50 replies to this topic

[sailman]

Looking into retro fitting the ice box with a refrigeration unit. I like the idea of the Isotherm self-pumping units, less power and no fan or pump. Anyone out there installed a system? What are people using? Ice box is ~9ft3

[Zac]

I like seafrost, just keeps working, air cooled.

[Chucky]

I have just ordered the water cooled version of the Isotherm ASU/SP system. I intent to sail my boat in the tropics and apparently the air cooled units fail in the hot environment. I have been advised to buy the water cooled version as it is more reliable and effective in the hot climates.

Good luck

[Zonker]

9 ft3 is a very good sized box. How's the insulation and your battery capacity?

If you are going to keep the boat in RI area then the air cooled systems are much cheaper and work just fine with typical air temps, especially if you locate the compressor unit down low in the hull (galley locker) where the cold water against the hull keeps the local air temps reasonable.

Not true that air cooled units fail in the tropics. The air really isn't that hot. The key is to follow the manufacturers directions about air vent sizes - and having pass through vents in the compartment really helps. The constant drain of the water pump really doesn't pay off until air temps are > 90F constantly. Which they really aren't especially if you consider night time temps.

We spent the summer in the Sea of Cortez with an air cooled NovaKool - no issues except it ran a bit longer. But the super sunny days meant the solar panels made lots of power. Our fridge is 2.5 ft3 with 6" of insulation all around except the door.

Keep the compressor cool and ventilated. Keep the box well insulated and you will do fine with an air cooled unit.

[sailman]

Zonker, Thanks for the info. I was looking at the self pumping vs the air cooled for power savings, what I can not find is an efficiency comparison between the two. I have three inches of insullation around the box except the lid.

[Moonduster]

A refrigeration system is really just a heat pump. In the end, the heat removed from the box is transferred either into the air inside the boat or the water outside the boat.

Water has a massive advantage in both it's ability to absorb heat and it's ability to hold heat. There's no comparison between a water cooled and an air cooled system provided the compressor, evaporator and ice box are reasonably well designed and built. The water cooled system will be far, far more efficient.

The other draw back of an air-cooled system is that it heats the air inside your boat - not really idea during the months when you need to be running the refrigerator.

All around, the water cooled system is really the better choice unless there's something quite difficult about getting access to a thru-hull

[sailman]

A refrigeration system is really just a heat pump. In the end, the heat removed from the box is transferred either into the air inside the boat or the water outside the boat.

Water has a massive advantage in both it's ability to absorb heat and it's ability to hold heat. There's no comparison between a water cooled and an air cooled system provided the compressor, evaporator and ice box are reasonably well designed and built. The water cooled system will be far, far more efficient.

The other draw back of an air-cooled system is that it heats the air inside your boat - not really idea during the months when you need to be running the refrigerator.

All around, the water cooled system is really the better choice unless there's something quite difficult about getting access to a thru-hull

Moon, Thanks for the thermo lesson...what I am not sure of is how efficient the self pumping unit is compared to the air unit. The self pumping unit relies on the boat's relative motion either underway or bobbing at the dock/anchorage. Not having a flow of water, like a pumping system, I am curious as to how well the unit will run compared to an air cooled unit. Will it have a longer cycle time with the boat at rest?

Boat shows are coming up so I will work up my list of questions and specs.

[Sailabout]

saying the water cooled will be far more efficient
What efficiency are we talking
watts input versus heat pumped?
no argument water will transfer the heat faster then air

[sailman]

saying the water cooled will be far more efficient
What efficiency are we talking
watts input versus heat pumped?
no argument water will transfer the heat faster then air

All I really care about is power consumption, both will refrigerate my ice box, which one will do it using the least power over the course of a day. Obviously water is a better heat transfer medium but if the water is not moving, as with the self pumping system, what does that do to efficiency? It has to drop it down, more than likely not to the level of an air cooled unit but is the difference worth the added expense?

[Moonduster]

I think the thing to focus on is the overall power draw of the compressor when it's running. My system was a low-runtime commercial compressor with a 1/2 hp DC motor. It drew about 22 amps when running but only ran for about 30 minutes per day in the tropics. A danforth-style sealed compressor is far less efficient but also draws a butt-load less current when running.

If your unit is at the low end, then adding a water pump will increase the energy consumption a lot - like maybe by one amp, which could be a 33% increase - but it won't decrease the run time by 33%, not even close. It's worth noting that the water flow necessary is really quite small because the thermal capacity of water is so high. Scouting around for a small, self-priming salt-water pump might find something interesting.

That said, the self-pumping scheme sounds a little suspect. Does it work at all when the intake filter has a little crud in it or can it not handle any resistance to flow at all? I'd ask for detailed test results and also whether you can put a pump in line with the self-pumping solution for situations where you're in glassy calm.

[Zonker]

Sorry I missed the part about the self pumping - I think the water cooled systems that use a separate pump rarely pay off in terms of electrical load capacity unless it's smoking hot all the time.

