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-   -   Will holding an ice cream bar in front of a car's AC vent slow or speed its melting process? (https://boards.straightdope.com/sdmb/showthread.php?t=799603)

SlackerInc 07-27-2016 03:29 PM

Will holding an ice cream bar in front of a car's AC vent slow or speed its melting process?
 
I'm spurred to wonder this by a "ten years ago" deal my wife just shared on Facebook. She was an undergrad then (this is before I knew her), and she and some friends were bringing the ice cream bars back to share with someone or something like that.

Anyway, one of the pics shows one of her college friends smiling sheepishly for the camera and is captioned "[friend's name] took one for the team and spent time and energy holding the ice cream bars in front of the air conditioning vent." So I wondered: the air coming out of that vent is cold, but is it below 32 degrees cold, even right out of the vent? I suppose it might be, but I'm not sure. If it's not, would it be better to move air that's, say, 40 degrees, rapidly over the bars, or keep them in air that's 70 degrees but more still?

And wouldn't the best idea of all have been to wrap them in a blanket or something?

engineer_comp_geek 07-27-2016 04:04 PM

The air coming out of your car's AC is not colder than 32 degrees.

I'm not an expert on automobile AC systems, but it's my understanding that they generally aim for an outlet temperature of somewhere around 40 degrees. I've also heard that as long as the outlet temperature is 40 degrees below the ambient temperature that the AC is considered to be operating ok, but I think some folks disagree with that. In other words, if it's 100 degrees out, the AC isn't going to be able to maintain a 40 degree outlet temperature, but as long as it maintains 60 degrees or cooler, it's not considered to be a problem with the AC.

Maybe one of our resident car experts can give some better info on that.

Anyway, the answer to whether it is better to have ice cream in 70 degree stagnant air or 40 degree moving air depends on how fast the air is moving. If the 40 degree air were still, then obviously the ice cream would melt less quickly in it than the 70 degree air. The faster the 40 degree air moves, the more heat it can draw out of the ice cream, so at some point it breaks even, where the ice cream would melt equally at 40 degree moving air as 70 degree stagnant air. Past that break-even point, the 40 degree air will always draw more heat out than the 70 degree stagnant air.

I'm bad enough at thermodynamics that I am not even going to attempt to calculate exactly where that break-even point is.

My gut feeling is that you don't need a whole lot of moving air to draw a lot of heat out of the ice cream, so most likely you are better off not putting the ice cream in front of the vent.

watchwolf49 07-27-2016 04:15 PM

The relative humidity of the air is important as well, my first instinct is that the water in the ice cream would quickly evaporate, only remaining in it's melted state briefly. What remains is still frozen. I can even imagine the solids remaining behind forming a crust over the frozen parts, slowing down conduction.

Depends on relative humidity ...

SlackerInc 07-27-2016 04:32 PM

Interesting! So far my hunch seems likely to be correct. But if so, I would bet you could not convince 90% of people this is true.

watchwolf49 07-27-2016 04:41 PM

Quote:

Originally Posted by SlackerInc (Post 19510868)
Interesting! So far my hunch seems likely to be correct. But if so, I would bet you could not convince 90% of people this is true.

Use a thermometer ... and you can buy my membership here ...

Machine Elf 07-27-2016 04:48 PM

Quote:

Originally Posted by watchwolf49 (Post 19510825)
The relative humidity of the air is important as well, my first instinct is that the water in the ice cream would quickly evaporate, only remaining in it's melted state briefly. What remains is still frozen. I can even imagine the solids remaining behind forming a crust over the frozen parts, slowing down conduction.

Depends on relative humidity ...

Even if the ambient air is extremely dry, the low temperature of the ice cream bar means that the micro-climate (at the immediate surface of the bar) will be at very high relative humidity. You won't see much moisture loss until the bar is melted and its temperature somewhere close to ambient.

