUCT' 'EXAMP'lE 2 - A: ','SWERSSTA tc REGA N _, RECT' ,ANGUILAR DUleT

-

AI , R , EQIUI ,VAL. E,NT' S.~Z,EE:C'TION QU,AN'T' I TY LE N:G'TIH, -{cf'm} (f't) (;n,')

:

\VTol ,A 540,0 '7 8 2 ,4W A B 3600 20 22,./' . ..W 'B 13 1 800 43~5 18. ', -W '1 3 _, 14 1 500 20 18W '1 . 4 _ 5 1 200 )'0 18'1 ,5 1 "'" 16 9010 20 6

16 - '] 7 600 20 '1 4 ,,,1 ' 7 ' ~ 18 300 20 '1 1

"

,AI:R EQIUIIIVI,A!L, ENT ~ND ICATED !R 'ECTSIEC,TI,OIN QIUANT!TY L,ENG'T'H R ,OUND S1ZE: 'sI,ZE

I(cfm) ( , . f t ) ('ii n.,) (ilnl~'),_\ 1 1 ' T o A 54,00 - 73 2 4 ..0 22 22x-W A IS 3600 2, '21 ~O 22 1 6.... .. x~

\ B 1 :3 1B o n , , .46 1 6~7 22 1 0.. , xl , ,W ' 1 : 3 _, 14, 1,500 25 1 , 6 ~ 1 20 x 10:W 14 .. , 5 1 200 25 15. ,5 18 x 1 0:15 . . 1 , 6 . 1 900 24 l4,.0 16 x 1016 ,_, 17 ' 600 24 1 2 7 14 x 10

I'1 , 7 - 8 300 ?3 1 0. 1 8 x 10Frict ion rate ::, e 15 in, wg/ '1 00 F "Equivo~en ft from fan to firs' ourl et = 78 f 1 'FrIction 10.5s = '78 x .1 5 = .,'12 in. wgTotal s otl c pressure requ ired by duct system ~ ,. 2 in ~wg

F r'i c ti on rat e ~ ,.15 in. w'g/100 tEqu ivo 1en I '~'~ rrom fan 'to f; rsr ou let = 73 ftF ric iion ~os S ::: 7' 3 x .,1 5/ 100 :: ~1 1 In. vvgToral stcti c pressure requ ired by duct system - .11 mwg

EXAI 'LE 3 - AN SW 'ER ,SVELOCI,TV R ED IU CTI10N _ ROUND 'DUICT

.. _ ..A , I R , I : 0 I ICIA'T' E , I O ACTUiALI FR CTI ;ON , EQUrVA,L'ENT I F 'R~C :T IONS!EC,TI0N QUA,NTI~T'Y V'E,L StIZE, V'EL RAT"E LENGTH L 1 O i S S ,

(i:n II 'w'g/(cfm) (fpm) (inI) ( f p l:m ) 1 . 0 0 f't) C f t ) (;n, wg)

, ..To . A ,5400 1 700 2 1 , 1 720 . , ' 1 5 78 ~ 1 2A '8 3600 1 1 6 0 0 20 1650 17 ' 20 03.. . . ,~1 8 ' 1 , 3 18'00 1 , 500 16 1 280 . , ' 1 0 " 2 ..0.. . :13 ,.' 14 l~OO 1 , 4 , 0 0 14 1 400 1 9 20 .,0414, J ' 1 1 5 1200 , 1 250 ' 1 4 , 1 1 '2 0 12 20 ~O 2'.15 1 6 900 1 1,00 12 1 1 50 . , ' 1 6 '2 0 .0,3

I 20 .0 ,1 6 ' 1 1 7 6 , 0 ' 0 950 10 1 100 17 :.. . 17' ' 1 1 8 300 800 8 860 '1 4 2 ,0 ~O 3. , ... ..

Due' fri c+ion toss =. .,35 ~'n,. wgRego'in ~ .75(.,18 - 04) = ~lO in. wg'T'o' o~ sto+ic pressure re-quired by duel sys+ern - , . ' 2 5 'in" wg

E,XAM PLE ,4 - AN SW 'ER SVELO CIT 'y R EDUCTIO N ,_ R ECTAN GULAR DU 'CT

'" , ~. _..

A.I R IIND~CATE,D RE,eT AC'T'UAL, EQUIIV,A,L,IEN'T' FR , ICT~O I 'N FR~CT' O IN'QU IANTITY VEL S~ZE V'EL LE]*4 IGTH RATE 'LOSS,I. I : I MIU'L TI ' ,: ", ',I, ':5ECT' I IO I PLIER(;n, wg,/

(efm) (fpm) (" ) ( f ' [pm) (ft) 100 ft) ( I wg)~n., rn,-Ta l A 5400 1700 _ 2 2 x 22 1 60 0 73 15 1 ,. 1 5 . 13