I like the Frigoboat ones that use an external keel cooler. I suspect the thru hull ones that rely on significant boat motion would tend to heat up the small amount of water in the thru hull fitting unless the boat is underway or in a choppy anchorage.

Otherwise, yeah they are probably a good idea for your large box size and colder waters.

Nonsense that an air cooled one will heat the inside of the boat to any significant degree. Assume 4A @ 12.3V, running 50% (that is a lot but typical for badly insulated boxes). That is 48 W x 50% run time = 24 W. But don't forget a refrigeration unit is really a heat pump and is capable of moving more than 2x the input electrical power. Lets' say coeff. of performance of 2.5.

So... 2.5 x 24 W = 60 W. Would you notice the heat output of a 60 W bulb in a well ventilated refrigeration compressor space? I think not.

[Moonduster]

Zonk,

Those numbers and your conclusion may make sense in Canada, but living aboard in the tropics is a completely different story.

[Zonker]

Please. We've been living aboard for 3 years on this boat. Having spent 16 months in Mexico, and then crossing the pacific that took 7 months I do have some feel for tropical conditions. We're living in Brisbane right now and yeah around Christmas it was hot. I just don't bother changing my location as we travel.

I said

we spent the summer in the Sea of Cortez with an air cooled NovaKool

. And it worked fine. Average day time air temps are around 35-40C. Run times of around 50% in hottest conditions we've ever been in - I was reading a book and was timing the on/off cycle during one hot afternoon. Water temps were in the 25C + range.

I guess a couple of things might influence your decision:

- do you live aboard? If so, eventually you have to haul the boat somewhere. Most of us live aboard during the 3 or 4 days (at least) for a regular sanding/painting/replace the zincs etc. Keel or water cooled fridges do have an issue when there is no water present but I honestly don't know if they can't be run or they just suck up power trying to keep the box cold.

- are you going to keep cruising the East Coast of the US, where average temperatures are fairly cool?

- do you have a spot where the compressor, if air cooled has good ventilation? We live on a catamaran, and the compressor is located in the bridge deck cabin, under a settee. It has a huge opening on both sides and there is a big hatch on the front side of the cabin. This ensures good ventilation. Then there some like friends of ours who mounted the compressor in a cockpit locker, with a tiny 3" hole for ventilation, and wondered why their fridge ran all the time...

- If you're convinced water cooled is best, I would much rather go for the keel cooled/thru hull type than the ones that have separate circulation pumps. They seem to have a limited life of a year or 2.

Richard Kollman, who is a refrigerator expert with a business in Florida published a rather interesting technical article analysing all this. Unfortunately he's taken it off his website (probably to push his book sales). But his conclusion was that air cooled ones are more electrically efficient than water cooled unless the AVERAGE air temp (day and night) was >80F. That really is not the case even in hottest Baja. It cools down at night, and it does so all over the tropics. He also had some concerns about electrolysis of some of the keel cooled ones. (Some model even have zincs).

Good slide show is still on his website. http://www.kollmann-.../slideshow.aspx

[ccruiser]

We have an Isotherm air cooled unit on our boat, which is located in Florida, with which we cruise the Bahamas and the Keys in the Summer - 80 degrees would be an unheard of joy, even at night. The unit works fine, maintains the box at whatever temp I set it at, and electrical usage is more than acceptable. The studies I have seen indicated to me that a circulating pump was unnecessary even in those conditions, and simply added electrical usage, so I went with the aircooled unit, and have been more than happy with it. We have the unit located in a locker under the main settee, with venting to help with airflow.

So what Zonker said +1.

[soma]

We ditched a Seafrost air and water cooled built i unit in favor of a Vitrifrigo air cooled, off the shelf unit. The Seafrost worked great in Maine, when the water temp was in the 50's. It was unimpressive in the tropics with water temps in the 80's. The Vitrifrigo is bigger, colder, and consumes less juice.

[Moonduster]

Ok. After stewing on why everyone is equally convinced of their contrary conclusions, I mucked around a bit with the numbers and think I understand. My big observation was that all the systems mentioned (Seafrost, Vitrifrigo, Isotherm, Frigoboat and Seafrost) are based on hermetically sealed, Danfoss compressors. Similarly, the Kollman web site is all about Danfoss-based refrigeration systems - planning, installation, maintenance and trouble shooting.

Danfoss compressors are both relatively inefficient and relatively slow at pumping heat. This is why even for modest sized boxes, 50% run time is typical. My experience is with high-end, belt-driven commercial compressors that have very, very low run time.

In terms of efficiency, if you look at a 50% duty cycle Danfoss compressor drawing 4 amps, you get 48AH per day. If you look at my custom-built solution, it drew 22 amps for about 24 minutes per day you get 8.8AH per day.

Note that these two system move approximately the same amount of heat per day. It's probably not the case that a direct comparison should be made between those two numbers because they weren't measured on identical boxes with identical insulation, identical gasketing and identical ambient air temperature - but bear with me anyway. If we do a direct comparison, we find that the Danfoss system uses about 5.5 times the energy - even if the thermal conditions surrounding the accounted for a factor of 2, the Danfoss system is still less than 50% of the efficiency of a nicely designed solution.

