Designing and installing a split fridge/freezer

[Kelpie]

I want to turn a locker into a fridge and possibly freezer space, probably using an Isotherm kit.
The simplest option would obviously be to adjust the thermostat depending on whether I want to use it as a fridge or as a freezer.
However I've seen reference to designing a 'split' unit where you can have both functions at the same time.
I can't seem to find out much information about how to do that- could it be as simple as putting a sheet of polystyrene across the space to divide it up, with the evaporator plate being in the colder (freezer) side?
Thanks

 

[KompetentKrew]

A fridge is all about pumping heat out of an insulated box.

When you arrive at the boat on a Friday night and load the fridge with a crate of warm beer, that is what you're doing when you turn the fridge on. If you turn the fridge off the following day then the heat will eventually creep back in - the insulation serves as a barrier against it, but is imperfect.

In theory, you can have a large, well-insulated box with an evaporator plate at one end, and some insulation in the middle, perhaps half or three-quarters height, to separate freezer from fridge. In practice, I think you'll find this impracticable, especially considering the power demands of freezing a lot.

My coolbox is 50cm x 30cm x 30cm (I find this is about 45 litres) and the evaporator plate is about 25cm x 37cm - it has a 90° bend in it, so it covers one of the end walls and 10cm of one of the long walls. The PO built a slot arrangement so that a perspex wall (⅓, ⅔ or full height) can be inserted to separate the evaporator plate from the rest of the fridge, but I'm not convinced that it makes much difference.

My observations are that stuff placed next to the evaporator plate gets colder than stuff placed at the far end of the fridge. The PO of my boat recommends those disposable ice-cube bags, which can be hung from the evaporator plate (he found some suitable metal clips) to provide the necessary for G&T. When I plan to leave a marina I freeze about 3 litres of water to act as a thermal sink, to save power when I'm at anchor - I have to start this about 3 days before I leave the marina, as the bottles take literally that long to freeze solid, even at full power and placed right next to the evaporator plate.

I have about 330Ah of AGM batteries, and can only run the fridge for a couple of days before I have to start conserving power - I don't have enough solar to keep up. I'm not even trying to freeze things, just keeping the beer and some ready-meals refrigerated.

In your position, or if I were starting from scratch, I would choose a "freezer compartment" style evaporator plate - e,g. the Frigoboat 130H / 160H / 200H (dimensions PDF). Anything put inside will be frozen easily and the rest of the coolbox will be chilled.

I don't know the details of my fridge's insulation, but I think you'd have to have a considerable amount, &/or a considerable amount of power, to justify a proper freezer. A keel cooler makes a significant difference.

Technical PDF: http://www.swingcat.co.uk/what/fridge_freezer_calculations.pdf

 

[sailaboutvic]

We have a fridge and right next to it a cool box the same size I did think of buying a bigger kit and converting the cool box into a freezer but the trouble and the cost wasn't worth it , I ended up buying a domestic fridge/freezer for less then a kit , it use 4A hour at around 50% ,it freezers everything ice/cream,meat,bread so no running off to buy fresh bread every day.
It does mean you have to find room to put it there not small but very happy with it , just wish we brought one years back .

 

[geem]

Have you considered making a fridge freezer? I built our fridge. The total volume is about 95 litres. I chose a Waeco unit. We use an eliptical evaporator plate mounted vertically inside the fridge towards the top. We use the space inside the evaporator plate as a freezer (about 10 litres). By installing a perspex shelf under the vertical evaporator plate and then installing 1” of flexible closed cell foam insulation we keep the cold in the bottom of the evaporator. The lid of the evaporator is another piece of flexible 1” foam.
When using the evaporator as a freezer the temperature in the fridge is about 4degC. Perfect fridge temperature. The fridge installation uses 100mm of insulation on the sides, 150mm on the bottom and about 75mm on the top. It works perfectly. We can use the whole space as a fridge simply by taking the foam insulation off the top of the evaporator and turning the temperature up. The whole space can be a freezer by leaving the foam lid off and turning the temperature down but we dont need to do this as the freezer is large enough for our needs even when crossing the Atlantic.
Good luck with you project

 

[Kelpie]

Have you considered making a fridge freezer? I built our fridge. The total volume is about 95 litres. I chose a Waeco unit. We use an eliptical evaporator plate mounted vertically inside the fridge towards the top. We use the space inside the evaporator plate as a freezer (about 10 litres). By installing a perspex shelf under the vertical evaporator plate and then installing 1” of flexible closed cell foam insulation we keep the cold in the bottom of the evaporator. The lid of the evaporator is another piece of flexible 1” foam.
When using the evaporator as a freezer the temperature in the fridge is about 4degC. Perfect fridge temperature. The fridge installation uses 100mm of insulation on the sides, 150mm on the bottom and about 75mm on the top. It works perfectly. We can use the whole space as a fridge simply by taking the foam insulation off the top of the evaporator and turning the temperature up. The whole space can be a freezer by leaving the foam lid off and turning the temperature down but we dont need to do this as the freezer is large enough for our needs even when crossing the Atlantic.
Good luck with you project

Thanks, I'm struggling a little to visualise exactly what you're describing though.

