Variable throat venturi scrubber-Venturi scrubber - Wikipedia

This is a continuation of application Ser. This invention relates to gas and liquid contacting apparatus and more particularly to a venturi type scrubber that is particularly useful for removing solid particles from a large volume gas flow such as that emitted from fossil fueled steam boilers or the like. Description of the Prior Art. The United States patent to Erni, U. The opposed side walls of the blocks forming the restricted section of the apparatus are not parallel so that a mere restricted orifice instead of a constant cross sectional area scrubbing throat is provided.

Ratio a -- 4. Operational Features and Characteristics In addition to the construction of the venturi of the present invention, wherein the scrubbing throats 72,72a FIG. The angle of divergence of the diverging portions of the venturi passage is equal to the angle of divergence Variable throat venturi scrubber the diverging side walls of the insert, and the latter are positioned in the diverging portion passage of the venturi passage, thus forming two parallel wall throats. There is no teaching of a venturi scrubber construction wherein the ratio of the width of the scrubbing throat to its length is constant throughout the range of adjustment of an insert in the venturi. There are no sharp edges Throaat contours through the throats 72,72a which would require additional power consumption for the fan that propels Ryka slip on gas through venturii assembly. The cyclone 20 is of the usual tangential inlet type well known in the art, the details of which are not critical to the present invention. These instruments include various flow meters and pressure measuring devices connected to various parts of the unit to monitor the operation thereof. Certain types of orifices throat areas that create more turbulence than a true venturi were found to be equally efficient for a given unit of energy consumed McIlvaine Company

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Robert Fischer. Three methods of maintaining a constant pressure drop for variable flow conditions are discussed below:. Air Flow Rate -- 3, cubic ft. As indicated in FIG. The diagram of FIG. Abrasion can be reduced by lining the throat with silicon carbide brick or fitting it with a replaceable liner. MEAN niA. Stated differently the aforestated insert and venturi passage side walls are parallel throughout the length of the throats 72,72a. With the aforesaid geometry, regardless of the adjusted position of the insert within the operating limits, the ratio of the width of each scrubbing throat formed by the insert to the depth of the throat remains constant throughout the range of adjustment. The resultant shape of the Russian teen models naked 60 is rectangular, Variable throat venturi scrubber its long dimension is somewhat too large for connection to conventionally shaped inlet ducts. Namespaces Article Talk.

Variable throat Venturi scrubbers are a special type of wet reducer applied for the treatment of humid flows containing very fine dust particles.

  • Environmental Protec- tion Agency, have been grouped into nine series.
  • Monroe Venturi Air Scrubbers are currently operating continuously at high efficiency in many types of industrial installations throughout the United States, Mexico, and Canada.
  • A venturi scrubber is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream.
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  • Venturi scrubbers are commonly used in pollution control systems as particulate control devices.
  • Effective design and testing define the success of a filtration system.

This is a continuation of application Ser. This invention relates to gas and liquid contacting apparatus and more particularly to a venturi type scrubber that is particularly useful for removing solid particles from a large volume gas flow such as that emitted from fossil fueled steam boilers or the like.

Description of the Prior Art. The United States patent to Erni, U. The opposed side walls of the blocks forming the restricted section of the apparatus are not parallel so that a mere restricted orifice instead of a constant cross sectional area scrubbing throat is provided. No specific relative dimensions of the venturi throat and major diameter of the insert are described and due to the fact that the parts are conical of circular cross section the resultant scrubbing throat has a constantly increasing cross sectional area.

The United States patent to Ueda U. Disposed in the throat is an elliptical insert restriction of variable width but the resultant flow passage throats are not of constant cross sectional area.

As seen in the diagram on FIGS. This results from the discontinuities in the insert in the throat and the turbulent losses associated with them. This non-recoverable pressure drop in Ueda represents a loss of energy and increased horsepower requirements for the fan or blower that forces the gas through the device. The United States patent to Willett, U. The resultant conical throat area decreases in cross section until the gas discharges from the throat in what amounts to a simple orifice.

