Dust extraction systems

F. F. L. Morgan. Dust extraction systems for surface grinding wheels. U.S. Patent no, 2 819 571. W ashington, DC Patent Office, 1958. 

For dust extraction systems, the concentration of solids usually is quite low. For this reason, the methods employed to calculate pressure loss are based on air-only conditions. Comprehensive information is available (ASHRAE, 1985 ACGIH, 1992) to assist the designer in estimating the pressure loss caused by pipe branches, ducts, elbows, etc. 

The concentration of particles inside the pipeline is extremely high and beyond the upper explosible limit (UEL). This forms a natural buffer zone or barrier for any explosion that may occur in a vessel at either end of the pipeline (i.e. flame propagation is unlikely). This could avoid the need for explosion suppression and/or isolation, which would be required normally for a dilute-phase or dust extraction system. 

Examples of local exhaust ventilation system are drawdown exhaust tables, slot hoods, dust extraction systems, and portable vapor and dust collectors. Care must be taken that the ventilation systems are utilized and maintained as designed. The condition of the filters and air flow rate should be checked periodically as well as the condition of the duct work, motors, belts, etc. 

Dust Extraction. Adequate dust extraction is necessary to maintain high-speed operation for extended periods of time. The entire compression area should be shrouded to minimize dust infiltration into other press areas. Effective dust extraction minimizes dust and oil contamination on the surface of the tablets, which could produce black specs. Insufficient dust extraction results in excessive material build-up on the lower and upper punches leading to tight punches. However, the proper balance of dust extraction without high levels of material loss must be determined. If the dust extraction level is too high material could be extracted from the die cavities and the recirculation channel. Furthermore, the dust extraction systems preferentially removes the fine particles. Therefore, if the granulation is a direct-compression blend where the active constituent is of fine particle size, minimum dust extraction levels combined with minimal recirculation may be necessary to prevent a loss of active constituents (resulting in possible low assay). 

Excessive material loss can be avoided by ensuring that the excess material stripper is flush against the die table. Otherwise the material will be sucked into the dust extraction system. 

If the insertion depth is too shallow relative to the fill depth, material will be pushed out of the die and lost to the dust extraction system. 

Central dust extraction systems should be interlocked with the appropriate air handling systems, to ensure that they operate simultaneously. 

Air should not flow from the room with the higher pressure to the room with the lower pressure, via the dust extract ducting (this would normally occur only if the dust extraction system was inoperative). 

Exhaust air discharge points on pharmaceutical equipment and facilities, such as from fluid bed driers and tablet-coating equipment, and exhaust air from dust extraction systems, carry heavy dust loads and should be provided with adequate filtration to prevent contamination of the ambient air. 

Where reverse-pulse dust collectors are used for removing dust from dust extraction systems, they should usually be equipped with cartridge filters containing a compressed air lance, and be capable of continuous operation without interrupting the airflow. 

There should be no failure of a supply air fan, return air fan, exhaust air fan or dust extract system fan. Failure can cause a system imbalance, resulting in a pressure cascade malfunction with a resultant airflow reversal. 

Doors of electrical cabinets should be cleaned free of carbon fiber debris using a vacuum cleaner prior to opening or, as a better option, using a factory installed dust extraction system with portable cleaning tool attachments. It must be remembered that if a portable vacuum cleaner is used, although it sucks in at one end it will blow out the other. This air exiting the... 

Dust is a hazard it tends to be generated by dry materials at all open transfer points, which must usually be safeguarded either by the provision of dust extraction systems, or by wetting the material. Combustible dust (coke, feed-grain) may be an explosion hazard. 

Some pultrusion lines include dust extraction systems for reasons of both environment and tidiness take-off systems that are used to handle and store the produced parts and/or tuimel heaters used for post-die curing operations that improve the physical and mechanical properties of pultruded parts. 

A combustible dust has a lower flammability concentration limit in air at 20°C of 0.9% by volume. A dust extraction system operating at 2m /s is found to have a dust concentration of 2% by volume. What minimum flow rate of additional air must be introduced to ensure safe operation ... 

Infra-red detectors have the ability to scan large areas and identify the wave length and pattern of radiation given out by a flame. They can also be used as spark detectors to protect dust extraction systems. 

Next, the level of exposure should be assessed, and this may include consideration of the lead in air levels, the blood-lead results and the degree of contamination of the hands and face. The assessment should include information about the control measures, including any dust extraction system used, its maintenance schedule and efficiency. Finally, a check should be made of any past records. 

Protection against rain was elementary. On many systems that had dust-extract systems, no further provision was made. On others a coarse weather louvre, often of plastic, was provided, as shown in Pig. A-40. Sometimes a partial weatherhood was provided, sometimes not. 

The use of dust extract systems only where essential... 

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