I don't know the Cruise system, but I checked out their website and am inclined to agree with their product design "philosophy", ie no proprietary components. (Most other desal system providers do as well, but they're not as honest about it.)
As you may know, desalination systems aren't rocket science -- they're basically a low pressure pump that supplies sea water at 10-30 psi to two filters of progressively smaller micron ratings to a high pressure pump which boosts pressure to 600-800 psi to the RO membrane. Salty water stays on the outside of the membrane, "fresh" water on the inside/core. Product water can be filtered or not, but once it's out of the membrane it should be "drinkable". The power for the high pressure pump in the gmp range you've mentioned is usually a low horsepower AC motor. (The Cruise 20 gph system uses a gasoline powered Honda generator, but you can use a marine diesel powered AC genset, if available). Power to the low pressure pump can be of the same voltage, but can be 12V off the boat's DC electrical system.
You can buy the components separately and build your own system, provided you have the knowledge to engineer the system. The big ticket item is the high pressure pump (about $1000-1200 if you're driving a 2.5" x 40" membrane), followed by the pressure vessel and membrane (~$500-650 from memory) and then the AC motor to drive the HP pump ($350-500). The filter rack is $80-100 and then there are various pumbing bits and pieces that make up the rest of the system. These include various pressure gauges, pressure shut off switches, hoses, connections etc., and probably add another $300-400 in total. (And don't forget the AC power source....Cruise has the small Honda generator - Price??, or you can use a marine genset $ thousands).
Companies such as Cruise are adding value through the engineering of the system -- they figure out which OEM components to put where to make X gal/hr at Y power consumption. From the looks of their website it appears as if they've designed a fairly simple system that will require some user input to work effectively. It doesn't seem to have the "push the button and forget it" features of some of the other "branded" systems".
One thing I am curious about is that the Cruise 20 gph system seems to have two 40" membranes -- my system has only one and will produce ~ 20 gph by itself. The difference may be in the power consumption of the HP pump -- the Cruise system seems to run on 120 V (from the Honda generator) at 13 amps, while my system runs at 240 V and 14 amps. My guess is that their pump isn't putting out the same pressure as mine and the production of the membranes is less????? You might ask for the specifics of the pressures -- you can produce potable water at 500 psi, but not much -- get the pressure up to 750-800 psi and production shoots up. (Note: I have an old system and the AC motor / pump are not in the best shape -- I get 18 gph at 240 V, motor drawing 14 amps, membrane input at 650 psi with an 80 deg F salt water supply).
Another thing about "simple" RO systems -- you may have to do more of the "work" yourself rather than having a sophistocated electronic control system do it for you. We have a fairly simple system on our boat and it requires the following inputs from the operator:
- turn on on the LP pump and visually check pressures at the either side of the filters
- if pressures are OK, turn on HP pump (at zero pressure to the membrane)
- with water flow confirmed, increase pressure to the membrane (requires turning a knob that constricts the exhaust water).
- with pressure up at production level, test salinity of product water.
- if salinity is good (we use a hand held $20 TDS meter), turn the valves that allow the product water to flow to the boat's freshwater tanks.
- measure the flow rate manually (requires a stop watch and 1 liter container, and some high school math).
- when product run is over, manually shut down the sytem by reversing the steps -- reduce membrane pressure, turn off motors/pumps, etc.
- if the system is to be shut down for more than a few days you'll need to flush the system with product water (note -- you can't use tank water for flushing unless all the water in the tank has come from the desal system. Clorinated tap water from municipal systems will destroy the membranes). On our boat I make and store 30 or so liters of product water to use for flushing and pickle-ing the system with biocide (for long term storage). You may want to ask your system supplier what you have to do to produce water and what the system does for you.
Sorry, but that's probably more than you asked for...... but two more points:
1. I don't think you can get high (20+ gph) out of DC systems. People I know who have DC systems run them for long periods at low (3-5 gph) production rates. Providing pressure that gets the most out of a 40" membrane requires lots of watts. On our boat it's 240V x 14amps = 336 watts. To get that power out of a 12V system requires 280 amps of draw from the batteries/alternators. That's a lot of amps, so if you want a high gph system I'd look to AC power sources that are independent of the boats 12 V systems.
Edit -- I think I missed some zeros in the V x A = W calculations above.
2. RO system output is very sensitive to sea water temperatures. In the tropics with 80-84 deg F water your production will be 2 X what it will be in northern waters where seawater temperatures can be in the 50-60 range or lower depending on latitude / location. Ask the system provider the water temperature their rating is done at and compare that to the water temps in your area.
One final thought -- I've found that having a desal system is a convenience, not a necessity, in most cruising environments. If you have access to shore based water, and you have reasonable tankage, there's really no reason to have a desal system. If you're crusing to far away places where you won't have access to local water suppies, it's a different story. We've spent a lot of time cruising the Caribbean. One season we relied on the RO system -- we burned a lot of diesel, put hrs on the genset and had very soft hair (women love RO water shampooing). Another season we paid 10-15 cents / gallon for our supply and didn't use the desal system. We burned less diesel, put fewer hours on the genset, and shampooed with harder water, which rinses quicker and so you use less of it. In my book, it's pretty much a wash economically and it all depends on how difficult it is to secure shore based supply.
Last edited by billyruffn; 07-02-2011 at 03:42 PM.