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Sifters used in milling industry and their dynamic behaviors

22 January 20207 min reading

“The dynamic behavior of a sieve is effective in determining both the quality and quantity of the product to be obtained and the tension and energy consumption of the mechanical system. In this respect, in addition to obtaining the desired quality product, minimum tension and energy consumption formation is essential. It must be ensured that the system enters the regulation until the start of the sieving process and the product flow begins, and also the tensions that occur during operation comply with the limits predicted in the design.”

Murat APAKHAN
Vice General Manager
Imas Machinery Hakkı EKEM
R&D Manager
Imas Machinery Ömer Sinan ŞAHİN
Konya Technical Universty ***

1. Introduction Wheat is one of the cereals, plants grown in the world since ancient times. Since its development, it has become one of the most important elements in the nutrition of people and has made a huge contribution to the development of civilization. It is known that wheat was identified and benefited around eleven thousand years ago. Wheat has been an important source of development both for humanity and animals and thus for the civilization since the time it was first grown. And this can be seen in the works of art remaining from many civilizations in the Near East, especially in ancient Egypt, as it is seen in Figure 1.

It is known that planar and conical stone mills, operated depending on human and animal power, were used in the periods following the grinding and use of wheat as food for humanity some eleven thousand years before today. With the beginning of the industrial revolution, and with the construction of a number of mills based on steam power, flour was produced in industrial sizes [4] and became widespread. Today, around 750 million tons of wheat is produced annually worldwide [1]. Annual wheat production in our country is around 20 million tons and Konya province has a 10% production share.

2. Sifter machines During the processing of wheat, there are many processing machines of different capacity and type to conduct cleaning, destoning, conditioning, husking, grinding, screening and handling.

Plansifter is a machine used in flour and semolina factories for the purpose of sorting the wheat, which is grinded in the roller mills, into sizes by sifting. In general, classical sifters consist of two sifting parts and one drive part located in-between these. Thanks to the centrifugal force produced by an eccentric mass in the drive part, the machine is enabled to make circular moves on a horizontal plane. Screening section is divided into parts called passage and the grinded product is sorted out from

The sieving section is divided into parts called passage, and the product grinded through sieves lined up one over the other in every passage, is sorted by being screened from top to bottom. Examples of modern sieves used today are shown in Figure 2.

Figure 1. Production of wheat [2] and flour [3] in ancient world

There are N type and B type types according to sieve frame types, sifters are named according to number of passages and number of frames in a passage. For example, in a system called 8x24 screen, there are 8 passages and sieve structure with maximum 24 frames in each passage. The required number of passage is determined by considering the sieving area calculated according to the type of product to be screened and the capacity of the plant. Sieving area value is taken into account rather than capacity in sieves.

As far as the drive modes are concerned, it is seen that the commonly used drive mode is the circular movement of the sieve machine in the horizontal plane by the centrifugal force of the mass mounted on a shaft placed vertically in the drive section. However, in today's inventory, there are different alternatives such as the use of timing belts instead of v-belts, placing the electric motor on the top or bottom of the machine. As a general rule, it is known that the machine performs the most efficient screening in the range of 240 - 250 rpm and 60 - 65 mm oscillation circle. Apart from this; rather than the general structure of the machine and the way it works, the factors such as the quality and tightness of the sieve cases in it, optimum cleaning of the sieve pores and the sieve tension are the factors affecting the quality of the sieved flour.

3. Determining dynamic behaviors The dynamic behavior of a sieve is effective in determining both the quality and quantity of the product to be obtained and the tension and energy consumption of the mechanical system. In this respect, in addition to obtaining the desired quality product, minimum tension and energy consumption formation is essential.

It must be ensured that the system enters the regulation until the start of the sieving process and the product flow begins, and also the tensions that occur during operation comply with the limits predicted in the design. In this regard, acceleration measurements and displacement measurements were performed on a xxxx type and xxx configured screen to determine the regime of the sieves and to determine the tension levels.

ACCELERATION MEASUREMENTS Acceleration measurements were performed on three axes perpendicular to each other since the first operation of the sieve and the maximum acceleration component was measured as 5.91 g. In sieve and similar dynamic systems, a certain period of time is required for the system to enter the regime and exhibit a stable dynamic characteristic. Table 1 displays the maximum compound acceleration values measured after obtaining stable dynamic behavior in sieves with different passages. As seen in Table 1, maximum compound acceleration values increased with increasing number of passages. The main reason for the increase in acceleration values can be attributed to the increase in dimensions along with the number of passages and in particular the increase in drive mass. On the other hand, the maximum compound acceleration value was measured as 3.72 g when the sieve was stopped.

Figure 2. Modern sifters used in present day [5]

• Measurements of screen displacement The movement of the sieve after starting the sieve was recorded and monitored time-dependently for 45 minutes. This time-dependent movement is presented in Figure 3. As shown in Figure 3, the circular movement started with the first movement of the screen. However, the movement obtained is rather irregular and it is seen that instead of the ideal circular motion of 60-65 mm diameter, a circular movement of 30-35 mm diameter is formed. During the first 10- minute period, the movement became clear. However, it takes a period of about 40-45 minutes for the sieve to enter the regulation and to move in concentric circles.

4. Results The measurements made on the sieve showed that the maximum compound acceleration value was dependent on the number of passages and that the value in 4-passage sieve was 5.9 g while the figure was at the level of 6.55 g in 10-passage system. It can be considered that this change in question is due to the increase of the moving mass value due to the increase of the number of passages and the change of the sieve geometry. In this case, it is concluded that the sieve designs should be optimized to perform the sieving process with the highest efficiency and to provide minimum acceleration values.

It was observed that after the sieve was operated, the stability was obtained within approximately 40-45 minutes and at the end of this time the sieve started to draw concentric circles in 60-65 mm diameter. In the sieve design, enabling the system to enter the regulation in a shorter period of time by improving the sieve configuration and geometry in order to shorten the time of entering the said regime will significantly increase both the sieve efficiency and the efficiency of the whole mill system.

References [1] https://www.zmo.org.tr/genel/bizden_detay.php?kod=30125&tipi=17&sube=0 (Date of Visit 03.10.2019) [2] https://www.egyptprivatetourguide.com/egyptian-facts/ancient-egyptian-agriculture-facts-agriculture-ancient-egypt/ (Date of Visit 03.10.2019) [3] https://www.atthemummiesball.com/tiger-nut-sweets/ (Date of Visit 03.10.2019) [4] https://www.tusaf.org/TR,80/tarihce.html (Date of Visit 03.10.2019) [5] https://milleral.com/tr/urun/mqp-kare-elek/24 (Date of Visit 03.10.2019)

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