Art or Science?
The choice between dilute phase pneumatic conveying and flexible screw conveying is not always clear
Everyone has rules of thumb when it comes to choosing a conveying system — rules based on bulk material characteristics, required flow rates and distances, source and destination, plant configuration, and other variables. But those with experience in all types of systems will tell you that the only real rule for choosing a conveying technology is that there are no hard-and-fast rules.
The fact is, most materials can be conveyed using any of several methods. No one can say with certainty, for example, that positive pressure pneumatic conveying is always the better choice for conveying high-temperature materials. The final selection comes down to balancing the pros and cons of each type of system for the particular application, with all its variables. That said, the following guidelines will help you select the optimum method for conveying your bulk solid products.
The first rule
The first rule is, ask the experts. While the science of conveying has advanced significantly over the past two decades, there is still an art to selecting the best overall system and engineering it to meet individual needs. The final analysis requires an intimate understanding of the material and process as they relate to the strengths and limitations of each conveyor technology. Consult with a specialist who does not have a vested interest in selling only one type of equipment. An expert will weigh each parameter and recommend the best solution for you.
A corollary to the first rule is, test before you buy. Testing will assure that the specified system will, in fact, convey your powder or bulk material the required distance without degradation or undesirable changes in product characteristics. A fully equipped, state-of-the-art testing facility will contain full-size systems that are easily reconfigured, as well as a full range of accessories and peripheral equipment (Figure 1). It will contain both pneumatic and mechanical bulk handling equipment to produce an objective recommendation based on your actual material. By verifying performance prior to fabrication, you can avoid costly misjudgments and delays in getting your system up and running.
A flexible screw conveyor (Figure 2), also known as a spiral conveyor, helical conveyor or centerless auger conveyor, consists of a flexible screw contained in a flexible or rigid tube that is driven by an electric motor. Materials are specified according to application, with the screw fabricated of spring steel or stainless steel, and the outer tube being of plastic or steel. It is a relatively simple design, and generally the most economical choice, with efficient performance, high reliability, and low capital and operating costs. When properly engineered and tested, it will provide excellent performance across a broad range of applications. There are also systems specifically designed to convey difficult-to-handle materials that tend to pack, cake, smear or fluidize, as well as fragile or brittle materials prone to breakage or crumbling.
Pneumatic conveyors (Figure 3) are also generally custom-engineered for each application and will satisfy a wide range of requirements. They move bulk materials that are suspended in a gas stream (most often air, but sometimes an inert gas) introduced by either a positive pressure blower upstream of material intake points, or by a vacuum pump downstream of material discharge points. Product is separated from the gas stream at the end of the line by filter receivers or cyclone separators, or sent directly into process vessels. These systems, which may be more complex than mechanical conveyors, can be integrated into process or production lines and will readily handle diverse products in the same equipment. Positive pressure pneumatic conveying is generally used to convey materials from a single source to one or multiple destinations, over relatively longer distances and with greater capacity than vacuum systems with similar size conveying lines. Vacuum systems allow easy pick-up of materials from open containers using wands, so are better suited to transport material from multiple sources such as storage vessels, process equipment, and rail cars to single or multiple destinations.
Choosing a system
The factors to evaluate when selecting a type of conveying system are:
- Material characteristics
- Material source and destination
- Conveying parameters
- Plant conditions