My first refridgerator..
July 16, 2010 3:54 PM   Subscribe

Physics question: A hotel room. An ice bucket. An air conditioner shooting up cold air. A fleece blanket. Put the fleece blanket over the bucket to trap the air. Hypothesis: The fleece blanket will trap the cool air around the bucket, keeping it colder longer. OR The fleece blanket will do what it always does, make things warmer. Which will be?

The fleece blanket is covering the ice bucket in such a way that cold air is shooting up into its enclosure and is prevented from escaping as much as possible. There's also a plastic bag over the ice bucket (but it has holes in it...) There is also a lid on the ice bucket.

This is for a short story I'm writing.
posted by amethysts to Science & Nature (21 answers total)
 
The fleece blanket should act as an insulator for the bucket and keep it colder for a longer period of time than if the fleece blanket were not there.
posted by dfriedman at 3:55 PM on July 16, 2010 [1 favorite]


Fleece blanket will do what it always does: keep something warmer, in this case the air in the room from being cooled by the bucket.
posted by rainy at 3:58 PM on July 16, 2010 [7 favorites]


Agree with dfriedman.

The reason that a fleece blanket makes you warmer is that it's insulating your body heat. The blanket itself does not produce heat (a form of energy). You produce heat and the blanket prevents it from leaving.
posted by jander03 at 3:59 PM on July 16, 2010 [4 favorites]


This isn't an electric fleece blanket, right?
posted by pullayup at 4:00 PM on July 16, 2010


Response by poster: This sounds like a consensus. It's not electric, it's just a regular cheap old fleece. Thank you for your help!
posted by amethysts at 4:02 PM on July 16, 2010


Insulates, not warms.
posted by b33j at 4:02 PM on July 16, 2010


Assumption: The cold air is at 33F or greater.

Now, here is the critical part: should the fleece blanket be heavy enough AND (the air conditioner blower unit be weak enough OR the air conditioner has other outlets), then less air will be moving past the ice bucket and the so the cold air (which still isn't colder than the ice, it's warm air as far as the ice is concerned) won't be melting the ice as much.

This is a problem of keeping air flow around the bucket low.

The third option, if you really want to keep it cold, is move the bucket away from the cold air and then put the fleece blanket over this.

I do a variant of this in hotel rooms not infrequently — dry ice in a cooler with a towel over the dry ice. Always put a towel over your dry ice if you want it to last longer. The lack of air circulation means that the air around the ice or dry ice becomes cold enough that melting or sublimation does not occur. I can keep a twenty-five pound block of dry ice for four days with a crap cooler and a towel.

If I take that dry ice home and put it in the freezer compartment as a test, it sublimates rapidly because cold air (but not as cold as dry ice air) is blowing past it.
posted by adipocere at 4:05 PM on July 16, 2010 [3 favorites]


The fleece blanket will trap the cool air around the bucket, keeping it colder longer.

Your bucket will stay coldest longest if you just throw the blanket over the top, sans air conditioner. The air from the air conditioner is probably somewhere between 55 and 65 ºF, while the ice is 32 ºF, so the effect of the flowing air will be to heat the ice up. What's more, since the air is flowing, heat from the air will be convectively transferred to the ice: a very efficient way of transporting heat energy. If you just put the blanket on top, heat can only be transferred by conduction through the blanket and through the sides of the bucket. Heat transfer by conduction is much slower than heat transfer by convection.
posted by mr_roboto at 4:09 PM on July 16, 2010 [2 favorites]


This is (it seems to me) the same question as, "when does a thernos know when to keep soup warm and soda cold?" It doesn't, it just isolates the stuff inside.

The temperature under the blanket wants to be equal to the temperature elsewhere with in its system. Perfectly isolated ice would never melt, it would always stay the same temperature. So, the ice covered by the blanket would melt much slower than uncovered ice. The air from the A/C is much warmer than the ice, but it is warmer than the air sneaking in under the blanket, at some point that A/C will help, but not if you could keep the bucket totally sealed off from the real world. --- this is my understanding of he 2nd law of thermodynamics but IANAP.
posted by Some1 at 4:09 PM on July 16, 2010


Best answer: Several problems with your hypothesis. First, a fleece blanket doesn't "make things warmer." It provides a barrier against the movement of warm to cold mass. So, the insulation it provides will slow down the entry of heat into the vicinity of the ice bucket. Second, you are ruining the insulative value of the blanket by putting it where "cold air is shooting up into its enclosure." The best insulation is dead air. That is the basis of the insulation in your walls. The dead air that is trapped there makes it hard for heat to move through the wall cavity to the cold surface of the outside wall. When you introduce movement, you reduce the insulative value. Next, unless you intend to inflate the blanket like a balloon, air has to be moving out of the area under the blanket in an amount equal to the amount that is blowing in. This air, because it is giving off some of its heat to the ice in the bucket, is colder than the air that is coming out of the A/C blower (vent?) and so you are actually warming up the bucket of ice faster than if the A/C wasn't blowing under the blanket at all.
posted by Old Geezer at 4:09 PM on July 16, 2010 [1 favorite]


Make sure you keep said fleece blanket (especially if it's dark colored) out of the sun!
posted by wayland at 4:11 PM on July 16, 2010


Response by poster: Wow. Interesting. As usual, I am glad I asked! This sounds like an excellent elementary school science project topic. I wonder if there would be some way to get a second bucket of ice and test it out...
posted by amethysts at 4:15 PM on July 16, 2010


Old geezer makes some good points which should be considered.
posted by dfriedman at 4:16 PM on July 16, 2010


I can keep a twenty-five pound block of dry ice for four days with a crap cooler and a towel.

Seconding this. I've kept food items cool in a car on a warm day with frozen water bottles or plastic bags full of ice, a towel, and a half-decent blanket -- no cooler at all. Insulate your cool items and they stay cool longer, the better the insulation.

What's more, since the air is flowing, heat from the air will be convectively transferred to the ice: a very efficient way of transporting heat energy.

I'm totally convinced of this, but I wonder if the equation changes at all if the blanket/bucket are wet, either externally wetted, or from condensation, or both...
posted by weston at 4:16 PM on July 16, 2010


Moisture from the air would condense on the bucket and drip onto the sheets. Depending on the humidity of the air, the bed could become quite wet.

Admittedly, that doesn't answer the question (which has pretty much been answered anyhow), but it's something you might want to keep in mind.
posted by Sys Rq at 4:25 PM on July 16, 2010


...aaaaaaaaaaaaand I totally misread the question.
posted by Sys Rq at 4:26 PM on July 16, 2010


In your proposed setup, you've essentially just jury-rigged a (rather inefficient) refrigerator.
posted by Sys Rq at 4:29 PM on July 16, 2010


fleece is not exothermic
posted by bottlebrushtree at 5:29 PM on July 16, 2010 [1 favorite]


What's all this about cold air shooting UP out of a refrigerator? Cold air is more dense than warm air and therefore sinks.
posted by DU at 6:05 PM on July 16, 2010


Response by poster: The way the AC vent is set up, the cold air is blasting in an upwards direction.
posted by amethysts at 6:40 PM on July 16, 2010


Old Geezer has it. The air the AC puts out is colder than room temperatuer, but warmer than the ice. So blowing it over the ice will actually warm it up. The best option is just to leave the bucket in the corner with the blanket over it.
posted by katrielalex at 4:19 AM on July 17, 2010


« Older Are there any boilerplate contracts for writers...   |   Making evernote and the iPad place nice Newer »
This thread is closed to new comments.