View Full Version : gravity coil

12-10-2008, 05:29 AM
Hi Folks

It's been a while since I have been around here. I see plenty interesting topics to read but now I have a problem and need some help.

I have been searching for design info on natural convection cooling coils (gravity coils) and come up blank on all counts.

I have searched heat transfer text books, ASHRAE Handbooks, manufacturers data sheets, web...

The only text book cases I find are

vertical or horizontal plates
vertical or horizontal cylinder

But, what are people using to calculate the duty of a fin and tube coil with natural circulation?

The closest I have found is a detailed review of natural recirculation systems by Harwell and the best match here is a tube bank.

Could it really be that there have been no studies on these coils.

I am expecting a correlation for the Nusselt number. This could be of the form

Nu = C (Gr Pr)^n

where C and n are determined by measurement.

Any info or feedback would be valued.

18-10-2008, 12:12 PM
Someone must know how to calculate a gravity coil!

US Iceman
18-10-2008, 04:13 PM
It's nice to see you back here and posting again.;)

I have never had to sit down and do what you are asking. Is this an academic exercise you are looking at?

My first impression would be based on the buoyancy effects due to air density differences. As the rate of air movement begins in the room the Reynolds number would reach a mean average, which could be used for a Reynolds number. Then you start the heat transfer calc.'s for a finned tube.

This is a really good question though. I do not think I have ever seen a study/report on this either.

18-10-2008, 10:37 PM
I seem to remember this information being in the old ASHRAE manuals from the 1950s.

19-10-2008, 06:42 PM
Someone must know how to calculate a gravity coil!

Hi nh3simman:)

I think that Dossats Principles had some information in the version around 1989


25-10-2008, 02:09 PM
Thanks guys,

My earliest ASHRAE books only go back to the 80's but I will check with my more senior friends. I did notice that good design info was dropped over the years.

I took on this task for a US manufacturer, thinking that it would be easily to find info. I should have checked the books first!

In light of the lack of design info, I have proposed a simple test using a coil, hot water and a short vertical section of duct. I will run it at my local Uni as a student project and have some interest from an Australian Uni to do similar.