Converting a 110v watermaker to 12v DC?

[Cheechako]

Originally Posted by diver61

All of the above posts have good points. I am a marine engineer with deep knowledge of water hydraulics and such.First of all you need to match a motor to the pump that will run at the same speed as the ac motor(1750 rpms) in order to match the gpm requirement for the hi pressure pump. Water in equals supply and the demand for the pump. Not enough water = low pressure. All other concerns (amp draw and hp ) don't matter if you cant feed the pump. If you put a smaller pump you still have the problem of water in and water out. It takes about 8 gals of salt water to make one gal of fresh water, the membranes are sized to fit the output of the hi pressure pump so you could get smaller membranes or take one out ,if it has more that one.All of this being said keep what you got and use as is or get a 12 lt system. Hope this helped some.GARY

Well, not really... it just has to reach adequate speed. He could put it on the main diesel engine and just run at a convenient speed. (which I have done)

[motion30]

so if my water maker currently works with one horsepower a hundred 10 volt motor so can I switch to a 12 volt 1 horsepower motor? I have the solsr to suppirt this. I guess the question really is is there a difference between an AC powered horsepower and a DC power horsepower?

[billknny]

Prvilege,

The answer you are looking for is No, but Maybe.

Unfortunately, for membranes to work properly and live a long and happy life, they have to be run very close to the manufacturer's specification for both flow and pressure. Either too high, or too low, will compromise the product water quality and the membrane's life span.

Now, reducing the flow rate, WILL cause your membranes to fail early because the salt side will have higher than normal salt concentrations and the membrane will clog with precipitated salt. The question that is not easy to answer is how much sooner... If they lasted 5 years instead of 10 I bet you'd be happy. If they failed after 6 months, maybe not so much.

You might try calling DuPont FilmTec technical support and explain what you want to do and they might have an answer.

[motion30]

the answer that eludes me is. is there a power difference between a 1 horsepower 110 volt electric motor. An a 1hp 12volt dc motor?

[Cheechako]

Well a HP is a HP right? The 12V will be a big load though. 1HP = 745 watts = 63 amps! and that's not a surge, that's steady amps for the duration you run your watermaker. More or less... You better have big cables...

[Privilege]

Thank you everybody for your input. The short answer is yes it can be done, but it's not practical, so to all intents and purposes the answer is 'no'.

This post is a few years old now. I ended up biting the bullet and going with a DC Spectra water maker supplying me 15 gph at 19 amps and am very happy with my decision.

[a64pilot]

If you have the Solar to support it, then you have the Solar to do it through an inverter and not buy another pump.
The efficiency loss isn’t very great, not as bad as people want to think it is.

[motion30]

Ok so as a follow up.... I in fact replaced the 120volt motor with a 1hp 12 volt motor, also the 110 volt low pressure pump with a 12-volt water pump On my sea pro watermaker. I used 1ga cable and the appropriate Breakers. So far I only have about 3 hours on this system but it is working absolutely perfect. the12 volt motor and low pressure pump together draw 81 amps max at 800lb producing 40 gallons an hour. I can reduce the water maker pressure to 700 lb still making 35 gallons an hour and drawing approximately 75 amps. With good water quality. I just completed this insulation and only have about 3 hours running time. During mid-day my solar panels easily support this system . I have now successfully completed a complete 12-volt conversion on everything that draws High amperage. I run the microwave through the inverter with the power draw of 105 amps. the hot water heater run through the inverter drawls 87 amps. and of course now the water maker. I couldn't be more happy with the SunPower 435 watt solar panels, I have seen them produce more than the rated capacity multiple times...... 4 panels routinely produced over a hundred amp hrs and I have seen the max of 136amp hrs

[Pizzazz]

Congratulations on the conversion. My understanding of the water making process is that you need a certain flow across the membrane in order to ensure it lasts a long time. However, few people have a clear understanding of what is a long time vs. the cost of replacing the membrane. I have read the Dow Filmtec documents in detail and basically they say that in small scale applications you can reduce the flow significantly, this will add scaling, etc. and you will need to replace the membrane in 1-3 years time. Well, a new membrane costs $200, so it is not such a big deal. A quick way to calculate the output is that at very low flow you would be able to recover 30-35% of the pump output. Once you get above 40-45% recovery, the quality of the fresh water will deteriorate, so you do not want to go too low.

Thus, it is a perfectly acceptable modification to reduce the flow (meaning weaker motor, smaller pump) and accept more frequent membrane replacement. Dow says so, my experiments show this as well. In your case, assuming the numbers are correct, you are getting 40 gph at 80A, which is 2 AHrs/gallon. Compare this to Spectra which uses energy recovery to achieve 1 AHrs/gallon but costs $10K and then consider how many replacement membranes you can order from Amazon, with two days delivery, to make up the difference. I think you did well.

SV Pizzazz

[motion30]

Pizzazz
Point taken thank you. I run to water maker at the recommended 800 lb. And the motor along with the low pressure pump draws 81 amps. This is brand new to me so I was adjusting the pressure to see what the difference in the power consumption actually is. it looks like the low pressure pump uses about 6 amps
and I do have a parts per meter gauge intrical in my system so I can adjust the pressure and look at the quality of the water. I did not know about the scaling and shortened life of the memory thanks for that info

[Cheechako]

a 1 hp DC motor must be pretty large..? Got pics of your setup? For some reason in boats the DC motors I've had have been short life. But maybe that's just because they are selected as poor quality to start by the manufacturer...

[motion30]

the motor is a little longer but perhaps a little smallein diameter than the 110 volt motor

[a64pilot]

By going with DC direct, you gain about a 5% efficiency over an inverter converting DC to AC, most of which may be lost in the long wire run to the DC motor.
DC or AC the Watermaker doesn’t care, PSI and flow rates will of course be the same.

Another possible issue with a DC motor pulling high amps is the voltage drop of the bank itself.

[motion30]

The 110v ac motor drew 13amps. So would be 130 tru the inverter vs 81 amps dc. My solar can support 81 amps without draining the batteries.

[RaymondR]

Originally Posted by Pizzazz

Congratulations on the conversion. My understanding of the water making process is that you need a certain flow across the membrane in order to ensure it lasts a long time. However, few people have a clear understanding of what is a long time vs. the cost of replacing the membrane. I have read the Dow Filmtec documents in detail and basically they say that in small scale applications you can reduce the flow significantly, this will add scaling, etc. and you will need to replace the membrane in 1-3 years time. Well, a new membrane costs $200, so it is not such a big deal. A quick way to calculate the output is that at very low flow you would be able to recover 30-35% of the pump output. Once you get above 40-45% recovery, the quality of the fresh water will deteriorate, so you do not want to go too low.

Thus, it is a perfectly acceptable modification to reduce the flow (meaning weaker motor, smaller pump) and accept more frequent membrane replacement. Dow says so, my experiments show this as well. In your case, assuming the numbers are correct, you are getting 40 gph at 80A, which is 2 AHrs/gallon. Compare this to Spectra which uses energy recovery to achieve 1 AHrs/gallon but costs $10K and then consider how many replacement membranes you can order from Amazon, with two days delivery, to make up the difference. I think you did well.

SV Pizzazz

good thinking 99.

That's 50 membranes for the cost of the Spectra. 50 by 1 year is 50 years and by 3 years 150 years.