Education needed on AC RW pump...

So, next AC related question.. a raw water pump for three AC units. A 9K, a 12K and a 16K BTU. That is what I have and will replace with. The RW pump supplies all three at the same time whether they are running or not. My current RW pump, a "Little Giant" magnetic drive (model 5MD-SC for the record) pumps 16.4 gpm or 984 gph. However, there are 3 units. Would the GPH be divided by 3 thereby bringing the water flow to any one AC system down to 328 gph? Given that my current units run fine, water flow wise at least, with that I guess it is enough? Having a hard time finding water flow numbers on the units I am looking at.

March who makes magnetic drive pumps for this service has three models from 5 gpm to 17 gpm. The smallest is suitable for one A/C unit and the largest is suitable for three. See https://www.marchpump.com/applications/mag-drive-pumps-marine-air-conditioning/

You are generally correct about splitting the flow by three, but different tubing run lengths make a lot of difference. The shortest run will get the most flow. You can put balancing valves (or maybe pinch the tubing with a hose clamp and a short length of dowell) in the shortest runs to make them all more or less the same.

Stick a bucket over the side in each outflow and see how long it takes to fill. Then adjust the flow to make them more or less equal if there is a significant difference. I wouldn't worry about a 30% difference.

David

Thanks, David. Generally speaking, does a 6,000BTU unit use the same amount of water, need the same water flow as a 16,000BTU A/C or does water flow requirement increase with size?

firstbase said:

Thanks, David. Generally speaking, does a 6,000BTU unit use the same amount of water, need the same water flow as a 16,000BTU A/C or does water flow requirement increase with size?

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Well, a 6,000 btu unit will probably USE the same amount of water as a 16,000 btu unit, but the 16,000 btu unit NEEDS proportionally more.

So the water flow requirement is based on the A/C unit size for the most part. As the water flow gets reduced the compressor discharge pressure goes up and the cooling capability goes down. Most modern marine A/Cs have a pressure switch that trips out and shows an alarm when it gets too high.

Again strictly speaking you should balance the flow among your three units so each gets the same flow per 1,000 btu. But nobody does that AFAIK. I suspect that there is a 3:1 safety factor in the 5 gph per unit flow requirement, even more if it is a small unit.

One thing you can do easily is to measure the discharge temperature of each system. If one is high it could be low flow, so check that with the bucket. But the low flow could also be caused by fouling inside the tubing which restricts the flow.

So come to think of it, before you start messing with flow balancing, run a batch of 50/50 Barnacle Buster or Rydlime through each system and let it sit for a couple of hours, then make sure to flush by turning the system on. That will remove any fouling and make sure that any flow imbalance is due to tubing diameter and length of run and not internal fouling.

But you seem to be obsessing a bunch about an A/C system that works ok now, right?

David

Of course it never hurts to carry a spare R.W. pump. They can be used in emergency as a bilge pump. Bt if the present system is working I would leave it alone with the exception of maybe the barnacle buster. I would probably buy the spare and keep it in the locker unless you are looking for a project. My guess is that there are probably more pressing issues to be addressed.

pete

djmarchand said:

But you seem to be obsessing a bunch about an A/C system that works ok now, right?

David

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Nope. My most pressing issue is making sure I have good AC next summer!
South Florida. Looking at replacing all three old (1987) AC units and just trying to 1) make sure I have an adequate pump/water flow for the new AC's and 2) learn a little about the system. Current system water flow is good, used Barnacle Buster on them all about a year ago.

Pete Meisinger said:

Of course it never hurts to carry a spare R.W. pump. They can be used in emergency as a bilge pump. Bt if the present system is working I would leave it alone with the exception of maybe the barnacle buster. I would probably buy the spare and keep it in the locker unless you are looking for a project. My guess is that there are probably more pressing issues to be addressed.

pete

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Again, I'm replacing the current AC units, all suffering different maladies, and thinking that I should at least look at replacing the pump as well. However, it seems to be working just fine. Not new by any means but doesn't seem as old as the ACs. As long as it will supply the water needed by the new units I will probably let it be. As suggested, buying a spare and keeping it for a variety of reasons is a good idea. I have spare bilge pump but...not the same thing of course.

