Marine Air conditioning

[mauruuru]

I have a six year old Mermaid 5200 BTU Reverse Cycle AC in the boat. It was working fine before I took it out of the water pre-Matthew. After relaunching one of the last tasks was to turn on the AC. It didn't cool though the pump came on, the compressor was running and the fan came on. Further investigation indicated exit water was cool not warm as usual indicating there was no heat exchange going on. So I started doing a closer visual inspection and I found that over 40% of the Al fins in the heat exchanger were gone--to dust. See the attached pictures.
\When I called Mermaid, they suggested it was due to electrolysis. The zincs on the boat were fine when I hauled it though I did replace them with new before relaunch. I also have a galvanic isolator on the incoming shore power. So my questions-as I prepare to ship this to Mermaid for repair/replacement:
Has anyone ever heard of electrolysis affecting an AC heat exchanger?
If its really a problem why would zincs be ok, why would isolator not protect?
What else could this likely be-since I (a licensed professional electrical engineer) am not buying galvanic action here?

 

[MitchM]

good diagnosis on your part ! did all this electrolysis take place between haul out for matthew, and re-launch ? or is it possible it's been going on for a while and just recently noticed? if it's rapid, i'd suspect someone in the marina near you now has a boat with a ruined AC ground system. ask mermaid if there is any particular grounding or bonding they recommend for the AC hot/ground feeding your AC system at your main AC panel to prevent this in the future. and let us know...

 

[mauruuru]

I can't honestly say how long it may have been going on since I haven't really done a close visual inspection of the heat exchanger before. I do know that the unit was cooling before I took it out of the water before Matthew and I know it has new zincs when I put it back in the water.

 

[thinwater]

(Chemical engineer with many years in the glycol/antifreeze/cooling system side)

I would check that there is only one ground (a 12v ground on the water pump and the 120v ground on the air conditioner must be the same) and that they are tight. But I'm also not getting the pattern of the attack, so I don't thing that is it. The thing is, the unit should only be wetted with distilled water, which isn't much of an electrolyte. Without an electrolyte, there is no galvanic corrosion, no way.

Is it possible there is a small seawater leak dripping or spraying on that side of the HX, and then running across the bottom? That's my guess. It could be VERY small, and even intermittent (only when cold). The salt dries and then tears it up.

 

[seadaddler]

Sea water spraying on it from leak in hose.
Nick

 

[kloudie1]

I had a corrosion failure on the shell of the compressor .. It took 16 years, and the only water it saw was condensate from the cooling coils.. The lower end of the compressor was immersed almost continuously for those years because the unit runs as a dehumidifier year-round . The lower part of my fins on the coil were immersed and they'd pretty much disappeared where they were in standing condensate water. Picture shows the bottom of the compressor and the pinhole that corroded through. Compressor still would run, but of course all the refrigerant had disappeared so it would not cool. After changing the compressor, I shimmed the base so that the water would not stand there while the boat is at rest.

 

[JamesG161]

Without an electrolyte, there is no galvanic corrosion, no way.

Without WATER there is no corrosion, at least the type on those pictures.

Not caused by electrical power. Note the pattern of Aluminum fin corrosion.

For you new AC...
1) Stop the water leak
2) Inspect the whole AC every time you change engine oil.
3) Do what kloudie1 said, make sure you drain pan fully drains.
4) Use an air filter and clean/change it.
5) When you clean the fins, use a AC coil cleaning spray.

Jim...

PS: I found "salt crystal" in my OEM 14 year old AC. The PO had remove the insulating panel at the back of AC enclosure, thus allowing Florida salt mist air into the return air. How stupid! You must isolate the return air as best as possible.
Also my new AC has a composite drain pan and is shimmed to drain.

 

[Ken Cross]

Without WATER there is no corrosion, at least the type on those pictures.

Not caused by electrical power. Note the pattern of Aluminum fin corrosion.

For you new AC...
1) Stop the water leak
2) Inspect the whole AC every time you change engine oil.
3) Do what kloudie1 said, make sure you drain pan fully drains.
4) Use an air filter and clean/change it.
5) When you clean the fins, use a AC coil cleaning spray.

