12 replies to this topic

[yogu77]

Dear All

I have a basic doubt, Apologies if the doubt is too basic to ask here.

When we see any heat exchanger design results from HTRI, how do we ensure that the design is adequate? Is it just by seeing the overall design margin?

Or Apart from the design margin, we have to check and make sure that tube side & shell side velocity are accepatable based on thumb rules of design? (thumb rule: tube side velocity -> 3 - 10 ft/sec, shell side velocity -> 2 - 5 ft/sec)

Will there be any impact if the design margin is still acceptable ( let's say 10%) but the shell side / tube side velocity is well below the thumb rule of design?

Thanks

[Zauberberg]

It all depends how much far away is your design from the "rules of thumb". If you get unreasonable results - e.g. something like 0.5 ft/sec tube velocity, such results should be suspected and discarded, because they don't make a lot of sense. HTRI gives you a warning if certain design is incorrect and/or uneconomical, if I remember well.

Since you will be working with the exchanger vendor(s) in any case, I wouldn't bother that much about software outputs.

[daryon]

Never used HTRI but have experience with HTFS Tasc.

First thing I look at is the value for U (overall heat transfer coefficient) this must be realistic, and is dependant on the quality of the fluid physical properties inputted into the software, and the exchanger geometry, as a rule of thumb you can check U values against the table below which contains typical value:

Hot-side fluid / Cold-side fluid / U (W/m².C)
Hydrocarbon (< 5 cP) / Hydrocarbon (< 5 cP) / 240 - 390
Hydrocarbon (> 5 cP) / Hydrocarbon (> 5 cP) / 100 - 180
Condensing Steam / Hydrocarbon (< 5 cP) / 300 - 500
Condensing Steam / Hydrocarbon (> 5 cP) / 100 - 300
Gas (at 20 bar ) / Water / 300 - 600
Gas (at 5 bar) / Water / 150 - 250
Gas (at 1 bar) / Water / 90 - 150
Hydrocarbon (< 5 cP) / Water / 250 - 650
Hydrocarbon (> 5 cP) / Water / 40 - 350

You should also check the following:
1) Pressure drop is within allowable limits typically 0.5 bar to 1.0 bar. It's good to maximise the pressure drop through an exchanger as it generally means high velocities and high film heat transfer coefficients.
2) Check fouling resistances are realistic in line with TEMA recomendations
3) I would normally check rhoV² values meet TEMA limits for inlet and outlet nozzle, program should flag up a warning if not met.
4) Check for vibration warnings, if there are vibration warnings you need to look at design carefully.
5) Check tube pattern - HTEX with 30° triangular pattern cost less per m² heat transfer area (due to highest tube densities) and transfer more heat per m² heat transfer area (due to the high shell-side heat transfer coefficients). But have high pressure drop for shell-side and can't be mechanically cleaned. 60° triangular pitch is rarely employed because heat transfer is comparatively poor whilst pressure drop is comparatively high.And is hard to clean. 45° rotated square pattern or 90° square pattern should be specified when shell-side fluids are considered heavily fouling and mechanical cleaning of the outside of the tubes is required. A general guideline is shell-side fluids with fouling resistances greater than 0.00035 m². °C/W should be specified with a square tube pattern. 90° square pattern would typically be employed where pressure drop limitations are constraining the design. Where pressure drop limitations are not constraining the shell-side heat transfer coefficients can be improved by specifying a 45° rotated square pattern at the expense of increased shell-side pressure drop. Removable bundles should always be specified with 45° rotated square or 90° square tube patterns.
6) Check tube pitch - A close pitch increases shell-side heat transfer and tube density at the expense of shell-side pressure drop. A open pitch reduces potential for shell-side plugging and provides easier cleaning of the outside of tubes. The minimum value for PT/ do is 1.25. When tube pitch (PT) is less than 1.25 times the tube O.D. (do) the tubes become too close together and HTEX tube sheet can become too weak. When mechanical cleaning of the tubes is required 45° rotated square pattern or 90° square pattern should be specified with minimum cleaning lanes of 6.35 mm.

Actually there is loads of other stuff I check(TEMA type selection, baffle cut and orientation, etc) but have run out of steam here, if you'd like I can send you some design guidelines I wrote for Shell and Tube Heat Exchanger thermal design. Let me know your email address.

[Lowflo]

There's some good advice here. I want to add to this a point that I consider to be one of the most important whenever you're running any simulation (HTRI, Aspen, etc) to design equipment. That point is that you should always run some sensitivity analyses before finalizing a design.

After you've gotten a reasonably good design, make changes to one design variable at a time and see how that affects the overall performance. This is easy and quick to do. It will also give you a very good understanding of the dynamics involved in heat transfer, as you contemplate why the performance changed so little or so much. For example, vary the diameter while holding the length constant. Notice the change that this makes to the total surface area,% overdesign, coefficients, pressure drop. If you have a vapor on one or more sides, then increase the pressure drop (lots of different ways to do this) and notice the huge affect this has on the HTC.

These sensitivity analyses result in a better optimized exchanger, and a much more educated you. There's no better way to internalize the dynamics of heat transfer than to run a series of sensitivity cases. If you don't have a lot of experience in exchanger design, you'll probably be in for some interesting and informative surprises. It's very rewarding to look at the results and figure out in you mind why they changed to way they did.

[yogu77]

Dear all

Thank you all for valuable advice

Thanks for your guidance.

[jcazenave]

I agree with Lowflo and since you mention Aspen.

There is a tool call Aspen Simulation Workbook which allows you to run the Aspen HX simulation from Excel (without the need of VBA programming). This is perfect to run sensitivity study, as you can create a table with all the cases you want to run and this is done for you.

Then in Excel you can create graphs etc ...

[rowanlim]

Thanks for the tips, daryon, I myself am learning how to use the HTRI & how to design the heat exchanger, currently lacking reading material (supervisor is away) so this thread is very useful!

[daryon]

I sent it via the messenger.

[JoeAxiom]

I sent it via the messenger.

Daryon,

I appreciate your assistance and the information provided as I too am working on a S&T heat exchanger application where your knowledge was helpful.
Thanks again!

[fatimah]

I sent it via the messenger.

hi daryon

include me too. Thanks.

[alert]Removed your email address from your post. If you want to exchange files please do so through the messenger system to protection your email address.[/alert]

[ccesarb]

Hello Daryon,

Could you please sent it to me also ?

[alert]Removed your email address from your post. If you want to exchange files please do so through the messenger system to protection your email address.[/alert]

Thanks

[Root]

Hi,

I'm interested to learn from your experience,

Thanks
Toor

[srfish]

That is good advice by daryon. The accuracy of the calculated U values he shows can be increased by using the equations in a workbook in the community download section. The name of the workbook is "Process Heat Transfer" by Art Montemayor. The information is also on www.gulleyassociates.com. The equations are in the "Calculation" section.

I would like to take exception to one remark and that is "Removable bundles should always be specified with 45^o rotated square or 90 ^o square tube patterns". U-tube heat exchangers sometimes have triangular tube pitch.