Duct & Vent Engineering

1 of 70

INDEX

Sr. No. Description Page No.

1 PREAMBLE 4

2 REFERENCES 4

3 TERMINOLOGY 4

4 INPUT DATA REQUIRED FOR DUCT DESIGN 6

5 HOLTEC STANDARD FOR DUCT DESIGN 6

5.1 Shape of duct 6

5.2 Duct angle 7

5.3 Velocity of duct 7

5.4 Diameter of duct 9

5.5 Compensation for bend and reducers 10

5.6 Material of duct 10

5.7 Calculation for pressure drop 10

5.8 Flanges 11

6 VENT WORK 18

6.1 Velocity 18

6.2 Duct Inclination 18

6.3 Vent points 18

6.4 Typical observation for design 18

6.5 Volumetric flow rate 19

6.6 De-dusting air quality 20

2 of 70


6.7 Pickup points 24

7 THICKNESS OF DUCTS 26

8 DESIGN OF BENDS 27

9 DESIGN OF ELBOWS 30

10 DESIGN OF REDUCERS 31

11 FLANGE CONNECTION 35

12 STIFFENER DESIGN ( POSITION & SPACING) 36

12.1 Circular ducts 36

12.2 Rectangular ducts 41

13 DUCT EXPANSION 42

14 DUCT SUPPORTS AND HANGERS 46

14.1 Loads on supports 47

14.2 Types of supports 48

14.3 Movable supports 48

14.4 Fixed supports 50

15 DAMPERS 52

15.1 Rectangular dampers 52

15.2 Shut off damper 52

15.3 Location of dampers 53

16 DO’S & DON’TS OF GOOD DUCT DESIGN 54

16.1 Curves in ducts 54

16.2 Transition pieces 55

3 of 70


16.3 Branches 56

16.4 Fan inlet 57

16.5 Fan outlets 58

16.6 Exhaust stacks 59

16.7 Access doors 60

17 PROVISION FOR INSTRUMENTATION 61

18 FLOW MEASUREMENT 61

18.1 Piezometric ring 61

18.2 Orifice plate 62

18.3 Venturimeter 62

18.4 Flow measurement calculation 63

Appendix 1: Page 65

Annexure 2: Page 71

4 of 70

DUCT & VENT ENGINEERING

1. PREAMBLE

This document covers the design of ducting.

Duct is a broad classification of ductwork used in industry for many diverse applications. For

the purpose of this document a duct is defined as metal shell conveying air or gases at high/

ambient temperature. This air or gas may be corrosive and may contain particulate with

varying degree of abrasiveness.

The document covers the following areas:

� Selection and sizing of Duct.

� Selection of stiffeners

� Selection of Duct Supports

� Other important considerations

2. REFRENCES

The basis for design has been taken from Holderbank Engineering, Canada (HEC) standards,

Holtec's Standards and from previous projects which have been running successfully.

3. TERMINOLOGY

The important technical terms are defined below:

Term Description Unit

Volume flow rate Volume of fluid flowing per unit time m3/sec

Gauge pressure Pressure applied by the fluid flowing through a duct mm WG

Pressure Drop Loss in total pressure between two points on account of friction, turbulence, bends, etc

mm WG

Draft Negative pressure measured in mm of water gauge. It can be natural, induced or forced

mm WG

Critical velocity Gas velocity at and below which the dust starts to settle down in the dust. The actual value of this velocity is dependent on

• Duct inclination • Nature of Dust( Particle Size)

m/sec

5 of 70

The important duct parts are defined below:

Part Description

Stiffener This is used for providing stiffness/rigidity to the ducts and to prevent

collapsing/deflection of the duct.

Fixed Support Structure rigidly fixed to the duct to take various loads of the duct.

Guide Support It is the support which allows the axial movement of the duct and at the

same time transmits the load of the duct to the other structures.

Compensator Compensators are the metallic or non-metallic members used in a duct

to take the thermal expansion and to isolate the vibrations.

Damper A mechanical device used to control the flow of gases. It is of two type

(1) Louver Type (2) Butterfly Type

Self Cleaning

Elbow

A duct in which there is a change in direction from upward to

downward direction, a U elbow is formed. To prevent accumulation of

duct at this bend, a special type of elbow is provided, called Self

Cleaning Elbow.

Manhole Opening in a duct provided for entry of maintenance personnel.

Cleanout Port Small opening with socket/ Plug arrangement in a duct provided to

clean out the accumulated duct.

Transition Piece It is a part of duct to change type of cross-section (e.g. circular to

rectangular & vice versa)

Guide Vanes Vanes installed in a duct (Especially in rectangular duct) to streamline

the flow of gases.

6 of 70

4. INPUT DATA REQUIRED FOR DUCT DESIGN

The basic data required for design of duct is:

1. Actual volume flow rate 'Q' (Am3/sec)

2. Total pressure in duct 'P' (mm WG)

3. Max. temperature of gases 'T' (0C)

4. Area of application

5. Dust Content

6. Type of Gas

5. HOLTEC STANDARD FOR DUCT DESIGN

5.1 Shape of Duct

Round duct is preferred for industrial ventilation, air pollution control and dust collecting

systems due to following reasons:

• Circular duct is naturally stiff, therefore requires less stiffening members for the same

cross section

• During erection, more flexibility is available in rotating the ducts for alignment.

