Vent Hole

[MechaWolf]

A unit is being designed out of aluminium, and will have a lid to cover the main enclosure, but not sealed. The box needs to have a vent hole in it to ensure there is no pressure build up inside the unit. The vent hole has to meet the following requirements:-

Maximum pressure decay rate of 70 mbars/sec.
Pressure decay of 0.8 bar in 27 sec.

The volume of air inside the box is 200000mm^3

Any ideas?

Thanks

 

[MechaWolf]

Sorry missed the wall thickness, the hole will go through a side wall on the unit of 8mm in length.

 

[drawoh]

MechaWolf,

This sounds safety related. Is anyone going to answer a question like this without seeing all the drawings, visiting the site, charging a fee, and perhaps putting his license on the line?

--
JHG

 

[handleman]

Wow, that seems like a pretty tight spec... You have no differential specified. Decay from 0.8 bar to atmospheric will be slower than decay from 1.6 to 0.8. Also, the decay rate will change as the differential changes. It may even be impossible to design a vent that will be below your 70mbar/sec at 0.8 differential but still vent from 0.8 to atmosphere within 27s.

Anything you are doing to intentionally limit the pressure reduction rate means that if something happens to increase the pressure buildup rate, you will end up with an unsafe overpressure condition. Sounds like you'll probably need an additional safety release valve that can vent gas at least at the maximum possible forseeable rate of gas entry.

Wow, only 200,000mm^3? So if it were a cube, just over 2 inches on a side? That's really pretty small.



-handleman, CSWP (The new, easy test)

 

[MechaWolf]

It is a small unit, with components inside, the volume is the air space inside the box. The pressure decay in required for a unit going from ground to altitude (exact level unknown), the only info supplied is the 70mbar/sec and the 0.8 bar in 27 secs.

Thanks

 

[handleman]

Ah, OK, so the maximum possible internal pressure is 15 psi? Definitely no issue with safety if that 200000 cubic millimeter volume is all you have. However, I'm thinking your decay spec is insufficiently defined to design this vent. You need to find out the test conditions under which this spec will be verified. If it won't be verified, who cares what size the vent is?

-handleman, CSWP (The new, easy test)

 

[MechaWolf]

Fair question and at the moment I really do not know, other than I need to prove that the hole is sufficient for the requirements.

Is there any formula for making a calculation on this?

Thanks

 

[handleman]

You can certainly calculate air flow rate through an 8mm long hole of a given diameter with a given pressure differential. One problem is that you don't know the pressure differential. If you assume that you are starting from a 0.8bar differential, you can calculate your flow rate as a function of pressure difference. Of course, you'll have to figure out if this is turbulent or laminar flow. Then, I think by using PV=nRT you can relate the amount of air in the box to the internal pressure. So, over time, your flow rate will decrease as the air flows out of the box and the differential decreases. Oh, and if you start out turbulent, you will have transitional and then laminar flow at some point. I believe you'll probably have to solve some hairy differential equations or integrals to get an analytical result. Once you're done, you may find out it's impossible to make a hole that is small enough to restrict the initial pressure drop rate to 70mbar/sec while still sufficiently venting 0.8 to atmosphere within 27s, especially considering that flow rate approaches zero as differential pressure approaches zero.

This is why I said you will need to find out the conditions under which adherence to spec will be verified. You could work on the math and fluids for a week and get a pretty answer, just to find out that the spec is poorly written and nothing like the actual requirement. GIGO. I feel your pain. Unfortunately, specs are not the same thing as requirements. A spec is just a representation of the underlying functional requirement.



-handleman, CSWP (The new, easy test)

 

[zekeman]

The box needs to have a vent hole in it to ensure there is no pressure build up inside the unit".

Please explain how can there be a pressure buildup given you are VENTING from atmospheric to atmosphere? Air is always going OUT of the unit.

 

[MintJulep]

I need to prove that the hole is sufficient for the requirements.

That's great.

Except that you have no requirements.

The information that you have provided is incomplete, thus no one can provide any useful guidance.

The general form of the orifice flow equation for air is:

Volumetric Flow = A Constant X Orifice Area X Orifice Coefficient X SQRT(Differential Pressure)

As you can see, it's not possible to fill in all the blanks with what you know.

 

[IFRs]

I'm betting there is a change in altitude which causes a delta P and the rate of change is based on the vertical speed of the craft.

 

[zekeman]

Yes, if the OP still wants to do the problem he will have to furnish time vs altitude data and then do it piecewise through all the regimes of fluid flow including possibly choked flow.