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Old 23-07-2014, 19:45   #1
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New Distillation type Watermaker, 'Slingshot'

I'm reading thru the June issue of Popular Science which contains a number of interesting short articles on the potable WATER shortages that exist in various parts of the world, and even those situations that are looming right here in the USA.

One particular article that caught my attention, and that I though might have boating implications, was one about Dean Kamen's new distillation type watermaker, 'Slingshot'. Remember Dean Kamen?....he is that inventor of numerous medical devices, and the Segway transportation machines, etc. While working on a new peritoneal dialysis machine project, he found he needed a large supply of 'pure water' even above the relatively good quality we get from many of our city tap waters in the USA.

Where does one find that quality and quantity of pure water at a reasonable cost? The 'distillation process' can get you that pure water even when the source might be sewage water, or chemical waste water, or seawater! But the process of distillation might need to be rather large scale, and it is normally expensive energy-wise. Kamen and his engineer's went to work on the problem, and what they have come up with they have chosen to call 'Slingshot', a vapor compression distillation machine.

The energy efficiencies for this watermaker look very good, likely exceeding that of most other methods including the reverse-osmosis ones we often utilize on vessels. BUT, still it requires a 'jump start' to initiate the evaporation stage, and a minimum amount of additional energy input to keep it running. As a result of their desire to get some of these watermaking machines into remote and impoverished areas of the world were energy input is not so readily available, they have developed machines that can run on solar power, and one of Dean's pet projects, the Sterling engine.

Of course our boats should have no problems with supplying these watermaking machines with enough power, either solar, or battery, or generator sourced. The point is we should be able to get a very high quality of water on board at a very reasonable expenditure of energy, and a very low demand of maintenance.

Brian Eiland
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Old 23-07-2014, 22:58   #2
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Re: New Distillation type Watermaker, 'Slingshot'

Thanks Brian! Very interesting.

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Old 24-07-2014, 04:03   #3
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Re: New Distillation type Watermaker, 'Slingshot'

Distillers and vapour compression has been around a long time. Vapour compression I'm familiar with was on a oil rig close to the coast and because the SW was boiled, no nasties survived from esterine water, pretty impressive multipath plate heat exchangers made it efficient, but not as efficient as Distillers useing waste heat. As an example on a tanker burning 40Tons of fuel we could produce 80T of beautiful fresh water, that was on a motor ship, once in port when we were hard up we fired the boiler to produce the heat, the equation became for each 2T of fuel we got 1 Ton of fresh water, that water was about $1/litre not good, we didn't do that again.

Efficiencies are easier with large machines, Vapour compression required pretty intense acid washing, expensive easily damaged equipment, daily attention, I nearly built a machine for a yacht one time cose I came by a free vacuum p/p. But now think that RO is the way to go.
Still it urks me that Power Stations in Aus do not use Distillers to make 2nd quality water from SW. I believe a combination plant using waste heat would be more efficient than a stand alone Power Station and stand alone RO (Reverse Osmosis) plant.
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Old 24-07-2014, 04:25   #4
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Re: New Distillation type Watermaker, 'Slingshot'

Reverse osmosis is not efficient and also very complicated and expensive. I think the reason it is popular, is a very small lightweight device can produce a large amount of water very quickly.

Moving on to a still which can be very efficient, consider, a glass panel 1 square meter. On the inside is a black fabric where seawater is pumped. A small electric vacuum pump reduces the pressure of the air inside the still to near vacuum forcing a very fast rate of evaporation, and pumping the water vapor through a heat exchanger, which uses the incoming sea water to cool the vapor down (and heat the incoming sea water) to drip fresh water output.

This system uses a small solar panel to power the vacuum pump and seawater pump, and enjoys a 10x efficiency increase over a traditional solar still. Normal solar stills have issues with water vapor and droplets reflecting incoming sunlight thus reducing efficiency. As well as relatively high temperature required for evaporation. Not in the vacuum solar still!

I would expect 50 liters of water per day using 1 square meter of sunlight in sunny tropical conditions, or in summer at 30 degrees latitude. For this reason, small boats such as mine with a single person could have a much smaller still than 1 square meter.

Considering in rainy conditions, there is plenty of water, this is the perfect complement, and I believe there are few conditions indeed which do not have any rain or sun for longer than a reasonable sized tank can store water.
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Old 24-07-2014, 04:44   #5
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Re: New Distillation type Watermaker, 'Slingshot'

To any one interested, the book Sailing the Farm by Neumeyer had a suugestion how a Solar still would work, I have to assume he was succesful, he produced enough water to feed his veg garden on board.

If you wish to add vacuum, then I would suggest a water ring pump would probably be best, but mind you in a fully fledged plant the extreme low pressure is achieved by the ~1000 fold reduction in volume between a vapour and the condensate, any eductor/pump is to take out the brine and incondensables. I think the boiling point of water with full 760mm Mercury is roughly 60 degrees, I would be very much interested in viewing a working plant, simple enough, small enough, trouble free, efficient for a small vessel. I haven't had my enthusiasm pill for a few years now.
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Old 24-07-2014, 04:59   #6
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Re: New Distillation type Watermaker, 'Slingshot'

The other problem with vapor recompression is boiling point rise; if the source water has a boiling point greater than 212F (or what ever the condensing temperature of water is at condenser pressure) the compressor has to overcome that hill first. As the boiling pool concentrates, the problem gets worse. If the water is drained to reduce the hill, then more water must be heated. For seawater the hill is about 5F or about 1.7 psi given away.

This tech will work on freshwater, but less well on seawater.

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