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Dust Control Handbook for Minerals Processing
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Why Dust Control?
After dust is formed, control systems are used to reduce dust emissions. Although installing a dust control system does not assure total prevention of dust emissions, a well-designed dust control system can protect workers and often provide other benefits, such as-
Types of Dust Control Systems
The three basic types of dust control systems currently used in minerals processing operations are-
Wet dust suppression techniques use water sprays to wet the material so that it generates less dust.
Airborne dust capture systems may also use a water-spray technique; however, airborne dust particles are sprayed with atomized water. When the dust particles collide with the water droplets, agglomerates are formed. These agglomerates become too heavy to remain airborne and settle.
Selection of a Dust Control System
The selection of a dust control system is normally made based on the desired air quality and existing regulations. Dust collection systems can provide reliable and efficient control over a long period; however, the capital and operating costs are high. Wet dust suppression and airborne dust capture systems, while somewhat less efficient, are less expensive to install and operate but also require careful selection and planning to be most effective.
The facilities that require dust control should be surveyed in detail before a dust control system is selected. Emphasis should be placed on the process, the operating conditions, the characteristics of the processing equipment, associated dust problems, and toxicity of the dust. The following is a list of information that may be required:
Dust Collection System
The dust collection system, also known as the local exhaust ventilation system, is one of the most effective ways to reduce dust emissions.
A typical dust collection system consists of four major components:
The following sections describe the design of exhaust hoods and ductwork used to remove dust from the process. Chapter 4 discusses dust collectors, fans and motors, which are used to collect the dust for disposal.
Principles of Airflow
Airflows from a high- to a low-pressure zone due to the pressure difference. The quantity and the velocity of airflow are related according to the following equation:
Q = AVWhere:
Q = volume of airflow, ft3/minAir traveling through a duct is acted on simultaneously by two kinds of pressure:
SP = Gauge Pressure - Atmospheric Pressure
Static pressure (SP) is a force that compresses or expands the air. It is used to overcome the frictional resistance of ductwork, as well as the resistance of such obstructions as coils, filter, dust collectors, and elbows.
SP is the difference between pressure in a dust and that in the atmosphere. When the SP is above the atmospheric pressure, it has a positive sign (+); when it is below the atmospheric pressure, it has a negative sign (-). SP is commonly measured in inches of water.
SP always acts perpendicular to the ductwalls and creates outward pressure when positive and inward pressure when negative.
Velocity Pressure (VP)
Velocity pressure (VP) is the pressure required to accelerate the air from rest to a particular velocity. It exists only when air is in motion, always acts in the direction of airflow, and is always positive in sign. VP is also commonly measured in inches of water.
Note: The relationship, as illustrated, is valid only when g=32.2 ft/s2 (gravitational acceleration constant) and P=0.075 lb/ft3 (air density). For other conditions, a correction factor must be used.
Total Pressure (TP)
Total pressure is the algebraic sum of SP and VP. It is the pressure required to start and maintain the airflow.
If the velocity of air flowing through a duct increases, part of the available SP is used to create the additional VP to accelerate the airflow. Conversely, if the velocity is reduced, a portion of the VP is converted into SP. These conversions, however, are always accompanied by a net loss of TP (in other words, te conversion is always less than 100% efficient).
When air enters a suction opening, the airstream gradually contracts a short distance downstream and, as a result, a portion of the static pressure is converted into velocity pressure. The plane where the diameter of the jet is the smallest is known as the vena contracta. After the vena contracta, the airstream gradually expands to fill the duct and, consequently, a portion of the velocity pressure is converted into static pressure. Both of these pressure conversions are accompanied by losses, which reduce the airflow. The amount of airflow reduction can be defined by a factor known as the coefficient of entry, "Ce."
"Ce", Coefficient of Entry
This represents the percentage of flow that will occur into a given exhaust hood based on the static pressure developed by the hood. It is defined as the actual rate of flow caused by a given static pressure compared to the theoretical flow that would result if there were no losses due to pressure conversions.
"he", Hood Entry Loss
Related to the Ce is the term "hood entry loss" or "he." It is defined as the factor representing the lost in pressure caused by air flowing into a duct. It is measured in inches of water.
The exhaust hood is the point where dust-filled air enters a dust collection system. Its importance in a dust collection system cannot be overestimated. It must capture dust emissions efficiently to prevent or reduce worker exposure to dusts. The exhaust hood-
The three general classes of exhaust hoods are-
Local hoods are relatively small structures. They are normally located close to the point of dust generation and capture the dust before it escapes. Local hoods are generally efficient and typically used for processes such as abrasive grinding and woodworking.
