Unformatted text preview: moisture content of 8 wt%
Analysis: Use a continuous sheeting or rolling dryer with stretched paper that passes over
steam-heated rollers. Exercise 18.10
Subject: Reasons for drying green wood.
Find: Best way to dry green wood to avoid distortion, cracks, splits, and checks. The green
wood contains from 40 to 110 wt% moisture (dry basis) and must be dried to just under its
equilibrium moisture content, typically 6 to 15 wt% (dry basis).
Analysis: To avoid cracks, splits, and checks, coat the ends with tar. To avoid distortion, dry
slowly and uniformly in a chamber with air at a moderate temperature and controlled humidity.
Use kiln drying in a closed container. Dry lumber machines better, glues better, and finishes
better. Exercise 18.11
Subject: Drying of wet coal.
Find: Types of direct-heat dryers that are suitable for drying coal to a moisture content less than
20 wt% (dry basis) before it is transported, briquetted, coked, gasified, carbonized, or burned.
Can a spouted-bed dryer be used? Does the use of air pose a fire and explosion hazard? Can
superheated steam be used?
Analysis: Fluidized-bed and fixed-bed column dryers are suitable, as is a spouted-bed and belt
or band dryer. Air does pose a fire and explosion hazard in the presence of coal dust particles.
Superheated steam can be used as the heating medium. Exercise 18.12
Subject: Drying of coated paper, films, tapes, and sheets
Find: Types of dryers for removing solvents from water-based or solvent-based coatings from
coated webs, such as coated paper and cardboard, coated plastic films and tape, and coated
metallic sheets. Typical coatings are 0.1 mm in wet thickness. Much of the drying time is in the
falling-rate period. Can infrared dryers be used?
Analysis: Yes, infrared dryers can be used. Their advantages over hot-air systems are thorough
drying that is fast and efficient. Infrared dryers are compact with simple installation and
operation, low maintenance, and accurate control. Exercise 18.13
Subject: Drying of finely divided solvent- or water-wet polymer beads of polymers such as
polyvinylchloride, polystyrene, and polymethylmethacrylate, made by suspension or emulsion
polymerization at high production rates
Find: Why rotary, fluidized-bed, and spouted-bed dryers are popular choices, using air, inert
gas, or superheated steam as the heating medium.
Analysis: Rotary, fluidized-bed, and spouted-bed dryers are good choices because they can
handle, at high production rates, low-density polymer beads very well, and can be designed as
closed systems with an inert gas or superheated steam if the beads are solvent-wet. Exercise 18.14
Subject: Comparison of air and superheated steam as heating media.
Find: Advantages and disadvantages of superheated steam compared to air, especially for the
drying of lumber.
Analysis: The use of superheated steam is viable for drying lumber. Of particular interest is the
use of superheated steam under vacuum conditions to lower the drying temperature so as to
avoid problems such as distorti...
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- Spring '11
- The Land