But the killer is really that the Danfoss system is a slow heat pump. Because the Danfoss removes heat from the box so slowly, there's not much heat being dissipated at any one instant in time. This allows a fan to easily keep up with the heat developed at any one instant.

However, the high-end system has 1/30 the run time - that suggests 30x the heat being dissipated while the system is running. This higher instantaneous heat load requires water cooling.

Another way to think about this is that Danfoss-based systems are so inefficient that water cooling can't help them. That said, it does seem to be the case that fan-based cooling is the best approach for a Danfoss-based system.

For what it's worth, the bible on marine refrigeration seems to remain Refrigeration for Pleasureboats by Nigel Calder, published in 1990.

[Cptnsiroco]

No one has said anything about cold plates, are these not a consideration?

[sailman]

No one has said anything about cold plates, are these not a consideration?

You mean something like this:

[Moonduster]

No. Cold plates aren't really a consideration. They do bring plenty of benefits and increase the overall efficiency of the refrigeration system, but they don't change the math about the efficiency or speed of the compressor or how best to remove the heat from the condenser.

[Jonathan Green]

Ok. After stewing on why everyone is equally convinced of their contrary conclusions, I mucked around a bit with the numbers and think I understand. My big observation was that all the systems mentioned (Seafrost, Vitrifrigo, Isotherm, Frigoboat and Seafrost) are based on hermetically sealed, Danfoss compressors. Similarly, the Kollman web site is all about Danfoss-based refrigeration systems - planning, installation, maintenance and trouble shooting.

Danfoss compressors are both relatively inefficient and relatively slow at pumping heat. This is why even for modest sized boxes, 50% run time is typical. My experience is with high-end, belt-driven commercial compressors that have very, very low run time.

In terms of efficiency, if you look at a 50% duty cycle Danfoss compressor drawing 4 amps, you get 48AH per day. If you look at my custom-built solution, it drew 22 amps for about 24 minutes per day you get 8.8AH per day.

Note that these two system move approximately the same amount of heat per day. It's probably not the case that a direct comparison should be made between those two numbers because they weren't measured on identical boxes with identical insulation, identical gasketing and identical ambient air temperature - but bear with me anyway. If we do a direct comparison, we find that the Danfoss system uses about 5.5 times the energy - even if the thermal conditions surrounding the accounted for a factor of 2, the Danfoss system is still less than 50% of the efficiency of a nicely designed solution.

But the killer is really that the Danfoss system is a slow heat pump. Because the Danfoss removes heat from the box so slowly, there's not much heat being dissipated at any one instant in time. This allows a fan to easily keep up with the heat developed at any one instant.

However, the high-end system has 1/30 the run time - that suggests 30x the heat being dissipated while the system is running. This higher instantaneous heat load requires water cooling.

Another way to think about this is that Danfoss-based systems are so inefficient that water cooling can't help them. That said, it does seem to be the case that fan-based cooling is the best approach for a Danfoss-based system.

For what it's worth, the bible on marine refrigeration seems to remain Refrigeration for Pleasureboats by Nigel Calder, published in 1990.

Good explanation, thank you for posting. What kind of compressor do you use?

[Masala]

Sailman,

Here is my experience with an IsoTherm self-pumping unit, with IsoTherm thru-hull condenser installed in the adjacent galley sink drain. I really like the quiet operation - no fans, no electric water pump, just the DanFoss compressor in locker with a bit of ventilation.

Boat motion is NOT required in order for the unit to operate. The condenser will happily dump heat into the water even in the most calm conditions, all it needs is a little convection in the water, and boats are never perfectly still in the water anyway. In fact, it works OK on the hard too, simply dumping heat into the air around the thru-hull. It's probably less efficient, but there is shore power on the hard, so who cares.

I installed mine in April, 2005. We ran it continuously for over a year, into mid-2006, and still use it regularly for weekends and vacation cruises. I think the compressor makes a little more sound than it did 7 years ago, but we've had zero maintenance to do on this system. It's worked hard at times (Southern France in July) but has performed well.

I selected an oval-shaped evaporator to pair with it, and we use this as a freezer compartment. Keeps meat frozen hard in the lower portion. Our ice-box is 33 years old and probably not that well insulated. The previous owner did some things to try to help it, but I'm not sure how much or how effective those were. The only data I have on power consumption is that the compressor is rated at 4 amps. If we run the fridge hard, if we've just arrived with a bunch of warm food and drink, then it runs up to 80% of the time. If we turn it down to keep food cool, it's less than a 50% duty cycle.

That's my experience as a very satisfied customer. I would readily choose an IsoTherm self-pumping system again.

Cheers,
Masala

[sailman]

Sailman,

Here is my experience with an IsoTherm self-pumping unit, with IsoTherm thru-hull condenser installed in the adjacent galley sink drain. I really like the quiet operation - no fans, no electric water pump, just the DanFoss compressor in locker with a bit of ventilation.

