Dust seal

It may be necessary to contain dust by enclosing a weigh hopper and using dust seals or flexible connections to seal openings. Figure 19 shows an arrangement where the top of the hopper is fixed to the stmcture, and the hopper must have an effective vent which minimizes even transient pressure... 

All presses equipped with punch-lubrication systems require oil and dust seals to prevent oil contamination in the product and dust contamination in the turret pimch sockets. These seals are normally double lipped, designed to strip oil on one side and powder on the other. As mentioned previously, some press designs use the lower pimch seals to retain the lower punches. 

Covers polyurethane foam pressure-sensitive adhesive tape used for weather stripping, dust sealing, damping, thermal insulation, sound absorption, and packaging. The specification was validated 6 July 1988. 

Screens The rate of dust generated by screens cannot be altered. However, properly enclosing the screen can reduce dust emissions. A complete enclosure that can be easily removed for maintenance and inspection should be used. Some screen manufacturers provide sheet-metal covers to enclose the top of the screen. These covers are effective when properly maintained. However, they do not provide a dust seal between the moving screen surfaces and the stationary chutes. 

Screw Conveyors Normally, screw conveyors are totally enclosed except at the ends, where emissions can be controlled by proper transfer chute design. To maintain a proper dust seal, a neoprene rubber gasket should be installed on the trough cover. Many manufacturers provide two-bar flanges and formed-channel cross members that make a continuous pocket around the trough. The flange-cover sections are set in this channel. Once the channel section is filled with dust, an effective dust seal is created. 

Polonium can be mixed or alloyed with beryllium to provide a source of neutrons. The element has been used in devices for eliminating static charges in textile mills, etc. however, beta sources are both more commonly used and less dangerous. It is also used on brushes for removing dust from photographic films. The polonium for these is carefully sealed and controlled, minimizing hazards to the user. 

Polyacrylate elastomers find limited use in hydrauhc systems and gasket apphcations because of their superior heat resistance compared to the nitrile mbbers (219,220). Ethylene—acrylate copolymers were introduced in 1975. The apphcations include transmission seals, vibration dampers, dust boots, and steering and suspension seals. Further details and performance comparisons with other elastomers are given in reference 221 (see also Elastomers, SYNTHETIC-ACRYLIC ELASTOTffiRS). 

Thermal drying has been studied in conjunction with a rail shipment of ca 1200 km from North Dakota to Illinois. Oil was appHed at 6-8 L/t to suppress dust loss, and cracks around the doors in the base of the cat were sealed to prevent ignition. Stable shipment and stockpiling were then possible (31). Thermal drying may be carried out to further reduce the moisture content as requited for briquetting or for mote efficient pulverizing and combustion. 

Gas flow in these rotary dryers may be cocurrent or countercurrent. Cocurrent operation is preferred for heat-sensitive materials because gas and product leave at the same temperature. Countercurrent operation allows a product temperature higher than the exit gas temperature and dryer efficiency may be as high as 70%. Some dryers have enlarged cylinder sections at the material exit end to increase material holdup, reduce gas velocity, and minimize dusting. Indirectly heated tubes are installed in some dryers for additional heating capacity. To prevent dust and vapor escape at the cylinder seals, most rotary dryers operate at a negative internal pressure of 50—100 Pa (0.5—1.0 cm of water). 

To prevent dust and vapor escape at the cylinder seals, a negative internal pressure of 50—100 Pa (0.5—1.0 cm of water column) is maintained. 

Dust entrained in the exit-gas stream is customarily removed in cyclone cohectors. This dust may be discharged back into the process or separately cohected. For expensive materials or extremely fine particles, bag collectors may follow a cyclone collector, provided fabric temperature stability is not hmiting. When toxic gases or solids are present, the exit gas is at a high temperature, the gas is close to saturation as from a steam-tube diyer, or gas recirculation in a sealed system is involved, wet scrubbers may be used independently or following a cyclone. Cyclones and bag collec tors in diying applications frequently require insulation and steam tracing. The exhaust fan should be located downstream from the cohection system. 

Three examples of ordinaiy gas seals are shown in Fig. 12-61. On direct rotary dryers, few gas seals are intended to be completely gastight, but by careful control of the internal pressure, generally Between 0.25 and 2.5 mm of water below atmosphere, dusting to the outside is prevented and in-leakage of outside air is minimized. 

Figure 12-61 also illustrates three basic types of trunnion rollbearing assemblies. Antifriction pihow blocks are the most common on modern diyers however, when the dryer load requires larger than a 12.7- to 15.2-cm-diameter bearing on the trunnion shaft, the dead-shaft antifriction bearing is substituted. This represents a considerable cost saving compared with the larger pillow blocks. They are completely sealed and continuously bathed in lubricant. Pillow-block bushings are less often used. The thrust washers are difficult to seal against dust, and they draw more power. Thrust roll mountings are depicted also in Fig. 12-61. These are usually dead-shaft.

Vapors (from drying) are removed at the feed end of the dtyer to the atmosphere through a natural-draft stack and settling chamber or wet scrubber. When employed in simple drying operations with 3.5 X 10 to 10 X 10 Pa steam, draft is controlled by a damper to admit only sufficient outside air to sweep moisture from the cylinder, discharging the air at 340 to 365 K and 80 to 90 percent saturation. In this way, shell gas velocities and dusting are minimized. When used for solvent recovery or other processes requiring a sealed system, sweep gas is recirculated throu a scrubber-gas cooler and blower. 

Electrode insulators must also be designed for a particular service. The properties of the dust or mist and gas determine their design as well as the physical details of the installation. Conducting mists require specif lowances such as oil seals, energized shielding cups, or air bleeds. With saturated gas, steam coils are frequently used to prevent condensation on the electrodes. 

At present, form-fill-seal is limited to products having reasonably free-flowing particles with low dust concentrations. Because heat seal-... 

Steel mills Blast furnaces charging, pouring CO, fumes, smoke, particulates (dust) Good maintenance, seal leaks use of higher ratio of pelletized or sintered ore CO burned in waste-heat boilers, stoves, or coke ovens cyclone, scrubber, and baghouse... 

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