Objective
To arrive at most appropriate and economical location and configuration of Preheater
stack.
Chimney
A chimney is a structure for venting hot flue gases or smoke from a boiler, stove,
furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or
as near as possible to vertical, to ensure that the gases flow smoothly, drawing air
into the combustion in what is known as the stack, or chimney, effect. The space
inside a chimney is called a flue. Chimneys are tall to increase their draw of air for
combustion and to disperse pollutants in the flue gases over a greater area so as
to reduce the pollutant concentrations in compliance with regulatory or other limits.
Industrial chimneys are commonly referred to as flue gas stacks and are typically
external structures. They are generally located adjacent to source of generation of
gases / steam/ dust etc., and are carried to it with ductwork.
With requirements of larger heights, durability and structural stability against lateral
loads due to wind and earthquake, industrial chimneys or stacks are normally built
in reinforced concrete or in steel. Refractory bricks are often used as a lining,
particularly if the type of fuel being burned generates flue gases containing acids.
The combustion flue gases inside the chimneys or stacks are much hotter than
the ambient outside air and therefore less dense than the ambient air. That causes
the bottom of the vertical column of hot flue gas to have a lower pressure than the
pressure at the bottom of a corresponding column of outside air. That higher pressure
outside the chimney is the driving force that moves the required combustion air
into the combustion zone and also moves the flue gas up and out of the chimney.
That movement or flow of combustion air and flue gas is called “natural draught/
draft”, “natural ventilation”, “chimney effect”, or “stack effect”. The taller the stack,
the more draught or draft is created.
Cost Comparisons of Chimney’s under following categories: An analysis of cost
done on following categories design. The Height of Chimney considered in all the
cases are different i.e. 110m,186m respectively.
1 RCC chimney: - Conical structure (ID= bottom – 8 m, top -5.3 m), Height 110 m,
without Brick lining inside flue because after installation of WHRS the flue gas
temperature will go down up to 110 deg. C, so brick lining not required.
2 Steel Chimney: - Self supported guided with RCC frame structure steel Chimney
dia. 5.0 m, height 110m & guided up to 106m.
3 Steel Chimney: - Guided cylindrical steel chimney from preheater tower with str.
Steel cantilever bracket (dia. 05 m, guided up to +170m)
4 Steel Chimney: - Guided cylindrical steel chimney from preheater tower with
RCC str. cantilever beam (dia. 05 m, guided up to +170 m)