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Old 06-04-2018, 18:55   #1
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Lightbulb Idea for $200 watermaker?

Hi folks.

I have this crazy idea for a vacuum desalination system. I was wanting to see if anyone can poke holes in my idea. lol

So I have been looking at the effect of vacuum on water and boiling point. At very low pressure, water will boil (turn to vapor) at relatively low temperatures, as low as 70 degrees if you can pull hard enough vacuum.

I also discovered that no matter how much vacuum you pull from a system, you cannot pull seawater higher than 33 or 34 feet and the rest will be pure water vapor. I figure if I make a 35 ft tall column from the deck, (attached to the mast for support), that should put it 40 feet above the water level. I would only need to drop a hose with a filter basket over the side for my water intake. (No pesky through-hulls)

It would be possible to use an electric pump for the vacuum. Worst case, I could take a cheap or old air compressor or a compressor from a fridge/freezer to make vacuum, (I've done it before) but I also had the idea to make it entirely energy-independent. I found a video on Youtube where someone made a hand-operated vacuum pump out of PVC.



So THEN, I figured that if a pump could be made to be driven by hand, why couldn't it be driven up and down by windmill? For centuries, windmills have drawn water up from wells so why couldn't one be used to drive a small vacuum pump?

Here is a rough drawing of my idea. Granted, this would only work in warm climates, but the sun on black pipe should be able to heat the water to 90+ degrees.

It wouldn't be 100 gph, but if I'm getting fresh water for free from wind power for a couple hundred bucks investment, I think I'll be pretty happy.

I found a comparable system in a much larger scale so the idea is sound.



All I have to do is run the exhaust of the vacuum pump into the water tank. That will pump vapor out of the vacuum down tube which will become water again once it is no longer exposed to vacuum. That's one reason I considered PVC pump rather than a motor-driven pump; to prevent contamination.

Ok. Let me know what you all think. Thanks.

Ron
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Old 07-04-2018, 09:18   #2
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Re: Idea for $200 watermaker?

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Old 08-04-2018, 07:53   #3
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Re: Idea for $200 watermaker?

I once did some back-of-envelope calculations on vacuum (flash) desalination. Since I run a power boat, I could assume a higher temperature for the source water (heated by the engine jacket water.)

Even with relatively hot water to start with , the sticking point was the cost of getting a high enough vacuum to flash the water to vapor. The pump was expensive, both to buy and in power consumption, and maintaining the vacuum was an issue. Then there were the mechanical issues of keeping the distilled water separate from the source in a moving boat. Maybe using a tall vertical tube would help address that.

This can be and is done at an industrial scale. It's done on cruise ships. I suspect it's not done at smaller scales for economic reasons, not because it doesn't work.
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Old 09-04-2018, 01:25   #4
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Re: Idea for $200 watermaker?

Thank you, Capt Tom for your reply.

Im no engineer I'm just kind of spit-balling the idea to see if anyone has been able to do such a thing.

I do a lot of fiberglass work, including vacuum-bagging. Ive got a vacuum pump that runs off 110. In a pinch, I have used a small air compressor by plumbing into the inlet side for vacuum. The best system Ive used is a venturi that uses positive air pressure to create vacuum. That little guy pulls a lot of cfm but it requires an air compressor and I would like to see this work without machinery.

I really want to try the manual vacuum made from PVC. It might get tiring to pump on that thing for an hour, but in a pinch, when the poop hits the fan, the motor is down and the batteries go dead, I like the idea that I could still make fresh water by hand.

Im in Ohio so its way too cold to try this, but come summer, I think Im going to see if I can draw pure water out of the chlorinated swimming pool. lol. Testing strips will tell the tale.
Too bad I don't have a hot tub.

I would be very curious to hear if anyone has tried this. It just 'seems' like such a simple idea.
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Old 09-04-2018, 03:49   #5
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Re: Idea for $200 watermaker?

Two big issues I see:
- That's a large contraption to be keeping on a small sailboat, particularly so since weight aloft is considered a bad thing on sailboats.
- Power demand is likely going to be higher than a standard reverse osmosis system.

PVC would be pretty akward but what if you used something like PEX tubing. I'm not sure if it would resist collapsing but it's pretty stiff. You could get a 70' length and pull the middle up with a spare hallyard. That way it could be stored in a hatch coiled up.

Preheating of water could be done with a simple solar shower or similar home built container. On a hot summer day, temps north of 110F should be achievable.

Also, do you need some sort of condenser? Otherwise, do you just wind up pumping moisture laden air into and out of the water tank before it can condense out as liquid water. Shouldn't be hard to incorporate some coils of tubing in a water bath.

Sounds like a fun science project but not sure how practical it would be.
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Old 09-04-2018, 04:26   #6
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Idea for $200 watermaker?

