Not bad, apparantly you've chosen a flow rate and section length so the time step corresponds to exactly one section length.
Because the greywater drain cross-section is so much larger than the freshwater, your Cmin/Cmax ratio ends up being about 0.24 (IIRC a 4-inch pipe with three 1-inch pipes inside). So a high efficiency of 87% doesn't really tell us how much energy we're saving. While your efficiency is 87%, it looks like you're still putting (72-55)*1.25*8.33=177 BTU/minute down the drain. Out of a total of 468.6 BTU/minute needed to heat 1.25 gpm from 55 to 100, that's nearly 38% of the heating.. Print out the data *during* the last shower, not 350 minutes later. I'd like to see what the greywater outlet temperature is while it's flowing. I think it's going to be a lot cooler than 72, but not sure. Better yet, print out the freshwater and greywater outlet temperatures *during* the ten time steps of the last shower.
After all, it is the temperatures out *during* flow that matter. The temperatures at the end of the stagnation period only tell us the initial startup point for the next shower. How quickly they change *during* the shower, and in what direction would be more telling.
After all, if the freshwater outlet during the shower really is at 94F, and the greywater really leaves at 72F, then you don't have conservation of energy (freshwater side picks up (94-55)*1.25*8.33 = 406 btu/min, while the greywater side gives off (100-72)*1.25*8.33=291 btu/min). That's a clue that something is wrong with these results.
Guess again. If your setup was restricted to just 5 feet long, would its performance be anywhere near as good as the GFX??? And that's the point. To get performance on par with GFX, you have to resort to something several tens of feet long.
Heck, If I had someone build a GFX that was 100 feet tall, I'm sure it's performance would put your setup to shame. But who has space for 100' of 4" pipe (vertical, coiled or otherwise).
Well, *you* can calculate using average flow, but the results are *NOT* meaningful. Just because you found a formula in a book, doesn't mean you can apply it to different situations, like intermittent and 'average' flow and still get meaningful results. Those ASHRAE formula are for calculating the steady-state performance of a heat-exchanger. Trying to apply them to 'burst' mode is a waste of time. The results do *not* mean anything. And they don't prove anything except that you don't know when to apply them.
But just to humor you, if the 'bursts' are 1.25 gpm, then the steady-state answer would be Cmin-Cmax=1.25*60*8.33 = 624.75 Btu/h-F. With an area of
78.4 ft^2 and U=10 Btu/h-f-ft^2, NTU=78.5*10/624.75 = 1.26 and E=55.7%.
Notice how the answer depends on the flow rate *when water is flowing*?? Not the average amount of water that flows during some arbitrary time period.
The fact that the two different flow rates give such drasticly different answers should be a clue that you're missing something.
The formulae you are using from ASHRAE are for steady-state, *flowing* heat-exchangers. The NTU and effectiveness assume *steady-state* conditions (i.e. a constant flow rate). So the efficiency of your system when water flows and has reached steady-state is only 0.45. But since your showers are less than the time needed to reach steady-state, even that number is useless.
Your other post with a step-wise simulation is probably much closer for this sort of transient behavior, but it too has some flaws. You posted the outlet temperature for the greywater as 72F while the outlet for freshwater as 94F. This is with constant 55F inlet freshwater and 100F inlet greywater. The fact that your freshwater is picking up more energy [(94-55)*flowrate] than your greywater is losing [(100-72)*flowrate] is a clue that something is wrong in your calculation.
Your simulation printed out the numbers after 350 minute stagnation period, not when there is flowing water. You should print out the temperatures
*during* the last shower, when there is actual flow. *That* is when there is energy flowing down the drain. Print out the numbers for fresh and grey water outlet temperatures *during* the last ten minute shower.
Calculate the energy removed from the greywater during those ten minutes and the energy being picked up by the fresh-water during those same ten minutes. Since the inlet temperatures are both assumed fixed (100F and 55F), if the energy picked up by fresh-water does not equal the energy given off by the greywater during those ten minutes of flow, then something is wrong with your calculations because energy must be conserved. (we're neglecting any ambient losses)
To find the true effectiveness for this non-steady-state operation, just calculate the amount of energy picked by the freshwater during the shower and divide by the total energy to heat that same water to the greywater inlet temperature.
