Originally Posted by SVI
I understand. But not to that effect. 8% is the recovery rate so the feed water in the 3rd is only 16% more salty.
Not the 8% recovery rate misunderstanding/myth again?
This 8% myth has screwed up more people in their understanding of how a Dow Filmtec SW30-2540 membrane works than perhaps any other single
technical issue except the ROSA software
(that is useless for marine
water makers by the way).
The 8% Recovery confusion/myth comes from taking the data provided by DOW for a continuously operated RO facility and then erroneously applying it to a marine
batch process water maker. What matters in a continuously operated RO facility is membrane life, due to the capital cost implications. So in this situation you put the maximum sea water flow rate through the membrane for maximum membrane life and powering that flow isn't a concern. But in a marine batch process, where pump size, flow rate and power usage is important, you don't use the same design criteria.
Rarely if ever on a Marina batch process water maker is it the case where you can increase your sea water flow rate and max out your fresh water production rate, which is what would happen if you design your system based on the 8% recovery rate. Typically we "over membrane surface area the system" as a way of getting more fresh water production without having to pay for the energy use. A more common recovery rate can be closer to 20%...but ROSA will tell you that you system will die as would the folks that don't understand the real life applications.
Here's some data from the SW30-2540 Membrane all at 800PSI, 68-degs F with 33,000TDS.
1.6GPM feed water = 21GPH of product water
2.3GPM feed water = 25 GPH of product water
4.2GPM feed water = 28 GPH of product water
Notice that the increase in product water output is not linear with the sea water input. So why pay for the power necessary to push 4.2GPM into the system? Well on a continous 24hrs/day 365days/yr system sure. But on a batch process, where you have different design criteria like maximizing product water output while minimizing power, you design it to what you want/need. If you wanted to maintain a 8% recovery rate on the above data points, you would need to decrease your system operating pressure to set the product water production at 8%.
Many people read the DOW SW30-2540 Spec sheet and fail to realize that they are looking at. 8% recovery isn't a limit. 8% recovery isn't their recommendation. All 8% represents is a data point for design, telling you that on a continuous running RO unit you can expect 7 year membrane life at a 8% recovery. But you can run at 20% if you want based on your application.
Here's what controls fresh water Production that you can easily control:
1. Membrane Surface area
2. Sea Water Flow Rate
3. Operating pressure of system (typically 800-900psi)
Now here are the items that vary fresh water production that you usually just live with:
1. Temperature (> temp = > production)
2. Salinity (> Salinity = < production)
All that for a simple "how much will I make question"....Rich, you are nuts.
Well yes and I have a pile of Miami Boat Show
credit card receipts that need to be entered into Quickbooks, so I would rather talk water maker tech specs than do that hell work any day!
We use a 4.2GPM pump feeding 3 SW30-2540 membranes in series for our 65GPH water maker. So you will mostly likely be between 55GPH and 60GPH with your set-up.