Desalination RO Membrane performance figures variations for 21 and 40inch???

[vas]

Vas,
Your figures seem pretty much correct and you have 535lph output from the pump, this should give around 27lph fresh water at 5% recovery rate from your 21" membrane. Not sure why you are getting 37lph from your system though??
One reason could be that the membrane is past it's useful life - what is the salinity level (ppm) of the product water?
You said in your first post that you bought the unit secondhand a few years ago so I am guessing that you don't really know how old the membrane is or how well it has been looked after. If so it may be a good time to bin the 21" anyway and start with new membranes that you can maintain as required and will always be a known quantity.
Your two options mentioned above are both viable means of increasing production, although some people say membranes should be plumbed together in parallel rather than series, but given the design of your endcap and manifold it looks like series is by far the easiest way to add another membrane. The downside of series connection is that the feed water to the second membrane will be a higher salinity so output may be less than optimal from the second membrane. The Dow Technical Manual does not really recommend one method over the other so it is probably not too important. Our two 40" membranes are in parallel and output is around 150lph as expected, never tried them in series so do not have a comparison.

From everything you have said I would suggest that the most cost effective means of getting the max fresh water from your system is to add a new 40" pressure vessel and membrane and to fit a new membrane into the existing 21" vessel. This setup should yield around 70lpm of fresh water. If you also add the VFD to increase the pump speed and flow then you could expect to get more fresh water.

Cris,

thanks again, you could well be right and salinity may be higher than it should've been. Yes bought the system s/h from ebay, sat on the garage for three years and only this summer I finished the installation and fired it tup. I have a handheld TDS meter and when I fired up the system it showed 150-180 when the RO I have at home (have crappy water on tap and I cannot stand it) is more. Further the salinity probe on the HRO system is not complaining, as per the book one minute after starting pressurising the system, the light comes up and water starts to flow.
Yes, parallel is out of the question, what I'll do is drill the extra hole for the product water of the external vessel to merge with the onboard one and be done with.
Going through the numbers with proper condition membranes I should:

without VFD: 21+40inch ~= 70lph
with VFD: 21+40inch ~= 85lph

For the sake of 120quid (which I can use in another project anyway!) I'll get the VFD and organise the extra membrane. Actually since the 21inch may be slightly buggered and overproduces, I'll keep that in for next season and see how it goes

Now, serious Q, where can I get a reasonably priced 800psi (well 1000 don't want it disintegrating at 805psi...) pressure vessel???
Don't want to order from US.
Alternatively and having access to lathes, is it feasible to built a pressure vessel, any drawings, experiences?
Regarding membranes I had a reasonable quote from lenntech a few years back, not sure they're still in business.

It looks to me that you don't need more water flow. It's the pressure that forces the water through the membrane. You currently have plenty of flow to achieve 800psi even with an additional 40" membrane. Increasing the flow rate won't give you any more product water with your 21" membrane. Fitting another membrane is the only solution that will increase your output. If you can't achieve 800psi once you have added the second membrane then you could consider changing the motor for a 2hp version

Sorry, but doesn't seem right. So you recon a 2GPM and a 5GPM pump that will achieve 800psi will produce the same amount of water from the same membrane? If so, there wouldn't be any point in using the 8% permeate rule to calculate output according to the volume of water that goes through the membrane. According to my calcs, my current setup SHOULD produce 26.75lph and with the VFD it should reach 33lph.

These motors are usually capacitor start motors. I don't think you can simply fit a VSD to this type of motor. You would need to change the motor as well.

two ppl who saw that and the specs of the motor say it will work with the proposed VFD (variable frequency drive) - you mention VSD I guess it may be done differently. Well frequency does influence the speed so should do what I need.

watermaking turns out to be rather confusing once you don't want to simply go and buy a ready made solution.

cheers

V.

cheers

 

[geem]

Cris,

thanks again, you could well be right and salinity may be higher than it should've been. Yes bought the system s/h from ebay, sat on the garage for three years and only this summer I finished the installation and fired it tup. I have a handheld TDS meter and when I fired up the system it showed 150-180 when the RO I have at home (have crappy water on tap and I cannot stand it) is more. Further the salinity probe on the HRO system is not complaining, as per the book one minute after starting pressurising the system, the light comes up and water starts to flow.
Yes, parallel is out of the question, what I'll do is drill the extra hole for the product water of the external vessel to merge with the onboard one and be done with.
Going through the numbers with proper condition membranes I should:

without VFD: 21+40inch ~= 70lph
with VFD: 21+40inch ~= 85lph

For the sake of 120quid (which I can use in another project anyway!) I'll get the VFD and organise the extra membrane. Actually since the 21inch may be slightly buggered and overproduces, I'll keep that in for next season and see how it goes

Now, serious Q, where can I get a reasonably priced 800psi (well 1000 don't want it disintegrating at 805psi...) pressure vessel???
Don't want to order from US.
Alternatively and having access to lathes, is it feasible to built a pressure vessel, any drawings, experiences?
Regarding membranes I had a reasonable quote from lenntech a few years back, not sure they're still in business.



Sorry, but doesn't seem right. So you recon a 2GPM and a 5GPM pump that will achieve 800psi will produce the same amount of water from the same membrane? If so, there wouldn't be any point in using the 8% permeate rule to calculate output according to the volume of water that goes through the membrane. According to my calcs, my current setup SHOULD produce 26.75lph and with the VFD it should reach 33lph.



two ppl who saw that and the specs of the motor say it will work with the proposed VFD (variable frequency drive) - you mention VSD I guess it may be done differently. Well frequency does influence the speed so should do what I need.
VSD or VFD it's the same thing.

watermaking turns out to be rather confusing once you don't want to simply go and buy a ready made solution.

cheers

V.

cheers

My pal has a CAT 247 pump running three 40" membranes. The motor is 1.5kw and running at 1450rpm. The CAT pump is rated at 13.6l/m at 1750 rpm. The membranes are over ten years old but they still produce 250 l/hr. This is the same set up as mine but I only have a single 40" and a 21" membrane. You don't need a bigger pump you need more surface area of membrane. It doesn't matter how much seawater you are moving, you are running the system pressure at 800 psi. More pressure will increase the flow rate of permeate. Less pressure will reduce it. It doesn't matter how much seawater you are moving through the system up to the limit of membrane surface area. If you can't reach 800 psi on your system because you have so much membrane surface area, you will need a bigger pump or a larger motor moving faster on your existing pump. You simply haven't got enough membrane area. Your old 21" membrane may be fouled which will reduce its surface area. My system originally had a pair of 21" membranes. It now runs a 21" and a 40". It will have another 40" installed soon when I can get around to it. This should push the output close to 230 l/hr. I understand that there is a limit on what can be squeezed out of thee 40" membranes in series before there is just salt left behind. A watermaker guy tells me that the last half of the third membrane in series suffers from salt build up. He believes that the two 40" and one 21" membrane set up will produce very close to a three 40" membrane system. Good luck