High chemial resistant metal sought

[Nestornotabilis]

As electrical heating of electro-galvanising bathes is waiting for an accident to happen and burn the works down we will be using metal tubes on the bottom of our process bathes. Through these will flow fluid (water) from a heat-exchanger. Therefore a high grade plastic such as PVDF is out due to poor temperature transfer.
The problem is that the bathes operate at temperatures of minimum 85 degrees C up to about 100 deg. C and are quite aggressive chemically. The pH value shifts between 0.6 to 0.9 and the main acids are combi H2SO4/HCL and HCl. There are more complex compounds involved but this will have to do due to legal requirements.
Titanium will hold out quite well in the H2SO4/HCl environment but not too good in the HCl. Hastelloy B3 is equally dissapointing in both. The material erosion is not much but we would prefer not to have to renew everything if not needed. Cost factor does play a role however, so mabee a compromise must be made.
Question is: Is there a material we should look into that can meet our requirements? or have we done that with the Hastelloy?

 

[hacksaw]

what is your tank made of?

 

[Nestornotabilis]

Tanks are made of PP polypropylene and are ideal it that respect as PP has a lower heat transfer coefficient than titanium. We want to heat the bathes with this system so the more heat transfer the better.

 

[stanweld]

We used Palladium stabilized Ti in a similar HCL application for a hydrometallurgical facility with good results.

 

[stanweld]

Tantalum will be acceptable in your environment as well. Check out Cabot Corporations web site. Very expensive though.

 

[BillPSU]

You may want to consider using PTFE pipe for this application. I don't know what the heat transfer qualities of this material but its chemical resistance and working temperatures meet your application.

Just a thought.
Bill

 

[Nestornotabilis]

I pretty sure PTFE will have a lower heat transfer than most if not all metals. The idea is to have the highest transfer possible to keep costs down. That said, using Hastelloy would be expensive any way, so our idea at present is to construct the system with titanium and live with the fact that it will need to be replaced after a period of time. We are not talking big money company here, just a small business on the grow but trying to keep its head well out of the red so cost is unfortunately an important issue.
Thank in any case, will look into all options offered.

 

[MikeHalloran]

A thin TFE coating can protect a metal tube without seriously impeding heat flow. Pinholes in the coating would be a bad thing.

If your heating coil is of relatively simple geometry, e.g. a very loose serpentine with large radii, with the ends bent up and terminated above the liquid level, you could protect the coil with thin PP or PE tubing, slipped on from either end. If the tubing is thin enough, it doesn't have to fit the coil closely; hydrostatic pressure will push it into intimate contact with the coil.




Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA