Understanding application of Psychrometric Principles

[Transient1]

Hi all,

I haven't looked at psychrometric equations since undergrad. I'm trying to lock down a method for determing if condesation could occur under a given set of conditions. The problem set up is that we have an oven for thermal cycling (-5 C to 76 C). There is a hole(*~4inches Diameter) in the oven that our test guys run a small (1/4 inch hose) through and then pump nitrogen in at 2 SCFH. The chamber is 4 Cubic Feet in Volume and the relative humidity is mainted at 55%. There is an unknown leak rate out of the oven. If I assume that it is less than 1 SCFH, then I could safely say that during the test the mixture in the oven is 1 CF nitrogen and 3 CF air/vapor mixture.

So I think I solve this problem as follows:

Since I know the relative humidity I can calculate the partial pressure of each gas. Then I can follow an example problem of calculating the specific humidity. Where I use the specific humidity and the initial temeprature (room temeprature) to determine what the dew point is. As I write this I have a thought that as long as any water vapor is present it will condensate in the given temperature range.

Thanks for your help.

 

[vpl]

Why do you feel there is leakage out of the oven? Oh, nevermind, you have a 4" hole with a 1/4" hose. duh.

So, what is your basis for assuming the leakage is less than 1 scfh?

Is the gas continuously being pumped in or is just pumped in for a certain length of time? If just for a while, then how long?

And this isn't graduate level homework, right?

Patricia Lougheed

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[monkeydog]

Spray insulation from the can will take care of that 12.5 sq in hole you have.

 

[jistre]

Hmm. If you're pumping nitrogen in at 2 SCFH with that 4" hole in the side, It's a fair bet that you won't pressurize the gas inside and pretty much 2 SCFH will be pushed out by the gas entering the oven, possibly even at a bit higher rate at the beginning as the gases initially inside the oven are first beginning to heat and expand out through your big hole.

 

[Transient1]

vpl,
Nope its not graduate homework. The hole is stuffed with a dense foam. I couldn't blow air through it and feel it on the other side. The basis for the leak rate is wishful thinking, and the fact that we haven't seen condensate forming. This question is more of a back verification. The gas is continuously pumped.

monkeydog,
I can't use a spray insulation, because we usually run wires along with the nitrogen hose, so we need to be able to run things in and out of the oven. Our test guys are finally taking a look at getting a better system in place for that.

jistre,
The interior gas won't be pressurized. What gas is pushing the 2 SCFH of nitrogen out? The air? So perhaps, that's a good thing.

I found some references suggesting to cyclee between room temeprature and a cylce hot several times while purging with nitrogen.

 

[jistre]

Ahhh. You didn't mention the foam originally, so all I was pointing out was that with a 4" hole, what you're pumping in is coming right back out. The foam changes that, though.

 

[monkeydog]

Transient1,
I have seen thermal chambers where the techs used (fabricated) a cork made of the spray foam, with a hole in the middle for the tubes, wires, etc, and cut in half to allow easy removal and replacement.
Just a thought.

 

[Transient1]

Thanks monkeydog, I'll be looking into it.