Despite advantages of the round configuration there are applications for industrial ventilation

and air pollution control systems where the rectangular duct must be considered.

Exception: Consideration should be given to the physical installation i.e., a short duct

between two rectangular flanges need not to be changed in cross section.

7 of 70

5.2 Duct Angle

It is also imperative that the duct is installed at the correct angle to prevent accumulation of

dust in the ducts carrying duct laden gases.

Typical values of Angle of Inclinations for Ducts and Vents are given below in Table 1.

TABLE 1: DUCT / VENT ANGLE in deg.

DUCT VENT Application

Upside Downside Upside Downside

Raw Meal 50 50 50 45

Coal 60 50 50 50

Clinker 50 45 50 45

Gypsum/Cement 50 45 50 45

Slag 50 45 50 45

Flyash 50 50 50 50

5.3 Velocity of Gas

The duct velocity (v) to be considered is given in Table 2.

Exception: If due to some constraints, the ducts are horizontal or sloped less than 450 ,then

the velocity should be taken in the range of 28m/s in order to prevent dust settling in the duct

and causing possible blockage of the system.

Avoid long horizontal ducts for dust laden gases.

The gas velocity for various inclinations and applications is tabulated as Table 2.

8 of 70

TABLE 2 : GAS VELOCIY IN DUCTS (IN M/SEC)

Inclination Dust Application

Less than 450 450-900

Raw Mill 28 22

• Raw Mill to Cyclone 22 20

• Cyclone to Mill Fan 22 20

• Bag House/ESP to Exhaust Fan 22 20

• Exhaust Fan to chimney 22 20

• Smoke gas Fan to Raw Mill 22 20

• Raw Mill Bypass 22 20

• Raw Mill Recirculation 25 22

Preheater

• Downcomer 22 20

• To Raw Mill 22 20

• To Coal Mill 22 20

Coal Mill

• Coal Mill to Bag house/ESP 25 22

• Bag house /ESP to exhaust fan 25 22

• Exhaust fan to chimney 25 22

• Smoke gas fan to coal mill 25 22

• Coal mill recirculation 25 22

Cement Mill

Cement Mill to Bag House/ESP 25 22

• Bag House/ESP to Exhaust Fan 25 22

• Cement Mill Separator to Fan 22 20

• Separator Recirculation 22 20

VENT

• Raw Meal 18 15

9 of 70

Inclination Dust Application

Less than 450 450-900

• Coal 18 15

• Cement/Gypsum 18 15

• Clinker 15 12

• Slag 15 12

• Fly ash 12 10

STACK

• Stack 10-16

Exception: Short ducts are usually sized according to Flanges of connecting equipment (from

cyclone etc.) even if the velocity of the gas is somewhat on higher or lower side.

5.4 Diameter of Duct

For designing the duct diameter, selection of correct gas velocity is very important. For dust

laden gases, Gas velocity should be selected in such a manner that it should always be more

than the critical gas velocity.

The cross-sectional area of the duct is calculated by the following formula:

Cross-Section Area (in m2) = Q (Volumetric Flow Rate in Am3 /sec)

V (Gas velocity in m/s)

It is important to take the volume floe rate at actual operating conditions considering

temperature and altitude. If it is given at NTP, then adjustment has to be made for

temperature and pressure at plant altitude. Pressure at altitude “h” is found out by the

following equation

P (Pressure in mm WG) =10342 e-0.0001255*h (Plant altitude in meters)

10 of 70

5.5 Compensation for bends and reducers

After routing of the gas duct has been established , it is important to check back through the

system to ensure that the pressure drop has not been considerably increased due to the

addition of bends, reducers etc.

5.6 Material of duct

For General Applications (For use in cement plants): Weld able quality structural steel IS:

2062

For High temperature and/ or Corrosive Applications: SS 304, 310,316

5.7 Calculation for Pressure Drop

The Pressure drop in duct can be calculated as per graph.

For change in cross section the pressure drop can be calculated by adding the equivalent

length in the graph after calculating the value of ξ.

Procedure for Calculation of Pressure Drop

1. Calculate total length of the duct = Lt

2. Identify various bends, reducers, venturi i.e. any change in the cross-section of the duct

including any bend etc.

3. Find the friction factor ξ for each of these bends.

4. Read α form the graph 1 from y-axis. The Gas Flow in cum/hour is on the x-axis. Read

the value from the graph for rolled sheet.

5. Calculate Leq = ξ / α*D for each of these bends etc.

ξ= Friction factor given in table 3

α= See Graph Below

D= Duct diameter in mm

6. Add sum of Leq to Lt.

11 of 70

7. Read value of Pressure loss per meter from graph 2.

8. Multiply per pressure loss with Lt to get the total pressure loss.

5.8 Flanges

In the process and vent ducts no intermediate flanges are to be provided between equipment

to equipment unless it is required for maintenance purposes.

