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Material handling and transporting
Material handling and transporting
9
Sylvia L. Schonauer 1,2 , Robert B. Fast y 1 Kellogg Company, Battle Creek, MI, United States; 2SSK Consulting, LLC, Bellaire, MI, United States
Introduction Cereal recipes include both dry and liquid ingredients. Understanding the ingredient properties and characteristics, the usage rates, the environment, and the equipment is vital to maintaining quality of the materials before and during processing. Improperly handled or stored ingredients can contribute to inefficient processing and inferior quality of finished food. Transferring both dry and liquid raw materials from storage to the processing area is the first step in a cereal process. Liquids are typically stored in tanks, totes, or barrels and pumped to the use point. Dry materials are typically transferred either pneumatically or via mechanical conveyors or feeders from silos, supersacks, or feed tanks. Transferring between different unit operations is also important. Again, liquids are typically pumped, while solids are transferred via conveyor or pneumatic systems. For dry ingredients, lump-breaking and sizing or screening operations in cereal processes are two separate steps, usually in close proximity to each other. Multiple pieces of equipment are often used for both operations, because they are frequently performed in a stepwise fashion. Coarse breaking takes place first, followed by coarse screening, then finer crushing, followed by finer screening, and so on until the product reaches the desired final size. Performing these operations in a stepwise manner minimizes the production of unwanted fines. Lump-breaking and sizing operations are required after cooking. Masses of cooked grain, when dumped from a batch cooker, are usually just that nondescript masses much bigger than the desired finished cereal pieces. Lump breaking is essential for obtaining smaller pieces of uniform size that can be dried, tempered, and processed into the final cereal shape.
Storage Material properties and usage quantity determine the type of storage and handling systems that an operation uses. Some of the key parameters to determine storage y
Deceased author.
Breakfast Cereals and How They Are Made. https://doi.org/10.1016/B978-0-12-812043-9.00009-6 Copyright © 2020 Cereals & Grains Association. Published by Elsevier Inc. All rights reserved.
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Breakfast Cereals and How They Are Made
and transfer equipment for dry ingredients include the angle of repose, the density, compressibility, porosity, flowability, hygroscopicity, and combustibility. These factors along with the usage rate will influence the equipment used. For example, if you are working with an ingredient of low density, your storage silo will need to be a larger volume than if the density was high. However, if your material is compressible, you will want to limit how large your storage vessel is so that you do not damage the bottom materials from excessive pressure. The angle of repose will play a part in how easily a material flows from a storage vessel or from a feeder and how high you can layer it on a conveyor without it sliding off. Another consideration is whether you are running your operation as a batch operation or a continuous operation. Many of the traditional cereal processes today are still batch or batch continuous and will influence how materials are handled. Silos, tote bags, or smaller bagged quantities are the standard method of storing dry ingredients. For smaller batch operations, dry ingredients tend to be stored in 50 lb or smaller bags to match the batch size. For large operations, the dry ingredients, particularly high usage items such as grains, flours, and sugar, are stored in silos similar to those shown in Fig. 9.1. It is good practice to include a magnet after the silo, before ingredients enter the process, to remove loose metal that may have come from postharvest processing. One of the key parameters for grain storage is the equilibrium moisture content of the environment, which is dependent on humidity and temperature conditions in the storage area. Stress cracks or fissures can occur in grains if the equilibrium moisture content of the environment is not maintained similar to the moisture content of the grain. Small cereal processing facilities may use supersacks or totes for their flour blends, sugars, fibers, and other ingredients as shown in Figs. 9.2 and 9.3. Minor ingredients such as minerals often come in bags.
Figure 9.1 Typical tank farm often used for grains, flour, and sugar. Photo Courtesy of Kellogg Company.
Material handling and transporting
183
Figure 9.2 Tote station often used for flour blends, fibers, or minor ingredients. Photo Courtesy of Kellogg Company.
Figure 9.3 Common supersack for lower usage ingredients. Photo Courtesy of Kellogg Co.
