Cement Fineness and Sturtevant Separator Internal Geometry

Cement Fineness and Sturtevant Separator Internal Geometry

by        Sam Fujimoto, Process Engineer

A study was initiated to investigate why Sturtevant Separators of different sizes performed differently.  This was prompted by an effort to improve cement blaine/325 ratios on certain units known to historically to be considered poor.  Although time did not permit an exhaustive and thorough study, the initial findings are sufficiently interesting to warrant this report.

Recognize that the recommendations in this report are geared specifically towards Sturtevants operating on finished cement and should not be applied to units operating on raw materials.  This is mainly due to the fact that fineness requirements are very different.  However the underlying operating principles should be the same and therefore one, with some thought and careful study, could modify the ideas presented here for raw grind units.

Background

Many within the company, for years believed that all Sturtevant separators are dimensionally the same.  For instance we’ve assumed that any two 20′ diameter units would be identical.  In addition we’ve all assumed that the larger units are scaled up versions of smaller ones.  Yet  we know from operating experience that performances vary and as a rule of thumb, the smaller Sturtevants seemed to perform better than the larger units.  Why is this so and why does a 20′ dia. Sturtevant in one plant perform better than another 20′ Sturtevant in another plant?  By studying the underlying reasons for this behaviour we hope to find ways to significantly improve separator performance at plants equipped with Sturtevants in cost effective manner.

What is Good Performance?

From a cement plant operator’s perspective good Sturtevant separator performance centers around two issues:

1. a) A lower separator bypass (or better efficiency). Depending on the mill circuit this usually translates into higher production rates.

1. b) A better blaine/325 ratio. This usually means a final product with a higher % passing 325 mesh.  In turn, this usually translates into better concrete compressive strengths which could also allow a small reduction in required blaine that will result in higher circuit production.

Unfortunately with Sturtevants these objectives are interrelated and in an opposing manner.  In other words one cannot affect separator bypass without affecting blaine/325 ratios and vice versa.  Depending on a given plant’s circumstances and objectives, they must choose between one or the other without compromising too much.

Internal Geometries

What has Sturtevant Supplied?

Generally speaking it is true that any two Sturtevant separators with the same basic diameter will be, for all intents and purposes, exactly the same with respect to external dimensions only.  However this is where our assumptions should end.  This is shown in Table 1, which indicates that internally many Sturtevants are different and this is what we have discovered during the course of the study.  Thus a survey of different Sturtevants was conducted to see if there was a pattern between internal geometries and performance.  This is summarized in Table 2.

Table 1: Basic Sturtevant Configurations (as supplied)

Table 2: Sturtevant Survey Results

Notes:

1. a) The above physical measurements were difficult to obtain and will result in minor deviations from Sturtevant specifications.
2. b) Actual 325 mesh values will vary for a variety of reasons. Values presented above represent a historical average on normal cements.
3. c) Bath FMA data was from last audit done just prior to conversion to OSEPA. However physical measurements were done recently since unit is still intact.
4. d) Bypass data was not presented since these values are highly dependent on separator feed rates which will differ because of different circuit configurations.
5. e) The dimension inside cone ID refers to the smallest diameter of the inner cone (upper half) where air travels through the return air vanes and cuts the corner upwards.

Interpretation

There are a number of observations that we can make from this survey and other literature.  However before discussing them let us quickly review how Sturtevants work.

Material fed is through the center of the unit around the drive shaft and is dispersed into the lower chamber by the lower distributing table.  Air, which is recirculated through the return air vanes, is pulled up through the dispersed cloud of material by the main fan.  Here air washes the cloud of fine particles and separates from the cloud.  This is sometimes referred to as the separating zone.  The heavier coarse particles are collected by the inner tailings cone and the lighter fines are lifted up through the selector blades mounted on the upper table and into the main fan.  The selector blades, depending on number and design, determine the final sizing of the finished product.  This is sometimes called the lift zone since fine dust must be lifted past the selector blades.  From the main fan, the finish product is collected in the outer cone and separates air out to be recirculated.

Upper Table Dimensions

Air that is recirculated around inside will naturally take the shortest path.  Although in the lower zone the machine disperses the mill product, as the air flow travels around the edge of the upper table it reconcentrates dust at this point since this is the shortest path.  We know this to be true from the wear patterns etched into these selector blades.  Then the selector blade must grade the particle by size to do any final sizing.  This is illustrated in Figure 1.  Also from Table 2, there is a clear pattern that appears to emerge.

Figure 1: Flow and Wear Pattern Across the Selector Blade

If the upper table is smaller than the inner cone diameter at the return air vanes, the air and dust is then given a straight, nearly unobstructed path up to the main fan.  This is illustrated in Figure 2.  As a result, some unwanted coarse particles will escape with the finished product, due in part to this reconcentration shown in Figure 1.  In Table 2, Bath and Brookfield have Sturtevants in this condition and traditionally have had 325 meshes less than 90% passing.

