LoadingSpouts

loading-spouts.ppt

Introduction

Introduction

Hopper Designs

hopper-designs.ppt

Explosion/Structural/Access Standards

explosion-structural-access-standards.ppt

Dry Products – Power Point

dry-products.ppt

Design Information

design-information.ppt

Particulate Collection Wet Scrubbing

Particulate collection wet scrubbing devices include spray chambers, cyclonic spray towers, impingement scrubbers, orifice scrubbers, mechanical scrubbers, venturi scrubbers(high energy), and eductors. Some, such as, spray chambers and cyclonic spraytowers are primarily for collection of large particles, generally 5 to 10 microns and larger. Eductors, impingement scrubbers, mechanical scrubbers, and orifice scrubbers generally are best for medium size particles, greater than 1 micron. Venturis are used for very fine particles, generally smaller than 1 micron in size. Of the aforementioned devices Sly Incorporated designs and manufactures impingement scrubbers (IMPINJET@), higher energy venturis, and eductors.  

VENTURI SCRUBBERS

 The venturi scrubber is specifically designed to collect particulate below 3 micron . Again, using inertial impaction as the primary mechanism, the venturi relies on accelerating the gas stream to very high velocities to produce sufficient kinetic energy values for sub-micron particles to be collected efficiently. The throat velocity for a 30″ pressure drop venturi is approximately 200 miles per hour, more than 4 times the whole velocity for an impingement plate. Since the mass of the particles is smaller the velocity must be higher.  To collect more of the fine particles, it is necessary to increase the speed (velocity) of the particles by increasing the speed of the air carrying the particles, as is done in the throat of a Sly VENTURI scrubber.  Higher velocity results in making the light particles “heavier” so that some of them travel in straight lines into the droplets instead of being carried around the droplet by the natural air stream.  This higher efficiency requires higher pressure drops.  For example:             VELOCITY                                  EFFICIENCY                        PRESSURE DROP            FEET/SECOND                      1 MICRON PARTICLES                     ” W.C.                         57                                            80%                                           1.5                       200                                           93%                                          12                       350                                           99%                                          30 Wet scrubbers rely on two mechanisms to accomplish the task of removing particulate matter from the air. These mechanisms are inertial impaction (the primary mechanism) followed by interception. Inertial impaction takes place when an air borne particle is unable to avoid colliding with an object in its flow path. This occurs due to the inertia of the particle and in relative terms its high kinetic energy. Kinetic energy is equal to one halftimes the mass times the velocity squared. Thus, in a gas stream containing a variety of particles of different sizes the larger particles with their higher mass have a higher kinetic energy and are more prone to be collected through inertial impaction.  Interception takes place when an air borne particle, following the air stream path around an obstruction, comes close enough to touch the obstruction and is then collected. This occurs when there is a high population of obstructions (water droplets) present and of similar size or smaller than the particles to be collected. Unlike inertial impaction, particle mass or velocity has little effect on interception.  A third mechanism, brownian diffusion, also has a minor effect. Very small particles, typically smaller than 0.3 microns exhibit essentially random motion, and do not necessarily follow stream lines. The collection of particles in brownian movement is random and will generally occur at very low velocities. As such, this mechanism does not affect the performance of wet scrubbers.  The Sly Venturi uses a “wet” approach design, which means the gas enters the scrubber in such a way that the first contact with water is on the converging walls of the scrubber. The water is distributed across the width of the converging walls using a pair of drilled pipe headers. The normal irrigation rate for venturi scrubbers is 8 gpm per 1000 CFM.  As the gas and liquid converge on the throat of the venturi ,both are accelerated. The water film is sheared by the acceleration of the gas into many fine droplets. The turbulent flow condition that exists in the venturi throat provides thorough mixing of the gas and liquid and producing large numbers of impaction targets for the particles.  The acceleration of the gas stream through the venturi throat also accelerates particulate matter, increasing the particle’s kinetic energy (expressed as 1/2 mass x velocity.  The presence of water in the venturi throat provides an abundance of targets for impaction.  Since kinetic energy is also dependent on mass it is easy to see why impaction type scrubbers have characteristic performance curves showing high efficiencies for larger particles.  Since mass is proportional to the cube of the diameter, a 1 micron size particle has 1/1000^th  the mass of a 10 micron size particle and 1/1000^th the kinetic energy when both are traveling at the same velocity. As with the Impinjet, the Venturi scrubber is sized and selected based on the outlet or saturated  gas flow volume.  The calculation is the same and requires the same input data.  The maximum flow conditions listed for the different sizes of venturi scrubbers is based primarily on the recommended shell velocities required in the cyclonic entrainment separator, not the venturi.  Sufficient velocity is required to establish a cyclonic flow pattern for the gas stream.  If the velocity through the separator is too high the collected droplets can easily be reentrained in the air stream. The specific pressure drop through the venturi throat is a function of the efficiency requirement and the particle size distribution. The same process used for determining the number of impingement stages in an Impinjet is used to calculate the pressure drop required of a venturi scrubber. In addition, we also calculate the apparent throat velocity to insure the scrubber selected will be operating within mid range of the adjustment of the throat damper.  As stated previously, the nominal irrigation rate for a venturi is 8 gpm per 1000 CFM. This can be increased for extreme particle loading conditions. Here too, the water can be recirculated from an external reservoir. The solids loading in the water should be similar to that recommended for the Impinjet not more than 10%. A continuous blowdown is always suggested to maintain a constant solids loading.  Since the processes that typically produce sub-micron particles are high temperature processes (500°F and higher), a pre-quench is recommended. The pre-quench is used to achieve saturation of the gas prior to entering the venturi throat. Adiabatic saturation of a high temperature gas stream usually results in significant changes in volume. If these changes were to occur in the throat the performance of the scrubber could be seriously compromised. In addition, the act of evaporation tends to repel particulate. This repulsion phenomenon is best dealt with prior to the throat.  Pre-quenches may be nothing more than a section of ductwork with two or three spray pipes installed to distribute water across the entire gas cross section. For very high temperature service, in excess of 1000^o F , the pre-quench is coupled with a water seal weir to provide for thermal expansion and contraction of the various system components. Normally the irrigation rate for a quench is determination to be 1.5 to 2 times the calculated evaporative loss. This provides more than sufficient water present to perform the desired function. And, using coarse spray distribution insures that for the most part none of the droplets will be completely evaporated. Complete evaporation of water droplets can leave behind a sub micron aerosol of the solids dissolved in the water, such as, calcium and magnesium salts typically present in hard water.