The locker I am converting would be top opening only, so any split arrangement would have to be side by side. I can't actually get to the boat just now to measure up, but I expect that it could be around 140l after insulating 100mm all around. That's based on looking at photos and extrapolating from a couple of known measurements, so could be out by a fair bit. Obviously I'm not going to buy anything until I've had a chance to measure up more accurately.

If I put the evaporator plate at one end, and put up a divider, would that create the desired effect? And if so, should the divider be fully sealed, insulated, etc?

An added complication is that we do already have a fridge, a self-contained 45l Waeco front opening unit with a tiny freezer box. I'd quite like to get rid of it, because it's not terribly efficient and awkward to use at sea. But we could keep it, and use the larger space as a dedicated freezer.I'm just a bit nervous about carrying that quantity of frozen food aboard in case we lose power.

(the planned useage is for liveaboard/bluewater cruising, mostly in the tropics, running almost entirely on a 870w PV array).

 

[geem]

Thanks, I'm struggling a little to visualise exactly what you're describing though.

The locker I am converting would be top opening only, so any split arrangement would have to be side by side. I can't actually get to the boat just now to measure up, but I expect that it could be around 140l after insulating 100mm all around. That's based on looking at photos and extrapolating from a couple of known measurements, so could be out by a fair bit. Obviously I'm not going to buy anything until I've had a chance to measure up more accurately.

If I put the evaporator plate at one end, and put up a divider, would that create the desired effect? And if so, should the divider be fully sealed, insulated, etc?

An added complication is that we do already have a fridge, a self-contained 45l Waeco front opening unit with a tiny freezer box. I'd quite like to get rid of it, because it's not terribly efficient and awkward to use at sea. But we could keep it, and use the larger space as a dedicated freezer.I'm just a bit nervous about carrying that quantity of frozen food aboard in case we lose power.

(the planned useage is for liveaboard/bluewater cruising, mostly in the tropics, running almost entirely on a 870w PV array).

Using this evaporator plate rotated so it can be loaded as a freezer from the top.
The advantage of having this 10l freezer is its super efficient as its inside the fridge. With two of is onboard we can carry enough frozen meat in the freezer for at least two weeks assuming meat every evening for dinner. We put all meat in to ziplock bags so it packs tight within the freezer compartment. We use the rest of the fridge space for normal cold fridge stuff. We have a separate small 42 litre top loading portable compressor fridge that we use as our warm fridge. Set at 10degC for fruit and veg that goes off quickly in the heat of tropics otherwise. This set up work perfectly for us

 

[Kelpie]

Thanks Geem, sounds a great setup and one that we could quite easily copy, using our existing front loader as a 'warm fridge'.

Next decision is whether to spend the extra on a water cooled kit... about £720 vs £450...

 

[mrangry]

My boat has a fridge and freezer compartment. They are two separate compartments with a 40mm hole joining them. I think the plate is in the freezer and the hole joining the compartments allowed the fridge to be kept cool from the freezer compartment. From what I can tell this was from factory (Beneteau 1992) and was engine driven by a Seafrost compressor. The system doesn't work and rather than repair it I am also looking at a more modern electrically driven set up that wont break the bank or the batteries so watching this thread with interest

 

[geem]

Thanks Geem, sounds a great setup and one that we could quite easily copy, using our existing front loader as a 'warm fridge'.

Next decision is whether to spend the extra on a water cooled kit... about £720 vs £450...

