Air suspension particle coating is a process by which thin coatings are applied to powder particles. The coatings can be formulated to act as permeable barriers to increase powder shelf life or to impart controlled release character.
The production of functional seeds with good physical characteristics depends on the properties of the materials used and the success of the process can be classified as formulation dependent.
Wurster fluid bed film coating is a dynamic process that is influenced as much by application conditions as it is by the materials used in the process. Drying capacity is inherently a central concern of the process and is influenced by atomization properties, solvent volatility, airflow and solvent/solid interaction.
With the knowledge of influence of process parameters on the coating performance factors, the coating process in Wurster coating was better understood. It is anticipated that this successfully developed functional coating seeds by air suspension coating can be optimized for quality end products.
The Wurster processing insert, patented by Glatt in 1992, provides significant improvements over conventional Wurster coating technologies through increased productivity, high spray rates, increased yields and applications with fine particles.
Wurster fluid bed film coating of particulate materials involves repetitive (cyclic) movement of core particles through an atomized spray region in a relatively controlled manner. Each cycle of the movement involves wetting followed by drying and it is a balance of this cycle that provides the appropriate quality and consistency in the product for the critical product parameters. Because this balance involves many critical parameters it is challenging to predict where the balance lies. Nevertheless, an understanding of the parameter relationships provides a predictive tool for filmcoating processes. In this work, filmcoating processes were looked at from an application perspective to impart application guidelines. Included in this evaluation are drying capacity factors such as atomization/droplet size, solvent volatility, airflow, and solvent/solid interactions involving both core and coating materials. The influences of filmcoat and core material properties are also considered. Findings and conclusions are supported by process observations and final product performance.
History and Chronological Literature Review of Fluidized Bed Systems
History (1, 2)
Scattered study to early observations of what is known today is fluidization can be found in published literature as far back on 1878.( Swarbrick J, Boylan J.C, “Fluid bed dryer, granulator and coaters, Encyclopedia of pharmaceutical technology , Marcel Dekker INC, New York , Volume- 6,171-173, 1992)
In 1940s the fluidized solid process was commercialized on a massive scale in the petroleum industry to effect intimate contact between the catalyst and hot vapors in the cracking of heavy hydrocarbons to fuel oil.
The concept of fluidized solids actually arose in the field of catalytic cracking process. The standard oil development company has a lot of contribution in the field of fluidization then Prof. W.K Lewis and E.R Gilliland carried out independent research on flow properties of powdered solids suspended in gases and developed the concept of fluid bed . (Othomer D.F,”Background,History and Future of Fluid bed systems”, Fluidization , Reinhold publishing corporation, New York , 102-115 ,1956). The first commercial plant using the fluid solids technique principle was put into operation in the year 1930 for nonpharmaceutical application but the process was first used for pharmaceutical application in the year 1960 by Wurster. The coating of tablets by spraying the coating solution into a bed of tablets suspended in a stream of warm air was invented by Dale Wursterwhose first patent for the method was filed in 1953 . Granulation of powder in a fluidized bed carried out in 1960 by Wurster . Then the 1980s have seen an explosion in the research, application and commercialization of fluid bed process. (Swarbrick)
2. Concept Of Fluidization (Swarbrick) (Banks, Michael, Aulton, Michael E, “Fluidized bed granulation – A Chronology ” , Drug development and industrial pharmacy , , 17(11), 1437-1463, 1991) (Ylirussi J., Rasanen E, Rantanen J., Mannermaa J.P “The characterization of Fluidization Behavior Using a Novel Multichamber Microscale Fluid Bed”, Journal of Pharmaceutical Sciences , , 93(3), 780-791,2004)
2.1. Introduction (Vazquez .E.R, “Optimization of drying end point measurement for the automation of Fluidized bed dryer using FT-IR Spectroscopy” M.Sc Thesis , , University of Puerto Rico , Mayaguez Campus, 17, 2004, (Lachman L, Lieberman H.A, Kanig J.L, Third edition “Granulation” , The Theory and practice of industrial pharmacy,Verghese Publishing House, Bombay, 58-59, 1991), (www.engr.pitt.edu/chemical/undergrad/lab-manuals ))
Many important industrial processes rely upon intimate contact between a fluid (liquid or gas) and a granular material. (8) In early applications, the fluid flowed through a static bed of granules supported on a grid. provided the material is suitable, great improvement in mixing and contact is achieved if the granule size is properly matched to the upward velocity of the fluid. The particles of material will be supported by the drag forces and the bed is said to be “fluidized”. The fluidized beds show following liquid or fluid like properties (7, 9)
Lighter objects float on top of the bed (i.e., objects less dense than the bulk density of the bed),
The surface stays horizontal even in tilted beds,
The solids can flow through an opening in the vessel just like a liquid,
The beds have a “static” pressure head due to gravity.
