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Drag conveyor with silos

Drag Conveyor Basic Design Fundamentals — Part I

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Q. How do you spec drag conveyors? — R.L.

A. Thanks for the question. We use drag conveyors on just about every project, and there are a number of features to consider with each.

Drag conveyors are commonly used for bulk material handling, specifically for moving flowable bulk materials horizontally, vertically and on various inclines. Drag conveyors can efficiently convey a variety of bulk materials with low to high densities and fine to large particle sizes. It’s possible to achieve high-capacity conveying rates with horsepower requirements that are lower than screw conveyors but higher than belt conveyors. Drag conveyors can be designed for high temperature and high moisture products. They are dust tight, water tight, and gas tight; and some models are easily reversible.

Some common materials handled with drag conveyors are:

There are three styles of Drag conveyors:

Flat Bottom

The conveyor trough/housing is designed with a flat bottom. This style will convey materials en masse with minimal conveyor wear, and they are gentle on the materials or products being conveyed. Flat bottom conveyors do not clean out as well as other drag conveyor styles.

Flat Bottom conveyers can be constructed with a divided pan if the incline is steep or if it will be used for vertical conveyance.

Round Bottom

The conveyor trough/housing is designed with a rounded bottom. This curved style provides better clean-out, and it works well for conveying products when there is a concern for contamination.

Round Bottom conveyers can be constructed with a divided pan for steep inclines.

Tubular

The conveyor trough/housing has a tubular design. This style is suitable for lower capacities and fragile or friable products. It is readily configurable for multiple direction changes and vertical conveyance. Tubular conveyors are not a good choice for high conveyance capacity. Clean-out is very good, and there is minimal contamination risk.

Each style is made up of these basic components:

  • Housing/Trough
  • Chain/Paddles or Cable/Paddles
  • Head Section
  • Tail Section
  • Drive and Motor
  • Accessories/Ancillary Devices

Product characteristics and the desired conveying capacity will determine which design and components of the drag conveyer style are selected for each project. Some of the product qualities to evaluate are:

  • Abrasive or non-abrasive
  • Corrosive or non-corrosive
  • Free flowing, sticky or non-free flowing
  • Density
  • Friability

The capacity of a Drag Conveyor is determined by the usable housing cross sectional area and the chain speed. Housing size and chain speed are sized based on the desired capacity in TPH, CFH or BPH. Chain speed is a critical factor in the drag conveyor design. If the chain speed is too fast, it can create discharge problems and product degradation, as well as accelerated housing, sprocket and chain wear.

Recommended chain speeds for common materials are:

  • 150 FPM
    • Free flowing dense products not prone to damage, such as grain, minerals, and powders.
    • Free flowing meals, products not prone to damage with a somewhat lighter density, as found in mixed animal feed or ground grains.
  • 100-125 FPM
    • Free flowing materials that are prone to damage with either low or high density, such as animal feed pellets, pet food, and seed.

Slug vs. Continuous loading characteristics also determine capacity. Allow for future expansion when sizing the conveyor housing and chain speed.

This is an important topic, and there’s a great deal of information to share. This overview of Drag conveyors covers basic design fundamentals, and we’ll expand on some of these items in our next issue.

Answered by Curtis R. Cook, Vice President of WL Port-Land Systems, Inc. He has over 36 years of experience in design, operations, and maintenance of bulk solids storage and handling facilities.