Marine A/C - please educate me

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ForceMultiplier

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First Draft
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21'4" Tolman widebody skiff
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?
 
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.
 
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.


David
 
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?
 
Do those heat ex changers need to be serviced for internal buildup




viewing this now


 
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The sea water side of the condensor/heat exchanger will need periodic descaling.
 
ForceMultiplier said:
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?
Click to expand...
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.

Ted
 
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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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.
 
ForceMultiplier said:
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?
Click to expand...

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 100-110°F.

-Chris
 
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.
 
ForceMultiplier said:
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?
Click to expand...

In a word, yes.

They are basically a heat pump.
 
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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.



David
 
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.
 
OldDan1943 said:
In a word, yes.
Click to expand...

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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ForceMultiplier said:
Yes...no... Dang, which is it?
Click to expand...

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.
 
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.

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
Click to expand...

I wold like to be able to cool my boat down fast too.
 
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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
 
Bud said:
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
Click to expand...

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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Yes it's a heat pump, just like the unit outside of most American homes. Residential heat pumps don't stop working at 100 degrees, they're just less efficient. As I mentioned already, the marine AC will work with 100 degree raw water, but you need more GPM flowing through the heat exchanger and energy wise it won't be as efficient.

Ted
 
So you think air will cool a condenser faster than submerging it in water?
 
Bud said:
So you think air will cool a condenser faster than submerging it in water?
Click to expand...
Don't know if you are addressing me with that question. In almost all cases liquid heat exchange is better than air. My Maryland home has geothermal which is a liquid cooled heat exchanger. Not only is liquid a better conductor of heat and cold, but ground temperatures are better than air temperatures most of the time.

Ted
 
To those who say my thinking is not correct, that is a very real possibility and will accept that.
Now my defense is as follows:

I did not say you could make your boat a refrigerator any more that you can turn your house or RV into refrigerator but you can make it daymned uncomfortable.

A house A/C, as someone said, can cause a 20 degree differential from the outside temp.
We cannot expect more from marine reverse cycle plant.

Maybe I was too simple when I said the marine units are basically heat pumps.
While I did not say the home heat pump nor the marine reverse cycle units were efficient, they work.

A lot has to do with insulation. I doubt if a boat nor RV are insulated as well as a house.

Right now, I am just north of Miami, the water temp is 80+F. It got too cold in the saloon so I had to turn the unit off. That is because the sun has gone down and the heating of the outside surfaces has been reduced.

If the RV AC become more efficient with more air passing over the coils, so be the marine units. The more water passing through the marine reverse cycle plants, the more efficient they become.

I used the simple comparison by saying the marine plants are basically heat pumps, giving up heat to the water or taking heat away from the water as necessary instead of home heat pump's interaction with the air.

With the home heat pump, as I recall, becomes inefficient when the air temp approaches 40F. I guess that is why home heat pumps include 'emergency heat strips' and the American Tugs come standard with 2 electric heaters.

All this being said, when the marine plants are in auto, they have the ability change from A/C to heat without intervention and adjust the fan speed too. Whereas home heat pump may be able to adjust the speed of the compressor and maybe the fan speed, it cannot automatically change from heat to A/C.

Again, if I am wrong but my explanation seems to satisfy my feeble mind.
 
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OldDan1943 said:
A house A/C, as someone said, can cause a 20 degree differential from the outside temp.
Click to expand...
I think that is in correct. My understanding is that the 20 degree differential is the difference between room temperature (air returning to the air conditioner) and air temperature coming out of the air conditioning vents. My old heat pump could pull the house down in the low 70s on a 100 degree day.

Ted
 
A 20 degree differential can be rather dramatic. Today I was working outside. I dont know the outside temp but when I came inside the temp inside was chilling. LOL
 
Most of the cooling takes place when the refrigerant evaporates (superheating). Latent heat of vaporization. Or latent heat of evaporation. Takes lots of BTUs to evaporate your refrigerant.


The refrigerant first has to be a liquid in order to evaporate. As long as the water is cool enough to change the refrigerant to a liquid (subcooling), your system will cool your living spaces. It is likely that your refrigerant, while under pressure, can be a liquid at 120 degrees or more.


A pressure-temperature chart for your refrigerant will tell you the subcool and superheat points, ranges.


The cooling or heat that you feel is sensible heat and is greatly affected by humidity. Dehumidification is a large part of the work done by your HVAC system.
 
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OldDan1943 said:
To those who say my thinking is not correct, that is a very real possibility and will accept that.
Now my defense is as follows:

I did not say you could make your boat a refrigerator any more that you can turn your house or RV into refrigerator but you can make it daymned uncomfortable.

A house A/C, as someone said, can cause a 20 degree differential from the outside temp.
We cannot expect more from marine reverse cycle plant.

Maybe I was too simple when I said the marine units are basically heat pumps.
While I did not say the home heat pump nor the marine reverse cycle units were efficient, they work.

A lot has to do with insulation. I doubt if a boat nor RV are insulated as well as a house.

Right now, I am just north of Miami, the water temp is 80+F. It got too cold in the saloon so I had to turn the unit off. That is because the sun has gone down and the heating of the outside surfaces has been reduced.

If the RV AC become more efficient with more air passing over the coils, so be the marine units. The more water passing through the marine reverse cycle plants, the more efficient they become.

I used the simple comparison by saying the marine plants are basically heat pumps, giving up heat to the water or taking heat away from the water as necessary instead of home heat pump's interaction with the air.

With the home heat pump, as I recall, becomes inefficient when the air temp approaches 40F. I guess that is why home heat pumps include 'emergency heat strips' and the American Tugs come standard with 2 electric heaters.

All this being said, when the marine plants are in auto, they have the ability change from A/C to heat without intervention and adjust the fan speed too. Whereas home heat pump may be able to adjust the speed of the compressor and maybe the fan speed, it cannot automatically change from heat to A/C.

Again, if I am wrong but my explanation seems to satisfy my feeble mind.
Click to expand...

More text didn't help. Still wrong.

Home units can certainly change modes. It's the thermostat doing the work, calling for one mode or the other, not the heat pump. Likewise bringing e-heat into play when necessary. Now, whether an install is properly configured, well, home HVAC installs are often done wrong...
 
ForceMultiplier said:
Yes...no... Dang, which is it?
Click to expand...

To give you an idea - this morning the "feels like" temperature in our area was 99 degrees at 6:30 am. My A/C's (16K btu in the salon, 8K btu in the aft cabin) ran all day long, non-stop. During the hottest period of the day, around 3-4pm, this is what my temps on my boat were. Water temps are around 85-87 degrees right now.
 

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