Jim...

PS: I found "salt crystal" in my OEM 14 year old AC. The PO had remove the insulating panel at the back of AC enclosure, thus allowing Florida salt mist air into the return air. How stupid! You must isolate the return air as best as possible.
Also my new AC has a composite drain pan and is shimmed to drain.

James G161 is right.

Ken

 

[MitchM]

checking for a good ground is a good idea.. but : "a 12v ground on the water pump..? "-- marine 120 V AC AC systems have a 120v AC water supply pump like a March $$$ or a Cal $. i have never seen one with a 12V .

 

[Bob J.]

Looking at the picture, I'd agree with Thinwater.
There has been a very small leak on the heatexchanger that over time rotted the aluminum fins away.

 

[mauruuru]

Pump and air handler/compressor are 120V AC no DC connections.
It must be salt induced corrosion.
I am constructing a box to mount replacement unit in and will ensure condensate drains out easily into a seperate container from which the condensator will pump

 

[Gunni]

sure looks like it took a long bath in sea water. Given the state of corrosion on the compressor and other parts I would not bother repairing, but instead replace the entire unit. With careful attention to raw water leaks and other saltwater sources. My compressor failed this year in the same manner as Claude's, there have been some product improvements like redesigned composite drain pans to keep the compressor dry, and marinized evaporators.

 

[mauruuru]

Well unit went back to Mermaid. I got a call from the repair tech who said he's never seen one so bad. Recommended against repair. They have no clue but when we negotiated for a new unit they insisted I replace control box too- just in case something strange is going on with it?!? At the beach this week but when I getback I will be doing some troubleshooting of the control.
And as I said in earlier post I will be completely enclosing the new unit unlike failed unit

 

[mauruuru]

Well, I have a brand new unit and am looking for advice from any of you familiar with AC systems. By the way the Mermaid people were very helpful and responsive though still had to replace the whole thing.
What I am thinking is spraying the heat exchanger coils with a clear coat--thinly! I know from my thermodynamics classes in engineering school that a black body is the most efficient radiator and so I am wondering why the coils are painted black outside of the heat exchanger body but left unpainted in the heat exchanger itself along with the fins. Anybody have a fact based opinion as to why I should just leave the metal unprotected?

 

[Gunni]

I have seen a lot of marine A/C units and what happened to yours is not normal. In fact those coils should be sweating distilled water - no cations to promote corrosion. It continuously washes itself. Somehow that unit came into contact with corrosive seawater. Most likely from your raw-water circuit.

 

[whatfiero]

Agree the salt water must have been sprayed on unit when running leeking hose maybe fixed and unit not cleaned afterwards

 

[JamesG161]

I am thinking is spraying the heat exchanger coils with a clear coat--thinly!

What the "fins" do is provide additions surface area to "conduct" heat , not much of heat removal due to "radiation".

If you coat the fins, remember the spacing between the fins must be open to allow air flow between them.

I am not sure why you would "coat" the fins. It was direct spray of most likely water. Stop the water, no need for a coating.
Jim..

 

[mauruuru]

In doing research I have discovered that salt air can be responsible for corrosion of the fins and coils.
There are several companies that make a coating- but no sales to retail customers. Also several large AC/heat pump manufacturers now sell residential and commercial units that are coated. One expert suugest a thin coat of poly clear coat so that is what I have done. We will see how it works. Additionally Mermaid suggests enclosing the unit and I fabricated an enclosure from duct insulation and a three inch high pan of starboard which will enclose the unit. The condensate will drain via hose outside the pan so no accumulation of moisture inside the enclosed unit!

 

[mauruuru]

Will post pictures once installed- later this week hopefully

 

[JamesG161]

In doing research I have discovered that salt air can be responsible for corrosion of the fins and coils.

for sure, but how are you getting salt air into your AC duct work?
If you stop the outside air leaks, no salt air on the coils.
Jim...

PS: I use a insulating board available at a local AC installer... "Duct Board"