Boat motion is NOT required in order for the unit to operate. The condenser will happily dump heat into the water even in the most calm conditions, all it needs is a little convection in the water, and boats are never perfectly still in the water anyway. In fact, it works OK on the hard too, simply dumping heat into the air around the thru-hull. It's probably less efficient, but there is shore power on the hard, so who cares.

I installed mine in April, 2005. We ran it continuously for over a year, into mid-2006, and still use it regularly for weekends and vacation cruises. I think the compressor makes a little more sound than it did 7 years ago, but we've had zero maintenance to do on this system. It's worked hard at times (Southern France in July) but has performed well.

I selected an oval-shaped evaporator to pair with it, and we use this as a freezer compartment. Keeps meat frozen hard in the lower portion. Our ice-box is 33 years old and probably not that well insulated. The previous owner did some things to try to help it, but I'm not sure how much or how effective those were. The only data I have on power consumption is that the compressor is rated at 4 amps. If we run the fridge hard, if we've just arrived with a bunch of warm food and drink, then it runs up to 80% of the time. If we turn it down to keep food cool, it's less than a 50% duty cycle.

That's my experience as a very satisfied customer. I would readily choose an IsoTherm self-pumping system again.

Cheers,
Masala

Masala,

Thanks very much for the first hand account of the isotherm. I found a used Nova Cool system sized for my ice box so I went with it. I will do the install this winter, including beefing up the ice box insullation.

Will Museler

[bert s]

2c idea....
Take your average air cooled unit and cut the hot gas line to the air cooled condenser. Manufacture a tall PVC hot water heater and install a coil of cuni tube inside it. Hot gas goes into the top and if it isn't fully condensed by the time it comes out the bottom, it runs thru the air cooled condenser. Cycle the air cooled condenser fan on head pressure. Wash dishes with the hot water that cooled your beer. What could be simpler? Any patent attorneys out there?

[stranded]

Why 9 cu ft / 250 litres ?
loaded that is a quarter of a ton
maybe blank half off for dry storage
how many mouths ?
danfoss bd 35 is good for up to 50 litres
bd 50........80 litres ... + X 3.....= 30 to 50 amps@ 12
sounds like you need commercial fishing boat gear
and a BIG diesel
BTW
why ask here ?
plenty of good installers in Brisbane
you in scarborough ?
talk to a prawnie

[sailman]

Why 9 cu ft / 250 litres ?
loaded that is a quarter of a ton
maybe blank half off for dry storage
how many mouths ?
danfoss bd 35 is good for up to 50 litres
bd 50........80 litres ... + X 3.....= 30 to 50 amps@ 12
sounds like you need commercial fishing boat gear
and a BIG diesel
BTW
why ask here ?
plenty of good installers in Brisbane
you in scarborough ?
talk to a prawnie

Wrong hemisphere, I'm in Portsmouth, RI USA. The size of the box is what it is, I will be taking the countertop off so I will be adding additional insullation as well during this install. I like the idea of the hot water heater "economizer" add on!

[stranded]

Sorry, It is Zonker who is in Brisbane.....

[Mogle]

Very happy with my Frigoboat that is water cooled. The fridge is 20 years old and still very effisient. I have had the yacht for the last three years where I have upgraded a few bits.

The electricity usage is a problem. When running I think it consumes 6.5-7 amp. Replaced the pump after it stopped. The new pump is more effisient. Next step would be to replace the compressor. Been told that the new compressor will bring it down to 4-5 amp overall. Personally I think the isolation is the key component.

[timothy22]

I have a dumb question.... If heat leaks into the box throughout the 24-hour day, how does a refrigeration unit that only runs for 30 minutes keep the box cool? I mean, it can only remove the heat that is in the box at the time it starts. Is it possible to build a box so well insulated that it only rises in temperature from 33 degrees F to 45 degrees F in 23 1/2 hours? I thought that holdover plates were a necessity to absorb a day's worth of heat so the monster compressor could pump it out in a short time.

[199.99]

I have a dumb question.... If heat leaks into the box throughout the 24-hour day, how does a refrigeration unit that only runs for 30 minutes keep the box cool? I mean, it can only remove the heat that is in the box at the time it starts. Is it possible to build a box so well insulated that it only rises in temperature from 33 degrees F to 45 degrees F in 23 1/2 hours? I thought that holdover plates were a necessity to absorb a day's worth of heat so the monster compressor could pump it out in a short time.

not sure with your 24h/30 min but the idea behind the holding plate is a eutectic fluid and latent heat. ie, something frozen doesn't increase in temp. until it is fully thawed.

[timothy22]

Yes, exactly- My understanding is that the small Danfoss compressor cycles on and off throughout the day, removing the heat a little at a time, so they don't need holdover plates. The eutectic fluid in the holdover plate absorbs heat throughout the day and uses a large compressor to remove it all in a short run time.

