Enclosure Transfer Rate

[cleagl]

Hi Fellow EngTips people. OK so here’s the problem. I am working on a thermal transfer problem. I am a mechanical engineer that hasn’t done thermal in a while, but here’s where I’m at…..
A 304 Stainless enclosure approximately 350mm W x 200 D x 900 H. The steel thickness id 1.5 mm. I need to determine if the change in pressure rate,(inside) due to exterior heating will exceed the capacity of a gortex vent on the enclosure.
I know all the physical parameters (start temp, volume, thermal conductivity of the air and the stainless steel, mass of the enclosure, etc.). I can calculate the change in pressure with a simple PV=nRT, but that doesn’t tell me anything about the rate of pressure change. I have calculated the transfer rate of the enclosure as: Qsteel=k x (surf area of enclosure) x ((Temp exterior –TempInterior)/thickness of the steel).
This gets me to the interior air side of the problem. I original conceived this as a convection problem, but now I am thinking it is a radiation problem. Here’s why…..I calculated the convective transfer rate of the air as: Qair=(h X delta T) x (Enclosuresurfacearea). Using this I can calculate the rate of change of the volume as : Vrate=((Vol of enclosure X Q)/(delta U of enclosure). Here we actually assume it is a volume rate since there is a vent, so the pressure will not increase, but the increase in pressure will be vented at a (vol/time) value.
The problem with the answer I got is that it gives me a very high rate of change that I don’t believe to be valid. What about radiation? Does it apply here? What do you think?
Any help is greatly appreciated.
-cleagl

 

[IRstuff]

Seems like you are spending WAY too much effort on the heat transfer part. What are the characteristics of the "Gortex vent?"

We once spent about 2 hrs during a design review mulling over the need to provide some sort of bladder, pressure relief valve, whatever. When we finally built the thing, 20 seconds after we pressurized it, it was completely equilibrated.

How well is this box really sealed? If this box were really well sealed, the walls would probably buckle. Rather than burning extraneous calories, figure out what pressure the valve won't pass. Figure out what the maximum possible pressure could be, i.e., at max temp. Do you have desiccant? Is the box outdoors? Can the valve suck in dew?

We've had a couple of systems that breathed quite a bit, and the seals tended to be constantly exposed to condensation that got sucked into the enclosure. After a couple of days, there would be standing water inside the enclosure

TTFN
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[MintJulep]

With no vent, if the temperature increased instantaneously by 45 degrees C the pressure increase would be on the order of 0.1 bar.

Alternately, at constant pressure the same instantaneous temperature change yields about a 10 liter increase in volume.

What is the vent capacity?

 

[cleagl]

Thanks for the replies. The vent is uni-directional and can pass 1.3 mL/min. The enclosure will be rated for IP 69k Ingress protection (The standard says something like 70 deg C water at 1400 psi, at 6” away for 3 mins, no leaks). The end user will clean the enclosure from the outside with high pressure water spray at approx. 90-100 deg C (although probably not 1400 psi). The vent should be able to withstand the increase in interior pressure, but there is no pressure rating. We have the opportunity to put one or two vents, or to upsize the vent, but I need to quantify it. We’re OK if the enclosure buckles elastically, but not plastically. Other thoughts on how to attack this?

Thanks again.
-cleagl

 

[LittleInch]

Mint Julep has given some ideas on pressure r volume.

Volume seems very high for your vent (128 hours to equalize.....) so can your vent stand the pressure?

If it can't then look at IRstuffs options (internal or external bladder, separate PRV)

Try seeing how much volume it takes to pump up to 0.1 bar (bending of the vessel) sometimes it's just easier to do tests than calculate.

My motto: Learn something new every day

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

 

[MintJulep]

The vent is uni-directional and can pass 1.3 mL/min.

At some pressure differential.

Lacking anything further, we can guess that the vent behaves similarly to an orifice. That is flow increases proportional to the square root of delta-p. But we probably don't need to worry about that for a first cut.

P1V1/T1 = P2V2/T2

First, find the delta-p that goes along with the 1.3 mL/min vent rate. That becomes P2.

Assume that the the ENTIRE heat flux onto the enclosure in one minute goes to heating the air inside the enclosure. Calculate how much the air temperature would increase in that minute. That's T2.

Solve for V2.

It's either greater than or less than 1.3 mL/min.

 

[IRstuff]

What's the pressure threshold to open the valve from the internal pressure?
How much air pressure differential can the valve tolerate in the other direction? Is there any air leakage at all?

Based on what's been given so far, the enclosure will likely oil can inward, since any internal overpressure will be bled off, leaving the enclosure with a deficient of air, once the box returns to normal temperature.

TTFN
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[chicopee]

You can turn you equation of state into a rate equation. d(PV)/dt= d(nRT)/dt