Central vacuum cleaning in mineral plants collects dust from multiple pickup points through a designed pipe network and filtration system. Even when effective local exhaust ventilation is installed, some material may escape from transfer points, inspection doors, feeders, conveyor galleries and maintenance activities.
Once deposited on floors, beams, cable trays and equipment surfaces, this dust can become airborne again through vehicle movement, vibration, sweeping or compressed-air cleaning. Depending on the material, it may create occupational exposure, mechanical wear, product loss, contamination or combustible-dust hazards.
A centralized industrial vacuum system provides fixed suction points through which deposited dust and small spills can be removed without deliberately dispersing them into the workplace. roots blowers can serve as the vacuum source where the system requires a substantial airflow at moderate or relatively high vacuum.
Roots blowers may also provide the pressure or vacuum needed to transport dust that has already been separated in bag filters, cyclones or process collectors. These are valid but distinct applications and should not be confused with the main fan of a local exhaust ventilation system.
Local Exhaust Ventilation
Local exhaust ventilation captures airborne dust at the point where it is generated. Hoods and enclosures draw a relatively large volume of air at a comparatively low vacuum.
Typical capture locations include:
- Crushers
- Screens
- Conveyor transfers
- Mills
- Dryers
- Bagging machines
- Bucket elevators
- Dump hoppers
- Truck-loading stations
The main air mover is normally a centrifugal process fan selected for high airflow and relatively low pressure. A roots blower is not usually the most appropriate machine for a large conventional dust-extraction network.
Central Vacuum Cleaning
A central vacuum system removes dust that has settled on equipment and building surfaces. It generally operates with lower airflow through each pickup point but with a higher vacuum than a conventional ventilation system.
Typical cleaning locations include:
- Crusher floors
- Conveyor galleries
- Transfer towers
- Grinding buildings
- Packing areas
- Silo galleries
- Filter floors
- Maintenance workshops
- Railcar and truck-loading areas
- Laboratory preparation rooms
The high vacuum makes it possible to transport dust and small pieces of material through comparatively small-diameter tubing over considerable distances.
Collected-Dust Conveying
A dust collector separates solids from a process airstream. The dust accumulating in its hopper must then be discharged and transferred to a silo, process return point, waste container or conditioning system.
A roots blower can supply:
- Vacuum for drawing dust from several collection hoppers
- Positive pressure for conveying dust after a rotary valve
- Air for transporting filter dust back to the process
- Vacuum for recovering product from spill-collection points
The dust must be separated from the air before the gas reaches the blower.
Dust Sources in Mineral Processing
A central vacuum or collected-dust conveying system may handle:
- Crushed ore fines
- Limestone dust
- Silica-containing dust
- Coal dust
- Cement and raw meal
- Fly ash
- Alumina
- Bentonite
- Gypsum
- Lime
- Dry mineral concentrates
- Kiln or furnace dust
- Filter cake fines after drying
- Metal-bearing dust
- Product spilled during bagging
- Dust collected from screens and conveyors
Material identification alone does not provide enough design information. Particle size, particle density, bulk density, moisture, abrasiveness, toxicity and combustibility must also be established.
Typical Central Vacuum Arrangement
A correctly designed central vacuum system may include:
- Multiple hose connection points
- Cleaning hoses and pickup tools
- Branch tubing
- Isolation valves
- Main vacuum header
- Primary dropout vessel or cyclone
- Filter receiver
- Secondary safety filter
- Gas-side Roots Blower
- Vacuum-relief valve
- Discharge silencer
- Dust hopper
- Rotary airlock or double-dump valve
- Dust container or downstream conveyor
- Pressure, temperature and filter monitoring
Dust-laden air enters the separator before reaching the blower. Coarse material is removed in the dropout vessel or cyclone, while fine dust is retained by the filter. The cleaned air then passes through the roots blower.
The system should not rely on the blower to tolerate mineral solids. Abrasive dust entering the rotor chamber can damage rotor surfaces, clearances, seals and bearings.
Pickup Points and Simultaneous Use
Central vacuum networks frequently contain many installed connection points, but only a limited number may operate simultaneously.
The required airflow can be estimated as:
Total Actual Airflow
- Qtotal = total actual airflow required at the vacuum source
- Nsimultaneous = number of pickup points operating at the same time
- Qpickup = airflow required at one pickup point
- Qleakage = air entering through closed valves, joints and unused connections
Sizing the blower for every installed point operating simultaneously may produce an unnecessarily large and inefficient system. Conversely, underestimating simultaneous users can reduce velocity in the branch lines and cause material deposition.
Automatic inlet valves, operator procedures and pressure-based controls can help maintain the intended coincidence factor.
Pickup-Air Velocity
The airflow at a pickup tool must be sufficient to entrain the expected material. Larger or denser particles require greater pickup energy than fine dust.
Pipe velocity is:
Pipe Velocity (Circular Pipe)
- vair = superficial air velocity, m/s
- Qactual = actual volumetric airflow at the evaluated pipe condition, m³/s
- Apipe = internal pipe area, m²
- D = internal pipe diameter, m
- π = 3.1416
Velocity must be high enough to prevent dust from settling in horizontal tubing. Excessive velocity, however, increases pipe wear, pressure loss, noise and particle-impact damage.
Air Expansion Under Vacuum
In a vacuum system, absolute pressure decreases toward the blower. The gas therefore expands, and its actual volumetric flow increases.
At any selected point:
Actual Gas Volume Conversion
- Qx = actual gas volume at location x
- Qreference = actual volume at the reference location
- Preference,abs = absolute pressure at the reference location
- Px,abs = absolute pressure at location x
- Treference = absolute gas temperature at the reference location
- Tx = absolute gas temperature at location x
- Zreference = reference compressibility factor
- Zx = local compressibility factor
The maximum actual airflow and velocity may occur near the vacuum source. Filter and blower sizing should therefore use the appropriate local conditions rather than the atmospheric pickup volume alone.
Glossary
Capture hood: Enclosure or canopy positioned to collect dust close to its emission source.
Capture velocity: Air speed needed at a pickup opening to draw dust into the duct.
Central filter: Shared filtration unit serving multiple vacuum-cleaning branches.
Duct conveying velocity: Gas speed used to keep collected dust moving through a duct.
Dust hopper: Vessel that stores collected solids beneath a separator or filter.
Filter loading: Dust mass captured per unit filter area over a stated operating period.
Local exhaust ventilation: Capture system that removes contaminants near their release point.
Vacuum pickup: Hood or nozzle that collects dust using sub-atmospheric pressure.