Water cooled to air cooled electric motor

[Diagos]

Hi All,

I need some pointers on replacing water cooled system with an air cooled system on a 2MW electrical motor driving a de-watering pump.

System as is:
Cooled water makes it way down the mining shaft for cooling and tool usage,from this cooling water line there are tap offs to each pump(4) for cooling of the electric motors. The water is at 10 degrees C and flow rate of about 6 l/s.

The issue now is that this water (after cooling the electric pumps) are dumped on the ground and eventually has to be pumped back up to surface. This results in significant unnecessary added costs to the system.

Proposed:
Convert to using ambient air underground at about 40 degrees C to cool the motor instead. Less water making its way under ground, thus cost reduction due to decreased pumping application.

At the moment I just have to figure out if this is technically possible. I am honestly not sure where to start, I would greatly appreciate it if anyone can point me in the correct direction.

Thank you in advance.

 

[LittleInch]

You can start with air flow required to cool the motor based on heat output and temperature rise of the air.

Given you're at the bottom of a deep shaft, I think you'll find out that while the water might cause you issues, it's much greater density and heat capacity than air is better than having to move vast amounts of air down the shaft and then back up again in ducts to avoid cooking the people and machines down there.

They are not, IMO, "unnecessary added costs to the system.", they are necessary added cost required by dint of the work being undertaken.

In a mine shaft, there is no such thing as "ambient air" You need to expend energy to get fresh air down and hot air out and both will increase to get additional cooling air and then hot air out. Is the "ambient" air at 40C to start with? That will need a lot of air to cool a 2MW motor


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[Diagos]

Thanks Littleinch

I have another water source down there that is at 25 degrees C in the water dams, perhaps a closed loop cooling system might work better utilizing that water?

 

[racookpe1978]

It "might" but I suspect the final answer will be "no".
A 2 Megawatt motor is very large, several of them in a closed (underground) shaft is a LOT of heat energy. (You have both resistance losses, magnetic cycle losses in the motor windings, and the work-energy of compressing the driven fluid and speeding it up to remove!) .

The problem with geo-thermal heat sinks (even in very small house heat pumps) is that the ground around the heat pump pipes becomes itself "saturated" with heat energy, and no longer removes the energy from the heated water. The no-longer-cooling-water returns to the motor, gets heated again, returns to the ground, heats it more, etc. This does not happen immediately of course, but underground, the region AROUND the heat-exchanger (the wet rock essentially) while very massive (which helps!) does not conduct heat energy very fast. Deep underground, the rock itself is hot (as you measured, 40 deg C is deadly at long times) and heats up the air itself, so the entire thermal region itself of air+rock+water is hot.

Long pipes are themselves heat exchangers. I would insulate the delivery pipes to the motor coolers to get cooler water to the motors. Try a VFD motor, maybe you not need as fast a motor-pump speed all the time as you think originally in the design. Reduce the number of pumps running when you can to reduce power wasted. The inlet water temperature to the motors is probably much higher than the inlet water temperature to the shaft. Try a refrigerator water scheme (check out the calc's to see if there are any positive money savings after the extra expenses.)

I am not sure if the energy and money to build a closed loop water system is worth the expense: Air-driven equipment deep in shafts delivers cool air to the head of the shaft, and that "cool" exhaust air from the air-driven equipment flows back away from the shaft head, which helps cool and the mine shaft and remove dust. The flow of cooling water from the motors is also removing waste heat, and - while dirty and muddy, may help overall. Or might be a cause of much discomfort and waste effort sucking up the muddy water and pumping it out. Can't tell.

 

[IRstuff]

So, we can run the math. Assume this motor is 90% efficient and that you use it 8 hrs a day, which results in 5.76 GJ. Divide by specific heat of water and you get 3.63E5 ºC*gal, so if you can tolerate a 10ºC rise in the water temperature, you'll need 36300 gals of water every day, which need to be brought back down in temperature every day.

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[gruntguru]

For air cooling, using IR stuff's example. If you can tolerate a 10*C delta T the 5.76 GJ/day will require 576,000 tonnes/day of air or 6.7 kg/s (5.5 m3/s).

The water begins at the surface. In theory you can get it back to the surface by adding only the energy required to overcome pipeline head-loss. Can the cooling water outlet from the motor operate under pressure? If so, supply the water jacket with the full head of pressure from the surface then add a pump after the electric motor to create the small increase in pressure required to get it back to the surface.

je suis charlie

 

[Diagos]

Thank you all so much for the great advice! All very valid points.

We are currently in the conceptual study of this project, only instruction I got to this point was that we need to reduce the amount of water being pumped out of the mine in order to reduce costs...

I will put a few ideas together and brainstorm them a bit in the team and revert back to this thread, I'm curious to see where this project will end up.

Thank all of you for the pointers and great replies.

 

[LittleInch]

Gruntguru has a great point there. Even if you can't sustain the pressure at the bottom of your shaft direct as cooling water ( you haven't told us how deep, but 1000m is not unknown), then can you use the water inlet as a hydro electric supply line and either generate electricity as you step the pressure down or use a fluid pump powered by the incoming water to pump some of it back up and then use the discharge water from the pump as low pressure cooling?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[moltenmetal]

Hmm: you're driving a de-watering pump with an electric motor that needs water cooling.

Why wouldn't you use some of the water you're "de-watering" as the coolant?

That water is being pumped out anyway- why not add a tiny bit more heat to it on its way out?

You can do that directly if the water is clean enough, or indirectly with one or two small pumps and a heat exchanger if it isn't.

 

[georgeverghese]

Watch out for electrical hazardous area implications if you switch from low TDS cooling water to air cooled for this electric drive. In an underground mine with poor natural ventilation, would guess you may have to go for a Zone 1 or worse still, a Zone 0 service electric motor if you go with air cooling - which may give your project manager / cost controller a rash on a 2MW drive. Also, if this a VFD application, you may need to go for an independant cooling fan if main drive speeds are expected to go less than 50% of design at turndown.

Talk to the plant electrical engineer about the current hazardous area service limit and these other concerns.

 

[georgeverghese]

Also beware of the fine print in the local hazardous area codes re taking credit for any forced ventilation and any supporting flammable gas detection devices - this is usually in the set of knowledge skills for a technical safety or process safety engineer.