Bulk Air Flow in a Duct. Hoods and Local Exhaust Ventilation. Inside a ventilation system. System Nomenclature

Transcription

1 Bulk Ar Flow n a Duct Hoods and Local Exhaust Ventlaton Essental pressure and low relatonshps Ar movement always ollows the pressure gradent Flow goes rom hgher (abs) pressure to lower (abs) pressure regons Ths s the eect o molecular-level collsons More energetc molecules collde wth neghbors, and ncrease the velocty o the surroundng molecules (and pressure) Flowng ar has momentum and vscosty momentum orces t to travel n ~ straght lnes vscosty orces t to low nto and ll the avalable volume the conlct o these orces nduces eddy currents n turbulent low Wth contnued collsons, added energy s gradually transormed rom the energy o low to the heat energy o ncreased veloctes o ndvdual molecules. Any energy nput s transormed rom ts ntal state to low energy and nally to heat. System Nomenclature Insde a ventlaton system Dagram Courtesy o S. Guey S = - V = + T = - Upstream (Exhaust) Sde S = + V = + T = + Downstream (Blowng) Sde

2 Ar as a lud Ar has mass, just lke any other lud Generally have a mxture o water vapor and dry ar Most Vent tables and charts reer to ar at Normal Condtons ressure 9.91 Hg Temp 70ºF Humdty lb HO/ lb dry ar (abs) Humd volume 1.5 t /lb dry ar Densty lb/t Enthalpy 5.54 BTU/lb dry ar Relatve humdty 50% Densty actor = 1 System Desgn Requrements In ventlaton systems, we want to specy: the volume o ar movng n the system branches the pressures that are requred to move ar n the system branches the an capacty needed to operate the system To compute ths n the system we apply: Conservaton o mass Conservaton o energy Conservaton laws Conservaton o Mass Used to compute the volume low rate (Q) and duct veloctes Conservaton o Energy Used to computer the system pressures and an horsepower requrements Conservaton o Mass Mass cannot be created or destroyed; so the total mass low n the system s just gven by the sum o the nputs (or outputs) an = = + + = Mass low rate n lbs/mn (or kg/s) 1

3 Conservaton o mass (Contnuty eqn.) Conservaton o mass means I we know the densty we can wrte eqns n terms o a volume low rate Q (t /mn).e. = ρ Q = ρ V A m n lbs/mn; Q n t /mn; _ n lbs/t so that 4 = + becomes ρ 4Q4 = ρq + ρq Ths general equaton s always true n the system Conservaton o mass- Volume Flow Oten the contnuty equaton uses volume low rate rather than mass low; assumng rho=constant Note that a volume low rate Q (t /mn) Can be wrtten as: Q = V A = ρ V bar represents the average velocty n the duct so that ρ4q4 = ρq + ρq becomes Q4 = Q + Q Ths equaton s approxmately true n most ventlaton systems Volumetrc Flow Rate The volume o ar lowng through a system past a certan cross secton pont Q Gven n Cubc Feet er Mnute (CFM) The amount o ar lowng through any pont has to be the same Volume o ar has to be the same, but the area and the velocty do not reman the same I you ncrease the area you decrease the velocty Q 1 Q Q Constant densty assumpton The constant densty assumpton requres that T, w, and are constant n the system. Ths s never strctly true ( s changes) but densty changes are oten small enough to be a useul approxmaton Example: say S = 10 H 0 n the duct, then densty change ~10/407= 0.05 or.5% Thus constant densty s a good assumpton

4 Densty at normal condtons Densty o standard ar = lb/t Ar densty aected by: mosture, temperature & alttude above sea level Densty correctons needed, when: Mosture exceeds 0.0 lbs water/lb o ar Ar temp outsde o F range ressure 8.4 < < 1.4 Alttude exceeds t relatve to mean sea level At other heghts: alttude bar = ( STD) exp 4,400 s Duct ressures v T = S + V.e. A statement o an energy balance Statc pressure ~ potental energy term Velocty pressure ~ Knetc Energy term Total pressure ~ total energy term Recall KE s _ mv so KE term s proportonal to V Densty eect on v At NT, V = (V/4005) or V n t/mn, V n nches H O V = 4005 OR V At non standard condtons V =1096 V = 4005 ρ actual Where d s the densty correcton actor: ρactual d = ρ NT V d Note that v becomes very small when V s small so the method s lmted to arly hgh veloctes above ~ 1000 FM Q: we can read +/-.005 H0, what s the error at V=600 FM? Densty correcton V Denstyactual lb / t V = Densty 0.075lb 50 F r essure = x x actual t (460 + t) 9.9 Densty actual = ar densty n lb/t t = temperature n o F ressure = pressure n nches o mercury Also gven by : Where _p s n eet o ar Note: 1 HO = 69. t o ar 4005 = V = 4005V = gδp (.17)69. (60 mn/ sec) 4

