Dryers heating systems

Heaters can be located in the space between the trms and the dryer housing, where they are not in direct contact with the product, and thermal efficiencies up to 3500 kj/kg (1500 Btu/lb) of water evaporated can be obtained by reheating the air within the dryer. Steam is the usual heating medium. The hi cost of heating electrically generally restricts its use to relatively small equipment. For materials which have a tendency to foul internal heating surfaces, an external heating system is employed. 

Decide on the envelope for the mass and energy balance. Some dryer systems have hot-air recycle loops and/or combustion or steam heating systems. It is not always necessary to include these to understand the dryer operation. 

Start-up Considerations It is important to start up the heating system before introducing product into the dryer. This will minimize condensation and subsequent product buildup on dryer walls. It is also important to minimize off-quality production by not overdrying or underdrying during the start-up period. Proper control system design can aid in this regard. The dryer turndown ratio is also an... 

Platen dryers. Drying systems have been considered that heat the surface of the veneer directly with heated platens (Loehnertz, 1988 Pease, 1980). A single veneer is placed on the lower plate in each daylight of the multiple daylight press and the platens elosed (100-200 kPa pressure on the veneer) so keeping the veneer flat... 

Drying, if required, is accomplished in flash dryers. Heat is produced by burner systems fired with coal dust. The coal is intensively dried in suspension in the vertical dryer tube and collected in dust-collection systems. 

For example, the dryer may be large enough to handle the intended capacity, but the heating system may be too small. 

For the correct estimation of capital investment a complete list of the equipment items is needed. Note that some units lumped in the simulation blocks or neglected as unimportant must be explicitly listed as equipment items, and sized accordingly. Typically these are storage tanks and reservoirs, furnaces and heating systems, special pumps and devices for the transportation of solids, dryers, filters, evaporators, centrifuges, cyclones, etc. These items should be added on the final Process Flowsheet Diagram. 

PPS is used in chemical process plants for gear pumps. It has found application in the automotive sector, in such specific uses as carburetor parts, ignition plates, flow control values for heating systems, and exhaust-gas return valves to control pollution. The material also finds uses in sterilizable medical, dental, and general laboratory equipment, cooking appliance, and hair dryer components. 

Solar dryers are generally classified [14] according to their heating modes or the manner in which the heat derived from solar radiation is utilized. These classes include sun or natural dryers, direct solar dryers, indirect solar dryers, hybrid systems, and mixed systems. 

Infrared heating systems are classified as open type, tunnel type, and closed type. The open type is the simplest system to complement, but its energy loss is the largest. The tunnel type installs infrared sources in tunnel dryers and is operated continuously. The closed type is operated in a closed room, and usually has the highest energy efficiency, as listed in Table 35.2, because the materials are not only heated by direct irradiation of radiative energy from infrared sources but also by the convective heating of air heated by the sources. 

FIGURE 52.12 Spin flash-dryer feed system. (1) Feed vat, (2) agitator, (3) feed screw, (4) feed pump, (5) rotor, (6) air heater, (7) drying chamber, (A) product inlet, (B) heating, (C) air inlet, (D) product outlet. (From APV Crepaco Inc., New York.)... 

Pulse combustion drying combines high economic of direct fire heating system with high intensity of drying of disperse material. Typical power consumption is 3,000-3,500 kJ/kg H2O evaporated (4,500 to -11,500 for spray dryer and 3,200-6,500 for drum dryer) [53,54]. The main advantages of pulse combustion drying are as follows ... 

R. Youngs, The improved energy efficiency of modem spray dryers. Heat Recovery Systems, 6(3) 216-223 (1986). 

Direct heating systems—For dryers containing flammable vapors, direct heating systems are too dangerous to be employed. In such cases, a separate hot air generator should be provided. 

Almost all dryers have to be provided with some type of heating system. Moreover, continuous-duty dryers require feeding and discharging arrangements as well as dust collection units. Therefore, it is necessary to consider some of the potential fire and explosion hazards associated with these systems and steps required to protect them from the hazards. 

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