It’s actually not a crazy idea. It can be a lot less expensive than reverse osmosis, assuming you optimize the process by using multiple chambers each operating at the ideal pressure / temperature, and harness waste heat (eg from Diesel engine). Per Wikipedia, 60% of the world’s desalinated water is produced this way:
https://en.m.wikipedia.org/wiki/Mult...h_distillation
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Old 10-04-2018, 00:07   #7
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Re: Idea for $200 watermaker?

Quote:
Originally Posted by Seafarer7 View Post
It’s actually not a crazy idea. It can be a lot less expensive than reverse osmosis, assuming you optimize the process by using multiple chambers each operating at the ideal pressure / temperature, and harness waste heat (eg from Diesel engine). Per Wikipedia, 60% of the world’s desalinated water is produced this way:
https://en.m.wikipedia.org/wiki/Mult...h_distillation
The problem with your example is that is a massive industrial system. No question it is technologically feasible but the real issue is the practicality of installing it in a small cruising boat with limited space and power.
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Old 11-04-2018, 14:05   #8
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Re: Idea for $200 watermaker?

Seafarer, thank you for that resource. That is awesome! I'm uncertain if I would even need a heat exchanger because I'm planning to pull to near-full vacuum and (If I read this right), I only need the water to be 70-80 degrees. That's why I was thinking black PVC for the seawater tube... idk?

Valhalla, I don't "think" I will need a condensor. I 'think' that as soon as the vapor returns to standard pressure, it will turn back into 70-80 degree water. That's why I was thinking of using a PVC hand pump or even a refrigeration compressor because it is designed to handle liquid. A standard vacuum pump 'may' not be able to handle the liquid. (In order for this to work, the vapor has to pass through the vacuum pump itself.)

I planned to just attach this contraption to the mast so it wouldn't be obtrusive. Plus, you would occasionally have to drain the tube anyway, so while underway, I would leave it empty. It's only 35 feet and most masts are taller than that.

For me, I have a bum ankle so climbing up and down a companionway ladder is not very practical. I will almost certainly need a catamaran. With a cat, I could make water underway... I think? lol

Ok; double-check my math. With a 3" tube full of seawater, that would be about 1.8 Cu. Ft. of water so about 13 gallons at 110 lbs +-. Wow, doing the numbers, that's probably not enough. If there's 13 gallons of saltwater, I imagine I could only pull 5-6 gallons of water out before I would have to dump the brine and re-fill.

Maybe a 4" tube would be better.... Yep. That would be 24 gallons at 195 pounds. Still less moment than a man climbing to the top of the mast...?

It may not be perfect, and RO may be easier. But again, I like the idea that I could make water without power if SHTF. And, I could rebuild the whole contraption for $200 in just about any country. I'm not going to have to tote around a broken watermaker, hoping I can find parts.

I also like the idea of having a windmill to do the pumping while I have a beer and watch it go roundy-roundy. lol

I'm just spit-balling here. I do appreciate you all humoring me with the discussion.

Ron
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Old 11-04-2018, 17:50   #9
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Re: Idea for $200 watermaker?

Quote:
Originally Posted by muskrat45335 View Post
It may not be perfect, and RO may be easier. But again, I like the idea that I could make water without power if SHTF.
Well the energy is going to have to come from somewhere, even if it is the food you eat ... so how much will you need? Here's a quick back-of-an-envelope calculation ... I'm not saying it is correct, but it is the sort of calculation you ought to think about:

1) How much force is needed for the pump?
Let's assume something like a bicycle pump:
pump area = 1 in^2
pump stroke = 12 in
atmospheric pressure = 15 psi
vapour pressure of water at 25C (298K) = 0.5psi
force needed for pump stroke: 14.5 psi x 1 in^2 = 14.5 lbs
That seems reasonable.

2) How much water vapour will we pump out on each pump stroke?
volume pumped in at 0.5 psi per pump stroke = 12 in x 1 in^2 = 12 in^3
volume pumped out at 15 psi per pump stroke = 12 in^3 x 0.5 psi / 15 psi = 0.4 in^3

3) So how much liquid water would this be if it *ALL* condenses?
1 mol of water = 18g = 0.018L
1 mol of water vapour (at 298K and 1 atm) = 22.4L
1 in^3 of vapour = 0.018/22.4 = 0.0008 in^3 of liquid

0.4 in^3 of vapour = 0.4 x 0.0008 = 0.00032 in^3 of liquid
That's not much ...

4) How long will it take to get 1 gallon?
1/0.00032 = 3125 pump strokes per in^3
= 3125 x 221 = 690625 pump strokes per gallon
= 690625/3600 = 191 hours pumping at 1 pump per second = 8 days!
This is looking bad.

5) But we need that gallon of water in one day not 8!
We could pump like crazy at 8 strokes per second! or ...
Use a bigger pump ... but:
8 in^2 and 12 in. pump stroke = 14.5 X 8 = 116 lb force to operate it
4 in^2 and 24 in. pump stroke = 14.5 x 4 = 58 lb force to operate.
And we still have to pump for 24 hours!

