Feed mill design tips
Jan. 8, 2013
Editor’s note: This is the first in a three-part series on feed mill design. Part 2 will focus on the design of the batching and mixing systems and will appear in the January issue of World Grain.
Whether planning to build a new feed processing facility or remodeling or upgrading an existing facility, many things must be considered. What is the purpose of the project? What type of end product(s) is to be delivered from the plant? What is required to make this product(s), and how much of each product is required per shift, each day, week or annually? What equipment is required, and what type of construction is required? What design parameters are needed to minimize production and utility costs?
We will examine these issues and others in these articles.
What is the need for the facility or upgrade? Is it to expand production of existing products, add new ones or serve a new market or sales region? Regardless of the purpose, determining the anticipated production and product mix is the first step.
Before determining required production rates, the product mix must be reviewed. This includes not only the types and amounts of products to be made, but what ingredients are required and what type of processing is required for raw ingredients and for final product production. A commercial feed mill may have an initial product mix similar to the following:
The next step is to identify and review all formulations needed to produce the products required. This includes large quantity ingredients such as grain or soybean meal to the smallest hand-added ingredients.
Once all the ingredients and amount of ingredients are known for all the individual formulas, the quantity of each ingredient required to meet the production requirements is determined. These ingredients are procured in sufficient quantities to support the production schedules. Depending on the amount of an ingredient required, it may be received in bulk, tote or bagged quantities.
The facility must be designed to handle and store each of the ingredients. Storage requirements will be discussed later.
It is necessary to determine each of the steps required for making each type of product. These steps will point out the equipment needed to accomplish each step. Costs, including labor, may be determined for each of these steps to determine the manufacturing cost for that step.
Thus, each step may be called a “cost center.” A group of cost centers must be used to make each product type, but not all are used for each product. An example of the cost centers required for making each of the types of feed in the preceding product mix is shown in Table 1 (page 64).
Production capacity needs to be based on initial requirements but should also be estimated for future increases or ultimate production goals. Usually the production rate is based on tonnes required and the desired time to produce them resulting in an average “tonnes per hour” capacity. Table 2 (page 66) shows a typical way to determine immediate and future production rates. The initial capacity is 240 tonnes in an eight-hour day.
The highest required rate is 48 tonnes per hour (tph), but this is an average. To allow for interruptions in production, an efficiency factor not exceeding 80% is recommended. Thus, the actual design capacity should be 48 tph/0.80, which calculates to 60 tph.
Although the initial design capacity is only 37.5 tph, the mill should be designed to accommodate the ultimate design capacity of 60 tph. The controlling factor in the design is usually the batching and mixing system. Other cost center equipment may be installed in increments such as grinding or pelleting systems.
The system for receiving bulk ingredients should have a minimum capacity of twice the production rate to allow for time variations in truck and rail car arrivals and unloading. The receiving system should have the smallest pit possible that is just steep enough to move the ingredients from the receiving grate opening to the receiving conveyor. Using this design, the bulk material fills the pit and backs up to the outlet opening on the truck trailer or rail car causing a flood flow condition. Since the only free falling material occurs at the initial opening of the vehicle outlet gate, fugitive dust is very minor and a dust control system is usually not needed.
Not requiring a dust control system on a bulk receiving pit is a substantial cost savings in installation and operation.
The bulk receiving system for the mill we sized above should initially be 30 tph x 2 = 60 tph, but must be able to handle up to 90 tph at maximum capacity. The cost to install a 90-tph system initially may be comparatively small versus replacing the original system or installing a second system later.
The bulk receiving system may require higher capacity if hours of operation are less than the hours of production for the facility, or ingredient receiving is done in large quantity over a much shorter time period. An example would be if a truckload of the ingredient is used daily, but that ingredient is received only one day a week. Much higher capacity may also be required if grain or another ingredient is received in unit trains with unloading time limitations. A 110-car grain train requiring unloading in 15 hours or less would need a receiving capacity of approximately 40,000 bushels per hour.
For ingredients received by bag or bulk tote, adequate warehouse space must be provided to store the ingredients until they are used. This requires human labor as well as mechanical handling systems such as forklift trucks or bag belt systems.
Ingredient sizing is accomplished by using a roller mill or a hammermill. A roller mill is one choice for grinding grains to smaller particle sizes. The coarseness and particle size range is determined by the number of roll pairs the product is passed through. For coarse grain grinding, a single pair of rolls may be used, but for finer particles a stand with two or even three pairs of rolls may be used. When using a roller mill to grind to smaller particle sizes, the particle size range will be narrow. A roller mill uses significantly less horsepower than a hammermill of the same capacity but will not work on soft or fibrous materials.
A hammermill may be used to grind ingredients to very small particle sizes, but the particle size range is wider and creates significantly more very fine particles than a roller mill. A hammermill may be used to grind most ingredients that require sizing including grains, pellets, soft ingredients and fibrous materials. When using a hammermill, an air assist system may be installed to pull air through the mill inlet and out through the screen. This system increases the hammermill capacity by 10%-15% using the same size mill drive and helping reduce the amount of unnecessary fine particles by helping get the screen opening size particles out of the mill sooner. The horsepower required for the added air assist system is small and much less than the power required for the same increase in capacity of a mill without an air assist system.
When designing a grinding system, the amount of ground ingredients required for each formulation must be determined. Generally, ingredients requiring grinding do not exceed 60%-65% of the batch weight. Based on this, the grinding system should be at least 70% of the design capacity for the facility. Significant energy savings may be gained by using the coarsest possible particle size. If the particle size desired is 600 microns, it is foolish to grind the material to a much finer size such as 400 microns.
In some cases, it may be appropriate to use larger capacity grinding equipment so that the system does not operate as many hours as the production line. These decisions are based on having sufficient ground products available when needed. In some cases, two grinding lines are installed, the first to support the initial capacity requirements with a second system added for the final capacity requirements.
If steam rolling of grains is required, a separate system must be used to do this. This system needs to include a boiler, steam chest, flaking roll, cooler/dryer and gentle handling and incorporation of the flaked grains into a blended product with the other ingredients. Rolled or flaked grains are major ingredients in textured feeds and are usually blended with other ingredients in a continuous system with high amounts of liquid molasses added just prior to bagging or loadout.