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Old 11-11-2007, 06:30   #1
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DIY Watermaker Motor Horsepower

I have a question for those folks who have built thier own watermaker.

I've been looking at the documentation for pressure pumps and most describe a horse power requirement for a given pump as:

HP = (GPMxPSI)/1460

It seems the mimimum flow for an RO membrane is between 1.2 and 1.5 GPM (the more the better). Dow filmtech membranes have a 1 GPM minimum requirement for brine discharge, so you have to put in Brine+Product from the pump.

So according to the formula, pumping 1.5 GPM into an RO membrane at 800 PSI will require .82 HP. Or about 613 watts. Or about 50 amps at 12.5 volts!

(Note: Dow's ROSA software produces several warnings in this configuration)

The thing I don't understand, is that there are manufacturers of watermakers that say that thier units only draw 27 amps to product 11 GPM. Are they running realy low flows over the membrane? Working backwards from 27 amps using a membrane pressure of 800 PSI gets you a feed flow of .82 GPM. Doing this voids the warentee on the membrane.

If you build a water maker, how much water is your pump pushing and how much does the motor draw?

Thanks in advance,

John
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Old 11-11-2007, 10:11   #2
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I don't know what your water needs are but 11 GPM is a serious watermaker. Mine produces 16 GPH requires 1.5 HP motor runs at 800psi and draws 12 to 13 amps @ 110vac. To do the same job with a 12vdc motor you would be drawing close to 90 amps. Are there some high output super efficient membranes out there that I am unaware of? Please let me know. I would be in the market. I use either the inverter power or the genset. Prefer to use the genset.
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Old 11-11-2007, 10:35   #3
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this may be of help, bottom right of page is a HP graph
http://www.catpumps.com/pdfs/Pump%20...0Pumps/231.PDF
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Old 11-11-2007, 11:00   #4
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I think you might have to do the calculations from the membrane cababilities. Depending on surface area, membranes will allow a given amount of flow throughput for a given pressure. The membranes I have are 2.5 in in Dia. by 24 in long and will allow 200 gal per day at 800 psi regardless of the available output of the pump. using two of them in parallel gives me 400g/day. I also have a Cat pump. The specs given for the pump are, I believe, for use as a pressure washer.
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Old 11-11-2007, 16:58   #5
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Thanks for the replies. I know this is a little on the technical side, but bear with me...

JusDreaming, I assume you're talking about the chart, there's no graph in the referenced document. Yes, that's where the formula I was quoting comes from. It's just to the left of the chart you refer to.

Lancerby, I was using the 11 GPH watermaker as an example of what manufactures say they can do. For what it's worth, my example is from EchoTec (ECHOTec. Marine Watermakers*- DC Watermakers for Yachts*12 or 24 Volt*(Modular)).

Let me go into my example a little further:
11 GPH = .1833 GPM
@ 13% recovery (the maximum recomended by the ROSA software for Dow Filmtech membranes), you have to put 1.4 GPM of sea water into the membrane to get .1833 GPM of product

From EchoTec's documentation they want you to run the unit at 900 PSI, and they say it will draw 27 amps @ 12.5VDC

So:
900 PSI * 1.4 GPM/1460=.86 HP or 634 watts (1 HP= 746 watts)
634 watts @ 12.5 VDC = 51.5 amps

How come the calculations say this unit should draw almost 52 amps?

In order to get a 27 amp draw, you'd have to run with only .73 GPM of feed water:

27 amps * 12.5 VDC = 378 watts or .45 HP
(.45 HP * 1460)/900 = .73 GPM

Doing so voids the FilmTech membrane warrenty.

However, Lancerby's watermaker seems to follow the formula more accurately:

16 GPH = .2666 GPM
@ 13 % recovery 2.05 GPM of sea water

2.05 GPM * 800 PSI/1460 = 1.12 HP or 838 watts or 7.6 amps at 110VAC
(maybe he has some other load for the other 5 amp, but at least it seems his motor is correctly sized)

Maybe the formula is only for AC motors, as apposed to DC motors?

Lancerby, I agree I'd rather do it on AC but I have more DC power sources available to me (Solar, alternator, wind, bat. charger).

I'd sure like to talk to someone who has DISIGNED a watermaker and has had it come out the way they expected, and knows why . There's something I'm missing for sure.

