(Slight off topic) Can you saw the back off a fridge?

[Crofter]

Not a house question, so hopefully I won't get reported to a moderator

 

I'd like to fit a small fridge to my boat but, as with most things marine, the markup on dedicated 12v equipment is crazy. To get a compressor driven fridge, or a kit of parts to build my own using slabs of insulation, costs over £400. By contrast, I can buy a mini countertop domestic 220v fridge plus an inverter to power it and still have change from £100.

 

The main downside I see to this is that the fridge, whilst very small, is quite hard to find space for. It's surprisingly hard to fit a cube that's half a metre a side into your boat.

 

So... other than voiding my warranty, what would happen if I tried taking one of these little fridges apart? I'm hoping that they are constructed such that the back panel contains all the clever stuff (radiator, compressor, and cooling plate for the inside), and the other five sides of the fridge are simply insulation in the shape of a box.

Could I simply cut the back of the fridge off completely, and then build a new insulated box to mount this in? This could allow to me fully utilise the rather awkward shape of a locker.

 

A related question- is a domestic fridge affected by being heeled over? Either for brief periods, or by dint of having been installed at an angle as part of a daft scheme to make it fit into a funny space?

 

Thanks for any ideas

[Jeremy Harris]

You may find it difficult to take apart whilst retaining the refrigerant circuit intact, best bet might be to see if you can take a good look at one first.  On one of my boats I made a cold locker, originally just an insulated locker with the bottom being the skin of the boat, so the water kept it cool.  That worked OK for things like milk and butter.  I later converted it to use a Peltier cooler, with a bit of 3mm stainless plate sikaflexed to the outside of the hull as the heat sink.  That kept the locker a lot cooler, but used a fair bit of power (around 2 to 3 A at 12 V).

 

If you have gas on board, then a camping fridge might be an option.  These are absorption fridges that use a small gas flame to keep the refrigerant circuit working, with no moving parts.  Sounds counter intuitive but they work OK.  We hired a yacht on the Norfolk Broads a few years ago that had one of these top loading gas fridges built into a cockpit locker (to keep gas out of the inside of the boat) and it was fine for two or three days worth of milk, butter, bacon etc.

 

 

Edited March 22, 2017 by JSHarris

[PeterW]

My beer fridge is a peltier unit and a fan and I reckon you could have the whole lot out and into a cool box the size you want in half a day

[Crofter]

Thanks for thse thoughts. I've looked into gas powered and peltier units and thry do have drawbacks- a compressor is definitely the way to go for best efficiency.

 

Potentially dafter follow up question:

There are time when I think it could be advantageous to have a freezer rather than a fridge. E.g. if I have a power surplus from a sunny day or access to shore power, I could store that energy by freezing ice blocks and then get several days use out of that using the unit as a cool box.

 

So the question is- what is actually different between a fridge and a freezer? Am I being overly optimistic thinking that by simply changing the duty cycle I could turn a freezer into a fridge?

[Jeremy Harris]

With a compressor-driven fridge there is no real difference between that and a freezer, and often fridge-freezers use the same cooling circuit for both.  It's simpler and easier to use a small freezer as a fridge than the other way around, but only because the freezer will have a larger cooling arrangement than a fridge, and the temperature can be controlled over a wide range with just a thermostat.

 

Many years ago, a friends old (1950's) yacht had a cool box with an ice compartment.  Back then it was easy to buy ice at pretty much any port, so all we did was put ice in the ice chamber and let it keep the cool box cold.  Ice would last a fairly long time, maybe three days if the weather wasn't too hot.  I'm sure that the phase change cold packs you can get would do a better job, as they start off at around -18 deg C, from having been "charged" in a freezer.

 

You could possibly make a tiny unit for just freezing these cold packs, then use those to keep the cool box cool.  Not sure of the best way to do this, but I would think a two stage Peltier cooler, cooling down a small, well-insulated, compartment that was only just big enough to hold a couple of cold packs, could be made to work OK.  Whether it would be worth doing is debatable, and depends very much on how long you'll be at sea and away from power.  Most of my sailing has been coastal, with lots of trips across the English Channel, and a couple of delivery trips down to the Med.  I can't recall being away from port for more than about 48 hours at a stretch, even on the delivery trips.

[Crofter]

Thanks as always Jeremy.

The way we have generally been accustomed to cruising is to load up the cool box with packs of frozen meat, and work our way through these over three or four days, eating the higher risk chicken first, then red meat, and finally things like sausages which I think pose less of a risk. For milk, we just use UHT. By day five we are generally down to store cupboard meals or, hopefully, able to restock from a shop somewhere.

My first thought had been to buy a little tabletop ice cube maker, and in times of power surplus make bags of ice cubes- but whilst this might work, my gut feeling is that I would need to give over a disproportionate volume of the cool box to ice, and not enough to the food itself. Whereas if I have a proper freezer, I can store that energy in the packs of food that I am carrying anyway, and/or the more efficient cool blocks.

 

It's not exactly top of my priorities list, but cruising with a toddler makes you aware of the need for slightly better hygiene, so we are looking to add a few mod cons this year.

