Possible to turn a piece of pipe/tube into an induction heater?

[Michael77]

I have been researching induction heating as of late and last week I found a site that showed a metal tube with two AC leads going to it (one at each end). The voltage was stepped down to something reasonable so that someone touching it when there is the hopefully unlikely grounding/neutral issue would not be fatal.

I tried to find the site again but couldn't. Then I found hundreds of sites showing an induction heater as a coil around a conductive object, which makes a whole lot more sense to me in terms of the word "induction" being used.

Perhaps the simple tube heater (intended to heat whatever is inside the tube) was actually a resistive heater, with the tube being the resistance? I just can't see how the resistance of a piece of pipe could ever be high enough to not cause an over draw dituation.

Thanks for any knowledge shed on this topic.

 

[Skogsgurra]

The tube (the one you first found) was either one of the numerous mistakes or myths that makes the web such an unreliable place to get information from.

First, there is no magnetic field inside a tube. If you think of the tube as a lot of strands connected to rings at the end of the tube, you will easily see that the individual magnetic fields cancel. Some magnetic field can be measured close to the tube inside wall, but it is not an efficient way of producing a heater.

Next, a heater working with small dimensions cannot use 60 or 50 Hz. A typical heater that is used to heat large steel pieces for forging presses uses 500 - 1000 Hz and smaller heating coils for welding use much higher frequencies.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

The tube could be just a resistor. Drop the voltage with a transformer down to whatever voltage is needed to dump 1kW across the tube. Tube turns red-hot.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Yes. like an electric oven. But long and narrow. Like attached picture?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Many years ago in Canada it was common practice to thaw frozen underground water piping by connecting an electric welder between the shutoff valve-stem at the water main and the water pipe where it entered the house.
These days too complications arise. Non metallic piping, insulated joints on metallic piping, possible damage arising from stray heavy ground currents; the list continues.
Mainly though, building codes now require domestic water services to be buried much deeper and freezing is no longer the frequent issue that it was years ago.


Bill
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"Why not the best?"
Jimmy Carter

 

[Compositepro]

If the wire are attached to the pipe, It is definitely being used as a resistance heater. The resistance of steel is much higher than copper but not so high that the voltage drop required to get high wattage is very high. The pipe heater requires high current at low voltage. A spot welder could be a suitable power supply. It supplies up to thousands of amps at a few volts. An arc welder supplies hundreds of amps at a somewhat higher voltage.

 

[Michael77]

Thanks for all the replies. You know the funny thing is, the web site was actually a company trying to sell this design as an induction heater, not just someone talking about it. I was nearly 100% certain that it had to be resistive.

I would love to turn my vessel into a resistive heater, just have to figure an economical way to step 115 / 220 down to say 5 volts or so. I guess an 'ol transformer it the only way to go.

By the way, I am a mechanical engineer, not electrical, so kindly excuse my ignorance here

 

[frankee]

Hi Michael,

It is common for induction coils for large heaters to be made from common copper water pipe. It's not clear in your post if this is what you were looking at. This units that I have worked with can carry currents of around 500-1000 Amps. I think you may be missing the science behind induction heating: the heat is not created by power transfer through the resistance of the pipe, but rather by the current around the pipe (bent into a coil shape) inducing currents into the target material (which mas be ferrous), and those currents heating the target. The induction of these currents is done at high frequencies, up to 10KHz, and the black magic in large induction heaters is tuning the heater to the optimum frequency. The coil design, how many turns of the pipe, and it's diameter etc., and the material being heated greatly effects the amount of heating. Adding to the complexity, as the target material heats up its magnetic properties decrease so there is a lot of tweaking to obtain an optimum power transfer.

Another interesting fact, induction heaters are one of the few remaining applications that still use vacuum tubes to create the oscillating signal required for the induction. Of course, these vacuum tubes can cost upwards to $1000 each, and are water cooled. Also, the copper piping used as the induction coil will also have water flowing through it since copper can not be heated to the same temperature as the target ferrous material (aka steel). It's a very interesting field, and I have a customer who uses induction heating while epoxy coating rebar, which is how I am familiar with it.

Frank

 

[rcw retired EE]

Thermon sells a pipe line heating/freeze protection system with a single power wire inside a steel heat tube. The tube is fastened to the pipe to be heated, sometimes welded or cemented for maximum heat transfer.

At the far end of the circuit, the single power wire is connected to the heating tube so the tube is the return circuit. Due to skin effect, most of the current flows on the inside wall of the heat tube, raising the resistance. A single phase power supply is connected at the near end.

It is a very practical and safe solution for heating long runs of pipe. Power supply points can be located much further apart than if normal heating cables were used. (15 miles, 25 kM)

But, I don't see how this design would help your application.

http://www.thermon.com/US/products.aspx?prodid=10

 

[biff44]

An induction heater is a coil of good conductive material, such as copper, that has an RF field imposed on it. If you place a poor conductor, like a piece of steel, inside of the copper coil, it will "induce" high RF currents to flow in the steel. Since the steel is resistive...it heats up.

They like to use copper tubing for the RF coil, since you can pass cooling water inside of it, and keep the copper from melting.

Not the sort of project a neophyte should tackle, as there are high RF voltages and currents involved--could be some safety issues.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[roydm]

Yes, we use a number of reactors made up of long lengths of 3/8 SS tube with 24VAC applied in the middle and grounded at each end. The transformer primary is fed from a 2 phase solid state controller.
It's not induction heat though, purely resistive.

 

[ScottyUK]

Biff: The frequency of interest is not necessarily RF - many induction heaters operate at a few kHz.


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If we learn from our mistakes I'm getting a great education!

 

[zzzjim]

I too have seen videos , big inverters ,tuned caps .

Probably burnt fingers , and so much RFI you best live on a ranch in USA. If you want to home brew this !