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 in to the atmosphere over a defined spread such that their concentration , on reaching the ground is within acceptable limits specified by pollution control regulatory authorities. Fast and economical construction of chimney is the need of the industries.    

Past few decades the usage of reinforced concrete chimneys in place of steel chimneys and brick masonry has become very popular due to their low economy and durability. Composite material like reinforced concrete is exceedingly suitable for chimney stacks. Brick chimneys are very heavy requiring expensive foundations. In contrast to the steel chimneys, the maintenance costs are minimum in the case of concrete stacks. The development of slip form method of constructing cylindrical stacks has resulted in rapid construction in the case of concrete chimneys. Concrete stacks with lesser maintenance costs are architecturally superior to masonry and steel chimneys.

LINING AND INSULATION
Concrete chimneys are invariably lined, primarily to concrete and reinforcement against thermal and chemical attack from exhaust gases. Secondly, the thermal-insulating property of a liner reduces heat loss from flue gases and thereby they remain buoyant and aid plume rise. Finally, a high temperature at the chimney outlet diminishes the likelihood of acid condensation.

Mainly four characteristics of a flue gas influence a liner's choice, viz. chemical composition, temperature, velocity and pressure. Gases flowing at a high velocity can cause liner abrasion, whereas a low velocity can lead to high pollution levels and possible cold-air ingress causing acid condensation at the chimney top. A positive gas pressure should be avoided since it aids penetration of flue gases into masonry liners, exposing shell concrete to possible chemical corrosion.

The use of present-day high-efficiency boilers has led to lower flue-gas temperatures, which has sharply increased the chances of acid formation and chemical corrosion. However, a lower temperature must be tolerated since a small gain in thermal efficiency can save considerable annual revenue, thereby readily justifying the use of a more expensive chemical-resistant liner.

A. Chemical Corrosion:-
Acid attack, which is of primary concern in chimneys, depends on the flue-gas constituents and liner surface temperature. On burning sulphurcontaining fuels(such as coal, oil, etc.), Sulphur is released which oxidizes to SO2 (S + O2 →SO2) and a small portion of SO3.this latter reacts with water vapour to form concentrated H2SO4 at a temperature called the sulphuric acid dew point which is 115° - 150°C .at a lower temperatures, corrosive acids, such as HCl, begin to condense and at 45° - 55°C the water dew-point is reached .

Acid attack, which is of primary concern in chimneys, depend Thus, acid and water dew-point temperatures play a significant role in the degree of corrosive attack and a designer should aim at maintaining the liner temperature away from them.

B. Mortars :-
A wide range of mortars are available and selection of the most suitable mortars depends on many factors including temperature conditions, type of chemicals present in flue gases, etc. The success of brickwork depends as much on its workmanship and mortar quality as it depends on the brick itself.

Cement:
Ordinary Portland cement can be used up to a skin temperature below 150°C and where chemical attack is not envisaged. High alumina cement is used to impart moderate chemical resistance but is not suited if continuous exposure to free acids is anticipated.

Sodium-silicate cements have superior resistance to both free acids and intermittent operations. Such cements have a service temperature limit of about 400°C. This mortar should not be wetted prior to putting a chimney into service, otherwise it will soften. This cement is destroyed by sulphation if the liner is exposed to a mist of H2SO4. Potassium-silicate cement has superior chemical resistance as compared to sodium-silicate cement, but it is generally more expensive. It does not weaken due to sulphation and can be used for service temperatures up to 1000°C.

Aggregates:

Ordinary sand can be used for low temperatures and where chemical attack is not envisaged. For higher temperatures, crushed firebricks are often used. A chimney has certain fixtures, fittings and features which are termed
"accessories". These are described below.

A. Cap:-

At the top of a chimney a cap is provided to prevent gas-entrained matter from falling into the space between the liner and the shell and also to protect the chimney top from chemical attack. This cap is often made of sectional grey cast iron of minimum 12 mm thickness laid over 5 mm thick asbestos However, cast iron may not be suitable for certain ceramic processes and for such cases, materials such as alloys have to be used. A cap must absorb the relative movement between a liner and a shell
caused by wind, temperature or other effects. A cap is often composed of interconnected segments, but up-stands for connecting these segments together should be avoided since they become the focus of corrosion. As a further protective measure, a cap should be painted with an anticorrosive paint.

B. Lightning Protection

Lightning protection is provided by creating a path of minimum electrical impedance from the chimney top to an earthling strip in the ground. At about 1500 mm from the chimney top, a circumferential conductor (coronal band) is provided which is lead-covered, stranded and of tinned copper. This is connected to air-terminal rods (which protect about 750 mm above the top of a chimney) and to tinned copper down conductors. These conductors (which are embedded in concrete) should not have an intermediate splice, they should be free from sharp bends, and should be connected to an earthling strip. During construction, temporary lightning protection must be provided and this could be in the form of a single conductor connected to a temporary airterminal rod at the top and an earthling system in the ground.

Vertical reinforcing bars near the chimney base should be electrically connected to a circumferential bar, which in turn must be electrically connected to a down conductor. Similarly, all steel pans and footing reinforcement should he connected to a down conductor. In the top reaches of a chimney, lightning protection conductors should be adequately protected from chemical corrosion by applying a lead coating or by other suitable means.

C. Aviation Warning Lights

Aviation warning lights are provided even during construction once the chimney reaches a height of about 45m above grade. Both temporary as well as permanent provisions should meet the stipulations of the Civil Aviation Department. These lights should be RED in color and be either of filament or neon type, the latter being generally more reliable. They should be of sufficient intensity (about 100 lux) so that they are conspicuous considering the back-ground illumination level against which they will he viewed. Such lights can have a flasher unit and it is preferable to incorporate an automatic alarm system which will be
actuated when these lights are on spare. Cables and conduits leading to these lights should safely withstand continuous exposure to the temperature experienced by concrete in which they are embedded.

Day markings are provided by painting the chimney in contrasting color bands for at least the top one-third of its height. Such bands should neither be too narrow nor too broad and could range from 0.75 to 3 m with the outer bands in the darker color. The paint used for such bands should be acid-resistant and should safely withstand the temperature expected at the chimney top.

D. Ladder

A ladder is provided for access up to the top of a chimney. It should be of sturdy construction, preferably with non-slip type rungs, and have a safety cage. A tower ladder safety device in the form of a galvanized rail carrier is also recommended and the ladder should be connected to the concrete shell by non-corrosive inserts and bolts.

E. Clean-out and Access Door

A suitable access door should be provided at the bottom of a chimney. When a metal liner is used, the opening meant for an access door is initially made large enough to serve as an access for liner cans which have to be transported to the chimney centre for erection. Two pairs of clean-out doors are provided in the shell and lining for access into a chimney and to clean the soot hopper. These doors should be suitably treated in order to be heat and acid resistant and should be of gas-tight construction. Such doors should be provided with a latch and should normally be kept locked.