During the manufacturing of two pasteurisers for Nestle Mossel Bay, alternative projects were discussed, including the screw feeder project. The objective for the screw conveyors was to transfer milk powder from the outlet of a vibrating sieve to the inlet of a magnet and metal detector which then fed into the packing hopper.
The project had some unique challenges to overcome. The factory layout has height constrains which forced detailed design consideration as well as having to transfer around a corner which resulted in us incorporating two screw feeders with one feeding into the other. This all had to be done while being able to convey at a volumetric flow rate of 9 tons an hour (product density of 500 kg/m3).
The specific client requirement was to have scrape detection between the rotating shaft and screw housing. This could only be done using two options; vibration analysis and electrical continuity testing. Vibration analysis had been used in another Nestle factory and the feedback from this, was that there were too many false events due to the sensitivity of the system, and it was a costly installation. As a result, the electrical continuity testing option was explored and the major challenge was electrically insolating the rotating shaft from the housing. By doing this, should the flighting touch the housing, a short circuit would be created, and this would give a signal to stop and inspect. The system still needed to remain hygienic which meant full stainless steel 316 material for all product contact surfaces. The bearings and gearboxes supporting and driving the screw conveyors were fabricated out of conductive material. Thus, a pedestal arrangement was designed through a series of iterations with the customer to allow for electrical isolation. Nylon was the choice of material to use as the main pedestal flange as it was non-conductive as well as robust enough to be able to take the load of the system.
The other engineering challenge faced was the fabrication of the conveyor body as each conveyor was 3m in length and needed to remain stiff while in operation to ensure that it did not flex which prevents scraping. A normal DN stainless steel pipe could not be used due to the length and thin wall thickness. Thus a stainless steel 316 schedule 40 pipe had to be used however the surface finish was not hygienic. We decided to use large hydraulic cylinders, where the inside of the tube was honed out to a 0.2 mm tolerance and the food-grade finish was achieved.
The final challenge to overcome was that of the installation. Nestle requested no items touching the floor underneath the screws to allow for cleaning and access. This led to a hanging installation from the roof of the building with specially designed rotating and tilting brackets to allow accessibility for maintenance and inspection without an external structure. When it came to maintenance and cleaning, the screw was designed to be removed and installed quickly and easily to minimize the downtime and effort required.
This project was an exciting design and development project which demonstrated Anderson Engineering’s expertise in doing large hygienic screw conveyors/
The project was awarded in January 2021 and the design phase took 2 months to complete in collaboration with the Nestle Mossel Bay projects team.
From the final approval of the design, the fabrication of the screw feeders took 7 weeks. Anderson Engineering then conducted a full simulation and factory set up in the workshop to ensure all was working before being dispatched to Mossel Bay. This process took a week and a half to conduct and extensive trials were run with the actual product that the Nestle Mossel Bay team sent through to us.
The installation has not yet taken place due to delays of third-party equipment being supplied however the factory test conducted exceeded the requirement in flow rate and all design objectives were achieved rendering the project a success.