 

A mini freezer takes double the power when running (150w) which maybe means a startup surge of 750w, but that remains doable with a low cost inverter. When running it as a fridge, it would obviously perform fewer cycles. Maybe that would actually be good for efficiency??

[Ferdinand]

2 hours ago, JSHarris said:

With a compressor-driven fridge there is no real difference between that and a freezer, and often fridge-freezers use the same cooling circuit for both.  It's simpler and easier to use a small freezer as a fridge than the other way around, but only because the freezer will have a larger cooling arrangement than a fridge, and the temperature can be controlled over a wide range with just a thermostat.

 

Many years ago, a friends old (1950's) yacht had a cool box with an ice compartment.  Back then it was easy to buy ice at pretty much any port, so all we did was put ice in the ice chamber and let it keep the cool box cold.  Ice would last a fairly long time, maybe three days if the weather wasn't too hot.  I'm sure that the phase change cold packs you can get would do a better job, as they start off at around -18 deg C, from having been "charged" in a freezer.

 

You could possibly make a tiny unit for just freezing these cold packs, then use those to keep the cool box cool.  Not sure of the best way to do this, but I would think a two stage Peltier cooler, cooling down a small, well-insulated, compartment that was only just big enough to hold a couple of cold packs, could be made to work OK.  Whether it would be worth doing is debatable, and depends very much on how long you'll be at sea and away from power.  Most of my sailing has been coastal, with lots of trips across the English Channel, and a couple of delivery trips down to the Med.  I can't recall being away from port for more than about 48 hours at a stretch, even on the delivery trips.

 

These days you buy ice at any Aldi for about 50p per kilo, or Tesco (including our local tiny Tesco Express) for about double that. No idea how that compares to the volumes @JSHarris used to purchase.

 

Good for cocktails.

 

There are also pretty effective 24 hour+ (depending on required temperature) flasks available now. 

 

The phase change cool bags I am familiar with are for insulin and are called Frio. I suspect there are bigger ones that are as good.

 

Or can you DIY a super cool box from vaccuum panels or Aerogel?

 

Ferdinand

 

Edited March 25, 2017 by Ferdinand

[Crofter]

Buying ice isn't much of an option up here- we tend to go for the best part of a week living on the anchor and visiting uninhabited islands or sea lochs in remote places. No houses, certainly no Aldi!

It's probably not much of a problem further south. I get the impression yachties tend to eat out much of the time, which again is a rare treat for us. Nearest we get is when a passing fishing boat gives us a few prawns!

[Jeremy Harris]

Certainly making a cool box with very good insulation would help a lot, but having made a home made cool box using a Peltier heat pump I did find that airtightness was more important that insulation thickness once you get more than around 75mm of PIR all around the box.  I found it difficult to make decent seals, and ended up using strips of wet suit neoprene with an over-centre catch on the lid that clamped it down tight to the seal.

 

It doesn't take a lot of power, in theory, to keep a well-sealed cool box at around 4 deg C (approximately fridge temperature).  If the box had top, bottom and sides with an internal area of 0.1m² each, insulated with 75mm PIR, and was airtight, then maintaining it at 4 deg C in 20 deg C ambient temperature would mean pumping out around 2.8 W of heat continuously.  More heat would need to be pumped out when cooling the box and contents down, but this could be reduced by pre-cooling stuff and using a frozen cold pack.  Peltier devices aren't efficient, but they do lend themselves to DIY stuff well, so a suitable Peltier device, operating at around 10% to 15% Carnot efficiency, would need around 25W to 30W. 

 

The key to making them work well is to have a large and efficient  heatsink on the warm side.  The closer the heatsink can be to ambient temperature, the lower the power the Peltier device needs.  The cool locker I made on one of my boats used a 3mm stainless plate, around 150mm square, Sikaflexed on to the underside of the hull.  The Peltier device was fitted in a hole in the hull behind this plate, and thermally connected by an aluminium block to an aluminium cooling plate in the bottom of the locker, with foam insulation squirted in to insulate it from the hull.  This worked well, as the warm side of the Peltier device was kept at sea temperature, so never more than about 15 deg C, even at the end of summer.  It might be possible to use a low power water pump and cooling block, like those used by the PC modders to cool over-clocked chips, connected to existing sea cocks.  Not sure how they would tolerate salt water, as I've never looked inside one.

 

I think making a well-insulated box or locker should be easy enough, but making the lid seal very well could be challenging.  That was certainly the major problem I had, and I found it made a tremendous difference to performance when  the lid seal worked properly.  I've still got my home made cool box somewhere, I'll see if I can dig it out and take a photo or two.  I built it around 18 years ago, but it was still working around 4 years ago, as we've used it on holiday trips lots of times, powered from a car 12V socket.

Edited March 25, 2017 by JSHarris

[Onoff]

I reckon.....you could run an Evolution saw around the top bottom and sides of a fridge and take a section out thus making it shorter front to back.....

 

Then glue together with expanding foam and pop rivet some side plates in to hold it all together.