Levels between two similar fluidized beds equalize their static pressure heads.
It has a zero angle of repose. (7)
Assumes the shape of vessel that contains it. (7)
A gas-fluidized bed may have the appearance of a boiling liquid. It has bubbles, which rise and appear to burst. The bubbles result in vigorous mixing and a generally horizontal free surface . (8) The motion of the bed varies with the fluid flow rate. At high velocities, particles may become entrained and transported by the fluid. (4)
Fluidization depends upon-
Velocity of air
2.2 Principle Of Fluidization Vazquez
The principle of operation of fluidized systems are based on the fact that if a gas is allowed to flow through a bed of particulate solids at velocity greater than the settling velocity of the particles and less than the terminal velocity for pneumatic conveying and equal to the minimum velocity of fluidization (V mf ), the solids get partially suspended in the stream of upward moving gas. The gas stream negates the gravitational pull due to weight of particles to enable the suspended state of the solid.
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The resultant mixture of solids and gas behave like a liquid and thus rightly solids are called Fluidized. The solid particles are continually caught up in eddies and fall back in a random boiling motion so that each fluidized particle is surrounded by the gas stream for efficient drying or granulation or coating purpose. In the process of fluidization there occurs an intense mixing between the solids or gas resulting in uniform condition of temperature, composition and particle size distribution throughout the bed.
2.3 Theory Of Fluidization (Lachman)
A. Batch Fluid Bed Systems
1)Top Spray Coating
2)Bottom Spray Coating (Wurster Coating)
3)Tangential Spray Coating (Rotor Pellet Coating)
B. Continuous Fluid Bed Systems
1)Top Spray Coating
2)Bottom Spray Coating
2.3.1 Phenomenon of Fluidization
Stages of fluidization:- The stages of fluidization is mostly based on the fluid velocity passing through the particle bed. According to Ridgeway and Quinn (1) the stages of fluidization can be summarized as follows.
1) Static bed
2) Expanded bed
3) Mobile bed
4) Bubble formation
5) Pneumatic transport
2.3.2 Role of Fluidization velocity (9)
A mass of finely divided solids is transformed into a fluidized bed by lifting action of gas passing through it. Thus three stages can be identified in the process of fluidizing a bed of solids basing on the velocity of gas flow through it. They include
1) Fixed bed or Static Bed
2) Expanded bed or particulate fluidization.
3) Mobilized bed
1) When a fluid is pumped upward through a bed of fine solid particles at a very low flow rate the fluid percolates through the void spaces (pores) without disturbing the bed. This is known as a fixed bedprocess .(9)
2) If the upward flow rate is very large the bed mobilizes pneumatically and may be swept out of the process vessel. This is known as Mobilized bed process .(9)
3) At an intermediate flow rate the bed expands. This is known as an expanded bed (9) .
4) After mobile bed formation if velocity is further increased the bed expands considerably with increase in voidage and bubble formation (1) occurs.
5) If further increase in velocity of air occurs, eventually the lifting force of passing air blows particle out of the bed altogether leading to Pneumatic transport .(1)
In the fixed bed the particles are in direct contact with each other, supporting each other’s weight. In the expanded bed the particles have a mean free distance between particles and the particles are supported by the drag force of the fluid. The expanded bed has the properties of a fluid and is also called a fluidized bed.
As shown in Figure below, the velocity of the fluid through the bed opposite to the direction of gravity determines whether the bed is fixed, expanded, or is swept out. This led to the development of the concept of minimum fluidization velocity (V mf ) at which the bed just begins to fluidize. Thus the primary concern is to measure and optimize the V mf for efficient fluidization.
