Radiator paint - how thick? 1

[Peter7307]

Painting a raditaor a darker colour like matte black is known to increase the rate of heat transfer from it. My question is how thick does the paint need to be before it begins to act as an insulating component?

 

[i278]

Peter7307,

Back in the late 30's when NACA was investigating painted finned cylinders for air-cooled aircraft engines, they found that with black enamel the best results came from coatings .002" thick, and by .005" thick the performance was about the same as unpainted cylinders. I don't remember the magnitude of the improvement, but it wasn't much.

Of course, that's a different situation and probably doesn't apply at all to yours, but it may be a good starting point.

Regards

 

[richdubbya]

The black paint used is to protect the copper fins from road salt and the elements. This is the reason you dont see aluminum cores painted. I agree that the paint is black to mitigate some of its insulating properties. It is a special paint that is very runny so it will not go on too thick and will penetrate down into the core. I don't know what your application is so I dont know if corosion is a problem. All the racing radiators I have built the cores remained unpainted by request of some customers and others by my judgement. I think if you could get a thin enough coat, black would help but I doubt if it makes a measureable difference and too much paint will cause problems especially on a high fin count core. It will not only insulate but impede airflow through the core.

 

[Metalguy]

Richdubbya,
I'd think that Al alloys need more salt protection than Cu.

 

[richdubbya]

That could be, All I know is I have seen copper/brass radiator cores with all the copper gone, all the fins eaten away by salt. Just brass tubes left. It is commonly known as fin rot in the radiator industry. I have never seen an aluminum core of equal age with any fin erosion at all. I know that copper finned radiator cores are painted for this reason. I owned a radiator shop for 12 years and witnessed this first hand.

 

[hydrae]

Remember your heat transfer,
conduction, convections and radiation
Painting something black only inproves the radiation portion of the equation, In order for black in improve performance it needs to shine on to some thing else, inside the core of the radiator it is shining on the fin next to it not doing much good, so I would expect only a tiny improvement if any.
Meanwhile the paint is acting as an insulator for the convection portion.
Hydrae

 

[Warpspeed]

Spot on hydrae, and if the radiator can "see" hotter objects like exhaust pipes or the sun, a black surface will absorb heat.

Black only works if the surrounding objects are cooler than the radiator core. If it is completely surrounded by equally hot metal surfaces, it will lose nothing through the mechanism of radiation.

The heat loss mechanism will be almost all through conduction into the moving air stream, and black is worth zip.

Corrosion protection is another matter entirely. Something like brown zinc chromate metal primer may be better, but it wold look absolutely ghastly.

 

[Metalguy]

Richdubbya,
interesting observation about the Cu fins being attacked but not the brass tubes--or at least the tubes aren't attacked as badly. But in general, pure Cu is more corrosion resistant than the brass alloys--the more Zn, the worse the corrosion.

Of course there are many other factors.

 

[Peter7307]

Many thanks to all those who replied.
The car is an MBG (hardly the pinnacle of technology!)and the radiator in question is the standard brass unit fitted to it at the factory.
The question came about and simply put those discussing it (including yours truly )had not even the vaguest idea.

Cheers , Pete.