12 of 70

13 of 70

14 of 70

15 of 70

16 of 70

17 of 70

18 of 70

6.0 Vent work

6.1 Velocity

Good ductwork design is to size the duct cross-sectional area for a velocity of 15-18 m/sec.

For clinker, the velocity should be taken at 12-16 m/sec.

6.2 Duct Inclination

To avoid dust accumulation in the duct work, it is recommended an angle of duct

inclination of minimal 50 deg. In limestone, slag and cement venting system and minimal 45

deg. In clinker venting system duct with low velocity encourages material to fall out. Duct

with high velocity encourage abrasion.

6.3 Vent Points

Venting system with an excessive no. of connected vent points cannot be

controlled/calibrated in a way that venting is effective. That is why it is recommended to limit

the number of vent points connected to one filter to at most 8 points.

6.4 Typical Observation for design

⇒⇒⇒⇒ Belt Conveyor

The distance between material feed chute and deducting hood should be minimum 1.5 times

belt width in conveying direction and 0.75 times belt width behind the feed chute.

Height of skirt plate to be 600mm from top of belt.

⇒⇒⇒⇒ Bag filter

Duct velocity between Bag filter AND Fan should not exceed 15m/sec.

⇒⇒⇒⇒ Bag Filter Fan

Duct velocity between Bag filter fan to atmosphere should not exceed 10 m/sec.

Frequently encountered ductwork problems are poorly designed branch entries, elbows and

size variations that hamper airflow and/or cause acceleration wear. Care should be taken to

ensure proper selection as per duct engineering manual.

19 of 70

6.5 Volumetric Flow Rate

Based on Volumetric Flow rate and selection velocity, duct diameter can be calculated based

on the table given in next page.

⇒⇒⇒⇒ Minimum duct diameter to be selected is 100NB

⇒⇒⇒⇒ For duct size from 100 to 300, pipes are to be used and the diameter mentioned in the

table is in NB.

DUCT DIA & X-SECTION FOR DIFFERENT VELOCITY AND VOL UMES

DIA 21m/s 20m/s 19m/s 18m/s 17m/s 16m/s 15m/s 14m/s 13m/s 12m/s 11m/s 10m/s

Mm GAS Volume m3/min (preferred range is 15-18m/sec expect for clinker. For clinker & dry fly ash

preferred range is 12-16m/sec.

100 593 565 537 509 481 452 424 396 368 339 311 282

125 928 884 839 795 751 702 663 619 574 530 486 442

150 1336 1272 1209 1145 1081 1018 954 891 827 763 700 636

175 1924 1832 1741 1649 1557 1466 1374 1283 1191 1099 1008 916

200 2375 2262 2149 2036 1923 1810 1696 1583 1470 1357 1244 1131

225 2979 2837 2695 2554 2412 2270 2128 1986 1844 1702 1561 1419

250 3711 3534 3358 3181 3004 2827 2651 2474 2297 2121 1944 1767

300 4994 4756 4518 4280 4042 3805 3567 3329 3091 2853 2616 2378

355 7483 7127 6770 6414 6058 5701 5345 4989 4632 4276 3920 3563

380 8574 8166 7757 7349 6941 6533 6124 5716 5308 4899 4491 4083

400 9500 9048 8595 8143 7691 7238 6786 6333 5881 5429 4976 4524

430 10979 10456 9933 9410 8887 8365 7842 7319 6796 6274 5751 5228

450 12024 11451 10878 10306 9733 9161 8588 8016 7443 6871 6298 5726

500 14844 14137 13430 12723 12017 11310 10603 9896 9189 8482 7775 7069

535 16995 16186 15376 14567 13758 12949 12139 11330 10521 9711 8902 8093

560 18620 17734 16847 15960 15074 14187 13300 12414 11527 10640 9754 8867

585 20320 19352 18385 17417 16450 15482 14541 13547 12579 11611 10644 9676

610 22094 21042 19990 18938 17885 16855 15781 14729 13977 12625 11573 10521

630 23566 22444 21322 20200 19078 17955 16833 15711 14589 13466 12344 11222

660 25864 24633 23401 22169 20938 19706 18474 17243 16011 14780 13548 12316

685 27861 26534 25207 23881 22554 21227 19901 18574 17247 15920 14594 13267

710 29932 28506 27081 25656 24230 22805 21380 19954 18529 17104 15678 14253

735 32077 30549 29022 27494 25967 24439 22912 21384 19857 18329 16802 15275

760 34296 32663 31029 29396 27763 26130 24497 22864 21231 19598 17964 16331

800 38000 36191 34382 32572 30762 28953 27143 25334 23524 21715 19905 18096

20 of 70

DUCT DIA & X-SECTION FOR DIFFERENT VELOCITY AND VOL UMES

DIA 21m/s 20m/s 19m/s 18m/s 17m/s 16m/s 15m/s 14m/s 13m/s 12m/s 11m/s 10m/s

Mm GAS Volume m3/min (preferred range is 15-18m/sec expect for clinker. For clinker & dry fly ash

preferred range is 12-16m/sec.

