Heat Exchanger Heads Design 3

[Sam Smith]

What is the criteria for selecting heads of heat exchanger? I read somewhere that design pressures are used as a criteria for the selection. For eg, torispherical heads are for design pressure less than 15 bar. Similarly, ellipsoidal heads are for pressures greater than 15 bar. Is there any reference for this?

 

[Krausen]

In general, most modern heads are 2:1 semi-elliptical (SE) due to their overall efficiency in strength of design & cost to fab. Hemispherical (hemi) heads are the best from a design standpoint, but costlier & often unnecessary. Torispherical (Flanged & Dished/F&D) heads are not very common in my experience & have inherent weakness in the knuckle regions due to tighter knuckle radius & higher forming stresses. An F&D head will require more material than a 2:1 SE head by code. But F&Ds can be helpful if you’re limited on head height or vessel clearances. This is a topic covered in most any Intro to Pressure Vessel courses.

https://www.pveng.com/home/asme-code-design/comparison-between-head-types-hemi-se-fd-and-flat/

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[SnTMan]

Sometimes the larger sizes are more readily available in tori' heads. You should be aware that not all of them are Code heads.

Regards

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[GD2]

Krausen is absolutely right. Hemispherical can take more volume. But do you need that for a HX?

GDD
Canada

 

[Snickster]

Heads types are usually determined by the type of heat exchanger you have such as fixed head or floating head per TEMA configurations. I never had to select whether the head was hemispherical, torispherical or whatever and needed to specify it on a data sheet when writing a heat exchanger spec as that is covered in the design standards - TEMA, ASME, etc. I imagine for small heat exchangers the head could even be made out of a pipe cap but refer to the codes.

 

[GinBread]

It's sometimes helpful to peruse websites of companies that produce something similar to what you're designing, just to get a general idea on what direction they went in. Obviously, you will be missing a few pieces of the puzzle that caused them to arrive at their final design. Here is one such company:

https://www.alfalaval.us/products/h...at-exchangers/shell-and-tube-heat-exchangers/

 

[Christine74]

Here's a summary of the various types of seamless heads used with shell and tube heat exchangers:


Semi-Ellipsoidal (aka elliptical or SE) heads: By far the most common type of head as they are typically the lowest cost option. ASME allows for various elliptical ratios but I've never seen anything other than 2:1 used.

Pipe End Caps: The ASME B16.9 geometry matches that of 2:1 semi-ellipsoidal heads but these are fittings so they are not manufactured from plate material. Most common for exchangers around 18" NPS and under but can be used for heads up to 30" NPS.

Hemispherical heads: Rarely used for exchangers with design pressure below 1500 psi but they become more common as the pressure increases. Required thickness is typically half of what's required for the shell cylinder so a tapered transition is usually required.

Dished/Dished Only heads: Spherically dished. Typically used for floating head covers but not all floating head covers use Dished heads.

Flanged and Flued heads: Occasionally used as expansion joints in fixed-tubesheet heat exchangers but they can also be used for annular distributors/vapor belts.

Flanged and Dished (aka torispherical) heads: These are rare but sometimes used for heat exchangers with low design pressures and/or large diameters. I believe the most common F&D configurations have a knuckle radius that's 6% or 10% of the crown radius.

Flanged only heads: Haven't seen these used before due to the higher required thickness but I suppose they could be used for low design pressures.

Flat heads: High required thickness but sometimes these are used as shell heads for certain configurations of single-pass floating head designs.


-Christine

 

[SnTMan]

Says it all

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Krausen]

I believe most hemi heads are not seamless but instead welded with multiple seams, just fyi. I do not have any direct experience with them on heat exchangers though & have also never dealt with one on a heat exchanger either

 

[MJCronin]

Christine:

You forgot about "80:10" heads ... as well as "90/17 Style 2:1 Elliptical Tank Heads" !!!!

"80"10"s are a head design that takes advantage of ASME rules to produce a product that is economical when an exotic material is desired

https://pveng.com/wp-content/uploads/2016/06/80_10FDHeads_10ksi.pdf

https://brightontruedge.com/products/custom-tank-and-vessel-heads/

It is also important to point out to the STEM newbies that SOME HEAD DESIGNS CAN BE USED FOR ASME CODED VESSELS ..... AND SOME CANNOT !!

As always ... this is my opinion only

MJCronin
Sr. Process Engineer

 

[0707]

Exchangers are designed according TEMA

luis