Putting containers of water in freezer

grumpy_codger

Section62 said:

grumpy_codger said:

CliveOfIndia said:

In very simple terms, it takes a lot more energy to keep air cold than it does to keep a more solid object cold. 

Is it a fact, not an urban myth? I don't see any physics behind this.

The fact is the opposite - the specific heat of water is about 4 times that of air.

You grossly underestimate the difference as specific heat capacity is per mass, not per volume. At 0C air is about 773 less dense. So, for a freezer the difference is about 3000 times.

However, you freeze a container with water once and keep it frozen. So, for a long time the relative difference can be negligible.

ETA: and as we speak about freezers, not fridges, the latent heat of fusion for water is much more important than the specific heat capacity. It's 334 kJ/kg  to freeze vs. about 4 J/kg°C to cool (i.e. about 160 J/kg to cool from +20C to -20C).

If the freezer door is a perfect seal and stays shut then the 'cold' loss through the cabinet will be more or less the same whether the freezer is full or half empty.  So the amount of work done by the compressor should be about the same in either case.

Yes, the amount is the same, but what I meant was saving from developing a fault. Wear is usually higher in starting regime  and  electric current is higher.

Section62

grumpy_codger said:

Section62 said:

grumpy_codger said:

CliveOfIndia said:

In very simple terms, it takes a lot more energy to keep air cold than it does to keep a more solid object cold. 

Is it a fact, not an urban myth? I don't see any physics behind this.

The fact is the opposite - the specific heat of water is about 4 times that of air.

You grossly underestimate the difference...

I don't think I did.  I just pointed out the difference in specific heat, which for water is about 4 times that of air.

grumpy_codger said:

If the freezer door is a perfect seal and stays shut then the 'cold' loss through the cabinet will be more or less the same whether the freezer is full or half empty.  So the amount of work done by the compressor should be about the same in either case.

Yes, the amount is the same, but what I meant was saving from developing a fault. Wear is usually higher in starting regime  and  electric current is higher.

If the 'cold' loss is more or less the same, and the work done by the compressor is more or less the same, then you wouldn't expect the compressor to have either more or fewer starts.  So no increase or decrease in wear?

Sarahspangles

Isn’t the difference simply that cold air, as a gas, will spill out when the door is opened and be replaced by warm air, whereas a block of a frozen solid stays put?

quartzz

Sarahspangles said:

Isn’t the difference simply that cold air, as a gas, will spill out when the door is opened and be replaced by warm air, whereas a block of a frozen solid stays put?

yyyyup. I think this is what we're saying. It'll act like a cold "battery"

Section62

quartzz said:

Sarahspangles said:

Isn’t the difference simply that cold air, as a gas, will spill out when the door is opened and be replaced by warm air, whereas a block of a frozen solid stays put?

yyyyup. I think this is what we're saying. It'll act like a cold "battery"

No, it isn't really anything like a battery.  It is simply that whatever - other than free air - is contained in the freezer will stay put, whereas the free air can escape.

If the freezer is filled up with inflated beachballs there will be less air that can escape when the door is opened.  Ditto blocks of polystyrene, bundles of newspaper, dead bodies, etc.

quartzz

That's what I mean, that's what a battery is. It's an object which stores energy, in this case it's a jug of water which is storing cold, and that cold can't escape like a gust of air when the door or lid is opened.

Inflated beachballs are still a battery, but rather than water storing cold, air is storing it - probably not as effectively.
a lithium ion battery could store cold, it's just better at storing other things. Dead bodies? depends where you live I guess?

grumpy_codger

Section62 said:

grumpy_codger said:

Section62 said:

grumpy_codger said:

CliveOfIndia said:

In very simple terms, it takes a lot more energy to keep air cold than it does to keep a more solid object cold. 

Is it a fact, not an urban myth? I don't see any physics behind this.

The fact is the opposite - the specific heat of water is about 4 times that of air.

You grossly underestimate the difference...

I don't think I did.  I just pointed out the difference in specific heat, which for water is about 4 times that of air.

grumpy_codger said:

If the freezer door is a perfect seal and stays shut then the 'cold' loss through the cabinet will be more or less the same whether the freezer is full or half empty.  So the amount of work done by the compressor should be about the same in either case.

Yes, the amount is the same, but what I meant was saving from developing a fault. Wear is usually higher in starting regime  and  electric current is higher.

If the 'cold' loss is more or less the same, and the work done by the compressor is more or less the same, then you wouldn't expect the compressor to have either more or fewer starts.  So no increase or decrease in wear?

Let's say the thermostat starts the compressor when the temperature rises 1C above the set value and switches it off when the set temperature is reached.

The heat transfer rate (J/sec) depends only on the temperature difference inside and outside. It's the same for empty and full freezer.

For a full freezer it takes much longer to lose/gain 1C - because of the higher heat capacity of the content (vs. the same volume of air). Similarly, it will take longer for the compressor to get the temperature back to the set value. It will work longer, but will be switched on less frequently (the total time of work over a long time will be the same).

FreeBear

p00hsticks said:

CliveOfIndia said:

It's not a new idea - even when I was a kid, the advice was to stick loads of loaves of bread in the freezer if it had a lot of empty space ( assuming you would eventually eat the bread, of course! )

Yes, we still use cheap sliced bread sometimes to bulk out the freezer if it's getting a bit empty - freeze it in four slices together and it defrosts really quickly when needed - even straight in the toaster.

My chest freezer is kept pretty full with paper

That reminds me, I really ought to defrost the darned thing soon...

CliveOfIndia

quartzz said:

That's what I mean, that's what a battery is. It's an object which stores energy, in this case it's a jug of water which is storing cold, and that cold can't escape like a gust of air when the door or lid is opened.

You're kind of right.  For all practical purposes, it's the air escaping that's the problem.  A simplified example :

Let's say the freezer capacity is 20 litres, and it's empty.  Switch it on and shut the door.  The compressor runs until all the air is down to -18 degrees or whatever.  At which point the compressor will shut off, and only kick in very occasionally to maintain -18 degrees.

If you open the door and leave it open for a couple of minutes, 20 litres of cold air comes out and 20 litres of warm air goes in - which has to be cooled down again.

Now, if you've got a 10 litre sealed container in there, you're only losing 10 litres of cold air every time you open the door, so you've only got 10 litres of warm air entering that has to be cooled again, instead of 20.

As I say, this is a simplified illustration.  You don't usually swap all of the cold air for warm air every time you open the door.  And depending on what the "object" in the freezer is, it will act to a greater or lesser extent as a heat sink.

But essentially you want to reduce the amount of "free" air in the freezer, so less heat is lost every time you open the door.  If you never opened the door it would make no difference if it was full or empty, the only heat loss would be that which leaks out gradually from the casing or whatever.

CliveOfIndia

Section62 said:

The bread 'trick' works because it fills volume in the freezer - bread has no magical properties.

Exactly.  The only reason bread is commonly used is because it's cheap, bulky and is something that's useful to have available anyway.  You'd get the same effect by filling the empty space with fillet steak, lobsters or caviar.  Or a dead body, as someone else said