Condensation on Aluminum Junction Box 2

[nlowrie]

I am not very knowledgable on condensation and vapor pressure topics so hopefully someone can help.

We have an aluminum j-box mounted outdoors with a circuit board inside of it. It is in a seal-tite 1/2" conduit system. I am a bit concerned about condensation forming inside and am considering installing a small brass screen breather.

Anyone have experince with this? I need to understand this enough to convince others that allowing an air exchange will prevent condensation, rather than promoting it. Others are in the school of thought that we need to seal this box totally tight, but I know that moisture will get in no matter what. Without some sort of good explanation my superiors won't budge.

The way I understand it is that moist air will get and stay in the box, then condense when the outside air temp lowers. The breather would prevent this by allow the moist air to escape and/or equalize with outside rather than condense?

 

[busbar]

 
The answer probably depends on local conditions, ambient temperature and humidity extremes, and heat generated by components on the circuit board. Local anecdotal experience should count for something, but that can be very hard to sell to your superiors.

Periodic visual inspection may be the practical solution. “Drain/breathers” are typically intended for use in hazardous locations. A thermostat-controlled enclosure heater may be another consideration.

 

[stevenal]

If it is truely sealed, no problem. But how can you tell, either at installation or later on? One way is to presurize the box, and alarm upon dropping pressure. The other option is to assume the seal is imperfect and that water in liquid or vapor form will enter and not leave without some help. This approach requires drains, vents, and heat. Your board may generate some heat. A thermostatically controlled heater might be needed in addition to keep the box above the vapor point. Keep components off the floor of the box, away from the drain. Place vents high and low, so convection past the heat source keeps the air moving. I speak from experience at the Oregon coast.

 

[ScottyUK]

Sealed boxes do indeed breathe through their door seals: they 'exhale' when the enclosure heats up and the air inside expands, and 'inhale' when the enclosure cools. If the enclosure is in a damp or wet environment this can allow moisture to enter as vapour which then condenses in the enclosure. In badly designed enclosures liquid water can collect on the door seal and be drawn in directly as the enclosure 'inhales'. Once the vapour condenses there is rarely enough heat energy to re-vapourise the liquid and force it back out.

Vents or breathers do improve the situation. We were experiencing major water ingress problems with IP67 stainless steel enclosures. We now install an IP55 breather in the base of wall-mounted cabinets, and have noticed a reduction in the moisture collecting in the cabinets so equipped. We use an anti-condensation heater as standard. IP55 does not sound that high a rating, but in the base of a cabinet it is less of a problem.

We are in a coastal environment, and the atmosphere around our plant holds a large amount of airborne water vapour from numerous steam traps. There are other additives in the air from our chemical complex neighbours, none of which are likely to help our cause. It is a pretty hostile place for electrical and electronic equipment.

Good luck!


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If we learn from our mistakes,
I'm getting a great education!

 

[nlowrie]

Thanks for your all you replies. A heater is impractical for this application, but that is definitely a consideration for enclosures with more space.

I think that there is a consensus here that some sort of air exchange is necessary. Makes me feel better about my thoughts here, especially the Oregon post, since this job is in downtown LA. There is a lot of fog and coastal influence there.

There has also been some talk of potting the board, which would work but requires further work and design difficulties.

I guess I could try to learn some thermodynamics(?) to really get down to the science of this one...

 

[mc5w]

You might want to try venting the enclosure to a known dry space that is indoors.

Another thing to try is a few cubic inches per hour of dry compressed air off of your plant air or even an aquarium air pump. There are also silica gel compressed air dryers that regenerate the silica gel either using a small amount of dried air at atmospheric pressure or electric heaters.

If you only need dry air for this application only then perhaps an aquarium air pump and some silica gel cat litter would solve your problem. A coffee can can serve as your silica gel dryer and with enough flexible air lines would be easy to change once per month.

Mike Cole. mc5w@earthlink.net

 

[peebee]

Various random thoughts on the problem:

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I don't see how a breather/drain hole could possibly prevent condensation, but at least it would permit any condensed water to drain out of the enclosure.

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Sealing the box wouldn't prevent condensation unless you sealed dry air in there. If you seal humid air (normal air) in the box, well, then as soon as the temperature drops the humidity will condense. Sealing won't help.

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Space heaters are usually provided for just this very reason, but you say you don't have space for that (and a bigger box is not possible?) so we'll forget about that for now.

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Maybe a chemical dessicant would help? This is the same thing as the silica gel mentioned by mc5w above. But, I'm not sure that you would necessarily NEED an active drying system as he mentions, simply dropping a few of the silica gel packets into your enclosure might be sufficient.

You're probably familiar with silica gel packets, although you might not realize it. . . . They are typically little paper packets marked "DO NOT EAT", that look like sugar packets, the kind you find packed in the box with new shoes, cameras, electronics, etc, and which are also permanently stashed inside hard disk drive enclosures, and which are also used inside waterproof camera housings for underwater photography to keep the lens from fogging.