Thus the gas discharges from the circular throat orifice into a low velocity chamber. There is no expander and no recovery of pressure with savings in the induction energy requirements. The Unites States patent to Hausberg, U. The scrubbing throat has a progressively increasing cross sectional area and there is no pressure recovery zone at the discharge for economizing in induction power.

The United States patent to Simizu, U. The shell is partially in the inlet to the venturi throat and partially in the throat itself in its lowermost position and completely out of the throat in its uppermost position. In the lowermost position, the gas flows primarily through the insert which thereby acts as a single orifice.

In other positions, the gas flows both through and around the insert before reaching the venturi throat and no constant cross sectional area passage is provided. Water is injected beneath the cone. There is no teaching of how to provide a scrubbing throat of substantial length that has a constant cross sectional area along its length.

In fact, the end of the cone and the diverging venturi wall form a simple orifice. The United States patent to Lorraine et al, U. There is no teaching of a venturi scrubber construction wherein the ratio of the width of the scrubbing throat to its length is constant throughout the range of adjustment of an insert in the venturi.

The apparatus of the present invention will be explained in connection with an installation for the scrubbing of flue or stack gases or the like to remove and collect particles, dust, etc. However, it will be understood that the general principles of the apparatus to be described in detail can be applied to other gas-liquid contact apparatus. One of the principle features of the present invention is the provision of a pair of relatively long scrubbing throats wherein the gas flows at a relatively high velocity in the presence of substantially atomized liquid, such as water, for providing a relatively long and effective contact path between the dust or other particles and the liquid, thereby producing a high collection or scrubbing efficiency.

Although the aforesaid relatively long scrubbing throats diverge, it is a feature of the invention that they have a constant cross-sectonal area along their length. Another feature of the present invention resides in provision of an adjustable venturi insert which provides the aforesaid pair of relatively long throats of constant cross sectional area, these throats having a constant ratio of width to length throughout the range of insert adjustment.

It has been found that holding of the aforesaid ratio of width to length at some predetermined constant facilitates a design which minimizes variations in the frictional losses that occur in the throats for various gas flow rates encountered, while maintaining optimum scrubbing velocities.

Another feature of the present invention is the minimizing of power losses in the scrubber, that is in minimizing the amount of energy required to accelerate the gas flow through the scrubber, throughout the range of adjustment of the aforesaid insert, while providing a scrubbing and particulate removal action that is superior to that of prior devices.

Briefly, the aforesaid features and advantages are attained under the present invention by providing a venturi passage having a rectangular cross-section. This passage has converging-diverging side walls that join to form a minimum area throat or restriction, which is also of rectangular cross section.

Within the venturi passage is what will be termed a "double diamond" insert, which in the form illustrated has diverging-converging side walls that are joined at a rectangular restriction representing the major cross sectional area of the insert. The other walls of this insert, which will be termed edge walls, are flat or straight and slidably fit the edge walls of the venturi passage, so that the gas flow occurs between the side walls of the venturi passage and insert.

The width of the major section or restriction portion of the double diamond insert is equal to the width of the restricted throat of the venturi passage, and the latter is very short and in fact, is preferably substantially contained in a plane.

The angle of divergence of the diverging portions of the venturi passage is equal to the angle of divergence of the diverging side walls of the insert, and the latter are positioned in the diverging portion passage of the venturi passage, thus forming two parallel wall throats. The included angle between the converging side walls of the insert and the diverging side walls of the venturi which can be referred to as the expander remains the same throughout the range of insert adjustment.

Long scrubbing throats of maximum width are provided when the insert is positioned in the venturi passage so that the apex of the diverging walls lies in the plane of the venturi throat restriction.

This provides two diverging scrubbing throats of constant width and hence of constant cross sectional area along their length. In this specificaion and in the appended claims, when referring to the scrubbing throats the term "cross sectional area" refers to a section that is perpendicular to the longitudinal axis of the scrubber.

