Liquid cooled resistor

[whyareallthehandlesu]

I am starting to consider the design of a liquid cooled resistor, and am interested to views on suitable materials mainly from a corrosion point of view.

The resistor will probably be some form of metallic tube, inside which the coolant will flow. I cannot determine the type of coolant with any certainty, since our customers will fill it with what they please, but it will typically be tap water, a mix of water and glycol based antifreeze (possibly with anti-corrosion agents), or sea water.

Since the tubing is a resistor, we would like the electrical resistivity to be as high as practically possible to keep the total length of the tube down. That propably rules out copper or aluminium based options.

So far, I have got as far as looking at SS 2507, and was starting to look at Ti based alloys, but I have little materials knowledge, so any pointers would be appreciated.

As a supplementary question, does anyone have any idea of the maximum power density practical (W/cm^2), with the above coolants, assuming that the exit coolant temperature will be around 90C?

For info, the switched voltage will be around 1100v, the current through the tubing will be around 200A, and the total tubing resistance needs to be around 5 ohms. So it needs to be quite a length, and I have to work out how to get sufficient flow through the pipe.

 

[EdStainless]

Some of the high alloy stainless grades have poor conductivity, such as AL-6XN.
You will want the tube wall as thin as possible, with larger diameter.
Though 200kW is a heck of a lot of heat. Do a simple heat balance and see how much cooling water it will take.

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[btrueblood]

200 kW = mdot*Cp*dT
Cp = 4.2 kJ/kg-C, dT = 50 C (90-20)
mdot ~ 1 kg/sec
or about 1L/sec, or about 15 gpm.

You'll want a line size of about 3/4" or 1" i.d. You could probably get by with 1/2" i.d., but the pressure drop starts getting pretty big. Realize that a conductive liquid (e.g. salt water) will change the conductance of your resistor.

 

[whyareallthehandlesu]

Thanks for the comments.

Btrueblood, the flow will probably need to be a little higher than you suggest, as the inlet temperature may be up to 45C, and the SHC may be as low as 3.3kJ/kG-C with 50/50 ethylene glycol mix. On the other hand the duty is intermittent, so the small amount of thermal mass may help a little.

EdStainless, thanks for the material recommendation. Looks to be nearly as high resistance as Incoloy.

Unfortunately it looks like the low current (200A) solution I was thinking about will be impractical due to the length of pipe required to get the resistance. So I will need to put a in step-down transformer. If we step down the 3ph to say 10v , then the current will be around 15kA at 0.7pf. In this case, the material resistance won't be such an issue. If we make the tubing the secondary winding on the transformer, is it possible to coil the SS or similar tubing with a pretty tight bend radius - perhaps 100mm?

 

[EdStainless]

How thin of a wall did you use? What about 0.020"? You are talking about low pressures and rather strong materials.
Let's see, 3/4" x 0.020" wall at 25gpm is 20ft per sec. This is not out of reality for these alloys. That is about a 84psi pressure drop in 100' of tube.

Small diameter with tight bends sounds like flow and cleaning problems to me.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm