Anyone having issues with water being too hot for your A/C to run?

[MarinAirNoWorky]

Originally Posted by CaptTom

Interesting. That's the opposite of what I would have thought.

Flowing air certainly removes heat more quickly than still air. That's why we have "wind chill" tables. In first aid classes I've also heard the same thing about cold water immersion. Moving around makes it worse.

When I want to cool down a cooking pot or something outdoors which has been in the sun, I don't just spray water on it. I run a stream of flowing water over it. Ditto for treatment of a first degree burn.

My understanding of icing on rivers is that it's the mechanical motion keeping ice crystals from forming, not that heat is being removed more slowly.

If this intuitive understanding is wrong, I'd love to learn the physics behind faster flowing water being a less effective heat sink than slower.

I feel the same way. If the temps. of the "pipe" is super hot, say 200 degrees, you'd think a faster flowing amount of water going through that pipe at 90 degrees would be better to remove heat. Get it in and out of there quickly.. unless the heat "molecules" (whatever you'd call them) can't attach themselves to the cool water because it's zipping by too fast? That doesn't make sense in my head but w/e...will fix it this weekend. It also wont make as much noise, which will be nice.

[CaptTom]

Originally Posted by MarinAirNoWorky

...unless the heat "molecules" (whatever you'd call them) can't attach themselves to the cool water because it's zipping by too fast? That doesn't make sense in my head...

I'm leaning toward trusting that sense in your head.

Heat is basically just energy; molecular motion. If there's a mass of water in a pipe at a certain temperature, it's going to absorb or emit heat through the pipe at a certain rate, based on the material the pipe is made of and the temperature outside the pipe. Initially, whether or not that mass of water is moving is irrelevant.

Of course, if the water inside the pipe is absorbing heat, it will increase in temperature. At some point it will reach equilibrium with the temperature outside the pipe, and no more heat transfer can take place. So we move that water along and bring in more water at the original temperature.

Obviously there's a minimum flow rate for efficient operation. I can't think of any physical process which would introduce a practical maximum flow rate. Maybe at extreme speeds there could be cavitation, but it doesn't seem we're in that realm. Maybe I'm missing something.

[Chotu]

The only situation where too much flow would seem to cause a problem to me would be turbulence or cavitation separating water from the surface of the heat exchanger

[sailorboy1]

Originally Posted by MarinAirNoWorky

Called the company. They told me that the GPH is too fast for the AC.. 1200 gph pump came with the boat but they only want 350 gph on it.

Is there really a chance that the water is flowing too fast and it cant exchange the heat because of that?

Doubtful in my mind this is the problem. Does the unit get all iced up?

But just throttle the outlet valve (you have 1 right)

Have you cleaned the fins of the unit to be sure it is getting good air flow.

[MarinAirNoWorky]

Originally Posted by sailorboy1

Doubtful in my mind this is the problem. Does the unit get all iced up?

But just throttle the outlet valve (you have 1 right)

Have you cleaned the fins of the unit to be sure it is getting good air flow.

Have not cleaned the fins, but it does have an airfilter in place that was cleaned over the weekend.

It does not get iced up.

I can throttle the inlet valve.. good idea.

[GordMay]

Many equipment manufacturers publish specific flow, and supply pressure requirements, to achieve turbulent flow.
Most heat exchangers are designed to generate turbulent flow.
Turbulent flow occurs when the velocity, in a given water channel, is high.
Turbulent flow, due to the agitation factor, develops no insulating blanket [thick boundary layer], and heat is transferred very rapidly.
Laminar flow develops an insulating blanket, around the channel wall, and restricts heat transfer.
However, too much velocity can cause erosion; and an appropriate balance, between heat transfer gain, and pressure drop penalty, needs to be considered.
This is complicated engineering/physics, best left to professionals - like the manufacturer.

[MarinAirNoWorky]

Sounds good Gordy. Will try and get it done this weekend.

[AKA-None]

You are in your car (water flow)
Your friend is standing on the sidewalk (heat exchanger)
He has a cup of coffee (heat)

When is it easier to get the coffee? (Heat exchange)
When driving rapidly (high flow)
Or
When driving slowly (low flow)

If the flow is really slow then the cooling fluid will reach equilibrium with the heat source. This will take time and heat exchange will slow as the two reach the same temp. Like charging a battery sort of, don’t get carried away by the analogy.