Fixed, Fluidized, and Mobilized beds
(a) Slow flow rate (b) Intermediate flow rate (c) High flow rate
Fixed bed Fluidized bed Mobilized bed
V 0 < V mf V mf â‰¥ Vo < U t V 0 â‰¥ U t
Figure -1. Fixed, Fluidized, and Mobilized beds. (9)
The fixed bed (a) occurs when the approach velocity or superficial velocity, ( V o ) , is much smaller than the minimized fluidization velocity,( V mf ). The fluidized bed (b) occurs when the approach velocity is intermediate between the minimum fluidization velocity and the terminal velocity. The pneumaticallymobilized bed (c) occurs when the approach velocity is much greater than the particle terminal velocity , (U t )
2.3.3 Determination of Minimum velocity of fluidization (V mf ) (8)
The determination of minimum velocity of fluidization plays a vital role in efficient operation of a fluidized bed system. Basing on the nature of size distribution of a solid bed V mf calculation differs.
Pressure drop with decreasing superficial velocity
Figure- 2 (a). Pressure drop with decreasing superficial velocity (8)
Pressure drop with increasing superficial velocity
Figure -2 (b) .Pressure drop with increasing superficial velocity (8)
e.g. if the solid bed contains solids of uniform density V mf calculation is done basing on the equation developed by Ergun (8)
If the solid bed is having wide size distribution of particles (i.e. bed contains solids with differing densities) Vmf determination is analogous to measurement of boiling point of a liquid mixture, the boiling point of mixture is not fixed but varies with composition. Also V mf can be determined using average particle size of the bed.
2.3.4 Role of pressure drop in Fluidization (8,9)
When a fluid flows through a bed of particles in a tube, it will exert a drag force upon the particles resulting in a pressure drop across the bed. As the fluid’s approach velocity is increased, pressure drop is magnified.
In an unrestrained bed with fluid flowing upwardly through it, a condition will be reached where, with increasing fluid velocity, the drag forces will cause the bed to expand. This expansion allows the particles to offer less resistance to the fluid flow. When the drag force is sufficient to support the weight of the particles in the bed, the bed is said to be fluidized . The fluid/solid system shows fluid-like properties, and the bed can be made to flow from one vessel to another.
The pressure drop across the bed, âˆ†p, then remains constant (even with further increase in the fluid velocity) and equal to the effective weight of the bed per unit area
When the unit is operated at pressures comparable to atmospheric, therefore Image is negligible compared with Image . As the gas velocity,V 0 , is raised beyond that required to bring the bed to a fluidized condition, i.e. beyond the minimum fluidization velocity V mf , the bed will begin to bubble. This condition is called aggregative fluidization .(8) If the gas velocity were increased excessively, the bubbles would grow so large that they would nearly or completely fill the cross-section of the tube pushing slugs of particles forming a slugging bed. (8)
If the fluid were more dense, (e.g. a gas at the high static pressure of a liquid), or if the particles were finer (20 to 100 µm) and less dens (<1400 kg m -3 ) , the bed would be able to sustain a degree of stable expansion, also known as particulate fluidization. (8) The bed would remain stable until the V mf had been exceeded by a factor of 2 or 3. In contrast, when using gas to fluidized a bed, the bed would collapse and reinitiate bubbling with further increase in gas velocity. A liquid fluidized bed usually continues to expand stablely with increasing velocity resulting in a non-bubbling fluidized condition known as aquiescent bed (8)
With finer, less dense and cohesive powders, it is very difficult to fluidize the bed at all, because the interparticle forces are then greater than the gravitational ones. The particles tend to stick together, and the gas passes through the bed by blowing channels through it. (Parameters To Be Controlled In Fluid Bed Systems (Aulton M.E., second Edition “Granulation”, Pharmaceutics “The science of dosage form design,Churchill Livingstone , Edinburgh , 373, 2002, Ansel C., Allen L.V.,Popovich N.G.,eighth edition “Tablets”, Pharmaceutical dosage form and Drug delivery system, B.I Publications , India , 193 and 243 , 2005)
The parameters that affect the final product processed through fluidized bed systems can be enumerated as below.
3.1 Apparatus Parameters
1) Air distribution plate Position of the air distribution plate influences the airflow pattern inside the body.
2) Shape of instrument body Annular base gives better product and fluidization.
3) Nozzle height in case of coater and granulator. It plays vital role as in coating, the atomized coating solution should not get dried before reaching the tablet surface.
