Principles of LEV Design. Duct friction losses Friction losses increase linearly with duct length increasing air density, typical form: L

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Cecilia Fletcher
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1 Typs of losss Entry losss, ducts and hoods Friction Losss: Fluid in motion ncountrs drag along th surfac Enrgy is ndd to ovrcom th drag forc Th drag forc is du to th fluid viscosity Dynamic losss Turbulnc and ddis in th flow Momntum losss du to chang in dirction Found in xpansions, contractions, lbows, junctions and hood ntris Duct friction losss Friction losss H f ar proportional to th kintic nrgy in th moving fluid Thrfor, losss ar proportional to v In gnral form: Wisbach-Darcy friction qn: H = f L D f 1 Losss factor f is function of v, R, and surfac roughnss Friction losss W us a simplifid form whr H f is proportional to v and L H f = Ff L 1 F f is dtrmind from charts and figurs g vnt manual or curv fitting For xampl in a galvanizd straight duct: 0 L H f =.38 ( ) 1.22 D 1 1

2 Duct friction losss Friction losss incras linarly with duct lngth incrasing air dnsity, typical form: L H f = Ff 1 100' Losss dpnd on th duct matrial and wall roughnss Losss incras with 2 (and also Q 2 ) Losss dcras ~ with squar of duct ara (proportional to 1/A 2.5 but approx 1/A 2 ) rincipls of LE Dsign To protct th workr s brathing zon: th contaminant is capturd clos to th point of rlas (local) and rmovd (xhaustd) from work aras prvnts contaminant migration to othr aras systms ar always mchanical volum flow rat is much lss than gnral or dilution vntilation Dynamic Losss - ntris Hoods ar th businss nd of th captur systm Th hood is th only plac whr you can captur th contaminant Hoods should Minimiz th loss of contaminant into th room Not intrfr with th work procss Minimiz nrgy losss into th systm Local Exhaust Hoods urpos Captur and rmov contaminant at th sourc Effctivnss dtrmind by Hood configuration & shap Th xtnt to which th hood ncloss th contaminant sourc Cardinal rul: nclos sourc to th xtnt possibl Amount of air flow into hood (i.. Q hood ) 2

3 Simpl Hood T1 = T2 + losss oint 1 oint 2 ~ 5 D D sh = hood static prssur Not: outsid T1 = 1 atm and 1 =0, so T1 gaug = 0 sh Hood ntry losss, sh 0 = T 1 0 = T 2 + losss 0 = sh losss = + losss sh 2 If w apply th sign convntion, v2 and losss ar + trms Th sh is th prssur ndd to: St th air into motion (v part) aka acclration loss Ovrcom any losss in th hood Not idally whn no losss, sh = ; that is all th potntial nrgy is convrtd into kintic nrgy of th moving fluid Hood static prssur Othr forms: sh = + losss Th losss ar charactristic of th hood shap and ar dfind as th hood ntry loss H (=ntry) = + H sh H is causd by dynamic losss and turbulnc in th inlt. Air can t follow prfctly into th inlt and contracts to a flow stram that is smallr than th actual duct cross sction -- known as th na Contracta. This crats turbulnc and nrgy loss na contracta F H = 0.93 About d/2 is max ffct 3

4 Hood ntry losss Entry loss is proportional to v Th loss factors hav bn tabulatd for various shaps sh = + F sh = + H H sh = (1 + F ) = F sh = 1+ F F = 0.93 F = 0.25 F = 0.50 F = 0.05 na contracta and F Hood Efficincy Hood fficincy can b valuatd in trms of th nrgy loss at th ntry Say w had a prfct hood, thn all potntial nrgy would b convrtd to kintic nrgy So: sh = v +0 (no losss) If w dfin C as th cofficint of ntry Qactual Actual flow C = Idal flow with no losss Q idal C rprsnts fficincy Rcall C = ratio of actual flow to idal flow Qactual C = Qidal 4005 So: ( ara) C = = 4005 Sh Sh ( ara) C rprsnts th fficincy of th hood it is unitlss and masurs how wll th hood convrts potntial nrgy ( S ) into kintic nrgy ( v ) in th flow 4

5 C is rlatd to F Rcall F givs hood loss in trms of v sh =1+ F 2 1 So if: C = thn C = Sh [1 + F ] So th fficincy of th hood is dirctly rlatd to th flow rat, and th squar of th fficincy is a masur of th ntry loss factor Efficincy for diffrnt hoods C dpnds on th hood gomtry Unlik H it dos not dpnd on Q th flow rat In class xampl Find sh and C for a plain nd duct with =2000 fpm (assum no friction loss and NT) Concpt of captur vlocity Captur vlocity: th vlocity at a point in front of th hood, that is ndd to oppos room air currnts and captur th contaminants It is an old concpt that only partially works Ignors: Mass gnration at sourc Turbulnc at th inlt Still widly usd Many formulas for diffrnt shaps Dallaall quations ar an xampl 5

6 Hood Dsign Dsign paramtrs for hoods Hood gomtry/shap Hood siz Q = volumtric flow rat (CFM) Q = A = air vlocity in fpm A = ara of fac in squar ft flow is not masurd dirctly dtrmind by masuring fac vlocity & knowing cross sctional ara of flow Airflow for captur hoods Dsign paramtr = Q To dtrmin th Q ndd, w can us th vlocity c ndd to captur contaminant at point x in front of th hood Control vlocity = c or captur vlocity Ndd to ovrcom opposing forcs Cross drafts Motion impartd by procss, tc. Captur Hoods Airflow for captur hoods Continuity of airflow Sourc: Dinardi SR. Th Occupational Environmnt Its valuation & Control (1998) 6

