WHAT IS A FLASH DRYER?

Flash dryer is a continuous drying plant used for drying variety of wet cakes by disintegrating wet cake into fine powder and dried by hot air to form powder. The whole process is fully automatic requiring no handling or human involvement. The feed rate is adjusted automatically as per the variation in moisture content in wet cake thereby achieving consistent product quality.

The principle of Flash Drying is to evaporate surface moisture instantaneously. The word "flash" is used, as the drying time is very less from milli to few seconds. It is well known fact that the surface area of wet lump increases as the size of lump decreases. The wet cake is disintegrated into fines to increase the surface area. The drying is instantaneous and the material remains at wet bulb temperature of air. Hence it is also called as "wet bulb drying".

The air velocities are similar to that of pneumatic conveying. The powder remains suspended in air and gets conveyed while drying. Hence it is also called as pneumatic dryer. The retention of a given solids particles in the system is very short, typically in seconds.

These make it ideal for drying heat sensitive, easily oxidized, materials that cannot be exposed to process conditions for extended periods.

Our flash dryer is energy efficient, requiring no intermediate material handling. It is a continuous production machine suitable for pasty / sticky wet cakes. These unique features make it ideal replacement for conventional tray and rotary drying.

WORKING PRINCIPLE OF FLASH DRYER

Figure 1 depicts working of Flash dryer

• Wet cake is fed in feed mechanism. The feed mechanism consists of balance tank, stirrer and screw feeder.
• The stirrer feeds the screw feeder. The screw feeder gives controlled feed at rated capacity to disintegrator.
• The air after passing through Air Filter is heated in Oil fired hot air generator or steam / thermic fluid based radiator.
• Hot air and wet cake get thoroughly mixed in the cage mill type disintegrator. Disintegration and drying of wet cake takes place in disintegrator.
• Wet cake is rapidly dried and about 50% drying take place in the disintegrator.
• Drying duct conveys the pulverized material to cyclone. Drying gets completed in conveying duct.
• The time taken by disintegrated powder to travel through drying duct is called residence time of drying.
• The material looses moisture that is absorbed by hot air. The temperature of air reduces while its humidity increases.
• Powder is further dried in cyclone. Separation of dried powder and air takes place in cyclone. Powder is discharged from cyclone through powder discharge valves.
• Fine particles escaped from cyclone are conveyed to bag filter.
• Bag filter recovers the fine particles. The air coming out of bag filter is dust free and confirms to pollution control norms.
• Two blowers, one at inlet & other at outlet, working on Push-Pull principle maintain the airflow in the system.
• Push-Pull principle ensures that the ambient pressure is maintained at feeding system to avoid dusting of material. Also it avoids extreme pressures (either positive or negative) in the system.
• The powder collected from cyclone and bag filter can be pneumatically conveyed to storage bin or bagging section.

• Air filter – HDPE, 3ply of sufficient area for minimum pressure drop.
• Flanged type construction for ease of removal for cleaning.
• Material selected for filter is water washable.
• Manometer is provided on filter for checking of blockage.

2)  HOT AIR GENERATOR

Hot air generator is designed and manufactured Transparent Energy Systems Pvt. Ltd. and have High Thermal efficiency of 93% on NCV & 87% on GCV that gives large saving in fuel cost.

• Optimally sized for the desired airflow rate.
• Low pressure drop.
• Settling vessel is provided to have highest powder separation efficiency.
• Cleaning window is provided with hinged door.
• Air purging system is provided to prevent bridging.

• Secondary disintegrator is provided on-line for further disintegrating the dried powder.
• It is belt driven to adjust the speed, which will control the powder fineness.
• It is an add-on type assembly. Hence can be used whenever required.

• Large filtering area of Bag filter – reduced pressure drop – large savings in power consumption of blower.
• Anti-stick coat on bags – reduced chocking.
• Low CAN velocity – low powder entrainment – reduce chocking.
• Bustle type construction – low powder entrainment – reduced chocking.
• Automatic air pulse jet system – clean filter bags on-line.
• Designed, manufactured & tested for
• Material.
• Airflow rate.
• Pressure drop at no dusting.
• Workmanship.
• Bag removal from top – maintenance in dust free atmosphere.
• Full SS construction (even on exhaust side).
• Bottom cone angle of drying chamber is 45° - powder slides.
• Air bleeding system linked with air temperature at inlet to bag filter for protecting the bags.

• Rotary air valves have very less gap between rotors and stators. This reduces infiltration of air thereby maintaining high powder-separating efficiencies.
• The rotary air valves has back pull type construction. This facilitates easy and quick removal of rotor without necessity of removing drive end.
• Rotary air valves are provided with compressed air purging system. This eliminates chocking of rotary air valve rotor by moist and hygroscopic powders.
• Rotary air valves are provided with oil seals and double Z ball bearings to avoid powder entering in bearings. This increases bearing life and reliability of rotary air valve.
• All Stainless steel construction instead of cast aluminum giving high abrasion resistance and long life.
• Safety grill at outlet is provided for safe operation.  

The Feeding Mechanism feeds the wet cake steadily in metered quantity. The entire feeding mechanism as depicted in Figure No. 2 it consists of following-

See Figure No.2

• Balance Tank

• The Balance Tank holds the wet cake and acts as a buffer tank.

• Stirrer

• Many materials are sticky / pasty having tendency of forming bridge if not kept agitated. Stirrer has special contour to keep the material agitated and force the wet cake positively on screw feeder. This guarantees continuous flow of wet cake to dryer as shown in Figure 3.
• It homogenizes the wet cake and increases uniformity of constituents in dry powder.
• Stirrer speed and blades are designed to suit wet cake properties.
• The agitation increases the flowability of wet cake. This reduces the power required for screw feeder to push the material.

See Figure No. 3

• Screw Feeder

• The screw feeder has pipe type construction with minimum gaps between screw and pipe. This ensures that there is no void space for material accumulation and material move ahead positively as shown in Figure 4.
• The screw feeder remains clean avoiding drying of material in voids. This avoids charring of material in screw feeder due to hot air and subsequent contamination of dried product.
• Angular contact bearings are used to reduce friction, wear & tear, thereby increasing life and reducing power requirement. Further, it eliminates need of thrust bearing making the assembly compact.

See Figure No. 4

• Distributor

• The material is discharged from screw feeder on rotating cage of disintegrator. If the pigment is very pasty / sticky the material falls as a lump on disintegrator rotating cage. The disintegrator will not be able to disintegrate this lump fast, and layer of wet cake will form on disintegrator cages and subsequently chock it. To avoid this, distributor is provided to distribute the material and feed it in small nodule shapes. Figure 5 further explains the distributor function.

See Figure No. 5A

See Figure No. 5B

2)  Cage Mill Disintegrator

The principle of Flash Drying is to evaporate surface moisture instantaneously. The material has to be disintegrated into fine particulate to increase the surface area for heat and mass transfer. Cage Mill Disintegrator does this and hence is the heart of Flash Dryer.

Figure 2 shows the assembly of disintegrator along with feeding system. The construction and working of Cage Mill Disintegrator is depicted in Figure 6.

