How do bottle doors work?

[rugrunner]

I have three slant shelf square top machines, two that I have only run long enough to make sure they cool and one that I have had running for several summers.
The one I use is always full of condensation and the other two have rust under the bottle door from, I assume, condensation.  I can't imagine they were designed to do this so what is the theory behind them and why don't mine work?

The two I don't use yet have broken inner glass with one crack (break would be more accurate) coming from the hole. The one I use has no cracks at all.

Thanks,

Bill

[rugrunner]

Is this a taboo question or just a dumb one?

[collecture]

Did you replace the bottle door gasket?
It will condensate up if it is not sealed well. Humidity also has an effect on it.

[rugrunner]

I haven't done anything yet I was trying to figure out what makes them work properly so I could
make sure that I fixed them properly. I guess the question should have been why do these doors all fog up?