Boiler Design: Top or Bottom Supported? 3

[RR79]

I have seen that most small boilers (below 100 ton/h of steam) are "bottom supported" design, while bigger boilers are "top supported".

I am trying to understand which variables impose the use of the more expensive top supported design, even in places without seismic loads.

I appreciate if someone could give references of criterias for selecting one or another design.

Kind regards,
Roger.

 

[boilerone]

RR79,

This might make it easier to understand why change from bottom-supported to top-supported as boilers get larger:

If you imagine making small and large boxes from the same thin wall material, such as paper, and leave them on the ground. Below certain size the box cans still be self-standing but a bigger box will collapse because of buckling by its own weight. You would normally want to increase the strength of the material but this method could be expensive. Then you come to think 'why can't I hang the box from the top just like hanging clothes?'. Bright idea and this why we do the same on boilers.

A boiler is also like a box and the strengths of the walls are relatively so thin and weak that even for small boilers they already need to have something called 'buckstay' (this concept came from trying to reinforce a barrel, but not exactly the same) to be able to keep the walls in straight shape first and next self-standing. At certain size/tonnage, it becomes more expensive to reinforce the walls than to start using cheaper structural steel and hang the boiler from the top.

Also, top-supported boiler has advantage of being able to easily and evenly transfer seismic/wind loads to the steel rather than resisting the stress by itself and cause leakage/explosion and injury.

Boilerone