9 replies to this topic

[aliadnan]

hello
i am a student of chemical engg.... i have to design a heat exchanger in which dowtherm A carrying heat from a gasifier jacket at 350C is to be cool in the heat exchanger to 250 using water which will vaporize and 30 psi steam will be produced ... now i am not able to find overall heat transfer coefficient for this system ... i also wanted to know is this system possible ... the dowtherm will be at 5 bar pressure ... and where the water should vaporize in the shell sid eor the tude side .... plz help me in this matter

[Art Montemayor]

Aliadnan:

All of those responding to your request for help are trying to do so, but you are making things very difficult and totally confusing when you post your query TWICE in the Student Forum (once as a Dowtherm A application and ANOTHER as an oil cooling problem) and then also post it in the Industrial Forum! All these posts – and their titles – are very confusing and clouding up the issue of what is it that you are asking. Please refrain from posting your query more than once. It doesn’t do you or anyone else any good to keep repeating what you need. If you do a correct and proper write-up of your query, people will understand what you are after and will respond efficiently. But when you scatter your queries all over these Forums, no one knows which is the thread that should be addressed and what has been responded - as well as not understanding when you change the scope of work: what is it that you are after – the cooling of an oil stream? or is it the generation of LP steam? These are two totally different scopes and confuse what you are trying to ask.

I am going to address this post in the Student Forum (where I believe it belongs, since it is such a basic and elementary engineering application) and delete the other queries. If you still want to post again, please indicate a different scope of work or description. This way, all the responses will be efficiently directed to this one posting and easy to read, compile, and compare as to value for you. I will respond to you knowing that you are confronting a typical student design problem that has been assigned for you to resolve.

I am confident that I know what your professor is asking you to do: you are to engineer a method whereby you can recover waste heat from a flow of Heat Transfer Fluid (HTF) used as a reactor coolant fluid. Specifically, you are to generate 30 psig saturated steam with this HTF which is identified as Dowtherm A. I seriously doubt that your professor allows you to simply use an estimated Overall U that you can obtain from journals, textbooks, or from an internet Forum without any reference or proof of applicability in this application. I suspect you have been assigned to generate the “U” that you employ. I also believe that your Prof expects you to analyze the proposal and detail out how you intend to control the specified results expected: the 30 psig steam and the 250 ^oC HTF produced. Assuming I am correct in my assumptions, I will proceed to explain what you are up against and how I can help you resolve your problem.

1. Before you do anything else, you should be able to prove or confirm that you can generate the required steam with the given HTF. Go to the internet and download an Adobe document from Dow which gives the thermal properties of Dowtherm A. I’ll anticipate you doing your homework and tell you that at 135 ^oC, saturated steam should be easily generated with 350 ^oC HTF. However, Dowtherm A has to be at approximately 6.8-7 barG of pressure to exist at 350 ^oC.

2. Knowing all the characteristics of both streams you can make a valued judgment on which side to place the HTF in the steam generator. I can almost bet it’s going to be on the tubeside. It’s up to you to justify and explain this important point.

3. The type of mechanical configuration used in the generator is another item you have to identify. It could be a kettle type of exchanger. You must take into consideration the temperatures, the stresses, the flow rates, the corrosion concerns, the pressures involved and other criteria to arrive at this identification.

4. You can easily make the heat balance and calculate how much steam you can produce; however, bear in mind the very large approach being asked of you: 135 ^oC steam exiting with 250 ^oC HTF exiting as well. This is over 100 ^oC of approach – something that you have to confront as to how you are going to control the HTF outlet temperature. I would caution you on believing that you can do this with a “special” overall “U” built into the steam generator. That is naivety and simply won’t work the first time. You must determine which of the two terminal temperatures is the driving one: the steam or the HTF? I suspect it is the HTF.

5. With the configuration and the flow and thermal conditions fixed, you can now get down to business and calculate the heat transfer film coefficients and ultimately, the “U” – clean and dirty versions. Find out and explain what determines and fixes the final, controlled HTF outlet temperature.

