GFX vs home brew

I keep wondering about the efficacy of a home brew system that is

  1. Not patented
  2. Not sponsored by the DOE
  3. is not reliant on the RAPID fall of water thru a vertical METAL tube.
Reply to
Robert Gammon
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That's because you are ignorant :-) Ignorance can be cured...

Nick

Reply to
nicksanspam

So a student is smarter, wiser than a PHD with a patent???

Sometimes yes, as the creative mind is at its peak in the early 20s.

However, physics clearly tells us that

  1. Metal conducts heat FAR more efficiently than plastics
  2. Water falls in a thin vertical film FAR faster than water that is flowing horizontally in a pipe.

OK, GFX doesn't help with heat recovery for a bath, but great for hot showers, dishwashing, clothes washing.

Reply to
Robert Gammon

Nick-

Your heat exchanger concept may or may not be better than the GFX but pissing folks off in a relevant ng is no way to "market" your device.

The GFX has some installation (retrofit) issues but after 30 years as an ME when I think of heat exchangers I rarely think of "plastic".

cheers Bob

Reply to
BobK207

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)...

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

  1. 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 $60

300' piece of 1" polyethylene pipe with a 50 year guarantee) and U = 10 Btu/h-F-ft^2 (with slow-mov>The GFX has some installation (retrofit) issues but after 30 years as

Think harder :-)

Cheers,

Nick

Reply to
nicksanspam

BAAMMMMM! Kick it up a notch.

Reply to
AstickfortheMULE

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.

Reply to
Robert Gammon

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. Nick
Reply to
nicksanspam

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 NTU=78.5*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

Reply to
daestrom

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.

daestrom

Reply to
daestrom

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.

Reply to
dold

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

Reply to
nicksanspam

Reply to
Solar Flare

No kidding. Somebody seems to need a definition of "greywater".

Reply to
Derek Broughton

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

blackwater == all water from toilets

Reply to
Robert Gammon

Here are some numbers for that. If 100' of 3 1" pipes (polyethylene, with a 0.07" wall thickness) has 78.5 ft^2 of surface with U = 10 Btu/h-F-ft^2,

10' has 78.5 Btu/h-F... 20 UPIPE=78.5'U-value of 10' section of pipe (Btu/h-F) 30 CFRESH=1.25*8.33'thermal capacitance of 10' of fresh water (Btu/F) 40 VGREY=10*3.14159*(2/12)^2'volume of 10' of greywater (ft^3) 50 CGREY=VGREY*62.33-CFRESH'thermal capacitance of 10' of greywater (Btu/F) 60 FOR SHOWER = 1 TO 1000'simulate showers 70 FOR M=0 TO 359'simulate 10 min shower + 350 min rest 80 IF M>10 GOTO 170'rest vs shower 90 TF(0)=TF(1)'move fresh water up from below 100 TG(0)=(100*CFRESH+TG(0)*CGREY)/(CFRESH+CGREY)'move greywater in at the top 110 FOR S=1 TO 8'pipe section (9fresh water in and greywater out) 120 TF(S)=TF(S+1)'move fresh water up 130 TG(S)=(TG(S-1)*CFRESH+TG(S)*CGREY)/(CFRESH+CGREY)'move greywater down 140 NEXT S 150 TF(9)=55'move cold water in at the bottom 160 TG(9)=(TG(8)*CFRESH+TG(9)*CGREY)/(CFRESH+CGREY)'move greywater down 170 FOR S=0 TO 9'rest 180 HEATFLOW=(TG(S)-TF(S))*UPIPE/60'heatflow into fresh water (Btu) 190 TF(S)=TF(S)+HEATFLOW/CFRESH'new fresh temp (F) 200 TG(S)=TG(S)-HEATFLOW/CGREY'new grey temp (F) 210 NEXT S 220 NEXT M 230 NEXT SHOWER 240 FOR S=0 TO 9'results 250 PRINT S,TF(S),TG(S) 260 NEXT S 270 E=(TF(0)-55)/(100-55)'effectiveness 280 PRINT E

pipe fresh water greywater section temp (F) temp (F)

0 93.32178 93.3218 1 90.556 90.55602 2 87.60851 87.60853 3 84.60492 84.60494 4 81.62819 81.62821 5 78.71178 78.71181 6 75.86447 75.8645 7 73.08838 73.0884 8 70.3852 70.38522 9 67.75679 67.75681

effectiveness

.8515951

The fresh and greywater temps are very close, about 6 hours after a shower, since the pipe time constant is much shorter (less than

8 minutes.) These results wouldn't change much with only a half-hour between showers. The effectiveness would be higher for shorter bursts.

