Duct design. King Abdulaziz University. Major losses in duct

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1 King Abdulaziz Univerity College o Engineering Mechanical Engineering MEP 5 Rerigeration & Air Conditioning June 009 Duct deign Air low in duct Major and Minor Loe in Duct Lo coeicient or ome itting Equivalent length or a itting Duct acceorie Preure diagram Duct deign Equal riction method Balanced Capacity method Air Flow in Duct Modiied Bernoulli equation Velocity air preure, P v ρv ρv p p gc gc gc p ρg P ρgl gc ρv p ρv g ρg g l Pv P Pv P 0 P 0 P Major and minor loe V V P v ρ 097 P v in in water and V in t/min 005. V V P v ρ P v in Pa and V in m/.9 ρgl g c Preure lo between and 3 Major loe in duct Moody Diagram L V ρ D g riction actor (Re, e/d) log 0 e.5 3.7D ReD F riction actor(re, roughne) e i the pipe roughne 5 6

2 7 8 Ue o preure lo chart -In Englih unit chart (Fig..), the y-axi i the preure drop per 00 t length o the duct -The chart ha parameter: preure drop, air low rate, air velocity, and duct diameter 3-Knowing two o thee parameter, one can get the other two parameter -The given duct ize i or circular pipe. Ue the equivalent table or equation to ind the ize o the rectangular duct Equivalent o a circular duct D e equivalent diameter 5/8 ( ab) ( a b) / D e.3 a and b are the dimenion o a rectangular duct 9 0 Fig.. and. correction actor A) Correction actor or denity and vicoity when T >00 F (38 C) ρ a C ρ 0.9 µ a µ 0. ubcript a reer to actual and reer to tandard condition P0 a C0

3 B-Correction due to duct liner Non-circular duct or typical galvanized duct i 0.0 or liner i between 0.03 and 0.06 ( Cro ection area) D h Wetted Perimeter ρvd µ D Roughne correction actor or commercial duct liner 3 Re Re ρvd µ h D h VD ν VDh ν Note on uing preure lo chart You can enter the chart knowing the CFM and preure lo to get duct ize in diameter and air velocity I you know the low rate and rectangular cro ection velocity, then calculate V and enter the chart uing V and D h Minor loe (loe in itting) ρv g P C0 C0P v Table to ind C o or dierent itting 5 6 Table.8 C 7 8 3

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5 Example.8 Compute the preure lo due to a round 90 diverging ection. The common ection i, and the traight through ection i 0. The low rate in the common ection i 00 cm, the low rate in the branch i 50 cm. c b Straight ection Branch ection Q Qb V 558 t/min Vb 73 t/min A Ab Q 850 Q b Qc 00 Qc 00 A Ab Ac Ac From ig..b C C b V C in 0 H O Vb Cb 0.6" 005 0b H 5 O Equivalent o a minor lo in term o traight pipe length Major lo L V L ρ D g Minor loe (loe in itting) V 0 Cρ g L C D Example.9 Compute L e or duct ytem hown in igure Duct acceorie - Damper (Parallel blade and oppoed blade) -Fire damper 3-Turnning vane (linear and aerooil) Fitting Decription Lo coeicient C o L e (t) L e (m) Entrance 5 Diverging branch Conical θ0 Table.0A 0.0 or D0 C b 0.6, Table.A, D Straight 5 diverging C 0.3, Table.A, 0.05 or D elbow C0.3, Table.8A, 0.08, D

6 Preure diagram Energy grade line EGLTotal preurep o Preure diagram Hydraulic grade line EGLtatic preure, P Preure diagram Duct deign method -Equal riction method -Balanced capacity method 3-Static regain method 33 3 Equal riction method -Select the run with the anticipated max. low reitance -Calculate the duct equivalent length (L e ) 3-From the known preure available or the upply duct, calculate the preure lo in inche o water per 00 eet (/L) -Size each duct ection uing the preure drop and the low rate in that ection 5-Calculate the total preure drop or each run. Ue damper when neceary Example. Size the duct ytem hown uing equal riction method. Total preure available ro duct i 0. water. The preure drop or each diuer i 0.0 water

7 Example. Available preure i 0. in H O (30 Pa). Lo in diuer i 0.0 in H O (5 Pa). Section Summary o reult or Example. Q (cm) 350 D (in) 0 V (t/min) / L L e L ( L Lent ) ( L Lt ) ( L3 Lwye L5 L90 L 3 boot L3 (0 30) (5 8) ( ) 60 t ) Preure loe in each run / L 0.*(00 /60) 0.063in HO per 00 t Run 3 0 ( inche water) Example. Balanced Capacity method Example. For ection inche o water 0 / L 0.039(00 / 7) From Fig.. D 8.7 or 9 inche V 69 t/min For ection From Fig.. D 5 7. or 8 inche V 5 0 t/min The dierence between the two olution i minor ection Equal riction method Balanced Capacity method 39 5 D 8 D 5 8 D 9 D Recommended air velocitie (/m) Recommended velocity Outide air inlet Fan outlet Reidence 700 School, public area 800 Indutrial area 000 Main duct Branch duct Recommended velocitie region m/ 00 t/min 7