I can tell you what would be an expected “U” – just like a bunch of other people on this forum and in others: it is going to be approximately 50 – 350 W/m² – ^oC. But I believe you have to calculate it.

I hope the above helps you organize a successful resolution to your problem. If I have been incorrect in my assumptions, then please correct me.

[aliadnan]

thank u very much Art Montemayor .... thanks for ur help .. actually i was not able to find the range of the overall coefficent which i can use a first guess to calculated the corrected coefficent ....
well can't i use a split ring floating head shell and tube heat exchanger for this system ....

[Art Montemayor]

Aliadnan:

I would not consider using a TEMA type AES or any other type of internal floating head exchanger for this service. That type of design is not practical, is very expensive, is costly to maintain, and a huge pain in the neck to extract and work with. These type of designs inevitably leak and allow commingling of the two fluids. There is no advantage, in your application, for this type of exchanger.

A TEMA AKU type of exchanger is much more suited for your application. Look carefully at the designs and study the various advantages and disadvantages between the types. That is what you are expected to do for your student problem in order to justify your selection of equipment. The kettle type of steam generator offers advantages very much suited to your application:

1. An inherent vapor disengaging space for the generated steam on the shell side – which is the preferred side for the steam;
2. An inherent ability to flex and adapt to high thermal stresses due to tube expansion and growth. The U-tube takes the high-temperature fluid very naturally and no expansion joints are required;
3. Both fluids are ultra-clean; they have to be, since they are engineered in almost closed-circuits. Therefore, the clean service suits the kettle design very well.

I hope this type of engineering analysis is what you are subjecting your problem to because it seems that your profs are expecting you to report on that.

Good Luck.

[aliadnan]

hey ART ... i did training in a fertilizer plant for 1 month and there i saw shell and tube heat exchangers in which 1500psi steam is generated in tube side and got gases at the shell side .. i have also seen shell and tube heat exchanger used for 575psi steam used as waste heat boiler to recover heat from the high temperature shift converter gas ... i have'nt seen any kettle type of exchanger for the steam generation in fews industires which i have visited (abt 3-4) ... why kettle type of exchanger i not used in those places since high pressure steam is being produced there ... coz the points u have given are very valid abt the use of kettle type of exchanger for steam production ..
reply

[Art Montemayor]

Aliadnan:

I don’t understand the reasoning behind your statements and the question asked (I also have a hard time trying to understand your writing. We have no words such as “i”, “coz”, or “abt” in English).

Let’s go back and review what you originally presented:
1. Two clean fluids exchanging heat;
2. One fluid at a relatively much higher temperature than the other;
3. One fluid changing phase (steam generation).
4. Both fluids at relatively low pressures (steam at 30 psig, Dowtherm at 100 psig)

You didn’t present any other basic data (such as flowrates) so, I recommended you analyze and justify the equipment you select for the application, adding that I believe the kettle unit is well suited for the application. I still stand by that recommendation.

You now return stating that because you haven’t seen any kettles employed in sites where you’ve been, you want to know why this is so. This, in my opinion, is irrational and non-engineering thinking. Only a non-engineer or layperson would ask themselves that type of simple question. Please consider that I’m not trying to insult you or demean your query. I’m trying to teach you something about the proper way an engineer goes about rationally thinking and justifying his acts so they result as logical and correct steps that lead to sound and well-engineered solutions. There are many different types of mechanical designs appropriate for steam generation (or heat recovery). The correct design depends totally on the specific application. There is NO ONE, IDEAL, or UNIVERSAL DESIGN. There couldn’t be – there are just too many different applications. That’s why you have a promising future as an engineer with a strong possibility of being hired and paid to practice as an engineer. If there were one solution to heat recovery or steam generation, the world wouldn’t need engineers. The problem would have been solved and all that would be needed would be minimum-educated technicians (paid at a lower rate, of course!).

