Fridge Question - Isotherm / Frigoboat compatibility?

magdalena

magdalena

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Yet another question about fridges...

I think I have settled on an isotherm with ASU, using water cooling.

The isotherm system of water cooling uses an enlarged skin fitting, and I have heard people speak of problems caused by internal fouling.

Frigoboat make a keel cooler that is external and much easier to clean (and seems to have a larger surface area).

Does anyone know if the connectors of the refrigerant pipes of the two types are compatible, and whether they use the same refrigerant?

If possible I'd buy an isotherm ASU and a frigoboat keel cooler, and just join them up. I have sent emails to the companies but had no reply.

Thanks,

Robin Hartley
 
I posted recently re fridges as we are looking for a new fridge unit.

We have settled (though not yet bought) the Isotherm ASU with an air cooled compressor.

The ASU clearly has many advantages over a normal evaporator plate and seems a good idea. As the ASU stores its 'coldness' (not the best way to explain it but I'm sure you know what I mean!) the compressor doesn't run need to run as often.

Isotherm recommended this set-up when I spoke to them (they said this would be more power efficient than a normal evaporator plate with the SU water cooled system).

In terms of compatability - I suspect they use different fittings for the SU and the Keel Cooler. The pipes are pre-loaded with refridgerant and you simply snap the fittings together - so it may be difficult to change the end fittings to suit the ASU compressor.

In terms of fouling the SU part - Isotherm say that say water (even at anchor) is moving through the outlet it shouldn't foul. (Waves will cause movement, and every time you drain the sink it should flush out the critters!). If you get to warmer waters free diving with a pipe cleaner should be easy if you do get fouling.

The keel coolers don't foul - but you do need another hole in your hull!

If you can't get answer from Isotherm and Frigoboat direct, try their distributors, Marathon and Penguin respectively.

Let me know how you get on and what you decide.

Jonny
 
Hmm, thinking about this the ASU vs Evaporator problem seems more subtle that I first thought.

ASU gives you the advantage of "storing" coldness (when you are using the engine) for future use. This is done by running the compressor hard and freezing a block of something-or-other.

The disadvantage is that when you are not using the engine and the compressor is running at normal speed, you still have that block of something in your fridge, and I understand that the fridge has to go to a lower temperature to work the plate, so your compressor has to work harder.

I wonder what the "cut off" is in terms of engine hours. ie when does the power saved by that stored cold outweigh the extra compressor time caused by having the plate? Or is there even a cut off time, given that the engine can only make the plate so cold and then it is done. If the plate gives only 2 hours power off per day, does that pay back the extra compressor working?

Suddenly an evaporator seems like a good idea, along with a supply of 50p tesco blue block freezer bag thingies to chuck in the fridge when I want to run my "old school" holding plate. In fact frigo seem to have a compressor speed selector in their electrics box, so I could even make my own ASU...

Thoughts welcome on this.
RCJH
 
From what I understand - when the 'block' of the ASU is not frozen, the fridge operates as a normal evaporator plate would - therefore the compressor runs very simalarly when the ASU is not frozen.

The ASU compressor also has an external control box allowing 'economy' settings etc. From what I have read the compressor sense increased battery voltage and therefore runs harder when the battery voltage rises (ie when the batteries are being charged by the alternator) - this also holds true for wind / solar charging - ie when there is spare power from wind / solar etc it will charge the batteries which will trigger the compressor to kick in and run the ASU plate down to frozen. Of course, the compressor clicks off if it senses the battery voltage falling (ie if it senses charging from solar then tries to pull too much power from the batteries).

The economy mode on the ASU, as I understand, allows you to run it like a normal fridge - ie if you want to charge your batteries without the fridge compressor sensing the charge and kicking the compressor in.

From what I understand the standard compressors (from both Isotherm and Frigoboat) aren't voltage sensitive, and therefore the compressor runs solely based on fridge temperature and doesn't run any harder when there is spare power (ie when the engine is running) than it would when the batteries were very low (say 11v).

I think the ASU seems like a good choice with this in mind. For me, its been recommended highly and sounds ideal - the only quandry is how to cool the compressor - air vs Seacock vs Keel Cooler??

Jonny
 
The way I interpretted isotherm's blurb was that when the ASU detected a high voltage indicating charging, it ran the compressor at a higher speed in order to cool the heating plate more rapidly.

This was why I was thinking about getting a Frigo keel cooled system to do the same, as you seem to be able to control the compressor speed.

This is what frigo US says about holding plates
http://www.frigoboat.com/sysdesign/aluminumevap.html

And this is the frigo instruction book that suggests you can vary compressor speed (look at the electrical bit near the bottom)
http://www.penguinfrigo.co.uk/pdfs/K35F.pdf

The more I research, the less sure I become!

Rob
 
See what you mean - what a quandry!!

The Frigoboat holding plates are (from what I've seen) recommended to be run from the engine directly (ie engine driven compressor - at least this is the only option you get on the Penguin website). I would imagine they are therefore an inherantly different design to the Isotherm ASU which is designed to be run from a compressor.

The Frigoboat compressor 'changeable speed' appears to be via a series of switches - I'm surprised more isn't made of this in the literature - perhaps its not an easy thing to change (more of a setting when installing the unit?)

One of the big things for me is going to be the ability to repair / replace the system without bringing the boat out of the water. The Keel Cooler needs another hole in the fridge, the seacock can get clogged (per your original post), but an air cooled ASU can be installed / replaced with the boat afloat. Iostherm reckon that provided the compressor is mounted where there is bilge access to allow for air to flow around the compressor and hot air to dissapait.