900 48095 45804 43514 41224 38934 36644 34353 32063 29773 27483 25192 22902

1000 59376 56549 53721 50894 48066 45239 42412 39584 36757 33929 31102 28274

1060 66715 63538 60361 57184 54007 50830 47654 44477 41300 38123 34946 31769

1120 74481 70935 67388 63841 60294 56748 53201 49654 46108 42561 39014 35467

1180 82675 78738 74801 70865 66928 62991 59054 55117 51180 47243 43306 39369

1250 92775 88357 83939 79522 75104 70686 66268 61850 57432 53014 48597 44179

6.6 De-dusting Air Quantity

The true volume that the ventilation system is required to handle must be determined first.

Therefore, it has to be determined how much vent air is required at each dust point.

Recommended standard volumetric requirements for transfer are given.

Before starting preparing drawing, draw a flow diagram depicting vent volume, of each vent

process, as per recommendation. Write vent volume at each rise points and total vent volume

before entering bag filter.

21 of 70

DEDUSTING AIR QUANTITIES

22 of 70

23 of 70

24 of 70

6.6 Pickup Points

The hood design at pickup points should provide ventilation of the duct points while

minimizing the amount of product introduced in to the duSt collection system. Improperly

designed hoods tend to increase the grain loading to the bag filter. Recommended standard

design practices for hood design are:

VERTICAL

VERTICAL

25 of 70

Sheet thickness of suction hoods: 3 to 4 mm

Intake velocity at hoods according to above table: V1 = 1.4 m/sec

Air velocity in deducting duct: V2 ≥ 15-18 m/sec

26 of 70

7.0 THICKNESS OD DUCT

Up to 300 mm diameter pipes are to be used. For venting work medium duty pipes are recommended

For fabrication of ducts! Vents above 300 mm diameter, the minimum plate thickness should be 6 mm. However depending upon the negative pressure and the type of material used the thickness of the duct is tabulated as below:

Duct Diameter (in mm)

Plate Thickness (In mm)

Raw Mill

Coal Mill (To take care of explosion)

Pre-heater & GCT

Cement Mill (Tube Mill)

Cement Mill (VRM)

355 - 560 - 5 6 - - 630 - 800 - 6 6 - -

900 - 1,180 8 8 6 6 6 1,250 - 1,700 8 10 6 6 6

1,800 - 2,000 8 12 ( Exception ESP

to EP Fan = 10 mm)

6 6 8

2000 & above 8 - 6 6 10

Fly ash Handling: For Fly ash handling, duct thickness on one size higher should be used. Say

instead of 6 mm to 8mm otherwise liner is to be provided with 6 mm thick plate.

27 of 70

8.0 DESIGN OF BENDS

Bends should have a minimum mean radius as indicated below:

Bend 1:

Use bend 1 for change in direction of duct.

Bend is fabricated as per following details

Description R=

Process Ducts D to 1.5 D

Vent 1.5 D to 2 D

Clean Air (Bag Filter Outlet) 1 to 1.5 D

46' -90' BEND DESIGNATION: BEND, 'D', ANG.



28 of 70

Bend 2:

Use for de-dusting pipes to change direction from upward to downwards.

This bend is applicable up to diameter of 500 mm only. For diameter above 50 mm use self

cleaning Elbow only.

29 of 70

Bend 3:

Use for self cleaning bends.

These bends serve to prevent dust accumulation with subsequent dust slides. These can be used

for de-dusting pipes as well as for process pipes.

For all plane faces which are exposed to pressure (under pressure), specify bracing irons when

preparing shop drawings for these bends.

30 of 70

9.0 DESIGN OF ELBOWS

The differentiation between Bend and Elbow is defined as follows:

• Use elbow only for vents

• Use elbows only up to 500 mm dia.

• Use elbow, when there is a space constrain

Elbow 1:

Use for V0 < 600

a = D * tg * V0/2

Elbow 2:

Use for V0 > 600

a = 2 * D * sin V0/2

b= 2 * D * (sin V0/2 – tg * V0/4)

c= D * tg * V0/4

31 of 70

10.0 DESIGN OF REDUCERS

Reducer 1

Due to pressure and flow conditions top angle should be between 300 to 600. However, no

general rule in this respect can be given and the angle is not shown in the drawing.

Calculation for Weight of Reducer 1

S = {( ( D – d ) / 2 ) 2 + L 2 } 1/2

Weight = t * S * (D + d) / 2 * 25.1 * 10 -6 excluding the weight of flanges.

32 of 70

Reducer 2

Due to pressure and flow conditions top angle should be between 300 to 600. However,

no general rule in this respect can be given and the angle is not shown in the drawing.


Weight = [ { ( ( B – b ) / 2 ) 2 + L 2 } 1 / 2 * ( A + a ) + { ( ( A – a ) / 2 ) 2 + L 2 } 1 / 2 * ( B

+ b)] * t * 8 * 10– 6 excluding the weight of flanges.

33 of 70

Reducer 3

Due to pressure and flow conditions top angle should be between 300 to 600. However, no

general rule in this respect can be given and the angle is not shown in the drawing.


Weight = [ { ( ( B – d ) / 2 ) 2 + L 2 } 1 / 2 * B + { ( ( A – d ) / 2 ) 2 + L 2 } 1 / 2 * A ) * t *

8 * 10- 6 + { ( ( d / D ) 2 + L 2 ) 1 / 2 * 3.14 * d} * t * 8 * 10- 6

Excluding the weight of flanges.

34 of 70

Reducer 4

Use for square duct joining circular bag filter fan inlet.

Reducer on the pressure and suction side of the fan should have as small a top angle as

possible.


Weight = [ { ( ( D – a ) / 2 ) 2 + L 2 } 1 / 2 * a + { ( ( D – b ) / 2 ) 2 + L 2 } 1 / 2 * b ) * t * 8

* 10 -6 + { ( ( d / D ) 2 + L 2 ) 1/2 * 3.14 * d} * t * 8 * 10-6

Excluding the weight of flanges.

Condition: D ≥ 4 (a*b)

35 of 70

11.0 FLANGE CONNECTION

Standard flange details are given below:

Sn.

D (Pipe OD) A B C

No. of Holes E F H J

1 125 127 3 171 6 10 207 12 2 150 152 3 197 6 10 232 12 3 180 182 3 222 6 10 262 12 4 205 207 3 248 6 10 287 12 5 230 232 3 273 6 10 312 12 6 255 257 6 298 8 10 337 L 40*40*4 12 7 280 282 6 324 8 10 362 L 50*50*5 12 8 305 307 6 349 8 10 387 L 50*50*5 12 9 330 333 6 387 8 12 433 L 50*50*5 14 10 355 358 6 413 12 12 458 L 50*50*5 14 11 380 383 6 438 12 12 483 L 50*50*5 14 12 405 408 6 464 12 12 508 L 50*50*5 14 13 430 433 6 489 12 12 533 L 50*50*5 14 14 455 458 6 514 12 12 558 L 50*50*5 14 15 480 483 6 540 12 12 583 L 50*50*5 14 16 510 513 6 565 12 12 613 L 50*50*5 14 17 535 539 6 591 16 12 639 L 50*50*5 14 18 560 564 6 616 16 12 664 L 50*50*5 14 19 585 589 6 641 16 12 689 L 50*50*5 14 20 610 614 6 667 16 12 714 L 50*50*5 14 21 635 639 6 692 16 12 739 L 50*50*5 14 22 660 664 6 718 16 12 764 L 50*50*5 14 23 685 689 6 743 16 12 789 L 50*50*5 14 24 710 714 6 768 20 12 814 L 50*50*5 14 25 735 739 6 794 20 12 839 L 50*50*5 14 26 760 764 6 819 20 12 864 L 50*50*5 14

36 of 70

12.0 STIFFENER DESIGN (POSITION & SPACING)

12.1 Circular Ducts

Ducts having a large diameter and a thin wall thickness may "Collapse" if subjected to external

over pressure (Internal under pressure).

On account of under pressure, if any, in the round ducts it will be necessary, to prevent

collapse, to brace the pipes with flat irons or suitable equivalent section.

The dimensions and mutual distance of which is determined on the basis of the monogram

enclosed in the following pages.

Based on Swedish "Tryckkarisnormer, 1973", a calculation basis has been developed which is

described below:

List of symbols:

P= under- pressure (bar), (1 bar= 10400 mm WG)

T= wall thickness (mm)

D= diameter of pipe (mm)

L= length of pipe or distance between 2 bracing rings (mm)

I= moment of inertia of bracing ring including part of the casing (mm4)

d= diameter, measured to center of gravity line for bracing ring (mm)

K= constant taking account of the E- module variation with temperature F

K (1500 C)= 1.05 K ; ( 3500 C)= 1.16

t= wall thickness of bracing ring (mm)

b= width of bracing ring (mm)

α = t/T

β=b/t

37 of 70

Formulae and calculation method

To examine whether a given pipe possesses the required resistance to “collapse” calculate

(100 * T ) / D L / D P * k

Now enter in the Annex 1, with 100 T/D on X – axis and up to the intersection with the line

concerned for p x K (interpolate, if required). Based on this intersection point the maximum

permissible value of L/D is higher than that calculated, there is no risk of “collapse”

Failing that, the pipe must be provided with bracing rings.

Determine the distance b/w the rings so that L/D becomes smaller than that read off above.

Now the necessary moment of inertia of the bracing rib can be determined by means of

formulae:

I > = 6 * 10 ^ -8 * p * k * L * d ^ 3

Estimate d

Now choose plate thickness for the rib and calculate αααα = t / T

Now enter in Annex.2 with I/T^4 as abscissa and up to the intersection of the line for the

selected value of α.

β can now be read off on the axis of ordinates and the rib width is found as

b= β *T.

