Refrigeration Replacement

[Dockhead]

I have two Isotherm SP reefer units on my boat, one top-loading freezer and one front-loading fridge.

They are both 17 years old and have given great service, but I think they are more or less end of life, and so I think I need to start thinking about replacing them. I have slow refrigerant leaks in both of them, and the special through hulls are degrading. There's not really any point in replacing just the through hull units, it seems to me.

We have some great experts on refrigeration on CF, so I thought I would see what kind of advice I could get, which might be useful for others as well.

One obvious approach would be to just put new Isotherm SP units in -- they are still made. The principle of transferring heat from the condensors right into the cold seawater I sail in seems logical to me. There's no circulating pump. And my reefers work great even when the boat is out of the water -- air circulates through the special through hulls by convection and the reefers work.

What else should I be considering? I am aware that some of the experts on here believe in air cooled condensors rather than seawater cooled ones -- but what would be the advantage of that, compared to the SP system, which seems to provide the best of both worlds?

I am also aware that some of the experts here believe in expansion valve systems instead of cap tube systems -- and certainly I would love to have that much control of the system, being able to see both high and low pressure side -- but do I really care that much, if a decent cap tube system will just work and work for 17 years like this one has?

I will be interested in any opinions and advice anyone has.

[a64pilot]

Air to air heat exchanger is the simplest, and doesn’t require another hole in the boat.
If it has a disadvantage its when it’s operated in a high ambient temperature, something I’m under the impression that you don’t.
Capillary tubes except for their penchant for clogging work fine in a properly designed system, and that is their problem, they are not as flexible in operation. As long as everything is as per a proper design, they are great. I think for example most House refrigeration is a cap tube?
But installing a system in a boat has so many variables from tubing length and ambient temp where the condenser is located to level of insulation that it’s easy to inadvertently get outside of the operating parameters of a cap tube system, where a thermal expansion valve will more easily accommodate variables.

My one bit of advice is go bigger than you think you need, if your system has more cooling capacity than you need, everything will stay cold, just your compressor off times will be longer, and you will use a little more power, but not as much more as you may think. The heat removed is the same, and a bigger compressor is not that much less efficient than a smaller one is. Plus very soon after you throw in a case of warm beer or fish to freeze, your system has the excess capacity to deal with it.

Undersized, even slightly and you won’t be happy, it will always be a battle to keep things cold, and it will take at least a day to cool down that case of beer, or freeze that big fish.
Plus your insulation is not getting better, it’s slowly losing its R value over time, so what works now, may not in five years.

It’s a lot like having a V8 or a 6 cyl in a vehicle, if everything else is the same, the fuel mileage isn’t much different, but when you need the extra power, it’s there. I’m also of the opinion that a cooling system that isn’t having to work it’s guts out to just meet the demand, will last years longer than one that runs almost continuously.

[Typhoon]

Hey DockHead

You already have an air-cooled system if you say the air passing over the condenser is working when you are on the hard.

If you are staying in the waters you are in and never plan to sail in the tropics then by all means , stay with what has worked.

Captube systems work at there best efficiency at a specific ambient temperature, this seems to match your cruising grounds perfectly .

Regards John.

www.coldeh.com

[RaymondR]

With the climate prevailing in the places you sail getting all the warm air you could from an air cooled condenser inside the boat looks like a good idea to me??

[Typhoon]

Originally Posted by RaymondR

With the climate prevailing in the places you sail getting all the warm air you could from an air cooled condenser inside the boat looks like a good idea to me??

Good luck with that , small Danfoss compressors give of less heat then a human being . Its a marketing myth.



Regards John

www.coldeh.com

[RaymondR]

Originally Posted by ColdEh Marine

Good luck with that , small Danfoss compressors give of less heat then a human being . Its a marketing myth.



Regards John

www.coldeh.com

I'm to lazy to do the conversion from electrical to thermal Watts but my experience of cold places is that every little bit helps.

[a64pilot]

Originally Posted by RaymondR

I'm to lazy to do the conversion from electrical to thermal Watts but my experience of cold places is that every little bit helps.

If you assume that 100% of the electricity consumed is turned into heat, then the average fridge that consumes 5 amps is drawing a little more than 60 Watts when it’s running.
The average human male at rest dissipates between 100 and 120 Watts of heat.
So when your fridge is running, it is radiating a little more than one half of a resting man, if it has a 50% duty cycle, then of course about 1/4 of a resting male.
The fridge is just not a source of significant heat, unless you can tell the difference when another person is in the salon.
http://large.stanford.edu/courses/2016/ph240/stevens1/

Now my BD80 compressor is in my Lazarette, and if it’s closed up, you can tell it raises the heat in the Lazarette, give it just a little airflow and it doesn’t of course.

