electric waterheating element for hard water

[jre]

What materials have proven to be the most resistant to the agressive action of hard water on domestic type electric hotwater element sheaths. The problem seems to occur after a layer of lime builds up and insulates the heat transfer between the element sheath and surrounding water thereby raising the sheaths surface temperature.Corrosion is then accentuated. I was going to experiment with 316 s/s but have since realised there are many more alloys available which may be better.

 

[mcguire]

The prevailing choice for the tank material in stainless is grade 444, roughly equivalent to 316 in corrosion resistance, but resistant to SCC. The criteria should be about the same for the element sheath. The next upgrade would be AL29-4C, which is used in high efficiency furnaces and will certainly resist all corrosion in this application.

 

[butelja]

Have you tried working with one of the major manufacturer's of imersion heating elements? Chromolox or Watlow come to mind.

 

[jre]

Thank you for your replies. I am in New Zealand and investigating why we cannot get domestic water heater elements that last.We can get 304 s/s and Incoly sheathed elements (at a price) but as they are machine made the element leg diameter is rather small to give a good watts/sq.in.ratio. Corrosion of the sheath and/or resistance wire failure occures prematurely because of lime scale buildup which raises the surface and internal temp.of the leg.I also believe that current element design of two or more element legs in parallel causes a hot spot between the legs which encourages the deposit of lime in this area. It soon builds up into a solid mass and
increases corrosion in this area.I am looking at a design for possible manufacture using a single tube of a material which will at least help solve the corrosion problem.