4) Positive and negative pressure operation
3.2 Process Parameters
3.2.1. In Drying Process
The following inlet air parameters are critical, and applicable in all processes of drying, granulation and coating.
As the inlet air temperature increases the rate of drying increases and vice versa. This approach to increase the rate of drying can not be used always because some materials are harmed by high temperature, e.g. Ibuprofen liquefies above 60°C temperature of inlet air should be optimized without any impact on product quality. If temperature is high, it leads to blistering. If temperature is low, soft spot can be formed.
Humidity in the inlet air should be as low as possible and ideally dehumidified air should be used for faster drying rate because as the humidity of inlet air decreases the rate of drying increases.
3) Air flow rate
Air flow rate should be controlled properly in order to get efficient use of drying air. As the air flow rate increases, the rate of drying increases but the cost of drying also increases. If drying air is allowed for sufficient time to remain in contact with the drying material, proper heat transfer and mass transfer takes place and thus drying cost decreases. Air flow rate should not be too fast or too slow but optimized to have efficient drying.
3.2.2. In Granulation Process.
Related To Spray Nozzle
1) Nozzle position in relation to material height.
Nozzle position is determined on bases of bed height and it should be placed suitably for better contact of binder with the powder to be granulated.
2) Spray rate.
It should be optimized otherwise poor wetting/agglomeration of the product will take place hindering the fluidization and quality granule formation.
3) Spray pressure.
It is important for proper atomization of binder solution.
1) Pressure drop across exhaust filters.
2) Outlet gas temperature.
The above two parameters give indication of the efficiency of the fluidization process. System’s level of efficiency can be drawn from measuring these two parameters.
3.2.3. In Coating Process
Related To Spray Nozzle
1) Distance of spray nozzle.
Efficiency of coating depends on the quality of the coating solution. The coating solution should not get dried before reaching the fluidized substances viz. tablet, particles, and granule surface.
2) Droplet size.
Quality of the coat depends on the droplet size. So it should neither be too big nor be too small.
3) Spray rate.
Flow rate should not be too fast or too slow, but should be of optimized rate for efficient coating.
4) Spray pressure.
Atomization of coating solution depends on the spray pressure, thus for proper atomization droplet size should be optimum.
1) Moisture content in processing compartment. Moisture should not be present in case of hygroscopic materials.
2) Method used for coating should be chosen on basis of the purpose for which it is used. e.g. SR, ER, etc.
3) Time of drying should be determined on bases of the product and quality of the coat desired.
3.3. Product Parameters.
3.3.1. In Drying Process.
1) Initial moisture content of material.
It should not be high otherwise it increases drying time.
2) Batch size.
It should be small and optimized based on feasibility.
3.3.2. In Granulation Process.
1) Granulating agent.
Type of granulating agent is based on selection of solvent to be used in binder solution. This solvent should be preferably aqueous as organic may cause explosions. Binder solution used to granulate the material should be used in optimum concentration so as to obtain good quality of granules. Temperature of granulating agent should not be high otherwise it will be dry before reaching to the powder surface.
2) Starting material.
Fluidization of starting material should be optimized for better contact with the granulating agent. If the starting material hydrophobic, hydrophilic granulating agent is to be used for better contact and granulation of material.
3.3.3. In Coating Process.
1) Coating agent.
Selection of coating agent should be done according to type of coating required e.g. Enteric coating, Sugar coating, etc. Solvent should be selected according to the properties of the coating agent. If solvent is volatile, it should be checked for inflammability. Concentration of granulating agent should be optimized for uniform spreading and droplet formation. Temperature of the coating agent should not be so high that coating solution get dried before reaching to the tablet surface.
2) Starting material.
Shape of tablets greatly affects the coating process. In case of powder coating the particles shape and density affects the coating process.
4. Classification Of Fluidized Bed Systems (Swarbick)
4.1. According To Process Applications
The fluidized bed dryers available for use in the Pharmaceutical industry are of two types,
1) Batch type Vertical Fluid Bed Dryer with Granulating option.
I.Reverse turning bed type
II.Rotating discharge type
2) Continuous type Horizontal Vibrating Conveyor Fluid Bed Dryer.
The fluidized bed dryers available for use in the Pharmaceutical industry are of two types,
1) Top Spray Fluid Bed Granulator,
2) Rotating disk Fluid Bed Granulator with Dryer option.