38 of 70

39 of 70

40 of 70

Calculation for Stiffeners for Ducts P under- pressure (bar), ( 1 bar= 10^4 mm WG) 665

T wall thickness(mm) 6 D diameter of pipe(mm) 2800 L length of pipe or distance between 2 bracing rings(mm) 6000 I moment of inertia of bracing ring including part of the

casing(mm4)

d diameter, measured to center of gravity line for bracing ring (mm)

K constant taking account of the E- module variation with temperature F K(1500 C)= 1.05 ; K ( 3500 C)= 1.16

1.16

t wall thickness of bracing ring(mm) 12 b width of bracing ring(mm) 80 α t/T β b/t

L/D 2.14 100*T/D 0.21 P*k 00.077

Check from Graph 1 L/D should be less than graph value 3 Tested for Acceptability of L OK

1>=6*10^-8*p*k*L*d^3 662376 I/T^4 512 Log natural 6.24 α = t/T 2 β=b/t 13.333 b= β *T. 73.5 SELECTED STIFFNER 80*12 (STIFFENER SPACING( FACTOR OF SAFETY=2.5) 2400s

Alternatively, a stiffener able to provide equivalent Moment of inertia can be used e.g. ISA 60*10

41 of 70

12.2 Rectangular Ducts

For Cement Plants rectangular ducts are not used often .The main areas are in Raw Mill and

Cooler. For this purpose the stiffeners have been standardized as follows:

For general applications, the guidelines are as follows:

All planes faces exceeding 1m² must be braced by means of flat irons or suitable equivalent

members so as to prevent collapses and vibrations.

Rectangular ducts do not possess the same strength in cross section or in length as circular

ducting. Great care must be taken to ensure that the duct is adequately stiffened to suit any

given application.

Refer Annexure 2 for typical calculation for the stiffener size

If the duct is refractory lined then the additional weight has to be considered when determining

plate thickness as stiffeners.

42 of 70

13.0 DUCT EXPANSION

Due to high temperatures of the gases, expansion of the duct will occur, Expansion joints are

used to absorb the expansion of the ducts.

The amount of expansion can be determined by the following formula:

∆L = L 0 X ∆t X 0.000012

43 of 70

Lo = original length of duct (mm)

∆t = temperature difference (in Celsius)

∆L = additional length of duct when hot (mm)

Typical calculations for expansion joints are given in Annexure1.

Basis of Selection:

1. For ∆l upto 150-200 mm, use one no. Non-metallic expansion joint or 2 nos. metallic

expansion joint.

2. Maximum no. of expansion joint in one location = 2.

3. If ∆l more than 200 mm provide additional intermediate Fixed and Guide supports.

4. Locate expansion joints to get maximum axial movement and minimum lateral movement.

Expansion joints are also used to avoid transmission of vibration and should be installed at the

inlet and outlet of large fans to avoid vibrations of fan housing being transmitted to the duct.

Features of Metallic Expansion Joints:

1. Longer life than non metallic joints

2. Rugged construction

3. Suitable for high temperature applications.

4. Require larger flange to flange distance for installation.

5. For high lateral movements generally two nos. expansion joints are installed adjacent to

each other.

6. Can accommodate vibration to certain extent.

7. Dust gets inside the metallic convolutes and offers resistance to the system. In such cases

thermal movements cannot be accommodated. This can result in increase of

reactionary/spring force exerted by the expansion joint.

8. Costlier as compared to non metallic joints.

44 of 70

Features of Non-Metallic Expansion Joints:

1. Fabric in nature. So highly flexible.

2. Requires less flange to flange distance.

3. Can accommodate higher lateral thermal movements in all planes like axial, lateral,

transverse, and radial. Also it can accommodate torsional movements, misalignments at

site, lateral shifts etc.

4. Suitable for vibrations in fan inlets/outlets and being fabric & flexible in nature, does not

transfer vibrations to adjacent equipments. Hence, no loss of efficiency or life.

5. Being flexible in nature it practically offers no reactionary / spring force.

45 of 70

CALCULATION OF EXPANSION JOINTS FOR DUCTS H Duct Length (mm) 8400 V Duct Length (mm) 4000 Included Angle between ducts degree 135

X Angle of inclination degree 45 T1 Initial temperature 10 T2 Max. Temperature duct required to withstand 350 Coff. Of Linear Expansion mm/deg. C 0.000012

AXIAL EXPANSION

Expansion in horizontal direction for H 34.27 Expansion in horizontal direction for V 11.54 Total 45.81 ADD FORMISALIGNMENT 10.00 TOTAL CALCUALTED AXIAL EXPANSION 55.81 THUS, AXIAL EXPANSION = 55.81 Type of joint Enter 1 for Metallic. 2 for Non-Metallic 1.00 No. of Expansion Joints 1

LATERAL EXPANSION

Expansion in vertical direction due to H 11.54 Expansion in vertical direction due to V 0.00 Total 11.54 ADD FORMISALIGNMENT 10.00 TOTAL CALCUALTED LATERAL EXPANSION 21.54 THUS, LATERAL EXPANSION = 25.00 Type of joint 1.00 Enter 1 for Metallic. 2 for Non-Metallic No. of Expansion Joints 1

RESULT NO. OF EXPANSION JOINTS TO BE PROVIDED ON DUCT

1

46 of 70

14.0 DUCT SUPPORTS AND HANGERS

Designing hangers or brackets for supporting a duct requires consideration of two important

factors.