With the aforesaid geometry, the insert can be adjusted to accommodate various gas flow rates while maintaining selected scrubbing velocities. With maximum flow rate, the insert may be lowered to its fully open position, or the insert may be raised to a position more effective for a reduced flow rate.

In the latter case the insert is raised, so that its maximum dimension or restriction approaches the venturi passage throat or restriction. With the aforesaid geometry, regardless of the adjusted position of the insert within the operating limits, the ratio of the width of each scrubbing throat formed by the insert to the depth of the throat remains constant throughout the range of adjustment.

In operation, the converging venturi inlet passage accelerates the gas, utilizing power most of which can be recovered in the expander section, and the gas reaches maximum velocity in the diverging pair of constant cross sectional area scrubbing throats without the introduction of power consuming turbulence other than the turbulent mixing of liquid and gas resulting from the high velocity in the throats.

These throats provide a relatively long path for water contact and particulate collection. The scrubbing liquid, such as water, is introduced above the venturi throat and is atomized during the passage of the gas through the relatively long, diverging constant cross sectional area scrubbing throats formed between the diverging wall portions of the insert and the diverging side walls of the exit passage of the venturi.

It has been found that with flow rates normally encountered in this service, liquid atomizing type nozzles are not required at the liquid spray pipes. It is sufficient to merely introduce liquid into the relativley high velocity air stream in the inlet passage of the venturi, thus forming droplets without requiring the use of high pressure pumps for the liquid.

Some of the liquid droplets impinge upon the side walls of the venturi inlet and upon the upper portion of the inlet.

The liquid is further atomized and sheared off at the venturi throat restriction and along the diverging, constant cross sectional area scrubbing throats. This atomization of the liquid, coupled with the relatively long length of the scrubbing throats, and the resultant relatively long length of the turbulent mixing zone , provides a high collection effeciency.

When the gas leaves the diverging scrubbing throats, it enters two gradually expanding passages so that pressure is recovered as velocity is reduced gradually, thereby minimizing the energy consumption to cause the gas to flow through the scrubber.

The advantages of the present invention can be obtained for a specific installation by designing the over all dimensions of the venturi and the insert to achieve a desired fixed flow rate through the constant cross sectional area scrubbing throats.

On the other hand the installation lends itself to adjustment of the insert within the venturi passage for accommodating the apparatus to various changes in initial flow rates encountered in service. These adjustments do not effect the collection efficiency and do not increase the frictional losses in the scrubbing throats, nor do the adjustments increase any turbulent losses that might occur in the expander.

In fact, if a unit is designed to efficiently handle a maximum flow rate wide open, and a lesser flow rate is encountered, closing the insert for optimum scrubbing will result in a decrease in frictional losses due to the design and principles of the unit. Another feature of the present invention is that it teaches how to determine a practical ratio between the width across the elongated scrubbing throats and their depth, which provides good compromiss between minimizing frictional losses, insuring adequate atomization and scrubbing action by the water introduced into the apparatus, without producing a unit that is excessively narrow and deep.

In accordance with the present invention, this ratio of throat depth to throat width will vary from a value not substantially less than one, to a more normal value of four, but this ratio may be eight or higher. However, ratios much higher then eight when measured with the insert fully open result in a rather narrow and deep apparatus, which requires a more complex transition to the ordinary inlet and outlet ducting of approximately square or somewhat rectangular cross sectional configuration.

In these diagrams the venturi assembly of the present invention is indicated generally at 10 and includes a venturi passage 12 with double diamond insert 14 disposed therein. The construction of the venturi assembly 10 is best shown and will be described in detail in connection with FIGS.

Referring to the overall layout of the system shown in FIGS. Perforated pipes 30 FIGS. The outlet of the venturi 12 directs the gases, which now incorporate the water that has been introduced into the venturi and has picked up particles of dust or the like, through an exit duct 18 and on to a cyclone separator The cyclone 20 is of the usual tangential inlet type well known in the art, the details of which are not critical to the present invention.

