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Old 23-09-2010, 23:18   #31
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Originally Posted by foggysail View Post
I have my doubts that a full fridge is any better than one nearer to being empty. Heat loss is a function of the temperature difference between the the two surfaces inside thefridge and outside the fridge times the conductance (1/Rfactor) . I doubt there is much radiated heat but there oculd be leakage due to poor gasketing.

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Whatever you do, do not blow a gasket! The calming effect associated a with a fridge full of aromatic hopp infused antifreeze is a palpable testimony to the rightousness of natures goodness, arrrgh, I need to go to the rail.

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Old 24-09-2010, 01:42   #32
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You don't want anything in the fridge that doesn't have to keep cold, water has to be made cold before it would be affective, then you still have to keep it cold. A full fridge is more efficient because it's less space to cool.
IMO, Your best bet is to do what we do. We have various sizes of styrofoam blocks cut to fit the fridge. They fill in the dead space and don't require cold to do it. Sheets of foam is okay but you will still be cooling the dead space unless you have a perfecy seal....
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Old 24-09-2010, 04:44   #33
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And finally we now keep a couple of beer mat towels in the bottom of the fridge. Changed every couple of days they stop the condensation building up and products sitting in a pool of water which leads to loss of heat through conduction.

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Old 24-09-2010, 05:45   #34
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Originally Posted by malovich View Post
Science, not theory:

A kg of air and a kg of of water do not store the same amount of energy per the same change in temperature. Thermal energy storage is a tough bugger to describe, generally expressed in units of J/kg*K which means the amount of energy absorbed (Joules) required to raise 1kg by 1 degree C. Higher numbers are better in this case:

specific heat of air at 0c: 1000 J/kg*K
specific heat of ice at 0c: 2050 J/kg*K
specific heat of water at 0c: 4187 J/kg*K
specific heat of aluminum: 910 J/kg*K
specific heat of chicken: 1470-1840 J/kg*K


This more exact description shows that losing cold air is even less of a big deal than I argued.

My point, which it does not seem that everyone understood, was that cooling down room temp air let into the fridge when the door is opened takes VERY little energy. Which is another way of saying that very little "cold" is contained in the air inside your fridge. Which means that when you open your fridge and let the air out, you are not "letting all the cold out", contrary to erroneous popular belief.

Air at room temp has a mass of about 1200 grams per cubic meter. That means that an entire 80 liter fridge full of air contains only 96 grams of air. Each gram of air only contains as much heat as 0.25 grams of water because of the specific heat properties of air which Malovich gave us. So the heat contained in an entire 80 liter fridge full of air is only equal to that contained in 24cc of water, which is about one and one-half tablespoons of water. In fact, your fridge will usually not be completely empty, so it will be even less in reality.

To put that into perspective, if you put one single twelve ounce beer at room temperature into your fridge, then cooling that down requires as much energy as opening the fridge and letting all of the cold air out of it, twelve times over. That is the worst case scenario, too, assuming a totally empty fridge. That also assumes an 80 liter fridge, about average I think for cruising boats.

So you gain practically nothing from filling up your fridge to reduce the amount of air in it. This is a complete waste of time. More likely you will actually harm the performance of your fridge, because with all that filler in there you will have to rummage longer to find things. If you leave the door open long enough while rummaging, then you will start to warm up the contents. If the air is no big deal, then the contents are very much a big deal, as they contain a whole lot of heat (or "cold", as the case may be).

Reducing the size of your fridge, however, brings big benefits. That is because the surface area of the fridge box goes up geometrically with the volume of the fridge. The amount of heat which is lost through the walls of a fridge is a linear function of the surface area of the fridge box. If your fridge were a sphere, the surface area would go down with volume at the rate of 4πr2.

Another very worthwhile thing, as others have mentioned, is improving the insulation of your fridge.

But reducing air volume inside your fridge to reduce heat loss by putting in some kind of filler -- forget about it.
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Old 24-09-2010, 05:52   #35
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Originally Posted by Event_Horizon View Post
When the door is shut the full fridge still admits heat at the same rate and is as equally inefficient as an empty fridge.
Exactly right. Adding thermal mass inside the fridge will not change the rate of heat loss, which needs to be made up by the compressor.

