Refrigeration Condenser in Water Tank

We cruised for 5 years on a steel ketch with very basic systems. It had a small refrigerator, maybe 3 cf, with a homemade rig - evaporator plate and belt driven piston compressor. The condenser was stainless tubing coiled in the fresh water tank which was around 80 gallons and integrated into the hull. The heat of condensation was transferred to fresh water and eventually through the hull to the sea. Very simple - no salt water pump, no through-hulls, no fans.

I’ll probably have to replace my Grunert compressor in the next few years. Much bigger refrigerator and freezer with holding plates. Capacity is around 5,000 BTU/hr and we run it about 2 hrs a day. We also have a large 750 gal water tank, also part of the hull. At 60% level around 5,000 lb of water, so it would heat up around 2 degrees a day. The hull is fiberglass so heat transfer to the sea would be slow but would eventually equalize at some intermediate temperature. I’ll have to dig out my old heat transfer books when we get back to our home port.

Has anyone seen or heard of such a system?

No but if Lin and Larry Pardey ever went to steel I would not be surprised if Larry built something similar.

Larry would never have refrigeration.

Your gain from the pump is only 2 deg, and you have 24 hours to dissipate that through the hull. Fiberglass isn't as good a conductor as steel, but it isn't bad. I'd be shocked it if didn't work. How much common area is there between tank and hull/seawater?

Do your calculations first and be conservative. We had 1/2 hp dc cold plate system that was water cooled. Once while on the hard, I plumbed one of the 75 gallon SS fresh water tanks to the system. Unfortunately, the heat transfer was pretty much absent and I raised the water temperature of the tank so much that I had an algae bloom. I later starting adding ice. It’s a amazing how much heat I needed to remove.

Well as an engineer I understand what you are talking about, but....

As Practical Sailor and others have said over the years, a battery is a better way to store energy/heat than a holding plate. Also a lower btu/hr system that runs at a 50% duty cycle rather than the 10% duty cycle of your holding plate system allows a small compressor which can be efficiently DC powered.

Frigoboat makes a keel cooled condenser based package that you could easily adapt by putting their bronze condenser in the bottom of your water tank so it would work effectively like what you have now. Frigoboat systems are much cheaper I bet than cobbling components together to try to duplicate what you now have.

I had one on a 6 cu ft fridge/freezer and it used 50-75 Ahs daily.

David

"As Practical Sailor and others have said over the years, a battery is a better way to store energy/heat than a holding plate."

I hope they defined "better".

I have lived with both systems and agree if a conversion is being done electric is quick & cheap.

But with a new build better box insulation can be built in and eutetic plates do not wear out and can have two circuits , engine powered for rapid refreeze or smaller 120v 1/6 hp unit for dockside ease of operation.

Were I setting up a simple budget cruiser today one of the modern super insulated ice cheats , with a DC setup would be modestly efficient and not expensive .

Even solar might work , sometimes.

Of course a Propane RV unit would be even easier to live with long term anchoring out.

DCDC said:

I’ll probably have to replace my Grunert compressor in the next few years. Much bigger refrigerator and freezer with holding plates. Capacity is around 5,000 BTU/hr and we run it about 2 hrs a day. We also have a large 750 gal water tank, also part of the hull. At 60% level around 5,000 lb of water, so it would heat up around 2 degrees a day. The hull is fiberglass so heat transfer to the sea would be slow but would eventually equalize at some intermediate temperature. I’ll have to dig out my old heat transfer books when we get back to our home port.

Has anyone seen or heard of such a system?

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I see your home port is New Orleans. Where will you be cruising? Imo, the success of your plan could have a great deal to do with sea water temperature. As an example, I installed geothermal heating and air conditioning in my house in MD. It works extremely well with a substantial electrical cost savings. There are plastic radiators (pipes) that go down into the ground hundreds of feet. Water is circulated through the system. Because the ground is in the upper 50s, there is plenty of differential to pull out heat or cold and still be efficient. Wanted to do the same setup in Fort Myers, FL. Unfortunately the ground temperature is too warm (mid 70s) to dissipate the air conditioning heat with a radiator system. I fear you could have that same problem if cruising in too warm water. Understand that the efficiency of the system is drastically impacted by the temperature of the medium you are dissipating the heat into. Simply, it may work, but the time required to cool the plates down could grow substantially.

Ted

Thanks for the feedback. We cruise the Bahamas, Florida and the Gulf Coast so the water is warm and heat transfer is limited but any cooling system has to reject heat to the environment at hand. Not a lot is required for the refrigeration system and we don’t use AC while cruising. As a retired chemical engineer, I am or used to be, well trained in heat transfer design and operation so I’ll check it out once we return home.

We have large battery banks and plan to add PV panels to reduce generator load so that is not a concern. Using a large water tank is worth a look to eliminate need for through hulls, strainer, hoses and raw water pump. Not sure I’ll try it as installation would be a challenge in our case, but for a new system it could be attractive.

I never heard of such a thing but it sounds neat.

pete

I've been on several boats where the refrigeration compressor and condenser were simply in the engine room. Condenser was air cooled with a little fan. Just rejected the heat into the engine room. There was enough natural ventilation in the engine room while dockside that the little bit of heat added was not really noticeable. With engines running, lots of air moving so no problem there. I guess at anchor after main engine hot shutdown that condenser will run a bit hot, but the owners had no complaints.

Just another option to consider.

I was going to suggest a sea water system like used for AC units, but the heat rejection rate for this would be very low and I am unaware of a really small low power pump designed for sea water. Using an AC sized SW pump would be overkill and eat a lot of unnecessary power. Small fans readily available in any size.

I'm not thrilled with rejecting heat into the FW tank. Cruising in the Caribbean our cold water was not really cold at all due to the tank being in the engine room. Water too warm to get a nice shower. I don't think we ever used the water heater!!

And yes, warmer it is the more bug growth you would get. With that big tank it would really be a shame to have that much water get fouled. Our small tank if it gets funky it is no big deal to dump it and hand clean the interior. We put deck hatches in it for that reason, easy cleaning.

Can you just fit a domestic unit? Would simplify things a lot. If you can obtain a European A+++ unit and run it off a small inverter then it will likely outperform your unit as well