current carrying capacity. 2

[zub]

Hi all,

Everyone knows the current carrying capacity of the busbars increases if the busbar is painted. WHY? it looks simple but i couldn't find any reasonable solution. Can you help me please?

zub.

 

[BJC]

Wasn't everyone. I have never heard that.

 

[buzzp]

I have not heard of that before either. But I am not into Power much. Seems like it would be the other way around since the paint will not allow the heat to radiate away from the busbar as easily as with no paint. Current capacity is only a function of heat.

 

[redtrumpet]

http://www.powellelectric.com/

Powell Technical Brief No. 24

"Surprisingly, a bus bar covered with insulation generally runs cooler than an equivalent bare bus bar, because the usually darker color of the insulating material is a better radiator of heat than the shiny surface of a bare bus bar."

 

[jwerthman]

I is quite common for high-current isolated phase bus duct conductors to be painted black to increase the emissivity of the material, resulting in better radiation of heat from the bus to the surrounding air.

 

[wvbht]

And not just any paint, either.
I remembered a chart I saw some years back. In the Third Edition of the GE SCR MANUAL, 1964, PAGE 280, Section 16.6, Radiation. This has do do with heat sink calculations.
Table 16.3
Material.... Emissivity

Anodized Aluminum.... 0.7-0.9
Commercial Aluminum (polished).... 0.05
Aluminum Paint.... 0.27-0.67
Commercial Copper (polished).... 0.07
Oxidized Copper.... 0.70
Air Drying Enamel (any color).... 0.85-0.91
Oil Paints (any color).... 0.92-0.96
Lampblack in Shellac.... 0.95
Varnish.... 0.89-0.93

"Table 16.3 indicates the wide variation in emissivity for various surface finishes. In free convedtion cooled applications, the radiation component of the total heat transfer is substantial, and it is therefore desiable to maximize radiation heat transfer by painting or anodizing the fin surface"
"Note that oil paints regardless of color improve surface emissivity to practically an ideal value (unity)".

It would appear that the ampacity of a buss bar could be increased by using the proper surface finish. I assume that the "oil paints" are regular oil-based paints.
It might be worth while for new installations to paint the buss bar(s); I will leave that calculation up to you, it's 3:45 AM, the calculation is lengthy and the assumptions are many, and I need some sleep.
I hope this will help.

 

[wvbht]

A PS to the message I just sent:
Section 16 gives detailed formulae and nomograms for the calculation and design of multi-fin heat sinks. I should think that the same could be used for buss bars, too.

 

[zub]

Thank you all. it is quite clear now.

zub.

 

[BJC]

jwerthman

When did they start painting isophase bus? I have been away from that stuff to long it looks like? Are they doing on smaller units such all the gas turbine units there building now?
The last units I worked with were in the range of 16- 25 KAMPS. They had force air cooling, air blowing out B phase and back in A and C. The cooling unit had the fan and a chilled water coil.
The bus was bare aluminum, when the covers were welded on thas the last time you saw it.
Outside of the bus was painted to look like the generator inside and the transformers outside.
Thanks
BJC

 

[buzzp]

Thats interesting. Learn something new everyday. I know emissivity is the measure of a the way a color or surface radiates heat (light) naturally but never thought it had anything to do with the ability to radiate generated heat.

 

[jwerthman]

BJC - I don't know when the practice of painting the bus started, but it's been around longer than I've been in this business (30+ years). I just worked on a renovation of a 35-year old 600 MW unit that has isophase bus just as you described, except the aluminum bus is painted black.

Most of the isophase bus that I have seen in the past 10 years on new combustion turbines has been painted, but not all. I really don't know why sometimes it is painted and sometimes not.

 

[motorman]

An interesting thread on this can be found at:

thread391-28018