Although perhaps not the most alluring equipment at the party, gravimetric feeders set the mass balance and throughput rate for every continuous OSD process. Feeding accuracy and response time are arguably the most important performance criteria affecting the ability of a continuous manufacturing system to maintain a controlled state of operation.
A loss-in-weight feeder generally consists of an inlet hopper with flow assist, load cells, a variable speed product conveyor system, and integral controls that regulate conveyor speed as necessary to maintain feed rate in weight per unit of time at a particular set point. Loss-in-weight feeders replace the traditional weigh/dispense operation in a batch pharmaceutical system. In continuous manufacturing, an array of feeders is arranged to simultaneously feed ingredients into a continuous mixer at a proportional feed rate in strict accordance with an approved product recipe.
Loss-in-weight feeders may be charged via gravity, vacuum transfer, or manually. Gravity filling is simple and convenient, but requires room height that is not always available. Vacuum transfer is a viable solution in areas with low vertical clearance, but height in the equipment stack up must still be considered for vacuum receivers, which will be mounted directly atop feeder inlet hoppers. In order to achieve rapid refilling of feeder hoppers, vacuum transfer systems often convey materials at high speeds over significant distances. The collision that occurs when material enters the vacuum receiver can adversely affect feeder performance if the receiver and feeder inlet hopper are not properly isolated. Manual charging is often employed for active ingredients or excipients used in very small quantities, such as lubricants.
Loss-in-weight feeders will almost always feed the unit operation immediately downstream via gravity. Therefore, feeders will typically reside on an elevated platform. It is critical that loss-in-weight feeders are mounted in such a way that their supports are isolated from potential disturbances, such as operators walking on platforms, or the aforementioned movement of an upstream vacuum receiver during refilling. Even the slightest disturbance, such as air movement from a nearby ventilation system, can adversely affect the performance of highly sensitive load cells in a loss-in-weight feeder.
Special provisions must be taken to protect operators if feeders are used to handle potent compounds. Feeders may be fitted with containment valves and spray devices to wet product contact surfaces, prior to disassembly for removal from the system, or at the cleaning station.
Loss-in-weight feeders will typically be cleaned out of place in a remote wash room. Feeders must be disassembled for cleaning so that difficult-to-access product feed screws and hopper flow aids can be cleaned thoroughly. Access must be provided around feeders to allow for setup assembly of feeder arrays, operator access, and disassembly and removal at the end of a production run. This assembly and disassembly of feeder arrays often results in undesirably long setup and changeover times. Preassembling feeder arrays in a more modular fashion, on a solid, mobile base, may represent an opportunity to minimize the durations associated with these nonproductive activities.
Feeders might not be the first machines to catch your eye when you’re checking out new continuous equipment, but designing your process to optimize their setup and performance may arguably be the most important measure an organization takes to ensure the overall effectiveness of a continuous OSD process.
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