6 replies to this topic

[David Sugiman]

Dear all,

I'm just wondering whether anyone have experience comparing shell and tube heat exchangers with other types of HE in the market nowadays.

Based on my understanding, majority of HEs in the chemical plants nowadays still utilizing shell and tube HEs, despite there are newer types of HE in the market nowadays.

Based on my understanding:
- Shell and tube HE: versatile for almost all applications. Behaviour is well known. U value is generally lower than other types of HE, requiring much higher surface area compared to other HEs.

Examples of other type of HE's:
- Spiral: high U value. Generally strong resistance towards fouling.
- Gasketed Plate HE: high U value. Poor resistance towards fouling. Need to replace gaskets when we clean the HE ( gasket price can be very costly), high U value.
- Welded plate HE: similar to gasketed Plate HE, but without gasket, making it a cheaper maintenance option but then cleaning becomes more difficult. Poor fouling resistance.
-etc

Sometimes, I am actually confused whether to use conventional HE, or to replace it with new types of HEs for construction of grassroot plants.
I have talked with HE vendor (like Alfa Laval) and they often suggest to use newer type of HEs (their HEs), rather than conventional shell and tube HEs, but at times I am still unsure whether this is a good choice or not.

I understand that it's a very general question. but could anyone share with me some comments on this matter?
Thank you.

[Art Montemayor]

David:

First, you have to identify just exactly what is the position you find yourself in, as a process engineer. Are you working for an engineering contractor or for an operating company (the end user or “client”). Are you representing the owner of a process that is being licensed and has to guarantee the process results?

All the above are important factors that enter into the decision of what type of heat exchanger to employ in a process. There are other, detailed factors that also are involved in the process but I’m not going into them since your question is a general one, and not specific.

I could recite a lot of more pros and cons for each of the heat exchanger types that you mention, but what value does that give to your general question? Ultimately, there is a Scope of Work with a Process Design Basis that has to be followed according to project and owner dictates. Often it is the latter that carries the priority in identifying what is acceptable and what is not.

If I don’t know your position or job description, I can’t comment on how you should go about deciding the type of heat exchanger. I can only guess that you are an engineering contractor’s process engineer and, as such, know very little about the detailed characteristics of the client’s process or the process specified for design and construction. Otherwise, if you were the client’s engineer you would know exactly what type of heat exchanger was allowed and demand the type that you select.

Am I correct? As I keep mentioning in the Student Forum, its all about making money. We are charged with designing and constructing profitable projects – not projects with equipment that we like or prefer for personal reasons or preferences. There has to be a strong, viable engineering reason for the basic decision – and it often lies in the client’s list of equipment specifications and engineering practices.

[David Sugiman]

Dear Mr. Montemayor,

Thanks a lot for your reply.

I'm actually an end user process engineer (although still a young inexperienced engineer with only two year experience).

I understand your point that it's all about money. In fact, recently we are requesting our engineering company to do comparison between shell and tube heat exchangers with other types of heat exchangers, in order to see how much cost reduction we can achieve. ( our old plants are utilizing a lot of shell and tube heat exchangers, but there are many places, other types can be utilized, when we are building a new plant).

For myself, I generally know my process condition well, but I still lack understanding of characteristics of each types of heat exchangers, which I wish to learn more, so that I can make a better decision later on in deciding whether the type of HEs is applicable or not.

And it's actually with this in mind, I raised the question earlier on.

Thank you.

Best regards,
David

[Art Montemayor]

David:

You are in a fortunate and opportunistic situation. You can set the standards and guidelines for your company - or play a strong role in doing so. There are no magic or quick solutions to knowing which is the optimum choice as a heat exchanger model. You basically rely on your process knowledge and experience - and very heavily on the latter - for a successful equipment application. This takes years of doing what you are presently doing: collecting wisdom, experience, and know-how from others to assist you in your early-year decisions. If you have a local, mature, experienced engineering mentor to guide and instruct you on your job, you have a great advantage. If you don't (like most young engineers today), then you have to resort to other means - like this Forum.

If you describe in detail typical heat exchanger applications in your recent engineering experience, complete with compositions, temperatures, pressures, and corrosive characteristics, then perhaps we can assist you with typical process analysis and recommendations on types of heat exchangers that we would employ in similar situations. I think this is the quickest and most efficient method of giving you the type of recommendations that you are seeking.