Handling of liquid ingredients has different considerations such as the viscosity, density, compressibility, melt point, and whether it is a suspension or solution. The usage rate and type of operation will also play a role in the type of tanks, pumps, and metering systems used to handle the liquids. High usage liquid ingredients such as liquid sugar and corn syrups are stored in bulk in tank farms. Often these various storage tanks require environmental control to preserve the quality of the ingredients. Headspace control, such as nitrogen blanketing of the tanks, is needed to prevent rancidity and off-flavor development from occurring. Some liquid ingredients, such as corn syrups, require thermal jackets to keep the material warm for easier flowability. Other ingredients such as liquid sugar or suspensions require agitation in the tanks.
184
Breakfast Cereals and How They Are Made
Transport and transfer One of the first steps in cereal making is the transfer or transport from raw material storage to the process area. Depending on the size of the operation and material use, dry ingredients may be transferred pneumatically or via feeders from silos, supersacks, or bag dump stations. Additional transfer steps occur throughout the process, typically via conveyors or pneumatic systems. When silos are in use for grain storage, the materials are often transported pneumatically. These systems need to be optimized for each type of grain. One can use dilute or dense phase transfer, where typically a pressure system is used. In the case of dilute phase transfer, a vacuum system can be used. Fig. 9.4 illustrates the layout of a pressure system. A vacuum system would have the vacuum blower on the other side of the receiver. Consideration of the air condition, air velocity, air volume, and loading factor should be made along with the piping system to maintain the equilibrium moisture content of surrounding environment. This will minimize breakage or stress fractures in the grain as it passes through piping elbows. In addition, the air velocity, volume, and loading factor need to be sufficient to keep the material suspended. This is called the saltation velocity. At the receiving end, there are various graders or sifters needed to separate the foreign material, clumps, or fines from the desired sized ingredient for processing. Unwanted material, such as stones, chaff, and fines from the material needs to be removed before processing to avoid equipment damage and to avoid compromising product quality. If a magnet is not in place before the transfer, the grader or sifter is a good place to install one. Liquids are typically pumped from large tanks, totes, or drums whether coming to the process or transferred during the process. A variety of pumps can be used for this transferdtypically positive displacement pumps such as lobe pumps, piston, diaphragm, or progressive cavity pumps are used. In some cases, nonpositive displacement pumps such as centrifugal pumps are used to move large quantities of low viscosity materials. The choice of pump used depends on the material characteristics, pumping distances, and quantities. Progressive cavity pumps are often used to pump viscous materials or materials with entrained solids. Sine pumps can also be used to
Filter air out
Air in
Silo
Blower
Rotary air lock
Figure 9.4 Typical layout of a pressure-based pneumatic system.
Receiver
Material handling and transporting
185
pump materials with entrained solids. Metering pumps such as gear pumps, diaphragm pumps, and peristaltic pumps can be used to deliver liquids. Gear pumps are used in situations where high pressures are needed in the process. As described further in Chapter 10, there will usually be some type of meter in the transfer line for measuring liquid flow. Well-set-up transfer lines will also include adequate temperature- and pressure-sensing devices to monitor the process. During the transport or transfer process, the key is to have minimum change in the functionality of the ingredient. For cereal grains, it is desirable to have no moisture loss or gain, minimize the breakage, and deliver an accurate targeted quantity. Liquids are largely immune to degradation during transport and transfer, but improper storage handling can lead to undesirable results such as fermentation, contamination, or mold growth.
Lump breaking Products from the cookers are typically dumped onto a moving belt. Before they are conveyed to a dryer, the cooked grain mass passes through a delumping equipment to break the loosely held-together grains into mostly single particles. Delumping is essential to obtain particles or agglomerates of grains small enough for good circulation of heated air around each particle for uniform drying. It may be necessary to accomplish delumping and cooling in steps to get good separation so that they are the optimum size for drying. In most cases, cooling takes place first, to stop the cooking action and remove stickiness from the grain surface. Cooling is kept to a minimum because, in subsequent drying, the product is reheated to remove moisture. Most coolingedelumping systems include screening devices. The most common are flatbed gyrating sifters or rotating-wire or perforated-drum screeners. Because lump breaking is usually needed just after material is dumped from a batch cooker, it is performed on a hot, sticky mass of cooked grain and other ingredients. Jam-ups easily occur in batches that have been overcooked or have a higher moisture content than the process calls for. In cases such as this, the breaker then turns into a powerful mixer that destroys the product rather than performing its breaking function. One feature usually employed in most breaking and sizing operations is the use of large volumes of air drawn through the equipment. In the breaking section, this helps to cool the product, form a slight skin over it, and thereby reduce its stickiness. The section also removes steam vapor, which is released as the cooling takes place. In the sizing section, the air also removes heat and moisture vapor and fine dust or particles from the product stream. Another aspect of proper breaking and sizing is the uniformity of the product feed to the units. It is not usually necessary to employ gravimetric feeders; well-measured volumetric feeding is sufficient. This is most easily accomplished by controlling the speed of the conveying belt ahead of the breaker or sifter. Speed control is also used in combination with adjustable gates mounted above the feed conveyors, which produce a uniformly thick layer of product on the feed belt. Overfeeding breakers and sifters is one of the pitfalls that can cause them to jam.
