wrote in message
...
daestrom wrote:

The GFX does well with its small surface...

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

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

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

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

How much surface area does your setup require?

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


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).

snip ASHRAE calculations for steady-state problem

We've been through this before Nick. You can't calculate Cmin or Cmax
using
a 24 hour 'average' flow rate.

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.


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.

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

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.

How long between showers?


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.

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.

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

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.

daestrom