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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-
  • Preventing or reducing risk of dust explosion or fire
  • Increasing visibility and reducing probability of accidents
  • Preventing unpleasant odors
  • Reducing cleanup and maintenance costs
  • Reducing equipment wear, especially for components such as bearings and pulleys on which fine dust can cause a "grinding" effect and increase wear or abrasion rates
  • Increasing worker morale and productivity
  • Assuring continuous compliance with existing health regulations
Proper planning, design, installation, operation, and maintenance are essential for an efficient, cost-effective, and reliable dust control system.



Types of Dust Control Systems

The three basic types of dust control systems currently used in minerals processing operations are-
  • Dust collection
  • Wet dust suppression
  • Airborne dust capture
Dust collection systems use ventilation principles to capture the dust-filled airstream and carry it away from the source through ductwork to the collector.

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:
  • Process flow diagram of the facility indicating items such as the type of material being handled, material flow rates, and the type of equipment
  • Major dust emission points and conditions that occur at these points during normal operations
  • Desired performance of the system
  • Drawings indicating equipment layout
  • Retention time of material in bins or stockpiles
  • Availability of electrical and other utilities
  • Areas requiring freeze protection


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:
Components of Dust Collection System
Components of Dust Collection System
  • An exhaust hood to capture dust emissions at the source
  • Ductwork to transport the captured dust to a dust collector
  • A dust collector to remove the dust from the air
  • A fan and motor to provide the necessary exhaust volume and energy
Each of these components plays a vital role in proper operation of a dust collection system, and poor performance of one component can reduce the effectiveness of the other components. Therefore, careful design and selection of each component is critical.

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     =     AV
Where:
Q     =     volume of airflow, ft3/min

V     =     velocity of air, ft/min

A     =     cross-sectional area through which
              the air flows, ft2
Air traveling through a duct is acted on simultaneously by two kinds of pressure:
  • Static pressure
  • Velocity pressure
Both SP and VP are components of a third kind of pressure:
  • Total pressure
Static pressure (SP)

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)


VP = ( V ) 2

4005

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)


                 TP   =   SP   +   VP
Total Pressure
Total Pressure

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).

Vena Contracta

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."


Vena Contracta
Vena Contracta

"Ce", Coefficient of Entry


Ce = [ Actual airflow ]

Theorectical airflow

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.

Exhaust Hood

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-
  • Encloses the dust producing operation
  • Captures dust particulates and guides dust-laden air efficiently
Types of Exhaust Hoods

The three general classes of exhaust hoods are-
  • Local hoods
Local Hood
Local Hood
  • Side, downdraft and canopy hoods
Side Hood Downdraft Hood Canopy Hood
Side Hood Downdraft Hood Canopy Hood
  • Booths or enclosures
Booth Hood Enclosure Hood
Booth Hood
 
Enclosure Hood

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 through the hood
  • Location of the hood
  • Shape of the hood
Of the above three factors, the rate of airflow through the exhaust hood (that is, the exhaust volume rate) is the most important factor for all types of hoods. For local, side, downdraft, and canopy hoods, the location is equally important because the rate of airflow is based on the relative distance between the hood and the source. The shape of the exhaust hood is another design consideration. If the hood shape is not selected properly, considerable static pressure losses may result.
Air-Induction Approach
Air-Induction Approach

Rate of Airflow - Two approaches used in minerals processing operations to determine the rate of airflow needed through a hood are-
  • Air induction
  • Control velocity
Air Induction - The air-induction concept is based n the theory that when granular material falls through the air each solid particle imparts some momentum to the surrounding air. Due to this energy transfer, a stream of air travels with the material. Unless the air is removed it will escape through all openings upon material impact, carrying the fine dust particles with it. For adequate control of dust emissions, the exhaust air volume rate must be equal to or greater than the air-induction rate.

