In fluidized bed coating, the pellets and pellets are «liquefied», the coating fluid is sprayed on them and the product is dried. The small droplet size and low viscosity of the atomizing medium ensure uniform application of the product.
Overhead spray coating is used for a wide range of coatings, from general purpose coatings to gastric juice soluble coatings. In overhead spray coating in fluidized bed (in batch and continuous systems), the particles are «liquefied» in a stream of heated air, which flows through a gas distribution grid into the product container. The coating fluid is injected into the fluidized bed through a nozzle in the opposite direction to the air flow (countercurrent, from top to bottom). With further upward movement of the particles in the air flow, drying takes place. To carry out the coating process, two types of fluidized bed systems are used: batch and continuous. These plants can also be used for the granulation process (see section 4.8). %. Glatt batch fluidized bed plant. The conical shape of the zone where the gas flow velocity is reduced allows even very fine products to be processed. Different variants of the gas distribution grids ensure an optimum gas flow over the product.
Glatt continuous fluidized bed coater. The coating method is particularly suitable for the creation of protective and colored coatings under the condition of high process throughput.
The product is continuously fed from one side of the plant and transported upwards by the air flow over the gas distribution grid. The plant is divided into different zones for different purposes: preheating, spraying and drying. The dry coated tablets are continuously removed from the plant.
Spray coating from below in a batch fluidized bed plant using the HS Wurster Glatt nozzle is particularly suitable for the production of pellets and pellets with controlled release of bioactive substances. When conducting the coating process with the HS Wurster nozzle, complete coverage of the particle surface can be achieved with a small amount of coating agent. The atomizing nozzle is installed in the gas distribution grille so that the atomizing agent moves in the same direction as the air flow. By using the Wurster HS nozzle and a gas distribution grid with different types of perforations, the coated particles are accelerated inside the nozzle and flow uniformly through the atomizing cone.

In a further upward motion, the particles dry out and descend again outside the HS Wurster nozzle to the gas distribution grid. There they are again directed towards the inside of the nozzle, where they are again accelerated by the atomizing liquid jet, thus forming very homogeneous films. As a result, particles of different sizes have a uniform film coating. Spray coating from below in a continuous fluidized bed system developed by Glatt is particularly suitable for the creation of protective and colored coatings under the condition of a high production rate. The product is continuously fed from one side of the plant and transported by the air flow upwards above the gas distribution grid. The plant is divided into different zones: preheating zone, atomization zone and drying zone, with atomization from below. Dry pellets and coated pellets are continuously removed from the plant. Spray coating from below in a fluidized bed system developed by Hiittlin. Thanks to Hiittlin’s Turbojet and Diskjet gas distribution units, the product flows in a defined direction throughout the entire container volume as well as in the spray area. Due to the defined geometry of the units, the air flow can be precisely controlled. Homogeneous product movement and mixing is achieved. Due to the 45° angle of air flow, the strain on the filters is reduced. There are no stagnant zones in the product container.
In fluidized bed apparatuses for coating, the nozzles that atomize the coating agents are of great importance. The nozzle has a third component called microclimate, which is what distinguishes it from the one used in conventional technology. The microclimate keeps the nozzle clean of solution and product. Thus, the distribution and droplet size remain constant and are reproducible. The nozzle is freely accessible during operation, so there is no need to interrupt the process in case of nozzle clogging. Solution losses are reduced due to the appropriate nozzle positioning. INN O JET has developed the AirCoater coating machine, which allows the coating to be applied with a high degree of efficiency. The special feature of this unit is the INNOJET Orbiter gas distribution unit of a somewhat unusual design. The flow of process air passes through the holes of the gas distribution unit, which is a ring of different diameters, assembled in a single structure with gaps between them. This assembly design ensures efficient, gentle and reproducible product flow. Application of the atomized liquid is carried out by a spray nozzle, which can be used to control the droplet size. Atomization takes place at 360°, which allows an even application of the coatings.