The watery and smelly sides of life.
Posts: 25
Joined: Thu Jul 02, 2009 8:00 pm

Re: Watermakers

Post by shiraz02 »

Just had a quick look at the Dow ROSA programme.
For seawater of 35000ppm product water ppm is :
membrane1- 250
membrane2- 325
membrane3- 425.

Most guidelines have 500ppm as max.

Smooth Cruiser
Posts: 583
Joined: Thu May 11, 2006 1:51 am
Location: Brisbane

Re: Watermakers

Post by Smooth Cruiser »

Not sure that I fully understand your 3 membrane system sean? From what you say the reject water from membrane 1 goes to membrane 2 and the reject from 2 goes to 3. If this is the case I don't see how you can generate enough pressure at the start of the chain to have a differential pressure of 750-800kPa across each of the 3 membranes? I would have thought all 3 membranes needed to be fed from the same high pressure feed source (ie in parallel). Maybe I am reading your posts wrongly though?
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Joined: Thu Jul 02, 2009 8:00 pm

Re: Watermakers

Post by shiraz02 »

The membranes are in series ( all boat RO are done this way to my knowledge) 3 is the most you can use to get any reasonable output from the last membrane. There is a flow rate and pressure decrease at each membrane. With a feed pressure of 850psi , pressure at membranes is:

At 850 psi ,product in gallons from membranes should be ( at feed flow of 4gpm):

When determining what pressure to operate the system you need to monitor the flow rate of membrane 1 so that it doesn't exceed the rating for the membrane, pressure is arbitrary really as you can run it higher or lower depending on salinity/temp etc so that you get the maximum flow rate of product from membrane 1 without damaging it.The common misunderstanding is that pressure is set, but really you change it to suit the situation, within the bounds of course of the membrane vessels and max membrane pressure. It is only membrane 1 that is vital to monitor for this reason as product from the next 2 is less.I have however incorporated flow meters to each of the membranes so I can monitor the health of each, as the last one will fail first. I have also got a flow meter for feed water.
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Re: Watermakers

Post by 44c »

mahnamahna wrote:I did a little experiment in the house kitchen sink, not really scientific but indicative, at full on I can fill a litre in 8 seconds. So that equates to 7.5 lpm or 450 liters per hour, so yes I guess the 25 liters per hour unit is about 1/20th the pressure of a house tap on full. 400ml per minute. Or to put that into perspective, in the same 8 secs I can fill a liter at home, I would get about 50ml on the boat or about the amount of cough medicine you have when you have a cold, it aint much.

So it trickles out. But I should be able to attach the product out to a 2 way valve, turned to test until I am satisfied it is good, then it can be redirected to go into the line direct to the water tank, without the need for a pressure reduction valve. I was thinking I could put a one way valve in the line so that water always stays in the line that way any push at the watermaker end results in water flowing into the tank at the other end (in the same way a hydraulic line works on the steering). Or is this simply not required? (confused by 44c saying yes can lift to tank and Jim suggesting no it just trickles out)

Thanks guys.
:lol: :) :) A little humour is always nice!
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