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Breakfast Cereals and How They Are Made
Most breakers are rugged units, built to withstand the pounding incurred in the breaking process. In simple terms, they consist of the following parts: (1) the main body, often equipped with a fixed comb or fingers, (2) one or more rotating shafts, on which breaking bars or projections are mounted, (3) a grid or screen device to hold the product in the breaker until the desired particle size is achieved, and (4) a drive train, consisting of a motor with or without a speed reducer. Representative types of lump breakers are shown in Figs. 9.5e9.8. Each unit is slightly different in design and construction. As a result, they handle different products with varying degrees of efficiency in breaking, generation of fines, and jamming tendencies (Feldman, 1987). The units are typically offered in carbon or stainless steel, with the latter preferred for food use.
Sizing Like lump breakers, sifters come in various sizes and shapes, each with its own unique application properties. The Rotex machines have rectangular single- or multidecked vibrating and/or gyrating screens. Azo screeners are different in that the screen in these units is cylindrical, horizontally mounted, and stationary. A rotating paddle assembly is mounted inside the screen, and it is this that feeds the product through the screen, from the inside to the outside.
Figure 9.5 Lump-breaking machine (Lump Abrador), designed to reduce agglomerated material that is friable and can be reduced in size by the impact of rotating crushing and impact fingers mounted on a rotary drum. The impact fingers pass through two sets of combs. Products up to 1 in. (2.5 em) in size can be reduced to particles as fine as 3D mesh. Photo Courtesy of Jersey Stainless, Inc.
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187
Figure 9.6 Lump-crushing machine. Dual counterrotating shafts are fitted with crushing fingers or bars. The machine can be fitted with special screens or plates to hold the product back until it reaches the desired particle size. Photo Courtesy of Jersey Stainless, Inc.
Figure 9.7 Heavy-duty crusher (Titan). This machine performs high-torque crushing at low speed to minimize the generation of fines, with split-grid construction for product holdback. The grids may be removed through the sides of the unit, obviating the need to remove the whole unit from the line. Photo Courtesy of Champion Products, Inc.
Figs. 9.9e9.11 show how a Rotex screener is built. In Fig. 9.10, the uppermost deck, on which the mixed-size material first alights, is the coarser of the two deck screens. The lower deck separates out midsize particles as overs and throughs. The circles under each screen deck in the sifter represent resilient balls, which prevent screen blinding. Fig. 9.11 depicts the path the material takes in flowing from the infeed to the discharge end of the sifter. This is represented by the circular black lines and arrows.
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Breakfast Cereals and How They Are Made
Figure 9.8 Medium-duty crusher (Gladiator). Pins mounted in the main body come in different combinations for different operations and are removable from outside the unit. The shafts are rotated toward each other for delumping and away from each other for granulation. The actual particle size depends on the holdback screen used. Photo Courtesy of Champion Products, Inc.
Figure 9.9 Gyratory sifter (Rotex screener). Photo Courtesy of Rotex Global, LLC.
Figure 9.10 Vertical cross section of Rotex screener.
Material handling and transporting
189
Figure 9.11 Rotex screener from above, with the cover removed. The lines and arrows indicate the path of material from the infeed to the discharge. Photo Courtesy of Rotex Global, LLC.