Rlationship btwn Q, c and X Assumptions Contaminant sourc at point X oint suction sourc Air flows into point sourc from all dirctions Captur vlocity Captur vlocity

Shap ~ 1/4 Cylindr Shap ~ Cylindr Shap - lan Ara 1/2 πxl (+ Sphr) Ara = h w Ara 2πXL (+4πX 2 ) Mor captur vlocity quations Exampl Extrior hood with plain round opning 12

7 Rlationship btwn Q, c and X Assumptions Contaminant sourc at point X oint suction sourc Air flows into point sourc from all dirctions Captur vlocity Captur vlocity quations oint Sourc lain Duct Flangd Duct Shap - Sphr Shap - ~Sphr Shap ~ Half Sphr Ara 4πX Ara 4πX -(πd 4) Ara 2πX Slot With Sids Fr-Standing Slot 5-Sidd Hood Shap ~ 1/4 Cylindr Shap ~ Cylindr Shap - lan Ara 1/2 πxl (+ Sphr) Ara = h w Ara 2πXL (+4πX 2 ) Mor captur vlocity quations Exampl Extrior hood with plain round opning 12 inchs in diamtr Round duct lading to hood 6 inchs in diamtr Rquird Q for systm = 1000 ft 3 What ar th vlocitis at th hood fac and within th duct lading to th hood? Sourc: log, pag 619 7

Dcrasing th Q ndd to achiv contaminant captur (rduc by ~ 25%) Improving th captur vlocity of a hood locity

8 Exampl A 4 x 8 flangd hood is drawing 500 ft 3 /min of air. What is th vlocity 6 in front of th hood? Effct of Flanging A surfac paralll to th hood fac to prvnt unwantd air flow bhind th hood Effcts of flang: Dcrasing th Q ndd to achiv contaminant captur (rduc by ~ 25%) Improving th captur vlocity of a hood locity profils (non-flangd vs flangd) Rang of captur vlocitis Sourc: log, 2002, pag 620 Sourc: log BA. Fundamntals of IH (2002). 8

Hood Typs Hood Typs Enclosing Hood contaminant sourc containd within hood xampls: lab fum hood glov box, paint booth good for: contaminants with high toxicity aras whr thr is a high cross draft

accss Light fixtur d h turntabl Sid iw (Enclosur transparnt) Captur Hood crats xhaust airflow in front of opning to captur & rmov contaminant dt captur vlocity or c a factor of how th contaminant is

9 Hood Typs Hood Typs Enclosing Hood contaminant sourc containd within hood xampls: lab fum hood glov box, paint booth good for: contaminants with high toxicity aras whr thr is a high cross draft potntial Airflow rquirmnts dtrmind by th product of vlocity x ara of nclosur Th mor complt th nclosur th lss airflow rquirmnt ndd Lss suscptibl to outsid air currnts Hings to improv ovrhad and sids accss Light fixtur d h turntabl Sid iw (Enclosur transparnt) Captur Hood crats xhaust airflow in front of opning to captur & rmov contaminant dt captur vlocity or c a factor of how th contaminant is disprsd room air currnts how far th sourc is from th hood opning Disadvantags May rquir larg airflow rquirmnts Subjct to crossdrafts Th ffctiv rach is limitd to ~ 1 diamtr or lss ws wt hs Lx Lx = gratst distanc from hood fac to sourc Hood Typs Rciving or Captur Hoods: utiliz natural movmnt of contaminant toward hood opning good for: canopy ovr hot procss (rang hood) radial arm saw hood not good for: fin particls high toxicity contaminants cold procsss Hood Slction Factors otntial for outsid air currnts natur of th procss which gnrats th contaminant otntial for contaminating th brathing zon (canopy hoods) Sourc: Dinardi SR. Th Occupational Environmnt Its valuation & Control (1998) 9

10 Enclosing Hoods Control vlocity = f = fac vlocity What vlocity across hood fac is ncssary for propr contaminant control? Natur of th procss and contaminants gnratd Hood shap and siz Magnitud of cross drafts Highr is not always bttr Gtting an vn distribution of airflow at th fac Making th booth dpr Using a baffl Us of filtrs/air claning dvics Eg. Lab fum hood Rcommndd fac vlocity: ft/min dpnding on Room air currnts Location of quipmnt in hood rlativ to fac Fac vlocity: > 150 ft. min Air turbulnc at hood fac Rvrs airflow Contaminants may xit at hood fac Swing grindr hood xampl Hood dimnsions; 3 ft high x 5 ft wid What Q is ndd to obtain th rcommndd fac vlocity of 150 FM? What Q is ndd if th opning is 4 x 6 ft? Slots, plnums & baffls Slot hoods with plnums Usful for procsss whr contaminant is rlasd across a larg surfac ara Dgrasing tanks lating tanks Wlding tabls Distributs airflow mor vnly across th surfac Slot hood A hood with a width-to-lngth ratio of 0.2 or lss urpos to provid uniform distribution of airflow lnum A larg chambr or compartmnt that distributs airflow 10

11 Slot & plnum END hr A B SLOT DUCT DUCT LENUM DUCT Sourc: Dinardi,

The pn junction: 2 Current vs Voltage (IV) characteristics

The pn junction: 2 Current vs Voltage (IV) characteristics Th pn junction: Currnt vs Voltag (V) charactristics Considr a pn junction in quilibrium with no applid xtrnal voltag: o th V E F E F V p-typ Dpltion rgion n-typ Elctron movmnt across th junction: 1. n