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-

_____
QE = 10 AU 3  RS2
D

Where:


Qind     = volume of induced air, ft3/min
Au       = enclosure open area at upstream end (point where air is induced into the system
 by action of the falling material), ft2
R        =   rate of material flow rate, ton/h
S        =  height of free fall of material, ft
D        = average material diameter, ft

Due to the approximate nature of the formula, Anderson recommends that-
Qind   =    QF
Where:
QE    =   required exhaust volume, ft3/min
The 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.


Control-Velocity Approach
Control-Velocity Approach

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/min

Vx    =     centerline velocity (i.e., capture velocity)
              at distance X from hood, ft/min

X     =     distance outwards along the hood axis, ft

A     =     area of hood face opening, ft2
Capture 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.


Location of Hood Using Air-Induction Approach
Location of Hood Using
Air-Induction Approach

The air-induction approach requires the hood to be located as far from the material impact point as possible to-
  • Prevent capturing coarse dust particles, which settle quickly
  • Capture only fine, predominantly respirable dust
  • Reduce unnecessary transport of coarse dust through ductwork and thus reduce dust settling in horizontal duct runs
  • Reduce dust loading (dust concentration) in the exhaust gases
  • Minimize subsequent cleaning and disposal of the collected dust
  • Prevent capture of valuable products, especially in ore-concentrating operations
The control-velocity approach requires the hood to be located as close to the source as possible to-
  • Maximize the hood capture efficiency for a given volume
  • Reduce the exhaust volume requirements
  • Enclose the source as much as possible
Shape of the Exhaust Hood - Sizable pressure losses may occur if the shape of the exhaust hood is not designed properly. These pressure losses are due to the mutual conversion of static and velocity pressures.
Effect of Flanged Opening


The following points should be considered in selecting the shape of the hood:
  • The exhaust hood shape with the highest coefficient of entry value, Ce, or the lowest hood entry loss factor, he, should be selected. Various values of Ce and he are described in the following table:
  • Wherever possible, the hood should be flanged to eliminate airflow from zones containing no contaminants.
This measure can reduce exhaust air volume up to 25%. For most applications, the flange width should not exceed 6 in.

Effect of Flanged Opening
Effect of Flanged Opening
Coefficient of Entry and Entry Loss of Different Hood Types
Reprinted by permission from the Committee on Industrial Ventilation, Lansing, VI, 18th Edition.

Ductwork

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

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:
  • A process flowsheet of the operation indicating-

    • - Type, size, and speed of the bulk material handling or processing equipment used
  • A line diagram of the dust collection system indicating-

    • - 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
  • A general layout of the facility showing-

    • -All equipment in the plan and elevations

      - The ductwork route and location of the exhaust hood

      - Location of the dust collector and the fan
  • A preliminary bill of material containing-

    • - 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
Proper ductwork design-
  • Maintains adequate transport velocities in the duct to prevent particulate settling
  • Provides proper air distribution in all branches to maintain designed capture velocities of exhaust hoods
  • Minimizes pressure losses, wear, and abrasion of ductwork thus reducing operating costs
Transport Velocities - To prevent dust form settling and blocking the ductwork, transport velocities should range from 3,500 to 4,000 ft/min for most industrial dust (such as granite, silica flour, limestone, coal, asbestos, and clay) and from 4,000 to 5,000 ft/min for heavy or moist dust, such as lead, cement, and quick lime. The table describes minimum transport velocities for different characteristics of dust.

Material Minimum Design Velocity (fpm)


Very fine, light dusts 2,000
Fine, dry dusts and powders 3,000
Average industrial dusts 3,500
Coarse dusts 4,000 - 4,500
Heavy or moist dust loading 4,500 and up

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-
  • Material buildup
  • Duct damage
  • Corrosion of ductwork
  • Duct leakage
Distribution of Airflow - Proper airflow distribution in each branch is necessary to maintain adequate capture and transport velocities in the system. If air is not properly distributed in a multiple-branch dust collection system, a natural balance will take place. For example, the exhaust volume will be determined by the resistance of the available flow paths, and the branch with the least resistance will carry the most volume. As a result, the desired airflow may not be achieved in each branch.