Your examples or prior experiences don’t apply in this case because:
1. We don’t have any basic data or scope of work regarding those applications;
2. You’re comparing a gas stream with a liquid stream as the sources of waste heat; this is like comparing apples to oranges. It’s not logical and certainly not engineering.
3. Just because you haven’t seen any kettles used as heat recovery units in your experience doesn’t mean that they don’t exist. Proof of this is that you can’t compare your 1-month training experience with my 45 years of hands-on field engineering in design and operations. I’ve seen hundreds of steam generators – and I’ve designed and operated some of them. All of the kettle designs I’ve used have been justified through engineering design logic and economics that was approved by the companies I did the work for. Obviously, the application made sense (and profit); that’s why they were applied.
4. Your question of generating high pressure steam is not applicable. You forget that the kettle shell side is primarily suitable for the lower pressures – not higher pressures! Again, try comparing apples with apples instead of oranges. Whenever you deal with high pressures you should be trying to employ the smallest diameters available – like piping or tubes.

I’m glad you agree that the points I’ve given about the use of kettle design are valid. However, I would be more pleased if you challenged my recommendations and tried to find some drawbacks or trade-offs with the design – because I can assure you that there are some. The important thing to bear in mind as a student is that you should be getting used to using your logical and common sense in justifying your solutions and be ready to employ that same logic to defend them. What is probably the worse thing for a student to do is to solicit the opinion of an experienced engineer and simply take that as a “solution” and not try to understand the logic and justification behind the answer. Engineering is not simply employing given or “accepted” formulas or equations to find a solution. As an engineer you must rationally prove that your reasoning and resulting calculations are the best answer – if not the answer. There are many ways to skin a cat – just as there are many ways to generate steam with a waste heat stream. It is up to the engineer to logically select the best method and device to do so. You just don’t employ a kettle type of exchanger because everyone else does it or because someone told you to do so; you do it because you are convinced and can justify your selection.

Don't forget what I said about your proposal of using a split ring floating head shell and tube heat exchanger. That type of tube bundles closure is absurd in this type of application. I don't know where you got that idea, but an experienced engineer would easily beat down that idea.

I hope this has helped to orient you.

[aliadnan]

I have another question ... u said that tubes are used for high pressures and shell is used when the pressure is not very high .. please can u expalin why this is soo because I don't have much idea about this ..

[Art Montemayor]

Aliadnan:

Thank you for asking those questions. It helps me help you when you are honest and forthright in asking your questions. I have previously assumed that you have already been lectured and taken courses in Mechanical Engineering and Strength of Materials. I have to assume that because I have no way of knowing just exactly how well you are being trained in Chemical Engineering and what curriculum you are required to pass. I have always suspected that the greatest majority of universities do not require that their graduate Chemical Engineers take certain Mechanical Engineering courses like Strength of Materials and vessel design. I consider this a great disservice to the graduate ChE and I believe it puts him/her at a net disadvantage when the first job opportunity in a production/process plant occurs. I normally do not like nor do I tolerate a thread to change subject or theme in the middle of a specific posted subject. However, this subject matter you’ve brought up is not only practical, but is very important for students to be aware of and to take action on while they are still capable of doing so. Here, I am referring to the question of mechanical strength and design. I will deal with the question of pool boiling later on.

For any young graduate – especially for any Chemical Engineering Student – out there, I strongly recommend you invest some of your hard-earned money and buy the following book:

“Pressure Vessel Handbook”
by Eugene F. Megyesy
Pressure Vessel Handbook Publishing, Inc.
P.O. Box 35365
Tulsa, OK 74135

This book used to have a foreword by Paul Buthod, Professor of Chemical Engineering at the University of Tulsa. This foreword should give you a strong clue as to why I strongly recommend Chemical Engineers to read, study, digest, and thoroughly dominate the information and data given in this book. You will never regret doing so. It probably is the most practical engineering book published in the USA. It will enrich your knowledge and further help you in your career like no other book. The book can be found at its own website and you can find it using Google’s search engine.

