RW pump for A/C units

[caltexflanc]

Power consumption is proportional to flow in those kinds of pumps.

?????

'splain please!

 

[STB]

Water will flow through plumbing in inverse proportion the the resistance of the path. So, depending upon the length and ID of the hoses, the configuration of the HVAC unit's heat exchanger, and the number of bends along the way, the volume flow rate may vary. So, if we've got three take offs it is possible that "too much" water flow will go to some units and too little to others.

Having a manifold enables one to tune this resistance such that, even though less water may flow in all, more water will flow to places that would otherwise be starved w.r.t. their needs.

Tuning this would probably require setting all units to run, generating the maximum workload and cooling demand, watching discharge temps at the units where it enters the discharge hose or outside at the thru-hull, letting it level out or get to the point where one or more get too hot, and then adjusting flow on those cooling well, with the goal of equalizing discharge temperature or, at least, keeping all well under limit.

A manifold is also good for service. What to do without one, if one springs a leak? Shut down the rest and close the seacock until it,can be capped off.

Simplicity and redundancy is why I prefer multiple thru-hulls and multiple pumps.

 

[RLKramer]

Install 2 of the same pump in parallel with isolation ball valves

Once you decide on the one single pump you need, I recommend that you install two pumps each one in parallel separated by ball valves and wired in to two separate breakers.

If one pump / motor should fail it's just a matter of switching the ball valves, isolating out the bad one and flipping the breakers so that the bad one is off and the backup one is on.

If you don't want to do extra breakers at least have the second pump wired and just change the wiring from the bad pump to the new pump.

When you go through the effort to plumbing the new pump It's not a lot more effort to plumb in a second one in parallel.

I can tell you from experience you'll be happy You did it when in the middle of the night it gets warm in your cabin and you realize the pump has failed.

 

[caltexflanc]

Once you decide on the one single pump you need, I recommend that you install two pumps each one in parallel separated by ball valves and wired in to two separate breakers.

If one pump / motor should fail it's just a matter of switching the ball valves, isolating out the bad one and flipping the breakers so that the bad one is off and the backup one is on.

If you don't want to do extra breakers at least have the second pump wired and just change the wiring from the bad pump to the new pump.

When you go through the effort to plumbing the new pump It's not a lot more effort to plumb in a second one in parallel.

I can tell you from experience you'll be happy You did it when in the middle of the night it gets warm in your cabin and you realize the pump has failed.

That's not a bad idea. In my case, accessing the pump was extremely easy; right under the floor in a walk-in, standup engine room. I proactively replaced the 25 year old Oberdorfer with a new one in about 5 minutes, if that. I am not the most adept, either. Then I sent it the old one to Depco for refurbishment, it came back like new and I kept it as a spare.
Personally I really liked the one easy-to-service pump and also easy to deal with manifold system for the 5 units we had on board.

 

[Mischief Managed]

?????

'splain please!

Sorry so slow to respond, just saw this.



Centrifugal pumps use more energy with the more flow they generate. If you throttle the flow after the pump outlet, the pump will draw less electricity than it will if it is running wide open. I know it may seem like throttling the flow would make the pump work harder, but it does exactly the opposite. They spin at a fixed RPM so if they move less water, they use less power. The flow to energy use curve is not linear though, if you throttle it down to zero flow, it will still draw some current.



The inverse is true with a positive displacement pump. If you throttle one of those down, the RPM will drop, the pressure will go up and the power consumption will go up. If you throttle it to zero flow, it will stall the motor and draw maximum current.


I hope that makes sense. If you have a jacuzzi you can sort of test this. Turn on the pump and then shut off all the nozzles. The motor will still turn at a fixed RPM with no strain.

 

[caltexflanc]

Sorry so slow to respond, just saw this.



Centrifugal pumps use more energy with the more flow they generate. If you throttle the flow after the pump outlet, the pump will draw less electricity than it will if it is running wide open. I know it may seem like throttling the flow would make the pump work harder, but it does exactly the opposite. They spin at a fixed RPM so if they move less water, they use less power. The flow to energy use curve is not linear though, if you throttle it down to zero flow, it will still draw some current.



The inverse is true with a positive displacement pump. If you throttle one of those down, the RPM will drop, the pressure will go up and the power consumption will go up. If you throttle it to zero flow, it will stall the motor and draw maximum current.