!B 3600 ' l / D O 22 14 '1 680 20 ~ '1 8 1 .20 .,04.. ,0:, ' XB I ,_ '13 ~ '1 800 1S O D 22 x 8 1 4 , 7 , 0 3 , 7 ' .2 ,0 1 ~35 ,.10. ' . .r13 ,_ 14 , 1500 l ~ ,O O 20 x 8 1350 20 ~ '1 9 1 . ,30 .05:'1 4 1 ,5 1 200 1 250 18 8 1 200 20 ~ '1 f 1 ~30 ~O,4. x 0ts . . 16 900 1 1 1 0 0 1 , 4 x 8 1 160 20 14 '1 .25 ,.0316 .. . 17 600 9'50 1 2 x 8 900 20 12 : 1 ~20 ~O31 ' 7 ' lO 18 300 800 8 x 8 6.80 1 20 ,. 1 2 1 , .'1 5 ~O 3~ '" _ i.. .. .. ,D- !i" '." ~ I! 45 -uc I' rl CTlo'n ~os s = ~ '" -, I t , wgR eg,(] in = .7 5 C . 16 -, ,.a 3) ~ . 1a .n II \Afg'Totol s ro lc pressure reoulred by duel system - .35 in. wg

7


9/9

EXAMPLE, 5 - ANSWERSEaUAL F R 1ICTIO N I _, 'ROUND DUCT

EXAMPLE 6 - A,N SW ER SEQUA'L FR ICT'IO N - R ECT'A,N GUILAR D 'U 'CT

-

AI R [EQUIV,AL,EN'T' ACTUAL,SIZESEC'TI'O:N ,QUi,ANT-TY LENGITH VEL(efm) (ft) (i :n.) (f(l'm)

To A 5 4 , 1 0 0 7B 2 4 . , 1 700A ~ B 3600 20 :2 0 1 650B ..I '1 3 : 1 B O O , 4 2 ' 1 6 1 2901 .3 ,. 14 1 500 20 14 1390:1 4 , 15 1 200 . 20 1 4 , 1 1 2015 - 16 900 20 1 2 1 1 , 5 01 1 6 , . 1 . 7 ' 600 ,2 0 1 a 1 '1 0 107 18 300 2 0 '8 870- ," .....

..

AI R EQUII~VALENT ACT'U,ALSIZ,IESECT' ION QU'.AN 'T I 'TY L;ENGT'H V'EL( c . f ' m ) C f t ) (i n.) (fp,m)

To A 5400 '"' 7'3 2~ 22 1 . 6010L X .:.A B , 3600 20 22 16 1 47 0"" .x.[B 1 'J ' 1800 37 ' 2 2 . '1 0 '1 180. I 1 1 X'1 3 "" 14 1 500 20 1 8 x 1 0 1 200

~ I4 .. . 1 .5 1 200 20 1 4 x 1 0 1 2301.5 .. . '16 900 20 1 2 x 1 0 1, 1 80' 1 6 ill 1 '7 6'00 20 8 x l0 1 08017 '.8 300 20 6 x 1 0 720.. .. .. .. ..

Frl,ction rate = .. 1 5 irr. wg/ ' lOO f tTotal equ1val~ntft:= 240 ftFrictJoln los s = 2.40 X ~ , 1 5 / 1 1 0 0 1 = ..36 In. w g.Re'glo'l n = .7'5 (. 18 ._ m047) = ~ 10 In~ wg'Totol stoti c pressure requ ired by duct .s y stern ....;.26 'in". wg

F'ri c+i 'O n r cre ~ .15 in. \lvg/ 1 00 ft'Total equ ivqlen t ft = 230 ftFriction [oss :: 230 x .15/100, ~ ~34 i n , wgRegain = .75(.,16 - .032) = ,.10 in , wgTotol s to ic p essure required by duct sy srern _ .,24 in, wg

I COMPA1R ISON OF D ES IG N M E,T'HO D SROUND DUe, I (in.] R E C ' T A N ' G '_ ' L A ' R 'DUC"'T ('. )1 . , . " , I .'. ..... '/ : . I. In~_

..

ST AT 'I ,C V:ELOCI'TY EQIIU,ALSEC,T' ION REGA' IN R E'D 'UCT I O N 'F R I CT 'I .ON""

To A. 24 24, 24A - B 22 20 20B 1 , . 3 1 . R 1 6 1 6'. .13 ~ 14 18 '1 4 1 L 1 ,'1 4 , 1 1 : 1 15 1s '1 4 14' 1 . 5 .. . 16' 1 6 1 2 1 21 6 .. . 17 14 1 0 1 01 .7 18 1 1 8 8.,

. ~ ~" v ..,_ _ .12 - [o's s 2t; . l os s 26 10.55n , wg In..wg In..wg. . . . . . ( . : "

.. .. .. .- ..

S'T,ATIIC V E L,rOC I 'TY ' EQUALS,ECTI ON R EGAI N R . E DUCTII ON 'R,I'C'TI,Q'NF!... : _ : ~ _ w ~~' T ' , a A 22 x 22 i 2 '2 x 2 2 22 x 22-A ,_ B 2 2 x '1 6 2 : ' 2 x 1 4 22 x 1 6.B .. 1 3 , 22. X 1 1 0 2 .2 x 8 22 x 1 01 . . 3 I I 14 20 x 1a 20 x 8 '1 8 x 10

.

14 15 1 8 1 0 1 ,8 8 : 1 4 x 1 0X x15 ,_ 16 '11 6 x 1 0 '1 ,4 x 8 1 2 x 1 0,'1 6 i 1 : 7 '1 4 x 1 a l. .2 x S . 8 x '1 017 i! 18 8 x 10 8 x 8 6 x 1a

~ _ ~ , :&~ ~1 1 loss ..35 ~ )',4 - I. I n ~ wg In 0 wg 0.55 ,. 1 n ~ wg oss'

." " .. ::~::.::: i::::~:::::::

Documents

Duct Calculator