The guy that is most knowledgeable here on the site is Tellie, he is in the expert category. But I will try my best here.
Three membranes in parallel will put you up against the minimum concentrate levels. I don't know if three in series is feasible, I know two are. My guess that three in series is fine but the product will lessen for each membrane because the salt
content will be higher.
40 gallons/hour is a lot of water and personally I think you can get that with two membranes operated in series even at lower temperatures. My suggestion is to download Dow's ROSA and run the program. I will comment that it took me a while to get meaningful results and I need to spend more time on it for my project
I am building a system that uses a Armitsu 307 pump with an output of only 3.2 gal/min. I am also going to turn the pump with a two HP 1725RPM motor. I was going to use a larger pump but I could not find a 6 pole 3HP single
phase motor; they are available only in 3 phase. The idea of 6 poles is the RPM
is less., abouit 1150 or so and the pump output is a direct fucntion of RPM
. If you get 1 gallon/min at 1000 RPM, you would get 2 gal/min at 2000 RPM. Now of course this is just an example, those partuclar RPMs will be found only in expensive special case motors such as servos.
Back to heat exchanger. I don't think one is needed plus they add too much complexity as noted by GordMay. A membrane will yield about 12% product and for simplicity sake, assume two in series have the same product output. So a 3.5 gal/min pump will give about .12(3.5)60= 25gal/hr. Two will give a total of 50 gal/hr. Three will add maybe another 20. Try ROSA and please keep us posted. There are more than a few trying to build there own systems and misery loves company.