[scottmax]

Eutectic plates cool down to around -20C when the compressor runs then cycles off depends on how big your compressor to how long this takes and how big your Eutectic tank is (not cold box). Think of a heat sink that will absorb the energy all day and that is the eutectic tank. This is great and works like a esky (or cooler what ever you like) with block ice.

If you wanted to run the area where the Eutectic tank is as a freezer to freeze food you run the compressor longer and cycle the temperature on the cold box temp. The heat transfer of the eutectic tank is not that great because of a reduced surface area compared to a evaporator plate which would be fare more efficient.

To save energy in refrigeration it is all about heat rejection, the more efficiently to can remove the heat the less power it will consume. Through hull fitting while a bit more risky are by far he the most efficient rate to remove heat. Water is 5 time more efficient at removing heat than air. There is a trade off and if you use the same system in the Caribbean as you may in the Arctic you may have some issues with compressor burn out or mechanical failures.

[stranded]

[ QUOTE ]

Yes, exactly- My understanding is that the small Danfoss compressor cycles on and off throughout the day, removing the heat a little at a time, so they don't need holdover plates. The eutectic fluid in the holdover plate absorbs heat throughout the day and uses a large compressor to remove it all in a short run time.

[ UNQUOTE ]


I have a 40 litre box with brine tanks that is driven @ 12v by a danfoss bd35

can pull down to 50 degrees below ambient

normal usage needs 2 hours twice a day

• 
  

  The interior cabinet and the exterior construction of the AUTOFRIDGE are white fibreglass the COLOUR is the best for hot climates and the integral strength of FIBREGLASS for use in rough conditions has been proven by the long-term popularity of fibreglass for boat manufacture.

• 
  

  The compressor chosen for use in the AUTOFRIDGE is the latest Danfoss BD35F unit and controller. This compressor has been selected because of its remarkable efficiency, and it is supplied as a spring-housed unit - suitable for use in any off-road conditions.

• 
  

  The EUTECTIC operation of the refrigeration system allows you, the user, the luxury of knowing that in ambient temperatures of up to 32 degrees Celsius, the MAXIMUM DAILY POWER CONSUMPTION of your fridge is on average only 20AH/12Volt. This of course means that you can easily determine how long you can remain stationary without flattening your battery.

The AUTOFRIDGE comprises a compressor driven system combined with the power-saving advantages of the eutectic refrigeration system - the 'eutectics' act as a cold storage compound for the refrigeration contents. The eutectic fluid surrounds the outside of the four walls of the interior cabinet acting as an 'ice bank', and, in an ambient temperature of up to 32 degrees Celsius, having initially pre-chilled the AUTOFRIDGE, it need only be run on its maximum setting for a total of 5-6 hours per day (preferably 2 hours in the morning and ~3 hours in the afternoon), and during this run-time, the fluid is frozen and the cabinet contents are thoroughly chilled. The AUTOFRIDGE can be turned OFF in between the run-times, and the eutectic fluid acts as a thermal phase change (cold storage) compound and provides the continuous refrigeration hold-over for NIL power consumption. Figures quoted assume normal usage of your refrigerator, e.g. opening/shutting the lid to use or replace contents.
The AUTOFRIDGE can be operated in any one of the following ways :-

• As a EUTECTIC refrigeration system – the AUTOFRIDGE is run for a total of 5-6 hours per day – split into two separate running periods of 2 hours in the morning and ~3 hours in the afternoon. The AUTOFRIDGE is turned "OFF" in the interim. Average daily power consumption for this mode of operation is ~20Ah/day @ 12V.
• 
  

  As a CYCLIC refrigeration system – the AUTOFRIDGE is set on a refrigeration setting (e.g. '2') on the thermostat and left to cycle 'on' and 'off' throughout the day. Average daily power consumption for this mode of operation is ~24Ah/day @ 12V.

• 
  

  As a CYCLIC freezer – the AUTOFRIDGE is set on one of its sub-zero settings ('5', '6' or '7') on the thermostat – the higher the setting, the colder the AUTOFRIDGE will cycle – and left to cycle 'on' and 'off' throughout the day. Average daily power consumption for this mode of operation is ~40Ah/day @ 12V.

As you probably are aware, the efficiency of your vehicle or marine electrical system can greatly affect the operation of any low voltage refrigerator. Whether to install an auxiliary battery can usually be determined by how you intend to use your AUTOFRIDGE or any other D.C. lights or appliances



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Most Efficient 12 Volt Refrigeration - Around Oz

Quirks have a fantastic Internet site, and you will find ... The AUTOFRIDGE has been designed and manufactured in Australia to suit Australian conditions.
www.aroundoz.com/a_clinic_archive/refrigeration/most... - Cached


9 cu ft ( 250 litres ) needs a bit more .......

[Ryley]

Sorry I missed the part about the self pumping - I think the water cooled systems that use a separate pump rarely pay off in terms of electrical load capacity unless it's smoking hot all the time.

I like the Frigoboat ones that use an external keel cooler. I suspect the thru hull ones that rely on significant boat motion would tend to heat up the small amount of water in the thru hull fitting unless the boat is underway or in a choppy anchorage.

