Temperature Rise 5

[Vladpl]

Hello all

I am working for a transformer company in Melbourne Australia.I mainly work on designs of various transformers and power supplies.

When designing a transformer I have to calculate tempereature rise (which is no problem). My problem is that if that transformer gets encased in an enclosure, that enclosure will have a certain temperature which I need to calculate due to the reasons that Australian Standard states that enclosure must not exceed 70C.

Please if any of you know how to calculate enclosure temp from a transformer temp rise reply to this thread. Thanks for your time in advance

 

[ko99]

Heat will be transferred from the transformer to the enclosure via conduction, convection, and radiation. It's difficult to calculate the enclosure temperature this way: it's very dependant on geometry, mounting details, materials, interfaces, etc.

It's easier if you can assume the heat is uniformly distributed over the enclosure surface AND you can ignore solar heating. Then you can estimate the deltaT from the enclosure to the external air using convection and radiation from the enclosure surface.



ko (

http://www.ecooling.biz)

 

[itsmoked]

ko99 that would be easier... bad results! But easier.

Our company mounted EVERYTHING in boxes for bulkhead and pole mounting in oil fields so we had to do studies on this.

If the air is just convection inside the box then essentially only the top surface and the top few inches of the sides are where it all happens. Hence the box acts like it is mostly insulated on the lower 3/4ths.

Also if you have direct solar on the box.... this is very, VERY, bad. This can get the box to 70C with no power going to it. You must provide shade, PERIOD.

What we found worked really well was in the standard Hoffman type box you have the panel dropped in on the studs right? You gasket the edges and the bottom. You borris-out a large hole at the bottom and mount a fan/s over the hole blowing into it. This sets up a strong circulation in the enclosure, air going in the bottom of the panel and shooting out the top against the box's inside top.

Set this up with a 100W bulb in the box. You will find the air totally stagnates with the top very hot and the rest of the box at nearly room temperature. Then energize the fan. In a few minutes the entire box surface will be warm. The top won't be any warmer than the sides.

Course the big negative is a fan that will eventually fail. We usually put a thermostat in the box to detect fan failure and signal for help or we tied a fan signal into the SCADA usually in the box.

The really high performance systems we would put the full Hoffman as described above entirely inside another one. We would then add fans to the external box which would circulate high velocity air on the outer surface of the inner box this allowed the inner box to reject more heat then pure external convection will. If you do that you can forgo the shade because the outer box remains within a degree or so of the ambient.

So to summarize if you are talking about an outer box Vladpl you should really do a test if you can. If not then worst case calcs would be calculate the energy dissipated from a horizontal flat surface the size of the box's top. It won't be pretty.

 

[Vladpl]

Thanks guys for helping me out on this one. Yes I see what you mean. I thought the best or easiest way would be to just make a transformer and encase it and then perform the tests. I will have vents in the enclosure to achieve chimney effect. If any of you have any formulas of how I could calculate these things please reply. Thanks again for your help on this subject.

 

[itsmoked]

Off hand I don't have the formulas Vladpl, but do look under "solar energy" stuff because chimney effects were always of interest and formulas exist for calculating chimney effects.

When I worked for the California Solar Energy Commission on judging solar home designs we used that information constantly. Some homes used it for driving ventilation. Black sheetmetal pipes. Heck, I use that on my garden shed.

 

[ko99]

For the record, I agree completely with itsmoked. My comments applied only IF you could ignore solar heating (ie, if your product is not outdoors).

The calculations are not trivial. There are many heat transfer paths but some may be negligible -- it really depends the specifics of your design. This is why I was hoping you could simplify your problem. Otherwise you'll need to calculate:
1. convection from the transformer to external air (via vents)
2. convection from the transformer to the enclosure
3. radiation from the transformer to the enclosure
4. conduction from the transformer to the enclosure
5. conduction from the enclosure to it's mounting structure.
6. radiation from the enclosure surface to the external air
7. radiation from the sun to the enclosure surface
Then put these together to find the encloure temperature.

Personally, I find it easier and more accurate to build a CFD model, or if possible run prelimary tests. But if you want to try calculating it yourself, go for it! You'll learn a lot. You'll need a heat transfer textbook. Try this for starters:

http://web.mit.edu/lienhard/

http://www/ahtt.html

ko (

http://www.ecooling.biz)

 

[itsmoked]

Nice list ko99!

Do we need to add convection from enclosure surface to external air?

 

[ko99]

Yes, definitely. Thanks

ko (

http://www.ecooling.biz)

 

[Warpspeed]

External air vents will be a mixed blessing. Entry of dust and vermin can slowly build up internal residues that can then become damp and promote all sorts of long term corrosion or electrical leakage problems. Think in terms of nests of ants, bees or spiders. Bronze fly wire quickly chokes with dust and cobwebs, so is not a solution. Also do not forget ingress of wind carried salt spray in coastal areas.

