pls comment

[Lc_shi]

Hi dear sirs
We use copper tube for evaporative condenser. As we all know,the thermal conductive resistance is mainly at tube outside. I consider if we can add copper fin to the tube (like normal cooling coil)and increase the outside surface and get more heat rejection. Is it feasible ? May be a silly thinking,don't laugh at me,only make your comment- thanks

regards
LC

[Argus]

I will confess that I am not an expert on Evaporative Condensers, but increasing the thermal conductivity without impairing air flow is always a good idea if you can achieve it.

But please consider one further point.

Evaporative condensers are prone to poor maintenance in the field. So much so that laws exist in this country (UK) and probably elsewhere, regarding their use. In this country, the laws demand notification to the local authorities and a maintenance system in place to counteract Legionella.
This means that the water system must contain biocide chemicals and other inhibitors.
So my point is to make sure that your materials in contact with the water are not affected by the water treatment chemicals in use and there is no cathodic reaction with the metals.
Most condensers, to my knowledge, have protective barriers on the metal parts.

I cannot say what chemicals are used. You will need to do some research. Water treatment is a separate discipline.

Hope this points you in the right direction.
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[US Iceman]

HI LC,

Secondary heat transfer surface on evaporative condensers is a bad idea. Please let me explain.

Evaporative condensers are difficult to keep clean even with good water treatment. Adding secondary heat transfer surface to the tubing (primary surface) increases the potential for very fast accumulation of minerals, etc.

The total dissolved solids (TDS) content in the water must be maintained at sufficiently low level to reduce the accumulation of minerals on the coils.

This is accomplished by water treatment chemicals and blowdown of the sump water.

Also, the actual water temperature must be low enough to prevent rapid accumulation. This is one of the reasons why evaporative condenser should be selected for operating conditions no higher than 35C.

Lower condensing temperature help to reduce the scaling problem. Conversely, higher condensing temperature make the problem worse.

Some HVAC manufacturers used evaporative condenser on AC systems back in the 1950's & 60's. They sized the condensers for 40C or higher condensing. Guess what??

After a few years, the coils were solid blocks of concrete!

In theory, the use of enhanced primary surface can increase the overall heat transfer per lineal meter of tubing.

Additional manufacturing concerns are:

bonding of the fin to the tube (fin root contact resistance)

bend radius may become too large if fins are included on the bends

Fin effectiveness (how much of the fin is useful)

Almost all of the evaporative condenser coils made here in the US are dipped in molten galvanizing after fabrication.

I would recommend you proceed carefully and perform very good operational tests.

Regards,
US Iceman

[Lc_shi]

Iceman
I'll use copper tube and copper fin. Per our application experience,the fouling and scaling is dpending on the water physical and chemical property. There is not much serious fouling and scaling in practical use. But for the Legionale ,I know very little.May be it's a real problem
thanks

LC

[Argus]

Iceman
........ But for the Legionale ,I know very little.May be it's a real problem
thanks

LC

It's a huge problem. Mainly caused by lack of maintenance.


As I said earlier, in the UK there have been people killed by it and there are laws on the installation and siting of cooling towers.


http://www.hse.gov.uk/legionnaires/

http://www.conduct.org.uk/

Chemical dosing in all towers is mandatory in this country. The following link is about a case where maintenance was suspended to save money. Seven people died as a result. The legal case is still going on.

http://news.scotsman.com/topics.cfm?...2&id=432642005

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