Robert Gammon wrote:

... physics clearly tells us that

1. Metal conducts heat FAR more efficiently than plastics

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

2. Water falls in a thin vertical film FAR faster than water that is
flowing horizontally in a pipe.

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.

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 $60
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.

BobK207 wrote:

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".

Think harder :-)

Cheers,

Nick