Dryers indirect-heat rotary steam-tube

Indirect-Heat Rotary Steam-Tube Dryers Probably the most common type of indirec t-heat rotaiy diyer is the steam-tube diyer (Fig. 12-69). Steam-heated tubes running the full length of the cylinder are fastened symmetrically in one, two, or three concentric rows inside the cylinder and rotate with it. Tubes may be simple pipe with condensate draining by gravity into the discharge manifold or bayonet-type. Bayonet-type tubes are also employed when units are used... 

Design Methods for Indirect-Heat Rotary Steam-Tube Dryers... 

Performance and Cost Data for Indirect-Heat Rotary Steam-Tube Dryers Table 12-22 contains data for a number of standard sizes of steam-tube diyers. Prices tabulated are for ordinaiy carbon steel construction. Installed costs will run from 150 to 300 percent of purchase cost. 

Design methods fir indirect heat rotary steam-tube dryers Heat-transfer coefficients in steam-tube dryers range from 30 to 85 W/(m - K). Coefficients will increase with increasing steam temperature because of increased heat transfer by radiation. In units carrying saturated steam at 420 to 450 K, the heat flux UT will range from 6300 W/m for difficult-to-dry and organic solids to 1890 to 3790 W/m for finely divided inorganic materials. The effect of steam pressure on heat-transfer rates up to 8.6 x 10 Pa is illustrated in Fig. 12-61. 

For indirect-heat rotary steam-tube dryers the values of heat transfer coefficients may range from 30 to 90 W/(m K) depending on the dryer rotational speed, flow rate of air (used to carry away evaporated vapor), and steam pressure used (Moyers and Baldwin, 1997 Vega et al., 2000). In units carrying... 

Rotary dryer, (continuous, indirect convection) atmospheric, steam tube, c/s including shell and flights, motor, drive, intake hopper, discharge. FOB 155000 at heat transfer area = 100 m with n = 0.75 for the range 40-700. L-rM = 2.2-3. L/M = 0.4. Alloy cost factors c/s X 1.00 s/s shell and flights X 1.25. 

Rotary drum vacuum filters, 11 345, 355-357. See also Drum filters Rotary dryers, 9 121-122 indirect-heat, 9 129 steam-tube, 9 129-130 Rotary fiber spinning process, 13 387 Rotary furnace technologies, 21 391 Rotary hearth furnaces, 13 177—178 Rotary impeller vane meters,... 

Equipment commonly employed for the drying of solids is described both in this subsection in Sec. 12, where indirect heat transfer devices are discussed, and in Sec. 17 where fluidized beds are covered. Dryer control is discussed in Sec. 8. Excluding fluid beds this subsection contains mainly descriptions of direct-heat-transfer equipment. It also includes some indirect units e.g., vacuum dryers, furnaces, steam-tube dryers, and rotary calciners. 

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. 

Conduction or indirect dryers are more appropriate for thin products or for very wet solids. Heat for evaporation is supplied through heated surfaces (stationary or moving) placed within the dryer to support, convey, or confine the solids. The evaporated moisture is carried away by vacuum operation or by a stream of gas that is mainly a carrier of moisture. Vacuum operation is recommended for heat-sensitive solids. Because the enthalpy lost with the drying air in convective dryers is large, their thermal efficiency tends to be low. For conduction dryers the thermal efficiency is higher. Paddle dryers for drying of pastes, rotary dryers with internal steam tubes, and drum dryers for drying thin slurries are examples of indirect dryers. 

The rotary dryers (direct-heat dryers and kilns) are controlled by indirect means, e.g., by measuring and controlling the gas temperatures in their two ends, whereas shell temperature is measured on indirect calciners, and steam pressure and temperature as well exit gas temperature and humidity are controlled on steam-tube dryers. It is not possible to achieve control by measuring the product temperature because not only this is difficult but also its changes are slowly detected, although the product temperature is used for secondary controls. 

An improvement over the standard rotary dryer is the steam tube rotary dryer. Here, two or three rows of steam tubes are located in concentric circles within the shell, which extend the full length of the cylinder. The tubes together with a series of small radial flights serve to agitate the material for uniform drying. These types of dryers were used in the polymer industries for heat-sensitive polymers requiring indirect heating. 

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