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Old 07-17-13, 05:09 PM   #31
ELGo
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Quote:
Originally Posted by AC_Hacker View Post
But the relationship is not linear, because area increases at roughly a square function, and volume increases at roughly a cubic function... and heat loss is proportional to area.
You are right with large numbers to focus on the exponents, but consider e.g. a rectangle that is elongated in height by 50%:

The surface area increases from
2xy + 2xh + 2yh to
2xy + 3xh + 3yh

While the volume increases from xyh to 1.5xyh.

Not that far off from linear in this range.

By the way, what is the temperature gradient in your fridge from bottom to top ? I'm wondering if extra external insulation on the top of the fridge would have much of an effect.

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Old 07-17-13, 06:34 PM   #32
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Quote:
Originally Posted by ELGo View Post
You are right with large numbers to focus on the exponents, but consider e.g. a rectangle that is elongated in height by 50%:

The surface area increases from
2xy + 2xh + 2yh to
2xy + 3xh + 3yh

While the volume increases from xyh to 1.5xyh.

Not that far off from linear in this range.
If the size of the refrigerators was determined by a chainsaw, this would be correct, but smaller refrigerators tend to become narrower, and sometimes shallower, too.

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...By the way, what is the temperature gradient in your fridge from bottom to top ? I'm wondering if extra external insulation on the top of the fridge would have much of an effect.
Yes, it would have a very significant effect.

My Freezerator, as most of the currently produced units, have the integral coils, and the outside surfaces become the heat radiating surface... the top, the two sides, and the back, all have integral coils. So to insulate would have a negative impact on efficiency.

Personally, I think that the integral coils feature costs the planet a huge amount of CO2. Only reason for it that I can imagine is that the coils are much less likely to be damaged during shipping. After shipping, it's down hill.

-AC
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Old 07-19-13, 04:41 AM   #33
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The current refrigerator we are using is consuming upwards of 5 kwh a day.
I think something is very wrong.
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Old 07-19-13, 06:19 PM   #34
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Just thinking here - and provoking AC to give us some thoughts (others too).

Clearly a square box is the simplest way to maximize volume with the smallest surface area (OK, I don't need smart *** comments on this really being a sphere!).

But if we were to built, from scratch, a 'fridge, how would we do it?

I bet we would build a modified rectangle, trying to be close to a cube, but wary of the deep recesses in back where "food goes to die" (quoting AC).

I bet we would choose an external "radiator" with stand offs and not one inside the metal cabinet - likely on the back (assuming an air to air compressor system).

I bet we would insulate with at least two inches of high performance foam (R10 per inch) and I bet we would forgo a freezer to maximize compressor performance with an interior temp of about 40 F.

Since the volume is small, how much water flow would it take to make this a water source compressor type fridge? I bet less than a fraction of a gallon per minute. (I know that as I use a rule of thumb of about 1-2 gallons/minute of water flow per 12,000 BTU of cooling.

Assume a 30 degree delta T with a house temp of 70 F (inside fridge temp of 40F).

Assume an outside 'fridge dimension of 4 feet tall, 3 feet wide and 2.5 feet deep (total surface area of 39 sq feet)

Given the above, what would it take to build this? I don't have my calculator available or I would calculate the net amount of BTUs needed to keep it at this design temp.

I bet a tiny compressor would be needed and a quart of water per minute (assuming an open loop system with a ten F delta T from water well temp to outflow temp.

Now, where could I get a fractional compressor real cheap . . . . (a compressor from a window AC unit would be far too big, same with auto and dehumidifier . . . .).

We all need a 'fridge and I bet this would not be a hard nut to crack!

Just thinking . . . .

Steve
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Old 07-19-13, 09:05 PM   #35
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Quote:
Originally Posted by stevehull View Post
Just thinking here - and provoking AC to give us some thoughts (others too).

Just thinking . . . .

Steve
Good thinking here.

I've done some serious looking into available compressors (probably no surprise there), and small refrigerator compressors are available in the 100 to 200 watt size. Trouble with them is that they use a very strange coaxial cap tube arrangement that seem to me to be difficult to hack, or duplicate. I wouldn't want to discourage anyone from trying, but I was not able to visualize an easy solution. If this cap tube thing could be overcome, the size of the compressor just might be perfect.

On the other hand, very small compressor-type dehumidifiers (as opposed to peltier-junction) can be found. The smallest that I have seen in the wild is a 25 pint per day... which was the guts for my first heat pump. I know that Sears sells a 20 pint per day, with a really small compressor... smallest I know of. The dehumidifier will have a conventional cap tube that will be easy to salvage and re-use.

So, if you're going to do this right, the compressor & condenser should sit on top of the unit, for best efficiency... just like the very first home refrigerators.



Sunfrost uses this kind of arrangement... and their efficiency is legendary... but they haven't brought a water-source unit to market.

I think a good place to start would be to find some kind of pre-made plastic container that would be the right size for the inside of the refrigerator, and build upon that.

Best,

-AC
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Old 07-20-13, 03:20 AM   #36
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I agree, stuffing the compressor and condenser up under the fridge is stupid.
Over sized passive coils along the back or top coils are best, it almost seems if their engineers forgot about convection.
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Old 07-20-13, 07:45 AM   #37
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Most of the newer (plastic) dehumidifiers are run by either a 4000 or 6000 btu compressor and use R22. The older (metal) dehumidifiers use R12 (ancient) or R134a (old) and have the size compressors in them you are looking for. I have seen a few with mini-fridge style compresssors in them that only draw 150 watts (including the fan). They run much more silently than the newer units. Check freecycle or craigslist in your area.
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Old 07-20-13, 01:23 PM   #38
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...Most of the newer (plastic) dehumidifiers are run by either a 4000 or 6000 btu compressor and use R22...
By "newer", you must be referring to newer used equipment.

R22 equipment is no longer allowed to be sold as new.

But the R22 used stuff is a very good fit for R290, and that makes it very attractive.

-AC
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Old 07-24-13, 08:05 PM   #39
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Default Freezerator Update...

I was looking about for a spare Kill-a-Watt, when I realized that I had left one attached to the Freezerator since March (1705 hours to be exact).

So I ran the numbers from the stored data (28.08 kw-h), and I am pleased to say that the average power use calculates to:

395.26 watt-hr/day

(about 1/12th of what oil_pan_4 is pulling from his refrigerator)

It did rise a bit (see photo below) because of the higher ambient temp, but I think that it is really quite respectable.

-AC

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Old 07-25-13, 08:52 AM   #40
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Very nice. That is about 1/3rd what my fridge uses.

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