Inspired by the success of my I decided that the next summer could be coming in a few weeks and that is time to find something for those long camping trips.
I did a lot of reading on the topic of induction heating to get my induction metal heater working and it is still a work in progress to get even more efficiency out of it.
The good thing about all this reading is that you also get a better understanding on magnetism in general.
So what is topic about?
We all have hot water in our homes, some use gas, other electricty or even wood to heat it up.
In areas with no gas mains supply or simply in your weekend home or camper you only have gas as an option.
This is where "my" idea comes into place.
I want to design a highly efficient water heater based on magnetic induction, similar to my other project but in "reverse".
The benefit would be that the actual heating unit can be sized to your actual needs and what is available in terms of electricity supply.
How do I think it should work?
Well, my "studies" showed that heat from induction needs high frequencies and strong magnetic fields to work.
I don't want to into all the details about magnetic fields, windings and resonance so for that check the link above to see what I could gather so far.
If you move a magnetic field over a conductor the resulting field changes will induce a current flow in the conductor - this current, if strong enough, will heat the conductor over time.
This idea started a few months back when a friend ran out of hot water because his gas bottle was empty again - not very efficient these cheap camping hot water systems...
So forgive me for not taking a video of my test but you can replay it at home:
Take a fast router that can go to at aleast 10.000rpm.
Take an old router bit and mount a strong neodymium magnet with epoxy glue centered on the bit so the poles face outwards to the spindle at 90°.
Mount the router with the bit so it face upwards (does not matter but makes the rest easier) - be sure it is tight and won't move around when you start the damn thing!
Create a mounting plate that can be fixed or clamped to the no upwards facing base of the router.
In the centre mount a piece of pipe - I had a 40mm copper pipe that only left about 2mm gap to the two magnets I joined together.
Make sure the pipe is properly centered around the router bit and fixed in place!!
If can vary the speed of the router start at the lowest and work your way up for a nice effect
You will see that within a very short time the pipe get hot, in fact red hot if you don't turn the router off or cool it!
What is the aim here?
To find a simple, yet efficient design that can be mounted or connected to basically any motor with a reasonable speed.
For camping use I was thinking of the motors for fuel pumps as they are strong and quite fast, plus they don't mind continious use - but does not matter as long as it fast.
I want to find and tests ways of making such a system efficient enough to be used as a continous hot water system with enough power that a good shower is possible.
Current design idea:
In the most simple form a stack of neodymium magnets on an axle, the magnets should be twisted so a spiral forms with opposing poles next to each other.
As an alternative the rotor of a DC generator or motor can be used but it should be the types with two rings for the connection, not the standard slotted design.
The actual generator needs to be able to accomodate the cooling, for us the water we actually want to heat
It also has to be able to get a huge current induced by the magnets while at the same time be low on mass with a high resistance to the eddy currents we want to induce - that why it is an induction water heater
Have to go a bit deeper here so get an idea how the system works - sorry...
Using a metal pipe around the magnets (will call the whole thing "rotor" from now on!) will heat it up nicely but is not efficient if we want water going through.
Several designs you can find on the net use quite complicated spheres of copper pipe, formed as a coil, around this centre pipe.
To keep the heat some enclose the whole thing with copper sheets - not very smart if you ask me as there is no benefit for the magnetic field and the copper trensmits the heat to the outside, insulation would be smarter...
But what if we actually create an inline heating system?
The rotor creates massive field lines in the pipe wich not only alternate by the spin but also by the placement of the magnets itself.
Using a coil type water circulation system that actually works would require to match the pitch of the copper coil to the pitch of the rotor and trust me you either need very small magnets or end up with a "bird cage" of copper pipes.
Keep in mind the produced magnetic field would rotate around the axis of the pipe any other way and it can't work.
To simplify the design of a heater core I was thinking of a relatively long rotor and pipe without copper pipes brazed onto the surface.
Instead I would use a second pipe, perferable something heat resistant but thermally insulating, that only leaves a small gap to the copper pipe around the rotor.
Capped at both ends and with added water connections on the end plates or outer pipe it should work fine.
To further improve efficiency there should be a spiral between the pipes, best from copper and brazed to the inner pipe.
This way the water has longer way to travel and more surface area for the heating at the same time.
Of course there need to be either a bearing on the end of the pipe for the rotor or both heater core and motor/rotor assembly must be properly fixed - keep in mind the rotor needs to be balanced to avoid vibration and damage to the bearings.
It needs to be tested if a magnetically inert material for the outer pipe works better than a galvanised pipe from soft steel - I am uncertain if a magnetic outer core that is not made from ferrite would work for those high frequencies as in my test the penetration trough the copper goes down the higher the speed of the rotor is - high frequency induction inside the pipe from the opposing magnetic fields generated by the rotor, at high speeds these magnetic fields start to cancel each other out in favour of the low electrical resistance of the copper and produce elecricity which adds to the already generated electricty from the rotor itself.
Find the right speed for the magnet configuration and just a copper pipe is glowing red in under 2 minutes!
My problems:
I only have a quite thin copper pipe or a 80mm pipe at hand.
For the thin one I have some magnets left for a small scale test setup but the big one would require a suitable DC core or at least a few hundred magnets, not to mention the problem of centrifugal forces on the magnets at high speeds and the safety concerns if the glue gives up (never use without water for the cooling LOL).
I hope someone finds interest in my design ideas and tries something out, I won't be starting building this project until my other one is performing to my satisfaction.
In my test I used a 50mm copper pipe, about 15cm long with a few rounds of 1/4"" copper pipe coil brazed onto it.
No external pipe or insulation.
Rotor was a router bit with two big magnets glued onto the top, I centered the bit in a cardboard pipe and filled it with resin to make sure the magnets are fully enlosed.
With low pressure a glass of water was filled in about 20 seconds and the temp difference was a bit over 18°
Considering only two magnets were used and that they only covered 5cm of the pipe it was quite good.
So a longer rotor with more magnets that cover the full lenght of the pipe enough flow and temp should be possible for a good shower.
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