Passive Dust Control System

The present invention relates to the provision of a material handling system having a dust containment housing that passively prevents dust from escaping from the housing during a transfer of bulk material from a first station to a second station. More particularly, the present invention relates to providing a region of low air pressure in a dust containment housing to cause air to flow from a region of high air pressure generated by a transfer of bulk material to the region of low air pressure inside the housing. Still more particularly, the present invention relates to the provision of a circulation compartment inside the housing to create a circulation path for an induced air flow inside the housing from a termination area of the transfer of bulk material back to the first station.

One of the most significant problems facing today's industrial society is air pollution. It is commonly known that air pollution is among the causes of many environmental problems, such as the greenhouse effect, and many health problems, such as silicosis and pneumoconiosis. As society has become aware of these environmental and health problems caused by air pollution, methods have been sought to reduce the amount of pollution released into the air.

One type of air pollution is dust emitted into the atmosphere. Dust is comprised of particles of a bulk material, such as hard or soft ores, that are suspended in the air. One cause of dust emissions is the transfer of a bulk material from a first station to a second station in a material handling system. The bulk material is typically transferred from the first station to the second station by causing the bulk material to fall from the first station to the second station or by projecting the bulk material from the first station to the second station.

A stream of bulk material is formed by the bulk material falling or being projected from the first station to the second station. The individual particles in the stream of bulk materials impart a frictional drag on the surrounding air. The surrounding air begins to flow in the direction of the material stream and attains a velocity approaching the velocity of the material stream. This is called an induced air flow. The bulk material is suddenly stopped and compacted at the end of the fall by impact of the material stream with a termination area of the second station. The velocity pressure of the induced air flow is converted to static pressure by the termination of its flow at the second station. A region of high air pressure is created by the static pressure. Air in the region of high air pressure must flow to a region of lower air pressure. As the air flows from the region of high air pressure to a region of lower air pressure, small particles of the bulk material are suspended in the air. In an open system, the movement of the air is from the region of high air pressure to a region of low air pressure is evidenced by a dust cloud billowing from the termination area of the second station.

One common dust control system is a housing enclosing a first station, a second station, and the material stream between the first and second stations. The housing guides the bulk material from the first station to the second station and prevents dust from escaping. A typical housing has an inlet near the first station to receive the bulk material at the first station and an outlet near the second station to transport the bulk material from the second station out of the housing. In addition to the inlet and outlet, a dust containment housing typically has several tolerance openings around moving and vibrating components of the first and second stations.

It is a problem that the primary function of the housing is to guide flowing bulk material from the first station to the second station and not to prevent dust emissions. When a region of high air pressure is generated proximate the second station by the termination of the induced air flow, the dust laden air in the region of high air pressure flows through leakage areas, such as the outlet and tolerance; openings near the second station to a region of lower air pressure outside the housing. The air flows out of the dust containment housing through the leakage areas carrying the suspended particles of bulk material.

One method for preventing air from carrying dust out of openings in the housing is to exhaust the induced air flow to a filtration system. A filtration system has a vacuum or fan to create a negative air pressure in a duct leading to the filtration system. An entry to the duct is attached to the housing. The air flows from the region of high air pressure flows into the duct having a negative air pressure and to the filtration system instead of out of the openings in the housing. In the filtration system, the air passes through a filter which removes the suspended particles from the flowing air. The filtered air is then released to the outside environment.

Filtration systems are not desirable for several reasons. A first reason that a filtration system is undesirable is the capital cost required to add the filtration system to a housing. One factor in the prohibitive capital cost of a filtration system is the need to incorporate a duct system leading to the filtration system into the housing. This increases the cost of the housing and causes many design and operation problems.

A second reason that filtration systems are not desirable is the high maintenance required to maintain the filtration system in proper working order. One cause of the need for high maintenance is that the bulk material caught by the filter must be removed from the filter using a shaker or other device. A second cause of the high maintenance is that the entire filter must be periodically replaced in order to maintain the same efficiency level for the filtration system.

A third reason that filtration systems are not desirable is the high energy consumption by the filtration systems. A great amount of energy is needed to create a region of negative air pressure in the ducts that will cause the air to flow through the filtration system. A further reason that filtration systems are undesirable is that an improperly working filter system will emit dust.

