Refridgerator/freezer design question

[jpendoley]

I have a huge icebox-15-18 cubic feet on my Pearson 424. Right now there is a seafrost system with a large holding plate that works as a fridge, but I want to remove it, reinsulate and have a separate fridge and freezer. I could reduce the overall size with blueboard insulation, but I am more inclined to reinsulate with vacume panels and blueboard to R40+ and design a 4 cubic foot freezer and a 11-14 cubic foot fridge. This would be instead of a spillover design and one large system. I'm not sure, but thought this would be more efficient in terms of amps consumed. Thumbnail estimate would be 35-45 amps per system per 24 hour period. Thinking I could support it with 480-700 watts of solar.
Ive designed a couple other spillover boxes that came out well-this would be my first effort with vacume panels.
Does this sound like a practical design?
Thanks for any suggestions and tips.
Jim

[RaymondR]

In the rebuild process myself wherein a very large ice box in an Island Packet is being reduced in volume and converted to eutectic with SS trays from steam tables in SS frames with wheels will roll along SS rails.

For the freezer I will be using the same type of trays with the evaporator tubes wound around the outside of the trays and insulated with polyurethane foam in a similar rolling arrangement inside the refrigerator enclosure and with it's own condensing unit.

I figure that with the small delta T between freezer trays and the fridge enclosure the power draw of the freezer should not be excessive.

Still pondering whether I should make the freezer trays lift out for cleaning, as the fridge ones will be, though.

[Bowdrie]

I'm guessing that your existing box is insulated with spray-on foam.
That procedure made a perfectly sealed interface against the fiberglass to prevent condensation.
If you strip that off and then try to put foam board and/or vacuum panels against the fiberglass you will be hard pressed to achieve the air-tight/vapor-tight integrity that's needed to prevent endless condensation.
Edit, at 18 cubic ft, that's ~135 gallons.
Look at a 50-gallon oil drum, how much volume to you need?
Since you want to remove the box, better results will be obtained by building a new box with much less volume.

[RaymondR]

The OP is considering using vacuum panels which should be well and truly sealed against imbibing any water vapor.

I considered using polystyrene sheeting but needed more strength to fix the SS rails to and support eutectic tanks so built a box from plywood and injected 2 pack expanding foam through holes to fill the space behind. The holes were then plugged with dowel and I am in the process of epoxy undercoating to seal any cracks or holes in the plywood before final coating the inner surfaces.

I also considered pulling the old box out and starting with a clean sheet but the amount of work in re-instating the counter tops decided me against it. It probably would have been quicker and less of a hassle but as a permanent live aboard living with half a galley without any counter tops whilst I continue to cruise would have been a real pain in the butt.

The project has consumed almost two years but most of that time has been spent cruising but I am hoping to finish this three month cyclone (hurricane) layover.

[rslifkin]

If you can divide the box up and run separate compressors for fridge and freezer, I'd definitely go that route. It'll make it easier to get enough total cooling power for a large box and also give better temperature control for each section.


If you go with separate compressors, I'd also plan for fans on the evaporators. It'll help efficiency, as it'll keep your evap temperature closer to box temperature. Especially for the fridge, that's a benefit, as the Danfoss / Secop compressors are more efficient in terms of BTUs moved per watt consumed when run at higher evap temps. Fans will also reduce the surface area needed for an evap that can remove heat quickly enough, meaning you lose less space in the box (and keeping air moving reduces frost buildup and keeps box temperature more even).

[jpendoley]

Originally Posted by rslifkin

If you can divide the box up and run separate compressors for fridge and freezer, I'd definitely go that route. It'll make it easier to get enough total cooling power for a large box and also give better temperature control for each section.


If you go with separate compressors, I'd also plan for fans on the evaporators. It'll help efficiency, as it'll keep your evap temperature closer to box temperature. Especially for the fridge, that's a benefit, as the Danfoss / Secop compressors are more efficient in terms of BTUs moved per watt consumed when run at higher evap temps. Fans will also reduce the surface area needed for an evap that can remove heat quickly enough, meaning you lose less space in the box (and keeping air moving reduces frost buildup and keeps box temperature more even).

I built my last box and insulated with blue foam boards. the result was good but got even better when I installed small muffin fan. Prior to that I had a stratification of air temperature in the box -some things would freeze that should not have frozen. With the fans the whole box remained very close to a uniform temperature-even the plates.
I will probably build a new box, but am going to see if I can keep the old outer walls, remove the inside walls, reinsulate then close it back up. Thats if I can fit enough solar to run two seperate condensers. If I can't, Ill reduce the box size-though I dont consider a fifty gallon drum to be too much space. I;d like to stay off the grid for a month at a time without the need to provision.

[Bowdrie]

Originally Posted by jpendoley

I will probably build a new box,
Ill reduce the box size-though I dont consider a fifty gallon drum to be too much space.

Ha ha, I used the "50 gallon drum" as a figure of speech.
A nice set-up might be 3>3-1/2 cubic for a freezer, and 6>6-1/2 for a reefer, that's ~75 gallons and that will hold a LOT of food and have room for VERY good insulation.
As an aside; in meats that are trimmed/wrapped well, you can expect to pack ~40lbs in one cubic ft., (a combo of steaks/chops/ribs/ground,) although in practice and due to air circulation needs actual amount is less.
Energy requirements, (for the same temps and insulation,) go up roughly in proportion to the surface area of the box, (X area exposed to Y heat).
For separately cooled boxes, (freezer/reefer,) with large amounts of insulation it is possible to get into an area where the heat transfer from the reefer thru the divider and into the freezer is greater per unit area than into the freezer from the outside, depends upon the TD and outside temps.
So, make the insulated divider somewhat thicker than you might think.