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Plate Type Heat Exchangers V/s Sthe


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#1 BhaskarL

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Posted 30 July 2007 - 12:03 PM

We are planning to replace an existing Shell & Tube HE with a Plate Type HE as the existing one is not offering the desired effect. The duty of the HE is attached. Since one of the fluid handled contains solids, can a PTHE handle it? I would like to know if similar retrofits have been carried out and what are the precautions to be taken while opting for such a change?

RegardsAttached File  Exchanger_Data_Sheet_1.xls   23KB   125 downloads

#2 Art Montemayor

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Posted 30 July 2007 - 01:02 PM


BhaskarL:

You are asking for consulting on this application. My frank and honest opinion is: Don't.

A PHE is a unique and specific type of heat exchanger. Like anything else, it has its attributes and its weaknesses. Solids contents in a liquid fluid is not one of its strong points; it is, rather, its prime weakness. If you proceed with your proposal, my opinion is that you will suffer both in process reliability and in economic setbacks due to continual maintenance in PHE clean-outs.

I realize that the attraction towards a PHE is very tempting. However, it has a specific niche in industry and I don't believe this is the one primarily because of the known solids contents. The unique heat transfer capabilities of a PHE are largely due to the rather high Reynolds Numbers developed due to the narrow and engineered flow cross-sectional areas that also promote turbulence and high convective heat transfer coefficients. However, when you add solids to the fluid you are promoting fluid flow pluggage that ultimately results in zero flowrate and an immediate downtime for clean-out.

Sorry for the negative opinion.



#3 wnovaes

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Posted 30 July 2007 - 01:57 PM

BhaskarL,

First, why are you thinking about to substitute a shell and tube heat exchanger for plate-frame heat exchanger? For your data, the needed heat exchanger size seems is not so big to justify a plate-frame heat exchanger and you now have a shell and tube heat exchanger installed. The plate-frame heat exchangers are very unfavorable for handle dirt fluids as in your case.
Second, what is the reason for the existing heat exchanger “is not offering the desired effect”? Try to assess its performance, looking for maintenance reporters for the last stop for inspection and cleaning and try to understand because the equipment is not work properly. In case of the heat exchanger is undersized, try to size a new approach using the existing arrangements with a new shell and tube heat exchanger in series or parallel and in the last case think about substitute for a new bigger one.

Best Regards,
WSN.

#4 JoeWong

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Posted 30 July 2007 - 08:04 PM

BhaskarL,
Mr. Montemayor and wnovaes have both highlighted PHE is NOT a good idea for fluid contain solid. Infact your statement "fluid handled contains solids" is rather misled people to slurry type fluid. As an engineer (i guess you are), please be specific. Please specify solid concentration (e.g. wt%, ppmw...) and particle size (micron, mm, etc). These parameters will give everybody better feel of the solids...

As wnovaes stated, spent sometime to investigate the factors of under-performed S&T HE if you have not.

There are other HEs available in the market which can handle solid much better than PHE. Examples are Spiral Heat Exchanger, Self-Cleaning type Heat Exchanger, etc. Personally i have experiences in slurry PVC (solid particles in mm level) using Spiral HE.

Please find below some articles related to both HE for the benefits of ALL :
    Spiral HE (Alfa Laval)
    Spiral HE (Sentry)
    Self-Cleaning HE (KLAREN)



Hope above help.

JoeWong smile.gif

#5 JBradley

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Posted 31 July 2007 - 08:11 AM

I've just had a quick look at your spec sheet - are you running the carbonate solution through the tubes??

If you are it's maybe worth taking the head off and seeing if your tubes are starting to block - it would explain why it's not fulfilling its expected duty.

Just a thought

#6 BhaskarL

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Posted 31 July 2007 - 11:40 AM

Thank you all for your opinions and as most of you have mentioned, even we had reservations regarding performance of the PHE vis a vis Amm carbonate solution, which has a tendency to crystallise at lower temperatures. The PHE will be installed in parallel with the existing STHE and only if the performance is satisfactory, it shall be lined up for total duty.
Thanks again for your comments,
Regards
smile.gif

#7 Chris Haslego

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Posted 31 July 2007 - 06:50 PM

I'll go ahead and ring in on this topic since it's a personal favorite of mine. A 20% by weight slurry of ammonium carbonate can be easily be handled in a plate heat exchanger if the following rules-of-thumb apply:

1. The particle size distribution shows that 80% of the particles are less than 70% of the interplate gap on the heat exchanger.
2. 100% of the particles are less than 90% of the interplate gap
3. The design velocity is below a point where erosion causes premature plate failure
4. The minimum wall temperature is above a point where crystal growth is expected.

Does your process fit the criteria above?

I'm aware of very specific experience where ammonium carbonate slurries of higher concentrations (above 20% by weight) are being used in plate heat exchangers with a minimum interplate gap of 4.0 mm. Based on your heat transfer duty, a 4.0 mm gap would fit perfectly.

There is one word of warning that I will offer in this particular application. I noticed that you're using cooling tower water as the service fluid. IF THE TOWER WATER GETS SIGNIFICANTLY COOLER AS SEASONS CHANGE IN YOUR REGION, the crystal growth could become a problem at that time while during the warmer season, the unit may operate perfectly. This is due to the plate wall falling below the crystallization temperature. This MAY be the cause of some of the problems in the current exchanger as well. The only way to combat this problem is to use a constantly controlled utility temperature or tempered water loop which employs a second water to water heat exchanger.

The advice given here in the forum is good, but a little broad in that plate exchangers have been used successfully in a wide variety of slurries in various industries. The key is that the particles have to be well dispersed and they must fit through the channels.

There was mention of a PVC slurry by another participate. That is a perfect example of a slurry which is NOT appropriate for a plate exchanger. If designed by a manufacturer with experience in the application, a plate exchanger would work well. The proper inter-plate gap must be used and the velocity of the slurry through the distribution zone of the plate must be carefully considered to avoid excessive plate erosion when the particles are somewhat hard.

In the kaolin industry, plate heat exchangers have been used for decades on kaolin slurries of up to 70% by weight. Plate heat exchanger manufacturers only needed time to find the design criteria. Luckily, the flexibility of the product made that fairly easy.

So my message is simple, don't rule out plate heat exchangers automatically if solids are present. Consult a reputable manufacturer for advice. They won't want a poor performing unit any more than you do, and don't underestimate the value of their experience.

---Stepping down off of my soap box---




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