New strategies for building fluid-bed dryers

Fluid-bed dryers and coolers form the output stage of the production process for a large number of products in both the chemical and the food industries. It is vital to handle the bulk materials in such a way that they can be processed further, stored or packed without any impairment to quality.

The degree of drying and the outlet temperature have to be adapted to the specific properties of each product. In addition to drying and cooling tasks, fluid-bed technology is also employed for roasting, agglomerating, temperature control and instantizing, in many applications interacting closely with vibration technology owing to the considerable advantages offered by vibration-assisted processes.

Figure 1: VF 48/6,4 vibrating fluid-bed dryer for milk powder

The criteria for selecting the optimum type of construction today extend far beyond purely functional characteristics. The quality of a drying plant is crucially dependent not just on its fundamental suitability for the intended drying task, but above all on a host of design details that are critical for compliance with hygienic standards, easy cleaning and straightforward handling of all cleaning-relevant components as well as simple adaptation to a variety of products and production conditions.

Figure 3: VR 150/15 natural-frequency vibrating conveyor dryer with 22 m² fluidizing plate for milk powder

In the last few years, VIBRA MASCHINENFABRIK SCHULTHEIS GmbH in Offenbach/Main has delivered numerous, truly pioneering drying plants based on the above-mentioned criteria. In addition to fluid-bed dryers which have to be cleaned thoroughly and completely (Figure 1), and which are consequently often equipped with an automatic CIP system, the firm's engineers have also developed a solution in which only the - bacteriologically highly sensitive - fluidizing plate of the fluid-bed enclosure requires wet cleaning. Figure 2 shows a natural-frequency vibrating conveyor dryer with a 22 m² fluidizing plate which integrates this solution. The fluidizing frame, which is subdivided into several sections, is clamped during operation between the air box and the dryer hood . When the hood is lifted off pneumatically, it can be moved in only a few seconds through a door at the end into the wet area on integrated rollers supported by ball bearings. By using pneumatic drives both to open the dryer hood and to close it again such that it is capable of withstanding vibration, manual intervention by the operator in order to clean the dryer is restricted to gently pushing the fluidizing plate into the wet area. There is no need to dismantle the elastic connecting sle eves for the supply and exhaust air and for feeding and discharging the product.

Figure 3: VF 60/10 VIBRAting fluid-bed dryer with fluidizing plate and drawer-type filter cartridges

Figure 4: The filter drawers and the fluidizing plate can be pulled out of the dryer enclosure when the end door is opened

Figure 5: Fluid-bed dryer with laterally installed filter cartridge

Easily accessible and exchangeable fluidizing plates, usually made of special perforated sheets with a selectable blowing direction and a defined pressure loss, are extremely important for all fluid-bed apparatus. VIBRA SCHULTHEIS has developed a proven drawer technique for this purpose that satisfies both hygienic and handling requirements. In many applications the dust filters are best integrated in the fluid bed, to enable the dust to be returned directly to the product that is to be dried and to facilitate a compact design for the plant as a whole. In order to achieve this, the filters are implemented as cartridges with an optional pneumatic cleaning device. To allow the complete apparatus to be thoroughly cleaned at regular intervals without any dismantling, the cartridge filters are mounted on mobile slide-in units, so that the drawer system also functions effectively when the cartridges are dismantled and replaced. Figure 3 shows a fluid-bed dryer on which both the slide-in filters and the fluidizing plate are designed as drawers. The filters and the plate can then be pulled out of the dryer enclosure, as depicted in Figure 4, when the end door is opened. The pneumatic docking device, which ensures gas-tight installation of the filter cartridges between the exhaust-air hood and the exhaust-air pipe, is another noteworthy feature.

Figure 6: VSK 25-7/7 VIBRAting fluid-bed dryer with an integrated screen and complete air-handling equipment, including a dust filter, for drying and simultaneously screening plastics granules

An alternative concept, in which the filters are mounted directly in the exhaust-air hood, is shown in Figure 5: the filters are installed in the hood laterally owing to a shortage of space. Regardless of the peripheral air-handling equipment, such as heat exchangers, fans and separators or filters, it is important to control the residence time and the bed thickness of the product stream directly on the fluidizing plate. Manually or pneumatically actuated weirs or gates, which can also be supplied in a special version suitable for discharging lumps, are provided for this purpose. The use of frequency-controlled vibrators additionally helps to optimize the drying or cooling processes. The development of vibrating fluid-bed apparatus integrating additional functions over and above the original thermal task was the outcome of calls for more compact machines suitable for use in confined conditions. Apart from the fluid-bed zone, these combined machines also feature a dewatering section and/or a screening section.

Figure 7: "Screen dryer" with hydraulically opened hood

Figure 8: The hood of the fluid-bed dryer with integrated screen can be raised with the aid of a lifting unit to facilitate rapid cleaning

Figures 6, 7 and 8 show machines of this type, also simply referred to as screen dryers. The machines illustrated in Figures 7 and 8 are designed to facilitate rapid cleaning, necessitated by frequent product changes: the hood is raised in the first example by a hydraulic cylinder and in the second with the aid of a lifting unit. Many processes, such as roasting hazelnuts, almonds or coffee beans as well as batch and long-retention-time drying of chemical and pharmaceutical products, demand long, precisely defined and reproducible drying times. Since these requirements cannot be met with continuously operating, rectangular fluid-bed dryers, VIBRA SCHULTHEIS has developed vibrating batch dryers to cater specifically for such applications. Figure 10 shows a plant consisting of two round, vibrating, batch fluid beds for roasting and subsequently cooling chopped almonds. When the set process time elapses, the product - which circulates continuously on the fluidizing plate - is discharged laterally and fed automatically to the next process step.

Figure 9: Two-stage, vibrating, fluid-bed plant for roasting chopped almonds

It is important to verify the theoretical design of all new applications by means of experiments. Assisted by extensive measuring technology and an electronic controller with a touch panel monitor, the laboratory fluid-bed dryer shown in Figure 13 supplies all essential information for designing a production plant. The broad spectrum of proven standard solutions is complemented by numerous special machines. The plants illustrated in Figures 10 to 12 are just a few examples. The fluid-bed dryers for detergents and polymer products depicted in Figure 10 and Figure 11 are fully insulated and feature a round inlet zone in which an agitator can be installed. The 15 m long, natural-frequency fluid-bed cooler shown in Figure 12 is designed for high vertical acceleration, to prevent the product that must be cooled from caking.

Figure 10: VF 60/8 vibrating fluid-bed dryer with welded insulation for detergent powder

Figure 11: VR 110/10 natural-frequency fluid-bed cooler, with a fluidizing plate area of 11 m² for polymer products

Figure 12: VR 110/10 natural-frequency fluid-bed dryer, fully insulated, for cooling rubber flakes

Figure 13: Laboratory fluid-bed dryer for calculating design data