The gyratory motion performs three functions in this type of sifter. First, it spreads the material across the full width of the screen deck. Second, it stratifies the material, causing the fines to sink down against the screen, where they quickly pass through the screen openings. Third, it allows the larger particles to float to the top, where they are conveyed toward the discharge. The bouncing balls underneath the screen are deflected against bevel strips and bounce continuously against the underside of the screen mesh. This action continuously cleans the screen mesh openings. The Azo Cyclone screener consists of a horizontally mounted cylinder of nylon screen cloth on a metal frame. A short motor-driven screw conveys the feed stock into the screening cylinder, where the whirling paddles, mounted on the same shaft as the feed screw, force the material through the screen opening. The natural vibration of the screen cloth performs the cleaning action. The paddles also have a lumpbreaking action on any soft, friable lumps in the feed stock. All of the units as shown or described are totally enclosed and relatively dust-tight in operation. Some of the questions that have to be considered in choosing a unit are as follows. The first consideration is matching the flow rate to the required square footage of the screen area, so that particle size ranges of each fraction from the screener meet the necessary specifications. Another consideration is the temperature of the infeed materialdis it warm enough to be giving off moisture vapor? If so, and if the sifter is mounted in a cool area, water vapor condenses on the inside with dust particles, so that the frequency and ease of cleaning become important. Still another important consideration is possible product degradation caused by the sifter. Sometimes the rubbing action of the product against the screen causes attrition or product breakdown. In some cases where this occurs, the product itself may have to be altered to withstand a sifting step. As was noted in the section on lump breaking, there are occasions in sifting wet or sticky materials when large volumes of air drawn through the equipment greatly aid sifter performance. Furthermore, the feeding of stock to the sifters must be uniform, so that screen areas do not become overloaded. Overloaded screen surfaces fail to make the desired separations; fines cannot pass through an overloaded screen and are carried over with the overs stock. The importance of proper lump breaking and sizing cannot be overstressed. The size of the finished product and its ultimate package weight may be a function of
190
Breakfast Cereals and How They Are Made
how well the lump breaking and sizing were carried out earlier in the process. This is particularly true of some flaked cereals, some granolas, and several hot cereal products, particularly instant mix-in-the-bowl hot cereals.
Reference Feldman, H., 1987. Selecting a lump breaker for gross size reduction. Powder Bulk Eng. 1 (6), 26.
9
Sylvia L. Schonauer 1,2 , Robert B. Fast y 1 Kellogg Company, Battle Creek, MI, United States; 2SSK Consulting, LLC, Bellaire, MI, United States
Introduction Cereal recipes include both dry and liquid ingredients. Understanding the ingredient properties and characteristics, the usage rates, the environment, and the equipment is vital to maintaining quality of the materials before and during processing. Improperly handled or stored ingredients can contribute to inefficient processing and inferior quality of finished food. Transferring both dry and liquid raw materials from storage to the processing area is the first step in a cereal process. Liquids are typically stored in tanks, totes, or barrels and pumped to the use point. Dry materials are typically transferred either pneumatically or via mechanical conveyors or feeders from silos, supersacks, or feed tanks. Transferring between different unit operations is also important. Again, liquids are typically pumped, while solids are transferred via conveyor or pneumatic systems. For dry ingredients, lump-breaking and sizing or screening operations in cereal processes are two separate steps, usually in close proximity to each other. Multiple pieces of equipment are often used for both operations, because they are frequently performed in a stepwise fashion. Coarse breaking takes place first, followed by coarse screening, then finer crushing, followed by finer screening, and so on until the product reaches the desired final size. Performing these operations in a stepwise manner minimizes the production of unwanted fines. Lump-breaking and sizing operations are required after cooking. Masses of cooked grain, when dumped from a batch cooker, are usually just that nondescript masses much bigger than the desired finished cereal pieces. Lump breaking is essential for obtaining smaller pieces of uniform size that can be dried, tempered, and processed into the final cereal shape.
Storage Material properties and usage quantity determine the type of storage and handling systems that an operation uses. Some of the key parameters to determine storage y
Deceased author.
Breakfast Cereals and How They Are Made. https://doi.org/10.1016/B978-0-12-812043-9.00009-6 Copyright © 2020 Cereals & Grains Association. Published by Elsevier Inc. All rights reserved.
