After last week's column, we received this correspondence:
We have a few machines where dryers have been derated. Do steel replacement dryers provide a cost-effective and technically sound solution for piecemeal replacement of a few dryers within a cast-iron group, or would we have issues with varying surface temperatures within the dryer section because of different heat transfer coefficients?
Mixing steel and cast iron dryer cans must be done with caution. I think I would do it only as a whole section. The closer to the dry end, the more it makes sense (dryers run hotter there and you can take advantage of the steel properties). You will also get more heat transfer, so if you have a machine that is dryer limited and has derated cans, you can get a double benefit.
Here is the problem with mixing them in the same section. Steel and cast iron have different rates of thermal growth for a given temperature change. If you put steel cans in a section that is gear driven, you will end up regulating the dryer temperature in order to avoid bagging or snapping between cans. If the section is felt driven you might get by with it because you are setting the surface speed, not the shaft speed.
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