Heating Coil in a tank 4

[ChemE2912]

Hi there,

We need to design a heating coil in a tank containing a heavy crude oil, in order to maintain it at 160 - 170 °F.

Can anyone provide us the equations for estimating the heat transfer coefficient on the oil side?

We intend to use a coil which is to be located at the tank bottom. The tank continuously receives production and the product is then pumped out for further processing. The heating medium is MP Steam (550 °F).

Any help is well appreciated.

 

[EGT01]

ChemE2912,

Might be easier to just suggest that you get a copy of "Process Heat Transfer", by Donald Q. Kern, McGraw-Hill Publishing. He has example calculations for tank heating coils of the "bundle" and "pancake" variety.

 

[Montemayor]

ChemE2912:

I've had the opportunity of employing, designing and operating both cooling and heating coils in industrial applications in the shape of Archimedes Spirals (pan cakes), single helix, and "trombone" type. As EGT01 suggests, I often resorted to Kern's classical "Process Heat Transfer". However, you are in for some frustration if you are going to dedicate yourself to finding film coefficients. I am going to assume that by "the heat transfer coefficient on the oil side" you mean the Overall Heat Transfer Coefficient, "U". If you are trying to calculate individual film coefficents, you are going to waste a lot of time and generate a lot of grey hairs - in my opinion.

First, you are confronted with a situation (to quote Kern, himself) where there is dearth of data in the literature on the transfer of heat to helical coils by free convection. That was in 1950 and, I'm afraid, still the situation from a practical standpoint. The effects of free convection in a tank are varied and difficult to predict due to many factors and the conditions for sediment, fouling, property changes, and proper condensate drainage in the coil are factors that can change the film one way or another. It is all theory when you finally obtain an equation. Poor Don Kern and his buddy McAdams did the best they could, but they never presented any hard, workable equation. All they were able to come up with were "suggestions" and correlations of offerings from others that they neither backed up or recommended for design use. They (like authors or today) never present a relationship or equation that they can firmly recommend or back up. But that's not their fault. The subject is so complex and lacking in research results that it can only be dealt with theoretically by them. The answer to designing a practical heating coil subject to natural convection is found in empirical data: a proven "U". The problem then becomes one of finding as similar an application as yours and using that "U". That's why Kern published his listing of approximate "Us" on page 840. There, you will find that for steam heating, the U can vary from 5 to 100 Btu/hr-ft2-oF. But be aware that the convection effects are not defined. So, in the end, you wind up using a conservative "U".

One important point to bear in mind: plan on having a mechanical engineer working with you to ensure that you can design and build a coil that produces positive condensate drainage to the outside steam trap without covering too much coil inside surface up with liquid. You want to have as much inside area available for steam condensation as you possibly can. Additionally, any convection that you can create or stimulate on the oil side will be of some help. I wish I could help you with more information; it's not a pretty picture to try to illustrate with powerful mathematical expressions.

Art Montemayor
Spring, TX

 

[stanier]

Suggest you contact Brownfin tube and get them to design and supply a heating system.

 

[rmw]

Tranter makes platecoil type material, for applications similar in nature to the one you describe, and I always found their technical manual for this product a good source for "off the wall" type "U" values of this type. (they may have a product that would do the job) That is where I would look first in your shoes.

It has been 10 years since I obtained my last one, so I do not know if they still publish it or not, and I did not check their website prior to making this post.

Good job Art. Especially the part about getting the condensate out.

rmw

 

[m777182]

In VDI Waerme Atlas (Springer 2002) page Gc1...Gc2 there are formulae to calculate heat transfer from a flow in the heat coil taking into account coil geometry, Re and Pr of the heating fluid. My version of the book is in German language but you may find an English edition.
m777182

 

[25362]

A qualitative comment: for large temperature differences as in this case, convection currents may be strong. However, depending on the tank size, the oil properties, the coil geometry, and the lots' volume, mechanically-aided circulation may be advisable.

 

[TangoCleveland]

If you want to get away from using MP steam in a crude heating coil, take a look at

http://www.infernotherm.com.

This company uses direct firing of gas in a firetube heater with a very low heat flux density on the outside. I've used these in asphalt service to 450°F with good results.

Larry

 

[chem55]

Hi,

I think this link would be helpful to you for typical U value.
Just take a look at

http://www.the-engineering-page.com/forms/he/typU.html

Cheers..!
Chem55

 

[saxon]

ChemE2912, If you can get your hands on one, there is an old Crane Engineering Data book (catalog vc-1900 copyright 1976, Crane Co.), that covers just what your looking for, "heat transfer through pipe walls" and "the design of pipe coils for heat transfer" plus a bunch of other important process piping engineering stuff. Try and get a copy. I have used it many times for designing pipe coils for cooling and heating and believe it or not, you can do the calculations manually with simple math as opposed to working in the realm of the higher level calculii.

Good Luck.
saxon

 

[TBP]

saxon - you're absolutely correct. There are a huge number of old publications with relatively simple, accurate calculations that provide the solution to problems just like this one. In many companies, the ability to peform elaborate computer gymnastics have come to mean more than fundamental engineering skills.

 

[Montemayor]

saxon:

I think you've gone directly to the core of identifying an acknowledged reference for this problem. I remember that I had this very catalong that you refer to and I concur with what you've recommended. My copy has been lost for some years now and I don't believe it's feasible to believe that I'll find it or that I'll come across another person who has a copy.

Can I request a copy from you? A xerox copy would be fine, and I'll compensate you for the cost & postage if you have the opportunity or occasion to make a copy. I consider this information very important and also very difficult to obtain. My copy disappeared when I loaned my book out to colleagues. Thanks for reminding me that this information exists.

Art Montemayor
Spring, TX

 

[TD2K]

Art, did you get a copy? I believe I also have exerpts out of the old reference Saxon mentioned.

 

[Montemayor]

TD2K:

Thanks for the kind note. I haven't received a reply to my request and would be very grateful for a copy. I consider this kind information very important and increasingly difficult to obtain as time goes on. I would like to compile this information into my heat transfer Excel workbooks as I am compiling all my past engineering information into electronic spreadsheet files.

Needless to say, I would be indebted to you if you could send me a copy to: 17002 Hillview Lane; Spring, TX 77379.

Allow me to compensate you in some manner, since I consider this a special favor. Thank you once again.

Art Montemayor
Spring, TX

 

[25362]

The Chem. Eng. issue of Dec. 24, 1973 brings a Plant Notebook article on Designing Heating Coils For Tanks By Nomograph by J. Otazo, Ethyl Corp., which unfortunately I don't have with me, that may be helpful.