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Old 05-27-2007, 09:51 PM   #1
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On the trip that I posted about----the one that someone moved to the Poop Deck because they must have thought that me talking about cruising all winter instead of truck driving was off topic----I was wondering what everyone's experience was with fresh water makers. On my last voyage, I took several cases of bottled water, which weighed a lot, and I had to break them up and distribute them throughout the Hard Knots, which was time consuming and disorganized. What does the water taste like after it's gone through one of these delsalinators? How much time does a manual pump take compared to an electric, and would anyone recommend one type or brand over another.

Thank you for your imput.

Robin Scott Johnson
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Old 05-27-2007, 10:41 PM   #2
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I can only give you a part of the answer to your questions. We had a PUR Survivor desalinator (it could be wired into the boat, which we did, or be hand operated) that produced about 4 liters an hour, a hair more than a US gallon. I think that it drew 4 amps. It really was not a great watermaker, and we should have put it in our liferaft or ditch bag instead. The water, however, was wonderful, completely tasteless and clear.

I've talked with people with larger, engine-driven watermakers that they've been happy with, though I never knew the make that they had.

Somewhere in my readings, I read about someone who used their pressure cooker to distill seawater. Put the lid on, attached a plastic hose to the steam vent, boiled the water, the steam went out the hose and was condensed (I don't know how) into a container. This is a more efficient way of distilling seawater, I think, though you must be careful to keep the boiling gentle so saltwater doesn't boil too high and contaminate the steam.. You could distill more than a gallon an hour if you're motivated. I don't have my pressure cooker anymore, so I can't try it. It's a worthwhile experiment, I think.
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Old 05-28-2007, 12:03 AM   #3
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Hello Robin,

Most modern desalinators use the process of reverse osmosis to remove salts from seawater - a simplified explanation is to use a pump to force the salt water through a membrame, leaving salt one side and water on the other (or with tiny quantities of salts on the other) .

The pump can be motor driven or hand operated. The higher the pressure combined with the density of the membrane will determine the quality of the end result. The very best will produce water that is 99.999999% desalinated and tasteless.

The use of a pressure cooker to obtain distilled potable water from sea water can be done - but it is not a straight forward process. If one is contemplating using a domestic pressure cooker then some engineering will be required - because a. Applying pressure in the process of boiling water results that the water boils at a higher temperature. b. Sea water boils at temperatures determined relative to the amount of salt in the sea water eg . water from the Dead Sea boils at a different temperature to the water of the northern South China Sea.

Therefore the domestic pressure cooker will require a control valve to limit the amount of steam

being produced by boiling. An accurate pressure gauge. A Temperature gauge. To condense the steam vapor a pipe will be run from the low pressure side of the control valve through a cold water bath and out to a container for distilled water.

Taste will probably be the cheapest method of finding out if you have got all your valve settings right. Remember also that what liquid remains in the pressure cooker will now be different after you have shut down the process.

Caveat - as seen above - it is probably much safer to have any modifications done by a professional engineer.
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Old 05-28-2007, 12:55 AM   #4
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I have no plans to install a watermaker as I have sufficient fresh water storage capacity on my boat. I visited people on a Maramu some time ago and I declined an offer of coffee in favour of a glass of water. I do not know if perhaps their tanks had been contaminated as some stage, but the water was a little brackish.

In the course of our conversation, I tactfully mentioned the salt taste. The couple who owned the boat tasted the water and determined it to be of the finest taste. My conclusion is that, as with home brewed beer, over time people get used to the taste even if it is unpleasant.

It may have been contaminated tanks, a maintenance problem with the screen in their reverse osmosis unit or something else relative to filtering which I no nothing about. Compared to filtered tap water, their fresh water was awful.

David
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Old 05-28-2007, 01:21 AM   #5
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Hi Robin,

I've owned two Spectra 380 Watermakers....each going with the cats I owned/sold. I'll probably get a third one for our next boat unless I can find something as reliable and hassle-free, yet cheaper. Power consumption wasn't a worry on the last boat and it was all solar and wind. The water tastes perfect and I've lived of it for quite a while. Even used it for the battery water over five years to no ill effect.

Only downside to a watermaker in my opinion is that you need to run it everyday to keep the membrane fresh. You also need to keep an eye on the sea-water around you for oil and diesel as you don't want to suck it in (lots of harbors do have this floating around). Also, if it's a little rough on a passage, our set-up would suck in an air-lock and we'd have to turn down-wind to run it. It's not a huge deal, just something to be aware of. You can back-flush the Spectras with the watermaker-water and go a few days without running it. We finally started using propylene glycol (spelling??) to store it and it was easier then the powder chem and even recommended by Spectra.

