SHAFTLESS SCREWS FOR VERTICAL CAKE CONVEYANCE – HOW HIGH CAN YOU GO?

The concept of conveying solids with shaftless spirals has been used in Europe since the 1950's primarily for the agricultural industry. The use of this technology for vertical conveyance has been limited in the United States compared to their use in Europe. However, within the past few years, shaftless screw conveyors have been increasing considered as a viable option for biosolids cake conveyance. Prior to 1999, the maximum vertical lift for a shaftless screw conveyor in North America was limited to 35 feet. Within the last three years, several large shaftless screw installations have been installed within the United States. The McAlpine Wastewater Management Facility, City of Las Vegas Water Pollution Control Facility, and Clean Water Services, Durham recently replaced their existing cake conveyance systems with shaftless screw conveyor technology. Two of these installations have pushed this vertical limit to beyond 45 feet. This paper provides a state-of-the-art look at shaftless screw conveyors, reports on the issues related to this technology, and provides valuable “hands on” design, control, and operating suggestions that can be used in subsequent projects. In addition, this paper describes some of the unresolved issues currently facing shaftless screw conveyor technology.