Finned U-Bend Area In HTRI 2

[dygorane]

In modeling an exchanger with low fin tubes, a question arose about how HTRI handles the u-bend area if the nozzle is after the bend. For the tube surface area in the bend, does HTRI use the finned area or does it use bare area? It appears that tube providers standard is to not fin the u-bends. If HTRI uses finned surface for the u-bend area, is the any setting to make is use bare area? The only option we could come up with is to place the nozzle before the bend to ignore the u-bend area entirely.

 

[gr2vessels]

dygorane,
Normally, the HTRI is ignoring the un-finned area of the bends and so you should do also. The bend area cannot be finned and the heat transfer of that area is negligible. AS far as configuration concerns, you could place the outlet nozzle behind the bend, but the inlet nozzle should hit the tubes near the tubesheet.
cheers,
gr2vessels

 

[dygorane]

Thanks gr2vessels

 

[rmw]

Actually I'd slightly reword what gr2vessels has so correctly said and say that HTRI ignores the bend area, finned or not. Then I'd agree with him that the bend area is normally not finned, but I wouldn't absolutely rule out some very low fin type of finned tubes being bent as finned.

In either case, the bend area is not considered as part of the heat transfer surface, nor is the portion covered by baffles or support plates and tube-sheets.

For what you are trying to do, only the exposed straight lengths of the tube is pertinent to your modeling attempt.

rmw

 

[Christine74]

A couple of points:

1) Low-fin U-tubes typically have fins in the U-bend region. Usually just the portion that fits inside of the tubesheet isn't finned, but occasionally they are unfinned at the baffle locations as well.

2) HTRI's effective tube length does include an allowance for added surface area in U-bends, as it should, particulary when the shell nozzle is located beyond the bend tangent.

I couldn't find any information as to whether HTRI considers the U-bend area to be finned or unfinned, but I would certainly assume the former. I'd check with HTRI to see if there's a way to manually override the u-bend surface area calculation.

-Christine

 

[dygorane]

First of all thank you rmw, christine74 & gr2vessels for replying to this thread.

I sent this query to HTRI, below I am pasting HTRI's reply. In case of nozzle beyond u-bend Xist assumes tube bend is identical to the straight length. So incase of finned tubes Xist assumes u-bend area as finned which is not the case always. If you are designing an exchanger and taking into account the u-bend area, proceed carefully.

HTRI Response:

Xist assumes that the U-bend portion of the tubes is finned.

Placing the nozzle before the U-bend automatically assumes a full support plate at the U-bend location, thereby removing the U-bend area from the calculations.

You could determine the U-bend area as a percentage of the entire effective heat transfer surface for a bare tube design and then consider this when evaluating your overdesign values with a finned tube case with a full support plate to capture the additional surface area provided by the bare U-bend.


Regards,
dygorane

 

[tkdhwjd]

Minor clarification...

The reason why most folks will say "HTRI ignores U-bed portion" is because it is a normal practice to locate the nozzle before the bend. Placing the nozzle at the bed can induce vibrations. As HTRI replied, locating the nozzle before the bend forces HTRI to not take the bend area into heat transfer calc.