182
Breakfast Cereals and How They Are Made
and transfer equipment for dry ingredients include the angle of repose, the density, compressibility, porosity, flowability, hygroscopicity, and combustibility. These factors along with the usage rate will influence the equipment used. For example, if you are working with an ingredient of low density, your storage silo will need to be a larger volume than if the density was high. However, if your material is compressible, you will want to limit how large your storage vessel is so that you do not damage the bottom materials from excessive pressure. The angle of repose will play a part in how easily a material flows from a storage vessel or from a feeder and how high you can layer it on a conveyor without it sliding off. Another consideration is whether you are running your operation as a batch operation or a continuous operation. Many of the traditional cereal processes today are still batch or batch continuous and will influence how materials are handled. Silos, tote bags, or smaller bagged quantities are the standard method of storing dry ingredients. For smaller batch operations, dry ingredients tend to be stored in 50 lb or smaller bags to match the batch size. For large operations, the dry ingredients, particularly high usage items such as grains, flours, and sugar, are stored in silos similar to those shown in Fig. 9.1. It is good practice to include a magnet after the silo, before ingredients enter the process, to remove loose metal that may have come from postharvest processing. One of the key parameters for grain storage is the equilibrium moisture content of the environment, which is dependent on humidity and temperature conditions in the storage area. Stress cracks or fissures can occur in grains if the equilibrium moisture content of the environment is not maintained similar to the moisture content of the grain. Small cereal processing facilities may use supersacks or totes for their flour blends, sugars, fibers, and other ingredients as shown in Figs. 9.2 and 9.3. Minor ingredients such as minerals often come in bags.
Figure 9.1 Typical tank farm often used for grains, flour, and sugar. Photo Courtesy of Kellogg Company.
Material handling and transporting
183
Figure 9.2 Tote station often used for flour blends, fibers, or minor ingredients. Photo Courtesy of Kellogg Company.
Figure 9.3 Common supersack for lower usage ingredients. Photo Courtesy of Kellogg Co.
Handling of liquid ingredients has different considerations such as the viscosity, density, compressibility, melt point, and whether it is a suspension or solution. The usage rate and type of operation will also play a role in the type of tanks, pumps, and metering systems used to handle the liquids. High usage liquid ingredients such as liquid sugar and corn syrups are stored in bulk in tank farms. Often these various storage tanks require environmental control to preserve the quality of the ingredients. Headspace control, such as nitrogen blanketing of the tanks, is needed to prevent rancidity and off-flavor development from occurring. Some liquid ingredients, such as corn syrups, require thermal jackets to keep the material warm for easier flowability. Other ingredients such as liquid sugar or suspensions require agitation in the tanks.
184
Breakfast Cereals and How They Are Made
Transport and transfer One of the first steps in cereal making is the transfer or transport from raw material storage to the process area. Depending on the size of the operation and material use, dry ingredients may be transferred pneumatically or via feeders from silos, supersacks, or bag dump stations. Additional transfer steps occur throughout the process, typically via conveyors or pneumatic systems. When silos are in use for grain storage, the materials are often transported pneumatically. These systems need to be optimized for each type of grain. One can use dilute or dense phase transfer, where typically a pressure system is used. In the case of dilute phase transfer, a vacuum system can be used. Fig. 9.4 illustrates the layout of a pressure system. A vacuum system would have the vacuum blower on the other side of the receiver. Consideration of the air condition, air velocity, air volume, and loading factor should be made along with the piping system to maintain the equilibrium moisture content of surrounding environment. This will minimize breakage or stress fractures in the grain as it passes through piping elbows. In addition, the air velocity, volume, and loading factor need to be sufficient to keep the material suspended. This is called the saltation velocity. At the receiving end, there are various graders or sifters needed to separate the foreign material, clumps, or fines from the desired sized ingredient for processing. Unwanted material, such as stones, chaff, and fines from the material needs to be removed before processing to avoid equipment damage and to avoid compromising product quality. If a magnet is not in place before the transfer, the grader or sifter is a good place to install one. Liquids are typically pumped from large tanks, totes, or drums whether coming to the process or transferred during the process. A variety of pumps can be used for this transferdtypically positive displacement pumps such as lobe pumps, piston, diaphragm, or progressive cavity pumps are used. In some cases, nonpositive displacement pumps such as centrifugal pumps are used to move large quantities of low viscosity materials. The choice of pump used depends on the material characteristics, pumping distances, and quantities. Progressive cavity pumps are often used to pump viscous materials or materials with entrained solids. Sine pumps can also be used to
Filter air out
Air in
Silo
Blower
Rotary air lock
Figure 9.4 Typical layout of a pressure-based pneumatic system.