I await your reply.
P.S. This is a subject I consider as very important for young engineers and I am in the process of putting together an Excel workbook on this very subject - selecting and operating the optimum type of heat exchanger for typical process applications. This is a very extensive and detailed subject that requires a lot of time and preparation. I am still working on it and hope to complete it before the year is ended.

[eilpar]

Hello,

When the Basic design is developed it is essential to check if the proposed exchanger type has been working satisfactorily in similar applications. Many designers/operators would like to play safe by repeating the existing design. Even within Shell & Tube exchangers there are several options available. The reasons for changing from existing design to a new design is normally for performance improvement (thermal / mechanical). The various factors such as temperature limitations, mechanical problems like vibration, corrosion, fouling, cost etc need to be addressed. It may be desirable to discuss with the manufactures the various issues to arrive at a correct solution. You may find general guidelines in text books or web sites.

Cheers

[David Sugiman]

Dear Art,

Sorry for my late reply.
Thank you very much. I'm really looking forward for completion of your work!

Anyway, recently I encounter one application that I would like to ask for your suggestion and recommendation:
Cold side:
Media: Circulated Cooling water (we are using open system cooling tower with minimized blowdown, main fouling material is mainly sludge)
Inlet temp: about 30 deg.C
Outlet temp: about 37 deg.C
Flow rate: about 1800 ton/hr.
Pressure: about 3 barg.

Hot side:
Media: Water (generally clean water, with minimum fouling)
Inlet temp: about 75 deg.C
Outlet temp: about 40 deg.C
Flow rate: about 300 ton/hr.
Pressure: about 4 barg.

For this application, we are currently using gasketed plate heat exchanger. My understanding is that if we were to use shell and tube HE, required size of HE will be very big (I'm not sure, but probably because of that the designer of the plant choose plate HE instead).

Due to fouling at cold side, we normally need to open up this HE for cleaning once a year, as we can't keep hot side outlet temp (which is the temp. that we would like to control).
One of the main setback is that after cleaning, we have to change all the gasket, and it can be very expensive (with only 2-3 years of maintenance cost, I can buy a brand new HE).

I wonder whether is there other type of HE that are more suited for these applications? (considering both investment cost, as well as maintenance cost).

Thank you very much

Best regards,
David

[Art Montemayor]

David:

What you have is a conventional, normal application for a plate heat exchanger (PHE) – with the exception that you are experiencing fouling that requires your maintenance. One of the tradeoffs encountered with all PHE applications is that you should only apply clean water (a minimum of fouling) due to the relative small passageways designed into the unit to allow it to have the efficient turbulence that yields good heat transfer coefficients. Another tradeoff is that of having to change out all the gaskets every time that you open up the unit. For low pressure, clean water, you normally use inexpensive rubber gaskets and can afford the change out. However, if you are having to use expensive gaskets, the change outs can start to take an economic toll after a couple of years.

What immediately puts your application in the PHE class is the size of the flow rates (8,000 gpm and 1,500 gpm – nominally) and the fact that you are handling water on both sides. However, as I said, if there is fouling and it is becoming expensive to deal with it, then you may have an application for a spiral plate exchanger (SHE). An SHE can easily handle the 75 psig rating and has only one gasket that is applied when you select the type that has a hinged cover plate. The hinged cover plate makes the SHE very easy to open immediately and to simply “hose” down any fouling existing in the spiral runs. The down time for clean out is probably the lowest in the processing business.

However, the SHE will cost more than the PHE – depending on the manufacturer and the materials of construction. I would seriously try to match an SHE against a PHE in this type of service. The materials of the spiral don’t have to be stainless steel. I don’t know if you have SS plates in the PHE or not, but if you do you may consider carbon steel for the SHE. The difference in capital costs between a PHE and an SHE may not be as much as we may believe for this application.

For this size of flow rate and fluids, I would try to stay away from a shell and tube design – especially if foot print is a big factor. This is a large S&T and cleaning it out due to fouling is going to take a lot of time and effort – although the gaskets won’t be as much as the PHE. Removing the heads and bonnets is going to require approximately the same structural overhead steel and trolley investment. This would not be the case with a hinged SHE. Be sure to give the SPE the benefit of being less demanding on structural and space requirements – as well as being much simpler and faster to clean out.

I recently discussed an application for an SHE in another thread, http://www.cheresour...amp;#entry15990. You may want to read what I offered for that application and the references I gave.

I hope this experience helps you.