13 replies to this topic

[merac]

Hi,

I would like to calculate the overall heat coefficient transfer of an air cooler from individual heat coefficientes (tubeside and air side) and foulings.

However I have the following problems:

1) I haven't found any information about the calculation of foulings . Does anybody know any equation for that? (depending on the velocity, temperature, etc)

2) Can I use the individual heat coefficient correlation that is used in conventional heat exchangers for the tube side in air coolers?
(ht=0.023*Re^0.8*Pr^1/3*Kt/Di where ht is the individual heat coefficient, Re is the Reynolds number, Pr the Pranldt number, Kt the thermal conductivity and Di inner diameter)

3) How can I calculate the individual heat coefficient for the air side?

4) How can I calculate the wall thermal resistance?

Any information about the U calculation of an air cooler would be very appreciated.
Thank you in advanced.

[ankur2061]

Merac,

Check out the following website for Air Cooled Heat Exchangers:

http://www.hudsonpro...nfan/index.html

This website gives almost all answers related to ACHEs.

Regards,
Ankur.

[merac]

Thank you for your reply but I have visited the website you recommended and I haven't seen any information/about how to calculate the Global Heat Transfer Coefficient from the individual heat transfer coeficients and foulings.

The only information about the global heat transfer coef. are the estimated values for diferent fluids
and situations but what I need is the formulas to calculate this value.

Maybe, I haven't looked in the right place. Where is this information ?

Thank you in advanced

[srfish]

You can find the equations in "Process Heat Transfer" by Serth. If you do not have ready access to it , let me know. Then I could dig it out of our software for Air Coolers.

[merac]

I don't have access to this book "Process Heat Transfer" by Serth.
Can you send me the chapter of air coolers please? I would really appreciate it. If you prefer to send it to my personal mail, please ask me.

Which is you software for air coolers?

Thank you in advanced.

[srfish]

There are over 50 pages in Serth concerning air coolers. That would take too long to scan and covert to .pdf.

Here is an equation for overall U:

U = [ (Ri + Tfoul)Aoi +wallr +Ro] ^ -1

where:
Aoi is the ratio of outside fin surface to inside tube surface
Tfoul is inside tube fouling
wallr is tube wall resistance

This is from our in-house software.

[Zauberberg]

GPSA Databook gives very good and simple approach for calculating individual film coefficients. If you don't have it, get it - is that good.

[sheiko]

merac,

The following link allows you to download a free textbook that will help you for sure. Dig it.
http://webwormcpt.bl...t-transfer.html

[Zauberberg]

Also look at "The Basics of Axial Flow Fans" from HUDSON website. It doesn't give the information about thermal design of fin-fans, but plenty of other design/operational data is available:

http://www.hudsonpro...te/flowfans.pdf

[merac]

Thank you srfish, but I've already known this formula to calculate the overall U.

What I didn't know was the formulas to calculate the individual heat transfer coefficients for the tube and air side (Ri, Ro).

[merac]

Thank you a lot sheiko, I have already download this free textbook and it seems it has a lot of useful information! I'm going to study it carefully!

[srfish]

You can find the tube side heat transfer coefficient in about any heat transfer text.

The air side heat transfer coefficient is usually the Briggs and Young correlation. It is from a "Chem.
Eng. Prog. Symp. Ser. 59"., 1963. You have to be careful and not apply this equation equally to imbedded fins and tension wrapped fins. There is a contact resistance that has to be evaluated for the tension wrapped fins.

[merac]

You can find the tube side heat transfer coefficient in about any heat transfer text.

The air side heat transfer coefficient is usually the Briggs and Young correlation. It is from a "Chem.
Eng. Prog. Symp. Ser. 59"., 1963. You have to be careful and not apply this equation equally to imbedded fins and tension wrapped fins. There is a contact resistance that has to be evaluated for the tension wrapped fins.

Hi srfish,

Related to the tube side heat transfer coefficient I have calculated it with different correlations:
-one is speficic for air coolers (tube side)
-the other is typical for the tube side of shell and tube conventional exchangers
and the result is totally different, reason why I had some doubts in applying the normal correlations for ht of shell and tube heat exchangers to air coolers tube side. The problem is that I haven't found much information about possible ht correlations in air coolers..

Related to the air side heat transfer coefficient, can you send me the Briggs and Young correlation, please?
I'm calculating the air heat coefficient with a correlation of the following book:"Industrial chemical process design, by Douglas L.Erwin" and it depends on the air cooler face area airflow(lb/hft2)

Thanks in advanced, I would really apreciate if you can send me the correlation for the air side you talked about.

[srfish]

Merac

I never heard of a tube side heat transfer specifically for air coolers. Why don't you use Seider-Tate and go on.

Sorry, the only Briggs and Young information we have is embedded in software.

Good luck