Help on Watermaker Design

[Projectboats]

Rain is blowing sideways here in Seattle this morning so I'm hunkered down and working on a watermaker for next season. One issue in the Northwest is the feedwater temperature. Assuming intake water at around 50 degrees, the product water is reduced to a lousy 54% of what it would be at 77 degrees. That's a huge penalty to pay in addition to the rain.
It occurred to me to capture waste engine heat through a carefully designed heat exchanger to raise the feedwater temperature a few degrees and increase production. (larger boat so 22KW genset will be running anyway)

The only challenge I see is the complete isolation of engine coolant from potable water.

Feasible?

Duncan
M/V Turning Point
Shilshole Marina
Email: duncan :at: projectboatcompany :dot: com

[goboatingnow]

Feasible yes. practical and cost effective no. easy solution , design a bigger watermaker to begin with

Dave

[Projectboats]

Thanks Dave,
I'm working with some design constraints. I have most of the components on hand and limited to a 3.6 GPM CAT pump., etc.
It's frustrating to have the roughly 40 gal/hr. potential cut almost in half by a few lousy degrees of intake water temp.
I know.....moving south would solve this but there are other limiting factors

thanks again, duncan

Did I mention it was raining?

[goboatingnow]

small pump, right on the edge of the membrane minimum flow rates limits really. yo will not see 40g/hr out of that .

try a 4' membrane or two 2' in series,

PS its always raining in seattle

Dave

[GordMay]

Safety considerations require a double walled heat exchanger for heating potable (drinking) water.
You MIGHT be safe enough with a single wall HEX on the supply side of an R/O unit.

Double Wall HEX ➥ http://www.heatexchangersonline.com/sidearm.htm

And ➥ Double Wall Brazed Plate Heat Exchangers

[foggysail]

Originally Posted by goboatingnow

small pump, right on the edge of the membrane minimum flow rates limits really. yo will not see 40g/hr out of that .

try a 4' membrane or two 2' in series,

PS its always raining in seattle

Dave

We do not know how he intends to drive the pump. But, think about this:

HP= 3.5*800/1450 = 1.93

A 2HP 12vdc motor is certainly a gutcha. Heck, even operating a 2HP motor on 120 AC requires over 12 amperes not including starting surge nor motor efficiency losses.

But he is well within the limits for a 2.5" membrane which has a minimum concentrate flow rate of 1 gal/minute. With two SW30-2540s in series, I think 40 gal of product is reachable but a trip through ROSA will provide better details.

Foggy

[Projectboats]

Foggy, Thanks for your help. I'm driving the pump with a 3 hp 240 volt motor. I intend on using 3 2.5" membranes. It seems to me that if I can increase product by around 20 gal/hr by simply adding a heat exchanger and a mixing valve for only a few hundred bucks that's a lot of bang for the buck. Am I missing something?

[foggysail]

ProjectB--

The guy that is most knowledgeable here on the site is Tellie, he is in the expert category. But I will try my best here.

Three membranes in parallel will put you up against the minimum concentrate levels. I don't know if three in series is feasible, I know two are. My guess that three in series is fine but the product will lessen for each membrane because the salt content will be higher.

40 gallons/hour is a lot of water and personally I think you can get that with two membranes operated in series even at lower temperatures. My suggestion is to download Dow's ROSA and run the program. I will comment that it took me a while to get meaningful results and I need to spend more time on it for my project.

I am building a system that uses a Armitsu 307 pump with an output of only 3.2 gal/min. I am also going to turn the pump with a two HP 1725RPM motor. I was going to use a larger pump but I could not find a 6 pole 3HP single phase motor; they are available only in 3 phase. The idea of 6 poles is the RPM is less., abouit 1150 or so and the pump output is a direct fucntion of RPM. If you get 1 gallon/min at 1000 RPM, you would get 2 gal/min at 2000 RPM. Now of course this is just an example, those partuclar RPMs will be found only in expensive special case motors such as servos.

Back to heat exchanger. I don't think one is needed plus they add too much complexity as noted by GordMay. A membrane will yield about 12% product and for simplicity sake, assume two in series have the same product output. So a 3.5 gal/min pump will give about .12(3.5)60= 25gal/hr. Two will give a total of 50 gal/hr. Three will add maybe another 20. Try ROSA and please keep us posted. There are more than a few trying to build there own systems and misery loves company.

Foggy