PNEUMATIC & AIR CONVEYING SYSTEMS IN PLANTS

Technology and continuous improvements have altered the equipment for delivery of  wheat/grain/flour. In milling industry, systems used in the removal of wheat -as a raw material- and flour -as a final product- from one point to another within the factory are generally divided into two groups as mechanical and pneumatical handling systems. While mechanical systems are especially preferred for removal of heavy and high-volume products, pneumatical systems stand out in the handling of lighter products with small particle sizes. Experts state that both systems have advantages and disadvantages and it is not possible to prefer one against the other.

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Pneumatic conveyors are generally used for transferring substances in granular/powder structure by means of the air stream moving in a transmission line or transferring unit loads in private carriers. The common working principle of pneumatic conveyor machines is transferring the kinetic energy of the air stream. Since air is a clean energy source and the system works in a completely enclosed environment, material losses and environmental pollution are minimized, and transfer operation is generally performed with zero loss. When a facility’s requirements demand high process rates for flour/grain (i.e. in the tens of thousands of pounds per hour), the method of choice is typically Dilute Phase Pnuematic Conveying. In this instance, outdoor storage silos transferring to indoor batching points are quite common. The bulk material is generally delivered via PD Pnuematic Truck or Railcar. Whether Truck or Railcar, the hoppers are pressurized to a certain PSI and then the material is systematically released into the transfer system by the operator.

More sophisticated systems can offer unloading and transfer without requiring an operator to be constantly managing the process, but generally speaking operator controls the unload sequence. Just like the larger Silo applications, the material is metered into the transfer tubing via a rotary airlock which allows the material to enter the conveying stream without the loss of the conveying pressure and air.
In facilities with lower transfer rate requirements and shorter distances, mechanical systems can be quite common. Facilities with these smaller requirements usually receive product in bags or bulk bags. In leiu of a Pnuematic Conveying System a mechanical system can usually handle the rate and accuracy requirements using Screw Conveyors, Bucket Elevators, etc. More common are simple Flexible Screw Conveyors comprised of a typically coreless auger “floating” in a plastic flexible tube. Similar to Rigid Screw Conveyors, these bolt to the discharge of the Bag Dump Station or Bulk Bag Unloader. The screw’s discharge delivers the product to an end point a short distance away. Rigid and/or flexible augers can deliver up to 25’ away in typical designs. These smaller systems are commonly referred to as “up and in” systems due to their short distances. In the same manner, small Bucket Elevators and other mechanical systems are utilized.

Pneumatic systems are used;
– At low pressure loss (300 mmSS) and low air velocity, aspiration and dust suppression systems, at destoning, descaling and ventilation of wheat;
– At medium pressure loss and air velocity (600 – 1000 mmSS), at conveyance of passages;
– At high pressure loss and air velocity (5000 mmSS), at storage and packaging departments of finished products

Pneumatic conveying, used commonly in food industry, is one of many ways of transporting products. It uses pressure force to create an unbalanced environment in the pipeline. This way, the product is pushed or sucked through the pipe from the feeding stations until the very end, where it is discharged. To design a functional and reliable pipeline, we must go through a lot of calculations and designing before we get to a suitable and satisfactory result. If a particular product (i.e. grains, etc) has real degradation concerns, dilute phase pnuematic conveying is not the selection of choice. The generally higher velocities associated with transferring in a dilute phase manner do not lend themsleves to handling products in a gentle fashion. In these instances, the product is typicaly handled with a Dense Phase Pnuematic Conveying System or a Mechanical Conveying System. A dense phase concept is designed with the goal of a high solids-to-air ratio. This allows the product to move in the tubing at much, much slower velocities when compared to a dilute phase systems. As such, these systems are quite desirable when transferring a product that has degradation concerns (grains and similar applications) or, that may have abrasion concerns on the handling equipment. Utilizing a lower air velocity, these systems can transfer friable products very effectively with little degradation. Additonally, should a process have a blended product, dense phase systems can deliver the product while minimizing product segregation. The larger the particle size difference, the more likely a dense phase concept may be required. If required, the dense phase concept systems allow for the same benefits that reside in dilute phase systems (see below: dust free, easy layout, robustness, etc).

