PVC Induction Heating Coil Design (early thoughts)

[RickDelta]

Hello Physics Forums Community! : )

prelude ...

My quest is to develop an alternative to the conventional method of "primer and cement" bonding of typical pressure PVC pipe and fittings.
I want to effect a molecular PVC (polymer to polymer) bond only ....... without any chemical priming or cementing.
To achieve this inductively, I must insert an "susceptor" between the interfacial contact area of the joining pipe and the receiving socket of the fitting.

early concept thoughts ...

This would need be only a very low power RF inductive heater (1K watt max. @ 380 degrees F for 3 second max. bond fusing "on" time).
The susceptor would be a very thin stainless steel micro-mesh fabric screen (think: the inductively bonded sealing foil under a plastic pill bottles' cap bonded to the bottle top ..... but in a screen form).

The pics below shows this micro-mesh fabric screen melted into the "butt end" of a PVC pipe.
This was done with a standard electric flat iron.
This was done to contain the water filtration's TAC polymer beads fluidized within the pipe (media tank).
No primer or cements! .... creates an indestructible bond at no cost or curing time (typically 8 hours)!

This pipe "butt end" polymer melt bonding led me to think ...... could I melt the pipe to socket connection like this as well!

The pic below now shows this "butt end", melted on screen, to its 2" PVC pipe, inserted into a 2"x 4" PVC reducing coupler.
The dark area (running from 2 o'clock to 4 o'clock) is the incerted micro-mesh screen susceptor (normally would encircle the entire O.D. of the 2" pipe ..... but shown here for contrast)

Now, from the outside of the assembled pipe and fitting, I would attach the inductive "work coil" onto the fitting's socket ...... completely encircling the socket.

The "work coil's" design ...

The work coil is to be of a "split ring" design (because it has to place itself around the already assembled pipe and fittings)

My thoughts are to make my work coil based on a segmented motor "stator" framework (see pic below)

The work coil would be mounted in a plumbers "ProPress" like "Jaws" fashion, ..... opening and engaging around the PVC socket.
This portable induction heating system's circuit board would be attached to the split ring work coil frame (about the size of a pack of cigarettes)
The circuit board to be powered by a supercapacitor's discharge.

My initial question in all this ...

Instead of the typical single inductor work coil, can the design be of an equivalent "distributed" multi-inductor/multi-capacitor (mini-tank circuits) surrounding the susceptor? ie: every mini-tank circuit is of each segment on the stator.

If so, would this not keep the very high tank currents off the circuit board ...... and away from the much lower exciter current?

..... your thoughts on this?

 

[Tom.G]

before investing in any hardware.
I suggest you research the specific heat, thermal conductivity, softening temperature, viscosity versus temperature of PVC.

From that information you should be able to determine how much energy is needed to adequately soften the PVC.

Add to that the energy needed to heat the metal mesh.

Also be aware that some fittings are slightly tapered. This may or may not be a problem.

Then at least triple the above total to account for system inefficiencies and any minor 'oops' for things you overlooked. (for instance outdoors in Winter with a cold wind)

Once you get some realistic numbers, you may find that a super capacitor discharge is not adequate.

Please let us know what you find.

Cheers,
Tom

P.S. How will you assure the correct positioning of the mesh during joint assembly?

 

[RickDelta]

@Tom.G
Hi Tom! ...... I hear ya! : )

Your correct in that the socket is tapered ..... and the pipe is not.

Only a third of the bond in a chemically (primer/cement) fused joint is actually a polymer to polymer bond .... the remaining void (empty) area around the pipe ...... is just the cement itself.

The susceptor screen is lightly coated with a PVC sticky power (plastisol).
This aids in retaining the screen in place and the surface of the metal screen itself has a slight "teeth grab" to it ..... further helping it to remain fixed.

