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Old 22-09-2010, 21:47   #16
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But I know what you mean: take two beers out at a time.

Now that is I can work with..layman's science..
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Old 22-09-2010, 22:17   #17
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So the 2 relevant answers are: taking up the empty volume with something will decrease the number of cycles per day therefor increasing efficiency by removing that moment before and after steady state cooling. The best filler would be a material with a high specific heat, water interestingly does serve this purpose very well, compressed hydrogen would be better but logistically useless and dangerous.

The second solution is insulating the walls, already mentioned with a great installation method. You could maybe get a better appearance with pvc sheet as a liner than foamboard. This will greatly increase efficiency but you will lose that potential storage space without being able to quickly/easily reclaim it.
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Old 22-09-2010, 23:10   #18
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You could maybe get a better appearance with pvc sheet as a liner than foamboard.
The stuff I've used has smooth PVC faces. Like paper foam board, but all PVC. It's a sign maker/tradeshow kinda thing. Really nice to work with. Strong, cuts with woodworking tools. Edges bevel nicely. Cements well. Scrubbable bright white.
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Old 23-09-2010, 01:15   #19
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So the 2 relevant answers are: taking up the empty volume with something will decrease the number of cycles per day therefor increasing efficiency by removing that moment before and after steady state cooling.
Yup, the less time that compressor needs to be running the better.

The higher internal heat capacity really only helps when opening and closing the door. When the door is shut the full fridge still admits heat at the same rate and is as equally inefficient as an empty fridge. 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.
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Old 23-09-2010, 02:53   #20
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Old 23-09-2010, 02:59   #21
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store liquid ... bottles of vodka work well ;-)
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Old 23-09-2010, 03:09   #22
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. Open a door on an empty refer and there goes all the cool.
Actually, not.

Check out the density of air, compared to, say cheese. If you were to let all the air out of your fridge and replace it with room temp air, it's no more work for the compressor than if you put in a small piece of cheese at room temp.

'All the cold' is not contained in the air, which weighs at most a few ounces. It's contained in the contents. As long as you don't warm up the contents, letting the air out from time to time is no big deal.

Adding thermal mass in the fridge (bottles of beer or water) won't change the energy consumption of the fridge. It might be pleasant to your compressor since it will even out the loads.
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Old 23-09-2010, 03:46   #23
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Theoretical opinion not science -

- You have a half full fridge - It makes sense to only want to cool half the volume regardless of what is in it or not in it.

- The energy equation is a mass equation - i.e. 3 kilos of chicken and 3 kilos of "air" probably take similar energy to cool - you just can't fit 3 kilos of (uncompressed) air in your box. Opening an empty reefer and spilling some cold air is a big deal because whatever volume it is you are immediately replacing 2 DEGC air with 28DEGC air and it has to be cooled. Chicken is a good heat sink. In this case the chicken warms slowly so the same volume full of chicken (full reefer volume) is not a big deal when you open the door because you don't transfer much heat from the chicken during the relatively short time the door is open. But imagine if every time you opened the reefer you took out 1 kilo of cold chicken and put in 1 kilo of warm chicken. Not good.

- The warm air that enters doesn't care where it gets it's energy deficiency from - it will take it from the evaporator coils or from the Chicken as it seeks equilibrium with the rest of the reefer contents - i.e. ice will melt and the chicken will start to thaw.

- Heat rises - we know that so a top loading reefer makes sense because less cool air volume is spilled when the lid is opened.

- Heat transfer takes place on the surfaces. Block ice is better than cube is better than crushed ice when trying to keep ice from melting. When trying to cool a fruity boat drink the opposite is true. Therefore a whole chicken is better than chicken parts is better than diced chicken when trying to keep it cold. And - OK - 24 oz beers are better than 16 oz beers are better than 8 oz beers.

- And finally what transfers the energy from the coils to the chicken? Right - the air.

So adding all this theory up means - We don't want an empty or half empty reefer. Air spills and needs to be cooled. Air is what cools the items so the items need circulation room when cooling but once cool if we could reduce their surface area they would stay cold longer. It all seems to make sense that we need to reduce the interior volume of the reefer.

- Construction of the box makes a difference - how is the evaporator coil arrayed? If it's along one wall of the box you do not want to isolate that wall from the groceries or the air.

Assume a top loader with the evaporator coils on the back wall. You want to reduce the volume starting from the front wall. I would reduce the volume with some sort of heat sink like an aluminum block. It would not lose much heat when the door is open and would facilitate cooling the air quickly when the lid is closed.

If the coils are in the bottom of the box I think the solution is tougher. In this case I think I would still use the heat sink but It would have to be stood off the floor and sides of the box so the air can circulate around the heat sink from the coils to the top of the box.

Look at the freezer section of the grocery store. Most of the items are in hanging baskets so the air can circulate.

An finally the door of the reefer needs to be as small as possible. Remember ice cream trucks? They had a big door - for loading - but built into the big door was a small door that the guy used to get the ice cream from. And it goes without saying the seals on the door must be as good as you can get.
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Old 23-09-2010, 08:38   #24
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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

As to the size of the filler, the same volume with greater surface area will cool the air faster and in turn warm it's self up just as fast, the same volume with less surface area will not chill the air as fast but it will chill the air longer. I have no idea which would be the most efficient. I can however assure you that adding more insulation to the walls will greatly increase the system efficiency.

In freezing scenarios, there is also something called latent heat. If ice were to be at 0c, it would require 334 kJ/kg of heat absorption to reach 0c in a water state. That's phase change for you and can be a minor (or major) hurdle for temperature change.

As a side note, I work at a company that makes commercial ice machines and we have a prototype machine that makes ice cubes 2.25x1.125x1.125 inches and 1 cube will last through 2 mixed drinks granted the drink will not be super cold but it will be generally cold much longer without getting watered down as quickly!
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Old 23-09-2010, 09:01   #25
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One CF regular with a similar problem filled the bottom of his fridge with styrofoam pellets. If you put them in plastic bags they would be easy to remove if needed and would still add insulation at the bottom as well.

Of course, if you had a Rocna fridge thermodynamics would no longer apply so all your problems would be solved



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Old 23-09-2010, 09:23   #26
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Of course, if you had a Rocna fridge thermodynamics would no longer apply so all your problems would be solved
I have two Rocna anchors. No impact on my fridge.
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Old 23-09-2010, 11:56   #27
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...1 cube will last through 2 mixed drinks granted the drink will not be super cold but it will be generally cold much longer without getting watered down as quickly!
Now there's a research job I want!
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Old 23-09-2010, 11:57   #28
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I have two Rocna anchors. No impact on my fridge.
You have to put them inside the fridge.
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Old 23-09-2010, 20:05   #29
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Quote:
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Science, not theory:

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
Thanks for the science

With the aluminum and the air having similar heat values it would appear that an aluminum filler would be no better than air except:

- The air is being lost to the system when the door opens, the aluminum is not
- The volume of one kilo air vs. one kilo aluminum is vastly different.

Reducing the fridge volume still makes sense does it not?
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Old 23-09-2010, 20:51   #30
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Reducing the fridge volume still makes sense does it not?
Reducing the air volume makes cent$.
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