I hope that makes sense. If you have a jacuzzi you can sort of test this. Turn on the pump and then shut off all the nozzles. The motor will still turn at a fixed RPM with no strain.

Thanks! Interesting....

but that aside, too high a capacity pump will begin to cause cavitation and other issues. That I have seen first hand in two different cases when boaters got much bigger pumps thinking otherwise (and because in each case, they found a "deal" on the way- bigger pump).

 

[Mischief Managed]

Thanks! Interesting....

but that aside, too high a capacity pump will begin to cause cavitation and other issues. That I have seen first hand in two different cases when boaters got much bigger pumps thinking otherwise (and because in each case, they found a "deal" on the way- bigger pump).

If the pump is too big, just throttle it with a valve on the output side. It won't be as efficient as a correctly-sized pump, but if you throttle it properly, it won't cavitate.

 

[Martin J]

I am a great fan of marinaire. I installed marinaire throughout the last boat, one pump to 2 units. They worked well for a year then, the ability for 2 units to control one pump became an issue, solved readily by marinaire. Always liked there aftersales service. Now with changes in their aircon programing I've just installed 5 units in the current boat. My boat has a sea chest, and to that is coupled a min 1500gph water filter 11/2" to 11/2 pipe work to a 2" manifold with 3/4 outlets supplying march individual pumps. I have no problems with it in 6 months of operation, The Improvements made by marinaire over the last few years have produced good reliable and quite units. However I would stick with multiple pumps for the extra couple of hundred per aircon unit.

 

[firstbase]

I was going to do this myself but I find that one unit doesn't quite fit and I need to do some carpentry to get it in. That's ok but I also find that my hoses are old and sticky so going to replace them them as well. Travelling a lot lately and just don't have time. So, I decided to get someone to install the whole thing. The installer said the pump I got is great. "Most people get the bare minimum pump". It's a March 1600 gph for 3 A/C units that require, per the specs, a total of 850gph "minimum water flow". He is adding a manifold with 4 outlets. Each of the 3 units will have its own flow control/gauge/valve. The fourth will be routed overboard so when the A/C's are "tuned" for proper flow any excess will go back to where it came from. This is either a really good idea or the winner of the 2020 winner of the Best Rube Goldberg Marine A/C Plumbing Design award. I will find out I guess.

 

[Mischief Managed]

He is adding a manifold with 4 outlets. Each of the 3 units will have its own flow control/gauge/valve. The fourth will be routed overboard so when the A/C's are "tuned" for proper flow any excess will go back to where it came from. This is either a really good idea or the winner of the 2020 winner of the Best Rube Goldberg Marine A/C Plumbing Design award. I will find out I guess.

The excess flow drain does not need to happen. Won't hurt the pump at all to just throttle the flow with 3 valves on the three A/C lines. It will use less electricity, produce less heat, and last longer if you throttle the flow. Centrifugal pumps operate with better efficiency when they have some pressure head and are not at max flow.

 

[rslifkin]

The excess flow drain does not need to happen. Won't hurt the pump at all to just throttle the flow with 3 valves on the three A/C lines. It will use less electricity, produce less heat, and last longer if you throttle the flow. Centrifugal pumps operate with better efficiency when they have some pressure head and are not at max flow.

Agreed. I also wouldn't throttle the total flow any more than necessary. If one unit needs throttling to keep it from hogging all of the water, then that should be done. But otherwise, it's unlikely that there will be enough flow through the units to cause a problem. And for the most part, they'll work better with more flow when in very warm water (or for heating, they'll be able to work in colder water before it starts to freeze in the unit).

 

[STB]

This is likely a Rube Goldberg winner.

The only case where this would make sense is if you have all of the valves closed, except for one unit. And, that unit's water path is so narrow at some point along the way as compared to the pump size that the pump can't move enough water to cool itself. In this case, one would need to have a path to relieve that situation, allowing the pump to move enough water to cool itself.

If you've got an extra above water thru-hull, I guess it can't hurt to plumb it that way. But, if not, I'd want to see the math that justifies putting another hole in the hull before I'd do that. Or, I'd set it up with that valve capped off, close all of the valves except the smallest unit, run it full on, and measure the temperature at the pump until it leveled off. If it wasn't getting to hot -- I wouldn't add the extra flow just to cool it.

With the valves open, unless he is adding electronic valves, even if the units aren't running, the water will be circulating, so this could only be a problem with valves closed.

If it is a problem without valves closed -- get a smaller pump. But, I can't imagine that.