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 without blast gates
  • Air balance with blast gates
Air Balance Without Blast Gates - This method, often called the static pressure balance method, provides a way to achieve the desired airflow (a balanced system) without the use of dampers or blast gates. Calculation begins at the branch of greatest resistance and proceeds from branch to main, through each section of main, and to the fan. At each junction of two airstreams, the static pressure necessary to achieve desired flow in both streams is matched and, thus, branches are brought into "balance." The static pressures can be balanced at the desired rate of flow by choosing appropriate sizes of ducts, elbow radii, etc.

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:
Hood-Entry Losses
Hood-Entry Losses
  • Hood entry
  • Special duct fittings
  • Duct friction
  • Air-cleaning devices
Hood Entry Losses - A loss in pressure occurs when air enters a suction or hood opening. This loss is indicated by the coefficient of entry for the hood (Ce). Several examples of entry coefficients are illustrated.

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"
  • As a fraction of the velocity pressure
  • In terms of equivalent feet of straight duct (of the same diameter) that will produce the same pressure loss as the fitting
Duct-Friction Losses - When air travels in a straight run of duct, pressure losses occur due to the friction between the duct walls and air. Many charts and graphs are available that give friction losses in straight ducts. However, most of them are based on new, clean ducts. The following chart, which allows for a typical amount of roughness, plots four quantities. If any two quantities are known, the other two can be read directly from the chart.

Duct Friction Losses
Duct Friction Losses
Reprinted by permission from the Committee on Industrial Ventilation, Lansing, MI, 18th Edition.

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:
  • All branches should enter the main at 30 angle; wherever possible, the velocity should match that of the incoming gas stream.
  • Duct size changes should be kept to a minimum. If needed, they should be gradual.
  • Wherever possible, a circular duct should be used instead of a rectangular duct to maintain uniform velocity distribution and prevent settling of material in the ductwork.
  • Wherever possible, flanges should be provided to minimize hood entry losses.
  • The centerline radii of all elbows should be at least twice the diameter of the duct.
The illustrations below provide examples of ductwork design.

Principles of Duct DesignPrinciples of Duct Design
Principles of Duct DesignPrinciples of Duct Design
Reprinted by permission from the Committee on Industrial Ventilation, Lansing, MI, 18th Edition.
    
Example Problem

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.

The Problem

Design a dust collection system for an industrial sand-handling facility.

Information provided
  • Process flowsheet and schematic of the dust collection system
  • Minimum transport velocity = 3,500 fpm
  • Necessary exhaust volumes
  • Description and materials

Process Flowsheet
Process Flowsheet


Details of Operation
Description Number Minimum Exhaust Volume (cfm)



Bag Machine Hood 1 800
Conveyor Transfer Point 2 300
Bag House Dust Collector 3 -
Fan 4  

Description of Material
No. of Branch or Main Airflow Required (cfm) Straight Run (ft) Number of Elbows Number of Entries





1-b 800 30 2-90 --
2-b 300 35 3-90 --
b-c 1,100 50 -- 1
c-d 1,100 0 1-90 --
d-e 1,100 30 -- --

After gathering the above information you can start to fill in the worksheet table:

Column Entry Explanation



1 1-b Section of system to be worked on from bag hood to Y junction
2 6.5 in. Based on minimum transport velocity and minimum air volume required using Q=VA formula A=0.27 ft2,
this gives a duct size of 6.5 in.
0.27 Duct Area
4 800 Air volume required, as calculated from Anderson, or others, or determined by past experience or testing
6 3500 Air velocity determined by Q/A = V
7 30 Straight runs measured from prints or on site
8 2 Number of elbows determined from schematic installation
9 14 Taken from table ___ for an elbow with a centerline radius of 2.0 times the duct diameter. This number
is multiplied times the number of elbows.
10 44 Sum of straight rum length (col.7) and equivalent length (col.9)
11 2.7 Frictional losses read directly from Figure ___, look at duct diameter vs. duct velocity and read frictional loss per 100 ft. of straight total duct length
12 1.19 Total duct length (col.10) x frictional loss per 100 ft. of duct (col.11) divided by 100
13 0.77 Velocity pressure of air in duct, VP = (V/P)2
14 0.5 Entry loss for hood (given)
15 1.5 The total exhaust hood loss. This represents the amount of energy required to get the air to flow into the hood (1.0 VP) plus the
specific hood entry loss (col. 14).
16 1.16 Product of cols. 13 and 15
17 1.16 The governing static pressure in branch 1-b (col. 12 + col. 16).

