Stacks and Draught

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Objectives :

To understand the function of stacks

To understand the procedure to calculate actualdraught levels in combustion systems

To appreciate pressure profiles in natural andforced draught systems

Stacks and draught

Stack 2

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Stack height depends on draught requirement ofconnected fired equipment. Stack draught should be sufficient to overcome the convection bank lossesand to create 2-3 mmWC under pressure in the topof the fire box

Function of Stacks

Stack 3

Controlled dispersion of flue gas in atmosphere

Stack height depends on emission standards

The flue gas path should not hit working platforms

Creation of draught

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parcel of air

Colder

Height Ambienttemperature

Adiabaticexpansion100 m -1°C

Adiabatic Expansion

Stack 4

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Design exit velocity 16 m/s

Plume riseCost effective

Downwash Plume rise

Stack exit speedHorizontal wind speed

Stack exit speed

Horizontal wind speed

Stack Aerodynamics

Stack 5

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Heig

htHe

ight

Heig

ht

Distance Distance

DistanceHe

ight

Distance

Unstable atmosphere (“looping”)

Neutral atmosphere (“coning”)

Stable atmosphere (“fanning”)

Inversion above stack (“fumigation”)

Characteristic Plumes from Stacks

Stack 6

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The more stable the atmosphere the

slower the dispersion

The more unstablethe atmosphere the

faster the dispersion

Stability and Dispersion

Stack 7

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Degree of turbulence depends on:

Wind speed

Surface roughness

Atmospheric stability

Turbulence in Atmosphere

Stack 8

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Actual draught = Theoretical draught - Losses

Theoretical draught =

ρa = actual density of ambient air (kg/m3)

ρ f = actual density of flue gas (kg/m3)

g = gravity constant = 10 m/s2

= effective stack/furnace height (m)

Theoretical draught = (mmwc)

Ta = ambient air temperature (k)

Tf = flue gas temperature (k)

v

3501 1

* *T T

Ha f

−

(N/m2)( )ρ ρa f g H− * *

H

Stack Draught Calculation (1)

Stack 9

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Loss = Friction loss + Exit loss

Friction loss = (N/m2)

f

Vf

D= friction factor

= stack diameter (m)

= flue gas velocity (m/s)

Friction loss = (mmwc)

Exit loss =

Exit loss = (mmwc)

(N/m2)

fHD

Vf f* * * *12

2ρ

0 325 2. * **V H

T Df

f

12

2* *ρ f fV

185 2. *VT

f

f

Stack Draught Calculation (2)

Stack 10

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Overview of simplified equations :

Theoretical draught =

Friction loss =

Exit loss =

(mmwc)3501 1

* *T T

Ha f

−

(mmwc)0 325 2. * *

*V H

T Df

f

(mmwc)185 2. *VT

f

f

Stack Draught Calculation (3)

Stack 11

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∆p Friction in conv.bank 5 mmwc

0,8m

35 m

4 m

10 m

V= 12m/s

200 °C

500 °C

800 °C

Required draughtfor burner 6 mmwc

Tamb = 20°C

Draught Calculation Exercise

Stack 12

Νο Friction in Radiant cell

Νο Friction damper

Calculate:1) Draught top convection

bank2) Draught top radiant cell3) Draught bottom radiant

cell4) Is required burner draught

sufficient

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Typical furnace static pressure (draught) profiles0 0

STACK

CVB

RC

BURNER

ID FAN

FD FAN

0 0

+6 +6 +6 +6

-6 -6-6-16

-4 -4 -4+6

-13 -13 -13-3

+200 +200+210

N.D. BURNER F.D. BURNER F.D. BURNER F.D. BURNERhigh deltaP CVB high deltaP CVB

ID fan

Stack 13

-12

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Manual Control Natural Draught FurnacesSTART

GOOD OPERATION

CHECK DRAFT

CLOSE STACK DAMPER

HIGH LOW

ON TARGET

ON TARGET

CHECK O2

CHECK O2CHECK O2

HIGH (3)

HIGH (1) LOW (2)

HIGH (3)LOW (3) LOW (3)

OPEN STACK DAMPEROPEN BURNER REGISTERS CLOSE BURNER REGISTERS

OPEN BURNER REGISTERSCLOSE BURNER REGISTERS

RETURN TO START RETURN TO START

RETURN TO START RETURN TO START

DRAFT

O2

TARGETS

Notes:• High Draft – Means Fire Box Pressure more Negative than Target• Low Draft – Means Fire Box Pressure more Positive than Target• High or Low O2 – Means O2 is above or below Target

Stack 14

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Coffee Break

Stack 15

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