Companies in the pet food industry store a variety of raw ingredients – from corn, soybean meal to poultry, cattle and swine meal. These ingredients may vary on their particle density, shape, size and distribution, moisture content, chemical composition, time at rest and temperature, among others. All those factors may affect how the material moves through the bins and silos, and may directly affect the final product quality. Handling solids is an integral part of many food manufacturing processes and deserves significant attention paid to it. Below a case study is presented of how Jenike helped a pet food manufacturer overcome solids flow problems.
Case study. The following case study demonstrates the impact of the flow pattern (funnel flow vs. mass flow) in the operation of a factory of dry, premium pet foods. The company stores raw ingredients in silos which hold approximately 180 to 230 tons, each.
The ingredients sometimes spoil, making it necessary to completely empty and clean the silos. Since only one silo may be available to store each ingredient, cleaning the silos frequently stops production. One of the silos was 4.5 m in diameter and 17 m high; the product flowed through a carbon steel 60-degree inclination conical hopper that ended at a 1.5 m diameter. It then went through a circular-to rectangular transition with vertical end walls that formed a discharge slot 0.3 m wide and 1.5 m long. A 0.2 m-diameter screw feeder discharged material from the silo.
Since production frequently consumed those ingredients, the operators kept the silo full. Consequently, those ingredients stored along the silo walls below the lowest material level remained there indefinitely and spoiled. Because of this, the silo had to be cleaned regularly – on average, 3 times a year but sometimes as often as every 2 weeks.
To clean the silo, workers removed the screw feeder from the silo bottom and then hammered on the silo walls to dislodge the material which fell to the plant floor where workers scooped it up with shovels. The entire job took three workers approximately 8 hours, and 3.9 tons of material were lost each time. The cost of labor and lost material was about USD 5,000 for each clean out. In addition, there is the lost production of sellable product.
The company contacted Jenike & Johanson and an engineer went to the plant to investigate the silo and the degradation problem. The engineer examined the empty silo and found that some material remained on the walls even after the silo had been cleaned. He also noticed that the conical hopper walls were rusty, indicating that the material was not flowing at the silo walls, and was operating in a funnel-flow pattern.
The engineer also found that material was dis charging only at the back end of the screw feeder in a narrow flow channel that extended upward to the top of the bin. The material flowed into this channel from the top of the silo, while the rest remained stationary. This is a typical flow pattern when using a constant pitch screw feeder.
After making these observations, the engineer took a sample of the material back to the Jenike laboratory to test and obtain its flow properties. Tests revealed that a rusty carbon steel surface at a 60-degree angle would not allow the material to move in a mass-flow pattern. However, the tests also showed that the material would flow on a sheet of ultra-high-molecular-weight polyethylene (UHMW-PE) at this 60-degree angle.
Jenike provided a report that recommended converting the silo to a mass flow pattern, so that all the bulk solid in the silo would flow whenever material was extracted by the screw feeder. The firstin, first-out material flow eliminated the presence of stagnate material in the lower portion of the silo. This also improved traceability of raw material in the lots produced.
To achieve a successful mass flow design, three modifications were recommended: first, the conical hopper had to be made steep and smooth enough to allow material to flow along the walls. Luckily, for this material and the operating conditions, tests indicated that installing a UHMWPE liner would achieve mass flow without changing the conical hopper’s wall angle. Second, the transition hopper also had to be made smoother. Here we recommended applying an epoxy coating to the hopper’s surface.
Finally, material had to be withdrawn from the silo outlet along the entire discharge slot to achieve mass flow. This did not occur with the existing constant-pitch screw feeder, which only withdrew material from its back end. Jenike recommended replacing the existing unit with a 0.25 m-diameter mass flow screw feeder. This type of feeder which has varying shaft sizes and pitches, extract an increasing amount of material in the direction of withdrawal and draws material from across the entire opening of the silo.
After the company agreed to the recommendations, Jenike’s engineers also checked the silo to make sure it could withstand the stresses caused by mass flow discharge.
After implementing the recommendations, which basically changed the silo flow pattern, material now flows through the silo in a mass flow pattern. The modified silo and feeder had been in operation for over a year and the company has not experienced material spoilage problems inside the silo since the modification was made. As a result, the company saved a minimum of USD 20,000 a year in silo cleaning costs plus they were able to maintain production, improve product quality, and enable lot tracking thereby offering substantial food safety improvements.
As mentioned before, having a mass flow pattern in equipment not only reduces maintenance costs but also diminish the effects of sifting segregation, improves blend uniformity, increases the ability to track raw materials and provides a steady discharge at a regular bulk density and a uniform and controlled flow.
Do not limit your operation accepting flow problems, know that these problems can be avoided or corrected.
