Dryers direct -heat rotary

Fig. 13. Partial view through a direct-heat rotary dryer.

FIG. 12-58 Alternative direct-heat rotary-dryer flight arrangements. 

Design Methods for Direct-Heat Rotary Dryers Direct drying in a direct-heat rotary dryer is best expressed as a heat-transfer mechanism as follows ... 

FIG. 12-59 Component arrangements of a countercurrent direct-heat rotary dryer. (ABB Raymond/Baiilett-Snow TM.)... 

In most cases, direct-heat rotary dryers are still sized on the basis of pilot-plant tests, because rarely is all the moisture to be removed truly Tree moisture, and residence time for diffusion is frequently needed. 

TABLE 12-19 Representative Materials Dried in Direct-Heat Rotary Dryers ... 

Operating costs will include 5 to 10 percent of one worker s time, plus power and fuel required. Yearly maintenance costs will range from 50 to 10 percent of total installed costs. Total power for fans, dryer drive, and feed and product conveyors will be in the range of 0.5 to 1.0 D. Thermal efficiency of a high-temperature direct-heat rotary dryer will range from 55 to 75 percent and, with steam-heated air, from 30 to 55 percent. 

A representative list of materials dried in direct-heat rotary dryers is given in Table 12-19. 

The thermal efficiency of steam-tube units will range from 70 to 90 percent, if a well-insulated cylinder is assumed. This does not allow for boiler efficiency, however, and is therefore not directly comparable with direct-heat units such as the direct-heat rotary dryer or indirect-heat calciner. 

Continuous Direct-Heat Rotary Dryers A Guide to Performance... 

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). 

McCormick, Gas Velocity Effects on Heat Transfer in Direct Heat Rotary Dryers, Chem. Eng. Progr. 58 57-61 (1962). 

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