(By the way - I don't profess in any capacity to be an expert on this - this is just from the research I have done in the last couple of months for replacing my own fridge).

Too much info can be worse than not enough - at some point you need to make a decision and stick to it - the downside of the internet is that you could spend years researching something and coming up with pros and cons for all systems!

Jonny
 
I have one of the seacock cooling units, one simply removes a seacock near the fridge, usually the sink drain, and substitute the one with the cooler. It has given no trouble in five years use.
The Isotherm unit is very simple to install, it comes charged with the coolant, and has clever connectors that do not loose the coolant when you screw them together. I dont think that these would be compatible with any other makes.
I think that the KISS principle is the one to follow here- keep it simple,sailor.
Have you thought about what you would do if your nice new fridge didn´t work. I imagine that both suppliers would say that the guarantee was void.
 
I've just been reading Mr Calder's book, and his thoughts were along the following lines:

Keel Cooler - no moving parts - but can get fouled (weed etc)
Seacock Cooler - less chance of fouling if boat is moving, but if the boat is sat in still water (marina) with no sink flushing then it can clog (thoughts of leaving the boat in the marina unattended for a couple of weeks?)
Air Cooled - no fouling, but need to ensure lots of air (preferably cool air from the bilge).

Compressors - use lots of current at start-up, so best if run constantly at low speed. Frigoboat and Danfoss have a 'smart controller' which tries to run the compressors at 90 - 95% utilisation (ie running the compressor all the time at low speeds). This avoids the start up current draw and also the compressor is more efficient when running at low speeds.
The Isotherm ASU senses voltage, 13.2V kick in and 12.8v drop out. Above 13.2 it runs the compressor hard to freeze the plate, and it stops when a) plate is frozen, b) voltage drops below 12.8v. When the plate is frozen it doesn't run - other than to keep the plate topped up if voltage is over 13.2v. If the plate defrosts the compressor runs a low speed fair continuously to keep the temp down.

The idea of continuous low speed running seems to be key from what the book says - more efficient and overall less power draw. All 3 systems do this (Frigoboat, Isotherm and Danfoss's own smart controller), however the ASU from Isotherm also has the voltage sensitivity built in (which sounds like a damn fine idea to me).

Hope this helps

Jonny
 
I replaced a defunct Isotherm with a coventional air cooled Frigoboat.
It struggled in the Ionian in July/August but is fine in the cooler Adriatic.
In fact once the thermostat packed up I shorted it out and switch the top loading fridge
on at breakfast and off at dusk. A couple of solar panels and an Adverc alternator
controller take care of the load out of a 210Ah combined domestic/starter battery bank.
KISS really works.
 
Well, I’ve consulted both Frigo and Isotherm, so here goes…

Isotherm

Isotherm say that ASU is the bees knees and saves lots of power. When not on engine power it cycles between -2 and -7 degrees C, taking about 2.5A with the compressor on low speed. This is about 36W at 12V. They also reckon that the thermal mass of the holding plate will result in fewer start and stops in a period of time, also saving power.

When the engine is running the compressor runs faster, they say 75% more powerful. It takes 5A which is 60W at 12V. It takes the holding plate down to -14 degC in 50 to 80 minutes depending on the plate.

They say that their skin fitting does not foul up except when anchored for a long period in warm waters, and if this is the case a coat of antifoul will protect it.

They finally claim a reduction in power drain from the batteries of about 40%,given two 2x40 minute engine runs per day, which I struggle to reconcile with the figures above.

Frigoboat

Frigoboat say that to get real benefit from a holding plate it needs to be big, and this needs a big (1hp, 750W) compressor to freeze it down rapidly (ie engine driven). They reckon that the savings made by ASU are marginal because the mass of holding plate is small (so that it can be frozen by a small compressor) so it doesn’t actually save you battery drain for very long.

They concede that it saves power, but reckon that if you spent the (substantial) holding plate money on extra battery capacity and used an evaporator system instead you’d actually end up with more power saved. You can charge batteries faster than you can cool holding plates (and you can’t run an ipod off a holding plate).

They went on to give several good fridgy rules of thumb:
Double the thickness of the insulation and you’ll have the amperage
Halve the volume of your coolbox and you’ll halve the amperage. Do this by putting polyurethane blocks in the base of the box – apparently it is an insulator and has a low specific heat capacity. Having said this, I always thought air had a low specific heat capacity, but I suppose air can escape when you open the lid.

Finally, Isotherm and Frigoboat do not use compatible fittings.

So which am I going for?

Well the main argument against holding plates seems to be that they are expensive and you’d be better off spending that money on a battery, but a Frigoboat water cooled evaporator system costs £658+Vat = £773, whereas an Isotherm ASU water cooled holding plate system costs £812, so I think it will be the Isotherm.

I still prefer Frigoboats keel cooler though. Oh well.

regards

Robin
 
We've just ordered the Isotherm 3201 (ASU with air cooled compressor) - came out at £650 inc VAT and Delivery from www.marinestore.co.uk

It doesn't surprise me they don't have compatable fittings!

I think Frigoboat's comments about extra money on holding plate buying extra batteries is a little far fetched when the Isotherm holding plate system is only £40 more than the Frigoboat evaporator system (£40 doesn't buy much battery!) - although maybe this is compared to their engine driven compressor/holding plate system which is very expensive?

Good luck with it all


Jonny
 
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