Purifier operation setup in most mill settings is seldom, if ever, properly tracked and recorded. The importance of being able to change operating conditions to harmonize with a specific wheat mix or recipe is critical to success. In many of the newer mill installations it is possible to use knowledge about a particular wheat mix setting to make changes automatically to previously successful roll or valve settings to minimize off-grade product during changeover and maximize mill performance.
In addition to changes in roll grinding, the purifier represents an important operation in maintaining mill balance yield and flour quality. Maintaining known purifier settings and consistent operation is as critical in today’s highly automated and controlled flour mills as it was in in the past. In this article we will review purifier setup documentation and performance measures.
How purifiers work
Purifiers function by allowing stock consisting of compound particles to be transported over reciprocating or vibrating screen decks. Air pulled up through the screen deck assists in the stratification and segregation of particles based on their aerodynamic properties (terminal velocity) influenced by composition (bran, germ and endosperm), shape and size.
The screens contribute to the sifting and separation based on size at the screen product interface. Material delivered to a purifier must fall in a narrow particle size range consistent with purifier setup and contain limited amounts of fines or flour as these may be lifted by the airflow and increase filter stock production. The up-to-date flow sheet serves as a record of particle size expectations and screen utilization.
Documenting Purifier Setup
The purifier detail shown in Figure 1 provides a potential alternative for purifier setup documentation. Documentation will require modification, depending on the manufacturer’s model and design. The purifier stock sources and screen size through which the stock has passed and is held over may be included close to the purifier inlet designating the stock and unit. Air duct settings for the balance of air between two sides of a double purifier may be shown in the center above the purifier. Setting for the false air inlet at either the head or the tail of the machine may be identified above the machine at either end where appropriate. The false air inlets allow for additional air to enter the chamber, creating the velocity needed to carry entrained material to the dust filter. Settings for each air chamber above a screen frame may be shown in the small boxes above the larger rectangles associated with each screen section. More often than not, there are four airflow channels above each sieve frame on a purifier’s top deck. Increased airflow at the head end allows for increased transportation and stratification after which air is reduced to facilitate separation by sifting. Gradually, airflow is increased toward the tail end to maintain lifting of lighter bran containing stock.
In each screen frame section the cloth size should be identified, preferably using the same nomenclature as used in the sifter flow, which is usually micrometer or micron. Some millers, however, prefer to list purifier cloth using GG nomenclature. Options and, more importantly, the settings for the over tail from each of the sieve decks should be identified as these settings may dramatically affect mill balance and quality.
The through stock from the bottom screen deck falls on to some type of valve, paddle or gate whose position may be altered to deliver stock to either the inside or outside channel of flow, which runs the length of the purifier. More often than not, there are four valves, paddles or gates under each sieve frame of the bottom deck. The inside or outside channels of flow may be divided at a prescribed location along the length of the trough to direct stock to an alternative from the inside or outside channel. A dark heavy line may be used to identify where the splits occur in a particular channel. To show the flow of the through stock, one may draw a line in the appropriate channel ending in an arrowhead pointed toward the established discharge outlet that has been properly labeled. When properly drawn, a single line should be shown for through stock under any given sieve frame in the deck. Alternatively, one may draw a short line and arrow indicating the valve, paddle or gate setting, showing how the stock is directed to either the inside or outside flow channel.
Measuring Purifier Performance
Moh’s ash, also known in some parts of the world as cumulative ash curve, may be used to measure and document purifier performance. A weigh off and ash analysis corrected to a common moisture base during optimal or target operation may serve as a baseline for future performance. When operating efficiently, the purifier should produce a significant amount of stock with an ash value (measure of bran contamination) lower than that of incoming stock. Failure to perform in this manner may reflect up stream settings issues or purifier setup such as improper screen selection, poor screen cleaner performance or incorrect air setting. Data for a purifier is shown in Table 1 (page 60) and is used to create the graphic shown in Figure 2.