Heat exchanger comparison

[Wayne Whitney]

Hello,

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?

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.

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. But absent those changes, does Design #1 win? By alot?

Thanks, Wayne

[SQLit]

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

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

[nicksanspam]

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.

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.

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

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.

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

Nick

[Bob]

More likely, the pressurized water is the cold water supply to the water heater. The other way, the shower temp would change as the pipes warmed up, etc, and special plumbing to the shower would be required.

Bob

[nicksanspam]

As I recall, the OP said he plumbed it "the other way," which likely requires shorter pipe runs.

Nick

[Wayne Whitney]

Indeed, particularly as I am going to be installing this horizontally rather than vertically. I could only fit maybe a 2' vertical unit in my crawlspace. I'm assuming an 8' horiztonal unit will be more efficient than a 2' vertical unit.

I guess my real question is this: design #2 is easier to build, so is design #1 enough better to be worth building? Of course, my plumbing inspector may not allow the single-walled design #2 and require the double-walled design #1. But I'm hoping he will allow design #2 since the positive pressurization of the potable water should mean any leaks in the single wall won't contaminate the potable water.

Thanks, Wayne

[Wayne Whitney]

I'm intending to just run the cold water supply to the shower through the heat exchanger. This is easier than running the entire cold water supply to the hot water heater through the heat exchanger. I don't really want to have the pressure drop for all the hot water outlets, as well.

As to the shower temperature increasing as the heat exchanger heats up, that is true. Of course, it is already the case that the shower temp slowly increases as the hot water pipes heat up and the stale hot water is drained out. If it becomes a problem, I will either switch to a thermostatic shower valve, or I'll put a mixing valve into the cold water supply to the shower, mixing preheated cold water with hot water to, say, 90 degrees.

How does a thermostatic mixing valve behave if the set temperature is above the temperature of both inlets, or below the temperature of both inlets?

Thanks, Wayne

[Wayne Whitney]

Hmm, thinking about this some more, if there is a small leak in the single wall, and water is being drawn through the outer pressurized region, could this suck water in from the unpressurized drain area? A sort of Venturi effect? Or is the 35 psi (minimum) of domestic water pressure avoid this?

Cheers, Wayne

[Bob]

heat exchanger,

flow).

pressurized flow

copper pipe.

sleeved in a

pressurized.

drainwater doesn't

horizontally

vertical unit in

build, so is

my plumbing

require the

design #2 since

mean any leaks

I'll just about guarantee that #2 will not be acceptable. A small leak between the two would not be visable to you - it will just go down the drain. Then, when the city shuts off the water for repairs, it will suck your sewage into the city water. #1 would leak outside where you will see it, and not offer a siphon path to the city water.

Bob