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Old 08-30-2018, 04:07 PM   #1
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Air Conditioning raw water plumbing options

My raw water pump flows into a manifold with valves and feeds three air conditioners / heat pumps. I reworked the plumbing during my refit, and it does work, but water flow is maybe marginal. Replaced the raw water pump for my charter boat today with a higher flow pump. The increased flow has the heat exchanger running cooler and the AC output temperature running lower ( colder air coming out the vents). I'm considering running the 3 heat pump raw water systems in series as opposed parallel as they are now. Conceptually, one heat pump should function better with higher water flow. I'm sure 2 units running in series would benefit also. With 3 units running, I feel confident there is still plenty of room for all the BTUs from all three units. As luck would have it, the highest duty cycle unit would be first and the lowest would be last.

Looking for thoughts and potifications from the peanut gallery. Does anybody have a system with the units in series? Can anyone point to a document that explains the theoretical advantages of one system over the other.

Ted
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Old 08-30-2018, 04:25 PM   #2
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Well, at least you'd know the flow rate through each part of the system. Of course the water would be hotter after going through each unit but if you move enough water and prioritize the order, I don't see why it wouldn't work just as you want it to.

My system is currently set up the same way as yours - 3 in parallel. It works ok but I doubt its been cleaned in a very long time if ever. Your results make me believe I could at least get some better cooling/heating if I clean out the exchanger loops.

Ken
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Old 08-30-2018, 05:08 PM   #3
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Series is a mistake. The last A/C in a series setup will perform poorly because it is seeing hotter water. Roughly speaking if you have 80 deg raw water temp then the first A/C will be 85 deg out, the second will be 90 deg out. So the third one will see water 10 deg warmer or 90 deg input and won't perform as well.


Assuming that you have sufficient flow (to hit 5 deg or less temp rise from each unit) and they are balanced properly, parallel flow is best.


David
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Old 08-30-2018, 05:54 PM   #4
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Ted

I would suggest you all two more pumps and solve your problem and have a back up plan so you can switch to a different feed is a pump quits. I know you have room in your bilge for a couple of extra pumps.
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Old 08-30-2018, 06:28 PM   #5
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I concur with Bigfish, but insist that the pumps must all be mirror polished SS housings.
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Old 08-30-2018, 06:36 PM   #6
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Don't see the need with a properly configured system, based on running 5 (count 'em, 5) units with a single pump and parallel manifold. One key is to have the pump sized correctly to the downstream plumbing; too much pump can cause cavitation.
As I have recommended here many times, give the folks at Depco Pump a call and discuss your situation, or consult a good AC tech. Depco even publishes an entire catalog focused on AC and RO: http://online.anyflip.com/ucrc/fxpq/mobile/index.html
But call them...
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Old 08-30-2018, 06:39 PM   #7
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We have 2 ac units with one pump hooked in series. Works great. not sure how 3 would work.
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Old 08-30-2018, 06:57 PM   #8
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Quote:
Originally Posted by djmarchand View Post
Series is a mistake. The last A/C in a series setup will perform poorly because it is seeing hotter water. Roughly speaking if you have 80 deg raw water temp then the first A/C will be 85 deg out, the second will be 90 deg out. So the third one will see water 10 deg warmer or 90 deg input and won't perform as well.


Assuming that you have sufficient flow (to hit 5 deg or less temp rise from each unit) and they are balanced properly, parallel flow is best.


David
Not sure I agree with you. Assuming the flow rate doesn't appreciably change, and each unit is seeing 1/3 of the flow when plumbed in parallel, each unit takes in 80 degree water in your example and puts out 95 degree water because there's only 1/3 of the water to absorb each units heat. While there may be less cooling for the 3rd unit, when all 3 units are running, a significant percentage of the time, probably most of the time, 2 units or less are running. If they run more efficiently because of greater flow, it may be an overall plus.

Ted
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Old 08-30-2018, 08:01 PM   #9
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Originally Posted by O C Diver View Post
Not sure I agree with you. Assuming the flow rate doesn't appreciably change, and each unit is seeing 1/3 of the flow when plumbed in parallel, each unit takes in 80 degree water in your example and puts out 95 degree water because there's only 1/3 of the water to absorb each units heat. While there may be less cooling for the 3rd unit, when all 3 units are running, a significant percentage of the time, probably most of the time, 2 units or less are running. If they run more efficiently because of greater flow, it may be an overall plus.

Ted
That's not how it works. What's this "greater flow" you are talking about? Hotter water moving by faster somehow? That achieves the opposite of your dream. Consult a professional.
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Old 08-30-2018, 08:15 PM   #10
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That's not how it works. What's this "greater flow" you are talking about? Hotter water moving by faster somehow? That achieves the opposite of your dream. Consult a professional.
If you have a pump producing 12 GPM and you feed 3 AC units in parallel, each unit gets 4 GPM. If the three units are in series, the flow through all 3 units is 12 GPM.

Ted
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