Heat transfer in subsea environment

[Thrusterman]

Steven, Everyone,
what would be the heat transfer in a cooling loop that is wrapped around an electrical submerged motor in a subsea environment? The motor is producing a significant heat (9kW) that is transported out of the motor casing into the cooling loop by a shaft driven centrifugal pump. The flow rate is (depending on shaft speed) at it's maximum say 230 liters per minute. The cooling loop (ID = 42mm) has a radius of 1.7 m and has 20 wraps before it enters the motor housing again. The fluid is some strange 50/50% water/mono ethylene glycol mixure. I need to know how time dependend the fluid temperature increases, stabilises or decreases over time when the motor is stopped. I have figured that this is highly depending on the seawater movement around the wraps of the loop. So I would like to ask if someone can provide me with guidelines or assumptions for seawater with a temperature of 4°C when
a) the seawter is not moving
b) the seawater is current driven with speed x (m/s)

Also, I guess I need a start temperature for the time investigation because the fluid properties (kin. & dyn. viscosities) are depending on the temperature. What would be a good starting point, the 4°C or the maximum temperature?

Much thanks!
Thrusterman
thrusterman@yahoo.com

 

[svanels]

To model the sea water behaviour looks a little difficult. I suggest to assume that the temperature of the pipe is maintained at 4[sup]o[/sup]C. To influence the temperature of the surrounding sea water, you will need a lot of heat.

For starting up I'd suggest to use the lowest temperature possible. The composition of the cooling medium indicates the possibility of freezing up.

Regards Steven van Els
SAvanEls@cq-link.sr

 

[Thrusterman]

Thanks for the fast reply, Steven.
The fluid is not likely to freeze at these temperatures. If I remember right it's about -20°C when its starts to get solid. But at 4°C it's already high viscose.
I have read that if the fluid is not (at all) moved around/away from the cooling loop that the surounding fluid heats up to the pipe wall temperature and that the heat exchange get's less and less efficient.
As a first pass I will keep the seawater temp at a constant 4°C at the cooling loop wall. Any more thoughts are highly appreciated.
Thrusterman

 

[wdl]

Thrusterman,
A somewhat belated response to your query, you've probably solved the problem by now. Be careful in using the ambient temperature as the wall temperature. The sea may be an infinite heat sink but your coil is not an infinite radiator. Previous experience with this sort of calculation, followed by testing, indicates that the wall heats up to some intermediate level which is going to depend on all the usual factors. My experience is with fairly big chunks of metal with much more heat input but the skin temperatures tracked bore temperatures albeit with a significant temperture drop across the foot or so of intervening steel. The original calculations assumed an ambient wall temperature with the result that heat transfer was less than projected with not too good results for the internals!