Contact drum dryer

Contact Rotary Ttabe Dryer (Contact Drum Dryer) [5.30]... 

Drum dryers for pastes and slurries operate with contact times of 3-12 sec, produce flakes 1-3 mm thick with evaporation rates of 15-30kg/m2hr. Diameters are 1.5-5.0ft the rotation rate is 2-10 rpm. The greatest evaporative capacity is of the order of 3000 Ib/hr in commercial units. 

Some characteristics of drum dryers are given in Table 6.8. In a given application, testing is usually required to establish drum speed, temperature, retention time, product characteristics and feeding equipment. The short time that solid is in contact with hot metal makes these units suitable for heat-sensitive materials. As illustrated in Fig. 6.4, many different feed... 

The rolls of a drum dryer are 0.6 to 3 m 2 to 10 ft) in diameter and 0.6 to 4 m (2 to 14 ft) long, revolving at 1 to 10 r/min. The time that the solid is in contact with hot metal is 6 to 15 s, which is short enough to result in little decomposition even of heat-sensitive products. The heat-transfer coefficient is high, from 1200 to 2000 W/m -°C (220 to 360 Btu/ft -h-°F) under optimum conditions, although it may be only one-tenth of these values when conditions are adverse. The drying capacity is proportional to the active drum area it is usually between 5 and 50 kg per square meter of drying surface per hour (1 and 10 Ib/ft -h). 

The drum dryer is commonly used to dry viscous, concentrated solutions, slurries or pastes on rotating steam-heated drums. It can also be used to dry concentrated solutions or slurries that become more viscous or pasty because of flashing or boiling off of moisture or of irreversible thermochemical transformations of their content that occur on their first contact with the hot drum surface. " ... 

Suction or perforated drum dryers are often used to dry fabrics, particularly nonwovens, and sometimes unbacked tufted carpets. These dryers usually consist of two or more perforated drums mounted horizontally in a compartment (Figure 38.8). Several two-drum compartments are typically linked together to form a complete machine. Fans draw air from the interior of the drums producing suction on the surface area in contact with the material. This suction holds the material to the surface of the drum permitting hot air to pass through the material being dried. A portion of the drum has no suction, which permits the material to transfer to the next drum without interference. 

Drying. The water used as solvent in the polymerization is removed from the polymerized gel by evaporation in continuous-operation, hot-air convection dryers or in contact dryers such as steam-heated drum dryers. In hot-air convection dryers, the rate of moisture removal depends on the heating gas temperature, humidity and flow rate, and the diffusion characteristics of water from the gel. Drying occurs in three general stages. For high water content in the gel, the drying rate is constant, as the rate is limited by heat transfer into the gel. At low water... 

Figure 33.11. At a contact time of 0 = 2 the maximum moisture content difference A< has been reduced to one fifth the value that is found without any reversing of the airflow, with the wettest material now being in the middle of the mass and the outer portion of the drier. In the suction-drum dryer, the radial moisture content profiles are essentially negligible after the fourth drum (three airflow reversals) [58].

Among the contact drying processes in the paper industry are steam heated centrifugal dryers and disk dryers. The types of convection dryers used are drum dryers, belt dryers or rack dryers. 

In sieve drum dryers (convection drying) the hot air is sucked through the fabric into the drum. The machine is a further development of the drum contact dryer. This drying principle provides a careful drying for tension-sensitive fabric qualities. 

Direct heat rotary dryer. The direct heat units are generally the simplest and most economical in operation and construction, when the solids and gas can be permitted to be in contact. In design mode, the required gas flow rate can be obtained from a heat and mass balance. Bed cross-sectional area is found from a scoping design calculation (a typical gas velocity is 3 m/s for cocurrent and 2 m/s for countercurrent units). Length is normally between 5 and 10 times drum diameter (an L/D value of 8 can be used for initial estimation) or can be calculated by using an incremental model (see Examples 21 and 23). 

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