duct pressure loss calculation

7/29/2019 Duct Pressure Loss Calculation

1/24

To find pressure loss

1.duct size is known.2.flow rate is known.

S.1 Compute the loss pressure for each branch of simple

Duct system shown in figure using loss coefficient

method.

Plenum

1

400cfm

50ft, 10in

200cfm

50 ft, 9in

120cfm

40 ft, 6 in

a2

3


2/24

Solution:

For section 1-a

(i)

for plenum exit loss

C = 0.85 ( from T-4 )

ft/min385.733)12/10)(4/(

400

A

QV

2

u

uu ===

22

ua-1

4000733.3850.85

4000VCH

=

=

For straight line 1-a ( 50 ft )

100

0.09

L

P

in10

400cfm=

( ) waterofin0.04550100

0.09P a1 ==

Plenum

1

400cfm

50 ft, 10 in

a


3/24

waterofin0.07350.02850.045a-1Runforlosspressuretotal =+=

section a- 2

( )( )ft/min633.79

129

4

280AQV

2

d

dd ===

waterofin0.002Pft/min633.79V

ft/min733.385Vfit

d

u =

=

=

For straight run

waterofin100

08.0

L

P

in9

cfm280=

waterofin0.0450100

0.08Pst ==

waterofin0.0420.0020.042-aRunforlosspressuretotal =+=

280 cfm

50 ft, 9in

a 2


4/24

Section a-3

( ) ( )ft/min611.154

126

4

120

A

QV

22

b

bb ===

0.48C

offtake45loss)(branch

0.833733.385

611.154

V

V

bu

b

=

==

o

( ) waterofin0161.04000

733.3850.48

4000

VCH

22

ubb =

=

=i

a

3

VU Vd

Vb

120cfm

40ft, 6 in


5/24

( )

losselbow45 oii

)2-Tfrom1DR

assume(0.37C==

waterofin0.00864000

611.1540.37

4000

V0.37H

22

be =

=

=

( ) losselbow90 oiii

waterofin0.0086He =

( ) )ft(40lossRunStraightiv

waterofin100

0.12

L

P

in6

cfm120=

( ) waterofin0.04840100

0.12P st ==

waterofin0.0813

0.0480.00860.00860.01613-aRunforlosspressuretotal

=

+++=

total pressure loss for section 1-2 is

pa28.75950waterofin0.11550.0420.0735P 2-1 ==+=

Total pressure loss for section 1-3 ispa38.54waterofin0.15480.08130.0735P 31 ==+= =

+==========================+


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11/24

Equivalent Length Method

S.2 Compute the loss pressure for each branch of simpleduct system shown in figure usingEquivalent Length

approach.

Section 1-a

ft85(35)50L a1 =+=

ftwater/100ofin0.09L

P

in10:dia

cfm400=

=

waterofin0.076585

100

0.09L

L

PP a-1 ==

=

Plenum

1

400cfm

50ft, 10in

200cfm

50 ft, 9in

120cfm

40 ft, 6 in

a2

3


12/24

Section a-2

ft55)5(50L 2-a =+=

waterofin1000.08

LP

in9:diacfm200 =

=

waterofin0.04455100

0.08P 2-a ==

Section a-3

ft90)10()5()35(40L 3-a=+++=

waterofin0.12L

P

in6:dia

cfm120=

=

waterofin0.10890100

0.12P 3a == =

The pressure loss for section 1 to 2 is

Pa30.00

waterofin0.12050.0440.0765PPP 2-aa-12-1

=

=+=+=

The pressure loss for section 1 to 3 is

pa45.9

waterofin0.18450.1080.0765PPP 3aa-13-1

=

=+=+= =

+======================+


13/24


14/24

TO FIND DUCT SIZE

Equal Friction MethodS-3 Select duct sizes for the simple duct system of shown

in figure using Equal Friction Method. The system is

for a residence. The total pressure for the duct system

is 0.12 in of water and the loss in total pressure for

each diffuser at the specified flow rate is 0.02 in ofwater.

Plenum

o

a b

150cfm

150cfm

200cfm

1

5

2

3

4

20ft

15ft

17ft

30ft

10ft


15/24

ft16730)105355(35101720Le123 =++++++++=

ft15230)555535(301720Le124 =++++++++=

ft16030)105035(1520Le15 =+++++=

Le123 is design line.

waterofin0.10.02-0.12P ==

water/ftofin100.5980.000598167

0.1

L

P

L

P 3-

design123

===

=

cfm500150200150cfm1 =++=

cfm350200150cfm2 =+=

line.ofallforequalisft100water /ofin0.0598L

P

design

=

ft/min660Vin,9.11Dcfm500cfm 111 ===

ft /min600Vin,2.10Dcfm350cfm 222 ===ft/min490Vin,5.7Dcfm150cfm 333 ===

ft/min525Vin,4.8D200cfmcfm 444 ===


+===========================+


16/24

Balance Capacity Method

S-4 Select duct sizes for the simple duct system of shownin figure using Balance Capacity Method. The system

is for a residence. The total pressure for the duct

system is 0.12 in of water and the loss in total pressure

for each diffuser at the specified flow rate is 0.02 in of

water.

Plenum

o

a b

150cfm

150cfm

200cfm

1

5

2

3

4

20ft

15ft

17ft

30ft

10ft


17/24

ft16730)105355(35101720Le123 =++++++++=

ft15230)555535(301720Le124 =++++++++=

ft16030)105035(1520Le15 =+++++=

Le123 is design line.

waterofin0.10.02-0.12P ==

water/ftofin100.5980.000598167

0.1

L

P

L

P 3-

design123

===

=

cfm500150200150cfm1 =++=

cfm350200150cfm2 =+=

2,3.1,lineforlinedesignisft100water /ofin0.0598L

P

design

=


ft /min600Vin,2.10Dcfm350cfm 222 ===

ft/min490Vin,5.7Dcfm150cfm 333 ===According to the theory, the pressure of all line are equal.