A second form of dust emission control is a dust suppression system. A dust suppression system can be used either in an open or a closed bulk material transfer system. In a dust suppression system, the bulk material is wetted with a surfactant such as foam or water. The surfactant agglomerates with the fine particles of the bulk materials. The agglomeration of the fine particles and surfactant increases the weight of the fine particles. The increased weight prevents the particles from being suspended in the air flowing from the region of high air pressure to the region of low air pressure. It is a problem that dust suppression systems are typically not as effective as filtration systems in preventing dust from being emitted. Further, the capital cost of a dust suppression system is comparable to the capital cost of a filtration system that has a comparable emission control performance.

Dust suppression systems that are as effective as filtration systems wet the bulk material with a mist having droplets approaching the size of the dust particles. However, these efficient dust suppression systems are as expensive to install as filtration systems and typically have higher maintenance costs. A high level of maintenance is needed to maintain production of the particle sized droplets in the mist and to assure that even very small particles of bulk material are agglomerated.

One disadvantage of a dust suppression system is that dust suppression systems have numerous problems in cold temperatures. In order to keep the surfactant from freezing, a dust suppression system must be heated or well insulated. The wetted bulk material can also freeze which causes plugging in the transfer system. These problems lead to excessive maintenance and extra cost to maintain efficiency in the dust suppression system in cold temperatures.

There is a long felt need in the art for an inexpensive and maintenance-free dust control system.

The above and other problems are solved and an advance in the art is made by the provision of a passive dust control system of the present invention. In accordance with the present invention, moving air carrying suspended particles of bulk material inside a dust containment housing is prevented from escaping through leakage areas in the dust containment housing. Induced air flowing within the dust containment housing is circulated inside the dust containment housing to prevent the air from escaping through leakage areas in the dust containment housing. A region of low air pressure is defined by the dust containment housing. As the region of high air pressure is generated inside the dust containment housing, the air flows from the region of high air pressure to the region of low air pressure inside the dust containment housing. As the air flows into the region of low air pressure, the velocity of the flowing air dissipates and the particles suspended in the air either remain suspended in slower moving air in the region of low air pressure or fall back into the bulk material at a second station.

In accordance with the present invention, a passive dust control system is provided in the following manner. A dust containment housing encloses a first station, such as a first conveyer belt and a second station, such as a second conveyer belt. The first station is at a first elevation and the second station is at a second elevation that is lower than the first elevation. Bulk material is transferred from the first station to the second station by dropping or projecting the bulk material from the first station to the second station.

During the transfer of bulk material, dust is generated in the following manner. A stream of bulk material is formed by the material falling or projected from the first station to the second station. The individual particles in the stream of bulk materials impart a frictional drag on the surrounding air. The surrounding air begins to flow in the direction of the material stream and attains a velocity that approaches the velocity of the material stream. This is called an induced air flow. At the end of the fall, the bulk material is suddenly stopped and compacted by impact with the second station. The velocity pressure of the induced air flow is converted to static pressure by the termination of the induced air flow at the second station. A region of high air pressure is created by the static pressure. Air in the region of high air pressure must flow to a region of lower air pressure. As the air flows from the region of high pressure to the region of lower air pressure, small individual particles of the bulk material are suspended in the flowing air. In accordance with the present invention, the air in the region of high air pressure flows into the region of low air pressure inside the dust containment housing. This prevents the air containing the suspended particles of bulk material from flowing out of the housing.

In a preferred exemplary embodiment, the region of low air pressure is defined by circulation compartment inside the dust containment housing. The circulation compartment substantially encloses the bulk material stream and a termination area of the bulk material stream at the second station. This allows the flowing air to circulate back from the termination area to a beginning point of the material stream at the first station. As the material falls to the second station, the region of high air pressure is generated. The pressurized air flows outward into the circulation compartment containing a region of low air pressure. Air continues to flow into the circulation compartment adding more particles of bulk material into the circulation compartment. Eventually, the air in the circulation compartment can reach a saturation point for the suspended particles. If the air in the circulation compartment becomes saturated with particles of bulk material, agglomeration will create an equilibrium at a certain concentration of suspended particles. In agglomeration, the smaller particles of bulk material collide and form bigger, heavier particles. The bigger, heavier particles then fall from the air in the circulation compartment and back into the bulk material in the material stream or at the second station.

Thus, the present invention prevents dust from being emitted from a dust containment housing of a material handling system without the devices and methods known in the prior art. These and other advantages of a passive dust control system will be apparent to those skilled in the art upon reading of the discussion below in combination with the accompanying drawings.

FIG. 1 illustrates a cross sectional view of a dust containment housing in the prior art;

FIG. 2 illustrates a cross sectional view of one possible preferred exemplary embodiment of the present invention; and

FIG. 3 illustrates an exterior view of one possible preferred exemplary embodiment of the present invention.

FIG. 1 illustrates a cross sectional view of a prior art dust containment housing 100. Dust containment housing 100 encloses first station 101 and second station 102 to prevent dust emissions from housing 100 during a transfer of bulk material 103 from first station 101 to second station 102 inside housing 100. In FIG. 1, first station 101 and second station 102 are represented by conveyor belt systems. However, other systems can be used by first station 101 and second station 102 to transport bulk material 103.

First station 101 is at a first elevation in housing 100. In first station 101, conveyor belt 110 rotates about head pulley 111 and a tail pulley (not shown) in a direction indicated by arrow 180. Head pulley 111 and/or the tail pulley (not shown) are rotated by a motor (not shown) to rotate conveyor belt 110. Idler pulleys 112 guide and support conveyor belt 110 between head pulley 111 and the tail pulley (not shown). Bulk material 103 is placed on conveyor belt 110 which transports bulk material 103 from a prior processing station into housing 100 in the direction indicated by arrow 182.

Second station 102 is at a second elevation that is lower than the first elevation of first station 101. In second station 102, conveyor belt 120 rotates about a head pulley (not shown) and tail pulley 121 in the direction indicated by arrow 190. The head pulley (not shown) and/or tail pulley 121 are rotated by a motor (not shown) to rotate conveyor belt 120. Idler pulleys 122 guide and support conveyor belt 120 between the head pulley (not shown) and tail pulley 121. Idler pulley supports 123 affix idler pulleys 122 to base 124. Impact bed 125 is positioned under conveyor belt 120 at a point that the bulk material 103 lands on second station 102 to add additional support to conveyor belt 120. Supports 127 affix impact bed 125 to base 124. Bulk material 103 is placed on conveyor belt 120 which moves bulk material 103 to a subsequent processing station outside dust containment housing 100 in the direction indicated by arrow 192.

Bulk material 103 is carried by conveyor belt 110 in the direction indicated by arrow 182. Conveyor belt 110 carries bulk material 103 into dust containment housing 100 through inlet 131. As conveyor belt 110 rotates about head pulley 110, bulk material 103 drops from an end of conveyor belt 110 in the direction indicated by arrow 183. Bulk material 103 then falls from the first elevation to the second elevation and lands on conveyor belt 120 of second station 102. Conveyor belt 120 moves the bulk material 103 that lands on conveyor belt 120 in the direction indicated by arrow 192. The bulk material 103 on conveyor belt 120 then exits housing 100 through outlet 132.

Housing 100 encloses first station 101, second station 102, and a chute defining a path for bulk material 103 to fall from first station 101 to second station 102. Upper skirt 140 encloses first station 101. First upper skirt side wall 141 and second upper skirt side wall (not shown) are substantially parallel to the longitudinal axis of conveyor belt 110 and are on opposing sides of conveyor belt 110. Upper skirt hood 142 is substantially parallel to the surface of conveyor belt 110 carrying bulk material 103 and extends from a top side of first skirt side wall 141 to a top side of the second upper skirt side wall (not shown) and is above a top side of conveyor belt 110. Inlet 131 is an opening in upper skirt 140 to allow conveyor belt 110 to carry bulk material 103 into housing 100.

Lower skirt 150 encloses second station 102. First lower skirt side wall 151 and second lower skirt side wall (not shown) are substantially parallel to the longitudinal axis of conveyor belt 120 and on opposing sides of conveyor belt 120. Lower skirt hood 152 is parallel to a top surface of conveyor belt 120 carrying bulk material 103 and is above the top surface of conveyor belt 120. Lower skirt hood 152 also extends from a top side of first lower skirt side wall 151 to a top side of the second lower skirt side wall (not shown). Outlet 132 in lower skirt 150 allows conveyor belt 120 to remove bulk material 103 from housing 100.

