Drying equipment drum dryers

The standard conventional finishing process has been modified to suit the purposes for different fabrics and garments. For example, tubular knits are frequently handled using specialized equipment to control tension and to get adequate padding. Some tubular knits are subjected to wet-on-wet padding, dried and cured in large drum dryers, and steam-treated to achieve a relaxed and nondistorted knit. 

The process of sublimation is analogous to the drying of solids so much the same kind of equipment is usable, including tray dryers (Fig. 9.6), rotary tray dryers (Fig. 9.8), drum dryers [Fig. 9.11(b)], pneumatic conveying dryers (Fig. 9.12), and fluidized beds (Fig. 9.13). The last of these requires the subliming material to be deposited on an inert carrier which is the fluidized material proper. 

Fig. I. Single-drum dryer (atmospheric). Dryers of this type may be dip or splash fed (not shown), or, as shown, equipped with applicator rolls. The latter is particularly effective for drying high-viscosity liquids or pasty materials, such as mashed potatoes, applesauce, fruit-starch mixtures, gelatin, dextrine-type adhesives, and various star dies. The applicator rolls eliminate void areas, permit drying between successive layers of fresh material and (omi me product sheet gradually. While single applications may dry to a lacy sheet or flakes, the multiple layers generally result in a product of uniform thickness and density with minimum dusting tendencies. (Bujiovak Division, BlawKnox Food Chemical Equipment. Inc)...

Fig. 2. Double-drum dryer (uttiiospheriei. Dryers of this type handle a variety of food products of widely varying densities and viscosities dilute solutions, heavy liquids, or pasty materials. A number of products can be dried successfully with this kind of configuration, inasmuch as exposure to temperature above the boiling point is restricted to just a few seconds. The movable dram permits effective control over product film thickness. Feed may be hy perforated lube mrngh. pendulum, or various special configurations. (Bufiovah Division. Blau -Knox hunt dc Chemical Equipment, Inc)...

Removal of 95% or more of the water from a material, usually a foodstuff, by exposure to high temperature by various means. Its primary purpose is to reduce the volume of the product, increase its shelf life, and lower transportation costs. Special equipment for dehydration includes tunnel dryers, vacuum (shelf) dryers, drum dryers, etc., in which the bulk product is exposed to a hot-air environment. Another method is spray drying, in which a liquid product is ejected from a nozzle into hot air dried milk and egg white are prepared in this way. The term dehydration is not applied to loss of water by evaporation or sun drying. See drying. 

When drying must be carried from solution to solid under low-temperature conditions, vacuum drum dryers are sometimes employed. Such equipment is similar to the single-drum dryer mentioned above, except that the drum must be enclosed in a chamber capable of evacuation. Furthermore, means must be provided for discharging the solids without breaking the vacuum. 

The drum dryer is a highly flexible equipment because its operational variables like the steam pressure, the drum rotational speed, the nip width, and the ratio of drum rotational speeds can be regulated independently. The steam pressure ranges from 2 to 7 bar, the drum rotational speed varies from 2 to 30 rpm, the nip width ranges from 0.05 to 0.5 mm, and the ratio of drum rotational speeds is from unity to 5. The feed can be pre-concentrated and preheated to reduce the drying load but there is a limit to the feed concentration beyond which the sheet may not form well. ... 

FIGURE 43.6 Rotary drum dryers (a) single-pass (Courtesy of Vadeb) (b) dryer equipped with disintegration device (Adapted from Imoto, Y. et ah, Dry. TechnoL, 11(7), 1495, 1993) (c) triple-pass (Courtesy of M-E-C Company) (d) cross-section of a triple-pass dryer. (Courtesy of M-E-C Company.)... 

In both of the processes just described, a crystallizer produces a solid and, following a solid-liquid phase separation, a dryer removes the moisture. In some cases, all three of these operations can be carried out in a single piece of equipment, a spray dryer or a drum dryer, but at the expense of increased utility cost because all of the solvent is evaporated. Such dryers are used extensively to produce dried milk and detergents. For these products, spray dryers are particularly desirable, because the drying process produces porous particles that are readily dissolved in water. Spray dryers can also handle slurries and pastes. 

The material comes off as flakes 1-3 mm or less thick. They are broken up to standard size of about g in. square. That process makes fines that are recycled to the dryer feed. Drying times fall in the range of 3-12 sec. Many laboratory investigations have been made of drying rates and heat transfer coefficients, but it appears that the only satisfactory basis for sizing plant equipment is pilot plant data obtained with a drum of a foot or more in diameter. Usually plant performance is superior to that of pilot plant units because of steadier long time operation. 

For handling large quantities of granular solids, a cascading rotary dryer is often the equipment of choice. If the material is not naturally free-flowing, recycling of a portion of the final dry product may be used to precondition the feed, either in an external mixer or directly inside the drum. Hanging link chains and/or scrapper chains are also used for sticky feed materials. 

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