Cartridge Heaters: Using a Firecracker to Heat Water?
With their obvious cost and size advantages, why would you not use a Watlow Firerod cartridge immersion heater instead of a screw plug immersion heater to heat water?
Screw Plug Style:
Cartridge Immersion Style:
The big concern is at 300 watts/in2, if improperly applied, a cartridge immersion heater could overhear, short out, and essentially act like a firecracker and "pop." However, water is an excellent thermal transfer material and that is why the stock immersion heaters are rated to 300 WSI. Still, nuclear boiling (film boiling at heater surface) is a potential problem. If you proceed with using Firerod cartridge immersion heaters, then you need to make sure the heater is fully wetted when energized. The Firerod should be positioned so that there is at least 2" of water surrounding the heated area of the Firerod, but preferably 3" or more. So long as the system is properly controlled and so long as the heater is "wetted", it should perform well in a water application.
With that said, there is a couple of additional concerns, which are:
The pins are copper. If the heater is allowed to run at greater than 1000F internal temperature, the pins can begin to soften and could potentially melt.
The end seal is silicone rubber. If the end seal is operated at higher than the maximum continuous temperature rating of 450F, the silicone becomes brittle and the seal integrity is damaged. The seal is at the lead end and may not come in contact with moisture, so this might not be a problem, but it is a potential area of concern.
Sludge build up can interfere with heat transfer, and scale build up on the Firerod sheath would definitely interfere with heat transfer and could be a problem.
Conclusion: if you can afford the cost and the extra space, the best way to heat water is with an screw plug style immersion heater. If you can't afford the cost or space, make sure you take extra care in the design and installation of your cartridge style immersion heater.
This article was authored by Dave Safford, who is the resident thermal applications expert at IES Technical Sales. More information on Dave can be found at: http://www.linkedin.com/in/davesafford
IES Technical Sales is a value added technical sales, distribution, and solutions provider serving the high technology vacuum, plasma/thin film, temperature, fluid handling and metrology markets.