Four walls of housing 100 define the chute that provides a path for bulk material 103 from first station 101 to second station 102. A front chute wall 162 connects a first end of upper skirt hood 142 and a second end of second lower skirt hood 152. Front chute wall 162 is also slanted inward from lower skirt hood 152 to upper skirt hood 142. Back loading plate 163 extends from a bottom surface of conveyor belt 110 to a top surface of conveyor belt 120 and from a first end of first chute side wall 164 to a first end of a second chute side wall (not shown). A tail box 165 on the exterior side of back loading plate 163 prevents dust from being emitted from a back side of the chute. First chute side wall 164 extends from a first end of first upper skirt side wall 141 to a first end of first lower skirt side wall 151 and from a first end of front chute wall 161 to a first end of back loading plate 163. A second chute side wall (not shown) extends from a first end of second upper skirt side wall (not shown) to a first end of second lower skirt side wall (not shown) and from a second end of front chute wall 162 to a second end of back loading plate 163. Bulk material 103 falls through the chute inside housing 100 from first station 101 to second station 102.

Dust is generated and emitted from housing 100 in the following manner. A stream of bulk material 103 is formed by bulk material 103 failing or being projected from first station 101 to second station 102. The individual particles in the stream of bulk material 103 impart a frictional drag on the surrounding air. The surrounding air begins to flow in the direction indicated by arrow 195 with a velocity approaching a velocity of the stream of bulk material 103. This is called an induced air flow. At the end of the fall, bulk material 103 suddenly stops and compacts by impact with a termination area of the second station 102. The velocity pressure of the induced air flow is converted to static pressure by termination of the induced air flow at the second station 102. A region of high air pressure is created by the static pressure. Air in the high air pressure must flow to a region of lower air pressure. As the air flows from the region of high pressure to the region of lower pressure small individual particles of the bulk material are suspended in the air.

The only region of low air pressure is outside housing 100. The air in the region of high air pressure flows through a leakage area in housing 100, such as outlet 132, to the region of lower air pressure outside housing 100. In FIG.1 the movement of the air out of outlet 132 is indicated by arrow 184. The flowing air carries the suspended particles of bulk material 103 out of housing 100.

The present invention illustrated in FIG. 2 provides a passive dust control system for preventing particles of bulk material 103 from being emitted from a dust containment housing 200. A region of low air pressure inside the housing is provided by the present invention. The air in the region of high pressure inside dust containment housing 200 flows to the region of low air pressure inside the housing. The particles either remain suspended in slower moving air in the region of low air pressure or fall back into the bulk material 103 in the material stream or at the second station.

FIG. 2 illustrates a preferred exemplary embodiment of the present invention. First station 101 and second station 102 are identical to first station 101 and second station 102 as described in FIG. 1. It is understood that first station 101 and second station 102 are depicted as conveyor belt systems for exemplary purposes only and that the present invention can be used with other transfer systems, such a hopper and receiving buckets. Housing 200 provides a circulation compartment 201 instead of a chute that is provided by housing 100. Circulation compartment 201 provides a region of low air pressure inside housing 200. Preferably, the region of low air pressure has a lower air pressure than the air outside housing 200. This forces the air in the region of high air pressure to flow to the region of low air pressure in circulation compartment 201 and prevents the air in the region of high air pressure from flowing out of an opening in housing 200.

Housing 200 substantially encloses first station 101, second station 102, and circulation compartment 201 which contains the dust from the transfer of bulk material 103 from first station 101 to second station 102 inside housing 200. Supports (not shown) attach to an exterior of housing 200 and hold housing 200 in place over first station 101 and second station 102.

Upper skirt 240 of housing 200 substantially encloses first station 101. First upper skirt side wall 241 and second upper skirt side wall (not shown) are substantially parallel to the longitudinal axis of conveyor belt 110 and are on opposing sides of conveyor belt 110. Upper skirt hood 242 is substantially parallel to a surface of conveyor belt 110 carrying bulk material 103, is above a top side of conveyor belt 110, and extends from the top side of first upper skirt side wall 241 to the top side of the second upper skirt side wall (not shown). Inlet 131 in upper skirt 240 allows conveyor belt 110 to carry bulk material 103 inside housing 200 in the direction indicated by arrow 182.

