Processing Magazine

Solving sealing problems in vacuum dryers for abrasive, bulk materials

September 1, 2008

Two challenges encountered by processors who have to dry abrasive slurries are contamination of product by packing flecks and the loss of vacuum leading to longer and longer drying cycles. For most, the greater the vacuum, the shorter the batch time, and the more profitable the process. As the product in the dryer is heated, the shaft grows in length. The dryer also undergoes thermal growth, and the shaft and vessel usually grow at different rates. This creates a thrust motion of the shaft, which can pull abrasive product into the seal cavity, in a wiping action. Packing in the stuffing box becomes abraded, and residue from damaged packing can contaminate the process material. Constant abrasion of packing material can also cause damage to the shaft, and eventual leakage. As the seal is compromised and atmosphere leaks into the process, the vacuum pump is taxed, requiring greater energy expense to run it. At the same time, the machine is prevented from holding full vacuum, so it takes more time to achieve the necessary product dryness. At best, this increases the batch time; at worst, atmospheric air contaminates the product and makes it less saleable - or not saleable at all. Mechanical seals can usually eliminate the problems related to packing wear, but they are not able to handle the thrust motion which results from thermal growth. One solution to this problem is MECO''s MECO-DR custom shaft seals. The MECO-DR seal is designed to accommodate thermal shaft growth and thus to effectively hold vacuum in a dryer. Product is excluded from the seal cavity, so abrasion damage to the shaft is prevented. Seal faces co-rotate with the shaft, engaging on a plane perpendicular to it. A low-flow air purge is used to maintain seal face pressure and to provide a monitoring tool to operators, indicating when seal face wear has occurred. The seals need infrequent rebuilding; split internal rotating parts are quickly replaced.