In the last week I have had three cruisers call me to help them with their DIY home built water
makers that they could not get to work, and all three of them made the exact same costly mistake...so I thought it would be valuable to warn others because the sound of "I just blew $1000 silence" on the other end of the phone
is painful to hear.
There are a flood of Cat 2SF35SEEL 3.5GPM Hp pumps
hitting the Ebay and Craigslist sites lately from owners parting out their failed XXX-Brand water
makers with energy recovery. The seller of the pump correctly told them that the pump was powered by a 1/2 Hp motor
and the pump seems like a great deal. They then buy a 1/2Hp motor
and sha-zam off they go to make water. Well...that is until they try to run the pump and it just stalls out and trips the breaker when they try to increase the pressure.
They spend days, weeks or even months thinking they have a RO Membrane blockage or plumbing
mistake. Then they pick up the phone
and finally call for help. I've helped out with enough of these DIY water makers to know that before I do ANY Troubleshooting or helping, I always ask the detail question about the equipment
to make sure it all matches up from an engineering standpoint....gulp...
They then find out that to run a 3.5GPM Hp pump at 800psi you need a 2.0Hp motor! Well holy smokes, they can't power that with their inverter
EU2000i like they planned. What's going on, the seller told them the pump was connected to a 1/2Hp motor...why am I now giving them the bad news that they need a 2.0Hp motor? As the guy this morning said, "the bastard lied to me, I'm going to report him to Ebay and get my money
back from PayPal". Well good luck with that.
The seller didn't lie, it's just that both he and the buyers didn't understand what they were selling/buying. On the energy recovery systems the cat pump is not ran at pressure, it is just feeding into the energy intensifier pump so the pump and motor set-up is never ran at load. This lets the 1/2 Hp pump run the 3.5GPM pump...but as soon as you try and increase the pressure on the pump and dial up your 800psi...boom...splat the motor stalls out and trips the breaker due to a basic pump GPM vs motor Hp mismatch for the Hp application. It worked like a champ at no load but now..well....you get it.
We could play with some equations to size the Hp pump with the right motor, but when you boil it down at the end of the day to what's readily available for a DIY guy, here are the Hp motor sizes needed to drive a GPM pump at 800psi.
0.5gpm = 1/3Hp
0.8gpm = 1/2Hp
1.6gpm = 1.0Hp
2.3gpm = 1.5Hp
3.5gpm = 2.0Hp
4.2gpm = 2.5Hp
Now note, if you want to be able to run on a Honda
EU2000i OR a 2000W inverter
, you can't go larger than a 1.0Hp motor.
Double Note, a Yamaha 2000 generator
WILL NOT start the low power
draw 1.0Hp motors like the ones we use with a capacitor start and capacitor run. The Honda will all day long. Sorry, your Blue Yami isn't as good as the Red Honda...
So that helps avoid the powering side mistakes
which are 1/3 of what I help DIY-ers with. Another 1/3 of the problem is their expectations of fresh water production. If I have heard it once, I have heard it 1000 times, that damn ROSA software
and calculations that the engineering types flock to like the black meteor at Mecca. It has as much use to a Marine
batch process water maker as garlic does in attracting dates. No one out there wants to publish this real world data and there is a well known water maker company out there that actually removes the pump manufacturers label from their pump to hide the pumps flow rate to make this data harder to figure out. Ask them these questions and they will say "that's proprietary". It keeps RO more black magic, but being a little contrarian to what is "normal" here we go.
This is a summary of fresh water production flow rates at 800psi, in 68-deg, 32K ppm (normal) sea water. Both from a single
and then double (in series) standard SW30-2540 membrane.
1.6GPM Hp Pump = 21GPH for membrane No 1 and then 13GPH from Membrane No 2 in series.
2.3GPM Hp Pump = 23GPH for Membrane No 1 and then 18GPH from Membrane No 2 in series
4.2GPM Hp Pump = 25GPH from Membrane No 1 and then 25GPH from Membrane No 2 in series.
Now can you run less than 1.6GPM into a 40" Membrane, well you can but I don't recommend it for two reasons:
1. The membrane life will be significantly shortened due to not having enough brine flow to wisk away the salts, changing the Ph and allowing scale to form on the membrane and then plugging them up.
2. The pumps cost the same...the motors cost about the same...so why make 10GPH and have your membrane die in half the time for the same amount of money
. Well the only answer would be because that is all you can power aboard your boat without a generator
...ok...so that is a valid reason, but knowing just how hard it is in the real world of cruising to power a 1/3 Hp or 1/2 Hp motor on DC...I just don't like to sell them and then have to deal with the consequences later of someone not being happy. So no, the client isn't always right and I send those folks to a few other water maker companies that sell those type of units. That's the advantage of loving what I do, not needing to do what I do. It lets me say, no thanks and not just chase every crazy dollar.
One more thing to notice is how the flow rates change (or don't change) with the accompanying change in sea water inlet flow. The membrane has a sweet spot and that's what we try to exploit in our standard units. Also remember that increased GPM flow of your pump costs you Hp (or Amps) so why use a 2.0Hp motor when a 1.5Hp will do the job? Why use a 1.5Hp motor when a 1.0Hp will do the trick?
I didn't start this epic post planning to cover this, but heck, my daughter just graduated from High School
tonight so I'm feeling happy. The final 1/3 of the DIY trouble shooting calls revolve around the issue of getting enough flow into the Hp Pump inlet. Piston pumps do not like to suck, they want to have a flooded head
of sea water pushed into them through the prefilters. The problem becomes when the DIY guy specs out his boost pump, he actually believes the manufacturers rated flow rate. Well, ok...I should disparage the manufacturers, so let me rephrase that. The DIY guy doesn't realize HOW the manufacturers are rating the flow rate from their pumps, there that is better. The manufacturers rate their pumps with a flooded head
of sea water on the inlet and with an open discharge on the other side. Sure some might give you a head lift
curve but once you get the boost pump from paper to the real world of a boat...boom, your 4GPM pump that you were using for your 1.6GPM Hp pump only gets 1.0GPM to the Hp pump inlet after the prefilter and line loss pressure drop. We literally have a PILE of R&D boost pumps that looked great on paper but then bombed out big time when we installed them on the test bench and simulated a real installation
. Not getting enough flow into the inlet of the Hp Pump will cause cavitation and can damage or destroy the membranes.
There you have it...
Little Thursday night DIY water maker tips session that will perhaps save me some troubleshooting calls down the road.