Otherwise, yeah they are probably a good idea for your large box size and colder waters.

Nonsense that an air cooled one will heat the inside of the boat to any significant degree. Assume 4A @ 12.3V, running 50% (that is a lot but typical for badly insulated boxes). That is 48 W x 50% run time = 24 W. But don't forget a refrigeration unit is really a heat pump and is capable of moving more than 2x the input electrical power. Lets' say coeff. of performance of 2.5.

So... 2.5 x 24 W = 60 W. Would you notice the heat output of a 60 W bulb in a well ventilated refrigeration compressor space? I think not.

I had a frigoboat keel cooler in my last boat. Freezer and refrigerator boxes right next to each other with a passthrough fan from the freezer to the fridge. It was amazingly simple, efficient, and worked better than I could have hoped. current boat has separate compressors for freezer (starboard side) and refrigerator (port side), both air cooled. I wish I had the keel coolers back - even when we went to bermuda. However, the frigoboat systems, whether keel or air, seem to be rock-solid, based on proven technology, and work very well.

[Weyalan]

My understanding is that at 9 cubic feet you are getting into the transition between the Danfoss BD35 and BD50 compressor (for air cooled, at least). That may not be a concern for you, but the BD50 will. I think, draw more current than the 35. We have 9 cu.ft and went with a BD50 set-up. If you want to minimise your electrical draw, think long and hard about your insulation.

[167149]

couple of things to bear in mind, danfos DC comps are principally for DX use (direct expansion) which equates to very little between the refrigerant and the product. Eutectic (holdover plates/ cabinets etc) are indirect expansion, refrigerant removes heat from eutectic which in turn removes heat from product which equates to a lag in efficiency of transmission(slower pull down etc). Larger engine drive systems certainly get the job done however.......you're stuck on the boat running em twice a day.....not everyones cup of tea. Larger AC powered systems consume power, gobs of it if you're thinking inverters. Water cooling a danfoss system via a pump is really an exercise in frustration as no one has really brought out a constantly rated DC pump to last the distance. Danfoss DC compressors do require a cooling fan of some description runnng over the electrics. Whatever system you run with, if the lid seal is defective your system will also be defective

[sailman]

Just began the project yesterday. Step one: taking off counter top. I found out how the top was attached. They took a piece of 1/2" marine plywood and screwed it into the upright 1/2" marine plywood. Then the formica was paid on top! So a quick run through with the saws all is in order. pics to follow.

[rattie]

2 bits, put a y join in the water hose with a couple of shut off taps, so you can fresh water flush. Got antifreeze in mine at the moment

[Joakim]

Danfoss compressors are both relatively inefficient and relatively slow at pumping heat. This is why even for modest sized boxes, 50% run time is typical. My experience is with high-end, belt-driven commercial compressors that have very, very low run time.

In terms of efficiency, if you look at a 50% duty cycle Danfoss compressor drawing 4 amps, you get 48AH per day. If you look at my custom-built solution, it drew 22 amps for about 24 minutes per day you get 8.8AH per day.

Note that these two system move approximately the same amount of heat per day. It's probably not the case that a direct comparison should be made between those two numbers because they weren't measured on identical boxes with identical insulation, identical gasketing and identical ambient air temperature - but bear with me anyway. If we do a direct comparison, we find that the Danfoss system uses about 5.5 times the energy - even if the thermal conditions surrounding the accounted for a factor of 2, the Danfoss system is still less than 50% of the efficiency of a nicely designed solution.

I find it very hard to believe there would be a big difference in efficiency for a Danfoss BD35F used in most marine systems compared to any other compressor. According to this Danfoss has COP of 2-2.5 for typical marine refridgerator enviroment: http://www.ra.danfos...dehc100d802.pdf

Under these conditions the theoretical maximum is around 5, thus it is impossible to find another compressor that would be much more efficient: http://www.energyboo...m/pdf/D1132.pdf

The efficiency of a compressor depends very much on the temperature difference. Thus having enough cooling is essential. If you really have had 5 times more power consumption from Danfoss, it must have been running very hot.

If you really only consume 8.8 Ah/day, you must have a small very well insulated box on you can't put much warm stuff into it. I had a 23 liter refridgerator box in my old boat with Danfoss BD35F. It run 1/6-1/10 duty cycle depending on the temperature (outside 10-30 C). While running it took 3.2 A thus it took 8-13 Ah/day. My current boat has a much larger box (100 liters??) not that well insulated with the same compressor. Also the compressor is installed under the box thus heating the box. It runs 1/3-1/6 duty cycles and take about 3.6 A while running thus 14-29 Ah/day.

[sailman]

Perhaps a crazy idea (I'm not an engineer, and didn't stay at a Holiday Inn last night), but has anyone ever turned the keel itself into a 'keel cooler'? Lead melts at 327.5C, Copper at 1084C, and Stainless Steel at 1510C. Could one insert a tube into the keel during the pour that could be connected to the fridge once assembled?

Theoretically, yes you could but it would not be very efficient.

