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Old 07-16-2019, 02:06 PM   #1
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Marine A/C - please educate me

Can someone explain to me how a "marine" air conditioner works as compared to a typical RV rooftop type or window unit A/C, please?
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Old 07-16-2019, 02:13 PM   #2
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Instead of an air cooled condensor, a marine AC uses seawater through the condesor to remove heat.

It requires a thru-hull and pump to circulate the seawater.

That is a simple explanation.
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Old 07-16-2019, 02:45 PM   #3
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To restate syjos' correct answer above:


RV rooftop A/Cs use air blown over a finned coil to condense the freon inside. Marine A/Cs use raw water pulled from outside the boat and pumped through a heat exchanger to condense the freon.


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Old 07-16-2019, 03:15 PM   #4
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Got it, guys. So, here in the southeast US, where the water temps are often 80+F this time of the year, a marine A/C would be practically useless, huh?
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Old 07-16-2019, 03:18 PM   #5
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Do those heat ex changers need to be serviced for internal buildup




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Old 07-16-2019, 03:23 PM   #6
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The sea water side of the condensor/heat exchanger will need periodic descaling.
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Old 07-16-2019, 03:39 PM   #7
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Got it, guys. So, here in the southeast US, where the water temps are often 80+F this time of the year, a marine A/C would be practically useless, huh?
Water is a multiple times better conductor of temperature than air. Stand in 60 degree air then immerse yourself in 60 degree water and see how long you last.

Was out cruising on the Chesapeake in a shallow bay this weekend. Water reached 85 and the air conditioning was still going strong. With good water flow (GPM), they should still cool at 100 degrees or more.

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Old 07-16-2019, 03:53 PM   #8
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The warmer the seawater, the worse the cooling. If the water is 90, then its tougher to cool thats for sure. same with heating, water close to freezing, heating is not going to be very good.

Mine still cools but not like I wish with warm slip water. I have thought if you could have a snorkel to pick up water deeper down, it will help. Lots of slips too shallow for that to help much I think.
I think my water pickup is about a foot under the water. I wonder if I picked up water in the deepest part of the bilge in the boat midsection it is about 2 feet deep, so an extra foot and a lot of extra hose and trouble to find out.

Look at a home AC unit, it has a very large condenser, maybe if the marine AC had a bigger one, I just dont know though.

I do know my compressor can get hot, and the cooling condenser refrigerant - water coil is close by so it is picking up some heat from that also.
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Old 07-16-2019, 04:09 PM   #9
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With refrigerants its a temperature pressure relationship.
At 60 PSI on the low side, you can get a nice cold evaporator, but if the heat exchange cant cool well, that means the low side PSI is going to be higher, and that means a warmer evaporator, its just the laws of refrigerant gas temp - pressure relationships playing out. The pressure of the refrigerant gas in a flowing system determines the temperatures, and vice versa.
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Old 07-16-2019, 04:16 PM   #10
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Got it, guys. So, here in the southeast US, where the water temps are often 80+F this time of the year, a marine A/C would be practically useless, huh?
Absolutely incorrect. Our water temps here in the upper Chesapeake are in the mid-80s just now, and our units are cooling fine. Air temps have been in the mid-90s, heat index between 100-110F.

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Old 07-16-2019, 05:18 PM   #11
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I believe most marine AC units will only offset 20F from water temp. So unless you water gets to 100F you're still likely to get a reasonable offset of cooling. Even with 85F water this past weekend we were still able to bring the four units down to 72F.
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Old 07-16-2019, 06:03 PM   #12
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Got it, guys. So, here in the southeast US, where the water temps are often 80+F this time of the year, a marine A/C would be practically useless, huh?
In a word, yes.

They are basically a heat pump.
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Old 07-16-2019, 06:22 PM   #13
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Yes...no... Dang, which is it?
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Old 07-16-2019, 06:38 PM   #14
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I think OldDan probably misspoke in his post above. I have run marine A/Cs in Galveston Bay with 85 deg water temps and it would result in 60 deg air out of the registers, and 75 degrees inside.


If we came back to the marina from sailing (those were in my sailing days), with the summer sun beating down at 3:00 and a 16,000 btu A/C in a 35' sailboat, it would slowly bring the temp down from 95 outside to maybe 80 over an hour. Then when the sun got lower it would finally drop to 75 or less inside. But 80 was actually comfortable.


