“The mill design starts with the flow diagram (grinding diagram). The preparation of the mill diagram is the selection and design of order, types, number, and specification of grinding units by taking into account technical data. The length of a flow sheet determine the flexibility of the miller has in making changes in flour yield and quality of flour produced to meet specific needs of different customers.”
Prof. Farhan Alfin
Head of Food Engineering Department
Although the aim of wheat grinding is very simple, the modern systems carrying out this simple process is very complex. The aim of wheat grinding is to break the wheat kernel and to separate endosperm from the bran and germ and then to refine endosperm and turn it into flour or semolina. Regardless of the system applied in the milling process, a part of the bran is mixed with the endosperm, and some endosperm remains with the shell and germ. In other words, it is not possible to separate bran, endosperm, and germ from one another. In the modern grinding system, the grinding and sieving processes are combined in an appropriate manner, and these mixed quantities are tried to be minimized. These processes are carried out in a gradual manner. At the end of each grinding, the classification is formed, and materials in different class are separated, and different processes are applied to each class. Thus, the bran and germ are effectively separated.
The grinding systems of flour mills have a continuous process, meaning that material moves from one machine to another just like in other food processing. But, this is a far more complex system. Because the material coming out of the grinding machine is divided into 4-5 materials in the next sieve and each is sent to a separate grinding machine. For this reason, the characteristics or working settings of a machine change the properties of the products. In addition, an instant stoppage in a device causes significant disorder in the entire system. For this reason, it is required to form an appropriate flow diagram compatible with the mill design and to choose an active machine. A high-quality flour from well-planned mills with high-tech machines can be obtained on the condition that these machines are configured in a good way.
Whether called the mill flow diagram or flow chart, this document, is the first stage in the mill’s design stages and is a valuable tool to manage mill successfully. The flow chart is a two-dimensional road map of the grinding process and draws the direction for any material coming from a machine to go in what direction and to which direction the material came from.
In the mill flow chart, the following information should exist at least:
a. The current diagram shows the locations of the machines to be used.
b. Flow direction, distribution, and quantities of material are shown.
c. Technical information is given about grinding machines, semolina purifier, sieves, and other used equipment.
The mill design and milling are like art based on experience rather than science. In contrast to sectors in which various factors determining the dynamics of the process are well known and formula and equilibriums allowing effective sizing and management of equipment define processes, in the milling, generally speaking, there are many factors affecting the grinding quality and the amount of the final product. For example, the results obtained from the mill (for example: flour yield and quality) vary according to the climate and humidity conditions, and the diagram varies according to the capacity of the mill. When planning a diagram, you must have information and experience with grinding. Information at the new literature on the planning of a mill diagram is not enough and is often not based on industrial experience.
As companies in the milling industries see the diagram as a competition element and consider as important in terms of grinding economy, they do not publish the diagram. Researches at universities and research centers are limited because in commercial scale mills many variables have to be tried on a large scale. Commercial mills do not permit such researches. Some of the information in sources is either old or limited.
When industrial companies design mills, commercial concerns, and technical data are taken into consideration. Before industrial companies offer a tender to investors, in order to gather these data, they want investors to fill out a survey. Generally, technical data can be classified into two main subjects: general technical data and technical data related to grinding diagram.
General technical data is the result of feasibility report on the weather conditions where the mill will be installed (temperature and moisture level), how much wheat will be taken at a time and which transportation vehicle will be used, how much it will be stored, wheat storage preferences, building height, product packaging, storage duration and delivery method and loaded deliveries, milling automation, preferences regarding to building type (hangar or multi-storey full closed).
The technical data for the grinding diagram can be summarized as follows:
• The capacity of the mill in 24 hours,
• Variety of wheat available and its characteristics,
• Product diversity, quality, and extraction rate.
In terms of the available wheat diversity and characteristics, the mills can be classified as specialized and multi-purpose mills. Specialized mills are designed for a single product (eg common flour, biscuit flour). In the multi-purpose mill, different types of wheat are milled separately, and the flours are blended, and various flours are obtained. This practice provides high added value. However, the requirement for investment, information, personnel, and technology are high as well. Based on the technical data related to the grinding diagram, the number of machines required in forming the diagram (such as unit grinding length, unit screening area and the use of semolina purifier) is calculated.
The mill design starts with the flow diagram (grinding diagram). The preparation of the mill diagram is the selection and design of order, types, number, and specification of grinding units by taking into account technical data that previously specified.
The length of a flow sheet determine the flexibility of the miller has in making changes in flour yield and quality of flour produced to meet specific needs of different customers . Although it is complex to design a flow sheet, the analysis of the flow chart is similarly complex. However, the analysis of the flow chart can be easy by splitting into several subsystems. The flow sheet of the milling process can be divided into flour main subsystems:
1. Break System: The break system break and open the kernel and separate the endosperm as much as possible with minimum bran and flour. In this process, it pays special care to leave endosperm as big as possible. So, it gets easier to separate this from bran at purifier. The break system consists of break roller mill corrugated and after each of these machines, there is sifting machinery. Based on the capacity of mills, there are 4-5 break stages in the break system. The final stages in mills with high capacity can be formed with two breaking machines with thin and coarse. After each stage, the material is sorted by sifting. Flour is separated at sifting. The material, which is coarse and consists of particles in which the bran and endosperm have not yet been separated, is sent to the next breaking machine. The medium size material is sent to the purification system of the plant for the separation of pure endosperm particles. These processes are repeated at each breaking stage and at the end of the last stage; the endosperm is removed with the residual bran.