So basically the change of flow rate vs heat exchange. What’s the slowest you can push the cooling fluid where it picks up the most heat but doesn’t linger so long that the coolant doesn’t carry the bulk of the heat away

Hopefully I’ve made myself clear, if not then carry on

[rslifkin]

Originally Posted by AKA-None

You are in your car (water flow)
Your friend is standing on the sidewalk (heat exchanger)
He has a cup of coffee (heat)

When is it easier to get the coffee? (Heat exchange)
When driving rapidly (high flow)
Or
When driving slowly (low flow)

If the flow is really slow then the cooling fluid will reach equilibrium with the heat source. This will take time and heat exchange will slow as the two reach the same temp. Like charging a battery sort of, don’t get carried away by the analogy.

So basically the change of flow rate vs heat exchange. What’s the slowest you can push the cooling fluid where it picks up the most heat but doesn’t linger so long that the coolant doesn’t carry the bulk of the heat away

Hopefully I’ve made myself clear, if not then carry on

That's not a good analogy at all for this. Slower flow means the output water will be warmer as they've gotten closer together in temperature. But faster flow and less temperature rise will lead to more total heat being removed from the condenser. Remember that we're not trying to get the output water warm, we just want to remove heat from the A/C condenser. Bigger temperature difference means faster heat transfer, so the colder the water still is when it reaches the far end of the condenser, the better the heat transfer.

That's also why they're typically plumbed as counterflow so the warmer output water is at the same end as the hottest refrigerant (to keep the average temperature differential across the length of the heat exchanger as high as possible).

[Chotu]

Originally Posted by GordMay

Many equipment manufacturers publish specific flow, and supply pressure requirements, to achieve turbulent flow.
Most heat exchangers are designed to generate turbulent flow.
Turbulent flow occurs when the velocity, in a given water channel, is high.
Turbulent flow, due to the agitation factor, develops no insulating blanket [thick boundary layer], and heat is transferred very rapidly.
Laminar flow develops an insulating blanket, around the channel wall, and restricts heat transfer.
However, too much velocity can cause erosion; and an appropriate balance, between heat transfer gain, and pressure drop penalty, needs to be considered.
This is complicated engineering/physics, best left to professionals - like the manufacturer.

Fascinating! and makes perfect sense!

A laminar flow would create a temperature gradient across the water, whereas turbulent flow would allow streams of new/colder water to break through and touch the heat exchange surface.

I was thinking more a situation where cavitation might occur from too much speed creating a situation where water vapor might then be in contact with parts of the heat exchange surface.

And Rslifkin’s post is 100% accurate

[sailorboy1]

It isn't the higher flow. Not matter what you think you aren't not getting much more extra flow through the system from the higher pump. My pump supplies 2 units and if I shutoff the flow to 1 the flow doesn't increase much through the remaining.

The only time I have had my AC trip on high pressure was because a loop got clogged up with shells etc. But you cleaned that, at least that is your belief and it may not really be clean. Is the water shooting out the outlet or just running out? You have a big oversize pump and it should shoot out with good force.

There is only 1 thing left and that is the water temp.

[gonesail]

i found a very small roof mount RV AC .. was so hoping it would work in my standard rectangular hatch but it was a couple of inches off. camper hatches have no correlation to boat hatches i guess. and i'm not ready to cut the deck to make it fit

[smj]

Originally Posted by gonesail

i found a very small roof mount RV AC .. was so hoping it would work in my standard rectangular hatch but it was a couple of inches off. camper hatches have no correlation to boat hatches i guess. and i'm not ready to cut the deck to make it fit

We had the same problem and ended up building a box over the existing hatch that was big enough to handle the AC cutout size.

[Chotu]

Originally Posted by gonesail

i found a very small roof mount RV AC .. was so hoping it would work in my standard rectangular hatch but it was a couple of inches off. camper hatches have no correlation to boat hatches i guess. and i'm not ready to cut the deck to make it fit

I did 2 of the most powerful RV air conditioners made and sliced right in. Lol.

Of course, boat wasn’t finished yet, so no big deal.

You could build an adapter flange if so motivated

[MarinAirNoWorky]

Originally Posted by sailorboy1

It isn't the higher flow. Not matter what you think you aren't not getting much more extra flow through the system from the higher pump. My pump supplies 2 units and if I shutoff the flow to 1 the flow doesn't increase much through the remaining.

The only time I have had my AC trip on high pressure was because a loop got clogged up with shells etc. But you cleaned that, at least that is your belief and it may not really be clean. Is the water shooting out the outlet or just running out? You have a big oversize pump and it should shoot out with good force.

There is only 1 thing left and that is the water temp.

It passed the "bucket test" first, before I spent any $ on cleaning or a person to come out.

The water temp is an issue, yes.. but my boat is the only one out of 10 that is having the issue. Funny enough, it's the only marin air unit AND it's the newest out of all of them. Part of me thinks it might be the tech side of things being too "safe" but who knows. The pump being oversized, I hope that's the issue, I just need it fixed.