Tuesday, June 10, 2014

Rethinking Water-maker raw water pre-heating

A while ago I posted about how I was going to try out re-plumbing the Kubota engine cooling system to pre-heat the sea-water in order to increase the output of our RO watermaker:

 Like all shade-tree ideas it is based on some sound reasoning:  RO membranes output is very temperature dependent.  And the warmer the input water, the more one can expect to get out of them.  So, last summer I re-routed things like this:

Good concept, bad idea.  Keep reading!

Note how the water flows through the Kubtoa engine 1st, THEN on to the watermaker!  Ha, Free Heat, Wow - really putting out the fresh water now..  This winter I am thinking about doing some more plumbing to the poor little Kubota, working to see if I can better manage its cooling and gets its operating temperate up (currently it runs around 160f or so, a bit too cool I think).  After thinking of several approaches the one I have settled on trying is a variable bypass of sea water around the heat exchanger.  Have located a sea-water safe 12v valve and will control it via the Arduino controller.

While I was at it I started wondering:  Just how much temp raise am I getting into the water maker using the above? Trying to measure it using an external IR thermometer did not give me too much of a raise.  And after all there are a few downsides to the engine-1st approach:
  • Increased hoses and complexity.  Starting to look like an early 80's GM car engine with all its hoses and such.
  • Health concern in case of faulre of engine heat exchanger, mixing coolant in with sea-water (I am not sure what the RO membranes would do and have been using low-tox antifreeze just in case)
  • Interjection of Zinc into stream of water passed on to the RO membranes, again not sure what all that does..
  • Lost fresh-water flush of engine when flushing out RO membranes after each run.

This last one kind of bummed me, as with the old plumbing every time I fresh-water flushed the RO system, the outflow would be passed down and fresh-water flush the engine as well.

So, Just for Fun I did some math this afternoon:
  • A BTU is defined as the amount of energy needed to raise 1lb of water 1f
  • A gallon of Diesel has 138,500 BTUs
  • The Kubota burns about .25 - .3 GPH of fuel
  • Seawater Pump pumps about 4GPM (per spec)
  • Rule of thumb says 25% of fuel burnt goes into the coolant (another 25% into work, and 50% out the exhaust).

So, lets do some math!
  138,500 BTU/Gal * .3 GPH  * .25(25% of heat into the coolant)  = 10,387 BTUs/hr of heat into the sea-water.
  10,387 BTU/hr = 10,387 Lbs*F/hr  (1 BTU warms 1lb of water 1f)

  4 GPM * 60 Min/Hr * 8.33 lb/gal   =   1999lbs/hr of sea water moved through the engine.
  (Sea water is actually heaver then this, but as this is the "back of the Blog" doodle, lets just use 8.33lbs/gal)

  Combine them:   10,387 Lbs*F/hr = 1999lbs/hr

  Solving for F we get:  F =  1999 / 10,387  = 0.19f temp raise.

Hum...  Less then 1 degree F raise.  Even if I captured ALL the waste heat out of the engine AND exhaust I would be looking at less then 1 degree F.

Bottom line:  It was a stupid idea and I will be taking it out next when I return to the boat this summer.

(And hey:  Math guys, dbl check I did this right.  I even worked to carry forward the Units;  my Physics teacher would be so proud!)

 4/8/2015:  Well, a Math Guy did dbl check my figures, and gave me an 'F'  (See comment below from Bruce).   Forest from Trees, did not do a simple 'sniff test' on that last formula - and the 'units' did not help me any as they were the same (and hence canceled).  So, Bruce, here we are - showing my work:

Combining them:  10,387 BTU/hr = 1,999lbs/hr  * Xf

or:  10,387 lb*f/hr  = 1,99lbs/hr * Xf

Solving for X -->    (10,387lb*f/hr) / (1,999lbs/hr) = Xf

the lb/hr cancel (See, units to the rescue!)  leaving us with:
  10,387f / 1,999 = Xf  

Xf = 5.19f     And even the units match!

So, bottom line:  I should expect a 5 degree F raise in the feed-water temperature into the RO membranes.. Using this table from Dow:

Temperature vs Rated Fresh Water Production

Degs F
Degs C
% of Rated Production









and starting with 50f water (perhaps a little warm here in the PNW), that 5 degree rise might get me an extra 7% output.   Perhaps worth it.   As of 2015 I had replumbed to remove this pre-heating.  I think I will take some actrual inlet / outlet measurments from the engine heat exchanger and see how those compair to the calculations - then deside what to do...

Thanks again Bruce, and like your You-tube clip of your rig: 



  1. Hello Viking Star! Congrats on being one of very few that can deal with RV/marine problems using science. Saw your watermaker post (6/2014) while looking carefully at your curves for charging batteries. Thank you! Now, I noticed that you have your final heating equation upside down....Should be BTUs/lbs H20 = deg rise F. Right? About 5 deg in your solution. Thanks for getting my gray matter warmed up. Bruce R. brucer@timelinedesigns.com

    1. Well, I did ask folks to dbl check me, didn't I!

      Thanks so much, see what you think about my 'revised' math :-)