Vicsage 07-27-2016 05:49 PM

Simple solution. Buy 2 identical ice cream cones, put 1 by the vent and 1 away from the wind. Watch what happens. You can eve buy more cones and try it at different fan speeds.

bob++ 07-27-2016 06:14 PM

In my youth I sold Ice cream on a beach. I had a soft ice cream machine and I was trained to pile all the ice cream up on top of the cone (except for pretty girls of course).

I would sometimes see a family about 500 yards or so away, send a small child to collect some cornets. If there was no wind, there was a good chance that they would arrive reasonably safely. If it was at all breezy, as it often was, the child would arrive with much of the ice cream running down their arms.

watchwolf49 07-27-2016 06:49 PM

Quote:

Originally Posted by Machine Elf (Post 19510909)
Even if the ambient air is extremely dry, the low temperature of the ice cream bar means that the micro-climate (at the immediate surface of the bar) will be at very high relative humidity. You won't see much moisture loss until the bar is melted and its temperature somewhere close to ambient.

We have air flow past the ice cream ... and if that air is dry we'll continually be picking up water vapor from the ice, more so if it's already melting. That was the key here, the ice cream doesn't appear to be melt even with 40F air flow over it. Something else has to be going on.

jz78817 07-27-2016 07:19 PM

it will speed up the melting. the simple fact that you're blowing air across it (regardless of how cold it feels to you) will increase the rate of heat flow.

SlackerInc 07-27-2016 07:40 PM

Quote:

Originally Posted by jz78817 (Post 19511228)
it will speed up the melting. the simple fact that you're blowing air across it (regardless of how cold it feels to you) will increase the rate of heat flow.

Except as stated upthread, there must be an equilibrium point. Surely 40 degree air moving past the ice cream bar at 1 FPS is melting the ice cream more slowly than stagnant 90 degree air?

jz78817 07-27-2016 08:27 PM

Quote:

Originally Posted by SlackerInc (Post 19511273)
Except as stated upthread, there must be an equilibrium point. Surely 40 degree air moving past the ice cream bar at 1 FPS is melting the ice cream more slowly than stagnant 90 degree air?

the stagnant air immediately surrounding the ice cream is not 90 degrees. air is a terrific insulator. Alton Brown even did a segment about this, but this is the closest I could find:

http://www.atozteacherstuff.com/pages/5881.shtml

nightshadea 07-27-2016 08:38 PM

ive had ice cream bars where the ice cream melted and the coating didn't and made a mess when I just bit half of it off .....

But if you've ever had a dreamsicle(or generic version) where its vanilla ice cream in a orange popsicle thw ice cream will melt first

SlackerInc 07-27-2016 08:44 PM

Quote:

Originally Posted by jz78817 (Post 19511363)
the stagnant air immediately surrounding the ice cream is not 90 degrees. air is a terrific insulator.

Okay, but in reality the air is not going to be perfectly still. So it's more like slower moving 90 degree, or 70 degree, air vs. faster moving 40 degree air. And I still think wrapping them in a blanket would be the best move of all.

Machine Elf 07-27-2016 10:12 PM

Quote:

Originally Posted by watchwolf49 (Post 19511171)
We have air flow past the ice cream ... and if that air is dry we'll continually be picking up water vapor from the ice, more so if it's already melting.

The ambient air will have to be very dry. According to the standard psychrometric chart, 90F air at ~12% relative humidity will generate condensation if cooled to 32F. So if your ambient relative humidity is more than 12% at 90F, then you're going to add moisture to your ice cream sandwich, rather than drying it out. There are places with humidity that low, but they're relatively rare.

If the AC's evaporator is only cooling the air to 40F, then it's not drying it out enough to prevent condensation when it's further cooled to 32F. Hope you like soggy ice cream.

Velocity 07-27-2016 10:15 PM

I am not a physicist, but I think that the airflow would speed up the melting. From the perspective of the ice cream, it's warm/hot air. It would be akin to how a hair dryer heats up your skin, even though it has convection.

Princhester 07-27-2016 11:40 PM

Quote:

Originally Posted by jz78817 (Post 19511363)
the stagnant air immediately surrounding the ice cream is not 90 degrees.