The scrubbed gases leave the cyclone 20 by means of an exit duct 22 and the gases are pulled through the system by a fan 24 of conventional design. The fan blows the scrubbed gases out of an exit duct 26 usually for release to the atmosphere. As mentioned, water is sprayed into the inlet passage of the venturi 12 by means of spray pipes 30 FIG. The pump 36 draws water from a recirculation tank In the cyclone 20 the water and entrained particles are separated from the gases flowing through the cyclone and dropped through the bottom of the cyclone.

The water is picked up in a manner conventional in the art by a surge tank 40 FIG. By means well known in the art, the dust particles are allowed to settle out from the water in the surge and recirculation tanks. Clarified water is thus recirculated by means of the pump 36 back into spray pipes 30, as previously described.

A control and indicator panel 46 is illustrated in FIGS. These instruments include various flow meters and pressure measuring devices connected to various parts of the unit to monitor the operation thereof.

The various pipes, etc. Construction of the Venturi Assembly. Referring to FIGS. A rectangular double diamond insert 14 of complimentary configuration is adjustably mounted in the venturi passage As seen in FIG. The flat or straight edge walls 58 of the venturi are welded to the converging-diverging side walls 52,56 to form a rectangular venturi passage.

The straight edge walls 58 are best seen in FIGS. The water spray pipes 30, previously described, project into the venturi just above the entrance throat 50, as seen in FIGS. In order to provide a connection to the inlet duct 16 FIGS. The converging side walls 52 and the diverging side walls 56 of the venturi join to form a rectangular restriction 60 FIGS.

The Insert. The insert 14, which has been referred to as a double diamond insert, has a special geometrical construction relative to the dimensions of the venturi 12 in accordance with the present invention. The insert is rectangular in section and is slidably received between straight edge walls 58 of the venturi, as best seen in FIGS. The insert has diverging, flow dividing side walls 62 FIG.

The insert has downstream converging sidewalls 64 which form interior obtuse angles with the sidewalls 62 closed by a cap member The walls 64 need not continue to a line junction as in the case of the flow dividing walls 62, because by the time the gases have reached the lower end of the insert their velocity is so low that it will have been reduced to the point wherein further controlled expansion is not required.

The insert is of rectangular cross sectional shape throughout its length and is closed at its edges by straight edge walls 66 best seen in FIGS. The diverging side walls 62 of the insert join the converging side wall 64 of the insert to form a maximum width, rectangular restriction indicated at The edge walls 66 of the insert slidingly engage the edge walls 58 of the venturi so that essentially all of the gas must flow between the side walls of the venturi and the side walls of the insert.

As best seen in FIG.

The diagram of FIG. OZ 9,a8E 6.? Sheldon would be at his whiteboard explaining to Howard the formulas and calculations for particle removal efficiency in a Venturi. However, due to the divergence of the entrance walls 52, the duct connection X,Y will be almost square. In the variable-flow Venturi, the throat is adjustable.

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The principal atomization of the liquid occurs at the rods, where the high-velocity gas moving through spacings creates the small droplets necessary for fine particle collection.

These rods must be made of abrasion-resistant material due to the high velocities present. All venturi scrubbers require an entrainment separator because the high velocity of gas through the scrubber will have a tendency to entrain the droplets with the outlet clean gas stream. Cyclonic, mesh-pad, and blade separators are all used to remove liquid droplets from the flue gas and return the liquid to the scrubber water. Cyclonic separators, the most popular for use with venturi scrubbers, are connected to the venturi vessel by a flooded elbow Figure 8.

Venturis are the most commonly used scrubber for particle collection and are capable of achieving the highest particle collection efficiency of any wet scrubbing system. The atomized liquid provides an enormous number of tiny droplets for the dust particles to impact on.

These liquid droplets incorporating the particles must be removed from the scrubber outlet stream, generally by cyclonic separators. Particle removal efficiency increases with increasing pressure drop because of increased turbulence due to high gas velocity in the throat. These high pressure drops result in high operating costs.

