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Fast temperature switching using Pletier and Induction heater

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LAN_403

Mates

Fri Nov 09 2007, 11:41PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

Hi guys, I need some helpâ€¦
For specific chemical reaction I built a specialized Peltier based device. In principle I have piece of glass lying on the ceramic surface of the Peltier unit which first gets pre-cooled to aprox. minus 20 C. First part of the reaction must be performed in freeze conditions. For the second part I need very sudden and fast temperature switch to aprox. 100 C. Iâ€™m using the polarity switch of the Peltier but unfortunately it is not quick enough. I can get better result by simply placing the glass on a heat block. But, moving of the glass is a bigger problem so I have to use the Peltier anyway.

My idea how to possibly get over the problem: By placing a small metal plate (like coin) between the peltier and the glass and use induction heater to heat up the coin (and the glass) instead of the peltier. I also have an idea how to avoid the heat losses by the contact of the peltier and the coin, by placing the coin on a small bedding leaving a gap between which can be filled by easy-to evaporate liquid (ethanol).

scheme

(Sorry the -100C on the heat sink is a nonsense !)

My questions:
Does it have a chance to be faster than the Peltier in heating up the glass?

Will also the Alu-heat sink get hot by the pulsating magnetic field (what about Cu)?

What is the best metal for the coin (some ferrite or plain Fe?)

Any links for construction of powerful induction heaters?

Thanks a lot...

Eric

Fri Nov 09 2007, 11:58PM

Registered Member #69 Joined: Thu Feb 09 2006, 07:42AM
Location:
Posts: 116

I think the fastest method to switch the temperature would be to pump 100C water or steam through a heat sink/block. Even if the block is at -20C to start with, the transition could be very fast.

Mates

Sat Nov 10 2007, 12:02AM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

Eric wrote ...

I think the fastest method to switch the temperature would be to pump 100C water or steam through a heat sink/block. Even if the block is at -20C to start with, the transition could be very fast.

Good idea but undoable in my configuration. Also the heating of massive Al block wouldn't be fast eneough...

thanks anyway

Nik

Sat Nov 10 2007, 12:08AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

You could put a resistive heater under/over the glass. That could be as simple as a thin sheet of metal and a high current low voltage power supply. When you need the heat just turn on the power.

Mates

Sat Nov 10 2007, 12:29AM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

Nik wrote ...

You could put a resistive heater under/over the glass. That could be as simple as a thin sheet of metal and a high current low voltage power supply. When you need the heat just turn on the power.

Good idea, but I'm affraid that would be just mimicking the peltier which is not strong enough. The pletier is 55W, (rated to 190C) so it works as a pretty efficient resistive heater. I'm not sure of getting better resisitve heater in this shape and power (must be plate-like). But I'll think about it - thanks for a hint

BTW: over the glass is not possible it needs to be heated from the bottom first (I've already tried very strong (250W) focused IR source)

...

Sat Nov 10 2007, 12:34AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You are not thinking enough power--you need to have the plate juiced with a few KW of power for a fraction of a second (you would want to use an external resistive heater for that). A pancake coil of type K thermocouple wire would work great for that.

There is no point in making a induction heater for something like this, since you can easily use direct electrical power. You would want an induction heater it you wanted to heat a block of metal inside that glass, or something along those lines.

Mates

Sat Nov 10 2007, 12:54AM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

... wrote ...

You are not thinking enough power--you need to have the plate juiced with a few KW of power for a fraction of a second (you would want to use an external resistive heater for that). A pancake coil of type K thermocouple wire would work great for that.

There is no point in making a induction heater for something like this, since you can easily use direct electrical power. You would want an induction heater it you wanted to heat a block of metal inside that glass, or something along those lines.

Any link for that K thermocouple wire? It will be also qiute tricky to make that pancake coil having the right properties for the system (the procedure has additional demands I can not describe all of them). However I'll try it as a possibility - thanks.

What about my questions?

Bjørn

Sat Nov 10 2007, 01:38AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Does it have a chance to be faster than the Peltier in heating up the glass?

Yes, you can heat it faster but it will not be very simple. Using a resistive heater will be much simpler and fast enough to shatter the glass and blow the whole setup to bits. It is just a question of applying enough power, by using a large capacitor you can get extremely rapid heating.

Will also the Alu-heat sink get hot by the pulsating magnetic field (what about Cu)?

Yes, the pulsating field will induce currents in conductors. The effect depends on the frequency and type of material. Search older posts and you will find several designs and more information.

ConKbot of Doom

Mon Nov 12 2007, 03:36AM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329

whats the reason to use thermocouple wire instead of something like nichrome?

...

Mon Nov 12 2007, 03:51AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

type K thermocouple wire is nichrome wire with a convenient braided glass insulation (good for extreemly high temps), available very cheaply surplus (ebay).

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