The Spectra is not cheap! Each one was around 5k...one from Miami and the other outta the Gold Coast in Oz. Spectra has stood behind its product and took care of the two small problems I had over the years - no questions, no hassles, prompt responses and resolutions.

These are 12v systems. We just didn't want an engine driven one on the first boat and that wasn't even an option on the second boat (two 4-stroke outboards). They'll put out around 6+ gallons an hour depending on your voltage level an use roughly one-amp-hour per gallon made. That's with one feed pump running. For our set-up we found it better to just run the one feed pump as when two were running we actually got a little under twice as much product water. This was due to the slight voltage drop running two motors with no engine/alternator pumping the batteries back up. Worked well and the second pump was looked upon as a back-up (of which we never used).

I do have a couple friends that made their own set-ups with parts from Australia for about 3k AUD. Quality membranes, etc....they just had to make their own pulley/belt/engine TO system to drive it. I want to say these put out a ridiculous amount of water for them and they could waste as much as they wanted (50-70 liters/hr??).

Those I have met that have had problems with their watermakers seemed to ignore them. You really have to run them often or store the membrane when not using it. Several yachts would just turn theirs off for months (years!!) and them wonder why the piece of crap wouldn't work at the flip of a switch!

Bottomline - it can be expensive water. Just depends on where you cruise and what you need. We like to be as self sufficient as possible and go to places where getting water isn't even an option. Quite often we'll fill up jugs for little island villages that are hurting for water. If it was sunny and windy we could run the Spectra from 9am to 3pm and be sittin' pretty without even dragging the batteries down. Nice to take two showers a day too!

Best - Jay
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Old 05-28-2007, 01:24 AM   #6
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I think that I wasn't very clear in my description of using a pressure cooker to distill water. Let me try again, first with a bit of an explanation of how most pressure cookers and canners work.

There are quite a few models and styles of pressure cookers, but they're all reasonably similar in their setup and operation. On the lid there are usually two holes. There are no valves that are mechanically closed/locked. That way lies disaster.

One hole has a metal or rubber plug in it - it is the weakest part of the cooker, designed to blow if pressure gets dangerously high.

The second hole is really a pipette, onto which the pressure weight is placed. It only allows as much pressure as the weight allows (some are adjustable to 5, 10, or 15 psi). Some cookers also have a pressure gauge.

When the water and food being prepared in the cooker boils and builds up pressure, the weight on the pipette will start bouncing around, releasing steam (pressure), and making a lot of noise. On simpler pressure cookers, this unsecured weight is the only instrument that allows pressure to build in the cooker. The bouncing and hissing is the signal for the cook to reduce the flame until the weight only (hisses) infrequently. You start timing, and when cooking is done, you remove from heat, and LEAVE THE WEIGHT ALONE until the cooker cools down a bit. Once it's cooled somewhat you can remove the weight, but if you do it too soon, you will get a nasty steam burn as the steam rushes out the pipette like a flame thrower. That's how food is cooked under pressure, or canned (bottled is the term in some places). Without the weight on the pipette the pressure cooker can be used as a regular pot. Because the hole through the pipette is quite small, there will be a bit more than atmospheric pressure, but not significantly more, and because the lid can be firmly secured, even if it falls off the stove in a seaway you won't lose all the food onto the cabin sole, just what can dribble out of the pipette. You should never pressure cook (weight on) while underway if there is any chance at all that the pot could be tossed off the stove; injury could occur if gauge breaks in the fall, or the weight is thrown off, directing a stream of super-heated steam at whatever is in its way.

When used to distill water that pipette no longer has its weight or gauge on it to exert pressure, it's just a metal nipple extending above the cover to let air and steam out. Attach a a long length of plastic or rubber hose to it, and run it through some cool water (as you might have seen with spirit stills. When the water in the cooker boils, the steam (above the boiling point of water, but not the temperature of steam under 5 or more psi of pressure) will escape through the tube, cool as it makes its long run through the cool water bath, and deliver distilled water at the other end. It's the simplicity of the pressure cooker that enables it to be used to distill water. Care must be taken, but no more than when cooking.
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Old 05-28-2007, 06:53 AM   #7
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" When used to distill water that pipette no longer has its weight or gauge on it to exert pressure, it's just a metal nipple extending above the cover to let air and steam out. Attach a a long length of plastic or rubber hose to it, and run it through some cool water (as you might have seen with spirit stills. When the water in the cooker boils, the steam (above the boiling point of water, but not the temperature of steam under 5 or more psi of pressure) will escape through the tube, cool as it makes its long run through the cool water bath, and deliver distilled water at the other end. It's the simplicity of the pressure cooker that enables it to be used to distill water. Care must be taken, but no more than when cooking. "