A dessicant will definitely saturate sooner or later and then it's essentially useless until it dries out again. But, even without an active drying system, it might still help in your situation, at least it would be better than nothing. Sooner or later the ambient conditions will return to a warm & dry enough level that the dessicant will naturally dry out.

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Does your enclosure house heat-generating equipment? Even a small amount of heat will be enough to prevent condensation. This will work even better with a little bit of insulation (but be careful with insulation -- you don't want your equipment to overheat).

This approach is commonly employed on some "lower-end" outdoor power switching equipment -- eg, S&C type PMH switchgear -- just the small amount of heat produced by current through the buswork, and a little bit of fiberglass insulation on the roof of the enclosure, is enough to keep condensation from being a problem.

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Can you provide a small 10-watt light bulb or similar small heat-producing device inside your enclosure to drive the moisture away?

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Can you provide insulating bats or other foamy-type substance to displace the air? If you can generally remove most of the air (by displacing it) then you will reduce condensation.

NOTE: you will DEFINITELY want some kind of vapor barrier around your insulation to keep condensation from forming within the insulation, if that happens, then you'll just rot & trash your insulation. If you can find small bats of insulation fully encapsulated in sheet plastic (or even small pieces of a closed-cell expanded foam, like what wetsuits are made out of, something that won't let the moisture get "sponged up"), and stuff them in there fairly tightly under fairly dry and fairly cool conditions, then this would go far to prevent the entrance of humidity and thereby prevent condensation from forming. A breather/drain tube would probably still be a good idea so that any minor condensation that did form could drain.

Similarly, potting your installation in epoxy or similar would prevent moist air from ever getting to your electricals, but this obviously would prevent you from accessing your electricals in the future.

Heat can be a problem with this type of solution.

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Hope this helps.

 

[ScottyUK]

Hi peebee,

The breather helps because it allows a path to the outside: consider a sealed box to IP67, which is a typical, but fairly high, rating for a European enclosure.

The box gets hot, and the air expands. The pressure inside the box rises, and air slowly bleeds past the seals to equalise the pressure.

The box cools, and where I work it usually gets wet too, and the air inside the box contracts. Air, plus any water on the sealing rings, is drawn into the box by the higher external pressure. Water is now inside.

The cycle repeats, but the liquid water does not get expelled because it is easier for the air to move past the seals than the water. This goes on indefinitely.

The breather helps by allowing air to pass reasonably freely, without creating enough pressure differential for water to get past the seals.

My theory of how water gets into seemingly airtight boxes. Throw stones at it if you wish!



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[digitrex]

I agree with ScottyUK. I have junction boxes previously installed without breathers and we had plenty of them with water ingress. The Junction Box manufacturer recommended to fit them with breathers and after we installed them, the problems are all solved.

 

[TMD]

I'm not real clear on how anti-condensation heaters "solve" the condensation problem. Suppose you close a junction box when it's 30 C and 100% humidity outside, sealing that atmosphere inside. As soon as the temperature drops below 30 C, condensation will occur. So, an anti-condensation heater must be capable of keeping the temperature of air in the enclosure above the maximum possible ambient. That may require some power if the range of ambients is significant, say -20 C to +40 C. In this case, a heater to raise the temperature by 60 C is needed. Am I missing something here?

 

[stevenal]

A heater doesn't eliminate the need for vents and drains, it supplements them. Convection keeps the air moving through the vents ensuring that even 100% humidity air entering will leave at a somewhat higher temperature and lower relative humidity.

 

[jacksonm]

Also don't forget that any seals or other parts of the enclosure which are plastic (most of which are hydroscopic to some degree) will also be happily sucking water into their structure on one side and expelling it on the other given a humidity differential.

 

[itsmoked]

stevenal this was nlowrie's only post ever, and was a year ago....

 

[ScottyUK]

stevenal hasn't posted since 9th August 2004...

Another bad day?



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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[stevenal]

itsmoked,

Were you addressing jacksonm?

ScottyUK,

I thought it was 1 Nov 04 in this thread, 9 Aug 05 in others.

 

[itsmoked]

Yes sir stevenal... sorry. I actually scrolled down the page and typed my answer only to notice I was suddenly in a different post altogether! By the time I got back to the correct post I had the wrong name.

 

[jacksonm]

Eh? Was there a message for me in there somewhere? What did it mean?

 

[itsmoked]

To totally beat a dead horse.. Yes. We don't generally start responding to year old posts..

 

[jacksonm]

Ok, maybe I misunderstood the use of this forum.

I was searching it looking for something else but was interested by the problem and replys here but thought there was some information missing - hence my reply. Now anyone (not nesessarily nlowrie) searching the fora for this subject will have (IMHO) a better understanding of this particular sealing problem.

 

[itsmoked]

Well you probably have a good point. Often peeps reply, even regulars, accidently to ancient posts. Usually by accident. Someone points it out. Our faces get mildly pink though since everyone's done it, no faces get red.