To choose the appropriate conveying system, a pharmaceutical manufacturer needs a thorough knowledge of the benefits and limitations of each conveyor type. A company must also understand the physical characteristics of the production line and the material to be conveyed. Flexible-screw, aero-mechanical, and vacuum conveyors are common in the pharmaceutical industry. The following provides an overview of each type of system.
Flexible-screw conveyors are often the simplest solution (see Figure 1). They incorporate a heat-treated and tempered carbon or stainless steel spiral that rotates within an ultrahigh molecular-weight polyethylene tube. The term flexible means that these conveyors can be curved to some degree: The user can direct them around obstacles between the inlet and outlet. These machines carry powdered, granular, and flaky materials with bulk densities as great as 150 lb/ft3. They operate at rates as fast as 40 tons/h across distances as great as 65 ft. Systems can be linked together to move materials across greater distances.
The flexible-screw conveyor’s simple design results in low initial cost, quick installation, and low maintenance. Pharmaceutical models can be quickly dismantled for cleaning. The system’s parts can easily be replaced. Abrasive products cause wear on these systems, but models with abrasion-resistant rubber tubes are available.
Flexible-screw conveyors should always operate while full of product. It is used to deliver product from storage or a bag-tip station to a weigh hopper or a vessel with a high level switch. The system maintains a constant head of material in hoppers by gently filling to the high level control. The in-flight product is constant, and flexible-screw conveyors yield accurate, repeatable batches.
The aero-mechanical conveyor (AMC) is ideal for transferring products along 10–85-ft distances at rates as fast as 120 tons/h (see Figure 2). The AMC consists of several evenly spaced polyurethane disks, which are attached to a wire rope and travel within parallel steel tubes. The disks’ high speed produces an air stream that entrains the product in airflow until it is ejected. This method of conveying facilitates capacities as great as 120 tons/h with low energy requirements and virtually no separation of mixtures.
The AMC’s major benefit is that its airflow shields materials from degradation. The material is separated from the air that carries it, and the unloaded air current is directed into the return section of the tube, where it is retained in the tube circuit. Because the air is recycled, the AMC eliminates environmental concerns.
One disadvantage of an AMC is that it requires a lot of maintenance. Rope tension must be adjusted regularly during start-up and checked periodically. AMCs with automatic rope-tension monitoring and adjustment are available. Rope life depends on the length of the conveyor, the number of starts and stops, solids loading, and routine inspection and tensioning. Properly maintained rope-and-disk assemblies can last as long as 14 years.
Vacuum conveyors carry products over long distances and tortuous routes (see Figure 3). Vacuum conveying is usually restricted to throughputs of about 10 tons/h and distances as great as 330 ft.
A vacuum conveyor uses air to move materials through an enclosed pipeline. It is easy to route, has few moving parts, is dust-tight in operation, and leaves minimal residue when emptying a product. Because air is sucked into the unit, a vacuum conveyor is the preferred choice for toxic or hazardous materials. Accidental damage to the conveying tubes will not cause product to escape to the atmosphere.
The motive force comes from an exhauster or a side channel, high-efficiency fan at the receiving end of the system. Air-powered venturi systems are a good choice for low-capacity conveying. Venturi systems impose little capital cost and are inexpensive to operate.
Vacuum systems are normally the only conveying choice for customers who want to suck material out of bags or open containers such as kegs and drums. These systems are also suitable for applications with multiple inlets.
Reverse jet self-cleaning filters clean the conveying air and return it to the atmosphere after use. These filters reduce maintenance and minimize product loss.
Flexible-screw, aero-mechanical, and vacuum conveyors are the most common devices for transferring pharmaceutical materials. Each has different operating speeds, distance limitations, maintenance requirements, and load tolerances. Understanding each system’s characteristics is essential to choosing the right system for a conveying application.
Keith Simpson is a marketing manager at Spiroflow Systems.