[/quote]

Hi Jeanne,

Thanks, for the detail - the system could be improved by having a 2ft length of a 1/4 inch bore - heat proof tube, this is attached to the "pipette" by a hose clamp. The other end of this tube is attached to a 10ft length of 1/4 inch outside diameter copper piping which is wound into a spring like coil - at the outlet, attach with a hose clamp a 3ft length of 1/4" bore plastic hose.

The metal coil is submersed in a bucket of cold sea water - and the plastic hose outlet into your collecting container. (the cold sea water will have to be replaced when it heats up too much)

However, as the pressure weights have been removed from the pipette, the sea water will boil at a temperature that will only produce about 480cc (1 US pint) of potable water per hour of operation.

Richard
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Old 05-28-2007, 10:40 AM   #8
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as the pressure weights have been removed from the pipette, the sea water will boil at a temperature that will only produce about 480cc (1 US pint) of potable water per hour of operation.
No pressure cooker on this boat, or I would test this. However, I have a great deal of experience in boiling away water (the positive spin to that comment is that I make a lot of reduction sauces), and it seems that even on a medium flame I can boil away several quarts of water in an hour. Hmmmm, need to try this some time.
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Old 05-29-2007, 03:28 AM   #9
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No pressure cooker on this boat, or I would test this. However, I have a great deal of experience in boiling away water (the positive spin to that comment is that I make a lot of reduction sauces), and it seems that even on a medium flame I can boil away several quarts of water in an hour. Hmmmm, need to try this some time.
With the Hard Knots, it would only be possible to do that in the calm of a doldrum or port, the boiling technique. Thank you for everybody's advice, I'll go with a Pur manual.
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Old 05-29-2007, 04:55 PM   #10
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While the pressure cooker method of distilling water does work wouldn't it take quite a bit of fuel per gallon of water?
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Old 05-29-2007, 05:16 PM   #11
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Ex-navy MM1(SS) here. Boiling sea water to make water is hugely in-efficient from a fuel standpoint.

Navy vessels do it (I used to make 4 to 8 thousand gallos per day) using steam from the propulsion plant, they also boil the water at a partial vacuum to lower the boiling point, but they have steam available in abundance; however, regardless of temperature at which it occures, the latent heat of vaporization is unchanged. It takes 1 calorie/g to heat the water from 0C to 100C it takes another 539 calories/g to convert it to steam at 100C, you have to remove that 539 cal/g to convert it back into water.

In an emergency if you have fuel and need water, sure, but for any other reason, look to other methods.
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Old 05-29-2007, 08:35 PM   #12
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In an emergency if you have fuel and need water, sure, but for any other reason, look to other methods.
I agree.
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Old 05-29-2007, 10:05 PM   #13
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I agree.
It has been a while, so I went and looked it up. These are rounded for simplicity

4.2 joules/calorie

Energy potential of LP gas =50MJ/KG

2.2 lbs/kg

So ... (20lbs in a cylinder) / (2.2 lbs/kg) * (50 MJ/kg) / (4.2J/c) = 108Mc/tank

Assuming boiling is even 30% efficinet energy transfer = 32Mc/tank transferred to the water

(32Mc) / (539c/g) latent heat of vaporization =

Result is 60,200 g of water converted to steam

assuming 100% efficinet condensation = 60 liters of water per tank of propane.

I'm thinking you're not going too far on that
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Old 05-30-2007, 03:53 AM   #14
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Ex-navy MM1(SS) here. Boiling sea water to make water is hugely in-efficient from a fuel standpoint.

Navy vessels do it (I used to make 4 to 8 thousand gallos per day) using steam from the propulsion plant, they also boil the water at a partial vacuum to lower the boiling point, but they have steam available in abundance; however, regardless of temperature at which it occures, the latent heat of vaporization is unchanged. It takes 1 calorie/g to heat the water from 0C to 100C it takes another 539 calories/g to convert it to steam at 100C, you have to remove that 539 cal/g to convert it back into water.

In an emergency if you have fuel and need water, sure, but for any other reason, look to other methods.
That's amazing. I never knew the navy did it that way.
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