Another UV Light Box (16x20)

[denverjims]

Congratulations ... Really nice design and well executed. Thanks for posting it.

Where did you get the light sockets ... and what are the part numbers or relevant.specifications?

Thanks Jim. 1000Bulbs.com. Search site for 'Levton 23351'. Note that it is for instant start type electronic ballasts like the Fulham I used. A different type ballast (like programmed start [& I think rapid start]) need a non-shunted 13351.
Jim

[polyglot]

Lovely build.

A note about the ballast temp though - that 70C specification is the max case temperature that they can survive and is absolutely no indication of how hot they will actually get nor does it say anything about the heatsinking required. The actual temperature attained is a function of not only how much power they dissipate but also how you heatsink them. If you put them in a sealed box, the temp will climb past 70C and they will fail sometime after it goes above 70C. If you put them in a well-ventilated box, they will probably stay within 10C of ambient.

If you can find out the thermal resistance of something (often printed with a theta symbol, and in units K/W that's kelvins per watt) then you can compute the exact temp rise from the dissipated power.

For a power supply like this, the power dissipated in the supply is = Load_Power * (1 - Efficiency) / Efficiency

For example with a 90%-efficient ballast driving 3x40W=120W of bulbs, Pdiss = 120 * 0.1 / 0.9 = 13.3W.

And say the ballasts have a thermal resistance of 1.2 K/W junction-ambient, then the internals of the ballast will be 1.2 * 13.3 = 16K (16C) above the air temperature that they're immersed in. Say it's a nasty 40C day with an infinite supply of fresh air (this doesn't happen; see below), the ballast will reach 56C, which being less than 70C, is acceptable. If you can find the thermal resistance value, you can determine whether additional heatsinking (and how much) is required.

Similarly, you can compute the flow rate of air required to keep the box internals below a certain temperature. Taking the heat capacity of air (1kJ/kg-K) and density (1.15 kg/m3 at 35C), you can figure out the fan capacity required. Say this box is dissipating 400W of power in total (bulbs and power supplies) and we want to keep the air exhaust within 10C of ambient, that means we need 0.4/10 = 0.04kg/sec of airflow, which is 0.035m^3/sec, or 35L/sec = 2000L/min = 74cfm, which corresponds to a fairly powerful 120mm fan running with NO restriction. You will need more fan than the specifications indicate, because the flow rate drops once you put it in a box and have to drag air around corners and through little holes.

So we have 35C air being inhaled and heated to 45C, which is then used to cool the ballasts. They're going to be 16C above ambient, which is now 61C; still acceptable.

If you get a bad answer (devices exceeding their thermal spec), adjust airflow rates and/or heatsink thermal resistances until you make it work.

[denverjims]

A note about the ballast temp though - that 70C specification is the max case temperature that they can survive and is absolutely no indication of how hot they will actually get nor does it say anything about the heatsinking required. The actual temperature attained is a function of not only how much power they dissipate but also how you heatsink them. If you put them in a sealed box, the temp will climb past 70C and they will fail sometime after it goes above 70C. If you put them in a well-ventilated box, they will probably stay within 10C.

Thanks for the explanation. Given that ballasts are enclosed in a small space in a fixture, I understand better now why fixtures are all metal. The fixture is a big heat sink necessary to keep ballast cool enough.

As to the calculations, I was able to follow some of that. I guess I got lucky as my exit air is not very hot so ballasts must be OK. My design was close enough to others, though, that I figured I would not smoke the ballasts or my house.
Jim

[Jody_S]

Thanks for the explanation. Given that ballasts are enclosed in a small space in a fixture, I understand better now why fixtures are all metal. The fixture is a big heat sink necessary to keep ballast cool enough.


Jim

Fixtures have to be designed to withstand a catastrophic ballast failure without starting a fire or injuring a building occupant.

It was common, with magnetic ballasts, to have to have gasoline or paint thinner on had when working on fluorescent fixtures, because every 3rd or 4th ballast had overheated to such a degree that it had leaked insulating tar all over the inside of the fixture. You needed a solvent-soaked rag to clean up before moving on to the next one.

[polyglot]

Thanks for the explanation. Given that ballasts are enclosed in a small space in a fixture, I understand better now why fixtures are all metal. The fixture is a big heat sink necessary to keep ballast cool enough.

Yep, and folded sheet metal is really cheap for mass-manufacturing purposes.

If you leave it on for an hour then power it down and have a feel of the ballasts, it should give you some idea. Say you test on a 20C day, add 15C to get the performance on a 35C day and if that answer is still OK, your build will be fine.

Also, running the air over the ballasts before the tubes will result in lower ballast temps since they dissipate only a small fraction of the heat.

[Robert Brazile]

Just a note to say that I built my own light box using Jim's as a model and am happy with the result. The build generally went well, despite a short moment of panic when I popped the circuit breaker as a result of a stupid wiring error with the switch. Slept on it, had a good think over a cup of tea in the morning, and realized my idiotic mistake. Put it right and all worked fine. Haven't put it into action yet -- have to pour some carbon tissue first -- but am really looking forward to it.







Jim, thanks very much for publishing your results.

Robert

[Cletus]

It sure is nice to see some great ideas circulating around and people using the info to build these (really nice) UV boxes. Back when I was contemplating building one myself, I kinda lucked into a ready-made Edwards Engineered box, which serves my needs fine. Certainly, had I gone down the DIY route, all this information would have been invaluable - especially polyglot's inspired knowledge of some of the power, temp and flow calculations required if you really wanna do it right!

Thanks Jim, Robert and Polyglot for posting all this great info (all of which is going straight into my little "UV Printing" archive) for those interested in the process, or thinking about putting one of these boxes together.

BTW - Sandy King published a very thorough and comprehensive study on Unblinking Eye pertaining to various UV sources and their use with various alt processes. It's a great read and something I'd highly recommend looking over if you're interested in this area of photography.

Here's the link: http://unblinkingeye.com/Articles/Light/light.html

[jp]

Looks great Robert!

[Tin Can]

Great, but I need it today. My partner is coming to try Carbon, we have the Sun, cold Sun.

Thanks for all the info, OP and everyone.

[Robert Brazile]

Thanks, all.

JP, meant to call you out as well, as I started on this path by looking at your box first, then when Jim posted his, took that into account also. It's so helpful to have the examples shared here to give guidance when embarking on one of these projects. Thanks to all who do so.

Robert