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Old 11-02-2007, 12:36   #1
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Economical Refrigeration

I am not sure what Forum to enter this in. But since the work I am doing is to reduce battery drain so my solar panels can handle it, perhaps this is the place.

I am retrofitting my refrigerator to reduce its demand on the batteries.

From Wings book "The Boaters Illustrated Handbook of Wiring" I have gleaned two suggestions for the refrigerator that seem easy to implement.

1) Put a fan on the condenser coil
2) increase the insullation

I will be adding 1.5" of insulation to the inside of the box in both the freezer and refrigerator areas.

But what is not clear in the text is what size of a fan to use on the condenser,

I have a 3" muffin fan that is 12 Volts and uses 120 ma. Is this the right size, do I need more, do I need less? I am not sure from the text if it needs to be a blower, or just a gentle circulation.
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Old 11-02-2007, 12:53   #2
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Hi Jim,

A muffin fan will do the trick. If you take a look at new 12v air cooled systems that have fans, you will see that all they are actually using is a muffin fan. Of course, the more air you blow across the fins, the more heat can be transferred away, but I think there is a point of diminishing returns when it comes to the amount of power it takes to blow the air across the fins. You could use an airplane propellor, but it wouldn't get you much extra cooling while at the same time costing you an arm and a leg in energy.
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Old 11-02-2007, 13:02   #3
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Thanks, that is what I suspected. I am planing a theristor to measure the temp on the coil and then have that operate a relay to operate the fan.
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Old 11-02-2007, 13:13   #4
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Very nice setup with the theristor to kick in the fan. I should add that the lion's share of you efficiency will indeed come from the insuation.

You want to get the best insulation possible, and make sure to encase it so that no moisture can get in, or you will end up back at square one with insulation that conducts heat. Not sure if you are already aware of that, but I figured I'd throw it out just in case.
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Old 11-02-2007, 13:18   #5
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My plan is closed cell styrofoam glued to the inside wall. closed cell styrofoam is not able to absorb moisture since all the bubbles are closed. I may try to put some sort of coating over the inside to protect the soft styrofoam from damage.
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Old 11-02-2007, 13:21   #6
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The reason for adding a fan is to achieve a more constant temp. everywhere in the box. All I've ever read indicates that gentle circulation is adequate. The turnover time (the amount of time it takes for all the air volume to pass through the fan) would still be short enough with a continuously running muffin fan to even out any temperature gradient.

The smaller draw of the muffin fan is in line with the goal of reducing the electrical efficiency of the unit, at well.

Adding insulation to the inside of the box will reduce the interior volume, a good move for a minimal system if you can live with a smaller box: less volume requires less energy to keep cool. As to the proper type of insulation, and the need/placement of any moisture barrier, I'll leave to others with actual knowledge.

Other general efficiency tips:
  • It's a cube with six surfaces: don't forget to insulate the lid
  • Using water-filled containers to fill air space: liquid is a better heat sink than air.
  • good seals on the lid
  • top-loading
  • some forethought into the arragement of contents to minimize lid-open time when you actually use it in the field
  • if you're adding insul. to the inside, you're likely sealing any drainhole, so won't need to worry about losing cold air down the hole and using power to cool the bilge

Now you know everything I know. It didn't take very long, did it?
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Old 11-02-2007, 13:24   #7
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What kind of adhesive is best for styrofoam (you mean styrene, as in cheap picnic coolers, probably?) and moist applications?
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Old 11-02-2007, 13:35   #8
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Thats not the kind of fan I am talking about. What I am talking about is one on the condenser coils to dissapate the heat and make them more efficient.

The glue I use for almost everything is "Liquid Nails" its water proof and will stick to anything. Actually I use the one called "Better than Nails"
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Old 11-02-2007, 13:37   #9
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"Using water-filled containers to fill air space: liquid is a better heat sink than air." That can't be good. Consider the physics of a refrigerator. You are removing heat generated by the random vibrations of the molecules in the refrigerator. Pack more molecules in it, and there's more heat energy to remove, so the frig has to work harder to stay cold.

Pack bottles of dense matter (water) in it, and there's got to be lots of energy to keep removing from the water to keep it cool. Put in empty bottles, or foam bubblewrap, and there's less air mass circulating around, so when you open the door, less cold air can fall out--and now you've gained efficiency.

But for a boat, where the lid is on top and there's no door? That shouldn't matter.

I'd also skip the thermistor (less stuff means fewer failures) and just tie the fan into the compressor relay, so that the fan is simply on when the compressor is running and the condensor coils are hottest. (Condensor--outside dumping the heat. Evaporator coils, inside sucking the heat out of the food in the box.) If you only had monel coils, you could water cool them.<G>
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Old 11-02-2007, 13:44   #10
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I found when I did my box, I didn't even need an adhesive. I cut out the panels and by the time they were jammed in (with an FRP face epoxied on over my moisture barrier), they stayed in place. All I did from there was to fill all gaps and cracks with spray foam insulation.

