Understanding Multiple Pneumatic Conveying Methods

Doc, my arm hurts when I do this. The surgeon recommended surgery. The physical therapist recommended weekly exercises. The chiropractor said it's actually my back causing the problem and recommended weekly adjustments. When it comes to a health situation, it can be very difficult to assess the different and often conflicting guidance and recommendations that physicians and other professionals offer. Each has his or her own perspective along with a financial agenda typically tied to a limited toolbox of available solutions. For many patients, working with one physician who is able to properly diagnose the condition and is also willing to recommend the most effective curative process from a limitless array of options would be ideal. When it comes to a material transfer situation involving pneumatic conveying powders and other bulk materials, many food, chemical, pharmaceutical, and other manufacturers often encounter a similarly confounding situation.

While a multitude of suppliers offer countless conveying equipment options in North America, many specialize in a single type of conveying equipment, though several of these options are effective in a wide range of powder and bulk material transfer applications. Though there is surely crossover where multiple conveyor types may suit a given application, one type is typically more efficient, more cost-effective, and/or an overall better choice. Some companies excel offering only one solution by intentionally targeting a very narrow segment of industry such as cement where a single solution is often effective across the entire market. The challenge arises when such a supplier is confronted with a conveying application that is not ideal for the single solution available. Given a financial incentive to specify the company's only available system, and a thought process that may be limited by specialized knowledge of materials and equipment, the engineering team may design clever custom accessories to make their system meet the requirements of the installation. Though it may perform admirably, it may also require more maintenance, more floor space, more careful attention during operation, and a greater financial commitment over the long term than another system more inherently suited to the application.

This installation shows a Gericke lean phase positive pressure pneumatic conveying system transferring powders to multiple inline sifters. Lean or dilute phase offers high throughput when particle integrity is less critical (image courtesy of Gericke USA).

Companies that offer expertise and experience in the design, manufacturing, installation, and maintenance of multiple types of pneumatic conveying systems serving multiple industries are well positioned to help determine which type of conveying system is inherently suited to a given application early in the research process. Their engineering teams and manufacturer's representatives understand how to use the characteristics of each type of system to their advantage - these characteristics become pros or cons based on the application, and the need to compensate for any shortcomings is virtually eliminated. These well-versed companies offer the ability to integrate and balance the capabilities of different systems as needed along with the willingness and financial incentive to specify exactly the ideal conveying system for the application from a single source.

Consider a food manufacturer that requires its workers to manually handle sacks of dried blueberries, a specialty flour, and fine sugar from storage and empty them into a batch mixer. Once mixed with a liquid binder, the thick mixture is discharged onto an open-air belt conveyor and transferred into a fluid bed dryer, which discharges the final product as a light density powder ready for filling and packaging. When the owner and/or plant manager tire of batch-to-batch inconsistencies, back injury complaints, worker's compensation claims, recruiting struggles, and managing combustible dust, material spillage, and constant cleanup, what options are available to automate the material transfer portions of the process with pneumatic conveying?

Lean Phase, Dense Phase? Positive Pressure or Vacuum?

Every type of pneumatic conveying system uses as gas such as air as to transfer powders, pellets, granules, and other bulk solids inside enclosed piping, hose, or tubing - as opposed to mechanical methods such as a belt conveyor, bucket elevator, screw conveyor, or vibrating conveyor. This eliminates or reduces the frequent material contact with rotating parts, increases transfer speeds, and minimizes both maintenance and risk of material degradation, while also protecting the material from contamination due to exposure to the plant environment. All types of pneumatic conveyors also operate automatically with varying levels of automation, transferring the materials from storage in a continuous flow to a feeder, hopper, mixer, or other equipment downstream at the push of a button. Moving heavy bags and sacks across the factory several times per day is eliminated along with the spillage, loss, and safety hazards.

Every type of pneumatic conveyor is capable of transferring a wide variety of different types of materials. The reason to choose a specific type of pneumatic conveyor is not only based on the material, but also its properties, such as bulk density, coarseness, and moisture content. Other important factors include the required throughput, conveying distance, and importance of particle size and shape at discharge, among others.

In lean phase--also called dilute phase--a rotary valve or eductor is set at the material pickup point to create a pushing effect that suspends the material inside the piping and moves it downstream. Depending on the material properties, suspending the solids in the airstream typically requires low pressures and high velocities to ensure consistent transfer. This works well for a wide range of throughputs but it can also adversely impact the particle size and shape, and the higher the throughput the greater the energy consumption.

