Conveyors: Pneumatic vs Mechanical
Pneumatic convey systems are used to transfer powders, granules, and many other types of dry bulk materials from one process area to another through negative conditions (vacuum) or positive conditions (pressure). These systems are comprised of four basic elements: an air source, a material feed device, a convey line, and an air-material separator. The primary difference between pneumatic and mechanical conveyors is that pneumatic systems convey material within a fully enclosed line, which greatly reduces the possibility of material loss.
The basic components of a mechanical convey system include a drive motor through a gear and/or chain reducer, connected to a drive device, such as a roller or a sprocket. The energy of the motor is transferred directly to the conveyor, which is in direct contact with the material. Mechanical conveyors can be opened or closed to the atmosphere, with hundreds of moving and working parts.
Determining the best method
The most important factor when selecting a type of conveyor is the form of material to be transferred. Identifying the key characteristics of material will help determine the best convey method for you. These characteristics include: hygroscopic, abrasive, explosivity, moist, sticky, or packable, high-bulk density, fluidisable, friable, mixed particle sizes, temperature sensitive, very sluggish flow and cross contamination issues. Other characteristics may need to be discussed with an engineer and equipment manufacture.
Ideal materials for pneumatic convey systems are fine, fluidisable, dry powders. These particular characteristics allow the material to be aerated (fluidized) and successfully pumped through a convey line. Some types of granular and pelletised materials are also good candidates for pneumatic conveying. A few materials commonly transferred with the use of pneumatic conveyors include wheat flour, cement powder, carbon black, starch, sugar, plastic pellets and resins, food products, coal fines, sands and metal powders, among many others.
Mechanical conveyors are well suited for heavy, granular materials that may be moist, doughy, and packable. Mechanical systems are more practical for materials with widely varied particle distribution; materials with both small and large particles combined. Materials commonly transferred by mechanical convey systems include whole grains, crushed rock, gravel, wet sand, and large food particles, among others.
Another significant factor when selecting a type of conveyor is the application. Application has several key areas to consider, including:
- Transfer Rate: Both pneumatic and mechanical systems can convey a few pounds to hundreds of tons per hour; however, the type of material and the convey conditions will determine the system.
- Distance: Generally, pneumatic systems are not used to convey short distances, as it requires the same amount of equipment to convey both 20 ft and 600 ft. The transfer of bulk materials over short distances can be accomplished with mechanical equipment even at the cost of some damage to the material or wear to the equipment. Both types of systems can be used for distances well over 1,000 ft if required, but the expense of installing the simple convey line (pipe) versus the structure for a belt is an important consideration. Long distances with multiple changes of direction and elevation are easily accomplished with a pneumatic system.
- Installation environment: Indoor or outdoor installations; certainly the enclosed pneumatic system offers some distinct advantages for most outdoor installations. Altitude; air is less dense at altitude and in some circumstances can have enough effect on a pneumatic system design rate to create a disadvantage in the form of larger equipment, especially the air source, when handling a difficult material.
- System clean-out and clean-up can be problematic for mechanical systems, such as a screw conveyor or bucket elevator. However, purging of material from a rotary valve or pressure tank and convey line are ordinary features of pneumatic conveying systems.
- Mechanical conveyors are more practical for heavy, granular materials, such as whole grains, crushed rock, gravel, or wet sand.
Compared to mechanical conveyors, pneumatic systems help simplify the routing of bulk materials transfer paths throughout a plant. For example, screw conveyors only run in a straight line and are limited on the amount of incline (based on materials). Additionally, screw systems require precise installation to keep alignment intact and take up a large amount of floor space. Pneumatic conveyors, on the other hand, are far easier to route. Because the convey line consists of simple pipe, they usually require minimum floor space and the line can flow in both horizontal and vertical directions.
Most pneumatic systems actually free up floor space if replacing a mechanical system, allowing for a high level of design flexibility in material routing. Convey lines can be easily adapted to existing equipment, and can be built to avoid or go around obstructions. Due to the straight route required by mechanical conveyors, any equipment in its path must be relocated during construction, adding expense. It is not unusual to combine different types of mechanical conveyors to create a single transfer system, e.g., a screw conveyor feeding a bucket elevator. However, when doing so, it is vital that the design rates are compatible to avoid overfilling or under utilisation of the next conveyor in the series. Overlooking this detail creates the possibility of spillage at each transfer point. Additionally, a complex dust system is often required to mitigate the escape of dust at the intersection of each conveyor.
One thing that all mechanical conveyors have in common is an abundance of moving parts. A multitude of belts, rollers, and bearings are used in their construction, and all of these parts need to be maintained. If proper maintenance is not completed it could result in failure, possibly leading to stoppage of the production line. Mechanical systems such as bucket elevators can fail if a single belt or chain in the system breaks. Even notwithstanding the possibility of part failure, the individual maintenance and inspection of all the working components of mechanical conveyors drives up the cost of operation.
Pneumatic systems rely on the airflow created by a fan or blower rather than the multiple parts that make up mechanical systems. The overall result of having so many fewer moving parts is less susceptibility to mechanical failure, which keeps productivity and efficiency working at the highest levels.
Another consequence that comes from working with an abundance of moving parts is a need for greater safety awareness. Gears, bearings, belts, and other parts all contribute to the chance that a worker could suffer injury from getting too close to the moving components. There are simply more opportunities for a hand or a finger to get pinched with all of these moving parts that need to be maintained and serviced. Pneumatic systems essentially eliminate this risk since all components are sealed to contain pressure or vacuum and thus have few exposed moving parts. Another way that pneumatic systems are safer is in relation to noise control. The motors, drives, chains, belts, and other moving parts that are essential for mechanical conveyor systems naturally create a noisier environment. The fewer working parts in pneumatic systems create a quiet operation, ultimately making for a more hospitable and safer working environment.
Energy and cost
However on these two parameters the pneumatic conveying scores less than mechanical conveying and therefore has limitations. You may find that the industry prefers to go for pneumatic conveying only when the situation demands, otherwise preferring mechanical conveying as the first choice.