Receiver
Material handling and transporting
185
pump materials with entrained solids. Metering pumps such as gear pumps, diaphragm pumps, and peristaltic pumps can be used to deliver liquids. Gear pumps are used in situations where high pressures are needed in the process. As described further in Chapter 10, there will usually be some type of meter in the transfer line for measuring liquid flow. Well-set-up transfer lines will also include adequate temperature- and pressure-sensing devices to monitor the process. During the transport or transfer process, the key is to have minimum change in the functionality of the ingredient. For cereal grains, it is desirable to have no moisture loss or gain, minimize the breakage, and deliver an accurate targeted quantity. Liquids are largely immune to degradation during transport and transfer, but improper storage handling can lead to undesirable results such as fermentation, contamination, or mold growth.
Lump breaking Products from the cookers are typically dumped onto a moving belt. Before they are conveyed to a dryer, the cooked grain mass passes through a delumping equipment to break the loosely held-together grains into mostly single particles. Delumping is essential to obtain particles or agglomerates of grains small enough for good circulation of heated air around each particle for uniform drying. It may be necessary to accomplish delumping and cooling in steps to get good separation so that they are the optimum size for drying. In most cases, cooling takes place first, to stop the cooking action and remove stickiness from the grain surface. Cooling is kept to a minimum because, in subsequent drying, the product is reheated to remove moisture. Most coolingedelumping systems include screening devices. The most common are flatbed gyrating sifters or rotating-wire or perforated-drum screeners. Because lump breaking is usually needed just after material is dumped from a batch cooker, it is performed on a hot, sticky mass of cooked grain and other ingredients. Jam-ups easily occur in batches that have been overcooked or have a higher moisture content than the process calls for. In cases such as this, the breaker then turns into a powerful mixer that destroys the product rather than performing its breaking function. One feature usually employed in most breaking and sizing operations is the use of large volumes of air drawn through the equipment. In the breaking section, this helps to cool the product, form a slight skin over it, and thereby reduce its stickiness. The section also removes steam vapor, which is released as the cooling takes place. In the sizing section, the air also removes heat and moisture vapor and fine dust or particles from the product stream. Another aspect of proper breaking and sizing is the uniformity of the product feed to the units. It is not usually necessary to employ gravimetric feeders; well-measured volumetric feeding is sufficient. This is most easily accomplished by controlling the speed of the conveying belt ahead of the breaker or sifter. Speed control is also used in combination with adjustable gates mounted above the feed conveyors, which produce a uniformly thick layer of product on the feed belt. Overfeeding breakers and sifters is one of the pitfalls that can cause them to jam.
186
Breakfast Cereals and How They Are Made
Most breakers are rugged units, built to withstand the pounding incurred in the breaking process. In simple terms, they consist of the following parts: (1) the main body, often equipped with a fixed comb or fingers, (2) one or more rotating shafts, on which breaking bars or projections are mounted, (3) a grid or screen device to hold the product in the breaker until the desired particle size is achieved, and (4) a drive train, consisting of a motor with or without a speed reducer. Representative types of lump breakers are shown in Figs. 9.5e9.8. Each unit is slightly different in design and construction. As a result, they handle different products with varying degrees of efficiency in breaking, generation of fines, and jamming tendencies (Feldman, 1987). The units are typically offered in carbon or stainless steel, with the latter preferred for food use.
Sizing Like lump breakers, sifters come in various sizes and shapes, each with its own unique application properties. The Rotex machines have rectangular single- or multidecked vibrating and/or gyrating screens. Azo screeners are different in that the screen in these units is cylindrical, horizontally mounted, and stationary. A rotating paddle assembly is mounted inside the screen, and it is this that feeds the product through the screen, from the inside to the outside.
Figure 9.5 Lump-breaking machine (Lump Abrador), designed to reduce agglomerated material that is friable and can be reduced in size by the impact of rotating crushing and impact fingers mounted on a rotary drum. The impact fingers pass through two sets of combs. Products up to 1 in. (2.5 em) in size can be reduced to particles as fine as 3D mesh. Photo Courtesy of Jersey Stainless, Inc.
