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Converted engine compressor driven freezer to electric.


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#1 Kick Ass

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Posted 17 January 2020 - 09:22 PM

Hi Guys

I have recently converted to electric with the evaporator plate and am wanting to make it more efficient. Currently it works well but could be better.

Should i be emptying the brine tanks? And possibly filling the tanks with foam?

Also currently only have evaporator plate mounted on the back face of the freezer box and have read it is more efficient to have it mounted on 2 faces. These evaporator plates can be bent so going to look into that.

Interested in your thoughts.

Thanks
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#2 Island Time

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Posted 17 January 2020 - 09:47 PM

Efficiency in thin plate refrigeration on an inconsistent DC supply is a contradiction. Sorry.

 

You have removed a >2 hp compressor and replaced it with a maybe 1/8th HP compressor. There is NO holdover ability other than thermal mass in the cabinet (food, water). Thin plate units run almost all the time.

 

So, that does not mean you can't make it better. 

Is the cabinet stainless or fibreglass? The ideal is a decent R rated insulation of about 75-100mm thick all around the cabinet, roof, walls and floor. With a vapour seal inside and out.  Make certain that the seals are airtight. If you need more insulation, remove the old eutectic tanks and replace with a good refrigeration foam.  Make sure there are no gaps, holes etc in the cabinet that are not properly sealed..

 

Cabinets, especially the old stainless ones, can be a real issue. Removing them can require lots of boatbuilding/cabinet making, and therefore lots of $$$$.

Sometimes is easier to make space for a box style unit off the shelf. Some even build those in!


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#3 Kick Ass

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Posted 19 January 2020 - 07:31 AM

Stainless brine tanks which is insulated with 100mm of foam.

Surely the brine absorbs alot of the energy instead of all the energy going into the box/food
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#4 Jason128

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Posted 19 January 2020 - 07:00 PM

I would think 100mm of foam is pretty good.
essentially you are trying to cool those brine tanks, which will take a lot to do initially.

I Tried to freeze 3x 10l bottles in the freezer at home before Christmas, it took the best part of a week to freeze them solid, kept the beer cold for at least a week in the esky too

Either drain them, or use them to your advantage if you can - you just need to get them cool before you load up the fridge. Setup it up so you can run it for 2-3 days before you set off, they will be nice and cold and help keep the temperature cool and stable. Once they are cooled down I don’t see any disadvantage at all in leaving them there. A 12v transformer do you don’t drain the batteries too much if you have shore power.?
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#5 Jason128

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Posted 19 January 2020 - 07:03 PM

Also a little pc fan inside the fridge blowing on the fan can make a huge difference for now much effort
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#6 Island Time

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Posted 19 January 2020 - 07:54 PM

Yep, the issue with full brine tanks is that they will absorb a LOT of energy. The electric fridge will take a LONG time to get them down to temp, possibly as much as 5 days, depending on capacity and temp required. If its a freezer, they should be frozen solid when the unit switches off. Then you have a defacto eutectic system, and the electric system will not need to cycle for some time. But most dont want to run it for 5 days before they go out! Personally I'd remove them.

100mm of foam is only good if it is dry and sealed, then, and only then, its as good as it's R rating... and don't forget the hatch seals!


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#7 CarpeDiem

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Posted 19 January 2020 - 08:05 PM

Can you plug it into shore power to get the brine chilled in advance? Or leave it on with solar?

Our engine driven one died a while ago and I put a new d1-30 in which had no way of attaching the compressor. So I am looking at doing the same as you.

What we do on the long weekends is throw 1kg of dry ice in the bottom. The fridge is still cold 3 days later. And we only put already chilled stuff in the fridge. That wouldn't happen if we replaced the brine with foam.

So my understanding is if you could get the brine precooled you'd have better results. This might be flawed for long trips (like rni duration).
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#8 Island Time

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Posted 19 January 2020 - 08:16 PM

Actually CD, it likely would. The main energy from the dry ice in your case cools the brine. The dry ice would last longer without the brine, and the whole unit may likely stay colder longer without the brine in the tanks. It would be interesting just to empty them, and see what the result is. Easy if you need to refill them...


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#9 BOIGuy

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Posted 20 January 2020 - 05:50 AM

So I've got the dreaded old stainless box with a brine tank somewhere in the woodwork that's had an electric unit installed, small 12v compressor etc with plate inside ice box.

 

Basically it actually works pretty well, may not be as quick as a perfect system but I don't live in a perfect world. The brine tank is probably cooling too but maybe this helps keep it all cool overnight when I turn it off so I cant here it running, food still cold in the morning. I don't waste a lot of time trying to make my world perfect, I go sailing instead.

 

I have a a solar set up which more than keeps up with the fridge and everything else, about 5 hours running till thermostat kicks in, can freeze stuff if its in long enough and I turn thermostat down a bit.

 

I wouldn't hesitate to use the same system again.

 

All the experts here that have these amazing brine tanks that "absorb a LOT of energy" need to have another look at how refrigeration works, (cooling is all about removing energy, not adding it).

The brine tank really doesn't make any difference in the long run, insulation is what affects efficiency.


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#10 Island Time

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Posted 20 January 2020 - 06:26 AM

While that is true BOI guy, to cool any thermal mass, which is what the brine tanks are, takes energy. To cause a change of state of a liquid to a solid takes a lot of energy. The insulation is what delays the cold from escaping the cabinet. If you are happy with a fridge, then it’s not too hard. If you want a freezer that keeps food at a proper safe temp (-15 to -21c) then that is more difficult. Then you need to maximise the use of the energy your system can produce.

for your info, the energy transfer difference between changing the temp of a mass of fluid, to changing the temp (same temp range) from fluid to solid (or solid to fluid) is about 40x more with the change of state.

Sailing vessels tend to have either a feast of energy ( engine running, maybe solar etc) or a famine - night time, no sun, sailing. So it makes sense to have a system that can use the energy when it’s there. 
of course, if you only do weekend trips, then a decent icebox with good insulation and a bag of salt ice works pretty well. All depends on how you use your boat.


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