Jim: For clarity, I followed your fan on the condenser. Just saying that to avoid confusion in the thread. Also, you can pick up FRP sheets (4'x8') at Lowes or Home Depot. These are normally used for "do it yourself showers" but make good liners to protect the insulation, or did for us.
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Old 11-02-2007, 13:46   #11
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I guess I need to clarify this:

I have a "refrigerator", not a chest. It has two doors on the front, one for the freezer and the other for the refrigerator area. So the top opening is not an option.

The second thing to clarify is that the fan I am talking about is NOT an internal fan, it is a fan on the outside condenser coils that dissapaites the heat generated.
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Old 11-02-2007, 13:50   #12
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I hadnt thought of FRP from Lowes. That sounds like a good idea. Since mine is a two compartment upright I will not insulate the floor of the freezer or the roof of the refrigerator but will make sure there is a drain on the bottom section so water does not collect there.

Thanks for the suggestions.
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Old 11-02-2007, 14:01   #13
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A refrigerator with doors!? Darn, forget about the insulation and fan, you've got the wrong machine on the boat. Even worse if it is not a marine design, because the compressors on "land" type frigs are designed only to be used when level (not heeled or rocking). Use that one in port, get a proper "chest" type to use unhooked.

Sean-
What kind of FRP do the big box stores sell?? Just thin stuff like Formica, or real FRP structural panels??
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Old 11-02-2007, 14:08   #14
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No, it is a marine refrigerator. Not propane. Its a marine refrigerator, but not for a small boat. its a 12v unit and will run all night on the hook but I run the engine at least 2 hours a day to keep the bats charged and have two 74 watt solar panels for when not running the engine.
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Old 11-02-2007, 16:27   #15
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Volume, Density, and its Effect on Heat Transference

Thanks for the clarification inre: fan placement, jimisbell . The question seems to become: will the electricity to operate the fan actually result in shorter compressor running time to the extent that it will yield a net decrease in overal power consumption? And it's a good question.

About filling dead space:
Quote:
Hellosailor say, quoting me initially:
"Using water-filled containers to fill air space: liquid is a better heat sink than air." That can't be good. Consider the physics of a refrigerator. You are removing heat generated by the random vibrations of the molecules in the refrigerator. Pack more molecules in it, and there's more heat energy to remove, so the frig has to work harder to stay cold.

Pack bottles of dense matter (water) in it, and there's got to be lots of energy to keep removing from the water to keep it cool.
(emphasis mine)

Yes, intially, the heat contained in the ambient-temperature water would be more than in an air-filled bottle of the same volume, for exactly the reason you state: there are more molecules per cc, each at a certain temperature, so more heat gets introduced into the compartment. Initially, that is. It's my understanding that once the 'frige has removed that heat, the water jugs become a better insulator than air-filled bottles would, so there's a net gain in efficiency over the long run.

If this is arguable, or is flat out wrong, I'd appreciate someone setting me straight.

A quick aside here while I review what I think I know: I've always read that the heat that gets into an insulated box mainly arrives by four means of penetration:
  1. through the sides (minimized by insulation)
  2. through the lid cut-out, or in jimisbell's box, the door seals (minimized by an air-tight seal)
  3. from introducing new items to be cooled; acceptable because it's the whole point of having the 'fridge in the first place though pre-cooling things in the bilge, or at least against the hull & below the waterline, before introduction into the 'fridge, can help)
  4. from opening the door, rooting around, and taking things out, which displaces cold air with warm air from the outside, equal to the volume of the removed item plus "splash out" or "slough" (in a front-loader) of the cold air from the compartment, which heat then must be removed by being captured by the evaporator and pumped out again.
This is where I have real doubt: I am not familiar with heat getting into the compartment by being generated from "random vibrations of molecules." Do you mean the water molecules bumping up against each other as a result of random boat motion and remote engine vibration? ASAIK a vibrating jug of water doesn't heat up of its own accord, i.e., from picking up the energy from vibration, so no heat is being generated that a system would need to "keep removing." Please direct me to a place where I can learn about this. I must have been asleep that week in my high school science class. I know I slept in history, b/c I know almost nothing about the War of 1812, except something about Dolly Madison running around the White House giving instructions to cut a portrait of Geo. Washington out of its frame to keep it from falling into the hands of the British.

We agree about minimizing (or minimising, for you UK subjects) dead air space to limit that warm air entry when the door is open. The question is, will a gallon jug of water, after being pulled down to the compartment temperature, and with its more dense and therefore more cold-conserving medium, function as a better insulator than empty bottles or bubble wrap, which is mostly air? According to my understanding, a solid would be even better than a liquid, but carrying around a dozen bricks for this purpose would be impractical for most people.

Anyone?

P.S.— Hellosailor, do you owe us a painting?
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