Any pump suffers from the pipe length and number of bends in the system.


I would use a pump for each AC unit. .

Several points:
Required water flow is directly related to AC BTU output. Water flows in the opposite direction from the gas it's cooling in the heat exchanger. The longer the length of the heat exchanger, the better the exchange, to a point. Haven't seen an AC manual that didn't specify the required flow rate (GPM) for rated output.

Pump flow outputs and flow rates are all based on optimal conditions. As stuff accumulates in the pre pump strainer, pump, distribution lines, AC heat exchangers, and discharge lines, flow will be reduced. Always over size the pump and associated plumbing to overcome some fouling. Understand that a reduction in flow doesn't mean a failure of the system necessarily, but a reduction of output until the system no longer has adequate output. Within reason, there is no such thing as too much flow.

As previously mentioned, frictional line loss reduces flow on longer water lines. This can be mostly eliminated by larger line sizes from the pump and reducing them at the AC unit.

Head pressure dramatically effects centerfusial pump's flow. Simply the height above sea level that the water has to be lifted to the AC unit is head pressure. As an example, the very popular March AC-5-MD pump has a flow rate of 16 GPM which could be fine for 3 AC units. However, if the units are 6' above sea level the flow drops to 15 GPM and at 11' it's 13 GPM. While these numbers might be marginally ok, they don't take into account frictional line loss or fouling of the lines.

It has been my experience that selecting a pump with up to 33% more flow than required (including head pressure) yields great results until the system is significantly fouled.

Ted

I have a single March AC - 5C - MD (17 gpm) that cools all three of my AquaAir 16k btu units. We've used the A/C all through S. Florida, the Bahamas, and all the way south to Grenada with no apparent overheating issues.

O C Diver said:

Several points:
Haven't seen an AC manual that didn't specify the required flow rate (GPM) for rated output. Ted

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Hard to believe and I wish you would prove me wrong but the units I have been investigating do not show water flow in their documents. Dometic, MarinAire, Flagship. Nothing online, I have downloaded their user manuals...nothing. Everything else but that.. Hope you prove me wrong but I can't find them and have searched. I find it very odd as well and keep thinking I am just missing.

All three units are no more than 3' above the waterline, the pump is center line about 2' below the waterline. going to confirm today but I believe the 16,000BTU and 9,000BTU are within a 10' hose run from the pump. The 6,000BTU unit, in the v-berth, is about 15'. I may be underestimating these a little but not much. I have no idea of where the length turns into an issue for me. Maybe those lengths are well within reasonableness, not sure. Will also confirm today but I don't believe there are any 90 degree turns in the lines. There are two 90 degree turns to get the water into the manifold right after the pump. After that, all are pretty simple runs.

I can understand the "each with it's own pump" thinking but these are not crucial to the boat and the thought of having 3 through-hulls and 3 pumps instead of 1/1 seems to present more potential issues than is necessary.

Anyway...it sounds like the pump I have is adequate for the new units and if I wanted to replace it, or have a spare, the March pump mentioned would be adequate at 15-17 gpm.

I know it may seem like I am "obsessing" but I am well aware that I don't know what I don't know. Just trying to learn.

I had one pump serve 5 AC units just fine on my old Hatteras. You have to make sure the output hose is a nice steady uphill rise (no dips) to the manifold and thence to each unit. Over sizing the pump too much will cause cavitation. I had friends who thought over sizing would be the ticket and learned this first hand.

My recommendation would be to call Depco Pump over in Clearwater and discuss this all with them. They are very knowledgeable and helpful. If you can afford to be without for a little while, they can likely rebuild your present pump and make it like new, as well as supply PM parts for it.

These are low pressure systems, you just need enough flow to fill them. Again, Depco or a professional marine AC tech can guide you on that.

