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Old 07-29-2016, 02:52 PM
Machine Elf Machine Elf is offline
Join Date: Feb 2007
Location: Challenger Deep
Posts: 10,552
Originally Posted by watchwolf49 View Post
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.
My claim is that if the ambient dewpoint is higher than 32F, then any mixture of ambient air and HVAC-cooled air will produce condensation on the ice cream.

HVAC evaporators have an operating temperature above freezing. If this were not true, they would frost up and block the airflow, just like the evaporator in a cheap dorm-room fridge. Given that this is the case, an HVAC system cannot get the dewpoint of its output air down to 32F. If you take its output air and move it across the ice cream, the air temp will drop to 32F - and the ice cream will develop condensation on it (this again assumes ambient dewpoint above 32F).

Originally Posted by watchwolf49 View Post
With RH's low enough, the evaporation will proceed at a much much quicker rate than condensation.
For a 90F ambient temperature, the RH needs to be below about 12% (i.e. the ambient dewpoint needs to be about 32F) in order to have neither condensation nor evaporation when the air hits the ice cream. This is exceptionally dry for the great outdoors. It's average for a place like Moab in June, but not for most of the US.

In the rare case of a 90F day with ambient RH below 12%, the air very close to the ice cream will be very close to 32F, so its RH will be very high, and evaporation will be exceedingly slow. Evaporation is an unlikely explanation for an observation of "the ice cream isn't melting;" if the ice cream isn't melting, it's more likely because it was very recently taken from a 0F freezer and its surface hasn't yet warmed up to 32F.

Originally Posted by watchwolf49 View Post
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 40F air is drawing energy out of the 32F solid keeping it frozen.
That would be an amazing rate of evaporation. The only way to achieve an evaporation rate like that would be a massive amount of heat, which isn't going to come from 40F HVAC air. Point an acetylene torch at an ice cube, and most of it is going to melt and drip away rather than evaporate.