5 Sgn Conventon or pressure Total pressure ( T ) and Statc ressure ( S ) Mnus sgn upstream o an ostve sgn downstream o an T = S + v Total ressure = Statc ressure + Velocty ressure Velocty ressure ( V ) Always has a postve sgn Seral low n a duct secton We use conservaton o energy to nd pressure + To Fan 1 T = 1 S 1 V 1 T = S + V Energy Balance says: T 1 = T + losses = + losses S + V S V Ideally s and V can be converted back and orth, but losses always occur Conservaton o energy Drecton o entropy Energy s proporton to pressure, so changes n pressure relect changes n energy (or power). On an absolute pressure scale, the ar always lows rom regons o hgher pressure toward lower pressure Ar low begns at 1 atm at potental energy proportonal to 407 w.g. On the upstream sde, the an creates a lower nlet pressure; t dgs a hole n the ar and ar alls nto the an On the downstream sde, the an creates a pressure hgher than 1 atm n the duct; t pushes ar out the exhaust The an energy must overcomng rcton, varous losses and restore the ar to atmospherc pressure at the outlet ower n duct lows Conservaton o energy says that the energy needed s the sum o the energy used to accelerate the low and the energy needed to overcome rcton and system losses Recall: T s proportonal to total energy In act: Q* T = power used n system ower s n Watts s n a and Q n M /s Converson: 1 H O=49 a; 1000 CFM=0.47 M /s Watts = T *Q* or T n H O and Q n CFM (Note: watts / ( H O CFM) = 49*.47/1000) 1 H = 745 Watts, so n prncple we also can estmate an horsepower! 5

6 1,000 cm 0 H ower = Q x T -1.0" +0.5" -0.5".08 H Change n Total ressure or Seral Flow ncreasng dsspated power as ar lows a b c d -1." +0.5" -0.8" 0.1 H -1.6" +0.5" -1.1" 0.17 H Reerenced to atmospherc -1.9" +0.5" -1.4" 0. H 0.06 H e +1." +0.4" +1.6" 0.5 H Types o losses Frcton Losses: Flud n moton encounters drag along the surace Energy s needed to overcome the drag orce The drag orce s due to the lud vscosty Dynamc losses Turbulence and eddes n the low Momentum losses due to change n drecton Found n expansons, contractons, elbows, junctons and hood entres Fan ower= 0. H+ 0.5 H Frcton losses Frcton losses H are proportonal to the knetc energy n the movng lud In general orm: Wesbach-Darcy rcton eqn: H = L D V1 Losses actor s uncton o v, Re, and surace roughness Frcton losses We use a smpled orm where H s proportonal v H = k V1 k s determned rom charts and gures eg vent manual or curve ttng For example n a straght duct: 0 L H =.8 ( ) 1. V D 1 6

7 Frcton losses Frcton losses ncrease lnearly wth duct length ncreasng ar densty Losses depend on the duct materal and wall roughness Losses ncrease wth V (and also Q ) Losses decrease ~ wth square o duct area (proportonal to 1/A.5 but approx 1/A ) Dynamc Losses - entres Hoods are the busness end o the capture system The hood s the only place where you can capture the contamnant urpose: To enclose or contan the source Drect the contamnate nto the system Mnmze the loss o contamnant nto the room Mnmze energy losses nto the system The more abrupt the change n drecton, the greater the separaton Hood Desgn Desgn parameter or hoods = Q Q = volumetrc low rate n cubc eet /mn Q = VA Flow nto a rounded entry Flow nto a plan duct entry ΔT = F x V or most components F depends on smoothness o turns V = ar velocty n pm A = area o duct n square eet low s not measured drectly determned by measurng velocty & knowng cross sectonal area o low 7

8 Hood Types Hood Types Enclosng Hood contamnant source contaned wthn hood examples: lab ume hood glove box, pant booth good or: contamnants wth hgh toxcty areas where there s a hgh cross drat potental Arlow requrements determned by the product o velocty x area o enclosure The more complete the enclosure the less arlow requrement needed Less susceptble to outsde ar currents Hnges to mprove overhead and sdes access Lght xture d h turntable Sde Vew (Enclosure transparent) Capture Hood creates exhaust arlow n ront o openng to capture & remove contamnant capture velocty or V c a actor o how the contamnant s dspersed room ar currents how ar the source s rom the hood openng Dsadvantages May requre large arlow requrements Subject to crossdrats The eectve reach s lmted to ~ 1 dameter or less dt ws wt hs Lx Lx = greatest dstance rom hood ace to source Hood Types Recevng or Capture Hoods: utlze natural movement o contamnant toward hood openng good or: canopy over hot process (range hood) radal arm saw hood not good or: ne partcles hgh toxcty contamnants cold processes Hood Selecton Factors otental or outsde ar currents nature o the process whch generates the contamnant otental or contamnatng the breathng zone (canopy hoods) Source: Dnard SR. The Occupatonal Envronment Its evaluaton & Control (1998) 8

9 END here Crtcal low condtons Increasng pressure upstream, e.g. 1.6 atmospheres Velocty not lmted atmospherc pressure atmospherc pressure Velocty lmted Decreasng pressure downstream, e.g. 0.4 atmospheres Crtcal Orces and ressures Upstream and Downstream 9