It won't be 90 degrees, but it won't be stagnant either. As the icecream cools the air immediately adjacent, that air will fall because it will become more dense, and it will be replaced by warmer air moving in from above and to the side.

watchwolf49 07-27-2016 11:42 PM

Quote:

Originally Posted by Machine Elf (Post 19511590)
The ambient air will have to be very dry. According to the standard psychrometric chart, 90F air at ~12% relative humidity will generate condensation if cooled to 32F. So if your ambient relative humidity is more than 12% at 90F, then you're going to add moisture to your ice cream sandwich, rather than drying it out. There are places with humidity that low, but they're relatively rare.

If the AC's evaporator is only cooling the air to 40F, then it's not drying it out enough to prevent condensation when it's further cooled to 32F. Hope you like soggy ice cream.

You may be going the wrong direction here. Let's assume the air coming out is 40F and at 100% RH. The air will start immediately mixing with the 70F air, the two will have a temperature above 40F, which means the RH of this mixed air will be below 100%. Evaporation will occur, and even if the air is then 100% RH, it is immediately push downstream being replaced by more air below 100% RH.

I understand you point, however it's invalid with the A/C unit. Even if our original air is 90F at 100% RH, the output will be 40F and 100% RH. The extra water condenses in the A/C unit and drains out the piping provide for this, not on the surfaces where the cooler air is going.

That doesn't mean the water is rapidly evaporating after it melts, and I have been specifying that this all depends on the RH. If the air is indeed leaving the A/C unit at 100% RH, then we may well see the melting first and not notice the evaporation. However I'm taking the OP at face value where we have a car full of young ladies exclaiming "It's like totally not melting". I'm only suggesting under certain circumstances this can be explained as I have above, The water melts but then quickly evaporates giving the impression that it's staying frozen, when it's actually "sublimating" instead.

One thing for absolute sure, ice cream in a 40F air flow will not stay frozen.

spamforbrains 07-27-2016 11:59 PM

My car has never melted regardless of how much ice cream is consumed inside it.

SlackerInc 07-28-2016 12:09 AM

Quote:

Originally Posted by Princhester (Post 19511778)
It won't be 90 degrees, but it won't be stagnant either. As the icecream cools the air immediately adjacent, that air will fall because it will become more dense, and it will be replaced by warmer air moving in from above and to the side.

Ah, good point. The plot thickens!

panache45 07-28-2016 12:10 AM

Quote:

Originally Posted by spamforbrains (Post 19511815)
My car has never melted regardless of how much ice cream is consumed inside it.

Only because your car is on a treadmill.

jtur88 07-28-2016 09:11 AM

This is similar to a question I've asked in a couple of forums, and never got an answer.

Is the heat exchange greater if I leave the refrigerator door open while making a sandwich, or if I keep wafting the circulation by opening and closing the door several times to take things out and put them back? In other words, was my mother right or wrong when she yelled "close the fridge, you're wasting electricity".

Machine Elf 07-28-2016 09:11 AM

Quote:

Originally Posted by watchwolf49 (Post 19511785)
I understand you point, however it's invalid with the A/C unit. Even if our original air is 90F at 100% RH, the output will be 40F and 100% RH. The extra water condenses in the A/C unit and drains out the piping provide for this, not on the surfaces where the cooler air is going.

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.

Machine Elf 07-28-2016 09:18 AM

Quote:

Originally Posted by jtur88 (Post 19512343)
This is similar to a question I've asked in a couple of forums, and never got an answer.

Is the heat exchange greater if I leave the refrigerator door open while making a sandwich, or if I keep wafting the circulation by opening and closing the door several times to take things out and put them back? In other words, was my mother right or wrong when she yelled "close the fridge, you're wasting electricity".

I think the buoyancy-driven convection associated with leaving the door open for a solid 5 minutes is going to warm things more than just quickly opening/closing the door a few times. Either way I think the cost of the energy involved is pennies.