Dilute Phase Pnuematic Conveying references the air-to-material ratio when transferring. Generally speaking, this is a low product-to-air ratio design that utilizes higher air velocities (and lower volumes of product) to transfer the product from one point to another within a facility. There are (2) types of Dilute Phase Pnuematic
Conveying styles:
• Pressure
• Vacuum

Generally speaking, Pressure systems provide better transfer efficiencies over longer distances. In a given application, Vacuum Systems typically will not be able to match the throughput of a Pressure System with all things being equal. However, Vacuum Systems are considered “cleaner” as the motive air is not delivered through the blower and into the product stream like a pressure system.

There are many advantages in applying dilute phase pnuematic conveying in grain/wheat applications. Below are some of the primary advantages:
•Dust free handling: With the material being handled in a closed system comprised of tubing (or, piping) from one point to another, these systems are virtually dust free when designed properly.
•Ease/Simplicity of layout: The layout of these systems is quite simple given the fact that the tubing is flexible in terms of final layout design. Care must be taken in order to minimize the distances (and turns, or elbows) of the systems.
•Robustness (Maintenance & Repair): Dilute Phase Pnuematic Conveying Systems are typically extrememly robust and can consistantly provide years of continuous, trouble-free operations. We have routinely witnessed system operating in their 4th decade with little maintenance & repair.
•Distribution: Pnuematic Conveying Systems alllow for a multitude of layout possibilities.
-Materials can be transferred from one point to multiple destinations.
– Materials can be trasnferred from multiple starting points to a singal destination.
– More complex systems can trasnfer from multiple starting oint to multiple destinations
•Ease of Control/Automation: Either Control Relay Logic or PLC (Programmble Logic Controllers) driven Controls Systems are common. The more complex the system, the more common you will see PLC drvien systems due to their robutsness and ease of operation.
•Security: The materials are received and transferred in closed systems (Silos, Tubing, Filter Receivers, Scale Hoppers, etc).

Of course, as with all systems, there are disadvantages when utilizing Dilute Phase Pnuematic Conveying for Wheat/Grain transport. Some of these disadvantages are:
•Inefficient Material Transfer: The Horsepower requirements are usually higher than mechanical systems when given the same operational parameters. Simply put, these systems cost more to operate when comparing on a Pound of Product delivered per Horespower.
• Explosion Protection/Safety Considerations: This key component is being reviewed and required per safety guidelines on a more continual basis than in the past. These safety systems add cost to the system.
•Distance Limited: Dilute Phase Pnuematic Conveyingis more limited when required to trasnfer products over long distances. Generally speaking, systems are typically under several hundred feet in order maximize throughput. A simple rule of thumb is to remember, “Distance and Turns (i.e. Elbows) can and will drastically diminish rate for a given application”

When a facility’s requirements demand high process rates for flour/grain (i.e. in the tens of thousands of pounds per hour), the method of choice is typically Dilute Phase Pnuematic Conveying. In this instance, outdoor Storage Silos transferring to indoor batching points (i.e. to intermediate storage or scaling/weighing for process requirements) are quite common. The bulk material is generally delivered via PD Pnuematic Truck or Railcar. Whether Truck or Railcar, the hoppers are pressurized to a certain PSI and then the material is systematically released into the transfer system by the operator.

More sophisticated systems can offer unloading and transfer without requiring an operator to be constantly managing the process, but generally speaking operator controls the unload sequence.

In some instances, the wheat/grain is supplied in smaller bags (50-100 lbs) or bulk bags (1,500 – 2,500 lbs). In this case the transfer is supplied via Bag Dump Station(s) (50-100 lbs bags) or Bulk Bag Unloading Station(s) (1,500 – 2,500 lbs). Just like the larger Silo applications, the material is metered into the transfer tubing via a rotary airlock which allows the material to enter the conveying stream without the loss of the conveying pressure and air.

In facilities with lower transfer rate requirements and shorter distances, mechanical systems can be quite common. Facilities with these smaller requirements usually receive product in bags or bulk bags. In leiu of a Pnuematic Conveying System a mechanical system can usually handle the rate and accuracy requirements using Screw Conveyors, Bucket Elevators, etc. More common are simple Flexible Screw Conveyors comprised of a typically coreless auger “floating” in a plastic flexible tube. Similar to Rigid Screw Conveyors, these bolt to the discharge of the Bag Dump Station or Bulk Bag Unloader. The screw’s discharge delivers the product to an end point a short distance away. Rigid and/or flexible augers can deliver up to 25’ away in typical designs. These smaller systems are commonly referred to as “up and in” systems due to their short distances. In the same manner, small Bucket Elevators and other mechanical systems are utilized.

We also suggest you to read our previous article titled "Cyclones and their Rotary Airlock Valves".

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