The plastisol melts out into the void areas enabling a full socket polymer to polymer bond

I'm hopeful the supercapacitor approach will work as the duty cycle is only 3 sec "ON" max and 60 sec. "OFF" (recover cycle).

....... another alternative approach for a susceptor I'm looking into is using Tuball 814 Matrix (graphene nanotubes in a PVC polymer base) in a paste or film strip.

https://tuball.com/about-tuball

 

[Baluncore]

This aids in retaining the screen in place and the surface of the metal screen itself has a slight "teeth grab" to it ...

How will you make the screen?

If it is a woven mesh, the cross-wires will not have good electrical contact, which will reduce the eddy currents. If the mesh has good electrical contact, it will not conform to the two surfaces being melded.

Teeth, imply that air will be trapped in the mesh and joint space as it melts, that air will be unable to escape.

The glossy surface of PVC does not have free bonds, as they are contaminated by the environment. Before you make the joint, you will need to re-activate the surfaces with a solvent that cleans contaminants from the surface.

 

[RickDelta]

@Baluncore
Hello! : )

"If it is a woven mesh, the cross-wires will not have good electrical contact, which will reduce the eddy currents" - Baluncore

Lets say, we make a wire mesh ...... where all of the continuous lengths of wire threads (going horizontal and vertical), don't even touch each other at any intersection!
........ your saying these wires cant be inductively heated??

 

[Baluncore]

........ your saying these wires cant be inductively heated??

There must be an area penetrated by flux, to generate the peripheral eddy current.
An orthogonal mesh, if it is woven with insulation between warp and weft, has virtually no area and so no significant eddy current or heating.

If, on the other hand, the mesh was made like chain mail, being thousands of small closed loops, then each planar loop would support an eddy current. Unfortunately, chain mail has a significant thickness, due to interlocking by loops on edge, so it will trap air in the joint. At the same time, the hot planar loops will not be in contact with the PVC, only those cold loops on edge will, so there will be no bonding. It will take a fancy magnetic field to heat all the loops.

Friction welding is used for assembly of long polymer pipelines. Maybe you can replace the spinning friction disc, with eddy currents, in small metal loops, that become embedded in the polymer.

 

[RickDelta]

@Baluncore
"There must be an area penetrated by flux, to generate the peripheral eddy current". - Baluncore

so, ...... your saying these metal "wires" can't be inductively heated??

 

[Baluncore]

so, ...... your saying these metal "wires" can't be inductively heated??

Not unless they are thin tapes.

 

[RickDelta]

@Baluncore
so, ..... then you cant inductively heat, lets say, ... a very thin sewing needle??

 

[Baluncore]

so, ..... then you cant inductively heat, lets say, ... a very thin sewing needle??

You can if the needle has a significant area compared to the area of the induction coil.

The laminations in a transformer are oriented so they will have minimum eddy current heating losses. Your mesh is similar.

 

[RickDelta]

 

[Baluncore]

That video shows a small coil with very close contact to the wire being heated. The diameter of the wire is greater than the separation between coil and wire.

You want to heat wires on the other side of the PVC, which will require a significantly greater coil size, and so coil area.

 

[RickDelta]

Agreed! : ) so, ...... we can inductively heat the wires?

 

[Baluncore]

Agreed! : ) so, ...... we can inductively heat the wires?

Not if they are far from your induction coils, which they must be, since the mesh is inside the PVC joint, and the induction coils are ranged around the outside of the pipe.

 

[RickDelta]

"Not if they are far from your induction coils ....."

You did read the proposed coil construction above ...... coils with flux concentrators ..... right?

 

[Baluncore]

You did read the proposed coil construction above ...... coils with flux concentrators ..... right?

No.
Induction heating coils have an air core, and operate between about 10 kHz and 30 MHz.

Induction coils cannot have a magnetic core, because the core would get hotter than the wires, and would screen the item that you wanted to heat.

What is a flux concentrator? How come it does not also get hot?

