DESIGN OF INDUSTRIAL CHIMNEY

The leeward side of the chimney is the compressive zone in which the compressive stress due to self load and wind has uniform value throughout the shell thickness.
The windward side is the tension zone. Due to self-load and wind, the whole concrete shell on the windward side is in tension which is completely borne by steel having stress t1.

SHAPE FACTORS FOR VARIOUS SIZES OF CHIMNEY:
STRESSES IN CHIMNEY SHAFT DUE TO SELF-WEIGHT AND WIND:

Purpose of analysis of stresses due to self weight and wind, it is assumed that the steel reinforcement is replaced by a thin steel cylinder and it is located at the centre of the thickness.
It is assumed that variation of stresses in the thickness of the shell is small and total compressive or tensile stress may be taken acting on the centre line of the shell thickness.

STRESSES IN HORIZONTAL REINFORCEMENT DUE TO WIND SHEAR:

Horizontal reinforcement is provided in the form of hoops which resists the horizontal shear due to wind.
Stress in steel, (t) is given by,
T= (P*S)/2AɸD1
P – horizontal shear force
S- pitch (mm)
Aɸ -horizontal reinforcement
D1- distance between reinforcement on both sides

STRESSES DUE TO TEMPERATURE DIFFERENCE:

Because of the high temperature difference in flue gases there is large temperature gradient.
The fall of temperature through the concrete shall cause severe stresses.
To reduce the temperature stresses, 10cm thick fire brick lining is provided, spaced slightly away from the concrete shell so that air gap of 8-10 cm wide is available between the two. The lining is supported at every 5-7m.

WIND PRESSURE FOR VARYING ELEVATION ABOVE GROUND LEVEL
Wind pressure:

Wind obviously causes obstruction to anything in its way. The intensity of wind pressure depends on wind velocity which in turn depends on the elevation above the ground. IS875 specifies wind pressure at various heights as given below:

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Vertical steel is provided to resist bending moment due to wind.

Horizontal steel is provided to take care of the horizontal shear and reduce the effect of temperature gradient.

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Inside temperature is higher compared to outer side. this causes differential expansion and hence stresses are induced in RC wall in both horizontal and vertical directions.
RELIABILITY OF CONCRETE OVER BRICK:
Brick chimneys are not suitable because they are restricted to limited height and and require huge foundations
Concrete chimneys are suitable for cases where the temperature difference between the outer shell and inner lining is 400 degrees.
In case of the temperature rising above 400 degrees, an additional layer of brick lining and an air gap is provided to take care of the increased temperature gradient.

SHAPE FACTOR FOR WIND LOAD CALCULATION
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REFERENCES

1. IS: 4998 – Criteria for Design of Reinforced Concrete Chimneys

2. IS: 6533 - Code of Practice for Design and Construction of Steel Chimneys

3. Tall Chimneys- Design & Construction by S.N. Manohar, Tata McGraw-Hill Publishing Company Limited.

4. Hand Book of Concrete Engineering, edited by Mark Fintel, CBS Publishers & Distributors

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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.
Eg. DHOKA System (from Austria)

METHOD OF CONSTRUCTION
FOUNDATION
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.

TEMPERATURE STRESS IN HORIZONTAL REINFORCEMENT

The inner surface is prevented from expansion and is therefore subjected to compressive stress due to temperature gradient, while the outer surface expands more than its natural expansion and is subjected to tensile stress. Since the horizontal reinforcement are located in the outer face, it is subjected to tensile stress.

INTERNAL LININGS

Acid Resistant
Fire Bricks

Coal-tar applied to Inner surface of
Concrete shell to
close shrinkage
cracks and as
protection to
concrete

Autocad drawings of chimney
Elevation of chimney
Elevation continued..
Continued..
Plan of chimney
Plan continued..
Plan continued..
DESIGN FACTORS
RCC chimneys are to be designed to sustain the stresses due to :
1. self-weight
2. wind load
3. temperature variation.
Wind load depends upon various factors like nature of terrain, wind velocity in the zone, height of building. it depends upon shape of structure. shape factor is one has to multiply wind pressure in the area to get design wind pressure on the structure.

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The fire brick lining is supported by reinforced concrete brackets provided at regular intervals of 4-6m.
Thickness of shell wall is kept to a min of 200mm at top and is increased to 300-400mm at bottom depending upon height of chimney.
an air gap of 80-150mm is provided between concrete shell and rick lining to reduce temperature gradient.

TYPES OF RCC CHIMNEYS
CHIMNEY OVERVIEW
In many industries chimneys are required to leave hot waste gases at great heights.
They are used mostly in thermal power plant eg, coal based thermal power plant, because of high amount of waste gases it lets out.
In order to avoid pollution on ground surface chimneys are designed very height.
RC chimneys are becoming more and more popular because of economy in construction and maintenance. maintenance cost of steel chimney is high and also brick chimneys become too bulky and costly when height of chimney is more than 30m.
AIM
To design industrial chimney which includes RCC chimney.
2
INDUSTRIAL CHIMNEYS

DESIGN OF INDUSTRIAL CHIMNEY
A Design project submitted in partial fulfillment of the
requirement for the award of the degree of
BACHELOR of engineering
IN
CIVIL ENGINEERING

SUBMITTED BY

BASITH AHAMED ( 113012103018)
MANISH KUMAR MANDAL (113012103056)
MOHAMED SALIM (113012103062)

UNDER THE GUIDANCE OF
Mr. MANOJ KUMAR ,ME
(DEPARTMENT OF CIVIL ENGINEERING )

VELTECH HIGHTECH DR.RANGARAJAN DR.SAKUNTHALA ENGINEERING COLLEGE,AVADI
OBJECTIVES
To analyze and design industrial RCC chimney.
To construct a stable superstructure considering in mind various load considerations.
To decide the size and structural parameters governing the chimney.

COMBINED EFFECT OF SELF LOAD, WIND, TEMPERATURE:

We have to consider 2 sections of chimney:
Leeward side
Windward side

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INDUSTRIAL CHIMNEYS

1. DEFINITION OF CHIMNEY
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 and suspended solids (ash) are dispersed into the atmosphere over a defined spread such that their concentration, on reaching 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.

RCC CHIMNEY
RCC chimneys of height 50-100m are commonly used.
The outer diameter of chimney maybe kept constant throughout or may be linearly varied. Also thickness of concrete shell maybe varied in steps or linearly.
The concrete chimneys are provided with lining of 100-150mm thick fire bricks ,so as to reduce temperature stresses in concrete shells.
PARTS OF RCC CHIMNEY
RC SHELL
BRICK LINING
AIR GAP
RC BRACKET
COALTAR LINING
SCOPE
To design chimney to sustain the stresses due to self-weight, wind load, temperature variations and also seismic effect.
To extend the durability and longevity of the structure.

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INDUSTRIAL CHIMNEY

3. CLASSIFICATION OF CHIMNEYS
a) Based on number of flues
Single flue (each boiler will have an independent chimney)
Multi flue (Single chimney serves more than one boiler; more flues are housed inside a common concrete windshield)
b) Based on material of construction
Concrete (Chimney); Reinforced/Pre-stressed
Steel (stack)
c) Based on structural support
Guyed stacks (used in steel stacks for deflection control)
Self supporting (cantilever structures)
d) Based on lining
With Lining : Lined chimneys/stacks
Without lining :Unlined chimneys/stacks

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INDUSTRIAL CHIMNEYS

5. 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

6. 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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DESIGN OF INDUSTRIAL CHIMNEY

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