Dense phase conveying sets a pressure vessel or sender at the material pickup point to create a comparable pushing effect but instead of suspending the material in the airstream, it is used to transfer the material in a series of pulsed slugs at low velocities, or as a continuous layer moving first in – first out as dunes. Transfer moves more slowly than in lean phase but also much more gently. For sensitive food ingredients and other delicate materials, the gentle nature of the material flow is often the key factor in selecting dense phase versus other conveying methods. Further, this low velocity transfer with dense phase helps mixtures stay mixed and blends stay blended inside the conveyor. Any disparity between the particle sizes and other properties of the different materials in transit increases the likelihood of the mix or blend separating as velocities increase.

What many process engineers and others looking for a conveyor often overlook is that both lean phase and dense phase pneumatic conveyors may be operated using either positive pressure or under vacuum. Positive pressure refers to the lean phase and dense phase conveying systems and their pushing effect described above. Conveying under vacuum, by contrast, involves setting a vacuum pump or blower at the end of the conveyor to create suction with a pulling action that draws the material through the piping. Vacuum conveying may be used to transfer the same types of bulk materials as lean and dense phase in positive pressure. It offers relatively lower throughput rates and shorter conveying distances but with the gentle conveying action needed to safeguard fragile materials. Conveying under vacuum excels at drawing multiple materials from multiple pickup points into a single destination. These systems typically require less equipment that can fit into tighter areas with lower ceilings than positive pressure conveying systems, and at a lower initial cost but also demand more energy during operation.

Positive pressure and vacuum conveying often suit comparable applications, yet they represent opposing philosophies that often lead conveyor manufacturers to specialize in offering one type or the other type. It is rare for a manufacturer to offer both approaches due to the costs and difficulties involved in maintaining a high level of applications engineering expertise and manufacturing ability in both worlds.

Taking another look at the process described earlier now armed with a greater understanding of the conveying choices available, automating the conveying process may begin by replacing the forklifts and associated material handling staff with automated bag handling and sack tipping stations. These automatically transfer the materials into the conveying process with far less physical effort and risk while capturing any fine particles inside before they could escape into the workplace as nuisance dust. To gently transfer the dried blueberries and the specialty flour, lean phase vacuum conveying may be specified. To transfer the fine sugar, a dense phase positive pressure pneumatic conveyor may be specified. Both conveyors discharge the three ingredients into day bins ready for an automated feeder to meter the precise amount into the mixer. After mixing, an enclosed screw conveyor automatically transfers the thick mixture into the fluid bed dryer, which continuously discharges the dry, light density powder into a hopper. This feeds a dense phase pneumatic conveyor that quickly transfers the finished product into another hopper atop a vertical form-fill-seal (VFFS) machine for filling and packaging. Though five different steps require material transfer, each step involves different conditions and requirements that may be met with four different conveying solutions for maximum efficiency.

Total Cost of Ownership

Maximizing efficiency is an important factor in pneumatic conveyor selection but it is only one of many important factors that need to be balanced and carefully considered. Cost remains a key factor in the decision-making process and very few process engineers could focus on peak efficiency without also achieving a return in a reasonably short payback period. Comparing the Total Cost of Ownership (TCO) of one conveying system to another considers the wide variety of factors to provide a long-term view of the overall investment beyond the initial purchase price.

Many conveyor buyers naturally focus on the initial purchase price and may be tempted to specify the pneumatic conveyor available for the lowest up-front cost, even given an understanding that it is not the most efficient or effective solution for the application. While lean phase positive pressure systems often cost less at the outset than dense phase positive pressure systems, for example, the high-velocity nature of the lean phase process typically demands more energy consumption and larger diameter piping with a greater footprint, and wears the system faster than other conveying methods. This tends to require more monitoring and maintenance to prevent unplanned downtime and shortens the overall lifespan. A comparable vacuum conveying system also may cost less at the outset than a positive pressure dense phase conveyor since it involves less physical equipment, simpler installation, and less operator training. Yet despite the gentle conveying action, vacuum conveyors often require a large amount of energy to generate its suction.

Calculating the TCO over the entire life cycle of each suitable conveying option for comparison enables those with a long-term perspective to see a more complete investment picture than those focusing on the initial investment. Similarly, a supplier that manufactures several different types of pneumatic conveying systems offers a broad, objective perspective when specifying the ideal approach for a given application.

Rene Meira Medina is executive vice president of Gericke USA, Somerset, NJ. Founded in 1894, the company designs and manufactures a range of pneumatic conveying systems, feeders, mixers, lump breakers, and other powder processing equipment. For more information, call 855 888-0088 x 805, email [email protected], or visit www.gerickegroup.com.