sizing for icebox conversion

[JimsT31Solace]

My first post. I am planning on converting the icebox on my Tartan 31 to a refrigerator and am looking at the Isotherm Compact Classic 2010 (uses a BD35F compressor for up to 4.4 cubic ft. My icebox is well insulated and is approximately 3.5 cubic ft. I am mainly constrained by the height I have avail for the compressor. Should I oversize the compressor? I'd rather not as the BD35F will have lower current draw. I have a dual battery bank (180 amp-hrs total) and charge with a 100W solar panel when not running the engine.

any thoughts or advice is greatly appreciated!

Jim
T-31 Solace

[Bowdrie]

Everything comes down to how many BTUs you need to remove from the box over each 24-hour period to keep the product in the box at the temp you wish.
A bigger compressor, (BD50,) might consume a few watts more than a BD35 when it's running, but it'll remove the heat faster and so it'll cycle fewer times, and or its cycles will be shorter.
Personally, I've always been suspect of the claims made by all the manufactures of refrigeration units.
Claims of capacity based on controlled/ideal conditions of ambient temp and product load frequently fall short in real world operation.
I always advise getting the highest capacity compressor/evaporator one can fit.
Very few people complain that their product is too cold when it's 90F outside.
No matter the compressor model, unrestricted ventilation is an absolute must, even if it must be provided by additional fans.
Perhaps none of this is what you want to hear, but refrigeration is a large expense, and you'd like to get "better-than-hoped-for" performance.
PS, welcome to the forum.

[wholybee]

Not only will a larger compressor not likely use more power because it will run less, you need to consider that the kit is sized for 3.4 cf, not just the compressor. It's the compressor, the evaporator, and a resistor that sets the compressor speed, all working together that makes it suitable for 3.5 cf.

The BD35 is big enough, provided the correct evaporator is used, and the box is well enough insulated. By well enough, that is a minimum of 4 inches of closed cell foam. Better insulated boxed have 6 inches. By far, the most difficult and expensive part is building an insulated box. As Bowdrie says, a larger compressor and evaporator is always better. But I am very suspect of your box being well insulated unless you built it yourself.

Unless you plan on running the engine everyday, you probably need more solar as well. I am guessing based on all the math I did for my box that your box (if well insulated) will require 30-35Ah per day. If not well insulated, about 100-150Ah per day. You solar panel will produce about 35Ah per day.

[JimsT31Solace]

Thank you Bowdrie and Warren for your prompt responses. I agree that a better insulated box will likely reduce my power draw much more than sizing up from the BD35F to the larger BD50F compressor. Kind of like anchors “bigger is better”. I can improve my current icebox and will look into that. Will bluefoam sheet material from Lowe’s work in this application? Lastly, I see Isotherm has an ITC smart controller they claim can reduce avg power demand by cooling “more” when I’m under engine power (with higher battery voltage sensed). Is this worth it or is it just a gimmick and to put my money instead into better insulation? Finally (sorry!) the Isotherm models are generally for freezer OR refrigeration but not both. Should I go to the full “both” even for 1-2 week cruises? I will also look at potentially increasing my solar from 100-200W.
Thanks again
Jim

[wholybee]

Yes, the blue boards from Lowes are fine. I used the pink ones from Home Depot. 2 part expanding foam works as well. You will also need to have a vapor barrier, and to ensure the foam is air tight from the box, that is no cold air from the box can reach the foam.

I don't know how well the ITC controller works. I doubt it actually reduces the power used, just shifts it to cool more when the engine is running, which if you run the engine is probably a good thing. I would go many weeks without starting the engine, so useless for me.

The question of Freezer/Fridge or both is one of construction of the box. The evaporator goes in a smaller freezer section, and a larger fridge section adjacent will be cooled but not frozen. This can be very difficult to get right. Often experimenting with insulation between the two is needed, and/or a small fan to circulate the air. Many isotherm evaporators can be bent into different shapes. A flat evaporator can be bent into an L, a U, or an O, depending on the need. Download and check the manual as they can only be bend in certain ways, so make sure it will bend and fit as you need it.

I also recommend a book by Nigel Calder, Refrigeration for Pleasureboats. It's old and much of it isn't relevant if you are buying a kit (I.E. how to build a system from parts) but there are sections on building a box, different types of insulation, calculating power needs and box sizes, etc.