[Moonduster]

Joakim,

The efficiency of the compressor does not depend on the temperature differential - the temperature differential affects the overall efficiency of the unit but not the compressor per-se. Compressor aside, the two important considerations are the efficiency of the heat exchanger and the efficiency of the ice box itself - the former has to do with how quickly you can get the heat out and the latter has to do with how quickly heat will re-enter. Clearly a water cooled heat exchanger is going to be far superior to an air cooled solution - especially if the air temperature is high relative to the water temperature as is often the case in the tropics.

As far as compressors are concerned, I don't know from where you got your efficiency numbers - but I do know that all commercial refrigeration compressors are belt-driven reciprocating-style, not hermetically sealed Danfoss-style compressors. I also know that I would never look to a manufacturer's claims of efficiency to select anything - but from what you've said, the Danfoss claims suggest there's 50% efficiency left on the table. You might give Nigel Calder's (only) excellent book "Refrigeration for Pleasureboats" for more information.

I'm not a refrigerator expert, but the guy who designed my compressor/exchanger/holding plate/eutectic solution was. The basic mechanism is an industrial single cylinder reciprocating compressor driven with a 1/3 HP motor. The holding plate is about 2 gallons and the ice box is around 8 gallons - that's a huge ratio of eutectic solution to cooled volume but it's really worth it if you're concerned about energy efficiency.

The icebox itself is insulated all around with 4-5" on all sides and the top and 8" on most of the bottom. There's also a reflective foil outer skin and an inside plastic vapor barrier. There's no point investing in a good heat pump if you don't also invest in an excellent box design. The design of the lid and choice of gasketing is also really important.

As you mention, what goes in and out is important, too. How the box is loaded, how often it's opened and scads of other considerations can have a massive effect on overall energy consumption. It's important to note that I don't drink beer - and I imagine that's probably the biggest heat load presented to any fridge on most cruising boats. Instead, I drink rum tonics and gin tonics and keep both the tonic water and the alcohol in the ice box. I'm also careful to keep the box as full as possible, using plenty of Bombay Saphire bottles (square profile) to load up with water when the fridge is low on food.

[Joakim]

the Danfoss claims suggest there's 50% efficiency left on the table. Y

I'm not a refrigerator expert, but the guy who designed my compressor/exchanger/holding plate/eutectic solution was. The basic mechanism is an industrial single cylinder reciprocating compressor driven with a 1/3 HP motor. The holding plate is about 2 gallons and the ice box is around 8 gallons - that's a huge ratio of eutectic solution to cooled volume but it's really worth it if you're concerned about energy efficiency.

The icebox itself is insulated all around with 4-5" on all sides and the top and 8" on most of the bottom. There's also a reflective foil outer skin and an inside plastic vapor barrier. There's no point investing in a good heat pump if you don't also invest in an excellent box design. The design of the lid and choice of gasketing is also really important.

No real heat pump can reach (even close to) the threoretical maximum COP. There might be heat pumps that are say 20% more efficient, but not twice as efficient. Danfoss compressors are used much in home refidgerators, which are nowadays very well optimized for low power comsumption. Industrial heat pumps have much higher power, but not much higher efficiency.

So your box really is very well insulated and quite small. Both boxes I have had had only aboout 25 mm (1") insulation and I suspect the current one is insulated only with airfilled plastic mold. A danfoss BD35F would certainly not take 48 Ah/day with your box. More likely about 10 Ah/day. Here are some values for about the same size, but much thinner insulation box: http://www.my-carava...lMatic-CB-36/70

Water cooling is of course more efficient. There are water cooled systems with Danfoss as well.

[Moonduster]

I've never bothered to calculate efficiency in absolute terms as it seems immaterial. That said, I wouldn't trust the Danfoss efficiency numbers as far as I could throw one of their compressors.

Home refrigeration systems are horribly insulated, as are the boxes you describe. In such cases, there's no need for a high-performance heat pump as it makes more sense to run continuously to balance the continuous influx of heat through the lousy insulation. It's for this same reason, I believe, that the evaporator approach makes more sense in this class of system rather than a large holding plate.

[Maxion]

You guys are all talking about compressor efficiency and how that relates to box design and air vs water cooled compressors, yet you are all forgetting that what typ of refrigeration system you need totally depends on how you sail.

For example, I live in Finland, and here in the baltic sea you're never more than 24 hours from a port. And I'd wager 99% of all sailors in the world sail in a similar fashion, with one and at most two days between ports.

Because of how we sail here, A danfoss type evaporator with a holding place is pretty much the smartest option.

When you're in port and hooked up to shore power the danfoss cools the holding plate. When you set off; if you've got a properly insulated box; the holding plate will keep the box cool for a long time. If you use any engine power to maneuver for anchoring the evaporator will cool the holding plate.

You're not really going to run it off of batteries much, if at all.

IMO a high-efficiency compressor and thinking of total power consumption becomes worthwhile only when you sail blue water, without access to shore power or the power the engine produces while doing mandatory maneuvers (such as entering port).

