Design Considerations for the Construction and Operation of Flour Milling Facilities
08 May 201420 min reading
Gregory D. Williams
President- Facility Engineering Services
Kurt A. Rosentrater
Assistant Professor - Iowa State University
Flour milling facilities have been the cornerstone of agricultural processing for centuries. Like most agri-industrial production facilities, flour milling facilities have a number of unique design requirements.
Design information, to date, has been limited. In an effort to summarize state of the art design procedures for flour milling facilities, an overview of accepted standards and procedures has been assembled and discussed. With this article engineers should become more familiar with specific design considerations for flour milling production facilities and develop appropriate references to expand their knowledge base. Educators may find this paper useful too.
Flour milling is as old as human history. Ancient farmers used saddle stones or querns to grind their grain into flour. In the middle ages, gristmills were developed that could grind larger amounts of grain into flour. These original mills were powered by wind, water, animals or even humans. Historically, each town had their own mill, and the miller would operate for a portion of the finished flour. This was the hub of each community, and remained that way for centuries. The development of the roller mill in the 1870’s started the growth of the modern flour mill and the consolidation of the flour milling industry.
Over time, consolidation has resulted in more technically complex facilities. Today’s flour mill integrates the building facility with the process into a comprehensive, efficient and highly automated structure. Because of this complexity, engineers must have an understanding of multiple technical areas in order to develop and operate functional projects. The purpose of this article is to discuss modern flour mill design and construction methods. This paper will discuss planning, life-safety considerations, food compliance regulations, and building design and construction.
A. OVERVIEW OF A FLOUR MILLING FACILITY
The modern flour milling process was developed in the late 1800’s with the advent of the modern flour mill. Over the decades improvements have been made to various aspects of the flour milling process, but the major elements of the process have stayed essentially the same since the inception of the roller mill. Others have described the complete details of the flour milling process (Owens 2000 and Posner & Hibbs 2004). Figure 1 shows the main building elements of a typical flour milling process, and Figure 2 shows the layout for a typical flour mill. Figure 3 shows a section of a typical flour mill with some of its major elements. The following paragraphs describe a general overview of the flour milling process.
The flour milling process starts with the receiving and storage of whole grain. Grain is stored in groups of steel or concrete silos laid out very similar to grain elevators. The grain is then moved through the cleaning system which is housed in the cleaning tower. A variety of cleaning equipment is housed in the multi-story cleaning tower, and cleaning is done with machinery using air currents, magnets and screens to separate the wheat from stones, sticks, other grains and undesirable elements. Equipment typically consists of separators, destoners, magnets, aspirators and other cleaning machinery. After cleaning, the grain is moved to temporary storage silos (known as clean storage) prior to tempering.
Immediately prior to milling the wheat is moved to tempering bins at one end of the milling tower where the clean wheat (or other grain) moisture content is adjusted to approximately 16%. The milling process is ready to begin. The flour milling process consists of the break system, purification or sizing system, the reduction system, and the tailing system. The mill tower usually consists of 4 to 7 levels with integral bins at each end of the tower. At one end are the tempering bins, and at the other end are the finished flour bins. Between the bin clusters are many levels housing the various systems that complete the milling process. The tower is usually constructed from precast or slip form concrete. Sometimes smaller mills (which are part of a larger process) are constructed using only steel. The major elements of the milling process are discussed as follows:
The break system is primarily comprised of roller mills. In this system the wheels in the roller mills run in opposite directions at different speeds and have a saw tooth configuration. The purpose of the process is to separate the endosperm form the rest of the kernel. To achieve this, the wheat is run through the roller mills up to five times. As part of this process, sifters are also used to separate the endosperm from the bran and germ which is typically a co-product known as wheat feed.
The purification system consists of purifiers, roller mills and sifters. Purifiers sort particles based on size, air resistance, and specific gravity. The roller mills further reduce the size of the particles. Almost no flour is produced in this operation and the material is either passed to the reduction system or sent back to the break system. The purpose of the purification process is to separate the small bran from the endosperm. The use of purifiers is decreasing in modern mills due to cost effectiveness and efficiencies in other portions of the milling process, however.
The reduction system consists of a series of roller mills and sifters in sequence. The roller mills in this sequence are smooth, resulting in a finer grind. At this point it is desirable to have mechanical starch damage to the wheat to improve flour quality. This phase reduces the endosperm to flour. This process is repeated up to 11 times to obtain the fineness required for the flour.
The tailings system is where the unwanted co-products of the flour milling process go. The products are turned into animal or pet food. These co-products are often sent out in bulk. There are fewer tailings (or co-products) for whole grain flour, and more for white flour, because more of the kernel is used for whole grain flours.