For best efficiency, think about what you want on your sandwich, then grab all those items in as few visits to the fridge as possible. Put the items back (in another single visit) as soon as your sandwich is made so that you minimize how much they warm up.

watchwolf49 07-28-2016 03:47 PM

Quote:

Originally Posted by Machine Elf (Post 19512344)
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 40F air is drawing energy out of the 32F solid keeping it frozen.

Gary T 07-28-2016 03:57 PM

Observation: The OP asked about an ice cream bar, not about exposed ice cream in a cone or dish.

Question: Air conditioning by definition dehumidifies. Why is it being assumed that the output air is at 100% RH?

SlackerInc 07-28-2016 03:59 PM

Just to be fair to my wife and her friends, I don't think anyone thought the AC vent would keep the ice cream frozen indefinitely. I think they just believed that it would melt more slowly that way, giving them more time to get it back to the apartment.

ETA: Gary is right; also, it was in a wrapper, and my wife's friend was holding it by the end of the stick.

watchwolf49 07-28-2016 04:28 PM

Quote:

Originally Posted by Gary T (Post 19513395)
[snip] ... Why is it being assumed that the output air is at 100% RH?

It seemed to me the most likely condition to produce a counter-example to my claims; and see, I was right on that score. I do have to add to my claim the condition of low initial RHs.

jjakucyk 07-28-2016 11:02 PM

Quote:

Originally Posted by Gary T (Post 19513395)
Observation: The OP asked about an ice cream bar, not about exposed ice cream in a cone or dish.

Question: Air conditioning by definition dehumidifies. Why is it being assumed that the output air is at 100% RH?

Because for an air conditioner to dehumidify it has to lower the temperature of the air passing through it below the dew point (100% RH) so the water vapor condenses and drains away. Cold saturated air pumped into a warm space dehumifies as it mixes with the ambient air. Or think about it like this, cool a small tank of air (something sealed tight, like a jug put in the refrigerator) until the water vapor condenses, then drain away the water. When you heat it back up the air inside will be drier than before. An air conditioner does the same thing just not to all the air at once.

In practice the outlet air is not exactly 100% because if it is then you'd see fog coming out the vents. It's still well into the 90% + range though, but since not all the air effectively touches the evaporator coil there's still some mixing going on to keep the outlet from being completely saturated.

Saffer 07-29-2016 02:25 AM

Quote:

Originally Posted by watchwolf49 (Post 19510890)
Use a thermometer ... and you can buy my membership here ...



Buy your membership? Huh?


Sent from my iPhone using Tapatalk

CC 07-29-2016 12:00 PM

Science teachers: this is an excellent question for students to explore in a science fair project. In fact, the thread can even be used as a component of the literature search. xo,
Dr. C.

Valentine_Smith 07-29-2016 01:30 PM

Ice cream bar(s), in a wrapper! Blanket is the way to go! Insulate, insulate insulate! Hell, they use to have special "ice cream bags" at the supermarket when I was a kid!

Machine Elf 07-29-2016 02:52 PM

Quote:

Originally Posted by watchwolf49 (Post 19513361)
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).

Quote:

Originally Posted by watchwolf49 (Post 19513361)
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.

Quote:

Originally Posted by watchwolf49 (Post 19513361)
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.

SlackerInc 07-29-2016 04:11 PM

Quote:

Originally Posted by CC (Post 19515508)
Science teachers: this is an excellent question for students to explore in a science fair project. In fact, the thread can even be used as a component of the literature search. xo,
Dr. C.

Cool! Be sure to report back your findings. :-)

sps49sd 08-02-2016 12:42 PM

Very little will evaporate from the ice cream.

There is an equation, but one would need the ice cream surface area, amount of air flow (mass flow rate), heat transfer coefficient and the heat capacity of the ice cream. (CpH of air is easily looked up).

Temperature of the AC air at the ice cream also needs to be determined; it will warm from mixing immediately upon leaving the duct. Air temp after the ice cream is also needed.

Experimentation would probably take less time. And there would be more ice cream around!


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