See Figure No. 6

• It consists of alternate rotating and stationary cages. The cages have close pitched, large number of rods.
• The material and hot air are fed at center of rotating cage.
• It instantaneously disintegrates wet lump in fine agglomerates exposing very high surface area for drying.
• Presence of highly turbulent hot air and intimate mixing of hot air with finely divided agglomerates results reduction of moisture very rapidly. The material looses its stickiness.
• Also strong centrifugal force totally prevents the material from sticking to the rods.
• The mixture of hot air and agglomerates move with high turbulence to the stationary cage. The agglomerates hit the rods of stationary cage with very high velocities and further disintegrate exposing their core to rapid drying.
• Hitting agglomerates dislodge any agglomerate sticking to stationary rods.
• The mixture of hot air and agglomerates undergo similar severe action of disintegration and drying in subsequent rotating and stationary cages.
• At the outlet of last rotating cage material is substantially dried and about 70% moisture is evaporated.
• Both the material and hot air move in radial outward direction. This avoids back mixing of semi-dry material with fresh material.
• The hot air and material moves co-current. The hottest air comes in contact with material having highest moisture. This avoids charring of material.
• These make it possible to use high inlet temperature of air making the plant energy efficient. Further it makes the plant size small reducing the capital cost of equipment, civil cost and so on.
• The rotating cages are dynamically balanced as per IS 1940 for smooth vibration free performance.
• The disintegration action depend upon rod gaps and impeller speed. To obtain powders of various mesh sizes and thus to increase flexibility of operation, the speed of disintegrator can be changed by variable speed drive or by changing pulleys if made belt driven.
• To avoid dusting of powder from gap between shaft and casing, air sealing is provided.
• The bearing block is robust, fabricated single piece assembly with single setting machining. This ensures prolong life of bearings avoiding maintenance and production loss.
• Cleaning windows are provided on casing that facilitates washing of disintegrator reducing down time.

Combination of cage mill disintegrator with secondary disintegrator make the plant ideal for fine powder applications in organic / inorganic pigments and dyestuff.

3) F. D. Fan and I. D. Fan

• Blowers are designed for the required duty point. This reduces the power requirement of blower.
• The blowers have high efficiency up to 80%.
• The blowers are tested for following parameters-

• Performance
• Vibrations as per IS 1940.
• Rise in bearing temperature.

• The bearing block is robust, fabricated single piece assembly with single setting machining. This ensures prolong life of bearings avoiding maintenance and production loss.
• Access door for inspection / cleaning facilitates maintenance / washing.
• Drain plug is provided for removal of condensate while the blower is running. This avoids plant stoppage increasing productivity.
• The flow rate of air is an important parameter for proper drying. To control the quantity of air, damper is provided at outlet of blower.
• To monitor the performance of blower and in turn that of dryer; manometer tappings at inlet and outlet are provided.
• The blowers are either belt driven or direct driven depending upon blower size and customer requirements.
• Guard is provided at air inlet and on belt to make the installation safe.
• The blower is mounted on rigid base frame and supports for vibration free performance.

4)  Drying cum Conveying Duct

• The bends provided are smooth and curved –

See Figure No. 7

• To reduce pressure drop in the system thereby saving power required for blower
• To avoid settling of powder as depicted in figure 7. On smooth surface powder does not get deposited easily than that on a rough surface. For heat sensitive materials this is very important as deposited material gets charred with time.
• Horizontal run of drying duct is averted to avoid settling of powder as depicted in figure 7.
• The drying duct also functions as conveying duct. The air velocities are similar to that for pneumatic conveying. This ensures that powder get conveyed properly without chocking the duct. Dryer runs continuously at rated capacity.
• Manholes / hand holes are provided for ease of cleaning reducing cleaning and down time of machine.  

5. Control Panel & Instrumentation

• All motors have starters, overload protection relays and necessary fuses / circuit breakers.
• Digital temperature indicators and controllers for air are provided at various points in the air circuit to indicate drying condition in the plant.
• Hot air generator burner is provided with its starter sequence programmer, flame monitoring and failure safety instruments.
• A variable speed drive is provided for material screw feeder that automatically adjusts feeding rate whenever moisture in feed material varies. This is done by a PID temperature controller which senses outlet air temperature at inlet of cyclone and gives signal to variable speed drive controller for increasing or decreasing the feed rate so that set outlet temperature of air is maintained.