John
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Old 12-11-2007, 02:38   #6
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Quote:
Originally Posted by svnakia
”... In order to get a 27 amp draw, you'd have to run with only .73 GPM of feed water:
27 amps * 12.5 VDC = 378 watts or .45 HP
(.45 HP * 1460)/900 = .73 GPM ...
... Maybe the formula is only for AC motors, as apposed to DC motors?”
John: There appears to be something seriously wrong with ECHTec’s claims.

Your assumptions and arithmetic all seem correct to me.
In fact it gets worse:

Motor HP = (V x I x Efficiency*) 746
HP = (12.5 x 27 x 0.85*) 746
HP = 0.384 HP (> 3/8HP but < 7/16 HP) [not 0.45 HP]

* A typical Motor might be about 85% efficient.
I don’t recall if the Motor HP = GPM Flow formula considers Pump Efficiency.


Single phase 120VAC motors use the same calculations as DC motors.
Only 3-Phase AC motors have reduced amperages ( 1.73).
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Old 12-11-2007, 07:10   #7
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Gordmay,
Excellent point! I'm sure the Cat Pump equation does not take into acount motor efficiency because it gives HP and not current draw. For a given HP a motor will draw more or less current depending on it's efficiency.

By the way, I sure don't intend or want this thread to turn into an examination of EchoTec. I've heard very good things about them and the one owner I spoke to is having results similar to those advertised. He has a smaller unit, not the one we're using in our example.

Taking a look at another unit, the Katadyn 160E (6.7 GPH @ 18 amps):

6.7 GPH = .11166 GPM
@ 13 % recovery = .86 GPM feed flow
(.86 GPM * 800 PSI)/1460 = .47 HP or 351 watts or 28 amps @ 12.5 VDC

Even more interesting is that Katadyn specifies the feed flow rate in thier documentation of 80 GPH, or 1.33 GPM which would require .71 HP!

Of course Katadyn uses energy recovery (similar to the clark pump used in Spectra watermakers, only on one stroke) so all the HP calculations do not apply to them.

John
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Old 11-12-2007, 16:30   #8
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Hi Svakia(John), finnaly someone in my shoes. Sort of. I have a leeson 3/4HP DC motor driving the Cat 2SF15SEEL with 2 40" mmbranes in serial. Don't ask me what ROSA worked out in my case cause, I gave up on those calculations long time ago. CAT pump is a 1.5 GPM pump. My brine flow meter is kind of not that accurate. It has 2, 1.5, 1 and .5 GPH markings and I am somewhere just above 1. My product is at around 24-30GPH. Up in new england and all alog the US east coast in cold water, we were making upwards of 30 and closer to 40GPH. But not here in the warm waters of the Caribbean.
Practically, put as large membranes as your space would allow and try to have more then 1 GPM across them. This will give you lots of water and will minimize the time it takes to fill the tanks.
While running, my system was drawing a wopping 80-90 amps. It would climb up due to a drop in voltage on the house bank. So it would start less, and then climb up.
What I do is wait for a nice sunny day, if the trades are up even better, and then I run the engine at idle at about 1500RPMs.
The solar panels + wind generator + high output alternator give me just enough to compensate the draw of the watermaker. And then a clowd comes by and there is a 20 amp draw on the house. 2 hours later, my house is at 12.0 volts and the DC motor is heating up. Definately run large wire to the motor. The fact that half of my house bank is over 4 years old, might have something to do with it.
My latest discovery was that my feed pump, a waterpuppy was drawing too much and was getting too hot, and blowing a 15 amp fuse. It was an old pump, one of the few original items on the boat. So as a quick solution, I added two valves to reroute my engine raw waterpump to feed the watermaker prior to cooling the heat exchanger. The result is that I cut about 10-15amps out of the watermaker power consumption, but the engine runs 180-200F and I am not even putting it in gear! I am ok with that and I am glad to save the amps any way I can.
If I was to do it again I would try to go with an engine driven pump. But that might be a whole different can of worms.
Good luck with the project.
Petar
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Old 16-09-2010, 04:49   #9
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Hello, do some clever man knows what type of high pressure pump a Katadyn 160E use. I want to build my own watermaker, I have no maney to buy one, I want to use a Leeson 12 volt motor. Regards. Willy
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Old 16-09-2010, 15:05   #10
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I called ECHOTec, they claim they don't use a Cat pump (didn't want to say what they currently use..)
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