[Dockhead]

Originally Posted by a64pilot

If you assume that 100% of the electricity consumed is turned into heat, then the average fridge that consumes 5 amps is drawing a little more than 60 Watts when it’s running.
The average human male at rest dissipates between 100 and 120 Watts of heat.
So when your fridge is running, it is radiating a little more than one half of a resting man, if it has a 50% duty cycle, then of course about 1/4 of a resting male.
The fridge is just not a source of significant heat, unless you can tell the difference when another person is in the salon.
Human Body Heat as a Source for Thermoelectric Energy Generation

Now my BD80 compressor is in my Lazarette, and if it’s closed up, you can tell it raises the heat in the Lazarette, give it just a little airflow and it doesn’t of course.

That sounds like a pretty convincing argument.

But what about the efficiency of the fridge itself? I understand that in Florida, where I used to cruise, the water temp is often the same as the air temp. But up here in the summer the water will be 10C or even 15C colder than the air. And besides that, air carries vastly less heat than water so you need to spend energy on a fan blowing vast quantities to cool the condenser.

So doesn't it make sense to dump that heat into the ocean, rather than the air? Plus fans make noise.

I'm not arguing this; I'm just asking what you guys think.

Concerning heat in the cabin -- YES, I would be glad to have 100 watts or whatever more -- every little bit is nice. But the cabin is usually over 20C even if it's 0C outside and the sea water is 2C -- how much sense does it make to be dumping heat into that? Won't that greatly increase the expenditure of energy of the reefer system?

[Pete7]

What will the next owners of the yacht want? and were will they sail. Chances are it will be south to warm waters, so they will want water cooled perhaps.

Pete

[Typhoon]

Originally Posted by a64pilot

If you assume that 100% of the electricity consumed is turned into heat, then the average fridge that consumes 5 amps is drawing a little more than 60 Watts when it’s running.
The average human male at rest dissipates between 100 and 120 Watts of heat.
So when your fridge is running, it is radiating a little more than one half of a resting man, if it has a 50% duty cycle, then of course about 1/4 of a resting male.
The fridge is just not a source of significant heat, unless you can tell the difference when another person is in the salon.
Human Body Heat as a Source for Thermoelectric Energy Generation

Now my BD80 compressor is in my Lazarette, and if it’s closed up, you can tell it raises the heat in the Lazarette, give it just a little airflow and it doesn’t of course.

Your blinding me with science . I love it !

This is a little blog we wrote about the advantages of water cooling.

https://www.coldeh.com/blog/avoid-th...ystem-problems

There is a lot more science here as to why some system designs require water cooling which I will not get into right now .

Maybe I will write a blog post for the tech hungry people .

Regards John.

[Dockhead]

Originally Posted by ColdEh Marine

Your blinding me with science . I love it !

This is a little blog we wrote about the advantages of water cooling.

https://www.coldeh.com/blog/avoid-th...ystem-problems

There is a lot more science here as to why some system designs require water cooling which I will not get into right now .

Maybe I will write a blog post for the tech hungry people .

Regards John.

Thanks; that's very useful and interesting.

I'm not a fan boy of Isotherm systems, but just for the record, I have not experienced any of these 5 disadvantages you mention.

The SP system uses deck or sink drain through hulls, so there are no extra ones.

I have not experienced any fouling problems. I get a barnacle or two once in a blue moon, but I just brush them out on every haul out. This might be different in warmer waters, but I wouldn't say that I in am exactly low fouling area.

No problem using the SP system on dry land - it is specifically permitted, and in my experience works well. I guess the deck drains form a powerful convection stack which really pulls the air through, and without any fan!

I can't say how well the system works in warm water, however, because in many tens of thousands of miles, I've never been in warm water! Most years I don't get below 50N. So it could be that this system has relative advantages which disappear in warmer climes.

But just out of curiosity -- why do air cooled systems work better in hotter climates, if indeed they do? I'm having trouble quite imagining why.

[a64pilot]

Best answer I have on water cooling is look at house systems.
Back in the 70’s ground source heat pumps were all the rage, based on the absolutely correct theory that water will more easily absorb more heat than air, plus in this case the ground is closer to the same temp all year round, so it’s warmer than air in the Winter and cooler in the Summer, just perfect for a heat pump.
My part of the world that is 68F. There is a lot more heat in 68f water to heat in the winter when air temps are close to freezing, and a lot less heat to Cool the house in Summer when the air temps are 90+f.
So it’s obviously way more efficient, right? So we should see most of not all houses using ground source heat pumps?
But we don’t, why?
The reason is mostly maintenance, there is far more maintenance required with water pumps etc, than a simple fan, more to go wrong.
Then pretty quickly it was discovered that a very large air to air heat exchanger coupled with a large fan, was nearly as efficient, and a whole lot less maintenance.
An air to air fridge with a large enough condenser, and a large enough fan can run as efficiently as an air to liquid one.