1) The additional stress of the support forces when combined with the working stress of the

duct must not increase the stress in the duct above the allowable limit.

2) The support should not restrain the stressed duct so it becomes too rigid to flex under

normal changes in working pressure, temperature & loads.

Many types of stresses are involved in any supporting structure. The more common types are

the following:

1) The internal pressure of the gas in the duct, along with the weight of the duct cause

tangentially and longitudinal tensile stresses in the duct.

2) Any radial force acting on a section of the duct causes bending stresses in the ring of the

duct as well as axial tensile stresses both of which act tangentially to the circumference of

the duct.

3) The radial force also causes radial shear stresses in the duct, and longitudinal shear stresses,

both adjacent to the hanger.

After proper analysis of the forces involved, the various stresses must be combines to

determine the maximum normal stress (tensile or compressive) and maximum shear stress.

Other loads and forces which also have to be taken into consideration are those caused by the

elements i.e. snow load and wind load and the seismic conditions (earthquake factor) determine

by the location of the plant also the increase weight of the duct due to dust “build-up” and the

addition of insulation.

The civil engineer needs to know all the conditions before designing a support. Its is therefore

the responsibility of the Mechanical Designer to first propose a routing for the duct, taking into

the consideration for the best possible location for the supports, i.e. utilizing buildings, etc. and

avoiding high towers if possible. After the routing has been established and location of the

supports determined, it should be discussed with Civil Engineer before continuing.

47 of 70

14.1 Loads on Supports

Loads to be taken into consideration when calculating the forces on a support are indicated

below:

� Weight of duct (plate, stiffener, flanges, etc.)

� Weight of dust (assume a 100 mm thick accumulation of duct on the entire inside surface

of the duct). Where short ducts , sloping less than 30 degrees Celsius from the horizontal

cannot be avoided, a dust load is to be assumed corresponding to the duct being half full of

dust over its entire length.

� Weight of insulation

� Weight of refractory if applicable

� Wind and snow loads

� Factors due to seismic conditions

� Forces due to gas pressure

� Frictional forces due to expansion

The loads can be described in two forms:

Dead Load: This includes:

• Weight of duct including insulation, refectories etc.

• Weight of stiffeners: Where calculations are cumbersome, approx. 10% of duct shell

weight can be taken as stiffener weight.

• Eccentric Load distribution: In most drawings, the calculated load from above is less than

the load considered. The main reason is the non-linear distribution of the load between the

two or more supports

Live Load: This includes:

• Weight of material build up

• Weight of gas: Mass flow in kg/sec, velocity in m/sec

• Force due to temperature change: 160000*Duct area*(Change in length/total length)

48 of 70

14.2 Types and location of supports

When selecting the location and type of support, expansion of the duct due to high gas

temperature must be taken into consideration.

If the ducting is not flexible, use expansion joints to compensate form movement of the duct.

Each duct section separated by expansion joints shall have only one fixed support, the other

supports must permit movement of the duct.

14.3 Movable Supports

NOT RECOMMENDED

The above two types of saddle sliding supports are not recommended because they create high

frictional forces, paint is scraped of due to movement, thereby causing corrosion.

RECOMMENDED

The above types of hinged support are recommended as the best way in which to support a duct

and also permit the duct to move in the direction of the expansion.

49 of 70

RECOMMENDED

The above type of hanger support compensates for vertical expansion.

50 of 70

14.4 Fixed Support

Fixed support can be as simple as a welded saddle type support or as complex as a structure

dictate what type of support should be used. A few of the more common ways are indicated

below.

BRACKET TYPE A

BRACKET TYPE B

51 of 70

BRACKET TYPE C: FOR CLEANING ELBOW

BRACKET TYPE D: FOR SELF CLEANING ELBOW

52 of 70

15.0 Dampers

All gas ducting calls for dampers to be installed in one form or another i.e. shut off or

regulating. They will generally be butterfly or louver types with Motorized to manual

operation.

15.1 Regulating Damper

Opposed Blade louver opening

- - - - - - - - - - - - - - - Butterfly valve

------------------------- Parallel louver Damper.

It can be seen from above that opposed blade louver dampers are preferred for regulating

because of the almost linear flow characteristics.

15.2 Shut off Damper

The type of damper used depends very much on what the term “Shut off” means for any

particulars system .For instance if the damper are required to “Shut off” a duct run, there by

allowing gases to be passed through another system, then standard butterfly damper could be

used.

If the damper is required to “Shut off” a duct run and permit maintenance to be carried out

while the system is still running then the safety of the damper. The dampers therefore are

usually the guillotine or disc valve type and can be very big and heavy and can require a

support system just for the damper itself.

53 of 70

15.3 Location of Damper

The general location of the dampers will be found from the flow sheet provided by the process

technology department. Care should be taken in the determining the final locations in the duct

system with particular attention being given to providing access for operating and maintenance

personnel, and the avoidance of dust “build up” on the damper when it is in the closed position.