Side, downdraft, and canopy hoods are larger versions of local hoods. They also rely on the concept of preventing dust emissions beyond the control zone. They are typically used for plating tank exhausts, foundry shakeouts, melting furnaces, etc. These hoods are generally less efficient than local hoods.
Booth and enclosure hoods isolate the dust generating process from the workplace and maintain an inward flow of air through all openings to prevent the escape of dust. These hoods are the most popular type in minerals processing operations because they are very efficient at minimum exhaust volumes. They are typically used for areas such as vibrating or rotating screens, belt conveyors, bucket elevators, and storage bins.
Design of Exhaust Hoods
The design of an exhaust hood requires sufficient knowledge of the process or operation so that the most effective hood or enclosure (one requiring minimum exhaust volumes with desired collection efficiency) can be installed.
The successful design of an exhaust hood depends on-
Rate of Airflow - Two approaches used in minerals processing operations to determine the rate of airflow needed through a hood are-
The air-induction phenomenon is of great significance in calculating exhaust volumes. The concept can be applied to many transfer points normally found in minerals processing operations because the calculations are based on variables such as the material feed rate, its height of free fall, its size, and its bulk density.
The exhaust volumes are calculated before the exhaust hood is designed and placed. An approach suggested by Anderson is the most commonly used in the industry today. It is based on the results of a comprehensive laboratory study made by Dennis at Harvard School of Public Health. In its simplified form, it is-
Due to the approximate nature of the formula, Anderson recommends that-
Qind = QFWhere:
QE = required exhaust volume, ft3/minThe most important parameter in the equation is Au-the opening through which the air induction occurs. The tighter the enclosure, the smaller the valve of Au and, hence, the smaller the exhaust volume.
Although Anderson's approach can be widely applied, it may not be appropriate for some special operations or situations. When Anderson's approach cannot be applied, the control-velocity approach should be used.
Control Velocity - In the control-velocity approach, the exhaust hood is designed before exhaust volumes are computed. This approach is based on the principle that, by creating sufficient airflow past a dust source, the dusty air can be directed into an exhaust hood. The air velocity required to overcome the opposing air currents and capture the dusty air is known as capture velocity.
Dallavalle investigated the air-velocity pattern in a space adjoining an exhaust/suction opening and developed the folowing equation to determine exhaust volume:
Q = Vx (10X2 + A)Where:
Q = exhaust volume, ft3/minCapture velocities for some typical operations are provided in the table on the following page.
Location of the Exhaust Hood- The location of the exhaust hood is important in achieving maximum dust-capture efficiency at minimum exhaust volumes. When the control-velocity approach is used, the location of the hood is critical because exhaust volume varies in relation to the location and size of the exhaust hood. The location of the exhaust hood is not as critical when the air-induction approach is used.
The air-induction approach requires the hood to be located as far from the material impact point as possible to-
The following points should be considered in selecting the shape of the hood:
The ductwork transports the dust captured by the exhaust hood to a dust collector. Efficient transport of captured dust is necessary for effective and reliable system operation.
Ductwork design includes the selection of duct sizes based on the velocity necessary to carry the dust to the collector without settling in the duct. From this information, pressure losses in the duct and exhaust air volumes can be calculated and used to determine the size and type of fan, as well s the speed and size of motor.
Before detailed design of the ductwork is begun, the following information should be available:
- Type, size, and speed of the bulk material handling or processing equipment used
- Exhaust hood and exhaust volumes required for each piece of equipment, each transfer point, and each duct network
- Each branch and section of the main duct, identified either by number or letter
-All equipment in the plan and elevations
- The ductwork route and location of the exhaust hood
- Location of the dust collector and the fan
- Length of each duct
- Number and type of elbows, transition, and taper pieces, etc.
- Number and size of "y" branches for each branch and main as identified in the process flowsheet
Note: The minimum transport velocity indicated in the table is for guidance only. The design velocity should be estimated by including a safety factor in the above minimum velocities. Estimation of safety factors should consider-
The system should be balanced to ensure desired airflow distribution. In other words, all branches entering a junction must have equal static pressures at the designed flow. Two methods available to balance the system are-
Air Balance With Blast Gates - This method uses blast gates to achieve the desired airflow at each hood. Calculation begins at the branch of greatest resistance, and pressure drops are calculated through the branch and through the various sections of the main to the fan. No attempt is made to balance the static pressure in the joining airstreams. The joining branches are merely sized to provide the desired transport velocities.