6) How much energy will it take to get one gallon:
energy needed per pump stroke: 14.5 lb x 12 in = 14.5 ft.lb
energy needed per gallon = 690625 x 14.5 = 10000000 ft.lb
by hand pumping = 10000000 ft.lb x 0.00032 = 3200 Kcal
. That's a lot of eating!
and for a comparison:
by electric pump = 10000000 ft.lb x0.00038 = 3800 Wh = 3800/12 = 316 Ah at 12V.


I can't be bothered to double-check this ... but before you build this hoping to get free water, you might want to try and find a mistake in my calculation.
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Old 11-04-2018, 21:52   #10
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Re: Idea for $200 watermaker?

Awesome! Thank you for that. Maybe you're right. That may be why no one has done this.

The only place I get hung up on your calculations is the molar conversion. I'm not a physicist, but I don't 'think' the molar calculation applies since we already accounted for the change in volume when we account for change in pressure. (I assumed 7.48 gallons / ft^3, standard day)

You may very well be correct. I just don't know.

I am also assuming that all of the air has been removed and we are left with 100% water vapor in the system. Maybe that's not feasible? Idk.

The pump I am looking at building is 1.5" diameter and I'll probably use a 24" stroke. That's about 25 pounds pressure and a volume of 42.39 in^3.

This is how my calculation went:

42.39 in^3 .. x.. ft^3.. x .........gal ..x.. 0.5 psi = 0.000109319 gal/stroke
...stroke....... 1728 in^3..... 7.48 ft^3... 15 psi
=> 9147.5 strokes/gal @ 3600 strokes/hr = 2.5 hours/gal

That would kind of suck to pump for 2.5 hours for a gallon of water, but it might keep you alive. If I could rig a windmill to do the pumping...?


So I guess the question is this: Does water vapor in a vacuum take up 30x the volume of water or 1244x the volume of water?


This is beginning to make my brain hurt.
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Old 11-04-2018, 22:17   #11
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Re: Idea for $200 watermaker?

At STP water vapor is 0.804 g/liter while liquid water is 1000 g/liter, so 1244 is the correct value at STP. At 0.5 psia vs 14.7 the volume in the vacuum state is 14.7/0.5 x 1244 = 36,574 liters of vapor at 0.5 psia = 1 liter liquid water at 25C and atmospheric pressure.

I've done vacuum distillation system installations. They take a lot of power. Can't imagine trying it human powered. Most commercial systems are put in where waste heat is available to allow a warmer water temperature and higher pressures.
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Old 11-04-2018, 22:28   #12
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Re: Idea for $200 watermaker?

One more thing, why do you need a 35 foot column? You only need that if you have a bottom open to atmosphere. Otherwise you can do this in a vacuum tank one foot tall. The headspace will be at vacuum, but the water will stay down at the bottom. A tiny control valve to let in as much additional water as you remove (which will be very, very little). All you need is a tank sufficient to keep the liquid from sloshing around to the wrong places when the boat is underway, and baffles can go a long way to helping with that.
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Old 13-04-2018, 11:22   #13
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Re: Idea for $200 watermaker?

Ok. Well, I guess that answers my question then. Thank you for clearing that up.

The reason I was thinking the 35 ft column is twofold:
(1) pvc pipe is cheap and can be found anywhere.
(2) Besides the pump, there would be no moving parts. It would be just my luck that I'd be in the middle of nowhere and the "tiny control valve" would bugger up. Where do you find one of those in a third-world country? Plus, the column would run right along the mast so it wouldn't be obtrusive.

You know a lot more about this than I do. It just seemed like such a simple idea at the time so I threw it out there.

So, then how many cfm would you need to be able to pump to get a couple of gallons an hour? (Like maybe a belt-driven pump?)

What got me thinking on this whole idea was a documentary I watched about Dean Kamen and the 'Slingshot' that he invented to bring cheap clean water to people in need. I 'thought' they said he figured out how to make it economical. idk. I'll have to watch it again.
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Old 13-04-2018, 11:41   #14
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Re: Idea for $200 watermaker?

Well, a vacuum pump should do it, but as mentioned they are expensive. Not sure how much the pump capacity would need to be.
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Old 13-04-2018, 14:51   #15
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Re: Idea for $200 watermaker?

1 gallon = 3.785 liters
1 cf = 28.3 liters

1 gph = 3.785 * 1244/28.3/60 = 2.77 scfm which doesn't sound like too much, but...

2.77 * 14.7/0.5 = 81.5 acfm (actual cubic feet per minute), so you need a vacuum pump capable of moving 81.5 acfm at 0.5 psi.

At 1 gph you still only need a small tank (say 5 gal), fill it up, then run until you have 5 gallons fresh water, fill it up again (and you could use a standard valve to fill it, no control valve).
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