A hermetically sealed inner enclosure will be much better, provided there is enough total exposed surface area to transfer out all the heat. Usually not a problem. The transformer itself will generate some internal air convection, but a very small fan to stir the air would make a huge difference if total size must be reduced to a minimum.

As previously mentioned, direct solar heat gain could be considerable on a windless summer day, and that can be a significant problem too.

My approach would be to enclose the transformer in a completely sealed inner hermetic enclosure large enough to allow sufficient total cooling surface area. Then enclose that into a rain proof external outer casing, that will form some sort of natural convection chimney. That would shield the internal enclosure from direct solar radiation, rain, and protect the internal enclosure from mechanical damage (including vandalism).

Your particular situation may not be this bad, but my own experiences with Telecom, and the railway industry, and various solar projects, suggest some interesting problems in a particularly Australian environment. (I am in Melbourne).

 

[itsmoked]

The company that I used to work at, created this product to deal with the problems we've been discussing in an industrial PC. Essentially to get the heat out, without evil fans, a large cast heatsink is used to conduct the inside load to the outside air. This works well. But it is a design headache to get the casting right.

http://www.digitaldynamics.com/pdfs/ts800.pdf

 

[ko99]

Conducting directly from the heat source (transformer) to a finned enclosure is a great way to cool the transformer but could be a problem if you're also trying to keep the enclosure surface cool, as Vladpl mentioned, I assume for safety.

ko (

http://www.ecooling.biz)

 

[Vladpl]

Thank you all for your inputs. It has been of a great help to me.

 

[Vladpl]

ko99 yes it is maninly for safety reasons due that the enclosure will be indoors and if someone would touch it they shouldn't get burned.

I know I could perform test on a trasformer and enclosure to measure the temperatures. This method would require lot of trial and error as the space is limited and bigger transformer is not an option. Also mounting a fan is not permited.

I thought there would be a few calculations I could perform and get a ballpart figure. It seems that the quickest and easiest way would be to go trough trial and error method since I can't even find any formulas around. Maybe I am looking in the wrong spots.

 

[ko99]

Vladpl,

The link I mentioned has the formulas. Conduction formulas are page 141, convection on page 424, radiation page 525. They are fairly general and may require a lot of brushing up, depending on your background.

It may seem a bit daunting, but as I said earlier, it can be simplified into just two heat transfer paths IF you can assume the heat is uniformly distributed over the enclosure surface (or a subset of the area, as itsmoked suggested) AND you can ignore solar heating. Then you can estimate the deltaT from the enclosure to the external air using convection and radiation from the enclosure surface.

If not, then I recommend a physical model (test) or a CFD model (software).


ko (

http://www.ecooling.biz)

 

[BrianR]

itsmoked,
Your post is the most practical and useful I have read on this subject. That's worth a star.

 

[itsmoked]

BrianR; Thanks! Glad to have provided some knowledge.

Vladpl; Now that you have finally come clean.... If you are boxing this xfrmer just for safety, why do ANY heating calculations? Just stick the thing in a box with a 2 or 3 inch clearance then cut out the bottom and top and replace them with expanded metal that you can't stick fingers through. The stack effect created by the 3 inch gap will increase the transformer's heat transfer to the air over just the transformer. Hence no cooling calculation!

 

[ko99]

Maybe I'm missing something (wouldn't be the first time...) but I thought Vladpl's concern was the enclosure temperature, not transformer cooling. They are closely linked of course, but I think he just wants to make sure the surface doesn't exceed 70C, a safety limit to avoid burning skin.

Either way, vents on the top and bottom have the best chance of keeping both the enclosure and the transformer cool. Immersed liquid (oil or flourinert) or conduction to a finned enclosure may result in a cooler transfomer but hotter enclosure.

ko (

http://www.ecooling.biz)

 

[itsmoked]

ko99 I'm probably missing something..

I thought he had a transformer he doesn't want people to touch for fear of being burnt...

Mount it in a screened area that prevents it being touched but allows it to be cooled directly..

What am I missing... sumptin no doubt.

 

[ko99]

Unless he has a lot of room, the enclosure itself can exceed 70C. Hard to tell, since we don't know anything about the environment and transformer type...

ko (

http://www.ecooling.biz)

 

[Vladpl]

Thanks you all for your input.

I can design a transformer with a low delta-t but the problem is lack of space. Transformer will be encased into a metal enclosure with vents. I am not worried if the transformer has a temp rise of a 80-90C. All I want to acheve that enclosure surface would be below 70C. Problem with people is that they try to fit 2 gallons in a 1 gallon drum.

Sorry about a long post like this and thanks again for all your help.