In each of these systems the "overs" of each sieve (particles not fine enough to pass through a screen) are directed to another set of rolls (i.e. back through the process) for further reduction, or to one of the residue streams.
At the end of the process the various runs of flour are blended and mixed to make the required grades of flour. They are then treated with the addition of malted barley, bleaching agents, enrichments, etc. before packaging or shipment in bulk.
Large mills have complex flows to control gradation to be able to produce multiple grades of flour. Other smaller mills are usually part of a vertical integration of a larger manufacturing process.
The finished product bins are where the inished flour is stored prior to packaging or bulk shipment. The warehouse is where the packaging takes place. Warehousing typically consists of single story, large square footage space, with loading docks and palletizing equipment. These structures are generally constructed from steel and metal panels or from precast concrete. All construction must be sanitary and easy to clean.
Utilities make the flour mill run, and air is a key system in a flour milling facility. It is used to convey flour from system to system, and to separate streams into components. Ten times as much air is required to move a single volume of flour. The air system consists of fans, blowers, compressors, and pneumatic conveying lines. Other elements of the air system consist of cyclones, dust collectors, and filters.
B. FACILITY PLANNING
Proper planning is an important aspect of long-term profitability for a flour milling facility. It is important for engineers of these facilities to minimize these costs to increase value for owners and shareholders. A major part of effective planning is considering items such as (1) Long Range Planning, (2) Grain Supply, (3) Economic Factors, (4) Regulatory Issues, (5) Location/Site, (6) Facility Expansion versus New Construction, (7) Technology, and (8) Facility Layout. Optimal planning should lead to lower life cycle costs. These are detailed as follows:
• Long Range Planning: Long range or strategic planning is a function of the strategic vision and objectives of an organization. It generally reflects the mission of the company and how it will proceed toward achieving its business objectives.
• Grain Supply: The primary ingredient in the production of flour is wheat. Wheat production is dependent on a strong local farming community or access to a rail or highway system. To manufacture flour, wheat will have to be shipped in or grown locally.
• Economic Factors: Economic considerations for the operation of a facility can have a major impact on its profitability and viability. Local issues such as grain types and volumes produced in a particular geographic location, the availability of transportation, and the number of existing facilities in a particular geographic area will directly affect the economic success of a flour milling facility. Global economic issues, such as long-term increases in population, which in turn increase demand, can also be a consideration. Additionally, the eating habits of the demographic population or the processing capabilities of regional industries may have an influence on production demand. Finally, Return on Investment, or ROI, should be a major consideration in the decision to operate a new facility.
• Regulatory Issues: Governmental and political issues can have a significant effect on the need for a facility in a particular region. Issues such as Good Manufacturing Practices (GMP) and identity preservation can have a major effect on international demand for products.
• Location/Site: Selection of an appropriate site is an important consideration for the profitability of a flour milling facility. Locations close to applicable transportation and infrastructure are essential for facility operation. Functional rail facilities are essential for most flour milling operations. Appropriately rated roads and highways are also essential for all operations with truck traffic. When searching for a new site the owner needs to consider the bearing capacity of the soil. On a sufficient site with good bearing capacity, pressures expected under the grain storage facilities can approach 6000 psf. Pressures under the mill can exceed 4000 psf. Additional site issues can include drainage, wetlands, water and other similar items can come into play.
• Facility Expansion vs. New Construction: Once a decision has to be made to build a facility in a specific geographic region, the owner and engineer must examine if a currently existing facility in the area can be expanded or upgraded first. If the discounted cash flow of the cost of the upgrades is greater than new construction then consideration should be given construction of a new facility.
• Technology: Like all industries, the flour milling industry is an evolving industry with changing technology. Senior management and engineers must be aware of new technological trends being developed within the industry. As technologies shift, companies must be flexible and make changes when they are appropriate.
• Facility Layout: Facility layout and design is a key consideration in the operation of a functional facility. The relative location between physical locations of the receiving and load out, or the possibility for double duty, can play a role in the operational costs of running the facility. The type of construction and the amount of available land can have major role in the physical layout of the facility. For example, a loop track for an 110-car rail shuttle loading system can require a vast amount of land. Issues such as explosion or fire safety can influence the physical layout of a facility as well. Finally, the budget that the facility owner has available can dictate the types and sizes of building construction and equipment that can be pursued, and can place severe restraints on the design of the facility. Additionally, the layout of individual pieces of equipment can have an effect on the total labor and power requirements of the facility.
