heat transfer enhancement issue

[Lc_shi]

Hi sirs
I read some articles on heat transfer enhancement tech issue. For example,apply a small component at the inlet of heat exchanger and cause turbulence at the boundary layer of tube inside. Have you any experience in this field? Is there any good method to increase the K value without cause more pressure loss?

thanks
LC

[US Iceman]

Hi LC,

From my past research any device you insert into the tube will cause an increase in pressure loss. The devices simply add resistance, which is how they create the turbulence.

I have seen twisted metals strips, devices made of wire, and others. They all work on similar principles to disturb the boundary layer. Another effect is the entrance length for fully developed flow. You loose part of the effective tube length until fully developed flow occurs.

Before you spend a lot of time of researching different enancements I would ask: what are you trying to accomplish?

If the film coefficients are greatly different (inside, outside, and wall resistance and/or fins) you only need enhancement on the area which has the lowest film coefficient.

If the inside film is refrigerant you already have good heat transfer film coefficients. If the outside of the tube has air flowing over it, this will be the lowest film coefficient.

The lowest film coefficient controls how high the overall U value will be.

Enahncing one film coefficient while another is very low does not contribute very much benefit.

A quick way of looking at this is: Find the film coefficients of the inside tube and outside tube. Divide them by each other. If the ratio is less than one, enhancement may help. If the ratio is 1, then no enhancements are necassary.

What I'm trying to say is you can spend a lot of money and time on enhanced heat transfer, but if the enhancement does not offer a significant benefit you just end up making an expensive heat exchanger.

[Lc_shi]

The lowest film coefficient controls how high the overall U value will be.

It's right. But the film coefficient are interacted. It's really a very complicated field. For example,evaporative condenser,if I want to enhance the overall U, is it first to improve the water dripping on tube outer surface or strengthen the inside convection heat transfer. The tube wall conductivity resistance is ignored usually. How do you think?

best regards
LC

[nh3simman]

But the film coefficient are interacted.

I could not put it better than the reasoning given by Iceman.

Try some numbers for yourself

1/(UoAo) = 1/(hoAo) + 1/(hiAi)

(approx, I ignore fin efficiency and tube conduction to simplify)

Ao/Ai is in the order of 20 for a fin and tube coil.
ho = 40 W/m2K
hi = 2500 W/m2K (boiling in evap coil)

calculate Uo with the above numbers.
Add 20% to hi and see the effect on Uo.


I saw that Guntner in Germany were claiming something like a 20% increase in coil performance due to tube side fins. I had the pleasure of a raft trip with Guntner down the Isa river and could not get clarity on questions. I guess that's the problem with too much Umpa, Beer and Eisbein!

The real benefit must come from the increase in area but I would still like to see some documented proof.

[US Iceman]

I saw that Guntner in Germany were claiming something like a 20% increase in coil performance due to tube side fins.

I take it the fins were internal to the tube and arranged in a helix. I've seen some of these before when we looked at them for DX shell & tube chillers.

If I remember right, the film coefficient does improve drastically, but when you make the surface ratio correction for the film coefficient you don't gain very much.

On a finned-tube exchanger the side that needs improvement is the air-side. Bu then you have issues of cleaning the fins if the infamous turbo tabs or other air turbulence devices or methods are used. Plus the static pressure jumps up causing air flow loss.

There are no free lunches that's for sure.

[nh3simman]

But then you have issues of cleaning the fins.. Plus the static pressure jumps up causing air flow loss...

Of course, cleaning tubes in a ribbed SATC tube could be a nightmare.

I always wonder if I'm missing something when so many companies are promoting these things.

Kays and London is probably the most comprehensive reference on the air side performance of fin and tube coils. The cutting of slits, louvres, turbulators seem to result in pressure drop with very little heat transfer gain. As you say, no free lunch...