Feasibility of using induction heater circuit for my coffee sample roaster.

Discussion in 'The Projects Forum' started by Viridian, Oct 7, 2014.

  1. Viridian

    Thread Starter New Member

    Oct 7, 2014
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    Hey All,
    I just joined AAC with the purpose of taking advantage of the extensive knowledge base here.

    I need to know if it is feasible to use an inductive heating circuit to directly heat the steel drum of a very small capacity coffee sample roaster. The roaster looks like this:

    Using induction might be more of a novelty than anything, but it could actually present some real advantages to the roasting process of coffee over and against using a resistive heating element to indirectly heat the steel roast drum. Heat transfer is very important in roasting coffee so turning the drum into the source of heat would improve efficiency, at least in theory.

    This video on YouTube was what got me to thinking about using induction to directly heat the roaster drum:

    Questions:
    1. Is induction a more efficient way to heat?
    2. Would it be feasible to surround the rotating drum with heavy gauge copper coils to directly heat it?
    3. How important would it be to tune the circuit to resonance to achieve peak efficiency?

    Bonus question:
    Would it also be feasible to use induction to rotate the drum? As I understand it, eddy currents cause the aluminum disk to spin in the electro-mechanical KW/hr meter like the utilities install for metering our power.

    Any thoughts are appreciated, even if it's to shoot holes in my idea.
    What do you think?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    1/ Induction heating is very efficient, but:
    2/ it will require a great deal of energy to heat a large drum, it is typically very localized as to the target point.
    3/ the frequency is usually based on the size or mass of the item to be heated within the coil.

    Having worked on a few induction heating systems, I don't think it would be the ideal answer in this case.
    The video shows a small system, anything larger will require all components to be water cooled.
    If it was pursued, it would require quite some experimentation to get the right combination and power level.
    Max.
     
  3. Viridian

    Thread Starter New Member

    Oct 7, 2014
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    @MaxHeadRoom, thanks for your replies. What do you think would be a more ideal answer? Heating a steel drum indirectly via a radiating resistive element seems wasteful and boring to me.

    I'm not averse to experimentation. :) Inductive heating would be unique to this process, I believe, and that makes it worth trying!

    2/ the drum may be only a few pounds and would not need to be heated quickly. Maintaining ~500F is the general target.
    3/ Could an anolog powerfactor meter work for indicating the correct frequency(unity)?

    Truth be told, there is a market for custom coffee sample roasters and the cool factor of this would be off the charts.

    Cheers!
    Viridian
     
  4. MaxHeadRoom

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    Jul 18, 2013
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    One problem I could see with this kind of application is maintaining an exact temperature, it is possible to switch power but the larger the mass, the larger the hysteresis?
    Unless there is previous examples out there, it would be a question of mocking something up and experiment, the smaller the target item the higher the frequency used, from say 700Khz for a 1" pipe to 60/120Hz for a large steel ingot.
    Max.
     
  5. williamj

    Active Member

    Sep 3, 2009
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    I've not "played" with induction heating. But it seems to me that if you induce a temperature into a pipe that has a circulating medium running through it. And then have that medium pumped through a heat exchanger that's placed in the roaster through the roaster's opening (heating the "inside" of the roaster and not the roaster itself) you might achieve what you are after. I have no idea if the transferred heat would be enough to reach the required temperature.
     
  6. MaxHeadRoom

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    Jul 18, 2013
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    The only thing I see is that HF induction heating is usually aimed at a very local source of heat induced into the primary (Inductive) item, coffee beans obviously would need a secondary source of heat such as a steel chamber or drum, this is not much different from heating the drum with a normal resistive heater, maybe the capacitive version may work, the same principle used to fast seasoning timber, the equipment is basically the same except the material is placed in the effective dielectric space, instead of the centre of the coil.
    Max.
     
    Last edited: Oct 8, 2014
  7. Viridian

    Thread Starter New Member

    Oct 7, 2014
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    As far as hysteresis, a tuned PID loop would be able to deal with that, would it not? Can a PWM circuit be controlled by an industrial PID and thus control the power? I've been doing some reading and kind of understand the function of these terms, but I would not be able to get very far on my own.

    I found this and have been reading it: http://inductionheatertutorial.com/
     
  8. Viridian

    Thread Starter New Member

    Oct 7, 2014
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    @williamj, Hot air is often used to directly roast coffee, it's called convective roasting. It works quite well and is how I have built all my roasters so far. I use a phase angle fired SCR to control power to an industrial air heater tube. I suppose that a nichrome wire in a tube could be heated via induction to heat an air stream.... Could it?

    But now I want to make a more traditional design, just with the difference of heating the drum/barrel via induction rather than through radiant heat transfer(inefficient) from a resistive heating element in close proximity to the drum.

