# Coolant gas.

1. Oct 16, 2009

### origen87

Hello all,
I am doing a research which involves hollow copper conductors. I would like to know about some kind of coolant or gas (like SF6) which can be passed through the copper tube to cool it, the temperature of the copper conductor can reach upto 100 degree C. The coolant should cool the conductor quickly so as the temperature at any point should not go high.

2. Oct 16, 2009

### Andronicus1717

I think more information is needed to get a good answer to your question... heat flux? is it localized? required response time? You mentioned SF6 so do you truly need a nonconducting material?

Water is a typical coolant for induction heating coils which sound similar to what little you've described from your setup. As for gases, helium is a good heat transfer fluid. Thermal conductivity is the material property you are interested in for gases. If the required response time is low you can make just about any gas work by adjusting the flow rate, SF6 included.

Last edited: Oct 16, 2009
3. Oct 16, 2009

### Topher925

I would look into using water. It makes for an excellent coolant.

4. Oct 17, 2009

### origen87

@Andronicus1717...
Well I actually dont have values for the heat flux n response time. But the setup goes lik this. It's a 1560 metre coiled copper tube, which will hav current upto 200A, so i^2R losses wil heat up the tube. As I am not using any other cooling means for the coil. I want to pass some gas or coolant so that this heat produced can be fully absorbed continously. I am not sure about the coolant being a conducting material or not,as the copper tube wil conduct, will it be a problem if conducting material is used inside the tube?

5. Oct 17, 2009

### Andronicus1717

That is a long tube! What is the inner diameter of the tube? Does the setup require only 1 inlet and 1 outlet or is there room for supplementary inlets and outlets along the length of tube?

With that length of tube you are going to run into large pressure losses due to friction along the entire length of the pipe.

6. Oct 17, 2009

### Andronicus1717

Are the tubes insulated? If they aren't, you may not even need a coolant gas? Free convection or forced convection on the outside of the tube may take care of the heat problem.
Just the resistances from first entry at http://www.stormcopper.com/CopperTubing.htm gives me about 4.5 Watts per meter of tube if my math is right.

7. Oct 17, 2009

### Q_Goest

Can you provide ID of tube, and estimate on heat flux per unit length? Also, is this in air or insulated?

8. Oct 18, 2009

### origen87

1 inlet n 1 outlet will dD=8.65mm and ID=8mm.6 such layers of coil are wound on top of each odr,between these layers we are planning to have NOMEX paper covering.And the hollow copper will also be covered with PICC insulation.

9. Oct 18, 2009

### Q_Goest

Annealed copper has a stress allowable of 6000 psi giving your tube an MAWP of roughly 450 psig. So let's assume we can allow a max 400 psi drop in pressure from inlet to outlet (I'll use 200 psi). Flow rate for this tube is affected not just by length but also by the turns, so let's use 4 times the 1560 meters (6240 meter equivalent length). Rough calculation for water inlet 40 F and 400 psig with outlet 200 psig and 210 F gives 0.04 lbm/sec. Heat that can be removed with this much water is roughly 7 kW.

If I use 4.5 W/meter as given by Andronicus1717, total heat rejection needed is roughly 7 kW.

Looks like water might work.

10. Oct 18, 2009

### origen87

according to my calculations heat loss due to i^2 r loss would be no more than 6kW.
but will there be problems because of water inside the conducting tube?and is there any related calculations to find out how much water can be absorbed by water?.and can continous expansion and contraction of water help keep the temperature of water low enuf?

11. Oct 18, 2009

### Q_Goest

Hi origen87,
If you calculate 6 kW and the value from Andronicus1717 gives 7 kW, then we're probably in the ball park. Without much better info, I wouldn't put money on a more exact figure.

I don't know enough about electricity to tell you how the water would affect the electric current, but I'd be surprised if there were an issue. That might be worth getting another opinion on.

Note that conservation of mass says that the mass flow rate of water going into your tube must come out of the tube, so there's no water being "absorbed". The flow is roughly 0.04 lbm/sec or 0.29 GPM.

Also, the water will gradually warm from 40 F to 210 F (aprox) as it flows along the tube. There is no expansion and contraction that affects the temperature. The 6 or 7 kW will heat the flow of water from 40 to 210 F continuously. How you get 40 F water or if the temperature can be higher than that will require someone to look at the details of what you're doing.

In the end, you need an engineer to look at the details of your design and determine how to make it work, but at least at this point, it looks feasible to use water at a few hundred psi to cool the conductor. If you'd like more detail than that, I'll have to charge you a consulting fee. \$

12. Oct 19, 2009

### origen87

hey thanks a lot man,,,the info u provided without consultation fee ;) was very helpful:)....

13. Oct 19, 2009

### cpooley

to origen's question:

Hydrogen is the best gas coolant by far because of high thermal conductivity, high sonic speed. It us often or usually used in large power plant generators because of low "windage" loss.

In GE enerators used in a plant my father worked in in '50s it was circulated through heat exchangers with blowers.

In small amounts not a fire hazard etc. Also if you want automatic leakage replacement upu can make an electrolysis cell using battery acid (10% H2SO4) and you only replace the water just like for a car battery.

14. Oct 20, 2009

### origen87

@Q_Goest
how did u come to that figure of 200-400 psi,which equation did you use?what was the flow rate you choose?
after all the calculations the total losses came upto 12kW,hence a flow rate of 38.1*10^-6 m^3/sec has 2 b used 2 cool the conductor(water at 25C)....if that flow rate has to be achieved the ID of the conductor should be increased to 30mm or more.

15. Oct 20, 2009

### Q_Goest

I'm using the Darcy Weisbach equation. If power goes to 12 kW, a more reasonable diameter might be 0.5 inches ID with a water flow rate of ~ 0.114 lbm/sec. This would give you a 100 degree F delta between inlet and outlet with a pressure drop of around 250 to 300 psi.

Note that hydrogen as suggested by cpooley is about as good a gas as you could get for cooling, but it doesn't come close to having the cooling capacity of water. DI water seems to be a good solution that's economical and safe.

16. Oct 20, 2009

### cpooley

Q Goest: Note topic was Gas Cooling. Of course H2O usually will be more effective. So would be sodium or the NaK eutectic. But he was asking re gas.

For temps over 500 C, decomposition of NH3 may be effective, if the products go somewhare, like into a rocket engine (1st Microlauncher Stage 3 design)

17. Oct 21, 2009

### origen87

dont you think Hazen-Williams formula would be a better option

18. Oct 21, 2009

### Q_Goest

Crane TP #410 is the bible in the industry. They recommend Darcy Weisbach.