Moving Material through a Process: A Guide to Selecting ...

Introduction

Selecting a conveyor system for a bulk material handling application is not always as straightforward as one might think. Moving material consistently, at a rate that is in line with production requirements, and in a manner that does not contribute negatively to the plant operating environment, such as dusting or increased maintenance, can be a challenging endeavor for process engineers and procurement personnel.

Often, conveyor systems are misapplied because of the overall plant’s desire to keep the number of unique pieces of equipment to a minimum while also adhering to spending limits. These factors are critical to consider when selecting a conveyor system for a material handling application; however, they should not be the first, or the only, evaluation metrics used.

The most effective way for a processor to evaluate all of the best options for a material handling application is to consider each material movement requirement from a blank starting point. Looking at an application with the mindset of “What is the best way to satisfy our material conveying need,” will put processors in a better position to make the right equipment selection prior to purchase. Taking this stance, there are several key considerations to evaluate for proper conveyor selection, including material, operation, environment, envelope, cost, and history.

Material

There are several important characteristics that make up the complete material definition, which should be understood in their entirety. Some of these are dynamic and can influence or may be influenced by one or more of the other characteristics, so it is always recommended to look at them together.

• Name of the material (many materials are known by more than one name)
  • An Esoteric or Trade Name (such as SnoMelt)
  • Generic Name (such as salt) or Chemical formula, which is defined & called by its primary ingredient or ingredients (sodium chloride, NaCl).
• Material form or state in which it must be handled
  • Solids (free flowing or semi-free flowing)
    • What is the loose bulk density? (lbs/ft³, g/cc, kg/m³)
    • Loose Bulk Density is commonly interchanged or confused with Specific Gravity but should not be. They are very different. Specific gravity is the weight per given volume of a substance, usually in its most natural, concentrated, unreduced solid form (no air pockets or interstitial spaces between particles). Loose Bulk density is the weight per given volume of the material in its reduced, free-flowing or semi free-flowing state.
• Solid Composition
  • Powder
  • Prill
  • Granule
  • Pellet
  • Fiber
  • Flake
• Particle Size
  • Symmetrical Solids are usually described in terms of their ability to pass through a screen of a certain size.
  • Asymmetrical Solids are usually described in terms of their geometric dimensions, minimum and maximum.
• Flowability – Flowability is perhaps the most important characteristic of any solid material to understand, yet it lacks a single standardized method of measurement across industrial disciplines.
  • Most often it is defined in somewhat arbitrary terms as Very Free Flowing, Free Flowing, Average Flowability or Sluggish.
  • Angle of repose is also frequently used as an indicator of flowability. It is simply the natural angle or rate of incline that is observed when material is metered from a single discharge point without vibration or any effort to settle or distribute the accumulating mound.
  • Some use a 1 to 4 or 1 to 10 indexing system in an attempt to quantify the flowability.
  • We recommended reading the following document to better understand flowability. It’s co-written by James K. Prescott and Roger A. Barnum of Jenike & Johanson and published in the October 2000 issue of Pharmaceutical Technology. It’s downloadable from the Jenike & Johanson website at: http://info.jenike.com/technical-papers/on-powder-flowability.
  • If the material is known or suspected to be a challenge because of its flowability, it may be best to send a sample to the equipment supplier for review.
• Abrasiveness
  • Like flowability, abrasiveness lacks a universal standard definition. However, there are two material factors that tend to determine the abrasive quality. First is the material hardness, which is measurable and can be defined using the Mohs scale or Vickers scale. The second factor is the particle shape. A hard particle having no abrupt edges (a sphere) is less likely to abrade a contact surface than one with sharp edges. Most often, abrasive materials are known to plant personnel, and experience tends to be the most valuable measure. In other words, if a material has been found to erode other plant process equipment, certain parallels can be drawn and educated assumptions made as to the way the material will affect proposed equipment.
• Material Temperature
  • Usually defined as a range (min. to max.)
• Moisture Content
  • Usually described as a percentage by weight.
  • Can provide an important clue as to flowability or cohesiveness.

Note: One of the most common misconceptions in regard to materials and their various handling characteristics is that they can be obtained from MSDS (Material Safety Data Sheets). However, this is rarely the case. As the name implies, the document is a declaration of any health, safety or environmental concerns, and while it may offer some vague clues, it seldom enables a sufficient understanding of the way a material is likely to behave while being handled, stored or processed.

Note: There is an excellent resource printed on a web site called http://www.tankconnection.com/, which goes to extensive effort to quantify the most important characteristics for hundreds of common solids, assigning each a code and providing a convenient code chart for interpreting a material’s unique character profile.

That chart may be found at http://www.tankconnection.com/products/understanding-dry-bulk/material-characteristics/.

Operation

Simply stated, operation refers to the function and performance that you need to satisfy. There are certain aspects of the operation that should include some detail and clarity.

Fundamentally, moving material through a process falls into two main categories. It is important for processors to understand the difference to ensure the right conveyor system is ultimately selected.

• Conveying
• Feeding

1. Conveying is simply moving material (or materials) from one or more pick-up points and delivering certain materials to one or more drop points. The rate at which this is accomplished is usually fixed and the delivery time, while important, fits comfortably within a minimum and maximum range. Conveyor systems are most often used as a refilling device for surge hoppers, feeders or process equipment.

• Specifying the operation of a conveyor consists of:
  • Defining the amount of material that needs to be moved and the window of time within which it must be moved.
  • In cases where there may be multiple discharge points, it is necessary to know the demand at each drop point.

If you want to understand more about conveyor components, feel free to View Details.