However if the upper table diameter is equal to or greater than the inner cone ID, then the air and dust must travel in a curve further out, due to the elimination of the unobstructed path.  This has two effects.  First the path length is slightly longer which improves the chances of separation to occur.  Second the path is farther away from the center where the centrifugal forces are stronger and therefore improving the separation forces.  This is the case with Woodstock’s 16′ separator and in Demopolis’s 20′ separator, where the finished product is usually greater than 90% passing 325 mesh.

Figure 2: Effect of a Larger Upper Distribution Table

However extending the upper table has two other effects.  First, from Table 2,  the extended tables relative to the diaphragm opening (or valves) are such that the diaphragm or valves overlap the table in plan view.  This appears to be a good thing in that it give a greater latitude and more effective control.  Whereas the smaller upper tables are smaller than the ID of the valves in the fully closed position.  Given that the air and dust reconcentrates at the upper table edge, clearly the diaphragm cannot be as effective if it doesn’t overlap this zone.

The second effect is more negative.  The larger upper table obviously will increase the internal resistance to airflow.  In keeping with the Qf/Qa principle this means that the separator bypass will increase (or efficiency will drop) which could result in a drop of production.  This was in fact the experience at Brookfield when they extended the upper table in an earlier attempt (shaft speed was 163 rpm).  Therefore to avoid this, the plant must also simultaneously modify the main fan to maintain internal airflow.  This could be accomplished by increasing the number of fan blades; or the size of fan blade; or the diameter of the main fan; or the speed.

Lower Table and Shaft Speed

Also indicated in Table 2 are the changes in shaft speed and lower table diameter and there is some controversy as to the effectiveness.  Within the company, Bath was one of the first to increase the separator speed and claimed a small but significant improvement in productivity which stemmed from a lower bypass or better efficiency.  However when Brookfield increased rpm from 163 to 190 or Demopolis who increased speed from 192 to 213 both plants could not discern a change in performance, (Demopolis reverted back to 192).  Upon a closer look we find that Bath at about the same time also increased the main fan diameter.  The increase in flow is roughly proportional to the increase in diameter cubed, hence it resulted in better performance (Qf/Qa).  So why didn’t Brookfield or Demopolis experience any changes?

The increased shaft rpm will increase internal air flows, however we’ve overlooked one thing.  The lower distribution table is connected to the same shaft and it imparts the outward velocity to the material, to disperse it in a cloud.  If the rpm increases, so does the material’s outward velocity and hence the residence time in the separation zone drops.  Other research indicates that the longer the residence time the better the separation efficiency will be.  Therefore making the speed change by itself will cause both an increase in internal airflow and a decrease in material residence time, which effectively cancelled each other out.  This resulted in no change.  However to compensate, we can do what Demopolis did and that is to reduce the lower table diameter.

Recognize that the outward velocity is roughly proportional to both the rpm and the size of the lower table.  To maintain approximately the outward velocity while at the same time increasing the shaft speed use this equation to approximate the new table diameter:

DIA2 = DIA1 X (RPM1 / RPM2)2

Where subscript 1 denotes an old value and subscript 2 denotes a new value.

For example, if the speed change is planned to change from 163 rpm to 190 rpm then the 7′ diameter table should be reduced to 5′-2″ using this equation.  This is not unlike what Demopolis did and they recorded a significant improvement.  However some caution is called for in applying this.  First of all the frictional and drag losses should be high but we cannot measure this and therefore the relationship is not as exactly suggested in the equation.  Second there is the danger that the one can make the table too small.  In which case the outward velocity is so small that it cannot disperse material properly.  Therefore it is suggested that the table be reduced in increments.  As a clue, study Table 1, where Sturtevant gives a range of rpm’s for each basic size.  This is likely the safe maximum and minimum shaft speeds.  Therefore use the lowest rpm given to determine the smallest lower table diameter.

SZ/LZ Ratio

In keeping with the idea that the longer the residence time in the separation zone, the better the overall efficiency (lower bypass), rules of thumb for internal dimensions were developed for Sturtevants and other similar designs.  One in particular is the SZ/LZ ratio which is simply the height ratio between the separation zone (SZ) and the lift zone (LZ).  The separation zone height is defined as the height between the lower distribution table and the midpoint of the return air vane opening.  The lift zone height is defined as the height between the lower table and the inside drum cover.  According to the Cement Data Book 1 (Duda), this ratio should be less than 1:1 for acceptable performance and less than 1:0.5 for efficient performance.  Note that the 16′ Sturtevant has a ratio of 1:0.32 whereas its larger cousins has a ratio of 1:0.76.  Little wonder then that the 16′ units have traditionally performed somewhat better than its larger counterparts.  Although there is little that we can practically do about this deficiency, it was mentioned to reinforce the notion that the internal geometries of different Sturtevants vary to the detriment of its performance.

Conclusions

In general to improve the % passing 325 mesh (to greater than 90%) on a Sturtevant separator the upper table should be enlarged to at least the same diameter as the inner cone ID at the return air vane level.  However one must, at the same time, take action with the main fan to overcome the increase in resistance.

To improve the unit’s efficiency (lower bypass) consider increasing the shaft rpm but coincidentally reduce the lower table diameter.  Some care should be taken when making these modifications so as not to make it too small.

Finally when comparing any two Sturtevants, do not assume that they are the same.  Chances are that they are not.