 

[RickDelta]

In induction heating, magnetic flux concentrators, also known as flux intensifiers or controllers, are materials used to focus and intensify the magnetic field generated by the induction coil, improving efficiency and allowing for more precise heating of specific areas of a workpiece.

https://www.inductoheat.com/an-objective-assessment-of-magnetic-flux-concentrators-2/#:~:text=March 26th, 2021. Magnetic flux concentrators (also,that of magnetic cores in power transformers.

Note: The segmented stator above are coil mounted "ferrite" cores ..... not iron laminations!

 

[RickDelta]

https://fluxtrol.com/magnetic-flux-control-in-induction-heating

 

[Baluncore]

It seems that a flux concentrator is actually a partial external magnetic core, designed to reduce interactions between multiple induction coils by screening.

Go ahead, design your coil geometry, include the flux concentrators, but don't expect to heat the thin wires, that are relatively far away on the other side of the PVC.

 

[RickDelta]

" ........ that are relatively far away on the other side of the PVC." - Baluncore

The gap is only 1/8 inch away ever!

 

[Baluncore]

The gap is only 1/8 inch away ever!

What is the thickness, t, of the wires?

 

[RickDelta]

.0065" (70 mesh)

 

[Baluncore]

0.125 / 0.0065 = 20

 

[RickDelta]

In my PVC "spin weld" prototype below .... the interfacial heating is largely restricted to just the immediate surfaces.
Its all done in less than 3/4 seconds!

(the joint is in between the arrows!)

 

[RickDelta]

0.125 / 0.0065 = 20

....... ok

 

[Tom.G]

If your mesh has the overlaps in electrical contact, you might be able to get enough induced current to get resistance heating of the mesh.

You obviously have done a fair amount of research on the project. (more than most of us have!)

I'm curious how you can ensure 1/8" separation between the exciting coil and the mesh with different pipe diameters.
1) Are you assuming a different coil assembly for each pipe size?
2) What about buried pipes in a ditch?
3) How awkward would it be having to slide the coil assembly along a 20ft. pipe; say between floors in a building?

Possible workaround: make the coil assembly as a hinged item, like handcuffs.​

Please keep us updated an your progress and results, both success' and failures. We like to learn too!

Cheers,
Tom

 

[RickDelta]

I'm curious how you can ensure 1/8" separation between the exciting coil and the mesh with different pipe diameters.
1) Are you assuming a different coil assembly for each pipe size?
Yes!
2) What about buried pipes in a ditch?
Not an issue.
3) How awkward would it be having to slide the coil assembly along a 20ft. pipe; say between floors in a building?
Not an issue! ...... The work coil is a "split ring" design (read above).
Possible workaround: make the coil assembly as a hinged item, like handcuffs.
Agreed! ...... my thoughts as well.

In the above pic,
See the raised circular rings at the top of the pic? .... there would reside the "work coil tanks" (multiple tank inductors and capacitors) within that raised area only.
Notice that the rings are "split" (think: spring loaded hinged wooden clothes pin to open the "jaws" around the pipe)

 

[Baluncore]

I'm curious how you can ensure 1/8" separation between the exciting coil and the mesh with different pipe diameters.

The PVC pipe wall thickness is typically 0.125", the mesh is between the two walls where the joint will be made, so the mesh must be at least 0.125" from the coil.

 

[RickDelta]

The PVC pipe wall thickness is typically 0.125", the mesh is between the two walls where the joint will be made, so the mesh must be at least 0.125" from the coil.

Good Morning Baluncore! : )
I'm a bit lost in your question : (

Schedule 40 PVC pipe is approximately 1/8"( 0.125") in wall thickness (3/4" thru 4" pipe sizes).

so, yes! .... agreed!

 

[Baluncore]

Good Morning Baluncore! : )
I'm a bit lost in your question : (

Good evening RickDelta.
That is not a question, it is a reply to Tom.G .
It may be morning where you are. It is evening here, 2100 hours.