Air circulation and condensation in custom enclosures for outdoor project

Question

I am designing 6 custom enclosures in mild steel to be placed outdoors for 5 months in London UK. They will be interconnected, only one will contains 4 power supplies, all with conformal coating and keeping cool (don't overheat after numerous testing). There will also be two Raspberry Pis inside. Up until now I thought I would use Gore vents to avoid condensation.

Unfortunately the enclosures won't be perfectly sealed, and the enclosures will be painted with a dark colour. What type of fans (or else) should I install in order to: - provide decent air circulation - avoid condensation (fan heaters?)

Should I add vents?

Also, as the boxes will be connected with holes through which I will run cables etc, should I install such vents in each one of them or just in one of them?

Answer 1

Vents do not prevent condensation. Condensation is prevented by keeping he temperature inside the enclosure above the dew point. The only way to do that is to keep the temperature inside of the box higher than the temperature outside the box. To some extent that will happen naturally for any box that contains something that generates heat. Everything that uses electrical power generates heat.

You need to determine how much heat is generated inside each box.

You need to determine the maximum safe operating temperature for each item in each box.

You need to determine how much box surface area is required to dissipate the internal heat at the temperature differential between inside and outside that is tolerable.

If the boxes are exposed to the sun, you need to calculate the heat gained by each box due to the sun.

You need to determine if there will be a problematic temperature difference between areas in each box. If that is a problem, a fan may be needed to circulate air in the box.

You may need to design each box individually according to what is inside.

Answer 2

If usage time is limited as you mention,

you can just use some desiccant to keep humidity within acceptable limits. Let's say your biggest box (180 l air Volume) will suffer a temperature difference of approx. 20 K during a day. It then will cycle 5% of its air content through openings of the housing. This is approx 10 l.

Let's say the incoming air has always 100% rel. humidity at an average temperature of 15 °C and you want to bring it down to 60% to prevent any condensation. So you have to remove 40% of 13 mg/l from the air, resulting in 52 mg water every day.

Given 150 days that's only 8 g water. If you use silica gel, you should not expect it to take up more than 10% water under non ideal circumstances, so 80 g silica gel should be sufficient to keep the box dry. Considering the dimensions of that box, you probably can put 10 times that mass inside to cope with eventualities and false assumptions.

Answer 3

I do not agree that ventilation does not play a role.

It also plays which one. When there is airflow then condensation evaporates faster. The holes will need to be done on the sides, and most importantly from below, so that moisture can drain. You can safely install a dehumidifier in each box to protect yourself. In a short time, you can determine how much moisture is formed inside the box and take action.

And I would put a small fan to blow air so that the circulation goes faster.