The installation of the damper is also important, i.e. that the blades face the right direction

when partially opened and do not cause turbulence in the gas flow.

AVOID THIS ARRANGEMENT

54 of 70

16.0 DO’ & DON’TS OF GOOD DUCT DESIGN

The design of gas ducting should be so that the gases can be transported through the system

without causing turbulence or excessive friction in the system, and also that there will be no

excessive dust “build up”.

This means that the principles for industrial ventilation should be used. Theses can be found in

many handbooks; however a few of the more important ones are shown below:

16.1 Curves in ducts

Curves in having a minimum inside radius equal less than as defined earlier should be provided

with the vanes to distribute the gas flow equally.

55 of 70

16.2 Transition Pieces

Transition pieces from round to rectangular or small to large diameter should be long enough

to avoid turbulence.

Recommended angle is 50 to 70 or 25mm change in dia to every 120mm length.

56 of 70

16.3 Branches

(However if no other choice add baffle plate)

To be avoided

However, if design dictates this

arrangement then install a collecting

hopper in bottom, of duct i.e. rotary

valve, double flap valve etc.

57 of 70

16.4 Fan inlet

Fan inlet curves to enter at 60 Deg.

58 of 70

16.5 Fan outlet

Fan dust laden gases avoid an arrangement where there is the possibility of accumulated dust

falling back on rotor of the fan.

The gas flow for case 1 is not so good as for 2 but there is less chance of dust falling on the

rotor.

59 of 70

16.6 Exhaust Stacks

When part of duct has to be designed for removal, allow for wedge type lift out section i.e.

sometimes in order removing a fan rotor, part of the gas ducting connected to the fan has to be

removed. Wedge type section can easily be slide out.

NOT RECOMENDED RECOMMENDED DESIGN FOR SMALL VENT UPTO 300 DIA.

60 of 70

16.7 Access Doors

All doors should have bolted access doors so that every section of the duct can be inspected

and cleaned.

A door must be installed close to each damper for inspection and on all points where dust

accumulation is expected.

Note: It is to be understood, that when detailing the duct runs, some of the straight lengths

should have a cutting allowance (say 150mm) and some of the flanges should be supplied loose

for filed welding. This obviously is to permit the erector a little latitude when installing the

system.

61 of 70

17.0 PROVISION FOR INSTRUMENTATION

In general following criteria can be used for provision of instrumentation:

All main ducts such as

• Mill to cyclones

• Mill to De-dusting equipment

• Preheater fans to mill

Need to have provision for pressure, temperature and Volume indications.

Ducts for

• De-dusting equipment to fan and stack need to have temperature and volume indications.

18.0 FLOW MEASUREMENT

18.1 Piezometric ring: Used for cooler fans inlet

62 of 70

18.2 Orifice Plate: Used for small dia. Ducts and short duct length : 1.0 to 1.5m dia.

18.3 Venturimeter: Used for large duct dia. & long ducting

63 of 70

18.4 Flow Measurement Calculation

Q = C * A * (2 * g * 144 * Pressure difference) / d]1/2

Q = Flow of Gases

C = Flow Coff.

Press. Difference in mm WG

g = 9.81 m/sec2

A = Cross section area of the ducting

d = Density of air / gases

64 of 70

Ducting : Analysis Statement : Typical Prj. Raw Mill Complex

Sr.No. Details Mill Outlet - Cyclone Cyclone - Mill Fan 1 Project I Drg. No. 9502-3612-005 9502-3612-005 2 Process Data • Volume - Normal 230,400 Nm3/hr 241,920 Nm3lhr. • Volume - Actual 408,700Am3/hr. 446,180Nm3/hr. • Temperature 95 Deg C 95 Deg C • Gauge pressure (-) 665 mm Wg (-) 865 mm Wg 3 Design Aspect

3.1 Duct diameter 2,800 mm 2,800 mm (Main duct)

1 ,500 mm at cyclone inlet

3.2 Gas velocity 18.44 m/see 20m/see (main duct)

12.18 m/sec (Cyclone inlet duct)

3.3 Pressure drop Considered 75 mm Wg Considered 75mmWg 4 Construction considerations

4.1 Plate thickness 6mm 6mm 4.2 Stiffeners • Size 2-1/2” x 2-1/2” X 3/8” 3” X 3” X 1/4” • Spacing 3,000 mm 2,200 mm (main duct)

1,400 mm at rectangular portion

4.3 Duct support One fixed at cyclone inlet.

One fixed at cyclone inlet.

One pivoted Four fixed from top at cyclone

One guide One pivoted 5 Special considerations

5.1 Expansion joints Mill outlet Fan outlet (pair)

Branch to cyclone (2 nos.)

Above cyclone inlet (4 Nos.)

5.2 Provision for instruments To be decided at site Access from all sides 5.3 Interconnections None None

65 of 70

Appendix 1

For general application, the selection of bracing can be done with the following graphs also:

Bracing for Pipelines

66 of 70


67 of 70


68 of 70


69 of 70


70 of 70


Duct & Vent Engineering

Documents

Transcript of Duct & Vent Engineering