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Re: GFX vs home brew

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Posted by nicksanspam on April 14, 2006, 11:28 am

But plastic is FAR cheaper, and metal doesn't help much with
a layer of crud and slow-moving water on both sides.

The GFX does well with its small surface...

...60% is not "great," IMO.

Here's what physics tells us on page 3.4 of the 1993 ASHRAE HOF:

1. E = (Thi-Tho)/(Thi-Tci) when Ch = Cmin and
     = (Tco-Tci)/(Thi-Tci) when Ch = Cmin, where

   Ch = hot fluid capacity rate, Btu/h-F
   Cc = cold fluid capacity rate, Btu/h-F
   Cmin = smaller of the two rates
   Th = terminal temp of hot fluid (F). Subscript i indicates
        entering condition; o indicates leaving condition.
   Tc = terminal temp of cold fluid (F)...

2. Number of Exchanger Heat Transfer Units NTU = AUavg/Cmin.

3. Capacity rate ratio Z = Cmin/Cmax.

   Generally, the heat transfer effectiveness can be expressed for a given
   exchanger as a function of NTU and Z: E = f(NTU,Z,flow arrangement).
   The effectiveness is independent of the temps in the exchanger.

   For any exchanger with Z = 0 (where one fluid undergoes a phase change,
   eg in a condenser or evaporator), E = 1-e^(-NTU).

   For parallel flow exchangers, E = [1-e^(-NTU(1+Z))]/(1+Z).

   For counterflow exchangers, E = [1-e^(-NTU(1-Z))]/[(1-Z(e^(-NTU(1-Z))],
                                 = NTU/(NTU+1), when Z = 1.

For instance, if we use 50 gallons per day of hot water in short bursts
and Cmin = Cmax = 50x8.33/24h = 17.4 Btu/h-F and A = 78.5 ft^2 (a $0
300' piece of 1" polyethylene pipe with a 50 year guarantee) and U = 10
Btu/h-F-ft^2 (with slow-moving greywater and crud outside and slow-moving
fresh water inside), NTU = 78.5x10/17.4 = 45.2, and E = 0.98.


Think harder :-)



Posted by AstickfortheMULE on April 14, 2006, 2:06 pm
BAAMMMMM!   Kick it up a notch.

Posted by Robert Gammon on April 14, 2006, 3:14 pm
 AstickfortheMULE wrote:

Nick uses a LONG, SLOW moving body of water to extract heat.  So in many
ways, it resembles a air conditioning condenser coil.  Both exchange
heat relatively slowly and rely on a LONG path to effect the heat
exchange.  Ok, Nick's will work, 300 ft of tubing surrounding a large
tube holding the greywater will transfer significant heat from greywater
to the potable water.

Nick's heat exchanger can be installed in almost any orientation, but
horizontal seems to be his desire.  His heat exchanger will need to be
cleaned out periodically of gunk, especially if toilets drain thru the
same heat exchanger.  He argues that his will extract more heat than the
GFX, and that may be true, but he will have a much larger unit (300 feet
of 1 inch tubing is more than 6 feet in length when stacked as a single
layer around a larger pipe that holds the greywater)

The beauty of the GFX, is that it is metal, it can be stacked or daisy
chained with pumps to extract even more heat in a smaller space.   The
illustrations on the web site show various parallel installations, but I
wonder about the option of taking two of the 48" units, placing them
side by side, and pumping effluent from the first to the top of the
second before the wastewater flows on to the city sewer/septic tank.

Think I'll write the GFX team about this idea.

Posted by nicksanspam on April 14, 2006, 3:44 pm

It also resembles a chair, if you wrap enough cotton gauze around both :-)

Not a good idea. It wouldn't make a good wheelchair either.

Physics clearly tells me so. She seems to lie to you. How fickle.

You seem confused. In this condition, many people read more carefully.
Some even stop talking and listen :-)

The 1" pipe would be in 3 100' pieces inside a 100' x 4" black plastic
corrugated drainpipe which can be in 1) a 2' diameter x 6' tall coil or
2) a 7' OD x 2' ID x 4" tall flat spiral under a basement ceiling, which
uses less floorspace.

Posted by daestrom on April 14, 2006, 3:56 pm

Actually, mis-applied formulae from a text book seems to be what is talking
to you again Nick ;-)

If you run your same calculations with a simple flow rate of 2 gpm (a
typical shower flow), what do your 'physics' tell you?

The fact that the answer is much different than when you run your 50 gpd
flow rate numbers should prompt you to pause and 'thick harder'.

While I agree that 'batch' flows that do not fully purge your apparatus will
give you some improvements, we haven't seenn any of your 'numbers' for that.
Quoting ASHRAE formulae that are intended for continuous flow when you
*know* you won't have that sort of flow rate is a waste of everybody's time.


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