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Induction heating - inline water heating

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Chobrutsky

Mon Dec 10 2012, 04:21PM

Registered Member #8448 Joined: Mon Dec 03 2012, 03:23AM
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Posts: 5

Hey all,

Some great threads here on IH, but none that seem to really address the following project.

I'm trying to make a hot water heater, i.e. for beverages so not like a shower or for a whole house. I'm thinking about using an arduino to output a variable PWM based on the water output temperature. I'm running into some trouble with the circuits/physics of the IH though, and looking for some expert help here.

I'm thinking about doing 10-100 turns of 105/40 litz wire around a .405" OD, .215" ID, black steel tube, lined with a 3/16" copper tube carrying water through it. The length of the heating tube could be up to a foot if required, but obviously less material would be better. I need to heat the water at a speed of about 0.07GPM from about 10C-100C.

What I'm really looking for is an inverter circuit that I can control with a PWM signal, and then maybe some advice on what sort of voltage/current/kHz requirements I should start experimenting with. Also, I'm hoping to run this from a convention US power outlet, if that makes a difference.

Thank you in advance, from the looks of it there is quite a bit of expertise here.

IamSmooth

Mon Dec 10 2012, 04:26PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

The project at the end uses an Arduino. You can modify the code and add a temperature input to customize it for your project.

Chobrutsky

Mon Dec 10 2012, 04:57PM

Registered Member #8448 Joined: Mon Dec 03 2012, 03:23AM
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I don't think that project is really quite along the same lines. He has quite a different set up which isn't exactly analagous. The code is not my issue. It's the hardware and circuitry. He has his wires wrapped parallel to what appears to be a cooling tube. I'm shooting to wrap the induction wire circuferentially around a long thin steel tube to heat it up very rapidly, then having that heat tranferrred to the copper tube inside carrying the water.

Thanks for the help!

Pinky's Brain

Mon Dec 10 2012, 07:31PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Or you could use a transformer and just heat the copper pipe ... with a meter of 1/8 inch tube you would need around 1.5 kA RMS :) (You need around 2 kW.)

More serious, I'd personally try wrapping 1/8 inch tubing in teflon and then wrap a coil of nichrome wire around that. Feed it with rectified mains and a buck converter or straight PWM. So at 170V you need around 15 ohm of resistance, a 4 oz spool of 18 SWG nichrome wire will get you a little below that.

PS. grounding the copper tube of course for safety, also you need a thermal cut off of course.

lightlinked

Mon Dec 10 2012, 07:54PM

Registered Member #2087 Joined: Tue Apr 21 2009, 08:32AM
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is what you want a separate inverter drive, ie induction part its own thing, controlled by an arduino as a PID?

Wolfram

Mon Dec 10 2012, 11:04PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

What advantage will an induction heater have over a resistive heater in this application?

More serious, I'd personally try wrapping 1/8 inch tubing in teflon and then wrap a coil of nichrome wire around that. Feed it with rectified mains and a buck converter or straight PWM.

Why not just use simple phase angle control?

Chobrutsky

Mon Dec 10 2012, 11:38PM

Registered Member #8448 Joined: Mon Dec 03 2012, 03:23AM
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Posts: 5

lightlinked wrote ...

is what you want a separate inverter drive, ie induction part its own thing, controlled by an arduino as a PID?

Yea that was the idea.

What do you all think about just using a single mosfet gated with a PWM from the arduino to produce a DC half-squarewave signal at like 100khz? I'm really not sure about how to go about estimating the heat I'll be getting out of this thing as a function of tube length and coils, other than just going for it and seeing what blows up first or melts.

I'm looking into the other suggestions posted to see what I can say about those...

Thanks again

lightlinked

Tue Dec 11 2012, 12:17AM

Registered Member #2087 Joined: Tue Apr 21 2009, 08:32AM
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if you were doing a resonant tank could you do zero cross switching with the arduino pwming enable at a low freq? i guess it depends how you want to tune it. maybe since the load stays the same, ie isnt something random being heated you could set it with a scope and set the freq with the timer registers

Chobrutsky

Tue Dec 11 2012, 12:27AM

Registered Member #8448 Joined: Mon Dec 03 2012, 03:23AM
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Posts: 5

lightlinked wrote ...

if you were doing a resonant tank could you do zero cross switching with the arduino pwming enable at a low freq? i guess it depends how you want to tune it. maybe since the load stays the same, ie isnt something random being heated you could set it with a scope and set the freq with the timer registers

From what I've read about induction heating, since my "work piece" is so small (.405" OD) to heat it efficiently i need to be over 100khz. How low are you talking for low freq PWM? My thought was that the PID would cause the PWM to go faster (higher freq) to heat more, lower freq to heat less. Does that sound right to you?

Chobrutsky

Tue Dec 11 2012, 12:30AM

Registered Member #8448 Joined: Mon Dec 03 2012, 03:23AM
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Posts: 5

Pinky's Brain wrote ...

Or you could use a transformer and just heat the copper pipe ... with a meter of 1/8 inch tube you would need around 1.5 kA RMS :) (You need around 2 kW.)

More serious, I'd personally try wrapping 1/8 inch tubing in teflon and then wrap a coil of nichrome wire around that. Feed it with rectified mains and a buck converter or straight PWM. So at 170V you need around 15 ohm of resistance, a 4 oz spool of 18 SWG nichrome wire will get you a little below that.

PS. grounding the copper tube of course for safety, also you need a thermal cut off of course.

That's not a bad idea. Is there something better than teflon to bridge the two? Teflon, while it can withstand high heat, to my known has terrible conductive properties. Do you think that'd be a problem? Do you really think I'd need 50ft of nichrome to pull that off? I think I need about 1600W. But I'm not sure how that translates into nichrome length/current/power/etc.

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