TYPES OF BOILER CHIMNEY

SELF-SUPPORTED CHIMNEY:

This type of chimney does not require any external supports and its design itself makes it possible to stand erect once it is bolted to the foundation.

The design and construction of the chimney conforms to IS:6533 – Part 1 & 2 – 1989.

Chimney is made of MS sheets and is conical at the bottom and cylindrical at the top and is designed for natural draught of flue gases from boiler to atmosphere.

The height of the chimney is important from the point of view of emissions control and environmental protection. Depending on the quantity of flue gases that it has to deliver to the atmosphere and sulphur content in the flue gas, minimum height of the chimney is calculated. It is also checked for local Pollution Control Board norms & regulations. 

The thickness of the chimney in various sections is arrived at first by the deflection calculations and further corrected for Corrosion Allowance. This is based on sulphur content in the flue gas and expected life for which it is supposed to last. For 10 years life 1.5 mm and for 20 years life 3.0 mm Corrosion Allowance is considered.

While calculating chimney load, the following is considered :

Dead load consisting of weight of chimney shell, permanent fixtures like ladder, platforms, helical strakes, flanges, etc.

Imposed load of 300 kg/m2 on sampling platform

Wind loads as per provisions in IS 875 (Part 3) 1987

Earthquake loads in accordance with IS:1893 – 1975

The chimney incorporates the following :

1. Base plate with gusset plate and base stool.
2. Inspection door with suitable handle near the bottom portion.
3. Access ladder : It starts from 3 meters from bottom of chimney and runs up-to the top. The ladder is supported from the chimney by structural members and ladder rung is located at a distance of 250 m from chimney. The entire length of the ladder is provided with protection cage for safety of person climbing it.
4. Platforms : This is provided for the purpose of flue gas sampling and inspection purpose. Number of platforms are decided by the height of the chimney. Platform length is not less than 800 mm and it has 1000 mm high railings. Toe guard support is provided at the bottom of the railing. Arrangement I provided on the platform for draining rain water.
5. Sampling port : This is provided as per PCB regulations.
6. Conical cowl with cowl stays : This provided for preventing rain water and other foreign objects from entering the chimney from top. The diameter of the cowl is 1.5 times the diameter of the chimney top section.
7. Aviation lights : These are twin type lights meant for aviation warning. Its fixtures are made of aluminium alloy casing with separate galleries to house two 100 W GLS on BC type bulb holder. The lamp fixtures have red domes. Aviation light point is connected to lighting circuit available in the boiler house with PVC insulated copper cable. The cable is of 3 core x 2.5 mm2 size. 
8. Lightening arrestor : This is made of GI with 5 prongs and is connected to two earth stations. The earthing conductor is of GI and is mounted on epoxy based insulators. Two earthing conductors run parallel to lightening arrestor and each is connected to independent earth stations. The earth stations in turn are interconnected with one another. Earth station is of GI plate conductor and is constructed as per IS:3043. Earth station is complete with excavation, filled with charcoal/salt, watering pipe, earthing conductor test link, cover and completed with masonry work.
9. Flue gas inlet is provided on the chimney at suitable elevation and as per size of the ducting connecting it to the boiler. It is provided with counter flanges to facilitate connection to ducting.
10. Helical strakes are provided to reduce wind excited oscillations for top 2/3rd height of the chimney.
11. Chimney bottom up-to 3 m height is insulated by 50 mm thick mineral wool insulation and MS cladding. This is provided for avoiding direct contact of operating personnel with hot chimney surface.
12. The chimney is given initial surface treatment including de-greasing, de-rusting, etc., and then a coat of primer is applied. The chimney is finally painted with two coats of heat resistant paint.
13. Foundation bolts of adequate length and diameter are provided along with anchoring channel on bolt base.

 Why an over-sized chimney /very low load boiler operation  is not recommended?

1. When the flue gas quantity that is being sucked to atmosphere is much less compared to maximum capacity of chimney, the suction effect is more. This in turn might affect the boiler by creating disturbance in combustion process and cause change the flame dimensions.
2. At much lower loads the effect of condensation of sulphur is more and as such the rate of corrosion enhances reducing material life.

STRUCTURALLY SUPPORTED CHIMNEY:

This type of chimney is supported by some adjacent / nearby structure for preventing its deflection. The supports are usually MS channels and these are connected to adjacent structure (usually a building wall) or a in certain cases a dedicated structure may be made for this chimney with which chimney is externally supported. By using this concept, the chimney itself does not carry entire deflection load and its weight and cost gets reduced. Since the weight of the chimney itself gets reduced, the foundation required for this chimney is much smaller compared to a self-supported chimney.

These type of chimneys typically find application in typical cases where there are space limitations like hotels and hospitals.

GUY ROPE SUPPORTED CHIMNEY:

This chimney is externally connected at suitable elevation by guy ropes in different directions (usually 3 ropes). The other end of the guy ropes is connected to some immovable fixed structures. These guy ropes prevent deflection of the chimney and keep it straight. They are made of steel wires and are sufficiently strong to support the chimney.

This chimney is quite light in weight but necessarily requires enough space around the chimney for tying the ropes.

As compared to a self-supported chimney, foundation required for this chimney is quite small.