*hint*, if the water outlet temperatures change a lot while the shower is running, you might reduce the time step to less than one minute intervals so as to get better resolution. This would make for better integration of the temperature versus time to get total energy. Too course a time step could lead to mismatch between greywater and freshwater energy calculations.
The issue with a 100 foot tall GFX is the required size of the potable water tubing and the pressure required to get a reasonable flow rate thru the 100ft stack.
As it is on a 60 inch GFX, manifolding is performed to limit pressure loss (coil height is about 27 inches each) with the base of each coil tied to the inlet water, and the top of each coil tied to the outlet.
The engineering drawings on gfxtech's web site clearing indicate an asymptotic behavior. Adding additional length brings lower and lower incremental benefit. Still with two S4-40s in series, pressure loss is about 2.5psi on a 2 gal/hr flow rate. and 80 inches of gfx recovery will get efficiency up another 5-10% over a 60 inch model and a 40inch height is easier, in many cases, to find a spot for.
daestrom is doing us a great service by pointing out the issues with the home brew system. I too do not believe that the home brew system proposed will work as well as a 60 inch GFX to recover waste heat from grey/black water and pump that heat to DHW and cold side showers.
I wasn't seriously recommending a 100' GFX. Just pointing out that 100' of any sort of piping takes considerably more space than a 5' GFX.
Yes, that is exactly how mine is constructed. The problem with piping the freshwater side in parallel is that the two coils form a sort of series-parallel flow heat exchanger. One heat exchanger cannot heat its outlet as much because it only receives already-cooled greywater from the other heat-exchanger. So when it's cooler freshwater outlet water mixes with the warmer water from the upper one, there is a reduction in overall efficiency. I can detect this when someone is in the shower by touch alone on the two coil outlets.
This is the same sort of thing that multiple-pass conventional heat-exchangers suffer from. Less than ideal, but a compromise of heat-transfer performance versus hydraulic performance (pressure drop).
I've toyed with the idea of restricting the flow through the lower coil to improve on this. Would increase the pressure drop some, but not as bad as the full series model. Some weekend project I may put a throttle valve in series with the lower coil and play around with different settings.
True, but you would need two 40inch heights, or a pumping arrangement. Since mine is installed in the main waste line for the entire house, pumping blackwater did not seem very attractive.
It could perform rather well. And if I know Nick at all from his postings over the years, it will cost less than my GFX did, even though I installed in myself. I'm just trying to keep Nick 'honest' by not letting him apply steady-state calculations to a transient system. But it does require more space to install, and may have some maintenance issues.
Hopefully when Nick is done building it, he'll post his performance numbers (good or bad). Direct measurements and experimentation are always better than theory.
daestrom "In theory, theory and practice are the same. In practice, they're different"
How about the fresh water outlet temp? Line 100 below accumulates the heat energy that needs to be added during the last shower...
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>9 GOTO 200'rest vs shower
90 IF SHOWER
The 4" tall spiral hung under a basement ceiling would be about 7' in diameter. The 6' tall coil would occupy a 2.7' floor circle. It seems simpler to install and might have better stratification.
10 PI=4*ATN(1)
20 D=4.25'pipe OD (inches)
30 L=100'pipe length (feet)
40 DI=2'coil ID (feet)
50 DO=DI+2*D/12'coil OD (feet)
60 CI=PI*DI'inner circumference (feet)
70 NT=L/CI'number of turns
80 H=NT*D/12'coil height (feet)
90 PRINT D,DI,DO,NT,H
4.25 2 2.708333 15.91549 5.636738
10 PI=4*ATN(1)
20 D=4.25'pipe OD (inches)
30 L=100'pipe length (feet)
40 A=D*L/12'pipe area (ft^2)
50 DI=2'flat spiral ID (feet)
60 DO=2*SQR(A/PI+(DI/2)^2)'spiral OD (feet)
70 NT=12*(DO-DI)/2/D'number of turns
80 PRINT D,DI,DO,NT
4.25 2 7.006704 7.068288
Particularly since GFX is non clogging and works with ALL sewer waters (grey and black).
