9 replies to this topic

[ChemG]

Hi all:
This is regarding the NGL recovery unit. I am comparing an existing plant(new) for my flow rate of feed. All the feed conditions to which the existing plant was designed match my requirement.
I have the Max UA values and duties for the heat exchangers in the existing plant.My question is, if the duty changes(considerably) and still if I maintain the UA value, will the heat exchanger work? My guess is there should be no problem.I donot have the heat exchanger configuration,or else I would have designed a heat exchanger roughly to compare.Plz suggest.

Edited by ChemG, 15 December 2009 - 04:58 PM.

[Joyy]

Dear ChemG,

You will need to atleast verify whether your exchanger passes the vibration analysis. You will need the existing exchanger configuration and a simulation software like HTRI or Chemcad to check this. Since you don't have the existing exchanger configuration, then ONLY for a PRELIMINARY check, you can compare the vapour/liquid rates of the original design with that of your case. This way you can say whether the vapour/liquid velocities at the inlet nozzles and inside the exchanger will less from the design case. And thus vibration may not exist. However, mind you that many of old exchangers designs have vibration problems with the original designs itself. Thus, this method is a preliminary check only.

And also you need to verify whether due to the change in duty and thus change in your utility load to the exchanger, whether the utility lines and control valve are adequate.

Best Regards,
Joyy

[ChemG]

Hello Joyy,
Thanks for the suggestion.After spending considerable time on this to find some alternative atleast for priliminary verification, there seems to be a lot of guesses with out a simulation software and I don't think there is room for this. As you said vibration analysis can only be done after knowing the heat exchanger configuration.This one is already built so I asked the client for HX spec sheets. Added to concern these are not normal shell and tube HX,these are plate fin exchangers.I had hard time to find 'U' values for the gas-gas heat tranfer in these type as well I had phase change.I was able to find liq-liq but the range is like 150-700 (too broad range to guess).But,thanks again and I will keep myself active in the forums.

[Zauberberg]

It's virtually impossible to analyze and predict exchanger performance with such a little input data you have. Apart from vibration issues, there is additional concern for maximum differential temperatures BAHX can withstand (50C in a steady state operation, max rate of change 3C/min during startup). By knowing the configuration of exchanger - which I doubt you will get if it is a BAHX - the whole engineering work might be useless. Probably the best thing you can do is to forward the new set of process data to the manufacturer (Chart, Linde?) and get their opinion.

Perhaps you'll find these materials useful as well: http://committees.ap...pemastndrds.pdf

[ChemG]

Hello Zauberberg,
Thanks for the suggestion and the material.

[Zauberberg]

Is this a cryogenic turboexpander plant? The exchangers you will usually see in these kind of units are either Feed/Product exchanger or Overhead Reflux Condenser which means there are no multiple streams feeding the exchanger like, for example, in LNG plants. That could simplify the evaluation process, but still if you don't know the exact exchanger geometry you will be in quite tough position when trying to predict future performance.

My experience with Chart and Linde is quite positive, and I would encourage you to get their opinion on the subject - if the equipment in question has been made by any of these companies.

Good luck,

[ChemG]

Yes this is a cryogenic turboexpander plant.I see that the equipment is manufactured by Chart.Yes there are no multiple streams feeding the exchanger. I will definitely contact CHART for re-rating of the heat exchangers.Thanks.

[Zauberberg]

Have a look at these resources as well: http://www.ortloff.c...l/nglpapers.htm

These people have designed the majority of turboexpander plants worldwide. For some time they have been the part of UOP but the original process licensing comes directly from the Ortloff's expert group. We operate their SCORE process here in Equatorial Guinea.

[S.AHMAD]

Hi all:
This is regarding the NGL recovery unit. I am comparing an existing plant(new) for my flow rate of feed. All the feed conditions to which the existing plant was designed match my requirement.
I have the Max UA values and duties for the heat exchangers in the existing plant.My question is, if the duty changes(considerably) and still if I maintain the UA value, will the heat exchanger work? My guess is there should be no problem.I donot have the heat exchanger configuration,or else I would have designed a heat exchanger roughly to compare.Plz suggest.

Hi,
Let us look into the heat exchanger equation:
Q = UA LMTD
If Q the duty change but if you are able to maintain UA ( I do not know how are you going to do that), LMTD will change. This means that the outlet temperature will change.
If we want to maintain the outlet temperatures, so the UA must change to accommodate the change in heat duty.
Logically, if the duty increases,we require bigger exchanger (bigger A). So UA shall be higher.

Regards

[David Southall]

Hi all:
This is regarding the NGL recovery unit. I am comparing an existing plant(new) for my flow rate of feed. All the feed conditions to which the existing plant was designed match my requirement.
I have the Max UA values and duties for the heat exchangers in the existing plant.My question is, if the duty changes(considerably) and still if I maintain the UA value, will the heat exchanger work? My guess is there should be no problem.I donot have the heat exchanger configuration,or else I would have designed a heat exchanger roughly to compare.Plz suggest.

Yes this is a cryogenic turboexpander plant.I see that the equipment is manufactured by Chart.Yes there are no multiple streams feeding the exchanger. I will definitely contact CHART for re-rating of the heat exchangers.Thanks.

Dear ChemG,

If the BAHX is a 2-stream exchanger, the answer in general terms to whether it will work for alternative conditions with the same UA is 'maybe' - that is, there is a greater chance it will than if the UA is different. This 'maybe' will only apply for thermal performance; hydraulic performance (i.e. pressure drop, maldistribution) is something else you would also need to consider.

EDIT: the 'maybe' will also depend on how similar the new operating case is to the original operating case.

When I worked at Chart (and if I remember correctly), their heat exchanger datasheets would provide you with information on what fins were used, numbers of layers, passage width and length, etc., with which you could make an estimate of thermal performance. If you have a layer drawing, then you could get distributor dimensions from that to check on hydraulic performance in more detail too.

If you did want to make your own estimate, then you could use the MUSE or PlateFin software from HTFS, or you could use methods from the HTFS handbook to do your own calculations. These would however only be an estimate, so I would recommend approaching Chart to re-rate for you - they will have their own proprietary fin data obtained from performance tests, plus the design details for your heat exchanger.


Good luck and Best Regards,


David.

Edited by southadc, 06 January 2010 - 08:09 PM.