Added thermal mass (like bottles of water, for example) will increase the load greatly during initial cool-down of the thermal mass. The heat (the "cold" in this case, that is, the absence of heat) stored in the thermal mass will however even out the compressor cycles. The temperature inside the fridge will change more slowly, and so the compressor will cycle less often. That might improve its efficiency slightly. But the compressor will not run LESS -- just less often. Because when the temperature does come up, the compressor will have to make up a greater amount of thermal energy, for a given number of degrees change, because there is more thermal mass in the fridge.

This might be some benefit, but it would be pretty slight, I think.

Quote:
Originally Posted by Event_Horizon View Post
The added "stuff" merely accepts thermal energy at a slower pace than "empty" volume (not to mention less of that cold air you worked so hard for is spilling out). Less heat energy gets in, less energy to pump out, less time running the refrigeration cycle.
You don't "work so hard" to produce cold air. See figures above. To cool an entire fridge full of air requires only as much energy as cooling a tablespoon and a half of water. It's true that substituting air with something which doesn't "spill out" will mean less of something to cool after opening a door. But in this case it makes very little difference.

This is why front-opening fridges work reasonably well. If "all the cold" were contained in the air inside the fridge, then we would only use top-opening fridges, even at home. In fact front opening fridges have similar efficiency in real life to top-opening ones. Because spilling the cold air is no big deal.
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Old 24-09-2010, 05:59   #36
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Quote:
Originally Posted by foggysail View Post
I have my doubts that a full fridge is any better than one nearer to being empty. Heat loss is a function of the temperature difference between the the two surfaces inside thefridge and outside the fridge times the conductance (1/Rfactor) . I doubt there is much radiated heat but there oculd be leakage due to poor gasketing.
Foggy
In that vein of thought, it is also a fact that the more often the refrigerator is opened the more outside heat can get inside and raise the temperature. Since the normal temperature of a "refrigerator" is kept between 35F and 40F large blocks of ice would not last long before they turn to water. The ice works in the freezer but really not very well in a refrigerator.
- - The beer/booze idea works much better on paper, but unfortunately all the calculating on paper causes a mighty thirst which results in excessive opening of the refrigerator which defeats the purpose.
- - So the best idea is the enjoy the beer/booze and not worry about the extra run time of the refrig to keep it cool. An alternative is to use the dense foam blocks to fill up any excessive dead space.
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Old 24-09-2010, 06:35   #37
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I'm glad my question wasn't totally stupid
Nice discussion and I have learned a lot, as I think many have new ideas for their own situations.
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Old 24-09-2010, 07:32   #38
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The only prob with the extra space is that it is air. As soon as you open it all the cold air escapes and is replaced with warm air, so when you close it the warm air has to cool again wasting power. Filling it up with foam will prevent that loss of air. You may as well somehow put the foam on as many sides of the fridge as possible to provide extra insulation in addition to reduction of airspace. That would be the ultimate solution I reckon.
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Old 24-09-2010, 07:46   #39
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How about putting empty plastic bottles in the fridge?
The air inside can't fall out, stays cool.
More permanent solutioons use foam or insulating the walls etc.
But plastic bottle are always around the boat somewhere... generally in the galley just next to the empty Whiskey bottle.........
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Old 24-09-2010, 08:54   #40
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Originally Posted by osirissail View Post
- - So the best idea is the enjoy the beer/booze and not worry about the extra run time of the refrig to keep it cool. An alternative is to use the dense foam blocks to fill up any excessive dead space.

Can we make that ROOT BEER????

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Old 24-09-2010, 15:30   #41
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This more exact description shows that losing cold air is even less of a big deal than I argued.

STUFF STUFF STUFF....

But reducing air volume inside your fridge to reduce heat loss by putting in some kind of filler -- forget about it.
Nothing you said in inherently wrong, save you conclusion. Your arguments are operating under the assumption that the cooling cycle operates at equal efficiency from start to finish.

It is obvious from this quote:

Quote:
The heat (the "cold" in this case, that is, the absence of heat) stored in the thermal mass will however even out the compressor cycles. The temperature inside the fridge will change more slowly, and so the compressor will cycle less often. That might improve its efficiency slightly. But the compressor will not run LESS -- just less often.
That you grasp the general concept behind adding filler (thank you for defining cold as the absence of heat, that part was nifty).