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 volume

- increase the resistance in branch 1-b by decreasing duct size or increasing air volume
Engineering 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.


WORKSHEET
Example Problem
Worksheet Example Problem


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-
Contact Angle of a Water Droplet
Contact Angle of a Water Droplet
  • Static Spreading - The material is wetted while stationary. The diameter and contact angle of water droplets are important factors in static spreading.

    • The surface coverage can be increased by reducing either the droplet diameter or its contact angle.
  • Dynamic Spreading - The material is wetted while moving. The surface tension of the liquid, the droplet diameter, the material size, and the droplet impact velocity are important variables in dynamic spreading.

    • The surface coverage can be increased either by reducing the surface tension or by increasing the impact velocity.
One of these two water spreading methods can be emphasized at the expense of the other, depending on the needs of the system. For example, both static and dynamic spreading of a droplet can be increased by reducing the surface tension and thus decreasing the droplet diameter. However, the impact velocity of smaller droplets decreases faster due to frictional drag and less momentum, which, in turn, reduces dynamic spreading. An optimum droplet diameter for maximum material surface coverage must therefore be determined.

Factors Affecting Surface Wetting

Droplet Size

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-
  • Reduced droplet diameter
  • An increase in the number of droplets
  • A decrease in the contact angle
Impact Velocity

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:

  • Plain Water Sprays - This method uses plain water to wet the material. However, it is difficult to wet most surfaces with plain water due to its high surface tension.
  • Water Sprays with Surfactant - This method uses surfactants to lower the surface tension of water. The droplets spread further and penetrate deeper into the material pile.
  • Foam - Water and a special blend of surfactant make the foam. The foam increases the surface area per unit volume, which increases wetting efficiency.

Advantages and Disadvantages
Advantages Disadvantages


Plain Water Sprays
  • It is probably the least expensive method of dust control.
  • Water sprays cannot be used for products that cannot tolerate excessive moisture.
  • The system is simple to design and operate
  • Water sprays cannot be used when temperatures fall below freezing.
  • A limited carryover effect at subsequent transfer points is possible
  • Usually, dust control efficiency is low, unless large quantities of water are used.
  • When good mixing of water and material can be achieved, dust generation can be reduced effectively.
  • Freeze protection of all hardware is necessary
  • Enclosure tightness is not essential.
  • Careful application at transfer points that precede a screen is required to prevent blinding.
Water Sprays With Surfactants
  • This method is used when surfactants are tolerated but excessive moisture is not acceptable.
  • Capital and operating costs are higher than water-spray systems.
  • In some cases, dust control efficiency is higher than with plain water sprays.
  • Careful application at transfer points that precede a screen is required to prevent blinding.
  • Equivalent efficiency is possible with less water.
  • Equipment such as the pump and proportioning equipment used to meter the flow of surfactant require maintenance.
 
  • Freeze protection of all hardware is necessary
Foam
  • When good mixing of foam and product stream can be achieved, dust control efficiency is greater than water with surfactants.
  • Operating costs are higher than with finely atomized water-spray systems.
  • Moisture addition is usually less than 0.1% of the material weight.
  • The product is contaminated with surfactants.
 
  • Careful application at transfer points that precede a screen is required to prevent blinding.


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.

