Can a heat transfer surface be too slick?

[hollerg]

I am debottlenecking a continuous dryer which fouls at high production rates. What impact would adding a diamond like coating or nitriding or polishing the 316 stainless steel have on the heat and hydrodynamic transport?

Adhesion (cohesion) of the sodium organic salt to a mill finished stainless steel is very high at the operating temperature 300 F. It is highly agglomerating and forms crust if not mechanically disturbed.

The dryer is built as a heated horizontal cylinder, containing a center shaft with a spiral of short blades that intermesh with stationary hooks, to clean the heated blades (~ 3 mm clearence).

If I only modify the cylinder interior, I imagine sliding but the hooks would result in surface renewal. Is there a negative to not coating the hooks, shaft and blade? Lack of convective turnover? increased stress on the hooks?

If I modify all surfaces, can I be too slick?

 

[dvd]

It seems like you need to try a sample of each regime of all possibilities within your existing dryer to determine what works best. I think that very smooth surfaces might foul more since you could adhere uniformly to the total surface versus having surface asperities. I think that to determine the effects on heat transfer you will need to do mocked-up experiments to determine the effect on drying time.

I haven't tried them, but I came across boron nitride release agents when researching a different type of sticking problem at high temperature. A simple release agent applied during cleanouts may be an alternative.

 

[MarauderX]

In short, yes, you can have a surface that is too slick. As you probably know a film will develop to prevent transfer that would be as good. While it can be a good thing for things such as sterile environments, it can be not-so-good for heat exchangers.

http://www.eypmcf.com

 

[TXiceman]

It you are boiling from the surface, slick is definitely BAD. You want a rougher surface to get the nucleation sites.

If it is to slick, how are you ever going to predict the boundary layer effect?

Ken

Ken
KE5DFR

 

[hollerg]

The application is drying solids. There is no nucleate boiling, as far as I know.