Coaxial loop water heating?

RickDelta

(prelude)
Hi!
My prototype PVC "coaxial loop" consists of the following:

A standard two (2) inch PVC DWV pipe 100 feet long .... with a 2" end cap on one end.
The other end is attached to a custom built hard copper "T" connector ... 2" (to this pipe) x 1.25" (side take-off) x 1.25" (center take-off) via a reducer fitting.
The 1.25 inch "center take-off" port is internally isolated from its surrounding 2" diameter "T" connector and passes straight thru it to the center of the 2" PVC pipe. Immediately clearing this custom connector this hard copper "center take-off" is continued with a 1.25" thin walled flexible tubing extending just a few inches short of the full length of the PVC pipe.
(A 1.25" pipe inside a 2" pipe approximately equates to the same pipe size area for both pipes)
The above description defines the makeup of this "coaxial loop" .... ie: "a pipe within a pipe loop"
This one and only loop is within a closed water loop heated by an in-line electric circulation heater driven by a standard circulation pump 15 feet away.

I had originally assembled the above components with 3/4" water feed and return piping. The circulation heater produced plenty of heat at is source, but I couldn't deliver the required heat (195 degrees F) to this 2" PVC to be thermo-formed. I had a 30 degree delta! I never let the heater exceed 210 degrees F, I

My question:
With the larger 1.25" feed and return piping (i'm assuming the 3/4" piping is the main reason for the large delta) will this be able to deliver the required temperature to this PVC pipe? (I'm seeking a 10 degree delta max).








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rick in Alaska

Not sure exactly what you are doing, but when I see 195 degrees and pvc, I cringe. Pvc gets very soft at that temperature, and if you have any kind of pressure on the pipe, it could be a major problem.
What are you trying to achieve?
Rick

RickDelta

Hi Rick!
I'm taking a 2" schedule 80 PVC pipe and expanding it to 4" diameter inside a 4" damaged cast iron sewer lateral pipe, to re-line the damaged sewer lateral ..... a pipe within a pipe.

rick in Alaska

I don't know about that.... You would have to heat the pipe up enough to make it soft enough, and then pressurize it to get it to mushroom out to the proper size. My guess is that if you could get this to all happen, the pipe would most likely not expand evenly, and be deformed on one side, but this is just a guess.
There are companies that install epoxy liners in piping to take care of what you are trying to do. You might want to check up on that.
Rick

RickDelta

No ..... it works great! I first bring the PVC pipe up to its "rubbery" state temperature (not expanding it yet) and thread it into the sewer lateral down past the trap then out to the municipal main. Then I raise temperature to its "forming" temperature and pressure (around 30 psi).
It evenly expands out to the inner wall of the lateral. While holding this pressure, I then cool down the heated water. This process changes the PVC on a molecular level affording significant "hoop" strength. The expanded PVC permanently forms tightly to the inside of the damaged lateral.
Note: the epoxy liners you mention cannot transverse a sewer trap, That's why these so called "NO DIG" company's show up with a back hoe to dig 6 feet down into your yard to remove the trap!..... my liner treads right tru the trap! ..... No excavation at all!

Gordy

I,was wondering also. Interesting!

ratio

That's awesome!
I think you're looking at a flow issue. Can you speed up the pump at all?

Mark Eatherton

Why mess with water. Why not kick in some steam? I seriously doubt that any AHJ would accept this method without seeing some sort of standards compliance report, but if no permit is required, have at it. Also, if the PVC is in contact with the 4" failed pipe, you will experience additional "loading" on this assembly, hence a higher delta T. May need to figure out some method of suspending the pipe within the failed pipe to avoid conductive losses to the exterior.

ME

RickDelta

@Ratio:
Hi! .... yea! I removed the Taco circulator and tried a "Jacuzzi" pump. Better delta, but the flow restriction caused the impeller in this pump to generate lots of heat! I saw the water temperature rising 15 - 20 degrees without even turning on the heaters!! YIKES!! : ( .... I think the 3/4" feed and return piping restricts the required flow too much.

@Mark:
Hello!
Schedule 40 PVC has long been an approved standard for sewer lateral repair/replacement (cast iron, clay, etc.).
Steam is too unwieldy for this application. It would be almost impossible to recover the spent condensate from the liner pipe. Remember, I'm inserting and expanding this liner from the ground level yard vent stack to a lateral 6-8 ft underground. All work is done from the only exposed end coming out of this vent stack. As I am blinded to the entire pipe liner insertion and expansion, the only way to know that the pipe has been fully expanded inside the damaged pipe is to meter the start volume of water in the 2" pipe and to stop the process when I reach the water volume of the 4" host pipe.

Boon

Above you said, "it works great!" ... but it is not working in this one instance so can you describe what changed between "it works great" (presumably the last time it worked) and what you're experiencing now?

hot_rod

A better heat transfer tube, maybe some finned or corrugated tube. I use CSST tube with the jacket stripped off for HX coils. With CSST or solar flex you get more surface area, and turbulent flow inside from the ruffles.

Also if you heat it with a torch you can anneal it and wrap it really tightly onto a coil. This 2" copper HX has 3/8 CSST coiled inside.

Expand the CSST to slide copper inside and braze it for transition connection.

RickDelta

@Boon:
Hello!
Initial testing lengths of 5 ft worked great. Longer test lengths of 10 ft work fine too, but required me to run source heaters at 210 to get a 185 degrees delivered to the liner pipe. My electric hairpin heating elements have to be cycled off on on constantly as they can produce tons of heating capacity. I never want the go above 212 degrees as a break anywhere in the system would flash to steam (explode). Now with my current test length of 100 ft, the delivery path of heat from source to liner pipe fails to reach its forming temperature (large delta T).
Note: This is a closed water loop with NO expansion tank in the loop! Heating the water from ambient to 200 degrees I am constantly "weeping off" the added water volume and pressure its creating.
While holding the forming pressure during cool down of the water the auto fill valve restores this expanded water volume as the water contracts back to ambient temperature as not to create a crushing vacuum within the pipe.

RickDelta

@HotRod:
I'm sure that would help! .... but the material and fabrication costs would go thru the roof making it doable .... but not practicable.
Also consider that this "inside" flexible pipe within the pipe must have a bending radius of the 4" trap its passing thru.
I'm using an inexpensive and disposable thin wall plastic hose to delver the heated water to the far end of the pipe liner.
I think the problem i'm having is in the volume capacity of the delivery feed and return lines.