The fluidized bed dryers available for use in the Pharmaceutical industry are of three types,
1. Top Spray Pellet with Particle Coating option,
2. Fluid Bed Bottom Spray Pellet with Particle Coating option,
3. Fluid Bed Roto Processor with Drug Loading & Coating option.
4. Huttlin Kugel Fluid Bed Coater
4.2. According To Principle
1) Spiral Granulator,
2) Bottom spray coating,
3) Tangential spray roto processor.
Classification according to principle
Figure – 3 Classification according to principle
5. Equipments For Fluidized Bed Systems (8,12-17)
5.1 Fluidized Bed Dryer
Fluid bed drying is most widely used technique for drying pharmaceutical powders and granulation. The direct contact between particles and air/gas is possible in fluid bed system. Here any type of inert gas or air is used. They can be designed in either batch or continuous type fluid bed dryer. Various innovations in fluid bed drying are discussed in section 7.
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The fluid bed drying operates under the principle of direct drying where direct contact between a heated gas/air and the product occur to effect heat and mass transfer. The hot air/gas used for drying can be generated by either steam coils or a combustion furnace. The holes in the perforated plate are covered with caps that prevent material form entering the plenum when the dryer is not in operation. The fan equipped in the upper part of the apparatus induce fluidizing air stream.
Hot air is fed in the material layer at rest from the bottom of gas straightening valve (e.g. perforated plate). The velocity of air is adjusted by means of a damper. When the velocity of hot air accelerates a bit, particle groups gradually come to make active movements gradually resulting in hot air pressure loss due to the material layers comes to be in proportion to the weight of material particles on the unit area of the straightening vane. With further accelerating the velocity of hot air, the particle groups undergo vigorous movement to mix with gas in all directions, resulting in suspension state.
If all the particles are fully supported by air/gas then a bed may be fluidized well. The solid become partially suspended in gas stream when the velocity of gas is greater than the settling velocity of the particles and less than the velocity for pneumatic conveying. The solids in this state are said to be fluidized and the resultant mixture of solids and gas/air behaves like a liquid. The angle of repose of gas-solid mixture is zero and it assumes the shape of the vessel that contains it. In fluid bed drying uniform conditions of temperature, composition and particle size distribution is achieved throughout the bed because of complete mixing between the solids and gas is obtained.
When the material layer approaches to a certain flow velocity (i.e. minimum air velocity for fluidization), it makes a minimal expansion causing particles moving and leading to minimum fluidization.
Various steps in fluid bed drying.
Figure- 4. Various steps in fluid bed drying. (12)
The material later expands to 1.2 to 1.6 times in height that was at resting position. 1 Product height in the fluid bed reaches between 100 to 500 mm (average 300 mm) depending on the application. The material layer behaves as it is liquid and shows an appearance as it is boiling. This state is termed as “fluid state”. This type of fluid bed can be formed within a certain range of flow velocity depending upon particle size, specific gravity and other properties of the material.
Uniform processing conditions are obtained by passing hot air (or other inert gas) through a material layer under controlled velocity conditions to create a fluidized state. Air is filtered after drying in multicyclones and /or bag filters. However, the use of bag filter is troublesome if the dryer is often used for different products because careful cleaning of the dryer is required.
The dry product overflows an adjustable weir plate and is discharged continuously through a rotary air lock. In fluid bed drying, airflow need not be the only source of heat. Heat can also be effectively introduced by heating surfaces (panels or tubes) immersed in fluidized layer. (14)
Fluid bed dryer can also be constructed with an integral cooling system thus avoiding the need for a separate cooler in those applications that require one. In fluid bed cooling usually ambient or conditioned air is used. Another option is cooling surfaces immersed in the fluidized layer. Conditioning of the air may be required to obtain sufficient product cooling in an economically sized plant and to prevent pick up of volatiles including moistures. (14)
Fluid bed dryer is suitable for powders granules, agglomerates and pellets, having average particle size normally between 50 and 5000 micron.
