Keeping water from freezing in outside open tank 1

[rcPRP]

I have a 40"x10"x125" that contains approx. 750 liters of water. the tank is open to air at the top, though completely protected from the wind.

Considering an outside temperature that can go as low as -30 Celcius in the winter, I need to keep it from freezing when the system is not in function (thus not warming the water by itself).

Presently, it is kept warm by an input of steam at one end.
I want to replace the steam system (which uses pumps, a gas boiler, piping and valves) by a simple electrical immersion heaters system. The heaters would be spread along the length of the tank. Water temperature will be controled with probes so the water temperature does not exceed 15C.

I want to calculate how much "watts" I need to do so. If someone could help me get started, I would be grateful.

 

[bimr]

This should help:

http://www.pfonline.com/articles/pfdsmith01.html

 

[Sefrick]

Justa tip, you may wanna use seperate heater banks. i.e., start up heaters to warm the water above freezing. then normal heaters to make up for ambient losses. you could probably program the start up heaters to cycle on and off in the event that the normal heaters can't keep up with ambient losses. just be careful to design so that cycling isnt too frequent

justa thought