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#25
07-28-2016, 03:47 PM
 watchwolf49 Guest Join Date: Dec 2009 Location: State of Jefferson Posts: 7,602
Quote:
 Originally Posted by Machine Elf If it's 40F and 100% RH, then when it hits the 32F ice cream it's going to cool further and leave behind condensate. If it's 40F and 100% RH and mixes with ambient air (at 90F and a dewpoint >32F), then the resulting blend will have a dewpoint above 32F, which will likewise cool further and leave behind condensate when it hits the 32F ice cream. The only way we're drying out the ice cream is if the ambient air has a dewpoint below 32F (welcome to Moab, UT), and/or the AC evaporator is operating at less than 32F (and so dehumidifying its output to a dewpoint below 32F). In the latter case, something is wrong with the AC.
I'm trying to explain the observation "the ice cream isn't melting". I think condensation on the ice cream wouldn't lead to this specific observation. If we could confine this flow from the A/C and stream it across the ice cream, then you would be correct; in all cases, water would condense on the ice cream. Unfortunately we are mixing two different air flows and much of what Bernoulli's Principle predicts will fall apart. With RH's low enough, the evaporation will proceed at a much much quicker rate than condensation. It's deadly to stand a few feet behind a jet engine at full throttle, not so much 100 yards away. The air flows mix and come closer to equilibrium by then.

That's one of the knocks on A/C, it dries the air out so much. If you bring up Moab, UT; then single digit RHs shouldn't surprise you (it's not the triple digit temperature, it's the double digit humidity that'll getcha).

All I'm saying is that if your ice cream stops melting when you hold it to the A/C vent ... it's because the melt is evaporating so quickly ... NOT because the 40ºF air is drawing energy out of the 32ºF solid keeping it frozen.

Last edited by watchwolf49; 07-28-2016 at 03:47 PM.