Going with Gravity

Published: September 2, 2020

As featured in World Cement’s BMHR 2020 Supplement
by Kevin R. Peterson & Adam Schrage

Air-gravity conveyors offer numerous economic, environmental and safety advantages. In terms of economic advantages, air gravity conveyors are cost-effective, simple to install, maintenance friendly and cause no damage to the product being handled. The environmental and safety advantages include being dust-tight to atmosphere, having a low energy consumption, no moving parts and low noise levels.

Typical air gravity conveyor

Vortex Global’s air-gravity conveyor (Aero-Slide™) provides an efficient way to transport dry, light-weight, easily fluidized powders (e.g. activated carbon, alumina, bentonite, calcium carbonate, cement, clays, detergent powders, fly ash, gypsum, lime, pulverized ores, silica sand, soda ash, and talc, among others).

The conveyor consists of a rectangular chamber separated by an air-permeable, porous media material. The media creates two separate chambers: a top chamber to handle the material to be conveyed and a bottom chamber to handle fluidisation air.

When installed, the air-gravity conveyor is sloped slightly downward (typically at an 8° angle). Fluidisation air is injected into the bottom chamber. The air permeates through the media to fluidise the light-weight material. With the help of gravity, it then flows like a liquid down the upper chamber. Typical applications for Aero-Slides are to convey material into or out of bins or silos. They can be designed with angled turns, distribution boxes, and multiple discharge points.

Traditional metering options
When establishing an air-gravity conveyor as a conveying method, consideration must be given to the types of shut o and metering gates used to control the flow of material through the conveyor. Typically, material is introduced from a bin or silo into the air-gravity conveyor by opening up a slide gate at the bottom of the silo. The fine material is then aerated to create a flow of material into the aero-slide.

Material flow through the conveyor is traditionally metered by three methods:

Vertically mounted slide gate with built-in air plenum
This type of gate utilises a square or rectangular blade that opens or closes the top chamber of the conveyor (the chamber material passes through). Upon opening, the blade is retracted up and out of the upper chamber. The further pass through the chamber. With the gate blade closed, the open plenum at the bottom of the gate allows conveying air to continue to circulate through the conveyor’s bottom chamber, thus evacuating any material trapped beyond the downward slope of the closed gate.

Square/rectangular butterfly gate
This gate incorporates a blade that mounts within the area of the upper chamber. As the blade is tilted (opened), it allows material to flow through the upper chamber. Tilting it more allows increased material flow. The blade never leaves the confines of the chamber when open or closed and may be subject to wear or sealing difficulties.

Drum valve
This valve features a rotating, cylindrical-shaped drum (blade) that incorporates a ‘v-notch’ bore. The design of the drum valve allows for a more precise metering of material through the conveyor and an improved material shut off in comparison with typical standard gates.

Many of these control valves were initially designed decades ago. While they still function for their intended purpose, upgrades have not been designed to address industry changing material flow rate demands of differing types and grades of materials. Other issues include guarding for employee safety and controlling the escape of micro-fine material to atmosphere.

A new approach to metering
For four decades Vortex Global has developed and manufactured numerous models of slide gates and diverter valves for handling dry bulk material. Realising the need for a newly-designed metering valve for air-gravity conveyors, the company has introduced the Aero-Drum™ Valve.

It is designed to be used in tandem with the Aero-Slide conveyor and offers a number of features:

  • Actuator mount: The valve can be operated through electric, manual, or pneumatic actuators. The mounting bracket is versatile and can be installed to allow the actuators to be placed on either side of the valve (in case a potential problem exists with current equipment creating an installation issue).
  • Drum opening: Instead of a ‘v-notch’, the drum opening is a parabolic curve that is mirror-symmetrical from either side. It allows the drum to be positioned in a full open or a full closed position with more precise intermittent material flow regulation or metering.
  • Seal material: The internal drum seal is made of needled polyester. This material provides increased rigidity and will not stretch once installed. It offers superior abrasion resistance which extends the service life.
  • Split inlet flange: Making it easier to service the seal material, the valve contains a split inlet flange. This flange can be independently removed from the valve body.
  • Dust-tight to atmosphere: The valve is sealed and gasketed to contain fine material and to prevent escape into the atmosphere where it may become a housekeeping or a health issue.
  • Safety guarding: Actuator arms and bearings are guarded for employee safety.
  • Rain guard: A rain guard (to shield the flange connections) is offered for outdoor applications.

Aero-Drum inlet valve: (top) closed position, (bottom) open position.

Besides being dust-tight to atmosphere, the Aero-Drum valve is a self-contained unit designed to prevent the internal packing of fine, dry, properly fluidised material. An advantage over a vertically mounted slide gate is that there is no bonnet area (the area the blade retracts into) that can potentially fill up with fine material to cause actuation problems.

The Aero-Drum valve is available in sizes from 6 – 24 in. (150- 610 mm) square or rectangular sizes, and can be modified for temperatures up to 400°F (250°C).

Construction materials include carbon steel and 304 or 316L stainless steel.

Case Study
Vortex encountered one application where air slides were being installed to transport very fine (4 – 6 m) material from two individual storage bins to a truck load out. The customer was concerned that a simple, vertically mounted air slide gate would not be able to completely seal material and thus create a cross-contamination issue. The Vortex drum valve was chosen to act as a flow control and a shut o valve for the application. The customer has been pleased with the choice.

ABOUT THE AUTHOR
Kevin R. Peterson is the Business Development Director for Vortex Global. He is a board member of the Industrial Minerals Association of North America and has shared hundreds of case studies and numerous articles with the dry bulk material industry.

Adam Schrage is the Engineering Manager for Vortex Loading Systems. A Michigan native, he designs and engineers individual loading and material handling projects and advises Vortex customers throughout North America.

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