Collision Between Dust Particle and Water Droplet Coalescence of Adhesion Between Dust Particle and Water Droplet
Collision Between Dust Particle and Water Droplet Coalescence or Adhesion Between Dust Particle and Water Droplet

Factors Affecting Collision

The collision between dust particles and water droplets occurs due to the following three factors:
  • Impaction/interception
  • Droplet size/particle size
  • Electrostatic forces
Impaction/Interception

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.

Particle Trajectories Around a Water Droplet
Particle Trajectories Around a Water Droplet

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.


Effect of Droplet Size
Effect of Droplet Size
Schowengerdt and Brown

Electrostatic Forces


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.


Oppositely Charged Droplet and Particle Attract Each Other Similarly Charged Droplet and Particle Oppose Each Other
Oppositely Charged Droplet and Particle Attract Each Other Similarly Charged Droplet and Particle Oppose Each Other

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
  • Electrostatically charged fogs
Finely Atomized Water Sprays

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:
  • Droplet Size- The nozzle's droplet size distribution is the most important variable for proper dust control. The droplet size decreases as the operating pressure increases. Information about the droplet size data at various operating pressures can be obtained from the nozzle manufacturer. For wet dust suppression systems, coarse droplets (200-500 m) are recommended. For airborne dust capture systems, very fine droplets (10-150 m) may be required. The fine droplets usually are generated by fogging nozzles, which may use either compressed air or high-pressure water to atomize water in the desired droplet range.
  • Droplet Velocity - Normally, higher droplet velocities are desirable for both types of dust control through water sprays. Information on the droplet velocity can be obtained from the nozzle manufacturer.
  • Spray Pattern - Nozzles are categorized by the spray patterns they produce:

  • - 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.

    Solid-Cone Nozzle Hollow-Cone Nozzle Flat-Spray Nozzle Fogging Nozzle
    Solid-Cone
    Noozle
    Hollow-Cone
    Nozzle
    Flat-Spray
    Noozle
    Fogging
    Noozle

  • Spray Angle - Each nozzle has a jet spray angle. The size of this angle is normally available from the manufacturer. A knowledge of spray angle and spray pattern is essential to determine the area of coverage and, therefore, the total number of nozzles needed.
  • Flow Rate - The flow rate of water through a nozzle depends on the operating pressure. The flow rate and operating pressure are related as follows:
Water flow rate
where K = nozzle constant
Spray Angle Water Flow/Pressure
Spray Angle
Water Flow/Pressure
From Bureau of Mines open File Report 145-82, Guide Book for Dust Control in Underground Mining, December 1981.

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:
  • It should be readily accessible for maintenance.
  • It should not be in the path of flying material.
  • For wet dust suppression systems, nozzles should be upstream of the transfer point where dust emissions are being created. Care should be taken to locate nozzles for best mixing of material and water. For airborne dust capture, nozzles should be located to provide maximum time for the water droplets to interact with the airborne dust.
Advantages and Disadvantages
Advantages Disadvantages


Finely Atomized Water Sprays
  • Water requirements are low-typically 5 to 20 gal/h per nozzle.
  • Tight enclosures are needed for effective system operation.
  • Moisture addition to the product is quite low-typically less than 0.1% of the material weight.
  • The system may not be effective either in highly turbulent environments or when the dust dispersion rate is more than 200 ft/min.
  • The material is not chemically contaminated.
  • Requires good droplet to particle size match for effective control.
  • The system can be economical.
 
Electrostatically Charged Fogs
  • Electrostatic fogs can be effective if the dust cloud carries predominantly positive or negative charges.
  • These systems are not recommended for underground coal mines or other gassy applications where explosions can be triggered by sparks.
  • The material does not become chemically contaminated.
  • Capital costs are high.
  • Moisture addition to the product is generally less than 0.5% by weight.
  • These systems require high-voltage equipment.
 
  • Maintenance of electrical insulation is critical for safe working conditions.

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.

Piping Design

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 32 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.

Instruments

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-


Pump HP = 1.40 x p x q

10,000

Where:

     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.