As appeared in January 2012 PBE

Copyright CSC Publishing

www.powderbulk.com

Special Section: Sanitary Choosing a hygienic feeder for your bulk solid material Jason Parpart and Steve Becker

Schenck AccuRate

When you select a bulk solids feeder for a sanitary application, such as processing food ingredients, the feeder must meet complex and varied hygiene requirements. When you combine these hygiene requirements with your application requirements, the number of factors impacting your feeder choice can become a little daunting. After discussing fabrication materials and manufacturing methods for hygienic feeders, this article describes a classification system identifying low-, mid-, and high-level hygienic feeder applications according to process variables and explains how feeder hygiene requirements vary at these levels.

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pecifying a hygienic bulk solids feeder isn’t a simple task. The number and variety of requirements that the feeder must meet is similar in scope to the explosion-proof environmental requirements that equipment in combustible applications must meet. Unlike with explosion-proof requirements, however, there’s no classification system to help you easily identify the requirements for your hygienic application or a bulk solids feeder that will meet them. In addition to hygiene requirements, a hygienic bulk solids feeder must also meet application-specific requirements related to the material being fed, the operating environment, cleaning and sanitizing requirements, cost, and more. Where do all these requirements come from? The US Food and Drug Administration (FDA) identifies approved

materials for hygienic feeder construction, and the US Department of Agriculture (USDA) and 3A Sanitary Standards Inc. establish the requirements regulating hygienic bulk feeders to help prevent contamination during processing and handling.1 These requirements, which address both the feeder’s design and construction materials, vary widely from one application to the next. As a result, it isn’t feasible for feeder manufacturers to design bulk solids feeders that exactly match every application’s specific hygiene requirements. Instead, they’ve designed multiple hygienic feeder models that meet different hygiene requirement levels based on whether the application can be considered dry (low-level), washdown (mid-level), or regulated (high-level). However, some basic criteria governing hygienic feeder design, including fabrication materials and construction methods, are common to all hygienic feeders. Understanding these criteria is critical when you’re trying to select the optimal feeder for your application. This article will help you better understand the broad spectrum of hygiene requirements and how they relate to optimal feeder design and also give you some ideas on how to select the best feeder for your application. Hygienic feeder design criteria Easy disassembly. Every good hygienic feeder starts with a good design — and certain design elements are desirable, no matter what the application’s hygiene requirement level. One of the first considerations is ease of disassembly. The best-designed feeders allow you to perform routine cleaning and maintenance chores without completely disassembling the feeder or disconnecting it

from the process stream on either the upstream or downstream side. Ideally, your maintenance personnel should be able to perform routine cleaning or repairs without moving the feeder or removing large components such as extension hoppers, as shown in Figure 1a. Sound feeder design can eliminate significant expense over time. For example, if you have a feeder that requires daily cleanout and the feeder’s design enables you to disassemble, clean, and reassemble the feeder in 15 minutes rather than 30, you can save 650 cleaning hours during the feeder’s 10-year life span. What’s more, you can save considerable time and money because your maintenance personnel won’t need as much training to learn to service a feeder with simpler, more intuitive disassembly. No flat horizontal surfaces. Another basic design consideration for hygienic feeders is to eliminate or minimize flat horizontal surfaces. Such surfaces typically trap 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 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, such as the domed hopper cover shown in Figure 1b, to facilitate draining and prevent these problems. Hygienic product-contact surface materials. In any hygienic bulk solids feeder, the construction material requirements for surfaces that will contact the process material (product) are more stringent than those for surfaces that don’t. As defined in 3A 81-00: Sanitary Standards for Auger-Type Feeders, product-contact surfaces 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.” All fabrication materials used in product-contact areas must be FDA-approved or -accepted. In most cases, feeder manufacturers prefer to use stainless steel for these surfaces because of the metal’s strength, corrosion resistance, and relatively low cost. The most common — and one of the least expensive — stainless steel alloys used is AISI 304 (1.4301 or 1.4303).2 In situations requiring higher acid resistance, particularly at elevated temperatures, manufacturers use AISI 316 (1.4401) or 316L (1.4404). In some applications, particularly for feeders that must handle corrosive or caustic chemicals, product-contact surfaces may be constructed from plastic or rubber. FDA-approved or -accepted plastics are inert and nontoxic and won’t add objectionable flavors or odors to the product. Typical plastics used in hygienic feeders are polyethylene, Ertalyte (polyethylene terephthalate [PET-P]), and

