Feeder Fundamentals: A Short Guide to the Basic Feeder Types by Kathy Hunter
Wednesday, June 03, 2009
Ingredient feeders are used to feed, dose, or meter raw materials in the manufacture of food products. Feeders are mainly used for dry ingredients, but can also be used to add liquid flavors, vitamins, or other liquid additives that require precise and accurate dosing. Typical processed foods that may require the use of feeders in their production include: cereals and breakfast foods, pasta, snack foods, confectionary/chocolate products, bakery products (including cookies, crackers, and cake mixes), and pet food and feed. Feeder controls hold the “recipe” for the food product being processed, making the use of accurate and reliable feeders critically important in the manufacture of quality food products.
Key food processing applications using feeders include:
- continuous extrusion for snack foods, cereal, pet food, or pasta
- feeding multiple ingredients into continuous or batch mixers
- coating drum, seasoning, and spicing processes that require careful addition of the seasoning ingredients
- major and minor ingredient-transfer and batch-weighing
- contained and accurate feeding of vitamins and other additives requiring careful handling
- sanitary feeding of expensive “inclusions,” such as nuts or cherries, in the manufacture of ice cream
- metering and totalizing of ingredient usage into a variety of processes to establish true end-product cost
Common Feeder Types
Good mechanical design, control, and weighing technology combine to produce an accurate and reliable feeder. The key mechanical component of any feeder is the feeding device used to move or “feed” the material. The basic dry material feeders are defined by the type of feeding device they use. The most common feeder devices are screws/augers, vibratory trays, and belts. Other feeding devices used by some feeder manufacturers include rotary valves, slide gates, oscillating plates, mass flow meters used as feeders, and bulk solids pumps (BSPs). Material flow characteristics help to determine the best feed method.
Screw Feeders / Auger Feeders
Screws or augers are the most commonly used feed devices. Some manufacturers offer both single-screw feeders and twin-screw feeders, widening the range of materials to be handled. Single-screw feeders handle free-flowing materials, such as semolina and other granular materials. Twin-screw feeders are used for more difficult materials that might be sticky, bridging, or floodable, such as flour. The “self-wiping” action of intermeshed twin-screw flights results in more accurate feeding of difficult and cohesive powders. Most feeder manufacturers offer a variety of screw/auger designs.
Vibratory feeders use a vibrating tray to move the material and are sometimes used by food processors to handle materials such as coated raisins. Sticky powders, however, tend to pack in the tray of a vibratory feeder. Both screw feeders and vibratory feeders use a hopper above the feed device to receive the material from a refill device of some sort, often a vacuum conveyor.
Weigh Belt Feeders
Weigh belt feeders are compact in height and do not require the use of a material hopper; therefore, they are a popular feeder choice for process lines that are tight on space. Weigh belt feeders are heavily used in the food industry for high-feed rates and to handle a wide range of materials, including fragile materials that might be damaged by a screw feeder. Weigh belt feeders employ a moving belt above a weighing device that continuously weighs the material as it moves along the belt. In addition to being used for either continuous or batch operations, a weigh belt feeder is often used as a meter in food processing plants. For example, weigh belt feeders are often placed below a silo in a “wild flow” application to totalize the quantity of peanuts passing into the process in the production of peanut butter. In addition, in many cereal plants, processors use weigh belt feeders to totalize the quantity of finished cereal being metered into the packaging line as a quality check.
Liquid Loss-in-weight Feeders
The recipe for most food products includes a liquid ingredient of some sort, such as eggs, oil, milk, or liquid vitamins. These liquid ingredients can be added to the process in a variety of ways, including use of a simple pump, a liquid flow meter, or a liquid loss-in-weight feeder. Processors use a liquid loss-in-weight feeder when the ingredient accuracy is critical. A wide range of pumps and tanks can be used with a scale for loss-in-weight feeding of liquids.
Sanitary Feeder Design
Sanitary design is very important when feeders are applied in the food manufacturing process, and the mechanical design must comply with FDA Current Good Manufacturing Practices (cGMP) for Food Equipment—21CFR Part 110.40. These FDA regulations require plant equipment to be designed and of such material and workmanship as to be adequately cleanable and properly maintained.
Gravimetric Feeders or Weigh Feeding
The gravimetric vs. volumetric control choice is generally made based on the level of accuracy needed in the process. In gravimetric or weigh feeding, materials are fed into the process at a constant weight per unit of time. The weight is captured by a scale or other weighing module. Gravimetric feeding combines monitoring of the feeding process with a feedback loop that enables accurate material control, producing accuracies of 0.25% to 1%, depending on the feeder and the material. There are two types of gravimetric or weigh feeders: loss-in-weight feeders and weigh belt feeders.
Loss-in-weight (LIW) feeders discharge the material with a constant weight per unit of time by weighing the entire feeder and material inside. As the material discharges, the feeder monitors and controls the “loss-in-weight” of the material. The feeder controls regulate the speed of the feeding device to match the desired feed rate or set point. The weighing control system compensates for non-uniform material flow characteristics and variations in bulk density and is the key to providing a high degree of feeding accuracy.
Weigh belt feeders use a moving belt mounted over a weighing module. The weigh belt feeder theory of operation is:
Mass flow = belt speed x material weight, or, lbs/minute = ft/min x lbs/ft.
The feeder controller compares the desired mass flow with actual mass flow (weight of the material on the belt x belt speed) and adjusts the belt speed accordingly.
Volumetric feeders discharge bulk material with a constant volume per unit of time by regulating the speed of the feeding device. The feeding accuracy is dependent on the uniformity of the material flow characteristics and the bulk density, and is generally in the 2-5+% range. By adding a scale and upgrading controls, most volumetric feeders can be converted into loss-in-weight feeders.
We would like to thank K-Tron Process Group for providing images for this article.