SQLit wrote:

"Wayne Whitney" wrote:

I'm interested in comparing two designs of water-water heat exchanger,
where one side is pressurized and one is not (gravity flow). The
pressurized flow rate will be 50-75% of the unpressurized flow rate.

Design #1: Unpressurized flow through 2" copper pipe, pressurized flow
through 1/2" copper pipe wound helically around the 2" copper pipe. I
guess the helical 1/2" coil should be soldered to the 2" pipe for
improved heat transfer?

That might help. The GFX is just tightly wrapped, enough so that the outer
pipe becomes oval, which distorts the inner pipe wall into a spiral (I think),
which spins the the drainwater and makes it hug the walls of the inner pipe.

Design #2: Unpressurized flow through 1.5" copper pipe, which is
sleeved in a 2" copper pipe. The space between the pipes is
pressurized; water enters one end and exits at the other through the
creative use of reducing tee connectors.

Ream out the Ts. That might be easier to build, and illegal according to
plumbing codes, with a single wall, but who cares? :-) Many codes are
designed by committees who want to sell stuff and keep people employed.

So which design is better? If Design #1 used 1.5" pipe instead of 2"
pipe, and Design #2 used a helical baffle between the two pipes to
direct the flow, I think there is no question that Design #2 is
better.

The bottleneck in both designs may be that the drainwater doesn't
completely cover the inside of the inner pipe.

Maybe I am missing something, How do you get the pressurized side to go
slower than the non pressured side?

It sounds like the pressurized water is the cold water supply to the shower
and the unpressurized water is the drainwater, a mixture of hot and cold
water with greater flow. This can be more efficient if the hot water temp
is the same as the shower temp, or you might change the plumbing to run
both the cold water supply to the shower AND the cold water supply to the
water heater through the pressurized side, so the flows are equal. That's
the novel part of the GFX patent.

Not knowing more about the delta t it is impossible to guess which would
work. Discounting efficiency completely.

Impossible for some people :-) The shower might be 105 F and the drain might
be 100 and the cold might be 60, but the heat exchanger efficiency does not
depend on these temperatures. Gary Reysa and I have been working on this with
a different and maybe more efficient approach, trying to make a counterflow
heat exchanger with stored stratified drainwater in a drum with PE pipe at
http://BuildItSolar.com.

Nick