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RF electrode active water cooling

Vereth

Hi,

I'm helping colleagues to design a RF (13.56MHz, a few kW) electrode and we argue about the water cooling.

In the current design, normal tap water is in direct contact with the electrode, flow in a plastic tube and then in a metallic tube. We dissagree on if we should consider water as a conductor (with resistance of 20-200 Ohm/m) or if it should be considered as a loosy dielectric.

 My consern if it is to be considered as a conductor is that if the plastic tube is too short, the impedance of the water line to ground will partly short the electrode to ground of strongly modify its impedance.

 Some colleague are saying that a minimum of 1.2m is necessary but they can't really tell me where they hear this. Any reliable source of information would be helpful to settel this debate.

 Thanks,

Vereth 

 

Parents

  •  Dear Vereth,

    Tawna's comment is certainly true. However, I happen to have some limited knowledge as my Dad worked for a number of years in a field where they relied heavily on the impedance properties of water. As I am sure you know, "tap water" impedance can vary dramatically depending on its source. Designs that are supposed to be robust that use water to cool them will need to precisely specify some DC impedance for their water from a supplier. In my Dad's case, it was a medical application and there was a major challenge to guarantee their vendors provided the proper level of water purity.

    Depending on frequency, the impedance of the water will change and may show a resonance. Hence, if you know your specific topology and water purity, you will need to characterize its impedance over frequency. A reference paper that provides one model of waters impedance in an application that may be useful is at URL:

    www.electrochemsci.org/papers/vol7/7043466.pdf

    I noticed that the resonance frequency of many of the impedance characteristics is much less than the 13.56 MHz frequency you are using and hence the impedance will be high - depending on the characteristics of your water.

    I hope this helps a bit anyway!

    Shawn

     

     

Reply

  •  Dear Vereth,

    Tawna's comment is certainly true. However, I happen to have some limited knowledge as my Dad worked for a number of years in a field where they relied heavily on the impedance properties of water. As I am sure you know, "tap water" impedance can vary dramatically depending on its source. Designs that are supposed to be robust that use water to cool them will need to precisely specify some DC impedance for their water from a supplier. In my Dad's case, it was a medical application and there was a major challenge to guarantee their vendors provided the proper level of water purity.

    Depending on frequency, the impedance of the water will change and may show a resonance. Hence, if you know your specific topology and water purity, you will need to characterize its impedance over frequency. A reference paper that provides one model of waters impedance in an application that may be useful is at URL:

    www.electrochemsci.org/papers/vol7/7043466.pdf

    I noticed that the resonance frequency of many of the impedance characteristics is much less than the 13.56 MHz frequency you are using and hence the impedance will be high - depending on the characteristics of your water.

    I hope this helps a bit anyway!

    Shawn

     

     

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