Naked Science Forum
General Science => General Science => Topic started by: Richard on 23/11/2010 12:30:03
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Richard asked the Naked Scientists:
Why is frost bad for a freezer? I thought snow/frost is an insulator, so doesn't it keep freezers cold longer? Why do people say it's bad?
What do you think?
The question was presented on the show.
http://www.thenakedscientists.com/HTML/questions/question/2869/
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I think this idea comes from the misconception that the coldest point in a freezer is inside the freezer box. It isn't.
When running, the compressor is actively trying to pump heat out of the freezer box via the freezer matrix enclosed at rear of the freezer box. If a layer of ice insulates this from the freezer's contents it will be ineffective at moving (pumping) the unwanted heat away.
To overcome the insulating layer of ice that has built up, the compressor will be pumping for longer before the internal temperature will drop enough to signal it to stop - making it less efficient overall.
However, frozen water can be useful in a freezer, if it is part full. Generally, the larger the thermal mass of a thermal reservoir (hot or cold) the greater its thermal stability and therefore the greater the efficiency of the system reliant on it - in this case the freezer.
Putting empty ice-cream tubs (or similar containers) with cold water (to freeze to ice) is recommended if your fridge is only partially full for long periods of time.
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I have always rationalised the fact that the full freezer is more efficient by noting the cold air that rushes out when you open the door; the cold loaves and tubs of ice cream stay in the freezer whilst you replace the cold air with warm air from your room. the less cold air to re-cool every time you open the door the less power needed.
but I do think P'corns is a better fuller explanation
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...the less cold air to re-cool every time you open the door the less power needed.
Very true as well. But a full-up freezer would be more efficient even if one never opened the door! [;D]
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You have a few issues.
In another chain, someone was saying that the optimum freezer temperature is about 0°F... perhaps up to 4°F, but not higher such as the freezing point of water, 32°F. So the ice may provide somewhat of a heat sink in the freezer (like a package of meat or a tray of ice), and may help stabilize the temperature when you open the door and add something more... or during a blackout.
But, it also insulates the freezer elements so that it takes more work for the freezer to freeze... in theory, at least.
It also wastes space which may be the biggest problem.
HOWEVER
Frost-Free freezers have a greater tendency to dry out and Burn the freezer contents.
If there is a trade-off from ruining my food due to dehydration and freezer burns vs defrosting once a year or so... perhaps it is worth it to defrost.
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In another chain, someone was saying that the optimum freezer temperature is about 0°F... perhaps up to 4°F, but not higher such as the freezing point of water, 32°F. So the ice may provide somewhat of a heat sink in the freezer (like a package of meat or a tray of ice), and may help stabilize the temperature when you open the door and add something more... or during a blackout. But, it also insulates the freezer elements so that it takes more work for the freezer to freeze... in theory, at least.
I thought I'd made all these points. [Can we work in Celsius, please?]
Frost-Free freezers have a greater tendency to dry out and Burn the freezer contents.
How do these freezers work?
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Frost-Free freezers have a greater tendency to dry out and Burn the freezer contents.
How do these freezers work?
Ok...
I'll try to use Celsius...
As long as you promise to use US Liquid Gallons for all references of volume :)
Good question about the Frost-Free Freezers.
I had always assumed they just dried the air (through a desiccator or something). But I was WRONG.
The notes that I'm seeing on the WWW indicate they have a heater element along the cooling coils.
Every 6-12 hours, they cycle through heating the cooling coils (inside the freezer) until the coils reach above Freezing (273.15K, 0°C, 32°F)
This doesn't mean that the inside of the freezer reaches above freezing, just the cooling coils (which would mean no ice on the coils).
Sorry, I don't know the actual temperature fluctuation inside of the freezer. But, it effectively prevents frost buildup on the cooling coils.
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Frost-Free freezers have a greater tendency to dry out and Burn the freezer contents.
How do these freezers work?
Ok...
I'll try to use Celsius...
As long as you promise to use US Liquid Gallons for all references of volume :)
Good question about the Frost-Free Freezers.
I had always assumed they just dried the air (through a desiccator or something). But I was WRONG.
The notes that I'm seeing on the WWW indicate they have a heater element along the cooling coils.
Every 6-12 hours, they cycle through heating the cooling coils (inside the freezer) until the coils reach above Freezing (273.15K, 0°C, 32°F)
This doesn't mean that the inside of the freezer reaches above freezing, just the cooling coils (which would mean no ice on the coils).
