Air pressure needed to keep rain out of an enclosure with openings on its top face.

[Argurios]

Hello apologies if the answer to this exists in another thread.

I would like to pressurize a box with air to prevent rain from entering said box through 4 slots, each 4mm wide and 84mm long, located on the top face. The internal dimensions of the box are 265mm High, 800mm Long and 90mm Wide.

My main question is how much pressure is required to keep rain out.

While trying to get some figures for "rain" i stumbled upon a Wikipedia page talking about how tents are rated for water resistance, the rating system uses mm of Hydrostatic head. So a well rated tent has a Hydrostatic head rating of 3000mm of water.

So can i deduced that "Rain" can exert a pressure of 29430 Pa. (1000 (density of water)*9.81*3(head in Meters) = Pressure (in Pa))? and if that is correct does the pressure in my box need to be at least 29430 Pa above atmospheric pressure?


Thanks for reading!

 

[LittleInch]

Forget pressure - think velocity. Small boxes with relatively large openings won't hold much pressure until you reach a very high air velocity. Tent rating is just a way of showing how good the tent is at resisting droplet velocity impinging on it, but has no real relevance here and is just an easy way to compare tents and test them.

The profile of your vents will also be important, flat cut slits will be a lot worse than slits with an upstand or curved upwards.

Far too many issues to think about without some drawings / sketches etc and a bit more thinking and investigation on your part.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[racookpe1978]

I don't think it will work.

A "common" thunderstorm will dump 1 inch of water in less than an hour. Worse storms are often found everywhere: Detroit just got hit more than 4 inch of water of rainfall. Wintertime, the snow fall up north will build up on top of your enclosure, then melt into the vents.

The 1 - 3 inches of rain hits every exposed surface: so, if your enclosure is 6 feet long x 12 inches wide, you get 3 inches of water x 12 x 72 to "blow off the vent" ..That's a lot of weight. (Yes, it does not all fall at once, but how are going to prevent ponding if the roof is not sloped away from the vents?)

 

[btrueblood]

Agree with LI: google and find the terminal speed of raindrops - it looks like this is approx. 10 m/s (but it varies depending on the drop size, and what about hail?). You need to have a flow velocity of at least this much to divert the drops from falling through the slits. That's a lot of air, just from taking a SWAG at it - are you going to run this full time? If it switches off when the rain stops (by what method), then how do you keep the standing water on the top of the enclosure from wicking back through the slits, like racooke suggests?

 

[bimr]

Agree with racookpe1978.

You will not be able to pressurize the box enough to keep the water out. If you have any significant pressure, the pressure will blow out the sides of the box.

It will be less expensive to install a rain shield above the box.

https://www.isa.org/standards-and-p...ptember/enclosure-purging-and-pressurization/

 

[1gibson]

Can you put shields above, and short walls around, the slots? Shields prevent direct entry, and the walls keep any runoff from dropping in.

/-\ roof
|.| walls (||) around the slot (.)

 

[Argurios]

Hi,
Thank you all for reply, I am sorry the question is rather vague.

Thank you for the advice I just couldn't get my head around what my main factor would be to keep rain out, air velocity makes a lot more sense now thinking about it.

Great point with the “ponding” problem, this shouldn't be an issue as top of the box is slopped away from the slots.

Snow fall shouldn’t be an issue however the enclosure will be used in Hong Kong, so monsoons are a major design consideration.

The box will have an actuated cover to stop rain entering when the air blower system is not in use.

The slot aren't actually vents they are laser apertures, so need a full view above them to carryout there intended job. Sorry it’s so vague but it’s not my design and I’m conscious of giving away confidential information.
The air to keep the rain out was intended to be supplied by a fan however is has to travel through 4.8meters of small diameter ducting first and flow around a number of obstacles inside this box.
Essentially I don’t think it will work either but I needed to do the math. I have a meeting with my manager today to discuss my issues with this design. The problem is the system is half built and I don’t think much can be changed.
Thank you very much for the help!

 

[mbt22]

Just because order of magnitude calcs are fun.

Assuming there's no other way, and 10 m/s terminal velocity is accurate.

Open area of 0.0013 m2, so 0.01344 m3/s or 60 kg/hr. Equivalvent area circular orifice is 0.041 m.

I make that a dP of about 2 mbar across the orifice.

Of course your lasers are now shining through a nice turbulent mess of water being blown all over the place, combined with whatever dust and muck the air has picked up on it's way through the box.

Matt

 

[IRstuff]

Terminal velocity of raindrops as described is only applicable in GENTLE rainstorms. A monsoon or typhoon could have 100+ mph -- 45 m/s wind-driven rain. Your box' holes are way too big for such an application. There are a multitude of factors can could make the situation worse in an outdoor installation, e.g., the fan inlet might get obstructed and air flow get reduced.

And what about bugs getting into the enclosure?

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[racookpe1978]

Well, the lasers are at least "straight" little diameter "lines" and the raindrops are (usually) slightly angled falling drops and sheets of water.

Try a "stacked chevron" series of 4 - 8 thin metsl chevrons perhaps 2 - 5 mm thick, each separated by about 1/2 to 2 inches vertically. (Some trial and error expected here!). The chevrons have a vertical hole in them so the laser can "escape" straight up into the air, but the rain is diverted sideways from each chevron on to the top of the one below it. Once diverted sideways, the rain water then slides off of the bottoms of the lower chevrons onto the sloped roof and away from the laser enclosure.

 

[kacarrol]

In the HVAC world, it's said that the terminal velocity of rain is around 2000 fpm, so a stack velocity above 2600 fpm should prevent rain from entering the stack. (See the ACGIH Industrial Ventilation Manual for further discussion).

I don't know if that helps, but hopefully it does.

K

PS fpm = feet per minute

 

[IRstuff]

MIL-STD-810 Method 506 uses 18 m/s (40 mph) for blowing rain

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[dhengr]

Argurios:
I would look at turning a lip/curb up on the 4mm x 84mm slots about 10mm, so that water can not run in from the top surface. Then, consider using what I think they call air-knives, a high pressure directed flow/jet of air, or liquid blow-off systems of various shapes and sizes, from the packaging industry. I would mount these on the outside or roof of your box, directed slightly upward, some mm above the top of the lip on your laser apertures/slots, probably about 100mm long over each slot. But, the exact size, location, air velocity, etc. to be determined by someone much more familiar than I am with these systems. The shape of your slot lips and location of the air stream could actually pull some vacuum on the slots and the box, further reducing the likelihood of water entering the slot/curb system. Since this is a laser, could you direct it out the side of the box, and then redirect it upward with a mirror or a prism?

 

[IRstuff]

Having holes in a laser system is just not a desirable design point. Everything we design that have lasers have sealed windows through which they fire. Alternately, the window could be the objective lens of the laser beam expander.

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[1gibson]

Any slots would need to be wide enough that they did not retain drops due to capillary action, which would make the slots big enough to let some lucky drops in.