HEAT TRANSFER IN HOTBOX 1

[Rahulvaland]

[highlight #204A87][/highlight][highlight #CC0000][/highlight]What is the reason of providing the HOTBOX in skirt-shell junction,

As per my knowledge it is provided to reduce the intensity of thermal stresses at shell-skirt weld junction,
But i have some doubts and i could not understood this phenomenon.

Please forward/attach if anybody has any reference/article.

 

[brkmech1234]

As far as I know, the term HOT BOX is misnomer. We don't use any box there. The skirt to head junction is treated as hot region(Hot Box) and will be analysed with a FEA to make sure that the stresses are under limit. Usually this will be specified by client for vessels with operating temperature more than 350 deg C.

 

[jtseng123]

Our common practice for years, is to provide hot box for design temperature 650 degF and above. It is to reduce thermal gradient so to reduce thermal stress. Hot box is extremely important if vessel is in cyclic service, such as coke drum and other vessels that require high temp regeneration process.
To learn more, read chapter 9 of "Pressure vessel design handbook" by Bednar, which gives you basic concept. If you want to learn more about the mathematical algorithm and how the thermal gradient and stress are calculated, which are extremely complicate, read ASME 7S-PVP-53, "Procedures for determination of thermal stresses in 3-shell junctions common to pressure vessel construction" by Gilbert, 1977. Not every company can have commercial FEA program to deal with thermal transfer through conduction, convention and radiation or to have knowledgeable people know what he is doing about heat transfer and thermal and mechanical stress and determine the life of the skirt junction, leave it so expert, but to put a hot box, it won't go wrong.