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.

[GordMay]

I highly recommend perusing the various expert posts made by Richard Kollmann.
Over the years, he addressed [nearly?] every marine refrigeration question, one could imagine.
➥ https://www.cruisersforum.com/forums...rchid=38343632

Including:
Smart ice box conversion refrigeration knowledge can insure satisfaction later
➥ https://www.cruisersforum.com/forums...er-216680.html

Design your own efficient Ice Box Conversion Refrigeration
➥ https://www.cruisersforum.com/forums...-197816-2.html

and many, many, more ...

Sadly, Richard passed away, at 89 Years old, on August 2, 2023.
➥ https://www.cruisersforum.com/forums...nn-275930.html

[mikeo16]

Hi Jim,
You should look at this as a total system which includes:
* adequate battery storage. If your current total battery capacity is 2 batteries totaling 180 amp-hrs, then you really only have 1 battery or 90 amp-hrs for your house as the other battery will be for starting the engine. Following the basic principle of not letting your battery go under 50%, then you only have 45 amp-hrs which you will find is not adequate.
* insulation - nobody ever has enough, but this is something you can continue to improve on as you become more aware of your energy consumption. Common areas for improvement are: seals around the covers and hinges; exterior of ice box, and interior (I put 1" in the inside which really helped a lot, if you can afford the slight loss of space)
* Solar - upgrading to 200 watts will be be more than adequate for summers in New England (100 watts will likely not be quite enough). If you typically only stay a couple of days in 1 location when cruising, then you just need enough to last you until you start the motor and go to your next location
* Monitoring power consumption - you will need to keep an eye on battery state of charge to avoid going below 50%
* monitoring box temperature - it is definitely nice to be able to view the temperature of your box - it changes with usage and also the ambient outside tempertures.

Regards - Mike (I can go into more detail when I see you at the shipyard)

[JimsT31Solace]

Hi Mike and Warren
thank you both for the detailed response, and Mike, I definately look forward to seeing your setup this fall. Basically the Tartan 31 icebox is a large "bucket" of just under 2 cubic ft before you get to a large grate, then another 1.5 cubic feet below the grate that we have to fill with block ice. So no "small" compartment dedicated for a freezer, so it may make more sense (unless I make major changes to the box) to create a large refrigerator down to the grate, and use 2" or 4" foam panels to reduce the volume below the grate but still be able to use it. I think the "smart controller" is gimmicky, since I motor every day I can just up the thermostat to cool things down, then if sailing only I can lower it to save power. Brian from Analgesic said that works for him. Yeah the 100W solar may not be enough, I'll experiment with it first.

Warren talked about needing a "vapor barrier" before the cold air gets to the foam. Is that to prevent moisture from seeping into cracks and creating mold?
Jim

[Madehn]

Count on running your compressor at a 50% duty cycle with a well insulated box. From that and knowing your amp draw you can size your bank and solar Remember that your solar will not produce theoretical amps. Figure on 85% efficiency on the panels plus another 5 % general loss. Then figure on max of 5 hours of good sun and another 3 hours of 40 % sun per day. That is on a good day. That should get you close.

[MikeHoncho]

I have the same unit in the same size box. My box was highly insulated by the previous owner when he replaced the countertops. He framed in the ice box (not the bottom) creating a couple inches of void space then filled it all in with two part foam. The lid had 3 inches of rigid foam on it but the underside of the counter top didn't have anything. I added 2 inches of rigid foam to it and installed a air gasket around the opening to keep air from mixing up so much when removing the lid. I have a 240amp/hr AGM and 250 watts of solar with a large controller. I am able to keep up with all of it while on anchor if the sun is out and bright. This summer we did a 2 week trip with multiple days of overcast with sailing (no motoring). I had to run the engine for a while before bed to get through the night with my current power loads (2 cpap machines). Its a little comparing apples to oranges due to different power needs but I believe you will be low on solar if you don't motor for a couple hours. You will be getting really low on battery running the refer over night with the battery capacity you stated. I didn't see what chemistry as that makes a significant difference on how much you can actually draw down. My AGMs can't go below 60%. At the worst, I got down to 70% during my trip. Remember it takes alot more time to get the LA batts from 90% to 100% as compared to them charging up to that 90% level. I bought a refrigerator controller and replaced the factory thermostat. This allows you to program a Delta so the compressor doesn't cycle on and off all the time. This made a big difference also. About $30 on Amazon.

[JimsT31Solace]

Thanks Mike. The batteries are lead acid so I can only go down 50%. I agree it looks like my solar is just not sufficient, thats a winter project too! I used a energy calculator and looks like I would need to run the engine for about 1/2 hr/day to keep up, I used a 50% duty cycle for the refrig at 4.5A (for 12 hrs/day). From what everyone is saying I'll have to add rigid foam to reduce the BTU loss.
I am grateful for all the responses, CruisersForum is really a wonderful resource!

[wholybee]

There is an excel sheet floating around on the Internet for building a refrigerator/freezer. You enter the dimensions of the box (freezer and fridge separately) the type of insulation used and how think, the ambient temperature and the temperature you want the box(s), and your compressor size. It will tell you how many Ah per day you will consume, and the duty cycle of your selected compressor.

I will see if I can find it.

[wholybee]

Instructions for the Spreadsheet
https://www.swingcat.co.uk/what/frid...lculations.pdf

The Spreadsheet is attached, but the extension needs to be changed to .xlsx because the forum software...

[JimsT31Solace]

Hi Warren, thanks I'll look at the spreadsheet to populate. I attached my power budget spreadsheet (from custommarineproducts.com who sell solar panels) as an FYI. It looks like I would need to run my engine for 36 minutes ea day to keep up (even with my 100W) but the estimate for the refrig draw might be inaccurate (I used 50% duty at 4.5A). I do like the idea of characterizing my current icebox with a cold "load" and measuring temp vs time "as is" and with additional foam insulation, that will give me an idea of how much improvement is possible. Good to use my electrical engineering degree occasionally!

Jim

[Bowdrie]

Have no idea of the insulation configuration of your box, that said, many a production boxes' were insulated by sprayed on foam, and as such the cured surface is rather lumpy/uneven.
If such is yours, simply placing some flat foam board against that surface will do little to add insulation value.
Other tactics need to be used, none of which are quick/easy.
I generally prescribe doing a BTU experiment with block ice to measure the heat ingress.
That will establish a base line to show you the effectiveness of the existing insulation/lid/door seals.