We have had water cooled on our last boat. Its a good system. Quiet and keeps heat out of the boats. The air cooled can be a problem in the Tropics if you dont have good ventilation around the condenser. Ours used to struggle when the air temperature was circa 30degC. The compressor run time would increase. In addition the compressor is close to our back cabin so the heat it put in to the boat was not welcome. We have since dramatically improved the ventilation around the compressor/condenser by installing additional ventilation grilles. The compressor and condenser now lose heat far more effectively and the addition of an extra deck hatch to the back cabin has solved the heat problem.
Prior to doing these mods I was considering conversion to water cooled. Its now no longer an issue.
I would normally suggest water cooled for its greater efficiency but in the Caribbean, the seawater is regularly close to 30degC. This is similar to the cabin temperature so apart from the addition of the condenser fan, the running loads are not that different. Hope this helps

 

[Kelpie]

We have had water cooled on our last boat. Its a good system. Quiet and keeps heat out of the boats. The air cooled can be a problem in the Tropics if you dont have good ventilation around the condenser. Ours used to struggle when the air temperature was circa 30degC. The compressor run time would increase. In addition the compressor is close to our back cabin so the heat it put in to the boat was not welcome. We have since dramatically improved the ventilation around the compressor/condenser by installing additional ventilation grilles. The compressor and condenser now lose heat far more effectively and the addition of an extra deck hatch to the back cabin has solved the heat problem.
Prior to doing these mods I was considering conversion to water cooled. Its now no longer an issue.
I would normally suggest water cooled for its greater efficiency but in the Caribbean, the seawater is regularly close to 30degC. This is similar to the cabin temperature so apart from the addition of the condenser fan, the running loads are not that different. Hope this helps

Thanks again, very useful.
The way our budget is shaping up, I would be hard pressed to justify a near doubling of the cost of the fridge project (we are upgrading a lot of systems and buying new gear prior to setting off later this year, all being well).
The compressor would most likely be going in a void between the nav station and the heads, so not a bad place to be throwing excess heat. The heads is permanently ventilated and a bit of warmth in there should be bearable, compared to a sleeping cabin.

 

[Graham376]

Using this evaporator plate rotated so it can be loaded as a freezer from the top.
The advantage of having this 10l freezer is its super efficient as its inside the fridge. With two of is onboard we can carry enough frozen meat in the freezer for at least two weeks assuming meat every evening for dinner. We put all meat in to ziplock bags so it packs tight within the freezer compartment. We use the rest of the fridge space for normal cold fridge stuff. We have a separate small 42 litre top loading portable compressor fridge that we use as our warm fridge. Set at 10degC for fruit and veg that goes off quickly in the heat of tropics otherwise. This set up work perfectly for us

Ours is Isotherm BD50F using the largest version of the evaporator shown above, mounted vertically rather than horizontally. Length of bungy across opening to stop food sliding out, no room for a door. Keeps food frozen indefinitely but need to take some out of the expanded polystyrene trays and bag it. Rest of the 170 litre box is at fridge temperature.

 

[Arcady]

If I put the evaporator plate at one end, and put up a divider, would that create the desired effect? And if so, should the divider be fully sealed, insulated, etc?

I had an American boat for several years with exactly this system. The divider was a sheet of polycarbonate perforated with circular holes.

On the plus side it is was a simple installation and it worked, but on the negative side it never worked well. Turn the ‘stat up too far and the freezer side would also freeze stuff in the bottom of the fridge. Turn the ‘stat down to prevent that and the food in the freezer was only just frozen. It’s possible I could have improved that by experimenting with a thicker, more insulated divider.

It’s also not efficient if most of what you keep is in the fridge. Running a deep-freeze just so the cold air overspill acts as a fridge is plain daft, although probably if you use the freezer all the time it might make more sense.

I ultimately sold the boat and the replacement boat had a dedicated (Waeco) fridge which I think consumed about 1/3 rd of the power of the combi unit.

 

[geem]

I had an American boat for several years with exactly this system. The divider was a sheet of polycarbonate perforated with circular holes.

On the plus side it is was a simple installation and it worked, but on the negative side it never worked well. Turn the ‘stat up too far and the freezer side would also freeze stuff in the bottom of the fridge. Turn the ‘stat down to prevent that and the food in the freezer was only just frozen. It’s possible I could have improved that by experimenting with a thicker, more insulated divider.

It’s also not efficient if most of what you keep is in the fridge. Running a deep-freeze just so the cold air overspill acts as a fridge is plain daft, although probably if you use the freezer all the time it might make more sense.

I ultimately sold the boat and the replacement boat had a dedicated (Waeco) fridge which I think consumed about 1/3 rd of the power of the combi unit.

 

[geem]

We had a similar system to the one you describe on our previous boat. It to was not a great set up until I experimented with the barrier between fridge and freezer. We eventually got it to work ok but it was never very efficient in freezer mode I think due to the energy loses from freezer section through the insulation in the heat of the Tropics.
The current set up where the freezer is truly inside the fridge is far more energy efficient. Having the evaporator function as the freezer with an insulated bottom and lid that effectively seals the freezer from the fridge with regard to air movement is a game changer. We have reproduced the set up on a few boats now and they all work very well and energy efficiently.