2. Purification System: The purification system consists of machines called as purifier that separate bran, pure endosperm, and composite particles in midsized coming from the sifting of the break system based on size, air resistance, and specific weight. Bran are delivered to feed materials, and composite particles are sent to either breaking system or sizing system based on the capacity, and the endosperm particles are also classified and sent to the reduction rollers. The use of the purification system is reduced in modern mills due to cost efficiency and efficiency in other parts of the grinding process.
3. Sizing System: The duty of sizing system is to separate bran, endosperm, and germ from endosperm particles attached to the shell and came from the purification system and to refine coarse semolina which is not very clean. Particles coming to the sizing system are cleaner than materials going to breaking system and coarser than semolina and are not as pure as semolina.
4. Reduction System: The reduction system also consists of a series of rollers and sifting machines after each roller. However, the surface of the reduction rollers is not corrugated as the breaking rolls, but it is smooth. The duty of reduction system is to refine pure endosperm particles coming from the sifting of the sizing system and the purification system and turn into flour at desirable fineness. In the reduction system, it is essential that the material is refined while controlling the damage of starch granules and the minimum abrasive of bran particles. This process is done again gradually, at the end of each stage a part of the flour is separated, and the larger material is sent to the next stage.
It is wrong to think that “The flow diagram will be kept constant during operation of the mill once it is designed in the mill design.” On the contrary, the current flow sheet should be reviewed from time to time according to conditions such as wheat characteristics, climate conditions, and flour quality and the required adjustment should be carried out. But, it should not be changed too often. An experienced person should know the overall diagram of the mill and must work on it first when it is necessary to make a change. In order to determine whether a change in the flow sheet is necessary, the products and the mid products are checked regularly after being visually analyzed. Generally, it is necessary to try to change the proportions of conditioning, breaking release , and purifier material before making changes in the flow diagram.
The objectives of changing flow diagram:
a. To obtain the highest flour yield from the mill.
b. To produce flour with the least possible ash.
c. To ensure that the characteristics of the flour are appropriate to the market requirements and purpose.
The flow sheet is regarded as the most important tool for miller, mill engineer, and even production and quality team to engage close communication. After the new mill is installed and in the process of operation or in the case of the appropriate use of the flow sheet of the previous mill, the flow sheet plays a critical role in the mill performance. In other words, the correct diagram is the key to success in grinding. In the success of mill design and diagram, it is crucial how equipment is in line with the objective and to use the equipment with the optimal setting. Mill diagram is one of the factors affecting the profit of the enterprise. Production costs play a role in the profit of enterprise, and mill performance is influential in production costs. Mill performance is evaluated with different parameters. However, the measurements used to determine mill performance, in general, can be listed as follows.
a. Total flour yield or extraction rate for specific ash content from milled wheat. The flour yield is the first parameter that defines the production cost of products.
b. The ratio of acquired high quality (first clear flour) in contrast to low-quality flour (second clear flour) from the mill.
c. The flexibility to mill many types of wheat. In some cases, it is required to grind hard, soft and even durum wheat. In this case, the flow sheet should be appropriate, and there should not be many problems in the transition from one wheat to another.
d. Flexibility to produce different products. The type of flour required from the mill may vary according to market requirements. It requires the flow diagram to be flexible. Although the flow diagram is as simple as possible, it becomes more complex to provide flexibility.
e. Energy efficiency. It is the amount of energy required to produce unit flour. This is directly related to the flow sheet.
f. Production stability. For high-capacity enterprises, flour with stability properties is very important. The flow diagram is of great importance for obtaining stable flour.
As long as materials in the mill work according to calculated flow amount at design, it is said that mill is in balance. Even if the mill is well-designed and suitable machines are selected, the mill’s performance is reduced and good results are not achieved if it is not precisely adjusted by an experienced person. In order to maintain the balance or even have a good balance, all the milling machines must be adjusted. To achieve optimum efficiency in the operation of all machines, all machines must be operated at the optimum setting, and all tools of the mill should be fed with the correct quality, size, and appropriate amount of material. The deviation in a machine affects the performance of subsequent tools because the balance of the material coming there is disturbed. For good adjustment of the machine, it is necessary to analyze the properties of the material well and to know where and how to adjust accordingly.
To reach the balance of the mill, the adjustment should be carried out by testing periodically when raw material or mid products are changed. For this, ash curve, granulation curve, and distribution table are found out. These provide useful information to the miller about the total performance of the mill, the quality of the raw material, and the efficiency of the mill.