Efficiency is NOT the only criteria here. If we recover 40% to 60% of the waste heat, we have made MAJOR strides in overall DHW production efficiency. Convenience, non-clogging, wife friendly are all MAJOR concerns.
Price is NOT the only factor either, but price and efficiency are Nick's main concerns.
Nick's will have to be connected ONLY to non-toilet drains. Nick's will have to be periodically cleaned of matter that goes down kitchen sinks and out the clothes washer drain.
None of these are concerns with GFX.
If we can afford one of these, either of these, these other factors, besides price and efficiency may well be MORE of a concern to the rest of us.
Well the efficiency of heat exchange factor is not the only efficiency. A purchased and installed product at 5% efficiency is much more economical than a well designed, thought out, project that will be implemented sometime after getting a 'Round Tuit
Efficiency of installation ease. Efficiency of installation time. Efficiency of product parts and pieces aquisition Efficiency of marriage after the home space displacement. Efficiency of trial and error costs. Efficiency of maintenance. Efficiency of computer time arguing about imaginary issues. Efficiency of home resale after the newfangled frankenstein paraphenalia is seen.
Le mieux est l'ennemi du bien. I'm better at thinking up things than getting round tuits.
I can relate to that, having spent about 40 hours in the last 2 weeks visiting various plumbing supply stores. It's been fun learning names of fittings, like "bullnose T" and "Dismukes crampon lifter."
Spouses care a lot more about what's on the lawn or in the living room than what's in the basement.
It might be nice to build more than one, with careful directions at
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And only empty the crud once a year, using a hose instead of a toothbrush.
Some issues are more important than others, eg daestrom's.
This leads me to make th>
Nah. Efficiency should be 8 vs 7, a lower price should give me more vs fewer points, convenience is TBD, and wives may like saving more money, or having more to spend in other directions.
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>9 GOTO 200'rest vs shower
90 IF SHOWER
Connecting a GFX to a toilet drain is not going to be very sensible, however, unless you heat your toilet water - makes the most sense when connected to drains that might have hot water - putting it in the blackwater stack just adds one more way to have a toilet flush make your shower uncomfortable, and reduces the potential efficiency by reducing the temperature differential across the heat exchanger.
GFX need NO cleaning EVER, even when hooked to WHOLE house SEWER.
If you wanted to clean a GFX, its simple to remove and HOSE it down also, but there is no need to do so.
Yours WILL need interior cleaning
This stands to reason that greywater output will rise in temp inyour heat exchanger as the time required to recapture the heat exceeds the amount of time that the greywater remains in the heat exchanger. Need higher surface area (i.e. longer, or larger diameter tubes).
Yes, there is no heat to recapture, UNLESS toliet supply is also hooked to GFX heat exchanger.
The point to adding in the toilets is to make installation simple. Locate the SINGLE sewer pipe in the basement that collects ALL waste water, and insert the GFX into the pipe.
Running toilet water to the mix of effluents processed by the system adds NOTHING to efficiency, but makes installation a BREEZE.
|| Efficiency of installation ease. || Efficiency of installation time. || Efficiency of product parts and pieces aquisition | | I can relate to that, having spent about 40 hours in the last 2 | weeks visiting various plumbing supply stores. It's been fun | learning names | of fittings, like "bullnose T" and "Dismukes crampon lifter."
This may be some of the best and most encouraging news I've seen posted to alt.solar.thermal!
I've admired Nick's tenacity in dealing with the math of solar issues; but deplored his seeming inability to "get his wheels on the ground."
Nick, bless you! Get out even more. Take some of those fittings home with you and play with 'em! Build at least rough prototypes of your ideas and take pictures to share...
I just realized my error, corrected myself and said yes, heat inlet water to toilets anyway to get a closer to a balanced flow, increasing the efficiency of the heat exchanger
You are unaware of the terrain I live in. Ruler flat!!!!
Sewage ejector == PUMP
Yes I could use either one WITH A PUMP as there is NO WAY to intercept the sewer ABOVE ground. No basements here except in high rise buildings, certainly not in ANY homes.
In the 1200sq ft living space I have now, the ONLY unit that will work is the GFX. There is no room for yours UNLESS it goes in the ATTIC!!!
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