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Old 07-27-2016, 02:29 PM
SlackerInc SlackerInc is online now
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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?

Last edited by SlackerInc; 07-27-2016 at 02:30 PM.
  #2  
Old 07-27-2016, 03:04 PM
engineer_comp_geek engineer_comp_geek is offline
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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.
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Old 07-27-2016, 03:15 PM
watchwolf49 watchwolf49 is offline
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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 ...

Last edited by watchwolf49; 07-27-2016 at 03:16 PM. Reason: It doesn't melt, it sublimates ...
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Old 07-27-2016, 03:48 PM
Machine Elf Machine Elf is offline
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Originally Posted by watchwolf49 View Post
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.
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Old 07-27-2016, 05:49 PM
watchwolf49 watchwolf49 is offline
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Originally Posted by Machine Elf View Post
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.

Last edited by watchwolf49; 07-27-2016 at 05:49 PM.
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Old 07-27-2016, 06:19 PM
jz78817 jz78817 is offline
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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.
  #7  
Old 07-27-2016, 06:40 PM
SlackerInc SlackerInc is online now
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Originally Posted by jz78817 View Post
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?
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Old 07-27-2016, 09:12 PM
Machine Elf Machine Elf is offline
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Originally Posted by watchwolf49 View Post
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.
  #9  
Old 07-27-2016, 10:42 PM
watchwolf49 watchwolf49 is offline
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Originally Posted by Machine Elf View Post
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.

Last edited by watchwolf49; 07-27-2016 at 10:46 PM.
  #10  
Old 07-27-2016, 03:32 PM
SlackerInc SlackerInc is online now
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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.
  #11  
Old 07-27-2016, 03:41 PM
watchwolf49 watchwolf49 is offline
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Originally Posted by SlackerInc View Post
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 ...
  #12  
Old 07-27-2016, 04:49 PM
Vicsage Vicsage is offline
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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.
  #13  
Old 07-27-2016, 05:14 PM
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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.
  #14  
Old 07-27-2016, 07:38 PM
nightshadea nightshadea is offline
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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
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Old 07-27-2016, 09:15 PM
Velocity Velocity is offline
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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.
  #16  
Old 07-28-2016, 08:11 AM
jtur88 jtur88 is offline
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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".
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Old 07-28-2016, 08:18 AM
Machine Elf Machine Elf is offline
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Originally Posted by jtur88 View Post
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.
  #18  
Old 07-28-2016, 02:57 PM
Gary T Gary T is online now
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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?
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Old 07-28-2016, 03:28 PM
watchwolf49 watchwolf49 is offline
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[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.
  #20  
Old 07-28-2016, 10:02 PM
jjakucyk jjakucyk is offline
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Originally Posted by Gary T View Post
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.
  #21  
Old 07-28-2016, 02:59 PM
SlackerInc SlackerInc is online now
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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.

Last edited by SlackerInc; 07-28-2016 at 03:01 PM.
  #22  
Old 07-29-2016, 01:25 AM
Saffer Saffer is offline
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Use a thermometer ... and you can buy my membership here ...


Buy your membership? Huh?


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  #23  
Old 07-29-2016, 11:00 AM
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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.
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Old 07-29-2016, 03:11 PM
SlackerInc SlackerInc is online now
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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. :-)
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Old 08-02-2016, 11:42 AM
sps49sd sps49sd is offline
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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!
  #26  
Old 07-29-2016, 12:30 PM
Valentine_Smith Valentine_Smith is offline
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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!
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