Naked Science Forum
On the Lighter Side => That CAN'T be true! => Topic started by: chris on 03/04/2017 11:10:57
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Elizabeth wanted to know whether this claim is a potential "mythconception" for the programme.
Hi Naked Scientists!
Is there truth to the saying that "a fridge is most efficient when empty, while a freezer is most efficient when packed full"?
Thanks!
Elizabeth
What does everyone think?
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The function of a refrigerator is to keep stuff cool. If there is no stuff, it isn't doing anything useful so its efficiency is zero. If you never take stuff out of a full freezer, it has no useful function so again its efficiency is zero.
Whatever the device, once it has cooled the contents, its job is merely to reject the incoming ambient heat. In this respect there is very little difference between a refrigerator and a freezer. The functional difference is that you open refrigerator doors more often than freezer doors, and you tend to replace items that have warmed up to ambient but not been completely used, so the fridge probably has to work a little harder in practice.
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According to some, they are lots of things than can influence the performance of a freezer based on its content.
See : https://www.thenakedscientists.com/forum/index.php?topic=43522.msg477756#msg477756
Some of it being true, in the end all of this has very little effect on your real life fridge or freezer, or combo.
A given appliance, in a given room, with a given number of door openings, is less efficient when empty : this is due to the fact that it runs often because it has no thermal inertia, and because the cooling system is not 100% efficient.
Thermal Inertia :
When empty, as you start the freezer, the system has to remove all the heat from almost no mass of air inside the freezer. This is quick, but as the freezer is not 100% insulated, this given "almost no mass of air" will become warmer faster than a larger mass of frozen good for a given insulation.
The freezer will need to start frequently for short runs. Is this the same as if it would start less often for longer runs? No.
Efficiency of the cooling system :
Real life heat transfer does not start right away when the compressor kick ins. The compressor needs to rebuild pressure. This takes so much energy that your fridge has to be on a separate breaker according to the current building code. So on a short run, the efficiency of the cooling system drops because of the priming period.
As you may know, a freezer transfers energy. The energy is taken from the freezer back to the ambient air where the freezer sits. In the case of an empty freezer, this will account for almost nothing, and won't really warm up the ambient air. But, your compressor, when running, releases (lots of) heat in the ambient room. As it runs often for inefficeint short runs (empty freezer, no thermal inertia), it will make the ambient air warmer, which will lead to faster loss of cold in the freezer and so on, and have the system start even more often.
A combination of an empty freezer in a poorly ventilated room, can have a significant impact on your energy bill, and you will notice the air in the room being significantly warmer.
As for a freezer / fridge combo, thermal inertia still applies, and as they have the same cooling system, using a active or passive techniques to transfer cool from freezer to fridge (forcing the cold air up or down)
Phil
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Well, that's a good question and a lot of people wants to know how can they make refrigerator more efficient while in summers.
The more stuffed your fridge shelves become, the harder it is to keep cool. Just like a crowded room, a full fridge will be naturally warmer than an empty one. Experts recommend turning down the thermostat by one degree if you've totally filled it. Nor will an empty fridge be very efficient
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The more stuffed your fridge shelves become, the harder it is to keep cool. Just like a crowded room,
That's not a great analogy.
People in a crowded room generate a lot of heat.
The stuff in your fridge barely generates any.
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Indeed one would hope that it is all dead and not fermenting.
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Most salad isn't dead.
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Now you've really upset vegans. I'm going to murder a sentient lettuce for lunch.
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A refrigerator's main purpose is to keep things chilled. Without anything to do, it is ineffective and has no efficiency. A full freezer has no utility if nothing is ever taken out of it, therefore once more its efficiency is zero.
Whatever the device, its sole function after cooling the contents is to reject the incoming ambient heat. There isn't much of a distinction between a refrigerator and a freezer in this regard. The refrigerator undoubtedly has to work a little harder in practice since you open refrigerator doors more frequently than fridge guide doors and you frequently replace items that have warmed to room temperature but haven't been fully used. (https://www.thenakedscientists.com/forum[url=spam[/url)