By Jason Parpart and Steve Becker, Schenck AccuRate
Specifying a hygienic bulk solids feeder is not a simple task. The wide variety of requirementsneeded for specific applications is similar in scope to explosion-proof environmental requirement variations. However, unlike explosion-proof requirements, a classification system does not exist to easily identify requirements for hygienic applications. The requirements from each producer vary as widely as the products they make. The material being fed, operating environment, cleaning and sanitization requirements, regulatory requirements, food producer requirements and perception, and cost are all significant factors in the design of a feeding solution. Therefore, in order to find a solution that is appropriate for a particular application, it is necessary for feeder manufacturers to offer feeders that cover a variety of hygiene levels.
General design guidelines follow.
Product-contact surfaces, as defined in 3-A Standard 81-00, are “Those surfaces which are exposed to the product directly and surfaces from which liquids or materials may drain, drop, diffuse, or be drawn into the product.” In most cases, stainless steel is the preferred metal for use in product contact. Its strength, resistance to corrosion, and relatively low cost make it an excellent choice in most applications. Typically, AISI 304 (1.4301 or 1.4303) stainless steel is used for metallic product contact surfaces. It is the most common stainless-steel alloy used and is one of the least expensive. In situations requiring higher resistance to acids, particularly at elevated temperatures, AISI 316 or 316L (1.4401 or 1.4404, respectively) can be used.
If it is necessary to use a plastic product-contact material in a hygienic application, a manufacturer should choose one with FDA acceptance. This ensures that the material is inert, nontoxic, and will not add any objectionable flavor or odor to the product. Typical plastics used in hygienic feeder design are polyethylene, Ertalyte (PET-P), and particular grades of nylon. In addition to their FDA acceptance, these plastics are popular choices due to their resistance to a variety of chemicals, ease of machining, and aesthetic appeal of their natural colors.
Similar to plastics, rubber or rubber-like materials should also have FDA acceptance when used in product-contact applications. Typical rubber or rubber-like materials used in hygienic feeder design are silicone, EPDM, and various urethane compounds.
|Figure 1: Surface roughness
(click image to enlarge)
Regardless of what materials are used in product contact, the surface should be smooth with no crevices or pits. If a feeder is to meet certain regulatory standards, specific surface finishes may be required. For example, the 3-A Sanitary Standard for Auger-Type Feeders, Number 81-00 dictates that all product contact surfaces must have a maximum surface roughness of Ra 32 μin. (0.80 μm) and be “…free of imperfections such as pits, folds, and crevices in the final fabricated form.” Surface roughness is defined as the total area of the peaks and valleys divided by the evaluation length of a section of material (Figure 1).
Non-product-contact surfaces are defined as any exposed surface that does not qualify as a product contact surface. As in product contact, stainless steel is also a popular non-product-contact metal. This is especially true in situations where corrosion resistance for the entire feeder is necessary, such as in washdown areas. In most cases, AISI 304 (1.4301 or 1.4303) will be used since the increased corrosion resistance of other grades is rarely needed outside the product stream.
Outside the product stream, metals such as aluminum or mild steel are widely used. However, in a hygienic application, it is important to protect these metals from corrosion. Preferred methods for achieving this are powder coating and plating with corrosion resistant metals such as zinc or nickel.
Plastics, rubber, and rubber-like materials are also used extensively as non-product-contact materials. Since they are outside the product stream, they do not require FDA acceptance. Even so, FDA-accepted materials should be considered if there is potential for them to enter the product stream during normal operation or cleaning. For all non-product-contact materials, care should be taken to ensure these materials will not be damaged by cleaning/sanitization solutions, heat and pressure of cleaning, or the operating environment.
Figure 2: 3-A approved feeder flexible hopper and feed screw
When manufacturing parts for use in a hygienic feeder, care must be taken to prevent crack and crevice formation, where material can accumulate. If a material is allowed to sit for too long, it may spoil and lead to contamination of other material. Lingering material can also foster excessive bacteria growth. Therefore, whenever possible, a onepiece design with smooth surfaces and large corner radii to ease cleaning is preferred (Figure 2).
When a single-piece design is not feasible, welding is the preferred joining method, as a weld can be ground and polished to the point where it is difficult to distinguish between the welded area and the base material.
|Figure 3: Industrial shaft seals (left) and 3-A-approved shaft seals (right)|
Gaskets and seals are commonly used to seal the interface between mating parts. For example, gaskets are commonly used in the interface between hoppers and covers to provide a water-tight or air-tight seal, preventing contaminants from entering the product stream while allowing disassembly for cleaning or inspection. Seals are similarly used in situations where a shaft must pass from a non-product-contact area to a product-contact area for devices such as feed screws or internal agitation devices (Figure 3).
Devices can also be implemented in a hygienic design to prevent unwanted substances from dripping onto the feeder and coming into contact with the material being fed. Drip rings are commonly used on the feeder discharge to isolate the material leaving the feeder from the exterior of the feeder (Figure 4). Any material on the exterior of the feeder that travels down the discharge nozzle cannot contact the material discharging from the feeder since it will drip off the end of the drip ring first.
