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Re: GFX vs home brew - Page 16

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Posted by Robert Gammon on April 20, 2006, 11:05 am
Robert Gammon wrote:

Warm water to drink is not good, BUT neither is really cold water
either.   Our preference is for cold water, especially in HOT climates.  
However we will usually drink MORE water at a time if the water is in
the 60-70F range than in the 40-50F range.   The really cold stuff, we
just sip every now and again.  A bit warmer and we'll gulp it down.

So in areas that have 40s year round average cold water inlet temp,
treat the WHOLE house to GFX treated water, raising cold water to 65F
helps us drink more water and has no significant affect on cold water
cleaning performance in the laundry.

Note we almost NEVER use the cold cycle in our laundry, so if Warm cycle
water temp changes from 85F to 95F, so what!!!

Posted by daestrom on April 20, 2006, 11:05 pm

Still seems messed up.  The water going into the bottom of the GFX is a
mixture of say 2gpm cold water (flow rate set by the shower head), and 4-5
gpm hot/warm water from the heater bottom (flow set by the pump).  In my
book that means the water going into the GFX is *warmer* than the non-pump
setup.  And that means the improvement in heat-transfer you get from the
higher flow rate is offset by the higher temperatures in the freshwater
side.  Warmer freshwater going in means warmer greywater going down the
drain.  And that's a bad thing.

If the circulating pump runs at 4 gpm and the shower at 2 gpm, then it is
crucial to know the tank bottom drain temperature.  Using the application
http://www.gfxtechnology.com/GFX-STAR.pdf    we see that we can expect the
efficiency to rise from 57% to 67%.  But that is based on the Tcold going
into the bottom of the heat-exchanger.  So if Tdrain-in is still our same
100F, and the bottom of the tank temperature at the recirc pump suction is a
warm 90F instead of 55F, then

Without pump
57% = (100F - Tdrain-out) / (100F - 55F)
Tdrain-out = 74.4 F
Tfreshwater out = (100 - 74.4)*2gpm / 2gpm + 55 = 80.6
Total energy gained by freshwater (2gpm*8.33 lbm/g)*(80.6-55) = 426.5

Withpump, first find Tcold into hx...
Tcold = (4gpm*90F + 2gpm*55F)/6gpm = 78.3F

Then repeat calculation for 67% efficiency and new Tcold...
67% = (100 - Tdrain-out) / (100 - 78.3)
Tdrain-out = 85.5F
Tfreshwater out = (100 - 85.5)*2gpm/6gpm + 78.3 = 83.13F
Total energy gained by freshwater (6gpm*8.33 lbm/g)*(83.13-78.3) = 241.5

In this situation, leaving the pump off results in a lower CDR (efficiency),
but more energy recovered from the greywater.  Isn't life just full of
wondrous things :-)

To get at least the same energy recovery performance with the pump running,
we would need to be sure the coil inlet temperature is at least down to...
0.67 = (100 - 74.4) / (100 - Tcold)
Tcold = 61.8F

And to get that with 2 gpm shower flow and 4 gpm pump flow, the pump suction
from the tank bottom must be at or below...
61.8F = (2*55F + 4*Tbottom) / 6
Tbottom = 65.2 F

But the tank bottom temperature is probably going to be close to the
heat-exchanger's fresh-water outlet temperature (that is after all where the
return water to the tank is coming from).  So unless the freshwater exiting
the heat exchanger is no warmer than 65.2F, it's a losing proposition.  But
if the freshwater out is that low, then you're losing anyway.

Now, I have no idea what the *real* tank bottom temperature is in this
situation.  But if its warmer than 65.2F, then leave the pump off while
showering.  Running the pump is still a great way to recover 'batch process'
energy and move it into the freshwater system.  And this is still a
significant advantage.  Perhaps a timer controlled in the bathroom that
inhibits the pump while someone is in the shower would be the way to go.
Then other 'batch' uses can automatically start the pump on the differential
temperature setup.

Improving 'efficiency' but at the sacrifice of temperature differential
isn't always a winning proposition.  Beware of salesmen and their numbers


Posted by Robert Gammon on April 21, 2006, 2:40 pm
 Your figures below are very interesting, particularly so for my application.

65F cold water inlet is darn near IMPOSSIBLE here, much less 55F This
thing, Nick's, and Power-Pipe ALL work MUCH better in cold climates than
warm climates.

So the efficiency does drop with rising cold water inlet temps, and Dr V
admits to the same in his notes on the web site.  The comparisons of
different regions of North America illustrate that fact.  

Hot side out with balanced flow of coil and waste water will have a peak
temp out of near 90F with cold side at 65F per his web notes.   I am
likely to have cold water in closer to 70F or even 75F, so the storage
tank and coil temp should rise to near 90F with a long shower and/or
clothes/dish washer in simultaneous operation.

Waste water will only fall to then about 85-90F on exit from the heat
exchanger.  that's good for a septic tank as that is what we will be
using.  Boiling water 200F or greater going down the drain will kill the
bacteria in the septic tank.

Hot water out of the dishwasher is/is not hotter than incoming hot
water, correct??  Internal heaters raise the temp to above 140F don't they??

daestrom wrote:

Posted by daestrom on April 21, 2006, 11:02 pm

I think that's an individual washer issue.  Many do have internal heaters to
heat water up to 140F.  I've seen a couple that recommend 120F hot-water
feed, then boost it internally to 140F.  But those with an internal heater
often have various cycles and one is an 'energy saver' mode that inhibits
the water-heater.  But remember, plain GFX won't recover any of that.  Only
a GFX-star or Nick's system would recover energy from a 'batch' process like
a dishwasher that fills and drains at different times.

For your climate, with very warm inlet water temperatures, it's a
double-edged sword.  Yes, it makes these sorts of heat-exchangers less
efficient overall, and they recover less energy from the greywater.  But on
the other side, it takes less energy to heat the water in the first place
from 70F to 120F than it would from 55F to 120F  (in the winter where I'm
at, cold water inlet runs about 35F-38F).

So the economics of such a system for you is probably not as attractive as
it is for one of us in the 'great white north' ;-)  After all, a 2gpm shower
where I am in the winter, without a heat-exchanger, the greywater is
carrying away energy at....

2gpm*8.33 lbm/g * (100-35) = 1083 Btu/minute

For you, it would run more like....

2gpm*8.33 lbm/g * (100-70) = 500 Btu/minute

So my GFX heat-exchanger running at just 50% recovers about 541 Btu/min and
can pay for itself in avoided energy costs in under three years.  Your's,
even if it runs at 67% can only recover 334 Btu/min.  If it cools greywater
outlet to 87F, that means greywater is now carrying away...

2gpm*8.33 lbm/g *(100-87) = 217 Btu/minute

And you'd be recovering (500-217) = 283 Btu/minute.  Makes the payback
analysis a lot worse for your situation than in mine.

But on the other hand, you need less energy without any sort of hx for that
shower than the energy I need even *with* a GFX heat-exchanger.


Posted by nicksanspam on April 22, 2006, 11:25 am

Dr. V says the (Canadian) NRC tested the GFX during the summer and
the Power Pipe during the winter without correcting for the water
temperature difference, which made the Power Pipe look better.

And FSEC biased their tests by evaluating the GFX as an alternative vs
a helpful addition to solar water heating, and didn't test it with
water heating panels that were reoriented for more winter sun,
which would have improved the results.

And the GFX still doesn't have an US Energy Star rating. Then again,
neither do water heaters nor clothes dryers.

There seems to be a lot of jiggery-pokery here.


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