particular nylon grades. Feeder manufacturers prefer these plastics because they’re resistant to many chemicals, easy to machine, and come in aesthetically appealing natural colors. For applications that call for rubber or rubber-like materials, silicone, ethylene propylene diene monomer (EPDM), and various urethane compounds are commonly used. Whether the feeder’s product-contact surfaces are made of stainless steel, plastic, or rubber, they must be smooth and free from crevices or pits. In some applications, the requirements establish a specific standard for surface finishes. For example, 3A 81-00 dictates that all product-contact surfaces must have a maximum surface roughness of Ra 32 microinch (0.80 micron)3 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 surface material, as illustrated in Figure 2. Suitable non-product-contact surface materials. Any exposed surface in a feeder that doesn’t meet the product-contact surface definition is considered a non-product-contact surface. (Product-contact surfaces are considered “inside the product stream”; non-product-contact surfaces are considered “outside the product stream.”) Stainless steel is also a popular choice for non-product-contact applications. This is especially true in situations where the entire feeder must be corrosion-resistant, such as in washdown areas. In most cases, feeder manufacturers will use AISI 304 (1.4301 or 1.4303) steel alloys since the increased corrosion resistance of other grades is rarely needed outside the product stream. Metals such as aluminum or mild steel are also commonly used for non-product-contact surfaces. However, in a hygienic application, it’s important to protect these metals from corrosion. In this case, manufacturers powder-coat or plate the surfaces 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’re being used outside the process stream, these materials don’t require FDA acceptance. Even so, you might want to consider using a feeder with FDA-accepted materials on these surfaces if there’s potential for dust or other materials from the non-product-contact surfaces to enter the product stream during normal operation or cleaning. Finally, it’s important when specifying a hygienic feeder to choose materials for the non-product-contact surfaces that won’t be damaged by cleaning or sanitizing solutions, cleaning heat and pressure, or the operating environment.

Figure 1 Features required or recommended on hygienic feeders a. Easy disassembly

b. Angles to facilitate draining

c. Smooth surfaces and rounded corners

d. Hygienic seals

e. Drip ring

Hygienic manufacturing methods. When fabricating parts for a hygienic feeder, manufacturers generally avoid introducing cracks, crevices, or other features in which process materials can accumulate. Food ingredients that catch in such areas can spoil and contaminate other material or foster excessive bacteria growth. To prevent these problems and make cleaning easier, manufacturers use one-piece designs with smooth surfaces and large, rounded corners wherever possible, as shown in Figure 1c. When a one-piece design isn’t feasible, feeder manufacturers typically join surfaces with a weld, which can be readily ground and polished so that it’s difficult to distinguish between the welded area and the base material. Gaskets and seals are commonly used to close off the interface between mating parts. For example, a hygienic gasket between a hopper and its cover is commonly used to create a water-tight or air-tight barrier that prevents contaminants from entering the product stream, yet still allows for disassembly to clean or inspect the feeder. Hygienic seals, as shown in Figure 1d, are used to create a barrier in the interface between moving and nonmoving parts. For example, when a feeder part — such as a feed screw — must pass from a non-product-contact area into a product-contact area, a hygienic seal is used to create a barrier that prevents contamination from the non-contact-product area.