Figure 4: 3-A-approved nozzle with drip ring
When designing a hygienic feeder, it is always desirable to make disassembly of the feeder as simple as possible. By eliminating the need for complete disassembly and providing the ability to disassemble the feeder from the non-process side, maintenance personnel will not be required to move the feeder or remove large components such as extension hoppers for cleaning purposes (Figure 5). Minimizing the amount of time necessary to clean the feeder can significantly reduce costs. Consider a feeder that can have the critical components disassembled, cleaned, and reassembled in 15 minutes compared to one that requires 30 minutes to complete the process. If the feeders are cleaned once per day, in a 10-year life span, 650 hours of cleaning will be saved by using the first feeder. Also, consider the time and money that can be saved when the maintenance personnel require lesssophisticated training to clean a feeder with simpler, more-intuitive disassembly.
|Figure 5: Non-process side feeder disassembly|
In order to reduce the potential for material build-up, it is important to minimize flat horizontal surfaces on the feeder. These surfaces will hold material and can allow pools to form from unwanted substances dripping onto the feeder or from feeder washdown. The pools can lead to excessive bacteria and/or fungus growth and potentially cause material contamination. Components with large, flat, horizontal surfaces, such as covers, can be replaced with components designed with watershed angles to facilitate draining, preventing these problems (Figure 6).
Low-Level Design Requirements—Dry
Figure 6: Domed hopper cover
The lowest level of hygienic feeder design is intended for dry, nonperishable food products and additives. Typical applications include basic food ingredients such as flour or sugar, which will be baked or cooked. This feeder would be placed in a dry environment, and the feeder would not be washed down for cleaning. Also, cleanout of the feed hopper would generally not be required between production runs.
As stated in the general design guidelines, the product contact materials of the lowlevel feeder are all FDA approved or FDA accepted. Product contact areas are manufactured to a smooth finish, and welds are cleaned. Motors typically meet an environmental rating of IP55 (or better) to prevent dust from entering the motor and for the motors to endure occasional wet cleaning.
Mid-Level Design Requirements—Washdown
The middle level of hygienic feeder design is intended for dry materials in a wet or wet clean-up area. Some typical applications include dried fruits, nuts, and spices, as well as applications similar to those of the lowlevel feeder. This feeder would be placed in a wet or dry environment, and cleaning of the feeder would typically be done via water washdown. The feed hopper could potentially require cleanout between production runs.
Product-contact materials should be FDA approved or accepted, and surface finish is generally the same as the dry feeder’s. In addition to the basic requirements of the dry feeder, exterior cracks and crevices are sealed with either silicone sealant or epoxy, both of which are FDA approved. Washdownduty motors and bearings are also used in construction of the mid-level feeder.
High-Level Design Requirements—Regulated
|Figure 7: 3-A-approved screw feeder|
The highest level of hygienic feeder design is intended for use in specific food applications requiring acceptance via requirements such as 3-A Sanitary, EHEDG, or USDA Meat and Poultry (Figure 7). Applications for such a feeder are typically perishable food products such as milk powder and cellulose. These products can be wet or in a wet environment. Clean-up is typically quite extensive and includes intense physical scrubbing and the use of chemical cleaning and sanitizing agents. These chemicals can be acidic, alkaline, or caustic.
At the very least, product contact materials used for this type of feeder must be FDA approved. Often there will be further testing a material must undergo to achieve this type of approval, such as the lactic-acid test for 3-A approval. Typically, surface finish is required to be a certain surface roughness or smoother, such as the 3-A Sanitary requirements mentioned earlier. Exterior cracks and crevices must also be eliminated, preferably by designing components without them, but they can be sealed if this is not possible. Special sealing requirements may be necessary, such as the use of sanitary tube fittings for
discharge nozzles and auxiliary device connections. Since the regulated feeder will undergo frequent cleaning, it also features washdown-duty motors and bearings.
Due to the perishable nature of the material being fed by this type of feeder, care must be taken to prevent material contamination and bacteria growth. Therefore, most of the design techniques mentioned in the manufacturing-methods section will be utilized.
When designing or specifying a hygienic feeder or feeding system, it is important to understand the perishability of the material fed, the cleaning processes used, and any applicable sanitary regulations. Equipment designed to the appropriate requirements and operated accordingly can reduce overall costs while improving the hygienic nature of the process.
Jason Parpart, ME is a mechanical design engineer with Schenck AccuRate and recently took the newly created position of CAD administrator with the company. He has a BS in Mechanical Engineering from Milwaukee School of Engineering.
Steve Becker is a mechanical engineering manager with Schenck AccuRate and was recently promoted to director of heavy-industry sales. He has an Engineering degree from the University of Illinois and an MBA from Lewis University.
Schenck AccuRate is an ISO 9001–certified manufacturer of volumetric and gravimetric bulk solids feeders, vibratory feeders, weigh feeders, solids flow meters, bulk bag frames, and control systems with more than 30,000 installations encompassing more than 9000 unique materials worldwide. For more information, call 800-606-9248, or visit www.accuratefeeders.com.
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Imholte, T.J. Engineering for Food Safety and Sanitation. Technical Institute of Food Safety, Crystal, MN, 1984.