Unfortunately you're wrong, having longer cycles less frequently doesn't "improve its efficiency slightly," it improves it a lot. The reason for this is as stated before, the evaporation cycle is like a car. The start up takes more gas than the steady state running of the thing. In the case of most refrigeration cycles however, the start up takes 2-3 times as much energy to get going. In the case of a 4amp compressor, the start up will draw 12amps-ish.

Basically, you're correct in that the general thermal energy added and detracted from the system may not be astoundingly different. It's just that running the refrigeration cycle for longer duration infrequently is MUCH more efficient than shorter durations constantly.
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Old 24-09-2010, 15:44   #42
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I noticed on my last trip through Antarctica... round the horn the wrong way in the winter with our Irwin...That if we left the fridge doors open the compressor unit never seemed to need to cycle too often....

Stands to reason that a cold sink internal to the fridge would have the same effect...The larger the better...or in other words the fuller with frozen goods the better.

Foam or Stuff unable to absorb and store cold will not have the same desired effect other then just plan turning it into a smaller fridge with awesome insulation property. and a quicker recovery time on lost cold air...It wont freeze anything any faster not counting that recovery time.
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Old 24-09-2010, 15:54   #43
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Originally Posted by Event_Horizon View Post
Nothing you said in inherently wrong, save you conclusion. Your arguments are operating under the assumption that the cooling cycle operates at equal efficiency from start to finish.

It is obvious from this quote:



That you grasp the general concept behind adding filler (thank you for defining cold as the absence of heat, that part was nifty).

Unfortunately you're wrong, having longer cycles less frequently doesn't "improve its efficiency slightly," it improves it a lot. The reason for this is as stated before, the evaporation cycle is like a car. The start up takes more gas than the steady state running of the thing. In the case of most refrigeration cycles however, the start up takes 2-3 times as much energy to get going. In the case of a 4amp compressor, the start up will draw 12amps-ish.

Basically, you're correct in that the general thermal energy added and detracted from the system may not be astoundingly different. It's just that running the refrigeration cycle for longer duration infrequently is MUCH more efficient than shorter durations constantly.
Well, if it were true that running the compressor longer and less often were MUCH more efficient than running it shorter and more often, then everything you say would be right. The only problem is, I don't see why in the world that should be so. The high current draw during startup is not it. It lasts for less than a second.
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Old 24-09-2010, 16:11   #44
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Well, if it were true that running the compressor longer and less often were MUCH more efficient than running it shorter and more often, then everything you say would be right. The only problem is, I don't see why in the world that should be so. The high current draw during startup is not it. It lasts for less than a second.
The reason is because of how the refrigeration cycle works (which I'm assuming is the vapor compression cycle). When it is not running the refrigerant just sort of sits there, warms up...tries to reach equilibrium... Your compressor needs to fire up and get it all flowing so the phase changes can start happening. This is a significant hump to get over. It is why when you turn on the AC in your car it takes a bit to start pumping cold air as opposed to spewing it out instantly. The compressor actually has to run for awhile before you start seeing any cooling.

Also, can you provide me with what evidence you have that shows this period is less than a second...because I'm pretty sure it is quite a bit longer.
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Old 24-09-2010, 17:34   #45
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Originally Posted by Event_Horizon View Post
The reason is because of how the refrigeration cycle works (which I'm assuming is the vapor compression cycle). When it is not running the refrigerant just sort of sits there, warms up...tries to reach equilibrium... Your compressor needs to fire up and get it all flowing so the phase changes can start happening. This is a significant hump to get over. It is why when you turn on the AC in your car it takes a bit to start pumping cold air as opposed to spewing it out instantly. The compressor actually has to run for awhile before you start seeing any cooling.

Also, can you provide me with what evidence you have that shows this period is less than a second...because I'm pretty sure it is quite a bit longer.
The startup load is usually 500 milliseconds or so, according to the electrical people who sell us equipment which is supposed to be able to deal with these loads. Which doesn't mean it's necessarily so, but that's the only data I have. I have an open mind about your theory, if you have fact and numbers which contradict what I know. Do you?
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