C. LIFE SAFETY DESIGN AND REGULATORY DESIGN CONSIDERATIONS
Once the layout has been determined by the owner and engineers, the facility detail design can begin. One of the first steps is application of the life safety codes. They are administered at state, local, and federal levels. Life Safety codes provide standards for the design of fire rated construction, occupancy, use, and egress of a plant facility. Additionally, there are code sections for the design of elements such stairs, working walking surfaces, and other detail elements of a facility. Flour milling facilities produce extensive amounts of explosive dusts. The designer must take the necessary steps during design to minimize the potential for deflagration. The National Fire Protection Association produces several documents that provide design criteria for limiting the effects of dust explosions (NFPA, 2013a&b, and NFPA 2014)
Other regulatory considerations for food processing facilities such as a flour mill include government regulations (NARA 2007) related to the production of food. Many governmental agencies have specific requirements for sanitary construction of buildings and equipment. A good source of design methods for sanitary design is Imholt and Imholt (1999). Additionally, during the design process it is necessary for the designer to occupational regulations for workplace safety. These items can include working surfaces, fall protection systems, and general worker safety.
Fire Protection Systems
The use of active fire protection systems in industrial facilities such as flour milling facilities can be used to increase allowable floor area or height as described in the various sections of the IBC. Type I construction typically does not require sprinklers for large areas. Other times, fire sprinkler systems are required for unlimited area single story buildings.. The need for fire sprinklers is highly dependent on the type of fire rated construction. Individual insurance underwriters often have particular requirements for sprinklers. Design of fire protection systems is discussed in NFPA 13 (NFPA, 2013c) and the International Building Code (ICC, 2012 a & b).
D. BUILDING DESIGN CONSIDERATIONS
Once the engineering team has determined the process layout, the functional process areas, and the functional building layout, the building system detailing can begin. In this section descriptions of the loads for and design of the major elements of flour milling facilities will be discussed. As part of this discussion, major design standards will be referenced and key aspects will be highlighted. This is not intended to be an all-inclusive discussion of all elements involved in the design process, but rather to highlight unique or significant aspects of the design process for flour milling facilities.
Loads on flour milling facilities arise from a variety of sources, including wheat and its components, flour, roof and floor live loads, equipment (including dynamic loads), dead loads, and lateral loads such as wind and seismic forces. Flour milling facilities store large quantities of raw grain and finished flour products in both bulk and bag form. Material handling characteristics and flow properties for common ingredients in the flour milling process are provided in Table 1. Before an actual structural analysis and design is undertaken, the loads on the total system must be determined. Items such as structural tower weights, equipment weights, snow, and floor and roof live loads must be determined. This information can be gleaned from equipment vendors and from the building code documents such as ASCE 7-10 “Minimum Design Loads for Buildings and Other Structures” (ASCE, 2010).
Grain Storage, Reclaim and Distribution Design
Whole grain storage is a major component of a flour milling facility. Storage may be of either concrete or steel construction. Typical concrete silo diameters vary between 25 to 35 feet or more with heights between 100 to 120 feet. Corrugated steel bins can also be used as whole grain storage. Corrugated steel bins typically will be larger in diameter and squatter. Blending and grading capabilities are desirable in a bin layout. Whole grain silos should be designed so that that some level of sanitation can be maintained. For example, beams should be dust shedded and attention should be paid to access for sanitation and clean out. In a flour milling facility, grain storage is often broken into two components: pre-clean and clean silos. Wall construction should be free of pits, pockets and other finish defects to inhibit insect infestation. For concrete silos this means that the inside of the bin should be trowel finished. Steel silos should avoid ledges.
Wheat cleaning towers are tall vertical structures that house cleaning equipment. They are usually constructed from concrete or steel. Height is important because they take advantage of gravity flow between the various cleaning unit operations. Cleaning towers are usually rectangular in shape and consist of 4 to 6 levels. Due to their height, most cleaning towers are constructed using slip form concrete methods, although it is possible to build a steel tower utilizing stick-built steel systems.
Tempering bins are usually somewhat smaller concrete or steel bins that are used for storing whole grains in the clean and pre-clean bins. The shape of these silos can vary depending on the size and shape of the structure that contains them. For example, if the tempering bins are contained inside a slip formed mill structure, they are integrally cast in the tower and are typically rectangular in shape. If the bins are contained inside a steel mill, then it is likely that they will be of round steel construction and separately installed in the mill. In other scenarios, slip formed concrete silos could be considered.
