Can an immersion heater cause flash boiling and burn out the immersion heater?

Question

I am currently working with an immersion heater and I am worried about using a too powerful heater and causing the water around the heater to flash boil thus allowing the heater to burn out due to lack of conduction to the water because of the air gap. I had heard that this was possible, but have never seen it in practice. Any help would be great. Thanks

To be more specific about what I am trying to do I am trying to maintain a water temperature inside a 15gal water tank made of plastic without any insulation at 75 degrees f. The outside temperature can range from 50degrees to -5degrees and the wind can be up to 85mph. From these calculations I figure I need about 4kw at the worst case. I am worried that if the heater switches on when it is say 40 degrees and no wind that because of the power it would burn up the heater.

Answer 1

I don't think you'd get "flash boiling" that is pretty rare on an industrial scale. I've seen it in the lab.

However, it is possible that if you get a boiling condition the immersion heater could be effectively surrounded with steam and insulated from the water. But that is unlikely.

For one thing boiling water will absorb around 1000 BTU's per lb so if it starts boiling the water will pull more heat out of the system than just heating water. I'd size the heater for the load and proceed on. I've sized imersion heaters for parts washing systems and never had a problem with that.

Answer 2

There's no theoretical reason it couldn't happen, but if you have a good sharp edge in there- like something scratched- that should provide a good nucleation point for boiling to start, and you shouldn't get too much supercritical water. You can see the effect of nucleation points when boiling water in a pan- boiling will start at tiny scratches in the side of the pan.

The immersion heaters I've seen have been pretty cheap and probably not very sturdy. I would check the duty cycle of the heaters.