I can't tell the difference (by hand) in the output temp and surface water temp on my AC. Maybe a more scientific test would show a few degrees but it can't be much. A lot of water at output and a lot of cold air from the unit. The same is true for when I run it as a heater.

I can see where a single pump would satisfy several units.

FWIW

pete

Pete Meisinger said:

I can't tell the difference (by hand) in the output temp and surface water temp on my AC. Maybe a more scientific test would show a few degrees but it can't be much. A lot of water at output and a lot of cold air from the unit. The same is true for when I run it as a heater.

I can see where a single pump would satisfy several units.

FWIW

pete

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You're measuring it the wrong way. You want to use an infared temp gun and measure the freon line from the compressor where it connects to the AC heat exchanger and the freon line where it leaves the heat exchanger. Think about it like this, if the heat exchanger is fouled with marine growth, the flow won't change much, the discharge temperature won't change hardly at all, but the freon discharge temperature and pressure will go up until the unit shuts down. If AC units had 2 or 3 times the length of heat exchanger, they would need a lot less water flow and you would see more of a raw water temperature differential. The shorter heat exchanger with a higher water flow allows the AC unit to work over a much broader raw water temperature range without freezing up in heat mode or over temping in air conditioning mode with high raw water temperatures.

Ted

firstbase said:

Hard to believe and I wish you would prove me wrong but the units I have been investigating do not show water flow in their documents. Dometic, MarinAire, Flagship. Nothing online, I have downloaded their user manuals...nothing. Everything else but that.. Hope you prove me wrong but I can't find them and have searched. I find it very odd as well and keep thinking I am just missing.

All three units are no more than 3' above the waterline, the pump is center line about 2' below the waterline. going to confirm today but I believe the 16,000BTU and 9,000BTU are within a 10' hose run from the pump. The 6,000BTU unit, in the v-berth, is about 15'. I may be underestimating these a little but not much. I have no idea of where the length turns into an issue for me. Maybe those lengths are well within reasonableness, not sure. Will also confirm today but I don't believe there are any 90 degree turns in the lines. There are two 90 degree turns to get the water into the manifold right after the pump. After that, all are pretty simple runs.

I can understand the "each with it's own pump" thinking but these are not crucial to the boat and the thought of having 3 through-hulls and 3 pumps instead of 1/1 seems to present more potential issues than is necessary.

Anyway...it sounds like the pump I have is adequate for the new units and if I wanted to replace it, or have a spare, the March pump mentioned would be adequate at 15-17 gpm.

I know it may seem like I am "obsessing" but I am well aware that I don't know what I don't know. Just trying to learn.

Click to expand...

Ok, you got me. My King Air, my Mermaid and my Webasto units all had flow requirements. I'm betting that a call to the manufacturer's technical support will yeild a GPM requirement.

For a comparison, the link below is the Webasto installation manual. Page 29 lists all their units by capacity with minimum and recommended flow rates. I'm not suggesting this replaces a call to the manufacturer, but should give you an idea on the dramatic flow requirement differences based on unit BTU output.

https://www.defender.com/pdf/WebastoFCF_Install_Manual.pdf

Ted

Have mentioned this before and will do so again:
If you want to see the impact on BTUs of cooling that water flow has, buy a digital readout air conditioning service thermometer ($20 +/-). Put the probe in a discharge vent with the AC running full blast. Once the output temperature stabilizes, vary the water flow by restricting the output of another unit sharing the same pump or the water flow feeding the pump with the thermometer in it. Simply when increased water flow no longer lowers the vent output temperature, you have reached maximum output of the unit (this assumes a clean heat exchanger). I can't think of a reason to want less than 100% of the units capacity.

Ted

We have the afore mentioned March pump and it handles 2-16 and 1-12k btu units here in south Florida. Very simple to repair just need to remember to oil them a couple times a year.

If you look at the Depco catalog they show the BTU capacity each pump is rated for.

Depco is the best pump supplier and rebuilder I’ve ever dealt with. Very knowledgeable people there.