Design for Pressurizing an enclosure to 0.1 inches water pressure

[markk2pe]

We are designing a launch pad that will have a square shaped base with a 10-15 foot height with a hole in the middle where the rocket flume will go through to the flame trench. It will have sheet metal panels all around to enclose the base which contains numerous electronic devices (motors, solenoids, switches, sensors, controllers, etc.). The launch pad has hazardous gasses (rocket fuel) on it so the base has to be kept pressurized at 0.1 inches water pressure (per NFPA 496) to keep any fuel fumes from seeping in and igniting from electronic device sparking.

Fresh air can be used to pressurize the launch base. The base itself has a few openings where cables, pipes, and flexhoses enter into the base. The gaps around these cables, pipes, and flexhoses will be sealed as best as possible, but there will be leakage there and any small gaps in the construction of the base enclosure may leak, maybe 10 to 20 % +/- (tbd, it's not built yet).

I'm thinking that we could use a large fan(s) and duct the output into a duct attach fixture to blow the fresh air into the enclosure. I don't know how to calulate the flow rate required/fan size to provide the 0.1" H2O pressure. I can estimate the volume inside the base and the leakage rate, but I'm not sure how to size the ventilation equipment. Any help or reference to an appropriate procedure would be greatly appreaciated.

Thank You in advance.

 

[LittleInch]

Well simply work out your flow needed.

Is it simply to account for leakage out of the various entries into this mysterious base or are you adding on "ventilation" to do some cooling?

Then just go look up some fan vendors to see what size you need.

The pressure is a "minimum" so work out what the system will withstand.

any sort of drawing or picture here?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[markk2pe]

I haven't figured out how to post pictures; do they have to be hosted somewhere else or can they be loaded directly into the forum?

I understand it may be "simple" for you to work these things out but I don't do these types of evaluations every day and I don't know what formulae to use, otherwise I wouldn't have not had to post on this forum. I'm looking for help.

There is no ventilation or cooling, there is only a need to maintain 0.1 inches H2O in the base.

 

[LittleInch]

for pictures to insert into a post click on the little icon two to the left of the smiley face.

You said you can estimate the leakage rate so go looking for some fans with double that flowrate and check out the fan curve to see what the delivery pressure is.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[GT-EGR]

One way to calculate an actual airflow needed to create that pressure is in ASHRAE fundamentals - crack area method. Essentially you total up all the crack lengths and widths - and then using those estimates plus some coefficients, and applying your target room pressure on them - the calculation tells you what the airflow through the cracks will be to create that pressure in the room.

It's very theoretical since trying to count small cracks in a room can be impossible - but if you are conservative, then you'll likely end up with more air than you need. This gives you your CFM target when selecting your fan. Then your fan pressure is the pressure you are trying to create in the room plus any losses associated with its inlet/ducting/overall arrangement.

And once you install the fan you can just slow it down via the VFD (or speed controller) or belts, or put in a relief damper and open/close it as needed to relieve some pressure if you have too much.

 

[danw2]

Caveat: I've seen purged enclosures, but I don't design them.

Hoffman's Specifier Guide (catalog chapter 13) on hazardous area enclosures lists pressurizing controls on the pages beyond the metal boxes.
Link

Every purged enclosure I've seen ran off compressed, clean and dry "instrument air". The panel itself is supposed to be NEMA 4 or 12; tight, not loose; it's not supposed to consume and leak air.

I am under the impression that a DP switch and/or a door switch is used to disable enclosure power in the event internal pressure drops below the minimum.