I understand the conventional situation of a young ChE, coming out of university and having to comprehend all the shapes, sizes, and complexities of an industrial complex. But as a professional engineer, you must undertake the necessity of knowing what happens to the specifications of a vessel (or a pipe) when you increase/decrease the pressure and temperature under which you want to operate it. You must understand and be able to predict what will be the net effect on the required vessel when such things happen. If you don’t understand this, you will remain ignorant of what it is that you are affecting by your design and decisions out in the field. From the standpoint of safety and practicability, that is not acceptable. For example, let’s just consider internal pressure in a vessel. From basic mechanical design we find out that for a cylindrical metallic shell:

t = PR/(SE – 0.6P)

where,
t = cylindrical wall thickness, inches
P = Design pressure, psi
R = Inside cylindrical radius, inches
S = Stress value of the metal material, psi
E = Joint efficiency of the welded sections, expressed as a fraction

The same basic equation (the “hoop” stress equation) applies to a pipe as well. From this well-known relationship, you can see that the allowable pressure for a given thickness of a given material will INCREASE as the radius (or diameter) gets smaller. This is a major effect and has an impact on your steam generator design. This clearly points out to you that you want to subject your higher-pressure fluids to a smaller diameter – such as that in the tubes – and not on the shell side. To do otherwise would cause the larger shell to have a large thickness and, consequently a large weight and greater cost. It is an impractical application – simply by engineering observation and deduction. This should explain to you why a fuel-fired steam generator (which can be built with the fire in-the-tubes or the steam-in-the-tubes) is always built such that high-pressure steam is generated within the tubes (and not in the shell of the boiler).

The same reasoning and logic is applied to the heads (or closures) on process vessels. How do we decide on the type of head? By logic and analysis we know that the hemispherical type is the strongest of all geometric head configurations. However, it requires a lot of manual labor and welding of seams. On a practical level a hemi-head is not justified on relatively small vessels. On vessels up to approximately 8- 10 ft (3 meters) in diameter, an ellipsoidal head of 2:1 dimensions is employed when dealing with pressures above 150 psig. Below that, ASME flanged and dished (F&D) heads are used. By readying and studying Megyesy’s book you will know and learn to reason why that is so.

Needless to say, this subject is very, very important to a ChE. It influences how you design a vessel and other process equipment such as heat exchangers. It affects how you design distillation towers, reboilers, reactors, as well as condensate drums and other pressure vessels. It determines how you will design, locate, and install such important internal devices such as trays, packing, distributors, insulation, platforms, ladders, manways, etc., etc. Why Chemical Engineering students are not subjected to this very important subject matter early in their studies is something that I fail to understand. I firmly believe it cheats the student from learning early-on what he also must employ early in his career.

Now to discuss your question on pool boiling. Yes, you should be applying the principles of pool boiling and the phenomena that come with it – like vapor binding. You are correct in assuming that you should use the methods of Kern or Bell to design your generator. A reboiler is nothing more than a generic vapor generator. In your case, since the liquid involved is water, the generated vapor is steam. Do not forget what I previously told you: “I’m glad you agree that the points I’ve given about the use of kettle design are valid. However, I would be more pleased if you challenged my recommendations and tried to find some drawbacks or trade-offs with the design – because I can assure you that there are some.” When you employ the rational methods of Kern or Bell you become aware of some of the trade-offs you have to confront and accept. Being limited to a maximum heat flux is one of them. This contributes to a relatively large diameter kettle. This is why kettles are inherently “obese”. There is a logical way to combat this by making the length of the U-tube bundle longer. But you have a practical limit dictated by the need for maintenance and extraction of the bundle on a yearly basis – at least for inspection. However, with the larger diameter of the kettle shell, you inherit a natural ability to disengage the generated vapor better because of the larger vapor space. Do not forget that you can employ more than 2-passes with a U-tube design. By increasing the tube passes, you increase the velocity and the tube-side heat transfer coefficient as well. This makes for a bigger “U”. However, you still must adhere to a maximum flux – usually from 8,000 to 14,000 Btu/hr-ft2.

I know I have given you a very lengthy reply to your 2 questions. However, I feel it important to discuss the subject of mechanical design because you and other ChE students seem to be without knowledge of the basics in that discipline. As you can now appreciate, mechanical design plays a strong role in your Chemical Engineering process design and even affects your basic scope of work. I hope you buy or obtain Megyesy’s book and study it well. You will be rewarded in your career for doing so, I can assure you.

Art Montemayor