Maybe it's worth adding 2 more $20 100' pieces of 1" pipe (altho that would be a tight squeeze), or a handheld showerhead that only runs when a button is pushed. Who sells them?

Nick

Reply to
nicksanspam

Oops. Fixing lines 100, 130, and 160 improves the effectiveness to 88%.

20 UPIPE=78.5'U-value of 10' section of pipe (Btu/h-F) 30 CFRESH=1.25*8.33'thermal capacitance of 10' of fresh water (Btu/F) 40 VGREY=10*3.14159*(2/12)^2'volume of 10' of greywater (ft^3) 50 CGREY=VGREY*62.33-CFRESH'thermal capacitance of 10' of greywater (Btu/F) 60 FOR SHOWER = 1 TO 1000'simulate showers 70 FOR M=0 TO 359'simulate 10 min shower + 350 min rest 80 IF M>10 GOTO 170'rest vs shower 90 TF(0)=TF(1)'move fresh water up 100 TG(0)=(100*CFRESH+TG(0)*(CGREY-CFRESH))/CGREY'move greywater in at the top 110 FOR S=1 TO 8'pipe section (9fresh water in and greywater out) 120 TF(S)=TF(S+1)'move fresh water up 130 TG(S)=(TG(S-1)*CFRESH+TG(S)*(CGREY-CFRESH))/CGREY'move greywater down 140 NEXT S 150 TF(9)=55'move cold water in at the bottom 160 TG(9)=(TG(8)*CFRESH+TG(9)*(CGREY-CFRESH))/CGREY'move greywater down 170 FOR S=0 TO 9'rest 180 HEATFLOW=(TG(S)-TF(S))*UPIPE/60'heatflow into fresh water (Btu) 190 TF(S)=TF(S)+HEATFLOW/CFRESH'new fresh temp (F) 200 TG(S)=TG(S)-HEATFLOW/CGREY'new grey temp (F) 210 NEXT S 220 NEXT M 230 NEXT SHOWER 240 FOR S=3 TO 9'results 250 PRINT 300+S;"'";S;TF(S),TG(S) 260 NEXT S 270 E=(TF(0)-55)/(100-55) 280 PRINT 410;"'";E

pipe fresh water greywater section temp (F) temp (F)

0 94.53323 94.53326 1 92.54844 92.54847 2 90.32916 90.32919 3 87.93309 87.93311 4 85.42671 85.42674 5 82.85161 82.85163 6 80.22588 80.2259 7 77.55527 77.55529 8 74.8424 74.84241 9 72.08957 72.0896

effectiveness

.8785163

Nick

Reply to
nicksanspam

[snip]

Question, How large a space does your heat exchanger occupy?

3 PE pipes inside a larger PE pipe is NOT very flexible. Bend radius for this configuration is measured in feet.

It not very thick, to be sure, but to collapse this into a practical shape, (100 linear feet of tubing in the rafters of a basement will NOT fit in most houses), you'll need to bend this into a coil of say about 5 or 6 feet in diameter, several feet high. You quoted some figures for the dimensions of the assembled unit, but now those dimensions as I recall them don't make sense.

Reply to
Robert Gammon

[snip]

Question, How large a space does your heat exchanger occupy?

3 PE pipes inside a larger PE pipe is NOT very flexible. Bend radius for this configuration is measured in feet.

It not very thick, to be sure, but to collapse this into a practical shape, (100 linear feet of tubing in the rafters of a basement will NOT fit in most houses), you'll need to bend this into a coil of say about 5 or 6 feet in diameter, several feet high. You quoted some figures for the dimensions of the assembled unit, but now those dimensions as I recall them don't make sense.

Reply to
Robert Gammon

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.

Reply to
Robert Gammon

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