New developments in plansifter design

by Emily Wilson
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Plansifters have a significant impact on the economy of a flour mill. In the processing of grain to flour, plansifters occupy a key position and are determined based on the plant's efficiency, product quality, space requirements and service or maintenance requirements.

The dimensions of plansifters have increased considerably in the past 40 years. Sieve areas have increased from 30 square meters to over 90 square meters per machine. New developments also have made it possible to double the specific load values, and the overall space requirement of plansifters in a flour mill has been reduced one-third.

The new NOVA sieves developed by Buhler AG, Uzwil, Switzerland, offer a significantly higher sifting efficiency. Unlike conventional sieves, NOVA sieves are equipped with stable and wear-resistant metal insert frames and new sieve cleaners. The sieve fabric fixed on the insert frames can be tensioned more tightly and without distortion, which is indispensable for achieving optimal sifting efficiency over a long period of time. The application of metal has reduced the dimensions of the frame profiles, gaining additional net sieve area per frame.

The new cleaners simultaneously clean the sieve fabric as well as the sieve trays underneath. The star-shaped cleaner lies directly on the sieve tray, circulating in an intensive sliding and tilting motion on a raised central foot from one cross-member to the next in the sieve frame field. The projecting nose ensures that the lateral discharge slots in the frame remain open, and it conveys the sieved stock from the sieve tray onward into the vertical channels.

Knobs or brush tufts on the surface of the cleaner thoroughly clean the sieve fabric. The roof-shaped geometry of the arms prevents product from building up on the cleaner.

The combination of metal insert frames and new cleaners reduces the height of each sieve by 10 millimeters. As a result, it is possible to install up to three more sieves per sieve compartment, providing sufficient space for the product to flow through. The higher number of sieves, larger net sieve area and metal insert frames provide up to 20% more sifting area, increasing the capacity of the plansifters.

Buhler's existing square plansifters can be easily retrofitted with NOVA sieves by exchanging the sieve stacks. In a mill remodeling project, this will increase capacity.

HIGHER SIEVE ACCELERATION. An essential factor influencing the capacity of a plansifter is the intensity of the gyrating motion. To obtain a proper sifting process, the machine and sieves are set in motion. The stock to be sifted must be conveyed, loosened, stratified and moved across the sieve.

A centrifugal drive, normally located in the center of the machine, gives the plansifter its characteristic gyratory motion. This motion allows the product to be conveyed in any desired horizontal direction with a low energy input. The intensity of this motion is called horizontal acceleration and is calculated on the basis of the rotary speed and the radius of gyration.

Over the years, the acceleration of plansifters designed for flour and semolina mills gradually has been increased by 20%. This increase has been achieved along with a higher specific plansifter loading.

Extensive field studies were made to determine the impact of an additional acceleration increase on the sifter performance at different throughputs and for different passages. In long and thorough field tests, it was found that the best results can be achieved for higher product throughputs by further increasing the acceleration by 20% to 25%. The higher acceleration can be achieved by changing the sifter rotary speed or the sifter gyration.

A higher acceleration results in a considerable increase in the dynamic forces inside the machine. The designs of square plansifters up to now have been unsuitable for withstanding these permanently higher mechanical loads.

NEW PLANSIFTER DESIGN. These improvements were taken into account in the development and design of Buhler's new MPAJ plansifter. In addition, improvements have been incorporated in the new design with regard to mechanical strength during operation, ease of installation, sanitation, safety and ease of operation. The steel support structure as well as the drive had to be resized and carefully redesigned to withstand the higher loadings.

In a tough test procedure, it was proven that the new plansifter has the required mechanical strength during operation to handle the 25% higher acceleration.

The sifter box can be dismantled into three sections for transportation, and can be bolted together on site with little effort. The suspension system uses glass fiber rods to guarantee high support safety. A large servicing door facilitates access to the drive area and the integrated motor. A light-weight, stable door reliably seals off the sieve compartments and can be removed in a few moves.

A method being used more often for pest control is thermal disinfection, which involves heating up the inside of the building with hot air of 50° to 60° Celsius for several hours. While Buhler's new sifter design is well-suited to this process, the nylon sieve fabric tends to lose some of its tension during heat treatment. To avoid this problem, remove the insert frames along with the sieve covers from the area exposed to the hot air during treatment and observe the recommendations of the fabric manufacturers.

All the wall surfaces inside the sifter compartments of the MPAJ plansifter are carefully insulated, preventing material build-up as a result of condensation and significantly improving sanitation. This also helps reduce the cleaning intervals of the sifter compartments.

The industry continues to demand increased grinding capacity in existing buildings. New plansifters with more sifting area and higher efficiency require smaller surface areas. Optimized flow chart designs for every case are possible with an appropriate range of machine types.

Buhler's studies and experiences in the field of grain mill design showed that a plansifter with 10 compartments may considerably improve flexibility and efficiency for a wide capacity range.

The range of types offered, which up to now have included 4, 6 and 8 compartments, has been increased to 10 compartments with the new MPAJ plansifter. This has resulted in a corresponding increase in the net sieve areas that are possible in practice, which range up to 90 square meters.

This new generation of plansifters has proven high efficiency in practical operation and fulfills the needs of today's grain milling companies.

Before retiring recently, Alois Keller led the research and development for plansifters at Buhler AG, Uzwil, Switzerland.