In this article we will discuss about how to design steel chimneys.
Steel chimneys also called steel stacks are common requisites in factories. These are provided for the emission of gases at suitable heights so as to minimize pollution. Chimneys are cylindrical and are designed to remain self-supporting. They are of sufficiently large diameter to permit the flow of burnt gases, and are of good height to discharge the same. Usually a chimney has a large flared lower part or base and is cylindrical for most of its height.
The maximum base diameter may be about 25 percent more than the diameter of the cylindrical part. The lower flared part may have a height one-eighth to one-fourth the height of the chimney. The vertical joints of the plates are generally single riveted lap joints while the circumferential joints are double riveted or triple riveted lap joints.
Steel chimneys are mostly provided with a lining. When self supporting brick lining is provided, it is usual to provide enclosing steel bands at regular intervals in the height of the chimney. Particularly steel bands must be provided below the level of each change in thickness of the lining. The brick lining is generally supported at intervals by shelf angles forming steel shelfs. These angles, besides supporting the brick lining also stiffen the stack against possible collapse due to wind.
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When brick lining is done, the lower region of the chimney is lined with fire bricks while the upper regions are lined with common bricks. High alumina cement lining and expanded shale aggregate linings are also in use. Independent continuous steel plate linings, carbon or corrosion resistant steel insulated on the outside of the lining are also used.
For design purposes, the self weight of the chimney and wind pressure are the factors influencing the design. In the case of stacks provided with lining, the additional load due to lining should also be taken into account while determining the compressive stresses. However, the stack should be designed to remain safe in the absence of the weight due to lining and when subjected to wind pressure.
Loads on Chimneys:
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The loads to be considered in the design of a chimney are:
(i) Dead loads due to weight of the chimney shell, linings, ladders platforms and other fittings
(ii) Live loads on ladders and platforms
(iii) Wind load
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(iv) Temperature effects due flue gases at high temperatures, and
(v) Seismic effects.
Simplified Method of Design of Stacks:
Let the effective wind pressure on the projected surface be p N/m2.
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Let the external diameter of the chimney be d metre.
Consider a section, h metres below, the top of the chimney.
Permissible Stresses:
Permissible tensile stress for the chimney section is 120 N/mm2 to make allowance for corrosive effect of gases discharged.
Permissible compressive stress ƒc may be determined from the following formula:
Let pc be the maximum compressive load per mm run of the circumference at the base-
