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Old 27-08-2010, 19:21   #1
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Insulation Between Fridge and Freezer

I am building a cold box that will have a freezer with spilover for refrigeration. The box is made of 6" of extruded polyurethane. Total volume is about 6 cu ft. Basicly one large box with a divider between the fridge and freezer. My plan is to make the divider out of 1/2" foam glassed on both sides. My question to those who have a spillover system is what thickness divider do you have? Is 1/2" foam too little? Too much? My thought is that if it is too little I can add more, too much and I can poke holes in it, maybe even install a fan. Thoughts?

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Mike
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Old 27-08-2010, 19:48   #2
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Your plan sounds very good to me! We have a half inch foam barrier with plastic laminate on both sides and an array of six one and a half inch diameter holes. We keep three of these holes plugged with wads of paper towel and sometimes open or close a hole for adjustment. Our adjustments are usually a result of the volume of stores on either side. At one time I thought of the wads of paper towel as temporary before I designed better means to adjust the openings, but they are convenient and I doubt I'll make any changes. I always have at least one low hole and one nigh hole open & convection serves me well without a fan for circulation. My freezer side is about 2 & 1/2 cubic feet and the refrigeration side is about 5 cubic feet. I'm using a cold plate with a Danfloss compressor w/fan for air heat exchage. This runs at a 3 amp average 12VDC. Take care and joy, Aythya crew
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Old 27-08-2010, 20:18   #3
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Thanks Capt. Thats exactly the kind of info I was looking for. I like the adjustable holes thing. The paper wads sound like just my level of high tech.

Mike
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Old 28-08-2010, 21:56   #4
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what Danfoss compressor

Captn - can you tell me what compressor package you are using? And how noisy is it?
thanks
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Old 29-08-2010, 13:55   #5
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I used some left over marine ply about 3/8" thick with a couple layers of thin glass over it to keep the moisture out. I punched out 2 4" holes near the top and bottom, then added a plastic rotating cap. I experimented with the opening sizes trying to make sure I had good cold flow and few pockets of freezing air. It worked quite well as long as the fridge wasn't too loaded. In the end I installed a small 12V fan that moves the air a bit more. The freezer is a bit warmer and needs a charge a bit more often but I found it a simple and effective solution to a cold plate fridge configuration.
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Old 29-08-2010, 14:11   #6
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Captn - can you tell me what compressor package you are using? And how noisy is it?
thanks
There's very little noise with these 12VDC units that are using air for heat exchange,- just the running of the small fan that blows across the heat echange coils. Of course, the water is more efficient for heat exchange, but you expend more energy pumping the water. I do like the Technautics unit of mine, but a number of manufacturers use the efficient Danfloss compressors. Take care and joy, Aythya crew
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Old 30-08-2010, 16:53   #7
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Very few boat icebox conversion Spillover refrigerators function properly because basic thermodynamic simple principals are ignored. A separating divider between a refrigerator and a high temperature freezer of +15 to +22 degrees needs insulating value of at least R5. One inch of extruded polystyrene with impact resistant material on each side will provide a temperature difference between separated areas of 20 degrees. To extend flavor quality of food stored for more than three weeks freezer temperature range should be zero to +15 degrees then divider will need to be insulated to R10.

In assembling a spillover system or any refrigerator or freezer think about incorporating these principals so they work for you instead of against you.
  • In a static state air is a bad conductor of heat, when air is trapped and canít move it becomes a good insulator. Heat conduction occurs when two object at different temperatures are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature. For air to be a good conduct of heat there must be movement and collision of its molecules, therefore there must be air movement inside box at all times.
  • The warmer air is the less dense it is, so it tends to rise and cooler more dense air sinks. Before home refrigerators were redesigned with fan cooled evaporators boxes were equipped with wire shelving allowing heated air to move naturally and slowly tumbling in box. With evaporator at very top of box air below it tends not to stratify in layers. Warm air to top and when cooled it drops.
  • Cooling is the process of removing heat so heat removal inside an area depends on heat conducting medium (air) being able to surround warmer item as much as possible and return to evaporator where heat is absorbed.

My Recommendation for a spillover refrigerator.
Most boat spillover boxes are side by side requiring pressurized air movement to maintain individual box temperature control. One thermostat controls freezer temperature by cycling compressor on and off. A second thermostat controls a very small refrigerator fan located in middle of divider, return air to freezer is provided by a hole at top of divider. Air circulation in freezer requires that rails be installed across box bottom and up both sides to insure lower product stays frozen.

Compressor must have capacity to handle planed worst case cruising climate. Evaporator would be installed in freezer covering as much wall space as possible and have equal maximum capacity of compressor.
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