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
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
, 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.