11 replies to this topic

[fahrudin]

Dear All.

What the meaning % over design of HE is negative?
is there we can't achieved target outlet temperature?
Please advice me

Regards,
Fahrudin

[GS81Process]

Negative % over design implies that the exchanger does not have enough heat transfer surface area to accomodate the required heat duty based on specified flow rates and temperatures.

Edited by GS81Process, 21 December 2011 - 04:30 AM.

[kkala]

GS81Process is right. Performance will not be as expected under specified conditions. These include fouling factors. In a few cases fouling factors (as specified in data sheets) control heat transfer; so a more frequent cleaning of the exchanger (resulting in lower fouling) could obtain expected performance.
Estimate overall heat transfer coefficient assuming 0 fouling factors, to see whether you have this possibility in theory. But even if this possibility exists, it may not be feasible to stop the exchanger every now and again for cleaning. However it is a situation that can be further investigated.

Edited by kkala, 21 December 2011 - 04:51 AM.

[fahrudin]

Dear GS81 & kkala.

thanks for replying.
to achieved expected performance, we shall increase heat transfer area.
we have plan to just replace existing tube bundle with twisted type tube.
for increase tube number
can we reduce the pith length until below than 1.25 OD tube?
caused in TEMA stated that tube pitch shall be minimum 1.25 of tube OD.

What the effect if I make tube pitch lower than that?
vibration will be higher?

please give the comment.

Regards,
Fahrudin

Edited by fahrudin, 21 December 2011 - 08:45 PM.

[sheiko]

Dear All.

What the meaning % over design of HE is negative?
is there we can't achieved target outlet temperature?
Please advice me

Regards,
Fahrudin

Correct interpretation will depend on your definition of overdesign...

Edited by sheiko, 22 December 2011 - 04:01 AM.

[GS81Process]

Dear GS81 & kkala.

thanks for replying.
to achieved expected performance, we shall increase heat transfer area.
we have plan to just replace existing tube bundle with twisted type tube.
for increase tube number
can we reduce the pith length until below than 1.25 OD tube?
caused in TEMA stated that tube pitch shall be minimum 1.25 of tube OD.

What the effect if I make tube pitch lower than that?
vibration will be higher?

please give the comment.

Regards,
Fahrudin

There are other possible ways of increasing the heat transfer rate, depending on what fluids you are using. One is to increase the circulating rate of the cooling medium. Another is simply to swap the bundle with one of a similar design but higher tubecount (if practical). Is the existing baffle spacing appropriate?

I'm assuming that your statement of % overdesign may be based on output from a heat transfer computer program. Since this is an existing exchanger, have you observed from plant data that the heat duty is not as high as you require/would like?

Edited by GS81Process, 22 December 2011 - 09:30 AM.

[fahrudin]

this is CFE (combine feed exchanger in Hydrocracking unit), as preheat before entering to Heater.
configuration in 2 paralel and 4 series (totally 8 unit).
Due to revamp project we will increase CIT (Coil inlet Temp of Heater) to reduce heater load/duty.

two option for this item is:
1. install 2 unit HE with same size with existing
2. replace existing tube bundle (plain tube) to be twisted tube (with same configuration 2x4)
expected just replace 2 unit bundle is enough.

twisted tube no need baffle, so pressure drop is lower compare the conventional HE.
first we try to simulate 2 unit bundle with twisted tube.
result is as I state above,% over design negative --> -5%

in my mind, I can increase heat transfer area by decrease tube pitch length.
but now tube pitch already minimum as mention in TEMA 1.25 of tube OD.
So, I am not sure and worry to decrease tube pitch.

Regards.
Fahrudin

[jcazenave]

hi

negative overdesign means that the surface is not big enough to achieve your required duty.

i am confused with your statement about Twisted tube and tube pitch. when using twisted tube, there is no tube pitch in the sense that every tubes touch each other at the bend. difficult to explain with words:)

if you want to replace your existing bundle with twisted tube, you need to do a proper calculation of the new bundle either by asking the manufacturer (like koch) or use a specialist tools that can model them like Aspen Shell & Tube Exchanger with the add in from Koch.

Kind regards and merry christmas

[ashrafnew2001]

Dear Sir

You stated in your previous posts that you wish to change the tube pitch or use a fin tubes in your HE and that is a good thinking because fin tubes as you know can double the heat transfer area of the outside surface of the tubes, but you must consider that there are some considerations and restrictions you must be aware of before choosing to use fin tubes of minimizing tube pitch for example:
1- You mentioned that your HE will be used in hydrocarbon services, generally in such services you must first check the fouling rate and corrosivity of the fluid which will be used in the shell side cause excessive fouling and corrosion will damage the outside surface of the fin tubes and minimize it life and as you know fin tubes costs 50-70% more than ordinary tubes, generally fin tubes should not be used when the corrosion rate of tubes exceed 2 mils per year.
2- As for the reduction of tube pitch what is the tube layout ? is it triangular or rectangular because there is restrictions for each type and as I mentioned above you should aslo consider the fouling rate of the shell fluid and the difficulty cleaning process in the future because when you minimize the tube pitch the HE fouling will deposit faster in the bundle and the downtime will be less beside that mechanical cleaning will be much more difficult.
Finally in my opinion you should stick to TEMA regulations regarding tube pitch value and good luck my friend.

[fahrudin]

Jcazenave,

we not have experience for using twisted tube,
We received print-out HTRI simulation using twisted type from KOCH.
at their simulation document tube pitch mention 22.6 mm.

Ashraf.

Sorry, I am not talking about fin tube. this is twisted tube.

Regards,
Fahrudin

[breizh]

This paper might be interesting to read.

Breizh

[kkala]

1. hx margin.pdf by braze contains useful advice from Heat Transfer Research to avoid excessive overdesign. However it is pointed out that Operations need some flexibility, which is promoted by design margins.
In the past 100% uncertainty in exanger design was occasionally possible, transformed into overdesign, as read in a book of ~1980 (probably by J P Gupta). This is much smaller today -- 5% according to local Mechanical Dept running b-jac, 10 - 20 % (or even higher) according to hx margin.pdf.
It is noted that hx margin.pdf defines the margin on the calculated area of clean exchanger. It is assumed not to be the case for reported margins here, referring to dirty exchanger.
2. Negative -5% overdesign is assumed to concern only the two exchangers, the bundles of which are to be replaced by pitched tubes. The other option is to add two "traditional" exchangers (total = 10).
Concerning the pitched tubes option, additional (say)15% has to be added to cover uncertainties of calculation (as above), so area should increase by ~20% . In previous exchanger applications, the area indicated by the software was increased by some percentage, this should be applied also to the new cases.
It is not known whether that missing ~20% in area can be counter-balanced by a reduced fouling factor in all 8 exchangers, still resulting in an acceptable schedule to clean the two exchanger trains separately. Calculation using reduced fouling factors can give guidelines.
3. Meanwhile the pitched tube bundle has to be evaluated versus the traditional one, even by visiting exchangers with pitched tubes elsewhere. http://www.twca.com/...oc/2001007.pdf'> http://www.twca.com/pdf/2k1ConfProc/2001007.pdf gives relevant data, but brown fintube seems to supply or promote such exchangers http://goliath.ecnex...00463-page.html. The web article reports that pitched tubes have low tendency for scale and have been used for crude heating. They can also be made to TEMA standards, so a pitch (whatever it may mean) out of TEMA recommendations may not be a good way out.