AST Dryer


The Scott Equipment Air Swept Tubular Dryer (AST) is a pneumatic drying system designed for rugged duty and versatility in many industries. The new, patented drying technology, utilizes adjustable paddles and air dams to create optimum air to solids heat transfer. Even slurries with dry solids content as low as 5% can be dried efficiently to as high as 99% dry solids in a single pass through the flash dryer.

The agitation provided by Scott's AST Dryer allows hot air to mix with the wet product entering the dryer inlet, reducing the drying time and breaking up lumps. Materials ranging from filter cakes, thixotropes, and dilatant slurries to powders and granules can be effectively dried in the AST Drying System.

Features:
  • Patented agitator design incorporates intense mixing, grinding and deagglomerating action
  • BTUs consumed per dry ton are 10-20% less than most existing dryers due to high heat transfer coefficients through velocity and agitation
  • Can process material ranging from pulverized powders with less than 1% moisture to granules with moisture content as high as 15%
  • Single pass drying even with slurries up to 95% moisture
  • Low capital cost and ruggedly designed for years of trouble-free operation
  • Custom designed to meet required specifications
  • The ability to adjust product retention time for hard to dry material
AST Dryer Operation

The Air Swept Tubular Dryer is a new concept in drying technology. The flow of material through the dryer is controlled, allowing the dryer to process anything from a fine, dry powder to a moist granular material. Below we have outlined the different operational stages of the AST Dryer to help you better understand the unique technology behind this innovative flash dryer.

Click Here for a larger view of the Dryer System Schematic

Feeding System

Provides Constant, Consistent Flow of Product

The purpose of the Feeding System is to accurately control the flow of product into the dryer. A constant and controlled feed to the dryer is important since a continuous system operates best at stable conditions. During steady-state operations, the feed rate is held constant as other parameters are monitored and controlled.

The Feeding System will be the Scott Cake Feeder, Scott Mixer Feeder or a pump (positive displacement or centrifugal), depending on the viscosity of the product to be processed. The feeder is equipped with a variable frequency drive to allow for a ramp up of product flow rate during dryer equipment startup.

Gas-Fired Burner

Direct or Indirect Fired Provides Efficient Heat Source

The function of the gas-fired burner is to be the source of heat energy into the AST Dryer. The burner heats the process air to an elevated temperature (400°-1200° F) and is responsible for controlling the energy balance of the system. The energy control is achieved by monitoring the process control thermocouple located at the discharge of the dryer. The drying system utilizes a PID loop (via the PLC) to maintain a set point by controlling the firing rate of the burner.

The burner is typically a direct-fired gas burner complete with flame supervision system, fuel train, etc. Other fuel options include propane vapor, fuel oil or an indirectly heated system.

AST Dryer

Mixes Product with Air Stream

The function of the AST Dryer is to effectively expose the product to the heated process air stream and retain the product until it has reached the specified moisture content. The AST Dryer has three distinct zones with differing operational functions.

Inlet Zone (Zone 1)

The Inlet Zone achieves size reduction (when applicable) and the majority of moisture evaporation. This is accomplished by a highly turbulent agitator area and efficient mixing area utilizing adjustable paddles. The agitator plates (with a high concentration of fixed blades) work to reduce the product into small pieces and provide a highly turbulent zone for efficient heat transfer. The smaller particle allows for more efficient heat penetration into the material and also allows the moisture to more readily escape to the surface as a vapor. High velocity achieved in this zone, 11,000-12,000 ft/min, further aids in the efficient transfer of heat to the material. This combination of velocity and agitation, working in concert with a high temperature differential, results in efficient and rapid evaporation. This zone of the dryer is sometimes considered the “flash zone” of the unit.

Further downstream in the first zone are a set of adjustable paddles that allow for back mixing. Product that is still moist and potentially sticky will continue to be agitated and back-mixed via the adjustable paddles until it can be carried into the next zone by the airflow. The back-mix process is continuous with some product being carried over as new product is introduced.

At the division between Zone 1 and Zone 2 is a set of air dams. The purpose of these dams is to classify the product. Material that is dry enough to be free flowing and easily conveyed by the air stream can pass on to the next zone.

Retention Zone (Zone 2)

The Retention Zone achieves product retention, mixing and drying. This zone is comprised of a series of mixing paddles, adjustable from conveyance to retention. The air velocities in this zone are considerably lower than in the inlet zone, this in combination with the adjustable paddles allows the product to be retained for the necessary time to complete the drying process.

At the end of Zone 2 is another set of air dams, which aid in product retention and provide a further means of classification.

Discharge Zone (Zone 3)

The Discharge Zone achieves additional retention, air and product discharge. This zone is similar to the Retention Zone; except this zone contains the air and product discharge and the mixing paddles are typically oriented to achieve discharge.

Product Collector and Exhaust Fan

Separates Product from the Air Stream Drawing Air through the System

Situated between the AST Dryer and the exhaust fan is the product collector, which separates the dried solids from the air stream. The industry standard is a baghouse.

The last component in the system is typically a centrifugal fan. The exhaust fan is located on the clean side of the collector and provides the process air required to do the work of pneumatically drying the material, as well as product conveyance through the system. The entire system operates under negative pressure, which makes for a clean, dust-free operation.

On larger systems, an inlet damper is provided to allow for airflow control, on smaller systems a variable frequency drive may be used for the same purpose.

AST Dry Feeder Options

The Feeding System controls the flow of product into the dryer, providing a constant and controlled feed to the dryer. Scott Equipment manufactures two feeders, designed for hard-to-feed products that are difficult to pump or screw-feed consistently.

Scott Cake Feeders

Scott Equipment Company has developed a versatile feeding system that a variety of process industries have com to rely on for their material handling requirements. The Scott Cake Feeder is durably constructed with a cylindrical barrel and vertical agitator. The insightful design of the vertical shaft provides a consistent flow of material for the horizontal discharge screw. The horizontal discharge screw may be fitted with a crew conveyor section or positive displacement pump for materials such as solid shortening, filter cake and wet pet food.

Features:
  • Bridgeless, trouble-free feeder for materials with consistency of a cake or paste
    • Carbonates
    • Oxides
    • Clays
    • Pigments
    • Mica
  • May be fitted with a screw conveyor discharge section or a pump discharge section (or a combination of the two when required for material cross-over and versatility)
  • Variable frequency drive for product feed
  • Can be supplied with a pump for thixotropic slurries/filter cakes
Scott Mixer Feeders

The Scott Mixer Feeder was developed as a trouble-free feeder for materials with a non-flowing consistency and a steep angle of repose. It was specifically designed to handle hard-to-feed products such as belt pressed sludges, wet granular materials, warm or sticky materials and fibrous materials. Its innovative design provides advantages over a live-bottom feeder with substantially fewer moving parts.

Features:
  • Bridgeless, trouble-free feeder for materials with a non-flowing consistency and a steep angle of repose
    • Belt/filter pressed slurries
    • Coal Fines
    • Fly Ash (Wet Sand)
    • Virtually any material with an angle of repose >60°
  • Superior to a live-bottom feeder due to lower maintenance required because of substantially fewer moving parts (two bearings and two drives are all that are required)
  • Fitted with a screw conveyor discharge section