Final interior temp obtainable is more a function of outside air temp, sun loads, and A/C tonnage than raw water temp.



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Old 07-16-2019, 06:41 PM   #15
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Lightbulb How marine A/C works

This may help explain marine A/C systems.

Air conditioners don’t really produce cold, they remove heat. As the phase-converting refrigerant passes through the evaporator coils, it gets colder but more gaseous (because it’s boiling), and absorbs heat from the surrounding air. A fan blows warm air from the cabin across the evaporator and discharges it, now cooler, through ducting back into the cabin. The evaporator/fan/ducting assembly is called an “air handler.” The warm air that’s pulled in from the cabin is called “return” air, while the “supply” air is the cold air coming out of the air handler.
Pressure equals heat, so to cool the gas on the downstream side of the compressor it’s run through a raw-water-cooled condenser, which returns the refrigerant to a liquid state. The colder the water, the more efficient the cooling, but most air conditioners will work fine to water temps into the 80-degree range, sometimes warmer. Then the refrigerant returns to the expansion valve and the cycle begins again.
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Old 07-16-2019, 06:49 PM   #16
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In a word, yes.
Except that's wrong.

I've used them, as have likely thousands of others, in higher water temps. Now, they won't offset as well, but they'll certainly get the interior cabin to comfortable temps in warmer waters. Do you think all of Florida boats are somehow not using these very same reverse-cycle heat pump units?

Now, what you may have to do in much warmer waters is install a higher capacity unit. With cooler waters the units will be able to transfer heat more readily from a smaller unit.

And note I said 'transfer heat'... not specifically calling out cooling.

Because these units can just as well be used to heat the interior using colder water. Same sizing applies, the colder the water (down to about 40F) the more a smaller unit might struggle to extract enough heat to make it worthwhile.
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Old 07-16-2019, 07:05 PM   #17
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Yes...no... Dang, which is it?
It's easy enough to pull up a google search and wander through a few results, if you want to learn more. Dometic/Marine Air has a lot of good info online.

Basically, like an RV unit it uses refrigerant to transfer heat. Take heat from one place (inside) and transfer it (outside). Leaving the air that remains cooler inside. This confuses some folks. You're not strictly putting cold air into the room. You're taking the air in the room and pulling heat out of it, lowering the temp in the process.

An RV uses air to transfer the heat away. On a boat sea water is used. Pull up sea water, run it through an exchanger that has hot refrigerant running through it, refrigerant that got heated up by being pumped under pressure and run through a unit inside the boat.

Where temperature comes into play is how much heat can be transferred into warmer water while cooling the passing refrigerant enough to make it worthwhile. 70F seawater is going to cool off the refrigerant right-quick, but 90F water won't take as much heat away using the same sized unit. This translates into it taking longer to 'make it cooler' when the water is warmer.
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Old 07-16-2019, 07:32 PM   #18
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It has always bugged me that a car AC has so many more BTU's than a boat.
A Boat is bigger has just as much or more sunload since it bigger, and well its bigger and it has lots more potential air leaks than any car.
And a car is so much smaller space.
And yet the AC capability is inferior in all boats by comparison.
People demanded fast cool downs in cars and got them.

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Car air conditioner BTU
According to electricity expert Michael Bluejay, the average refrigerator uses 488 BTUs per hour in normal use. Your car Air Conditioning is about 5 Tons (60,000 BTU) at highway speeds.Jul 18, 2014
I wold like to be able to cool my boat down fast too.
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Old 07-16-2019, 07:46 PM   #19
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my a/c units here in the Florida keys work fine with water cooled condensers in 89* canal water. my refrigerator and freezer also have water cooled condensers and they work fine also. Just a question to the nay sayers Do you think air cooled would be better?
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Old 07-16-2019, 07:54 PM   #20
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my a/c units here in the Florida keys work fine with water cooled condensers in 89* canal water. my refrigerator and freezer also have water cooled condensers and they work fine also. Just a question to the nay sayers Do you think air cooled would be better?
Bud
They probably do work better due to the evaporative cooling effect of water in the air hitting the condenser and then evaporating. Its a proven fact, that is how sweat cools you down for example.
When water evaporates off a surface it takes the heat away with it.
The condenser is hot, so it recieves a greater cooling effect from humidity in the air.
A submerged one will have no evaporative cooling effect like a car does.
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