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Re: GFX vs home brew - Page 4

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Posted by Robert Gammon on April 15, 2006, 1:16 am
 
Derek Broughton wrote:

greywater == all water going down a drain EXCEPT water from Toilets

blackwater == all water from toilets

Posted by dold on April 14, 2006, 6:19 pm
 

Horizontal is how all of my waste plumbing is arranged.  I don't have any
place to put a vertical drop without adding a pump.


I have never cleaned out the drains in this house.


"stacked" implies a lot of vertical drop.  Pumping requires energy.

--
---
Clarence A Dold - Hidden Valley (Lake County) CA USA  38.8,-122.5

Posted by Robert Gammon on April 15, 2006, 12:50 pm
 Stacked or daisy chained does NOT mean increased height.  It merely
means we hook up two shorter GFX lengths in series, pumping effluent
from the outlet of the first one to the inlet of the second one.
Overall efficiency rises.  For instance we could get two S4-40s and set
them on the wall parallet to each other.  Inlet for the first one is
from house sewer.  Outlet of first is pumped (100W power used) to inlet
of second, outlet of second is piped to city sewer/septic tank.  One
BIG advantage of this system in a septic tank is that you can put
BOILING water down the kitchen drain, something you cannot do with a
plain ole septic tank.

One issue with this configuration is water pressure drop  GFX Tech will
argue for manifolding, that is, connect potable water supply to coil
inlet on BOTH GFX units and tie the coil outlets from BOTH units to a
common pipe to hot water heater and cold side of showers,   That
produces a pressure drop of about 2-3 psi  A full series connection
with potable water to the coil inlet on the one connected to city
sewer/septic tank, its output then connected to coil input of the GFX
attached to the house sewer, and the coil outlet then connected to
house hot water and shower cold side.  This produces a pressure drop of
5-6 psi.

In my configuration, the inlet water pressure will be a constant 65psi
So dropping to about 60psi is no big deal, and the water temp to the
house rises.  Effluent temp in both series and parallel configs drops
by at least 20F, maybe much more

In 40 inches of vertical height, we get 80 inches of heat recovery.


Posted by daestrom on April 14, 2006, 3:49 pm
 

But the conductivity of the pipe wall is only a minor factor in fluid heat
transfer.  In almost all situations, the conductivity of the film layer
*next* to the wall is the dominant factor.  Just look at the R values for
two conventional water films versus that of 1/16" of Cu or 3/16" of plastic.
When conducting heat through a wall, the two films and wall material are in
series so it is appropriate to just sum the R values. (we'll neglect the
calculation accounting for the wall being cylindrical and just *assume* flat
plates)

Forced convection water films R values range from 0.02 m^2-K/W to as low as
0.0001 m^2-K/W.  For a flow of about 2 m/s through a 3 cm pipe, we get a
Reynolds number of about 6.4e4.  For water around 20C, that gives us a
Nusselt number of about 350, and a heat transfer coefficient of about 7000
W/m^2-K (or an R value of 1.44e-4).  Cu has an R value of about 0.0025
m-K/W, or about 5.0e-6 m^2-K/W for a 2mm thick layer.

So the total R value for heat transfer across a water-water heat exchanger
tube might run about 1.44e-4 + 5.0e-6 + 1.44 e-4 = 2.93e-4 m^2-K/W

If the PEX has a conductivity of only 1/10th that of copper, and is three
times thicker, we would have about 1.44e-4 + 1.5e-4 + 1.44e-4 = 4.38e-4
m^2-K/W.  Worse, true.  But still about 67% that of the Cu.

And that is with rather optimal surface conditions and relatively high flow
(~2.1 m/s is a common 'rule of thumb' design flow rate, it balances between
poor film coefficients and excessive erosion).



But the flow through a flooded horizontal pipe means a much thicker film
layer.  The novelty of the GFX design is that the water film formed by
having a small flow rate of say 2 gpm flowing over the inside surface of a
3" diameter pipe.  This means the total thickness layer in the GFX flow is
about the same or *less* than the boundary layer thickness in conventional
pipe flow.  So the average thickness between the bulk of the water and the
pipe wall is about 1/2 that of the flow layer.  This reduces one of those
two film coefficients by an order of 2.  This could be...

1.44e-4 + 1.0e-5 + 7.2e-5 = 2.26e-4 m^2-K/W  (assuming twice the thickness
of Cu since it is double wall design).

With the high velocity of the water film on the drain side, overall heat
transfer could even be a bit better than this.

Flow in a horizontal pipe could be done in two ways.  Flood the pipe
completely.  But then you have issues of venting both sides of the drain
line, and the bore of the pipe would result in very low velocities and
correspondingly poor film coefficients.  Or leave the pipe only partially
filed (like most current drain lines) and then you only have a tiny surface
area coming in contact with the drain water.

While not the *best* possible performance, like many designs it compromises
between getting better heat transfer coefficient, material costs, ease of
maintenance and installation.


For a total surface area of just (4 in)*pi *60 in /144 = 5.24 ft^2, 60% is
pretty 'great'.  How much surface area does your setup require?


We've been through this before Nick.  You can't calculate Cmin or Cmax using
a 24 hour 'average' flow rate.  When water is flowing, (say 16 lbm/minute or
960 lbm/hr), your Cmin=Cmax = 960 Btu/h-F.

So *while* the water is flowing, you might see NTUx.5*10/960 = 0.818.  And
*that* would give you about E = 0.45.

By using an average flow rate that includes long periods when there is no
flow at all, you make it seem as though the heat exchanger is much longer
than just 300'.  If you want to get your kind of performance with the
existing surface area and U, you would need to reduce the flow to 0.034 gpm
and keep it there all day/night.  To get your kind of performance at 2 gpm,
you would need about 55 times longer tubing (~3 miles).

When you look at it that way, GFX's 0.60 performance in a 60" tall package
starts to look pretty good.

daestrom


Posted by nicksanspam on April 14, 2006, 8:52 pm
 

It might be 3 vs 4", but it's still poor overall performance.


There's no requirement... 300' of 1" pipe is a convenient design choice.


Sure I can :-) You might enjoy calculating E if 50 gpd of hot water flows
in 1 second 1.25 gpm bursts, then 2 second bursts, and so on.

My shower is 1.25 gpm, so a 10 minute shower fills the 1" pipe.


How long between showers?


I disagree, altho that might happen with continuous hot tub water exchange.

Nick


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