Lower skirt 250 substantially encloses second station 102. First lower skirt side wall 251 and second lower skirt side wall (not shown) are substantially parallel to the longitudinal axis of conveyor belt 120 and on opposing sides of conveyor belt 120. Lower skirt hood 252 is substantially parallel to a top surface of conveyor belt 120 carrying bulk material 103, is above a top side of conveyor belt 120, and extends from a top side of first lower skirt side wall 251 to the top side of the second lower skirt side wall (not shown). Outlet 132 in lower skirt 250 allows conveyor belt 120 to carry bulk material 103 out of housing 200 in the direction indicated by arrow 192.

Instead of defining a chute as shown in FIG. 1, to provide a path of bulk material 103 from first station 101 to second station 102, dust containment housing 200 defines a circulation compartment 201 to enclose the falling bulk material 103. Circulation compartment 201 is defined by five compartment walls. A top side compartment wall 261 extends outward from a first end of upper skirt hood 242 over second station 102. A front side compartment wall 262 connects a first end of lower skirt hood 252 to a first end of top side compartment wall 261 and is slanted inwards from lower skirt hood 252 to top side compartment wall 261. Back side loading plate 263 extends from a bottom side of conveyor belt 110 to a top side of conveyor belt 120 and extends from a first end of first compartment side wall 264 to a first end of a second compartment side wall (not shown). First compartment side wall 264 extends from a first end of first upper skirt side wall 241 to a first end of first lower skirt side wall 251, from a first side of upper compartment wall 261 to at least conveyor belt 120 and from a first end of a front side compartment wall 262 to a first end of back side loading plate 263. The second compartment side wall (not shown) extends from a first end of the second upper skirt side wall (not shown) to a first end of the second lower skirt side wall (not shown), from a second side of upper compartment wall 261 to at least conveyor belt 120 and from a second end of front side compartment wall 262 to a second end of back side loading plate 263. The passive dust control is provided by circulation compartment 201.

FIG. 3 illustrates an external view of housing 200. As shown, housing 200 completely encloses the transfer point of bulk material 103 from first station 101 (shown in FIG. 2) to second station 102 (shown in FIG. 2). A dotted line representation of head pulley 111 is provided to show the end of first station 101 (shown in FIG. 2). Housing 200 encloses circulation compartment 201(shown in FIG. 2) which extends outwards from first station 101 (shown in FIG. 2) over second station 102 (shown in FIG. 2) forming a circulation compartment. Air inside housing 200 flows from a region of high pressure generated by the transfer of bulk material 103 into the circulation compartment instead of flowing out of outlet 132.

As illustrated in FIG. 2, circulation compartment 201 provides a region of low pressure inside housing 200. The air carrying suspended particles inside housing 200 flows inside housing 200 instead of flowing out of housing 200. The circulation of the air occurs in the following manner. A stream of bulk material 103 is formed by bulk material 103 falling or being projected from first station 101 to second station 102. The individual particles in the stream of bulk material 103 impart a frictional drag on the surrounding air. The surrounding air begins to flow in a direction of the stream of bulk material 103 from first station 101 to second station 102. The flowing air has a certain velocity pressure. This is called an induced air flow. At the end of the fall, bulk material 103 suddenly stops and compacts by impact with a termination area of the second station 102. The velocity pressure of the induced air flow is terminated and is converted to static pressure by its flow termination at the second station. A region of high air pressure is created by the static pressure.

The air in the region of high air pressure must flow to a region of lower air pressure. Circulation compartment 201 provides a large amount of space in housing 200 having a lower air pressure than the air pressure generated by bulk material 103. The air in the area of high air pressure flows from the generated region of high air pressure inside housing 200 into circulation compartment 201 as indicated by arrows 204. This prevents most if not all of the air in the region of high air pressure from flowing out of a leakage area, such as outlet 132 near second station 102.

In the preferred embodiment, circulation compartment 201 is above the termination area at second station 102 and is between first station 101 and second station 102. As air flows from the region of high air pressure into the region of low air pressure, the air move towards first station 101. This allows the air to again become part of the induced air flow caused by falling or projecting of bulk material 103 from first station 101 to second station 102. A circular flow of air in housing 200 is created by the air flowing in circulation compartment 201. This reduces the air that flows through housing 200 from inlet 131 to outlet 132.