The proper amount of liquid must be injected to provide adequate liquid coverage over the throat area and make up for any evaporation losses. If there is insufficient liquid, then there will not be enough liquid targets to provide the required capture efficiency. Venturi scrubbers can be used for removing gaseous pollutants; however, they are not used when removal of gaseous pollutants is the only concern. The high inlet gas velocities in a venturi scrubber result in a very short contact time between the liquid and gas phases.

This short contact time limits gas absorption. However, because venturis have a relatively open design compared to other scrubbers, they are very useful for simultaneous gaseous and particulate pollutant removal, especially when:. To maximize the absorption of gases, venturis are designed to operate at a different set of conditions from those used to collect particles. The gas velocities are lower and the liquid-to-gas ratios are higher for absorption.

For a given venturi design, if the gas velocity is decreased, then the pressure drop resistance to flow will also decrease and vice versa. Therefore, by reducing pressure drop , the gas velocity is decreased and the corresponding residence time is increased.

Liquid-to-gas ratios for these gas absorption applications are approximately 2. The reduction in gas velocity allows for a longer contact time between phases and better absorption. Increasing the liquid-to-gas ratio will increase the potential solubility of the pollutant in the liquid. The primary maintenance problem for venturi scrubbers is wear, or abrasion, of the scrubber shell because of high gas velocities.

Particles and liquid droplets traveling at these speeds can rapidly erode the scrubber shell. Abrasion can be reduced by lining the throat with silicon carbide brick or fitting it with a replaceable liner.

Abrasion can also occur downstream of the throat section. To reduce abrasion here, the elbow at the bottom of the scrubber leading into the separator can be flooded i. Particles and droplets impact on the pool of liquid, reducing wear on the scrubber shell. Another technique to help reduce abrasion is to use a precleaner i. The method of liquid injection at the venturi throat can also cause problems. Spray nozzles are used for liquid distribution because they are more efficient have a more effective spray pattern for liquid injection than weirs.

However, spray nozzles can easily plug when liquid is recirculated. Automatic or manual reamers can be used to correct this problem. However, when heavy liquid slurries either viscous or particle-loaded are recirculated, open-wear injection is often necessary. Venturi scrubbers can have the highest particle collection efficiencies especially for very small particles of any wet scrubbing system.

They are the most widely used scrubbers because their open construction enables them to remove most particles without plugging or scalding. Venturis can also be used to absorb pollutant gases; however, they are not as efficient for this as are packed or plate towers. The ability of venturis to handle large inlet volumes at high temperatures makes them very attractive to many industries; consequently, they are used to reduce particulate emissions in a number of industrial applications.

Venturis are also used to control fly ash and sulfur dioxide emissions from industrial and utility boilers. The particles that impact liquid drops can be easily separated from the bulk of the gas stream by collecting them in a cyclonic centrifugal or chevron impaction mist eliminator.

Some Venturi scrubbers are designed by well-intentioned engineers to shortcut these laws of physics in an effort to produce a competitive product. The laws of physics, however, require that a properly designed Venturi scrubber must include three critical elements: a converging inlet, a defined Venturi throat and an expander section.

Whenever shortcuts in design exclude any of the three critical elements, the Venturi scrubber will be less effective and will consume excess energy without achieving the intended PM collection efficiency. This article highlights the important design factors for wet Venturi scrubbers and explains why shortcuts in design create Venturi scrubbers that are less effective at capturing PM.

In the wetted approach Venturi scrubber as shown in Figure 1, the gas is introduced in a radial fashion and scrubbing liquid is provided to completely wet the inlet section. The gas is introduced in such a way that, once it leaves the inlet gas nozzle, it never contacts a dry wall. The only surfaces the gas can touch are already wetted with a liquid film so solids deposition does not occur.

Venturi scrubbers can be furnished as either fixed-flow or variable-flow devices. In the fixed-throat Venturi, gas flow must be constant to maintain a steady differential pressure and collection efficiency.

