Milling Operations: Dusters and finishers

by Jeff Gwirtz
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flour wheat
 
Dusters or finishers used in flour milling processes are similar in design to scourers in typical wheat cleaning systems. Typically, they are vertically or horizontally oriented, with a high-speed shaft with paddles or bars inside perforated sheet metal cylinder. Most often they are used in hard, spring and durum mills on bran and shorts after last break rolls as well as shorts from the reduction and residue systems. They are used in similar positions in soft wheat mills but also may be found between break scalp and break rolls to facilitate endosperm removal as well as scalp sifting. The photo below shows a horizontal bran duster with the inlet at the end of the machine along the top and an outlet hopper positioned under the perforated sheet metal screen collecting the through stock and the discharge at the opposite end of the machine for the over tail of the perforated sheet metal screen. Typical screen openings are round and 0.6 to 1 millimeters for shorts and fine bran dusting, and 1 to 1.5 millimeters for bran dusting.

Some duster designs include features such as a screw feeder or distribution disc or baffle, polygonal screens, a baffle system along the cylinder or other retention system to increase endosperm stock attrition. Dusters also may include an exhaust collection attachment point to provide dust control.

Sifters and Dusters
Horizontal (right) and vertical (left) bran dusters. 
Photos courtesy of Jeff Gwirtz.
 

Duster stock

Inlet stock for the bran duster generally consists of broad pieces or bran that have not been splintered during roll grinding yet retain some good endosperm for removal. Shorts from the break system may contain small pieces of bran with some retained endosperm. Shorts from the reduction system also contain small pieces of bran with significantly more endosperm than break shorts and at times smaller pieces of flattened endosperm. Tail stock from either the bran duster or shorts duster should not be significantly reduced in size or splintered due to the duster operation. When viewed from a distance and compared to the inlet stock, properly dusted tail stock should appear darker in color due in part to the removal of adhered endosperm particles. Less endosperm should be present on dusted stock upon closer examination with or without a magnifying glass. Through stock should be tan to creamy in color with small specks of bran present. Dark colored stock with excessive bran specks may indicate material has been retained too long in the dusting chamber or the bran is too dry and brittle. Excessive roll grinding resulting in small splintered bran pieces with little endosperm to be recovered may also result in darker than normal through stock.

Through stock that is too free of bran specks or whiter may indicate poor roll grinding allowing excessive endosperm to reach the duster or finisher, and it should be corrected. Large pieces of bran may indicate a poor screen seal or fit in the machine or perhaps a hole in the screen that should be repaired or replaced. Through stock represents approximately 4% to 8% of mill load and may be sieved on its own sifter section or combined with appropriate stock to a shared sifter section such as a secondary break or suction filter sifter section.

Flour cloths generally will be smaller than those found in primary reduction sifters to prevent introducing excessive bran powder into the flour stream. Duster flour makes up 1% to 4% of the total mill load or about 40% to 60% of the through stock. Duster flour has an ash content of 1% to 1.75% and protein content of 11% to 16%. Low quality middlings over the flour cloth makes up about 10% to 25% of the through stock and are generally directed to a secondary reduction system (C5, C6) or low grade (C7 or C8). The coarsest particles of bran making up 25% to 40% of the duster through stock are directed to mill feed.

Duster
Figure 1: Duster flow position examples.
 

Duster adjustments

Unfortunately, duster operation in most mills is not very well controlled or monitored. An operative miller would be advised to monitor duster operation periodically by examining incoming stock sources, monitoring through stock production gradient across the length of the duster, measuring rate of the through stock production and quality (ash and protein) of the duster through stock and or duster flour. Some suggestions to provide measurable operational data include:

  1. Install by-pass valves to allow maintenance while running and measure input load.
  2. Install valve to measure through stock production or duster recovery overall.
  3. Sieve test through stock using specific screens are flow dependent (or perhaps 350 and 118 micron screens over a Pan); to assess bran release per unit of recovered endosperm.
  4. Analyze (moisture ash and protein) flour generated in process or test sifting depending on availability of separate flour stream.

Duster shaft speeds are generally fixed and seldom employ variable speed drives. It is suggested that equipment manufacture information provided with the machine be consulted before making any changes. Unlogged emergency repairs may result in operation of the duster at non-conforming speed and sub-optimal performance. Operational speed should be checked and set to target.

Amperage draw may reflect under or overloading at the optimal speed. Excessive amperage draw may indicate overloading, excessive retention or close beater bar or paddle tolerances. Changes in paddle configuration or angle can reduce or increase retention impacting duster power consumption. Low amperage draw may reflect an opportunity to increase loading and/or retention to increase through stock production.

Balance on the high-speed rotating shaft is important. Removal or addition of beater bars or paddles should be reviewed with the equipment manufacturer to ensure the shaft remains in balance and that the product is properly moved through the machine. An out-of-balance shaft will increase vibration, energy consumption and bearing and drive wear. Changes in paddle configuration or angle can reduce or increase retention. Through stock removal is generally highest at the inlet end of the duster and decreases dramatically toward the discharge end of the machine. The stock discharged through the screen at the beginning of the duster will contain greater amounts of endosperm and somewhat less bran.

Toward the discharge end, the through stock will contain increasing amounts of small bran pieces and less endosperm, resulting in darker through stock. Shifts in the quantity and quality of the through stock may reflect grinding operation at the secondary breaks and/or shorts production from the sizings operation and the three sub systems of the reduction process. These shifts should not be ignored and assignable causes determined and monitored for future reference.

Duster
Figure 2: Duster screen wear on the inside (left) showing rounding on edge of perforation and sharp edge of the openings from the outside of the screen (right).
 

Duster screen condition

Dusters often are ignored until product can no longer move through the machine or a hole develops in the screen. The screen, however, is subject to wear as stock is propelled against the sharp edge of a new perforated opening, where there is more sheer or cutting action due to the impact of stock on the sharp edge.

Over time, the impact edge of the perforated screen opening wears and becomes rounder, reducing the shear and cutting action, which results in less through stock output and more endosperm being retained in the feed stream.

In the case of screenings of a mill feed hammer mill, such screen wear has a considerable impact on throughput, energy consumption and heat generation. Similar but smaller shifts may or may not be readily noticeable, thus the advice to closely monitor duster operation.

With some duster designs, it may be possible to change machine rotation or flip the screen to present a sharp edge for the stock to be dusted to impact. In some duster designs, this may not be possible. Figure 2 (above) shows the worn screen surface found inside a duster screen compared to the sharp unaltered side of the opening on the outside of the screen. Operation of dusters have a significant impact on quantity and quality of flour recovered from wheat. Design and operational factors should be explored to optimize the performance of these often-ignored machines. Replacement of worn duster screens have been reported to have six- to eight-week payback for the investment of screen media and labor.

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