Material handling and transporting
187
Figure 9.6 Lump-crushing machine. Dual counterrotating shafts are fitted with crushing fingers or bars. The machine can be fitted with special screens or plates to hold the product back until it reaches the desired particle size. Photo Courtesy of Jersey Stainless, Inc.
Figure 9.7 Heavy-duty crusher (Titan). This machine performs high-torque crushing at low speed to minimize the generation of fines, with split-grid construction for product holdback. The grids may be removed through the sides of the unit, obviating the need to remove the whole unit from the line. Photo Courtesy of Champion Products, Inc.
Figs. 9.9e9.11 show how a Rotex screener is built. In Fig. 9.10, the uppermost deck, on which the mixed-size material first alights, is the coarser of the two deck screens. The lower deck separates out midsize particles as overs and throughs. The circles under each screen deck in the sifter represent resilient balls, which prevent screen blinding. Fig. 9.11 depicts the path the material takes in flowing from the infeed to the discharge end of the sifter. This is represented by the circular black lines and arrows.
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Breakfast Cereals and How They Are Made
Figure 9.8 Medium-duty crusher (Gladiator). Pins mounted in the main body come in different combinations for different operations and are removable from outside the unit. The shafts are rotated toward each other for delumping and away from each other for granulation. The actual particle size depends on the holdback screen used. Photo Courtesy of Champion Products, Inc.
Figure 9.9 Gyratory sifter (Rotex screener). Photo Courtesy of Rotex Global, LLC.
Figure 9.10 Vertical cross section of Rotex screener.
Material handling and transporting
189
Figure 9.11 Rotex screener from above, with the cover removed. The lines and arrows indicate the path of material from the infeed to the discharge. Photo Courtesy of Rotex Global, LLC.
The gyratory motion performs three functions in this type of sifter. First, it spreads the material across the full width of the screen deck. Second, it stratifies the material, causing the fines to sink down against the screen, where they quickly pass through the screen openings. Third, it allows the larger particles to float to the top, where they are conveyed toward the discharge. The bouncing balls underneath the screen are deflected against bevel strips and bounce continuously against the underside of the screen mesh. This action continuously cleans the screen mesh openings. The Azo Cyclone screener consists of a horizontally mounted cylinder of nylon screen cloth on a metal frame. A short motor-driven screw conveys the feed stock into the screening cylinder, where the whirling paddles, mounted on the same shaft as the feed screw, force the material through the screen opening. The natural vibration of the screen cloth performs the cleaning action. The paddles also have a lumpbreaking action on any soft, friable lumps in the feed stock. All of the units as shown or described are totally enclosed and relatively dust-tight in operation. Some of the questions that have to be considered in choosing a unit are as follows. The first consideration is matching the flow rate to the required square footage of the screen area, so that particle size ranges of each fraction from the screener meet the necessary specifications. Another consideration is the temperature of the infeed materialdis it warm enough to be giving off moisture vapor? If so, and if the sifter is mounted in a cool area, water vapor condenses on the inside with dust particles, so that the frequency and ease of cleaning become important. Still another important consideration is possible product degradation caused by the sifter. Sometimes the rubbing action of the product against the screen causes attrition or product breakdown. In some cases where this occurs, the product itself may have to be altered to withstand a sifting step. As was noted in the section on lump breaking, there are occasions in sifting wet or sticky materials when large volumes of air drawn through the equipment greatly aid sifter performance. Furthermore, the feeding of stock to the sifters must be uniform, so that screen areas do not become overloaded. Overloaded screen surfaces fail to make the desired separations; fines cannot pass through an overloaded screen and are carried over with the overs stock. The importance of proper lump breaking and sizing cannot be overstressed. The size of the finished product and its ultimate package weight may be a function of
190
Breakfast Cereals and How They Are Made
how well the lump breaking and sizing were carried out earlier in the process. This is particularly true of some flaked cereals, some granolas, and several hot cereal products, particularly instant mix-in-the-bowl hot cereals.
Reference Feldman, H., 1987. Selecting a lump breaker for gross size reduction. Powder Bulk Eng. 1 (6), 26.