The material containing very fine, light powder (low density) or highly elongated particles may require vibration for successful fluid bed drying. (14)
Most fluid bed dryers are single pass system where the process gas passes through the bed only at once and is exhausted to atmosphere. Depending on the economic feasibility of the operation, some systems can be designed for recirculation or recuperation. The larger particles that fall out of bed are introduced back into the bed or propelled along the length of dryer by vibrating action. Vibrating systems decrease power requirements for fluidization and thus reduce operating cost. (15)
In indirect fluid bed processing, a tube or series of plates are incorporated into the drying chamber. They are arranged in such a way that products remain in intimately contact with the heated surface. The heat energy is transferred by means of conduction. Here steam is used as a source of energy. The cost of operation is low because steam is low cost energy source. (15)
A fluid bed dryer is capable to use almost any heat source. As the temperature of the process gas is increased, the volume of air required is small and the unit required is also small. (15)
With the correct design, fluid bed dryers can withstand at extremely high temperatures, providing the potential for calcining. Incorporation of refractory lining the box, drying chamber or expansion chamber is required with these designs. With independent control of airflow and temperature, the dryer can be divided into several different zones. This design is useful for sensitive products or where altering the inlet temperature can benefit the process. The advantage of this design is that drying can take place at the maximum desirable rate in each stage by maintaining efficiency and preventing damage to the heat sensitive materials. The width of dryer ranges from 12 to 57 inches and length ranges from 10 to 50 feet. The bed depth is about 3 inches. Dryer capacity is dependent only on retention time produced by speed of conveying, which generally ranges from 5 to 25 feet per minute.
The fluid bed dryer can be operated in either open or closed cycles. Using a solvent recovery system & an inlet gas like nitrogen as the drying medium, the operation of the fluid bed dryer can be carried out in a closed cycle. A cyclone or febric collector, and a condenser to remove the solvent clean vent gases. The cooled, saturated gases can be heated and utilized further. (16)
5.1.3 Types Of Fluid Bed Dryer (8,17)
(1) Batch Type Vertical Fluid Bed Dryer With Granulating Option.
In batch-type dryer, the drying chamber is equipped in such a way that it can be removed from unit to permit charging and dumping. The dryer is capable of drying 5 kg to 200 kg material with an average drying time of about 20 to 40 min.
Batch fluidized bed dryer
Figure-5. Batch fluidized bed dryer (6)
I. Reverse Turning Bed Type (17)
In this equipment, by turning the gas dispersion plate (the reverse turning bed) in 90° direction with the control motor, all the dried material can be discharged at once.
II. Rotating Discharge Type (17)
Dried material is discharged by opening the discharge gate equipped at the side of the Dryer. As the perforated plate is used as the gas dispersion plate, the gas inside the equipment whirls and pushes the dried material out from the discharge gate.
Characteristics Of Batch Type Fluidized Bed Dryer (17)
The residence period of the dried material can be controlled which results in uniform drying. It is most suitable in case where an accurate control of the residence period is required at the decreasing rate drying zone. Small destruction of particle occurs therefore suitable for granular or crystallized material.
Easy operation can be achieved by an automatic control of material feeding, drying discharging etc. When multiple stage system us adopted, the exhaust gas heat can be used efficiently.
(2) Continuous Type Horizontal Vibrating Conveyor Fluid Bed Dryer. (17)
The dried material is moved to a next during chamber through a gap at the bottom of the partition plate and after finally dried, the material is discharged over the overflow gate. For large volumes of materials, a continuous dryer is more suitable than a batch type. The continuous Fluid bed dryer which is suitable for pharmaceutical use is horizontal vibrating conveyer dryer shown in figure-7.
The heated air enters the chamber below the vibrating conveying deck. The air then passes through perforated conveying surface and enters into the wet bed of material and causes fluidization of the particles.
Due to vibrating movement of the conveyer, a fluidized bed of uniform density and thickness is maintained in any given drying zone.
Residence time in any drying zone is dependent on
(1) Length of the zone
(2) The frequency and the amplitude of the vibration
(3) Use of dams
Horizontal multiple chambers fluidized bed drying and cooling system
Figure -6. Horizontal multiple chambers fluidized bed drying and cooling system (12)
Heat Transfer Unit Built In Continous Fluidized Bed Dryer (17)
Heat transfer units such as tube or plate, are built inside the equipment. These unit supplies 60-80 % heat necessary for drying. Thus the quantity of hot air is decreased, reducing the power consumption and operating cost. The equipment becomes compact. The auxiliary equipment can also be miniaturized.
Characteristics Of Continous
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