Rotary DryersCoolers and Calciners

2Rotary Dryers

Barr-RosinAs a member of the GEA Group, Barr-Rosin is one of theforemost companies in the field of industrial drying andcooling. Extensive experience has been gained with a widerange of thermal processing equipment designed to meet thedemands of modern day process industries worldwide. Withengineering and sales offices in Maidenhead (near London)and Montreal (Canada), as well as GEA offices around theworld, a complete service is available - from initial designthrough to final commissioning. Each Rotary Dryer installationis custom engineered to suit the clients specific requirementsand complements the well known range of Barr-Rosin fluidbed, ring, flash, column and Rosinaire paddle dryers.

The Cascade DryerDespite the introduction of new technology, the longestablished Rotary Cascade Dryer is still widely regarded as theworkhorse of many process industries. The robust yet simpleconstruction combines flexibility with reliability, enabling thistype of dryer to handle a vast range of materials and tooperate continuously under the most arduous conditions. Thedesign also permits the use of the highest possible dryingtemperatures and in contrast to other dryers is not sensitive towide variations in material size, moisture content orthroughput.

Principles of OperationIn its simplest form, the cascade dryer consists of a slightlyinclined rotating cylinder, fitted with a series of peripheralflights arranged to lift, distribute and transport the material.The flights are designed to suit the particular handlingcharacteristics of the material, which may vary with increasingdryness.

The principle of operation is based on showering, or cascadingthe wet material through a hot gas stream, flowing either co-current or counter-current to the solids. The hot gas inducesthe evaporation of the moisture. The heat lost to the materialand evaporation of water vapour reduces the gas temperaturerapidly, such that it leaves the dryer at a comparatively lowtemperature.

The efficiency of the dryer is largely dependant on thedifferential between the inlet and exhaust gas temperatures,although the heat transfer rate is also influenced by therelationship between the design of flights and the speed ofrotation. However, irrespective of the gas and materialtemperatures the drying (or residence) time may be critical, asthis is governed by the rate of diffusion of water from the coreto the surface of the material.

For special applications, the rotary cascade design is adaptedto provide a shortfall, or Cruciform, lifter configuration.Similarly, Rotary Cascade Coolers and Combined Dryer/Coolers areavailable as required.

Feed

Air Heater

Support Rollers

Typical Co-Current Dryer Installation

3The choice of dust collectionequipment will vary accordingto the application, but inaddition to the range of highefficiency and high capacitycyclones, a bag filter, wetscrubber or scrubber/condensercan be supplied as required.

To Atmosphere

Dust Collection

Dust Discharge

Drive Unit Product Discharge

4Co-Current PrincipleCo-Current dryers are the most widely used and areparticularly suitable for drying materials containing a highmoisture content, which are heat sensitive or have a tendencyto stick or cake.

The wet material is in contact with the gas at its highesttemperature, which rapidly evaporates surface moisture. The initial heat transfer rate is high, causing an immediate and considerable drop in gas temperature, which preventsoverheating of the material and the dryer shell.

The final product is in contact with the gas at its lowesttemperature, enabling the moisture content to be readilycontrolled, usually by maintaining the dryer exhaust gastemperature at a pre-set value.

Counter-Current PrincipleCounter Current dryers are more suitable for materials thatmust be dried to very low levels of moisture, where the lasttraces of moisture are difficult to remove, or where an elevatedproduct temperature is desirable. They are also usedeffectively as combined dryer/preheaters.

However, since the final product is in contact with the gas atits highest temperature the counter-current dryer is oftenunsuitable for heat sensitive materials.

Although this system can be more efficient, moisture which isto remain in the product, is not so readily controlled.

Heating SystemIn both co-current and counter-current systems, the material isin direct contact with the hot drying gases, which are normallysupplied as products of combustion from an oil, gas or solidfuel fired furnace in the temperature range 250C to 1000C, as required.

However, for low temperature and heat sensitive applications,or where contamination must be avoided, indirectly heated aircan be supplied via an electric or steam tube type exchanger.

The dryer normally operates on a two fan, balanced draughtsystem, whereby the gas inlet is maintained under slightlynegative pressure to minimise air in-leakage.

When drying (and/or preheating) materials which areunaffected by heat or exposure to a flame, an oil or gas burnercan be arranged to fire directly into the drum.

The combined effects of the very high temperature (800C-1300C) and the radiation from the flame, ensures maximumthermal efficiency while reducing the size of dryer and thecapacity of the exhaust system.

In this case the quench (or dilution) air is induced around theburner by the exhaust fan, thus preventing overheating of thedryer shell.

Direct Fired Dryers

Time

C

Gas Temp.

MaterialTemp.

C Gas Temp.

MaterialTemp.

Time

5Co-Current ApplicationsWet feed in contact with the hottest drying gases suppliedfrom an external source, where heat transfer isby convection.

Suitable for filter cakes, minerals, fertilisers,beet pulp, de-greased bone, floatation concentrates, coal/coke,clays, phosphates, animal feeds, germ, stillage, sludges.

Wet feed in contact with the hottest drying gases from aninternal burner, providing higher thermal efficiency, whereheat transfer is by convection and radiation.

Suitable for heavy ores, crushed rock and stone, sand, slag,refractory materials, limestone/clay separation.

Counter-Current ApplicationsFinal product in contact with the hottest drying gases suppliedfrom an external source, where heat transfer isby convection.

Suitable for silica gel, sugar, chemical salts and crystallineproducts (low temperature range) ammonium nitrate, ores and minerals, pigments, removal of floatation reagents.

Final product in contact with the hottest drying gases suppliedfrom an internal burner, providing even higher thermalefficiency, where heat transfer isby convection and radiation.

Suitable for roadstone, sand, gravel, crushed rock, limestone,slate, pigment fixing, combined drying/pre-heating andcalcining.

Gas Recycle and Integrated systemsFor even greater thermal efficiency, or when potentially hazardousmaterials are processed, exhaust gas recycle is often employed.On direct fired systems this facility provides for the recycling ofa high proportion of the dryer exhaust gas back to the air heater,as dilution. The high humidity creates a safer, inert environment,by displacing most of the oxygen with water vapour.

In addition, substantial energy savings are achieved due to the recovery of heat from the dryer exhaust, while greatlyreducing the final off-gas volume.

As a result, further dedusting, heat removal or odour treatmentof the humid off-gas can then be more economically achievedby integration of a wet scrubber, scrubber/condenser orevaporator system, with optional regenerative thermal oxidiser.3

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Vapour

Rotary DryerGas Heated Evaporator

RegenerativeThermal Oxidiser

6Other Rotary Systems

Direct Fired CalcinersDirect Fired, Refractory Lined Calciners are also supplied for hightemperature applications where it is necessary to change thestate of the material. Since this process usually requires along residence time, the length to diameter ratio is often inexcess of 10:1. In most cases an oil or gas burner fires directlyinto the discharge end of the unit. The material is heated inthree ways - by radiation from the burner flame, conductionfrom the refractory lining and convection by contact with thehot gases.

Indirectly Heated Units

Indirectly Heated Rotary Dryers and Calciners are available fordusty, finely divided materials and low to mediumtemperature calcining applications. The rotary drum ispartially enclosed and externally heated by a series of burnersmounted in an insulated stationary chamber or jacket.Alternatively, the products of combustion from an externalheater may be recirculated to increase thermal efficiency. Sinceheat transfer is by radiation and conduction via the cylinderwall, the internal flights are arranged to distribute the materialaround the inner periphery of the drum. The entrainment offines is minimal as a low air flow is only required forremoving the water vapour, little or no de-dusting equipmentis therefore necessary.

Rotary CoolersThe Cascade Cooler always operates in counter-flow, but isotherwise similar to the cascade dryer, the hot gases beingsubstituted by ambient or chilled air, which is in direct contactwith the material. In general this type of cooler is used forlower temperature, coarse product applications or inconjunction with an indirect system.

For dusty and/or high temperature applications, where spaceis limited, or where the material must be cooled to atemperature approaching ambient, Indirect Rotary Coolers canbe supplied. These require either a continuous or recirculatedwater supply and are available in two forms - external sprayand sectional coolers. Either system may be used for coolinghot products discharged from kilns, calciners or ovens attemperatures up to 1000C.

External Spray Type Water Coolers are of simple design andoperate on the principle of heat transfer through the rotatingcylinder wall, which is continuously irrigated by a series ofexternal water sprays mounted above the unit. The water ispartially evaporated as steam, while the excess is collectedfrom the underside of the unit and recirculated.

The Sectional Cooler offers an efficient indirect system, whichhas the advantage of presenting a much larger cooling surface,while maintaining the water in closed circuit. The sectionaldesign ensures that the hot material is in continuous contactwith the water-cooled walls of the vessel. The individualproduct chambers also ensure gentle action thus avoidingdegradation of the material.

Rollers

Cooling JacketProduct Chamber

Trunnion

Water Sprays

Large Cooler

Trough

7Typical Installations

Direct-flametype dryerwith trommelscreen

Oil fired co-currentdryer withbag filter

GEA Group is a global engineering company with multi-billion euro sales and operations in more than 50countries. Founded in 1881, the company is one of the largest providers of innovative equipment and process technology. GEA Group is listed in the STOXX Europe 600 Index.

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GEA Process Engineering

GEA Barr-Rosin

GEA Canada Inc., Barr-Rosin Division92 Prevost Street, Boisbriand, Quebec J7G 2S2, CanadaTel : +1 (800) 561 8305, Fax: +1 (450) 437 6740Email: sales.barr-rosin.ca@gea.com

GEA Process Engineering Inc., Barr-Rosin1600 OKeefe Road, Hudson WI, 54016, United StatesTel : +1 (800) 561 8305, Fax: +1 (450) 437 6740Email: sales.barr-rosin.ca@gea.com

GEA Process Engineering Ltd., Barr-Rosin Division48 Bell Street, Maidenhead, Berkshire, SL6 1BR. United KingdomTel +44 (0)1628 641 700, Fax +44 (0)1628 776 118Email: sales.barr-rosin.uk@gea.com

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