Finished Flour Bins
Similar to the tempering bins, the finished flour bins are smaller sanitary bins of either square or round shape (figure 4). Slip formed bins tend to be square or rectangular and steel bins are typically round. Shapes can vary however. These bins hold the finished flour, which is typically pneumatically conveyed into the bin. These bins can be either steel or concrete construction, and must be mass flow in order to properly function. Because of the final nature of the finished product, these bins must be sanitary in construction. Finishes must be smooth, free of pits, ledges and other surface defects. Welds must not only be structurally strong, but must also seal all joints and connections. With pneumatic discharge and filling it is possible that the bin could develop internal pressures. Concrete bins must be designed according to ACI 313-97 “Standard Practice for the Design and Construction of Concrete Silos and Stacking Tubes for Storing Granular Materials” (ACI, 1997) and steel bins must be designed according to API 620 “Recommended Rules for the Design and Construction of Large, Welded, Low-Pressure, Storage Tanks.” (API, 2013).
- Construction Methods
Both concrete and steel construction are used for the construction of the mill portion of a flour milling facility. Typically, large mills are constructed using precast and slipform concrete, or just slipform alone. Slipform construction consists of cast in place concrete walls that are extruded continuously over the entire height of the structure. Floors can be constructed using both precast and steel beams. Precast beams can be set during the slip form and steel beams are attached to inserts that are embedded during the slip form (figure 5). Floors can be topped precast or cast in place concrete which is set after the slip form is complete.
At times the mill processing floors are constructed between the mill tempering and final product bins using precast elements. When this occurs, beams are attached to slip wall inserts and supported by intermediate columns. Floor and roof elements are added to support the structure. Precast beams can consist of inverted tee, spandrel, and rectangular beams depending on the floor elements, which are usually hollow core plank with a bonded structural concrete topping.
All the elements are held up by precast columns. Double tees are usually used as structural elements for the roof, and the structure is enclosed by insulated precast wall panels which can be either a double tee or flat plate style construction. Built up roofing is then applied over the roof system. For design of precast elements the reader is referred to the PCI Design Handbook (2014).
Smaller, or mini mill, frameworks are usually built using steel construction. Some of the smallest mills are skid-mounted and incorporated into a larger configuration. Other mills are constructed using piecemeal or stick-type construction to form the structure. Typically, a structural steel configuration consists of closed tube columns and wide flange beams with bar joists. The walls are enclosed with insulated metal panels, although precast concrete can be used as the enclosure. When a bar joist ceiling is used, an insulated metal panel drop ceiling needs to be installed. Sanitary construction must be followed and could include curbs, enclosed shapes, and shedded beams. See the section on sanitary construction for further commentary.
Another design concern is floor vibrations and dynamic forces from the processing equipment. Care must be taken during the design of the floors for vibration. As a first step, the operating natural frequency of the equipment should be compared to the natural frequency of the floor. More than one natural frequency is possible, and so several modes will have to be investigated for each floor. When the natural frequency of the equipment and the floor match it is possible to have resonance. Dynamic forces will increase loading on supporting elements. An extensive discussion of vibrations and dynamics occurs in Clough and Penzien (1975).
- Slip Wall and Bin Construction
Storage of tempered grains, flour, and co-products are accomplished using bins built integral to the mill structure (figure 4). Almost all mill towers are rectangular. Concrete mill structures are fairly large and the walls typically need stiffening using vertical wall pilasters (figure 6) that extend the entire height of the structure. Bins are constructed integrally with the outer mill shell, and extend about the upper portion of the tower.
Packaging and Warehouse Construction
Once the wheat is milled and turned into flour it is stored in bulk and either shipped or packaged. In this section we will look at preferred construction methods for food grade warehouse construction. Food grade packaging and storage warehousing can be constructed from precast, tilt-up, or steel construction. Each has particular features that are discussed in the following paragraphs.
When structural steel construction is used, steel frames are often constructed using closed shaped tubes as columns for sanitation. Primary beams are constructed from wide flange shapes and the secondary roof framing is of bar joist construction. The walls of these types of facilities are typically non-load bearing precast and tilt-up or alternatively, insulated metal panels. Standard metal building panels could be used, but insulated metal panels are preferred over standard metal building metal panels because of their greater cleanliness. For further sanitation, a USA space or suspended ceiling should be added to enclose any mechanical piping in the facility.
A precast warehouse usually consists of precast beams and columns with nonload bearing precast wall panels. Column and beam lines form the interior frame work.
Inverted tee or ledger beams run over the columns and precast double tees are used for the roof construction. They typically are used to span from beam to beam. Flat or double tee wall panels are used to enclose the wall (figure 7). Occasionally, wall panels can be load bearing. Curbing is added at the base of the wall where it attaches to the floor slab. Additionally, any ledger beam shelved should be filled with grout to form a slope to prevent dust and other materials from accumulating.
This article summarizes design procedures related to the construction, planning, and operation of flour milling facilities. In particular, the life safety, layout, planning, and structural provisions were discussed. Toward that end, standards, procedures, and methods of design and construction were discussed. Both engineers as well as educators should find this paper useful. Please feel free to contact the authors for further information.
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