In order to relieve the region of high pressure generated by the induced air flow, circulation compartment 201 must have a volume that is large enough to create a region of low air pressure that has a lower air pressure than any leakage area in dust containment housing 200. In order to assure that dust laden air does not escape from dust containment housing 200, the preferred embodiment of circulation compartment 201 is sized to have a volume that creates a flow resistance that is at least ten times smaller than the flow resistance friction loss of air flowing through dust containment housing 200 hereinafter referred to as flow-through induced air.

To properly size dust containment compartment 201, the frictional loss of the flow-through induced air must first be calculated. One manner for determining the frictional loss of the flow-through induced air is to determine the total power of the flow-through induced air as shown in Chapter 13 of "Industrial Dusts" and then use well known fluid mechanics equations to determine the frictional losses of the flow-through induced air. See, P. Drinker, Industrial Dusts (2.sup.nd Ed., McGraw-Hill 1954). One way to find the total power of the air flow in the system is given in "Industrial Dusts" at page 234 of chapter 13, equation (1) for the total power of flow-through induced air is:

where:

AHP=total power of the system;

Q=total flow rate of the air flow;

V.sub.a =the velocity of the air; and

h.sub.s =the system resistance.

Q, V.sub.a, and h.sub.s, are typically unknown. However, it is known that the number of unknowns can be reduced as shown in equation (2) from page 234 of Chapter 13 of "Industrial Dusts".

where

k.sub.s =the resistance for air flow through an enclosure; and

A=the cross sectional area of the stream of bulk material.

Substituting Equation (2) into Equation (1) the equation for total power becomes:

The cross-sectional area of the material stream can be measured to determine A. The coefficient, k.sub.s, can be found in commonly available references such as Chapter 6 of Industrial Ventilation, A Manual of Recommended Practice (American Conference of Industrial Hygienists 13.sup.th Ed.). Q.sup.3 must still be determined. One method for determining Q.sup.3, is given in Chapter 7 of "Plant and Process Ventilation." See, W. C. L. Hemeon, Plant and Process Ventilation (2.sup.nd Ed. Industrial Press 1963). In chapter 7, an equation for Q.sup.3 is:

where:

Q=total flow rate of the air flow;

H.P.=horse power of the falling material; and

A=the cross sectional area of the stream of bulk material.

A can be measured as stated above and H.P. can be calculated. As discussed in chapter 7 of "Plant and Process Ventilation", H.P. is calculated using an equation dependent on whether the bulk material is falling or is being projected and whether particles in the material stream reach terminal velocity during the fall from first station 101 to second station 102. See, Plant and Process Ventilation, at 125-136. Once H.P. of the stream of bulk material is found, Q.sup.3 is determined from the stream of bulk material and is substituted into equation (3) to determine the total power of the flow-through induced air in the system. The frictional losses of flow-through induced air are then determined using K.sub.s for leakage orifices and circulation compartment 201 is sized to have a volume such that the flow resistance of the recirculated induced air is ten times smaller than the calculated flow-through induced air frictional losses. The suspended particles of bulk material 103 either remain suspended in the air inside circulation compartment 201 or fall back down into bulk material 103 at second station 102 as flow of air dissipates inside compartment 201. Agglomeration also begins to occur as more suspended particles are added to the air inside circulation compartment 201 by the flow of air. In agglomeration, smaller particles of bulk material suspended in the air collide and form bigger, heavier particles of bulk material 103. The force of the flow of air is not sufficient enough to keep the bigger, heavier particles of bulk material 103 suspended in the air and the bigger, heavier particles fall back into bulk material 103 at second station 102. The agglomerated particles that fall back are bigger and heavier and do not become airborne at a later time. The bigger particles are carried out of housing 200 by conveyor belt 120 with the rest of bulk material 103.

The present invention relates to providing a passive dust control system. Although a specific example of a passive dust control system is disclosed for example herein, it is expected that persons skilled in the art can and will design alternative passive dust control systems that are within the scope of the following claims either literally or through the Doctrine of Equivalents.