Stacks and Draught
CONFIDENTIAL
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
CONFIDENTIAL
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
CONFIDENTIAL
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
CONFIDENTIAL
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
CONFIDENTIAL
The more stable the atmosphere the
slower the dispersion
The more unstablethe atmosphere the
faster the dispersion
Stability and Dispersion
Stack 7
CONFIDENTIAL
Degree of turbulence depends on:
Wind speed
Surface roughness
Atmospheric stability
Turbulence in Atmosphere
Stack 8
CONFIDENTIAL
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
CONFIDENTIAL
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
CONFIDENTIAL
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
CONFIDENTIAL
∆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
CONFIDENTIAL
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
CONFIDENTIAL
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
CONFIDENTIAL
Coffee Break
Stack 15
CONFIDENTIAL
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