Posted by nicksanspam on April 18, 2006, 7:02 pm
I've learned that further discussions with you would be pointless.
Posted by daestrom on April 18, 2006, 8:52 pm
No, I don't think this is the way to go. When using warm water to heat cool
water, the smaller the temperature difference along each section, the better
as far as entropy goes.
That is one of the reasons why counter-flow hx are so good. The warm water
is never more than a few degrees above the 'cool' water. Where the 'warm'
water is the hottest, the 'cool' water has already been heated up to almost
the same temperature. And where the 'cool' water is the coldest, the 'warm'
water has already been cooled to almost the same temperature.
Compared to a parallel flow, where the 'warm' and 'cool' water start out
with vastly different temperatures and aproach each other over the length of
With your 'infinite' tank idea, you end up with very cold greywater, but the
freshwater is hardly warmed at all. Making the two tanks nearly equal, you
approach the limits for a parallel flow system (e.g. grey and fresh leave at
the same temperature, about (100+55)/2 = 77.5F.
I take it you've considered what the pressure drop would be with the flow
rates? Check the greywater side pressure drop carefully. Although you have
larger x-section, the *available* driving pressure for a shower drain is
just a few feet of water.
Posted by nicksanspam on April 19, 2006, 11:17 am
Suppose we take a shower and collect 100 F greywater in the upper part of
a $0 100'x4" black plastic corrugated drainpipe coil containing 3 $0
100'x1" pieces of black plastic polyethylene pipe, with bidirectional
plug flow, like this, viewed in a fixed font like Courier:
| --------->--------------------------------> hot water to shower
| | Tl |
--------- sewer ---------
| | Tg | out | 120F |
| | | | |
| | | ^ | |
| | | | | |
| |1" |4" | | tank |
| | | | | water |
| | | | | heater |
| | | | | |
| | | | | |
| | | | | |
| | |---- | |
| | | | 55F |
--------- P ---------
| ---- Tc |
-----------------| <- |-------------------< cold water supply
Now we disable the water heater and run a slow, low-power pump P
(eg Grainger's $20 4PC86 (Taco 003-BC4-2) 1/40 HP 120V 0.43A pump)
if Tg - Tl < 5 F and Tg - Tc > 5 F, and enable the water heater again
when Tg - Tc < 5 F...
Is this a GFX-Star? It's hard to tell from the website description.
20 UPIPEx.5'U-value of 10' section of 3 1" pipes (Btu/h-F)
30 CFRESH=1.25*8.33'thermal capacitance of 10' of fresh water (Btu/F)
40 VGREY*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 CSERIESRESH*CGREY/(CFRESH+CGREY)'caps in series (Btu/F)
70 RC=CSERIES/UPIPE'combined time constant (hours)
80 EXPF=EXP(-1/60/RC)'exponential factor
90 FOR SHOWER = 1 TO 100'simulate showers
100 FOR M=0 TO 89'simulate 10 min shower every 90 minutes
110 IF M>9 GOTO 220'rest vs shower
120 IF SHOWER <100 GOTO 150
140 PRINT 400+M;"'";M,TG(9)
150 TGT=TG(0)'save original Tg(0) for later Tg(1) calc
160 TG(0)=(100*CFRESH+TG(0)*(CGREY-CFRESH))/CGREY'move greywater in
170 FOR S=1 TO 9'pipe section (9<->fresh water in and greywater out)
190 TG(S)=(TGT*CFRESH+TG(S)*(CGREY-CFRESH))/CGREY'move greywater down
210 NEXT S
220 IF (TG(0)-TF(0))>5 OR TG(0)<60 GOTO 290'no pumping
230 PUMP=PUMP+1'pump in fresh water at bottom
240 IF SHOWER>49 THEN HEAT=HEAT+CFRESH*(TF(0)-55)'gain from gw
250 FOR S=0 TO 8'shift fresh water up
270 NEXT S
280 TF(9)U'move cold water in at the bottom
290 FOR S=0 TO 9'rest
310 TF(S)=TFINAL+(TF(S)-TFINAL)*EXPF'new fresh temp (F)
320 TG(S)=TFINAL+(TG(S)-TFINAL)*EXPF'new grey temp (F)
330 NEXT S
340 NEXT M
350 NEXT SHOWER
360 SHOWERGYP*10*CFRESH*(100-55)'50 showers with no GWHX (Btu)
370 PRINT HEAT,SHOWERGY,HEAT/SHOWERGY,PUMP
231607 234281.3 .9885854 1458
Looks good on paper, with 98.9% heat recovery :-)
Posted by Robert Gammon on April 19, 2006, 12:31 pm
This is so very very close to a GFX Star it isn't funny
What the Dr argues for is a el-cheapo electric water heater that is used
solely as a storage tank. The inlet to the pump is thru a check valve
that ties to the drain connection of the water heater/storage tank. Hot
out of the heat exchanger then goes to TWO places.
1. To a tempering valve to limit scald risk. the other input of the
tempering valve is the output of the normal hot water supply (electric,
NG, LP, or same inputs tank less)
2. Cold in on the water heater/storage tank.
Hot Out of the water heater/storage tank goes to Cold in on the normal
Nick's figures and the Power-Pipe folks argue that the heat recovery is
equivalent to a 12-18KW electric heating element (for a 60 inch GFX).
In testing of the the GFX done at at least a couple of universities,
they found that the upper heating element in an electric water heater
NEVER TURNED ON in ANY of their testing.
The heat recovery of a GFX when used in this configuration jump 15-20
percentage points and becomes an almost level 65-75% Course we will
have 2KW/day losses in that storage tank. But with a CONSTANT input ot
the normal Hot water heater of 85-90 F, it will merely LOAF along to
deliver the HOT water needed.
One of the reasons for the higher heat recovery is that the flow rate
thru the coil LEAPS. The Taco pumps will move up to 20Gal/hr depending
on model to 20 feet. More realistically a Taco 006 or Taco 008 will
delvier upwards of 10Gal/hr at 10 feet of height. Now we have 2x-4x
MORE flow thru the coil than is flowing in the greywater. The graphs on
the GFX web site illustrate what happens with higher coild flow rates.
GFX Star controls the pump via one of two methods
a. Timer - showers at KNOWN times EVERY day
b. Differential temperature controller -sensors on coil and inlet to the
heat exchanger (GFX or Nick's) will trigger the pump when temp
difference exceeds a set point - i.e. 2 or 3 degrees
Posted by nicksanspam on April 19, 2006, 12:47 pm
I'm still not clear on that, after talking with Carmine again. One diff
might be beneficial stratification in the greywater drainpipe, vs full
mixing in a conventional greywater tank. At any rate, with 98.9% heat
recovery, we might heat 50K Btu/day of water with 550 Btu/day, eg
a 7 watt night light burning 24 hours per day :-)
Dr. V got US Customs to sieze Power Pipes at the border, based on a
theft-of-trade-secrets charge, but they seem to have gotten around that.
We also discussed some testing techniques that were biased against GFX.
Maybe a lot more, with greywater plug flow.
That isn't part of my scheme, but another circulating pump could increase
the velocity through the coil and the conductance inside the coil...
That's quite different. No timing for me, and I'd turn on the pump when
the gw-fw temp diff at the output is LESS than 5 degrees.