AC Heat Exchanger

[Iwannarelax]

Hello all,
Is there a such thing as an inline heat exchanger I could use before the seawater gets to my AC unit? The seawater temp is high and the ACs just aren't getting cooled down enough. If so, how would it work? I've read about using another pump to push another cooling liquid through the heat exchange to then cool the seawater and make that portion a closed loop, but not sure how that works.
At first I was looking at water chillers, then mini-split units, and now heat exchangers.

Thanks in advance.

[sailorboy1]

And how would you cool that HX?

Have you cleaned your AC evaporator and descaled the SW side?

[Bowdrie]

The heat has to go somewhere, what do you do with it?
Where will you send it?
At each stage of heat transfer efficiency is lost and power requirements go up.
There are a couple of schemes to pre-cool the outside water before it reaches the condenser, but they all lead back to the first two sentences I wrote.
Probably the least expensive and less intrusive method is to have oversize condensers installed.
For example: have the condenser on an 8K BTU unit replaced with one from a 12K BTU unit.
Increased water flow, (bigger water pump,) can have some effect, and would probably the easiest/fastest/cheapest way to make an "initial stab at the problem" so to speak

[SailingHarmonie]

You can violate the rules of thermodynamics. You can not make heat "disappear" you can only move it from one place to another, and adding to it at the same time.

Most marine AC units I am familiar with are rated to work with seawater temperatures of up to 95F. Is it hotter than that where you are?

If not, following the advice above and making sure your system is clean and has the proper water flow will return more benefits than hoping for magic.

[Outlaw7]

I caution you on using a larger pump. To fast of a water flow does not allow the heat to transfer efficiently your unit's manufacturer should have in it's manual the recommended water flow.

[SailingHarmonie]

Originally Posted by Outlaw7

I caution you on using a larger pump. To fast of a water flow does not allow the heat to transfer efficiently your unit's manufacturer should have in it's manual the recommended water flow.

This is just wrong. The faster the water flow the better the heat removal--always. No AC maker specifics a MAXIMUM water flow, just a MINIMUM. There is, of course, a practical maximum to the water flow rate, but not because the heat exchange "efficiency" goes down.

Source: Chemical engineer who designed heat exchangers.

[Statistical]

Originally Posted by Outlaw7

I caution you on using a larger pump. To fast of a water flow does not allow the heat to transfer efficiently your unit's manufacturer should have in it's manual the recommended water flow.

That is not correct. The heat capacity of water is per unit of volume/mass so a certain amount of watts per gallon. More gallons per hour means more heat rejection capacity. That is universally true.

Now supersizing the pump may not do much good. Water has friction and that increases for a given pipe size with velocity. if you simply double the output of the pump without increasing the size of the lines you will not move twice the water in fact you may not move much more water at all. On the other hand it is possible the pump is marginal and thus the AC struggles with higher water temps in which case slightly increasing capacity may improve things.

Short version is how much heat can be rejected is based on the fluid being used, the temperature of the source (condenser), the temperature of the sink (raw water ambient), the size of the heat exchanger, and the flow rate.

[GordMay]

Originally Posted by SailingHarmonie

This is just wrong. The faster the water flow the better the heat removal--always. No AC maker specifics a MAXIMUM water flow, just a MINIMUM...

Some, including I [slightly informed, non-expert], would disagree:
ie:
Maximum Velocities in Heat Exchangers
➥ https://www.deppmann.com/blog/monday...angers-part-1/

Minimum Velocity in Heat Exchangers
➥ https://www.deppmann.com/blog/monday...angers-part-2/

Basically, you're trying to get the most contact [turbulent vs laminar flow], for the best amount of time [residence time], to transfer the most energy, between the mediums.

[JoeRobertJr]

Originally Posted by GordMay

Some, including I [slightly informed, non-expert], would disagree:
ie:
Maximum Velocities in Heat Exchangers
➥ https://www.deppmann.com/blog/monday...angers-part-1/

Interesting. My guess was that the issue was turbulence at higher flow rates interfering with the heat exchange. Gord May's first link, however, sites concerns about wear on the unit.

What’s the concern if you exceed the maximum recommended velocity? When the manufacturer’s maximum is exceeded, you may experience a significantly shortened tube bundle life. The failure would be caused by erosion of the tube material, damage to the mechanical joint where the tubes are attached to the tube sheet, or vibration damage due to excessive velocity at the tube bend.

[GordMay]

"Slight" turbulence enhances heat exchange [vs laminar flow] - hence, the occasional use of 'turbulators'.