PP 43 =

( ) ft90301053510Le3 =++++=

waterofin0.0503890L

PP

design3

=

=

( ) ft753055530Le4 =++++=


18/24

ftwater/100ofin0.07176waterofin100.717675

0.0538

L

P 3-

4

===

ft/min560Vin,1.8D

200cfm


P

44

4

4 ==

=

=

Section a-5

o4o5 PP = .. equ:(1)

b4aboaa5oa PPPPP ++=+

b4aba5 PPP += equ:(2)

ft75LL 4b4 ==

ft22517LL 2ab =+==

waterofin01315.022100.598LL

PPP 3-222ab ==

==

waterofin0538.075107176.0LL

PPP 34

4

4b4 ==

==

waterofin0.06690.013150.0538PP a55 =+==

( ) ft10530105015La5 =+++=

water/ftofin10637.0ft105

waterofin0669.0

L

P

L

P 3

a5

a5

5

==

=

ft/min489Vin,7.5D150cfmft,water/100ofin0637.0L

P55 ==

==

+========================+


19/24

S-5 Design the duct system of shown in figure by using

the equal friction method. The velocity in the duct attached

to the plenum must not exceed 900 ft/min and the overallloss in total pressure should not exceed about 0.32 in of

water. Total pressure losses for the diffusers are all equal to

0.04 in of water. The lengths shown are the total equivalent

length of each section. Use English unit.

For main duct velocity = 900 ft/min

cfm1 = 100+250+250+100+100 =800

Plenum

o a

b

cd

1 2 3 4

5

6

78

9

55 ft 25 ft 40ft 45ft

95ft

100

ft

100

ft

80

ft

100

ft

250 cfm 250 cfm

100cfm

100cfm 100 cfm


20/24

from chart,


P

ft/min890Vin,9.12Dcfm

ityduct velocmain

design

11

1

=

==

lines.allforequalisL

P

design

From chart,

ft/min870Vin,12.2D700100100250250cfm 222 ===+++=

ft/min780Vin,4.10D450250100100cfm 333 ===++=

ft/min700Vin,5.9D350250100cfm 444 ===+=

ft/min530Vin,9.5D100cfm 555 ===

ft/min660Vin,2.8D250cfm 666 ===

ft/min530Vin,9.5D100cfm 777 ===

ft/min530Vin,9.5D100cfm 888 ===

ft/min660Vin,2.8D250cfm 999 ===

+===========================+


21/24

S-6 Design the duct system of shown in figure by using

balance capacity method. The velocity in the duct attached

to the plenum must not exceed 900 ft/min and the overallloss in total pressure should not exceed about 0.32 in of

water. Total pressure losses for the diffusers are all equal to

0.04 in of water. The lengths shown are the total equivalent

length of each section. Use English unit.

For main duct velocity = 900 ft/min

cfm1 = 100+250+250+100+100 =800

Plenum

o a

b

cd

1 2 3 4

5

6

78

9

55 ft 25 ft 40ft 45ft

95ft

100

ft

100

ft

80

ft

100

ft

250 cfm 250 cfm

100cfm

100cfm 100 cfm


22/24

from chart,


P

ft/min890Vin,9.12Dcfm

ityduct velocmain

design

11

1

=

==

ft2609545402555Le12345 =++++=

ft2458045402555Le12346 =++++=

ft220100402555Le1237 =+++=

ft1801002555Le129 =++=

ft15510055Le18 =+=

waterofin0.2496260100

096.0P 5-o ==

6-o5-o PP =

6-dd-o5-dd-o PPPP +=+


096.0L

L

PP 5

design

5-d ==

=

waterofin0912.0PP 5-d6-d ==

waterofin1014.1800912.0

LP

LP 3

6

6-d

6

===

ft/min690Vin,2.8D

250cfm

114.0ft100L

P

66

6

6 ==

=

=


23/24

7-o6-o PP =

7-cc-o6-dd-4-cc-o PPPPP +=++

water.ofin0432.045100096.0L

LPP 4

design

d-4-c ==

=

waterofin0912.0PP 66-d ==

waterofin1344.00912.00432.0PPP 6-dd-4-c7-c =+=+=

3

7-c

7-c

7

10344.1

100

1344.0

L

P

L

P ==

=

ft/min600Vin,6.5D

100cfm

0.1344ft100L

P

77

7

7 ==

=

=

9-o7-o PP =

7-cc-bb-o9-bb-o PPPPP ++=+

7-cc-b9-b PPP +=


096.0L

L

PP 3

design

c-b ==

=

waterofin1728.01344.00384.0PPPP 7-cc-b9-b9 =+=+==

water /ftofin10152.1150

1728.0

L

P 3

9

==

ft/min700Vin,0.8D

250cfm

1152.0ft100L

P

99

9

9 ==

=

=


24/24

9o8-o PP =

9bb-aa-o8-aa-o PPPPP =++=+

9-bb-a8-a PPP +=

waterofin024.025100

096.0L

L

PP 2

design

ba ==

=

waterofin1968.01728.0024.0P 8-a =+=

twater /fofin10968.1

100

1968.0

L

P 3

8-a

==

ft/min700Vin,2.5D

100cfm

ftwater/100ofin1968.0ft100L

P

88

8

8 ==

=

=

+=========================+

duct pressure loss calculation

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