facturers typically design their feeders to fit applications at the low, middle, or high end of the hygiene requirements spectrum. Low-level (dry) design requirements. The lowest level of hygienic feeder design requirements covers applications that process dry, nonperishable food ingredients and additives; operate in a dry environment; and don’t require wet cleaning or hopper cleanout between production runs. In the feeder’s product-contact areas, manufacturers must use materials that are all FDA-approved or -accepted. In addition, they must manufacture all product-contact surfaces to a smooth finish and clean the welds. And the feeder motor must typically meet an environmental rating of IP554 (or better) — that is, it must have water- and dust-tight seals to prevent moisture and dust from entering the motor. Mid-level (washdown) design requirements. Mid-level hygienic feeder design is intended for applications in which dry materials are processed in a wet operating environment or that require wet cleanup or hopper cleanout between production runs. Some typical applications include processing dried fruits, nuts, and spices. Feeders for midlevel applications must meet the same requirements for product-contact materials and surface finishes as lowlevel feeders. Additionally, any exterior cracks and crevices must be sealed with FDA-approved silicone sealant or epoxy. The feeders also require washdown-duty motors and bearings.

Manufacturers can also add devices to prevent unwanted substances from dripping onto the feeder and coming into contact with the material being fed. For example, they commonly install drip rings on the feeder discharge to prevent product-stream contamination; any material on the feeder’s exterior that travels down the discharge nozzle will collect on the drip ring and fall off before it reaches the material discharging from the feeder, as shown in Figure 1e. Meeting design requirements Although there isn’t a formal system that classifies different hygienic feeder applications into clearly defined hygiene requirement categories, from a practical standpoint these applications can be divided into three levels. Manu-

Figure 2 Defining surface roughness Evaluation length Mean line

Surface profile

Peak area Valley area

This smooth-surfaced screw feeder meets 3A 81-00 requirements for handling dried milk and other products hygienically.

High-level (regulated) design requirements. The highest level of hygienic feeder design is intended for specific food applications regulated by standards established by organizations such as 3A Sanitary Standards Inc. or the USDA. Such feeders are typically specified for applications that process perishable food products such as milk powder and cellulose. In addition, the products are commonly wet or processed in a wet environment, and feeder cleanup is quite extensive, including intense physical scrubbing and the use of acidic, alkaline, or caustic chemical cleaning and sanitizing agents. At the very least, manufacturers must use FDA-approved product-contact materials for this feeder type. In some cases, the materials must undergo further testing to receive approval. Typically, requirements establish a maximum surface roughness. Manufacturers must also eliminate exterior cracks and crevices, preferably at the design stage. If that isn’t possible, the surfaces can be sealed with FDAapproved materials. The feeder might also be subject to special sealing requirements, such as using sanitary tube fittings for discharge nozzles and auxiliary device connections. Finally, since high-level feeders undergo frequent cleaning, they must also have washdown-duty motors and bearings. Due to the highly perishable nature of the material handled in feeders regulated by high-level hygiene requirements, feeder manufacturers must be especially careful to eliminate features that promote material contamination or bacteria growth (such as flat horizontal surfaces that allow material to collect or pool). PBE References 1. For information on these organizations’ hygienic standards, visit their websites: US Food and Drug Administration (FDA), www.fda.gov; US Department of Agriculture (USDA), www.usda.gov; 3A Sanitary Standards Inc., www.3-a.org. 2. AISI designates a steel manufacturing standard of the American Iron and Steel Institute (www.steel.org), and the numbers following this designation identify specific steel alloy grades and compositions. 3. Ra is roughness average in microns. 4. IP55 (Ingress Protection rating 55) is an international rating relating to seals designed to protect motors from moisture and was developed by the International Electrotechnical Commission (IEC, at www.iec.ch).

Jason Parpart ([email protected]) is systems administrator at Schenck AccuRate, 746 East Milwaukee Street, Whitewater, WI 53190; 888-742-1249, or 262-4732441, fax 262-473-2489 (www.accuratefeeders.com). He has a BS in mechanical engineering from the Milwaukee School of Engineering. Steve Becker (steve.becker@sar inc.com) is director of sales and marketing at Schenck AccuRate and has a BS in engineering from the University of Illinois at Urbana-Champaign and an MBA from Lewis University, Romeoville, Ill.