Holding plates

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Graham_Wright

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www.mastaclimba.com
I would welcome some feedback on refrigeration options.

Frigoboat seem to have gone away from holding plates "because the fridge contents can thaw out!???" and recommend continuously cycled conventional refrigeration.

Didn't make sense to me but I lacked the courage to ask (at Excel).

It seems to me that, if you can really build up a reserve of "cold" while the engine is running, and if a thermostat can either warn or run the compressor straight off the battery when the temperature rise requires it, that would be an ideal solution.

Any help with my decision please?
 
In the days of low efficiency compressors, the amp/hrs required to run a decent fridge was outside the capability of most boats. Thus running the engine every so often to freeze down a holding plate made a lot of sense. However, this does allow a considerable fluctuation of the temperatures inside the fridge.

Modern danfoss compressors tied in with a keel cooler and a 6" insulation can hold power consumprion down to less than 3 amps/hr which comes within the capabilities of most boats, and provides a much more stable temperature.
 
That is true but trying to keep the fridge temp down overnight in the Med. with no solar charging or engine available is the main priority.

We have a so called "high efficiency" fan assisted evaporator which Waeco rate as their most suitable device for our circumstances but I'm strongly considering swapping this for one of their "Gel Tank" holding modules as the fan assisted beast requires defrosting every three days or it just blocks it's matrix and fools it's own thermostat resulting in a dangerous rise of internal fridge temperature.

Steve Cronin
 
position of the evaporater and air flow will make an enormous difference to its capability

I am not a fan of air cooled systems, or ones that require their own fan to circulate the cold air. both require additional power that a well designed system should not need.

I have seen a small computer fan being used, but that was to move cooling air from a freezer section into a fridge using just the one evaporator.

Recommend you talk to http://www.penguinfrigo.co.uk/
 
I have a water-cooled, holding-plate unit by isotherm and so far I'm very happy with it.

Its control Unit is supposed to automatically take advantage of periods when batteries are well charged to freeze down the holding plate.

Of course, if batteries run too low, the fridge will run in low-power mode or even switch itself out, but this would be true of any refrigerator!

If anything, mine tends to be too cold! (although rated as a refrigerator, it actually freezes food in the lower part of the coolbox).
 
I spoke to Frigoboat about latent heat but we weren't on the same wavelength.

They did say that there exist two grades of brine with different freezing points (-18 was one but I've forgotten the other.) The latent heat (in a negative sense I suppose) should ensure that the plate temperature remains at the target temperature until the heat sucked out of the compartment exceeds the latent heat. Only then will the temperature rise. If, at that point (or a determined safe maximum), the compressor switches on, I can't see that the holding plate solution represents any danger not present with a simple compressor fridge.

Am I missing something?

Keel cooling seems a great idea to me. The space occupied is minimal but of course there is another hole in the hull.
 
For my Atlantic circuit I fitted a eutectic plate system from Penguin Refrigeration. System worked brilliantly eventhough the insulation was less than ideal. The compressor was sea water cooled via a bronze stintered plate.

Batteries, of 300ah, charged via a 32w solar panel, 105a alternator, Adverc controlled. Plus the use of shore power, usually via the Honda genset.

The new boat has a "blast chill" system. It may work in the Baltic, but it 'aint gonna get the beer cool in Bequia.
 
Not sure if you are enquiring about refrigeration for a freezer or a refrigerator or for both. The following is what we did and it only works if you have both and is oriented towards our own situation where we may be away from shops for 3-4 weeks. But the systems thinking behind it applies to all situations even if just refrig or freezer alone.

We installed a powerful dc powered freezer unit with vertical reciprocating compressor, water cooled condenser, and in the freezer box holding plates (the holding plates are actually build into the walls so do not intrude into the space). It can, as you say, be managed so it only runs during other engine run times eg during charging or when motoring (if during charging we run it after the charge controller has completed its bulk charge phase so that the full alternator output can continue to be utilised by the surplus being taken by the refrig compressor). By usually running during charge times one can keep the battery size down - we were then able to cope with a little over 500 AHr with less than 50% discharges to preserve battery life. The freezer holds holds at -18C but will hold over for several days if necesssary with the only penalty of a temperature rise from the -18. A side benefit of the holdover, which was important to our sailing, is that you do not lose all your food pretty much immediately if a refrig or electrical fault.

For the frig we have quite a big one but with only a thermoelectric unit in it which we mostly run on low output.

So we work this way. All meat, fish we catch, frozen vegetables, bread, etc goes in the freezer (including milk, but we normally use powdered milk which wife and I are both happy with), it is our prime cold storage and we take from it what we need each day for thawing (in the frig if possible as that adds cooling to that). We run the frig at above normal frig temperatures to keep the current draw down but is fine for cold drinks, salads, butter, etc. If for some reason we want it lower temperarily we keep frozen containers of water in the freezer (usually 2 x 2 litre containers) and just throw those in the frig which drops it down to close to less than 5C quickly.

Now, for example, people will tell you that thermoelectric, or whatever, is inefficient and quote current draws etc for all sorts of refrigeration units but the nub is actually not so much to do with that at all. The big controllers of energy demand is how good the insulation is and how much warm stuff you are putting into the compartment (including the air each time you open and close it). An easy way to see this is so is to consider a box with perfect insulation and a low powered refrig unit. If the box is filled with things to be frozen or dropped to a frig temp then the temp will draw down at a rate according to the capacity of the refrigeration providing unit. If the box is never opened the box will reach target temperature and never again require any more refrigeration (or current draw). Of course the world is not like that but all the above serves to show that there is much more to effectiveness than quoted current draws and efficiencies.

It is clear that one has to consider ones own usage pattern. The above is how we considered ours (10 years ago now) and is a surprise to many people but was at the time (and probably still is) the method that best suits our own situation. The other extreme is that if one has an energy frugal boat people will give all sorts of ideas greatly concerned about minimising energy use whereas is possible the best solution for a particular user could be to manage the boat without refrigeration. Blind advice from others saying that you should go for this or that method because of some simple quoting of current draw, or even of efficiency, is very bad advice unless they know your usage pattern.

Of course with unlimited energy, big MoBo, continual running generator, etc then can just do what is easiest /forums/images/graemlins/smile.gif.

John
 
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