Regeneration gas aircooler thermal expansion.

[fdomin]

I´m designing the regeneration gas aircooler of an NGL plant.
API 661 ask for methods of restraint relief for multipass bundles with temp. difference larger than 200°F, which is the case of this equipment.
I have tried multipass bundles (4 to 6 passes) with different tube pass configurations as side-side, crossed, a side-side/crossed mix, but in every case the first pass delta T is aprox 285°F. The second pass delta T is 100°F.

Our mechanical engineer said differential thermal expansion will damage the bundle, specially at header/tube union and a single pass cooler should be used.

On the other hand I had checked similar equipment of recognized companies (Smithco) and found even larger temperatures differences. (Gas is cooled from 550°F to 120°F in a 2-pass split box header bundle with 360°F & 70°F delta T for each pass).

I´m reluctant to design a single pass cooler as the heat transfer coeff inside the tubes will be very low, even lower than air, thus generating a very large cooler.

If someone has some experience on this matter it will be helpful.
Thank.

 

[speco]

fdomin,

You already know that you need a split header because of the large temperature difference through the cooler. The question is, what is the best way to do it. My suggestion is to use either a 4-row/2-pass arrangement or possibly a 6-row/3-pass one. In the second case, I would suggest that the front header should be split between the first and second passes, since the largest temperature drop will likely occur in the first pass.

Regards,

Speco (

http://www.stoneprocess.com)

 

[thermmech]

fdomin,
A process engineer should specify all the possible operating cases for this aerial cooler including turndown, and low ambient temperature cases.
An experienced thermal designer should then investigate which operating case is the governing one on the basis of calculated mean metal temperatures of tubes in separate rows/passes.
You might need more than one split.
Regards,
S.

 

[speco]

thermmech is right. Regen gas coolers usually have more than one operating case. Often there is a low-flow case with a large cooling range, and a higher flow case with with a shorter cooler range. Also, there is usually a case where a some water is condensed, creating freezing problems in cold climates. Often, these coolers have a warm-air recirculation system for winter operation.

Speco

 

[fdomin]

Thanks for you answers,

I´m aware that multiple split headers may be required.

Regarding other operating cases, I´m checking low air temp condition, but this is a warm climate location and freezing will not be and issue (but hidrates could form).

 

[thermmech]

I would advise you against having unusual row-pass configurations, especially side-to-side - as the tubes with very different process fluid temperatures will be placed in the same tube row and cooled with the air of the same temperature.
Cheers,
S.

 

[dcasto]

We use horizontal splits on our regen coolers. On the small units of say 2 to 3 MMSCFD of regen gas, we use split headers. The hot gas goes in top, runs over the top 2 rows and the reverses and comes back the bottom 3 rows and out. We tilt the cooler a degree or two to aid in draining the water.