Reminds me of the discussion I had with my Brother after he bought his first 911. He asked why more automobiles weren’t air cooled. I smiled and told him that all automobiles are air cooled. He said no, they all have radiators filled with coolant. So I asked him what cooled the radiator?

Bottom line, an air to air system with a large enough condenser and a good fan is very efficient, even up to ridiculously high air temps.

Add in an oversized condenser to deal with abnormal high temps, and an thermal expansion valve to deal with variable loads and a good sized receiver drier that can hold a little excess refrigerant and you have a system that is both efficient and can accept a wide variation in cooling demands.

See if a manufacturer can control cabinet size, insulation, temp the system is in etc., then a capillary tube system can be sized just right for those conditions and the system run trouble free and very efficiently for decades.
However a Marine conversion system has to be flexible enough to deal with different sized boxes, incorrectly mounted evaporators and a wide variety in R values, plus a lot of us have our boxes close to the engine room, so we get a lot of excess heat from there when the engine is on.
That is when an oversized condenser and fan, and a thermal expansion valve, and a receiver drier that can hold excess coolant becomes effective.
But it is more expensive to manufacture.

[Typhoon]

I agree with everything you just said pilot but I will add one thing . Captube systems have no receiver , they have very small amounts of refrigerant . These system have to maintain a balance flow in volume between the condenser and evaporator . It’s some thing like a chain and sprockets on a bike . This design limits the size of the condenser the system can use . This is bad in warmer climates so some designers will go to water cooling to try to get there condenser temperatures down which is key to striking that crucial balance . But the water is warm in the tropics , advantage lost.

[Dockhead]

Originally Posted by a64pilot

Best answer I have on water cooling is look at house systems.
Back in the 70’s ground source heat pumps were all the rage, based on the absolutely correct theory that water will more easily absorb more heat than air, plus in this case the ground is closer to the same temp all year round, so it’s warmer than air in the Winter and cooler in the Summer, just perfect for a heat pump.
My part of the world that is 68F. There is a lot more heat in 68f water to heat in the winter when air temps are close to freezing, and a lot less heat to Cool the house in Summer when the air temps are 90+f.
So it’s obviously way more efficient, right? So we should see most of not all houses using ground source heat pumps?
But we don’t, why?
The reason is mostly maintenance, there is far more maintenance required with water pumps etc, than a simple fan, more to go wrong.
Then pretty quickly it was discovered that a very large air to air heat exchanger coupled with a large fan, was nearly as efficient, and a whole lot less maintenance.
An air to air fridge with a large enough condenser, and a large enough fan can run as efficiently as an air to liquid one.

Reminds me of the discussion I had with my Brother after he bought his first 911. He asked why more automobiles weren’t air cooled. I smiled and told him that all automobiles are air cooled. He said no, they all have radiators filled with coolant. So I asked him what cooled the radiator?

Bottom line, an air to air system with a large enough condenser and a good fan is very efficient, even up to ridiculously high air temps.

Add in an oversized condenser to deal with abnormal high temps, and an thermal expansion valve to deal with variable loads and a good sized receiver drier that can hold a little excess refrigerant and you have a system that is both efficient and can accept a wide variation in cooling demands.

See if a manufacturer can control cabinet size, insulation, temp the system is in etc., then a capillary tube system can be sized just right for those conditions and the system run trouble free and very efficiently for decades.
However a Marine conversion system has to be flexible enough to deal with different sized boxes, incorrectly mounted evaporators and a wide variety in R values, plus a lot of us have our boxes close to the engine room, so we get a lot of excess heat from there when the engine is on.
That is when an oversized condenser and fan, and a thermal expansion valve, and a receiver drier that can hold excess coolant becomes effective.
But it is more expensive to manufacture.

Very well argued and sounds completely logical to me. Thanks.

[Dockhead]

Originally Posted by ColdEh Marine

I agree with everything you just said pilot but I will add one thing . Captube systems have no receiver , they have very small amounts of refrigerant . These system have to maintain a balance flow in volume between the condenser and evaporator . It’s some thing like a chain and sprockets on a bike . This design limits the size of the condenser the system can use . This is bad in warmer climates so some designers will go to water cooling to try to get there condenser temperatures down which is key to striking that crucial balance . But the water is warm in the tropics , advantage lost.

And that sounds convincing. I read some things about cap tube systems -- maybe on your site -- which made me desire a different type of system.