Chimneys
• Chimneys are tall and slender
structures which are used to
discharge waste/flue gases at
higher elevation with sufficient
exit velocity such that the gases
DEFINITION and suspended solids (ash) are
dispersed into the atmosphere
OF over a defined spread such that
their concentration, on reaching
CHIMNEY the ground is with in acceptable
limits specified by Pollution Control
Regulatory Authorities.
• In a coal based power plant,
flue gases from each boiler are
fed to a chimney, for dispersion
into atmosphere.
Main Codes:
IS: 4998 – Criteria for Design of
Reinforced Concrete Chimneys
IS: 6533 - Code of Practice for Design
and Construction of Steel
DESIGN Supplementary Codes:
• IS: 456 – Code of Practice for Plain
CODES and Reinforced Concrete
• IS: 800 – Code of Practice for use of
Structural Steel in General
• IS:875 – Code of Practice for
Design Loads for Buildings and
Structures (Part-3 : Wind Loads)
• IS:1893 – Criteria for Earthquake
Resistant Design of Structures
Based on number of flues
CLASSIFICATION
• Single flue (each boiler will
OF CHIMNEYS have an independent
chimney)
• Multi flue (Single chimney
serves more than one boiler;
more flues are housed inside a
common concrete windshield)
Based on material of
construction
• Concrete (Chimney); Reinforced/Pre-
stressed
• Steel (stack)
Based on structural support
Classification • Guyed stacks (used in steel stacks
for deflection control)
OF CHIMNEYS • Self supporting (cantilever
structures)
Based on lining
• With Lining : Lined
chimneys/stacks
• Without lining :Unlined
chimneys/stacks
Multi Flue
Chimney
Steel
Chimney
8
Guyed
Steel
CHIMNEYS
Lining
• The top clear inside diameter is governed
by minimum escape velocity of flue
gases at top op chimney.
• Height is governed by gas plume
dispersion at top of chimney. The height
CHIMNEY should be such as to ensure required draft
PROPORTINING and exit velocity of flue gases such that
the Ground Level Concentration (GLC) of
pollutants, after atmospheric dispersion,
is within the limits prescribed by the local
pollution regulatory standards
• Structural considerations : A taper of 1 :
50 to 1:100 is usually selected to suit the
concrete shell diameter at foundation
top, required from design for
wind/seismic loading considerations
INDUSTRIAL CHIMNEYS
DESIGN LOADS
• Dead Loads (weight of chimney shell & lining)
• Wind Loads (as per IS:875; Part-3)
• Seismic Loads (as per IS;1893)
• Temperature Loads (depends on flue gas temperature)
Note: wind and seismic loads are not considered to act simultaneously as both are
environmental loads
LOAD COMBINATIONS
• Dead load + Wind load
• Dead load + Seismic load
• Dead load + Temperature load
• Dead load + Wind load + Temperature load
• Dead load + Seismic load + Temperature load
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INDUSTRIAL CHIMNEYS
Strakes
• Strakes are discrete vertical plates attached on top portion of
Chimney to break the formation of Vortex shedding (very
common practice in Indian Chimneys)
• Provided in helical form for top 1/3 height.
• Projection from chimney surface = 0.12D; Height 1 t0 1.5m
where, D = top diameter of chimney
Shrouds
• Metal skin cover, with perforations, attached to top of Chimney.
• Gap between chimney and shroud = 0.12D
• Area of openings/Gross area of shroud = about 30%, is most effective
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INDUSTRIAL CHIMNEYS
Chimney fixed
with strakes at
top to avoid
Vortex Shedding
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INDUSTRIAL CHIMNEYS
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INDUSTRIAL CHIMNEYS
Temperature Loads
• Inside surface is at Flue gas temperature, T
• Outside surface is at atmospheric temperature, T0
• T can vary from 100 to 1750C
T0can vary from 20 to 450C
• Differential temperature,
Tx= T – T0
Concrete to be designed for this temperature differential.
If the temperature difference is very high, then internal lining is
required to be provided for flattening the temperature gradient.
Temperature stress, ft = (α EcTx)/2
α = coefficient of expansion of concerete
Ec= Concrete modulus
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INDUSTRIAL CHIMNEYS
LINING OF CHIMNEYS
• When the gradient Tx> 400C, lining will be required
to reduce thermal stresses in concrete and steel.
• Materials used for lining are:
Acid resistant bricks (very common)
Acid resistant Fire bricks (very common)
Mild steel liners
• Thickness of lining bricks will be decided by the
limiting thermal gradient and design temperature.
• Bricks are built over corbels (brackets from
chimney shell), which are generally at 10m height
intervals.
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INDUSTRIAL CHIMNEYS
INTERNAL
LINING
UNDER
CONSTRUCTION
Acid Resistant
Fire Bricks
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INDUSTRIAL CHIMNEYS
INTERNAL
LINING
UNDER
CONSTRUCTION
Acid Resistant
Fire Bricks
Coal-tar applied to
Inner surface of
Concrete shell to
close shrinkage
cracks and as
protection to
concrete
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INDUSTRIAL CHIMNEYS
FOUNDATIONS
• Generally circular raft foundations are
provided. Pile foundations are also
common.
• Diameter and thickness of raft foundation is
governed by combined vertical loads and wind /
seismic loads.
• Stability factors govern the design,
• F.O.S (overturning) > 1.5
• F.O.S (sliding) > 1.5
• Foundations are taken deeper to get additional soil
weight on raft to assist stability.
• Gross bearing pressure under footing should be
compressive
• i.e. “loss of contact” is to be avoided or limited to a
maximum of 1/6 of raft diameter. 20
INDUSTRIAL CHIMNEYS
METHOD OF CONSTRUCTION
Following two methods are very common:
• Jump form
• Slip form
Jump Form: Construction is in stages of about 1.5 to
3.0m lifts
Slip form: Continuous construction
Formwork keeps moving upward at low speed as the
concreting continues.
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