In the variable-flow Venturi, the throat is adjustable. When reduced gas flow conditions are encountered, the position of the throat damper can be changed to hold constant differential pressure, and collection efficiency can be maintained.

The physical mechanisms affecting PM collection in Venturi scrubbers include inertia inertial impaction , diffusion, electrostatics, Brownian motion, nucleation and growth, and condensation. While all of these mechanisms affect collection, the predominant phenomenon is inertial impaction.

When capturing a particle of a given diameter and density in a gas stream of a given viscosity, two main variables affect the collection efficiency allowing, as stated earlier, that impaction is the predominant mechanism :.

For inertial impaction to occur, the dust particle must run into a liquid droplet. How does this occur? The greater the relative velocities momentum between the dust particles relative to the liquid droplets, the greater probability that the dust particles will collide with the liquid droplets and be captured.

To illustrate: Someone drives a car down a road, and the air is filled with flying insects. The faster the car travels, the more likely that insects will hit the windshield. In this analogy, the bugs are the dust particles, and the car is the liquid droplet collector. A Venturi is a well-known device for accelerating a fluid stream to a high velocity and returning it to its original velocity with a minimum loss of energy. This is why a Venturi is an integral component in high-velocity wind tunnels.

It is therefore only natural that the Venturi was chosen as the most efficient method for contacting a gas and a liquid for maximum PM removal from the gas.

Liquid drops can be created in two different ways in the scrubber. The most common is simply to allow the high-velocity gas to atomize the liquid. This consumes some energy as fan horsepower. When these drops enter the throat area and encounter the high-velocity gas stream, they explode into thousands of smaller droplets atomization. The second method is to atomize the liquid using spray nozzles. In this case, the energy used to atomize the liquid is provided by pump horsepower.

No substantial energy savings are realized by using either technique, but the high-pressure nozzle technique is limited to applications in which a clean liquid stream is fed to the Venturi, limiting its application when recirculating scrubber liquid. As the gas exits the scrubbing throat, it carries with it all the liquid droplets, which have now achieved a velocity nearly that of the gas stream see Figure 2.

In the expander section, the gas is slowed as the cross-sectional area increases. Some of the kinetic energy from the liquid droplets transfers back to the gas stream, resulting in a recovery of part of the energy required to accelerate the gas to throat velocity.

This energy regain is what distinguishes a Venturi scrubber from any other type of wet scrubber. Figure 3 is a typical pressure profile for a Venturi scrubber. As the gas accelerates, the pressure in the gas stream decreases to its lowest point in the throat. As the gas begins to slow down in the expander section, pressure rises and reaches a level only slightly lower than the pressure at the inlet.

The difference between A and D inlet and outlet pressures , or the pressure drop, represents the energy expended in the scrubbing process. If the expander section is omitted, the energy expended is greater and shown as the difference between A and C. The Calvert equation can be used to predict the actual pressure drop for a given throat velocity. The Calvert equation states:. At saturated conditions, the pressure drop is equal to the power required to accelerate the liquid to the gas velocity.

This is not percent accurate because it does not account for:. After the required pressure drop has been computed, the Venturi scrubber must be sized. Usually, all dimensions of the scrubber derive from the size of the scrubbing throat. Therefore, a designer can use either the Calvert equation or empirical velocity-versusdifferential-pressure curves for a given scrubber to size the throat for a given saturated gas flow rate.

Note that using the saturated gas flow rate, rather than the hot inlet gas flow rate, is imperative for sizing the throat. As a hot flue gas stream enters the throat, it is immediately quenched to its saturation temperature, and the flow rate is reduced substantially.

If the throat were sized on the unsaturated hot gas flow rate, it would be far too large for most applications, and the required collection efficiency could not be achieved. A fan must also be selected which will handle the required gas flow rate. The fan is sized on the actual gas flow rate at the fan, not necessarily the saturated gas flow rate, particularly on the forced draft side.

Finally, pumps, piping, duct work and tankage must also be designed to complement the design parameters of the Venturi scrubbing system.

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