Sorry, I don't know the actual temperature fluctuation inside of the freezer. But, it effectively prevents frost buildup on the cooling coils.
Does this also include the single deep freeze storage units?
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Does this also include the single deep freeze storage units?
Yes. You can get frost free versions of them too. When we bought one the sales guy told us that they are more prone to producing "freezer burn". His advice was that if you don't need to open the door frequently, just go with a manual defrost version. We did, and we hardly ever have to defrost it.
Presumably a layer of ice on the heat exchangers reduces their ability to remove heat from the air in the cabinet. In effect, it's like operating the compressor with a smaller heat exchanger than it's designed for, so the compressor has to run longer to lower the temperature to the point where the thermostat turns the compressor off.
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Does this also include the single deep freeze storage units?
Yes. You can get frost free versions of them too. When we bought one the sales guy told us that they are more prone to producing "freezer burn". His advice was that if you don't need to open the door frequently, just go with a manual defrost version. We did, and we hardly ever have to defrost it.
Presumably a layer of ice on the heat exchangers reduces their ability to remove heat from the air in the cabinet. In effect, it's like operating the compressor with a smaller heat exchanger than it's designed for, so the compressor has to run longer to lower the temperature to the point where the thermostat turns the compressor off.
I guess to reduce freezer burn is to prefreeze the food then vacuum pack it
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Maffsolo:
Freezer burn occurs when water sublimes out of frozen food starting a process sort of like freeze drying. If the food is in a container that has airspace the water re-condenses as frost inside the container. Once I found this out I tried packing some soft meat (deer burger) into zip locks and pressing out all the air. Without air spaces, ice can't sublime. I tried one package that was 5 years old it was still pretty good. So, do the vacuum pack before freezing, and when you can't eliminate air spaces, eat it within a few weeks for the best flavor.
Steve
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Maffsolo:
Freezer burn occurs when water sublimes out of frozen food starting a process sort of like freeze drying. If the food is in a container that has airspace the water re-condenses as frost inside the container. Once I found this out I tried packing some soft meat (deer burger) into zip locks and pressing out all the air. Without air spaces, ice can't sublime. I tried one package that was 5 years old it was still pretty good. So, do the vacuum pack before freezing, and when you can't eliminate air spaces, eat it within a few weeks for the best flavor.
Steve
Steve I have had some bad luck with the standard brand name zip lock bags. I found that they some times leak! Not on the zipped part but where the manufacturer has sealed the zipper part to the bag. This has happened on several attempts on different purchases of the bags.
So I bought the vacuum heat sealer food saver brand and the machine.
I started to begin my seals of nice Delmonicos. The process was simple but after I had them sealed I found that they weren't sealing either.
I found that moisture from the meat had gotten between the area inside the bag where the heat seal band rests to seal. That little spot did not seal.
After that I made the bags place the meat in them, dried the upper neck of the bags and set them all upright in the freezer for a couple of hours.
After that I took them out and pulled a vacuum and they all sealed.
Just to make sure I did a second seal just above that. Placed them all in the freezer for the duration and I had no problems after that.
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We discussed this question on our show
Dave - The problem is, because it’s a good insulator.
The way a freezer works, you have a load of pipes at the back, you compress a gas in the pump, that makes it hot and loses the heat out of the back of the fridge or freezer. It then pipes this condensed, compressed room temperature gas into the freezer, it then expands, evaporates, gets cold, and so the coldest bits of the fridge are where this expanded gas is flowing through.
The problem is that wants to get the heat from the fridge, butif you've gota great big layer of ice, that's going to insulate the cooling part of the fridge from the contents of your fridge, so the fridge is going to be warmer, which means that the actual fridge-freezer is going to work harder to keep cold, which means it gets even colder, it means you get more ice that will build up so it’ll go horribly wrong until the fridge just conks out.
Chris - And you have a big bill.
Click to visit the show page for the podcast in which this question is answered. (http://www.thenakedscientists.com/HTML/podcasts/show/2011.01.16/) Alternatively, [chapter podcast=2986 track=11.01.16/Naked_Scientists_Show_11.01.16_7771.mp3](https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.thenakedscientists.com%2FHTML%2Ftypo3conf%2Fext%2Fnaksci_podcast%2Fgnome-settings-sound.gif&hash=f2b0d108dc173aeaa367f8db2e2171bd) listen to the answer now[/chapter] or [download as MP3] (http://nakeddiscovery.com/downloads/split_individual/11.01.16/Naked_Scientists_Show_11.01.16_7771.mp3)