Water-cooled induction heating coils -- dangerous voltages on the water? - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: June 07, 2015, 06:46:07 am
content: Induction heating coils are often hollow copper tubes with cooling water running through them, for obvious reasons. However, real-life water is far from a perfect insulator, so I've often wondered if this requires special precautions. Obviously, the water within the copper itself is not a concern, but presumably the input and output water hoses will have a significant AC voltage across them? The input water is grounded, so the output water is at a dangerous voltage? Does pouring it down the drain threaten some sort of short circuit? Is forming a closed-circuit system with a pump a big no-no?Or do we note that standard tap water has apparently a conductivity* of 5-50mS/m, which means a 1m length of 1cm^2 cross section water hose has a resistance of 200 kiloohms "worst case", which leads to a negligible current flow even at high voltages. I know I sort of just answered my own question maybe, but wanting to understand if this is all there is to it. Is corrosion on the inside of the pipes somewhat accelerated, even if there's no danger to life/limb per se?Please discuss :-)* Note for all you people who incorrectly use mS instead of ms as an abbreviation for milliseconds: see above for theproperuse of mS.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2015, 07:27:45 am
content: Not a problem, even for fairly bad tap water. I wouldn't recommend going out of your way, to, like, use sea water, but as long as it doesn't break down (arcing, creepage and tracking are a different matter underwater than through air) and doesn't dissipate too much as direct losses, you're fine.As for keeping voltages down, water circulated to the pump should always be passed through grounded connectors at some point, so the circuit is completed from within the coil side.Water cooling coils is easy -- you don't have to worry! But water cooling electronics can be tricky. The old SCR and tube units required high voltage DC across the water. Large coils of hose were usually used to help mitigate conductivity, but the connectors still eroded badly over time, even for good water quality. Even for modern MOSFET/IGBT designs, you may have to be careful -- it's tempting to, say, grease some TO-247s directly onto water-cooled heat spreaders, and get the full manufacturer's rated 400W or whatever out of them! But you either have to deal with the DC on the water, or insulate the heatsink, in which case you're back to the same damn problem almost all heatsinking meets with: thermally conductive electrical insulators.Tim

author: SeanB
author_rank: Super Contributor
timestamp: June 07, 2015, 11:15:58 am
content: Standard is to use at a minimum ion exchange softened water, preferably triple distilled afterwards and vacuum degassed, and then to make every part in contact with the water a non corroding metal like stainless steel ( you pickle it internally to get the protective coating that passivates it) or anodised aluminium, oxide layer thickness depends on withstand voltage. then you have the feed and return in non conductive pipe, either glass ( best) or PTFE, or some non leaching plastic ( HDPE, PE, acrylic) to provide standoff. then your pump and radiator is similar, preferably also insulated from ground ( magnetically coupled pump, sealed radiator) so that the syatem does not collect cantaminants from the air. Will handle 50kV and 200kW of heat for decades, though there you want a conductivity measure that turns off the system if it drops to something bad.For cooling electronics you would be better off using a silicone or other synthetic oil ( don't mix them though) to transfer heat to a larger cooling surface, inside a sealed enclosure or a tube with either thermosyphon or active pumping. That way you have a non conducting liquid, though you lose the ability of the large phase change energy of water for cooling hot spots by boiling.

author: IconicPCB
author_rank: Super Contributor
timestamp: June 07, 2015, 10:33:47 pm
content: One of my first jobs ( in '79 ) was with a company called inducto therm . They made RF heat tretment equipment as well as 50Hz induction melting furnaces.Water was used as the cooling medium. It is quite easy to calculate the leakage current in the water circuit as one can measure and control the ionic content of the water in closed loop systems alternatively provide sufficient ( excess length of water hose ) to ensure safe conditions exist.Either conductivity ( disolved solids in water ) is measured and bulk resistance is used to calculate the length of water circuit required to establish safe operation or a minimum worst case conductivity is assumed and circuit length calculated .

author: SArepairman
author_rank: Frequent Contributor
timestamp: June 08, 2015, 12:22:33 am
content: many places install a conductivity meter that alerts you to replace the coolant

author: aargee
author_rank: Frequent Contributor
timestamp: June 09, 2015, 06:59:52 am
content: I worked on Inductively Coupled Plasma Spectrometers waaaay back in the '80s.They were running 1500-2000W at 27MHz into a water cooled load coil with nothing special about the coolant connections. We were more interested in making sure the chemicals in the water didn't corrode the copper coils or the little fan cooled radiator supplying the water.From memory tap water and standard ethylene glycol (as used in car radiators) was used.

author: G7PSK
author_rank: Super Contributor
timestamp: June 09, 2015, 03:19:43 pm
content: Use 100% ethylene glycol. Using glycol that way also stops corrosion plus you never need to worry about freezing.

author: SArepairman
author_rank: Frequent Contributor
timestamp: June 09, 2015, 04:09:36 pm
content: Quote from: aargee on June 09, 2015, 06:59:52 amthe icp is a really nice machine huh? I love how it has like a 4 inch thick piece of glass to block the UV..

author: calexanian
author_rank: Super Contributor
timestamp: June 09, 2015, 08:51:32 pm
content: Typically the water supply to the coil is isolated with some rubber tubing, then both sides pass thought a length of metallic tubing which is in turn ground bonded.