    Let's assume that the barrel/drum is 4" diameter and 6" long and 0.125" thick wall. After watching several videos it makes me hopeful that it could be evenly heated to 500F using induction. Max are you saying that it would be difficult to heat the barrel evenly over its length?

    What are the factors I need to take into account? Is my grip on reality weakening?
     
  9. Viridian

    Thread Starter New Member

    Oct 7, 2014
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    @MaxHeadRoom, I'm not sure what you are saying here but I think what you are saying is what I'd like to attempt: I want the drum to be the heat source.
     
  10. MaxHeadRoom

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    Jul 18, 2013
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    It may be quite possible, Just I have never come across HF Induction heating for this application, some experimentation may prove it works, what are the exact dimensions of the drum you envisage?
    I have never tried capacitive heating either, although coffee beans as a medium may not be the best idea.
    Max.
     
  11. Viridian

    Thread Starter New Member

    Oct 7, 2014
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    I'll get the exact dimensions ASAP.

    Capacitive Heating? Sounds like another rabbit hole to go down.
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It uses the same principle and tank circuit, just that the medium to be heated occupies the dielectric space instead of in the coil.
    http://www.radyne.com/food.html
    Max.
     
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  13. Viridian

    Thread Starter New Member

    Oct 7, 2014
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    Can you please say more about this?

    effective dielectric space? Do you mean in the air around it?
     
  14. MaxHeadRoom

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    The HF heating consists of a tuned Tank, a coil and a capacitor bank, for induction heating the steel object is placed within the coil, in dielectric heating, the non-metallic object is placed between the capacitor plates.
    Similar in principle to a Microwave oven.
    Max.
     
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  15. wayneh

    Expert

    Sep 9, 2010
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    I think this is a technology looking for an application. Why the complexity?

    Efficiency does not come from how the heat is made, it only comes from preventing the heat from escaping - insulation. The beans don't care what made the metal hot, they know only temperature. Heat transfer into the bean will depend on the rotation rate, maybe the loading of the chamber, the use of internal baffles, but not on the source of heat to the metal.

    You said that heat transfer was important. I get that, but isn't it really the time-temperature profile that you want to achieve? Of course you need heat transfer to make that happen, but the goal is temperature control in the bean at every moment, right?
     
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  16. Viridian

    Thread Starter New Member

    Oct 7, 2014
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    Yes, you are correct in all you've said. Nice way to pull focus on what is happening.

    Yes, insulation is important. Yes, I want to control the time-temp profile of the coffee beans.

    resistive heat element --> roaster drum --> coffee beans
    inductive heating of roaster drum --> coffee beans

    I guess I'm asking two things:
    1. Is it possible to heat a metal tube of a certain size to a given controlled temperature using induction?
    2. Does inductive heating represent a less "lossy" way of getting energy as heat into the coffee beans?

    My goal is to be able to bring one pound of beans to a temp of 440F in about 11 minutes from an ambient temp of 70F using 1KW of power for heat. I've been able to do this using a fluid bed air roaster design(ceramic insulation was key).
     
  17. MaxHeadRoom

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    Jul 18, 2013
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    I would think it should be possible to design a PID loop to control the HF design, just as you would a resistive method etc.
    There are still losses as the coil and often the capacitor bank are generally water cooled in the process, as well as the switching semi conductors.
    Max.
     
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  18. wayneh

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    Sep 9, 2010
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    1. Probably. I defer to the experts.
    2. I don't think so. You have circuitry handling high currents and generating inevitable losses. This lost heat does not end up in the beans. A heating element insulated all the way around except for where it touches the roasting chamber seems to me a more efficient mechanism. Eliminating as much heated mass as possible will help direct the heat to the beans instead of the equipment.

    I see the challenge as minimizing thermal mass and tightening the control loop, so that you can achieve tight control versus the desired profile. A chemical engineer would ask 1) What is the heat capacity of the system - how many watt-hours does it take to raise or lower the temperature of the system, and 2) What heat fluxes (in and out) are needed to achieve the desired temperature ramps. The heating element must of course deliver at least that much power.
     
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  19. MaxHeadRoom

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    Jul 18, 2013
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    The big advantage with the HF heating method would be that the rotating drum does not have to have contact with the stationary coil, as it probably would with the resistive method to be efficient, where power would have to be transferred to the drum if the heater was attached, otherwise it would rely on radiated heat for a fixed htr.
    Max.
     
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  20. wayneh

    Expert

    Sep 9, 2010
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    I agree there could be some geometric or mechanical advantages, and in minimizing the total amount of mass that gets heated.

    There's a piece of lab equipment that rotates a chamber (usually a glass sphere) in a bath of hot oil or in an insulated chamber with a heating element. The controls keep the bath or the chamber temperature tightly controlled. You have to heat the chamber or the oil, but otherwise these would be as efficient as you want them to be. Just insulate.
     
    Last edited: Oct 9, 2014
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