Dessicant Sizing

[Tunalover]

Folks-
Given a sealed electronics enclosure, if one knows the outside air temperature, pressure, and humidity profiles, is there a way to analytically predict how much moisture will condensate within? The seal is a conventional linear O-ring in an O-ring gland of traditional design. Also given are the inside surface area and volume. I dug out my old thermo textbook but it contained no problems remotely close to this one! Everywhere else I've looked, including a Google search turned up nothing. TIA for your help!


Tunalover

 

[IRstuff]

Oddly, I'm working on a similar problem. However, your question seems to be different than the ones I would ask.

> What temperature range do you want to have non-condensation?

> This establishes the dewpoint, which dictates the amount of water allowable at room temp

> Which determines the RH at RT

> Which determines the steady-state desiccant demand

> What is your max temp and how much leakage, both outflow and inflow

> Which determines the amount of potential moisture introduced into the structure

> Which determines the amount of moisture to be removed at any given time

TTFN

 

[Tunalover]

IRStuff-
I'm trying to determine the amount of dessicant to use in the enclosure and how long it will last until requiring recharge. I know the time history of outside RH, temperature, and altitude. I just need to know how much condensate will form inside the enclosure. Do you know of any papers on the subject where someone actually comes up with NUMBERS?


Tunalover

 

[tombmech]

I'm kind've with IRStuff on this one. A simple Psych chart will tell you everything you need about the air-water vapor mixture properties. It will even assist with modeling the process. However, you still need to estimate/measure the factors that he points out.

Condensation occurs when heated air, containing a higher moisture level, is cooled beyond the dewpoint temperature of the air-water vapor mixture. Now, that may seem like stating the obvious, but the rest is up to you.

Out of IRStuff's list, I would count airflow into and out of the enclosure, and heat transfer through the enclosure's surface as most important - and totally dictated by your specific situation, your estimates, and your measurements.

 

[MintJulep]

How sealed is sealed?

If you can safely assume that there is no air exchange with the outside then the problem is relatively straight-forward.

If there is air exchange things get more complicated.