• Reduced running cost.
• Continuous production machine without human touch.
• Ideal replacement for tray drying of wet cakes.
• Extremely sticky /pasty wet cake can be handled.
• Particle size as fine as 500 mesh is achievable.
• Low residence time – minimum exposure to heat – retains original product quality.
• On line particle size reduction-eliminating need of separate pulverizer.
• No product degradation or charring.
• Proprietary design of disintegrator ensures uniform mixing of wet cake and hot air giving consistent and homogeneous powder quality.
• Fully Automatic – needs little running attention.
• Negligible material dusting.
• High volume processing.
• Large filtering area of Bag filter – reduced pressure drop – large savings in power consumption of blower.
• Measurement of pressure drops at various places facilitates easy trouble shooting and maintenance.

APPLICATIONS OF FLASH DRYER

Our Flash Dryer is a versatile machine and ideal for wet / pasty press cakes. We list below industries where it can be used.

• Chemicals - Inorganic & Organic
• Pesticides and Agricultural Chemicals
• Dyestuff and Pigments – Organic & Inorganic
• Fertilizers
• Mineral Ore Concentrates
• Technical Ceramics
• Food Industry
• Pharmaceutical Industry
• Agricultural extracts 

HOW WE MAKE FLASH DRYING PLANT HIGHLY ENERGY EFFICIENT?

We ensure lowest drying cost by designing and manufacturing energy efficient Flash Drying Plant. Our Flash Drying plant is highly Energy Efficient due to following-

• Fuel cost is dominant in drying cost. We use 93% efficient Hot Air Generator manufactured by our associate company. This reduces the fuel consumption thereby reducing drying cost.
• The unique cage mill disintegrator with feeding system allows high inlet air temperature without charring the material. This increases the thermal efficiency of dryer decreasing drying cost.
• The use of high inlet air temperature reduces the plant size thereby reducing the capital and civil cost.
• The individual components and sub-assemblies are tested as per "Quality Assurance Plan" and then send to site. This reduces possibility of malfunction of components and sub-assemblies thereby reducing downtime of Plant. Typical Quality Assurance Plan is enclosed for reference.
• We use high efficiency blowers having efficiency up to 80%. This reduces power consumption and drying cost.
• We design and select following components for low air pressure drops-

1. Air filter.
2. Cage Mill Disintegrator.
3. Duct and bends.
4. Cyclone.
5. Bag filter.
6. Hot air generator.

• Provision of on-line secondary disintegrator eliminates the need of separate pulverizer. This eliminates wastage of material and manpower required for intermediate handling. This increases production and reduces drying cost.
• The plant is fully automatic requiring minimum manpower.
• On-line cleaning system like pulse-jet for filter bags cleaning and knockers minimize the down time. This increases productivity reducing drying cost.

There are many minute aspects to reduce the power consumption, increase production yield, and reduce down time. These aspects are explained in detail under the head – "Main Components of Flash Dryer" and "Other Components of Flash Dryer".

HOW WE MAKE FLASH DRYING PLANT EASY TO OPERATE AND DIAGNOSE PROBLEMS?

1)   Easy to Operate

• Fully automatic machine consisting of following

• Appropriate safety interlocks.
• PID temperature controller for ensuring constant air inlet temperature. Steady state of burner giving minimum maintenance of burner.
• PID temperature controller with variable speed drive for screw feeder ensuring constant air outlet temperature.
• Audio alarms for any deviation from the set parameter.
• Continuous monitoring / recording of vital parameters.

• Parameters like airflow rate, fuel firing rate, temperatures and so on, are set once for all for a given material properties at the time of commissioning. Hence it is easy to operate in subsequent runs.

2)  Easy to diagnose problems

• The dryer is easy to diagnose for problems due to following facilities-

• Continuous visual indication.
• Manometers for pressure drops of critical components.
• Viewing windows at strategic locations.

• In our service manual exhaustive troubleshooting chart is given. Prior to handing over the dryer our service engineer educates the operators by simulating various operating conditions.

This makes the Flash Dryer user friendly.

COMPARISON BETWEEN TRAY DRYER & FLASH DRYER

RUNNING COST COMPARISON BETWEEN FLASH AND TRAY DRYER

 COMPARISON OF OUR FLASH DRYER AND SPIN FLASH DRYER

See Figure No. 8

RUNNING COST COMPARISON