Note: Choosing the branch of greatest resistance is critical in this method. If the choice is incorrect, any branch or branches having a higher resistance will fail to draw the desired volume even when their blast gates are wide open. To prevent this error, all branches that could possibly give the greatest resistance must be checked.
Selection of Balancing Method - Both of the above approaches are common. However, air balance without blast gates normally is selected for processes where highly toxic materials are exhausted so that possible tampering with blast gates will not affect airflow. Air balance with blast gates is selected when exhaust volumes cannot be properly estimated or the system requires some flexibility in varying exhaust volumes.
Note: In the air balance without blast gates method, although calculations are time consuming during the design stage, airflow in the field need not be measured and balanced. In the air balance with blast gates method, the design calculations are fast, but considerable efforts are required in the field to measure and adjust the blast gates to achieve the balance.
Irrespective of the method selected, additional hoods should not be added once a multiple hood layout is completed and balanced because they may alter the airflow and make some other hoods totally ineffective. A comparison of both balancing methods is provided in the table on the following page.
Pressure Losses- Pressure losses occur when air travels in a duct. To overcome these pressure losses, power is supplied by the fan and motor. The higher the pressure losses, the greater the motor horsepower requirements.
Pressure losses in a dust collection system occur due to the following:
Losses from Special Dust Fittings - When air travels through the various duct fittings, such as elbows, "y" branches, enlargements, or contractions (tapers), pressure losses occur. Pressure loss across these fittings is expressed in one of two ways"
Losses from Air-Cleaning Devices - In addition to pressure losses in the ductwork, the losses in the dust collector must also be known. Although the pressure drop for dust collectors varies widely, data are usually available from manufacturers. More information on dust collectors can be obtained from chapter 4.
Points to Note in Ductwork Design/Layout - To minimize pressure losses, the Industrial Ventilation Manual recommends the following guidelines for ductwork design:
This example, which illustrates the balancing of ductwork, is provided to aid understanding of the detailed ductwork design procedure. In the example, the balancing of ductwork is based on the air balance without blast gate method, and the resistances are based on the equivalent foot basis. This approach is one of several available for balancing ductwork; however, an understanding of this approach should facilitate understanding of other approaches. Information on other approaches can be obtained from the sources provided in the references.
Design a dust collection system for an industrial sand-handling facility.
After gathering the above information you can start to fill in the worksheet table:
Repeat this procedure for each branch circuit, when you reach a junction of two branch circuits the balanced pressure method requires that governing static pressure at the junction be within 5% of one another. If this is not the case, as in our example, then design parameters must be altered to achieve balance Several things might be done:
- lower the resistance in branch 2-b by increasing duct size, or reducing air volumeEngineering and economic judgment should be used to make this decision; for instance, whether you can use additional air volume or your fan cannot handle the static pressure may dictate the way in which you choose to balance your system. In the example we chose to increase the resistance in branch 1-b to achieve balance. This step may require several trial and error attempts until you become familiar with the process.
Continue filling in the work sheet using the governing static pressure column to keep a running total of pressure (note that pressures in series circuits are additive; in parallel circuits they are not). The governing pressure in that branch is used.
Finally, as the air exits the exhaust stack of the fan, the velocity pressure of the air is converted back to static pressure and results in the recovery of that energy which is subtracted from the governing static pressure of the system.
Wet Dust Suppression System
Wet dust suppression systems wet the entire product stream so that it generates less dust. This also prevents dust from becoming airborne. Effective wetting of the material can be achieved by-
The surface coverage can be increased by reducing either the droplet diameter or its contact angle.
The surface coverage can be increased either by reducing the surface tension or by increasing the impact velocity.
Factors Affecting Surface Wetting
Surface wetting can be increased by reducing the droplet diameter and increasing the number of droplets. This can be achieved by reducing the surface tension/contact angle. The surface tension of pure water is 72.6 dyne/cm. It can be reduced from 72.6 to 28 dyne/cm by adding minute quantities of surfactants. This reduction in surface tension (or contact angle) results in-
Surface wetting can be increased by increasing the impact velocity.
Impact Velocity can be increased by increasing the system's operating pressure.
Note: A droplet normally travels through turbulent air before it impacts on the material surface. Due to the frictional drag of the turbulent air, the impact velocity of the droplet is less than its discharge velocity from the nozzle. Moreover, small droplets lose velocity faster than large ones. To cover the greatest surface area, the best impact velocity for a given droplet diameter must be determined for each operation.
Types of Wet Dust Suppression Systems
Wet suppression systems fall into three categories:
Airborne Dust Capture Systems
In this approach, very fine water droplets are sprayed into the dust after it is airborne. When the water droplets and dust particles collide, agglomerates are formed. When these agglomerates become too heavy to remain airborne, they settle.
Factors Affecting Collision
The collision between dust particles and water droplets occurs due to the following three factors:
When a dust particle approaches a water droplet, the airflow may sweep the particle around the droplet or, depending on its size, trajectory, and velocity, the dust particle may strike the droplet directly, or barely graze the droplet, forming an aggregate.
Droplet Size/Particle Size
Droplets and particles that are similar in size have the best chance of colliding. Droplets smaller than dust particles or vice versa may never collide but just be swept around one another.
The presence of an electrical charge on a droplet affects the path of a particle around the droplet. When particles have an opposite or neutral charge, collision efficiency is increased.
Types of Airborne Dust Capture Systems
Airborne dust capture systems can be simple or quite complex. Basically, they fall into two broad groups:
Finely atomized water sprays are normally used at transfer points without excessive turbulence or when the velocity of dust dispersion is less than 200 ft/min. The optimum droplet size, water usage, relative velocity, and number and location of nozzles depend on the conditions at individual transfer points.
Electrostatically Charged Fogs
Electrostatically charged fog uses charged water droplets to attract dust particles, which increases collision. The atomized water droplets are charged by induction or direct charging.
Design of a Water-Spray System
The spray nozzle is the heart of a water-spray system. Therefore, the physical characteristics of the spray are critical. Factors such as droplet size distribution and velocity, spray pattern and angle, and water flow rate and pressure all vary depending on the nozzle selected. Following is a general discussion of these important factors:
- Solid-cone nozzles product droplets that maintain a high velocity over a distance. They are useful for providing a high-velocity spray when the nozzle is located distant from the area where dust control is desired.
- Hollow-cone nozzles produce a spray patter in the form of circular ring. Droplet range is normally smaller than the other types of nozzles. They are useful for operations where dust is widely dispersed.
- Flat-spray nozzles produce relatively large droplets that are delivered at a high pressure. These nozzles are normally useful for wet dust suppression systems (i.e., preventive type systems).
-Fogging nozzles produce a very fine mist (a droplet size distribution ranging from submicron to micron). They are useful for airborne dust control systems.
A knowledge of the water flow rate through the nozzle is necessary to determine the percentage of moisture added to the material stream.
The following factors should be considered in selecting the nozzle location:
Water Flow and Compressed Airflow Rates
Once the nozzle is selected, its spray pattern and area of coverage can be used to determine water flow rate and/or compressed airflow rates and pressure requirements. This information is normally published by the nozzle manufacturer. These must be carefully coordinated with the maximum allowable water usage. Water flow rates will be highly variable depending on the size and type of material, the type of machinery, and the throughput of material.
The piping should be designed so that each nozzle receives water or compressed air at specified flow rates and pressures. Drains must be provided at the lowest point in each subcircuit of the piping system to flush the air and water liens in winter months. Heat tapes and insulation must also be provided at locations where the temperature may drop below 32º F. The heat tracing tape should be able to provide approximately 4 watts per linear foot for water pipes up to 2 in. in diameter. The pump and other hardware, such as valves and gauges, should also be heat traced and insulated to prevent freezing during winter months.
Pressure and flow gauges are recommended to monitor system performance. These instruments should be located as close to the point of application as possible. Liquid-filled pressure gauges and rotameter-type flowmeters are satisfactory and quite inexpensive.
For situations where it is desirable to activate wet suppression systems only when the material is flowing (for example, if the belt conveyor is running empty, water sprays need not be on), a solenoid-activated valve may be installed in the water line. The solenoid can be activated by instruments such as the level controller or flow sensor. This measure will reduce water usage, reduce maintenance and cleanup, and reduce or prevent freezeup problems.
Pump and Compressor Selection
An appropriate pump and compressor (where applicable) should be selected once the airflow and waterflow rates and pressure are determined.
An approximate method of determining the proper pumping energy for water at 40:1 efficiency is-
p = pressure drop in water lines, psig
q = water flow rate, gal/min
An approximate method of selecting a compressor is by assuming that-
One horsepower of compressor can provide approximately 4 std ft3/min of compressed air, at 100 psig pressure.