Need help with heat transfer...

[ryanbabs]

I am actually a EE, but I'm helping my co-worker who is a ME try to figure this out (I haven't had too much heat transer experience) ...

We have a 25,000 ft long pipe with an O.D. of 18" and I.D. of 16.875". The temperature of the water inside the pipe is 65 degrees F, and the ambient temperature of the air was decided to be 20 degrees F. We also figured a 15 mph wind hitting the pipe. I figured laminar airflow as well, although I'm not sure if turbulent would be more accurate. I have looked through my old school books, and even my FE prep book, and not much there. Any help appreciated.

Also, we figured the thermoconductivity of the piping to be 46.43 BTU//hr/ft/F (SCH 40 iron), and water in the pipe, although actually there will be a 15% propylene glycol mixture.

 

[ryanbabs]

Sorry guys, I wasn't clear...I'm trying to determine what the exit temperature would be around. Any guidance is appreciated.

 

[Montemayor]

ryanbabs:

Instead of telling us your background and that of your buddy, why don't you tell us the vital engineering basic data? Tell us what you are trying to do and what are your parameters. For example:

1. You say there's water in the pipe; then you say "although actually there will be a 15% propylene glycol mixture". Just what exactly is there going to be inside this pipe?
2. What is this fluid being used for? What are its pressure and temperature at the inlet?
3. What are the design fluid conditions at the end of the pipe?
4. Are there any pipe elevation changes in the the 4.7 mile trajectory?
5. What is the fluid's flow rate?
6. Can you insulate?
7. Can you heat trace?

In short, you've given us the typical type of academic problem statement: just a general, simplified description without any actual real data. Is this an academic problem or an academic test or example?

If you are serious, please help us help you out by giving us all the rest of the story.

 

[Ashereng]

Ryan,

Is this a homework question/academic problem for you/your friend?

One reason why I ask is that typically, in industry, water pipes are either burried, or insulated and usually heat traced in areas of 20°F temperature, for that long a run.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[ryanbabs]

No, this is NOT a homework problem...I graduated 2 years ago. This is not a typical problem I work on.

I am trying to help out my co-worker whose actually out of town...all the information I have given is all he has given me. I've never done a heat trace. He was working on that part, and asked me to do this.

The flow rate is 2-1/2 ft/sec.

The project is for a Water Treatment Plant. As far as I know, there is no insulation, and I remember asking him about that. I am also assuming that the pipe is not buried. I am trying to solve this using water, although there is the mixture (forgive me for not totally understanding what's exactly in the pipe...15% glycol and I assumed the rest to be water?).

All I'm asking is for a little direction. I'm not asking you to do my work. I've solved for the Reynold's, found the heat transfer coefficient (h), Nusselt, etc. Where I am getting confused is solving for the heat transfer rate and the exit temp, as they are dependent on each other, based on the equations I have.

Also, I am confused on how to "tie-in" the heat transfer coming from the pipe to the environment (from the external air flow) and from the heat transfer from the flow. I assumed to sum these up, but I'm not sure this is correct.

 

[25362]

A short-cut method I've seen used for cases like this one:

1. Assume the pipe-wall temperature equals that of the water stream.
2. Assume all the heat transfer resistance is in the air film (boundary layer) at the pipe wall.
3. Take the air film temperature as an average of the pipe-wall and the bulk air temperature. In this case (65+20)/2 = 42.5^oF.
4. Calculate Re = D.v.[ρ][÷][μ] and Pr = C_p.[μ][÷]k (most probably around 0.7)
5. Estimate the htc "h" from h = (k/D)(C)(Re^m)(Pr^0.33)
where k = thermal conductivity of air; D = pipe external diameter; for 40,000<Re<400,000 take C = 0.0266, and m = 0.805. A rough estimate gives h = 25 W/(m².K) for air at 15 mph. Please note that the value of h for still air is much smaller (about 3-4 W/(m².K)
6. Estimate the heat loss per unit length from Q/L = (h)(3.14)(D)([&Delta;]T)
where [&Delta;]T would be 65-20 = 45^oF = 25^oC. In this case it would be around 920 W/m, at least for the first pipe section.

To get a more acceptable end temperature you may divide the pipe in sections and renew the computation using the lower water temperatures resulting from the estimated sectional heat losses.

I'd appreciate comments from the experts.

 

[stanier]

http://www.pipeinsulation.org/

This site allows you to download suitable free software

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[MortenA]

Depending on how important this result is i think you should consider if calling upon professional help isnt more appropriate?

It seems like this is teory that you and your friend have not earlier experience with and not much background to start with?

Either you should take the time to study the subject throughly (take a course or classes or read a standard textbook on the subject (Kerns Process Heat Tranfer is one on them but a rather heavy one) - or call upon a consultant imho.

BTW Perry sec 10 also give some methodes for establishing outisde heat transfer coefficient for different geometries.

Best regards

Morten

 

[25362]

For references I suggest you pay a visit to the ChE issues:
Sept. 1993, Oct, 1993 both articles under the heading Calculate the heat loss from pipes by A.R. Konak, and Dec. 1995, with Spreadsheets for heat loss rates and temperatures by Butch Bornt for the iterative model procedure to speed up the analysis.

 

[ryanbabs]

Thanks a lot for your help 25362...I actually found some of his old notes from college, and I figured it out (hopefully correctly).

Morten, yes, you are correct, I have not had much training in heat transfer, as a lot of us EE's have not. My FE exam prep book only had 2-3 chapters on it as well.

Maybe I need to get one off E-BAY?

Thanks guys.

 

[ryanbabs]

...that was 2-3 pages, not chapters.

 

[JLSeagull]

Download John H. Lienhards' IV and V textbook A Heat Transfer Textbook from

http://web.mit.edu/lienhard/

http://www/ahtt.html