[Joakim]

Home refrigeration systems are horribly insulated, as are the boxes you describe. In such cases, there's no need for a high-performance heat pump as it makes more sense to run continuously to balance the continuous influx of heat through the lousy insulation.

That may have been the case in the past (maybe still is in US?). In the EU many new refridgerators go into A+++ energy class, which requires quite low consumption. E.g. this uses 75 kWh/year, which is equal to 17 Ah/day on 12 V, despite 346 l (almost 100 gallons) volume and big door. http://www.pricerunn...te-Product-Info

[Moonduster]

Joakim,

You seem to enjoy quoting manufacturer's claims, but you don't supply any information about what they mean. Sure, I could run a household refirgierator on no electricity if the temperature of the house was, say, 4C. The 75Kwh/year is meaningless without tremendous additional context. Go ahead and find the ISO standard by which this is measured and let us know the ambient temperature of the room and the time the door is oper per day and the thermal mass of the contents of the unit when this number is "computed".

In my experience, these numbers are like car fuel consumption ratings - a reasonable basis for comparison between two items, but not practically obtainable.

Further, there's just no reasonable way to achieve outstanding insulation with the wall thickness of a typical home refrigeration unit. There's no way that one could keep a ~350L box at a reasonable internal temperature when the outside surface is in the 25-30C range without using vacuum panel insulation and achieve a 30mm wall thickness.

It's really clear you have a strong liking for Danfoss compressors. That's fine. But you are delusional if you believe they are the ultimate in heat pumps. There's a world of evidence to the contrary - just look around.

[Joakim]

Moonduster,

There are clear EU directives that must be used for measuring the consumption. The ambient temperature is 25 C and inside temperature depends on type of the compartment. You can find it all from here: http://eur-lex.europ...017:0046:EN:PDF

The Bosch refigerator is 186 x 65 x 60 cm while 350L would be e.g. 155 x 50 x 45. Thus there should be room for at least 50 mm insulation. PU has thermal conductivity of about 0.02 W/m/K and there is 3.5 m2 area. For 20 C temperature difference and 50 mm PU the heat loss through insulation would be 28 W. For COP of 2 that would be 14 W electrical power, thus 28 Ah/day for 12 V. Since this is a recent energy efficient model, it most likely has something better than PU. Vacuum panels are below 0.01 W/m/K. Using 0.007 W/m/k would equal to 10 Ah/day with the earlier assumptions.

I haven't studied that directive in detail, but typically EU directives give meaningfull values. E.g. all the cars I have had have consumed very close to the claimed EU average. On longer trips without heavy traffic I get easily 20% below them.

I don't have a strong liking for Danfoss compressors. They just seem to be almost the standard for marine and home use and I never had a problem with them.

[stranded]

It's not just about performance numbers,

longevity and reliability can outweigh a lot of the arguments used here

I have had good service from a danfoss driven fridge


and as someone else has said, less than 10 days, ice or dry ice ( or frozen beer cans )

longer periods and temperatures above 25 c / 70f call for refrigeration

up to 80 or 100 litres, a danfoss bd 50 can do the job for a good box, above that, ( OP = 250 litres/ 9 cu ft ) you are looking at an engine driven compressor

car air con compressors will do the job, and are compact , and can be hung on the motor like an alternator if engine driven,

or also be driven electrically

No one seems to have mentioned the different types of refrigerant gases, and their specified temperature ranges


as someone else said, how and what the box is used for has a large effect on power consuption

[Joakim]

up to 80 or 100 litres, a danfoss bd 50 can do the job for a good box, above that, ( OP = 250 litres/ 9 cu ft ) you are looking at an engine driven compressor

Danfoss has bigger units as well. These are up to 400 litres
http://marine.dometicgroup.com/en/products/cooling_units_488.php
Water cooled:
http://marine.dometicgroup.com/en/products/cooling_units_491.php

[stranded]

the difference between consumption for a bd 35 and bd 50 is not a linear increase

because a fan or fans are required

both units will use about 10 to 15 ampere hours and 15 to 20, respectively,

depending on the box and how demands are managed.



my point is, you then start to move to a need for a larger compressor,

either electrically or mechanically driven via electric clutches ( more drain )

and pumps if a water cooled condenser is used

[sailman]

Progress update:

Finally got the tops off! They were screwed into the marine plywood uprights and the ice box fiberglass flange and then formica was placed on top covering the screw heads! Fun process chiseling out formica to get at the screws that could not be cut. In the end though the tops are off and templated for Corian replacements. The insullation is not bad, some voids will be beefed up with spray foam.

Open question: I have no way of confirming this or doing anything about it, but, would it be safe to assume that the bottom of the icebox is foam insullated as well?

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 Sink top.JPG   281.03K   20 downloads

[sailman]

Temperatures finally warmed up enough to start again. I got the evaporator in and refrigerant lines run, next job is to mount the compressor and wire it up. New countertops are done, 1/2" maple so that it will line up with the marine ply in the back. The transition from maple to ply will be hidden under the sliders behind the stove.

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