Criteria for Fabric Selection

The basic criteria for fabric selection are temperature, inlet loading, particle size distribution, gas composition, abrasion, static charge, release and efficiency requirements.   1.         Temperature:   Match inlet gas temperature with the fabric capable of continuous operation at or above the maximum temperature the system might experience. 2.         Inlet loading:   Inlet grain loading (normally expressed in grains/dry standard cubic feet) is especially important when the loading is very light.  Light loadings can make it difficult for the media to operate with sufficient dust cake. 3.         Particle size distribution:    Finer the distributions often require using higher efficiency media. 4.         Gas composition:   The designer must make sure that if corrosive compounds or hydrocarbons are present in the gas stream, that the media selected will hold up in that environment. 5.         Abrasion:   When the dust is abrasive, the designer might look at using heavier felt or adding wear cuffs to the bottom of the filter bags on hopper entry inlets. 6.         Static charge:  Explosive dusts and gases or dust that can develop a static charge in the duct system require some time of static grounding.  This can come in the form of a braided grounding wire sewn into the filter bag and attached to the tubesheet.  Grounding can also be done through the bag material such as epitropic (using carbon or graphite fibers throughout the bag, or by using 3-4% stainless steel fibers throughout the bag.  In these cases it is important to make sure the bag collar (top loaded bags) are fabricated out of the same material so that there is conductivity to ground. 7.         Release:   A properly designed dust collector has two (2) functions.  The first is to collect the dust, the second is to release the dust off of the filter bag after it is collected.  In the case where the dust is oily or sticky, the fabric may require a treatment to the filtering surface in order to aid in the release.  This can take the form of modifying the surface with treatments such as singing or glazing.  It can also mean a coating such as a PTFE bath or even a membrane.  There are also specialty fabrics that treat the fiber before the felting process providing a longer lasting surface than typical coatings. 8.         Efficiency:   Last but certainly not the least important is collection efficiency.       Expressed as a mass loading (gr/dscf), the required efficiency is determined by the environmental engineer based on EPA, State or regional regulations that coincide with the particular plants allowable discharges to the atmosphere. We have tried to give you an overall view of fabrics and the criteria used for the correct selection used in various types of air pollution control equipment.  This information should be used as a guideline.  There can be multiple choices for any particular application.  The ultimate choice depends on the experience of the designer.

Case Study – Mayville Limestone (Scrubber)

Mayville Limestone located in Wisconsin contacted Sly (Bill Harding) regarding the replacement of their existing wet scrubber on their rotary limestone dryer.  In discussions with the customer it was determined that they did not know the emission level of the existing unit, particle sizing or whether it was currently meeting the existing regulations. The system includes multi-clone precleaners, hydroclones (for water recirculation) which were wearing out and a large settling pond. Since no inlet data existed we convinced the customer that the only way to safely approach this project with any guarantees on outlet emissions was to do a simultaneous inlet and outlet test for loading and particle size distribution.  Since the existing scrubber was only operating at a 3” pressure drop we felt the efficiency was very low.  Test results indicated a very high inlet loading (22 gr./cf) and a mean particle size of 11.5 microns. During the interim the customer was cited by the DNR and required to file a plan of action.  At this point the discussions involved a new venturi running at a 40” wg pressure drop to meet an outlet emission requirement of 0.03 gr/dscf or a baghouse.  Also at issue was the replacement of the multi-clones that were experiencing high maintenance.  The customer perception was that wet scrubbers were always used on this application and had not considered a baghouse.  Since the dew point was relatively low (110 deg. F) we felt a baghouse was a viable option.CConsiderationsNew venturi wet scrubber:Pros:    Less expensive than baghouse            Eliminate hydroclones since pond was large enough            Less maintenance than baghouse                         (bag replacement)Cons:   Higher energy costs (40” wg pressure drop)            Higher energy costs due to                         pump for water supply.            Must have pre-cleaner.            Pond maintenance.  New baghouse: Pros:    Eliminate pre-cleaner by using                         high side inlet.            Lower energy costs (lower pressure                         drop/ no pump).               Lower emission rate.Cons:   More maintenance due to bag changing.            Higher capital costs. The competition on this project for the baghouse option was DCE and MAC.  DCE offered a collector at a 7:1 A/C based on not using a cyclone and no high inlet or classifier on the collector.  They stated to the customer that the high inlet loading would make the collector more efficient.  MAC offered a high inlet at a 5:1 A/C.  MAC’s high inlet eliminated a few bags and installation of a target plate forcing the air down to the hopper.  This type of inlet does not allow for settling that reduces the load on the filter bags. Sly offered a 4:1 A/C and a high inlet that provides a large section of the collector to bring the air into with full partitions on either side to eliminate bag wear.  This inlet section runs the full width of the collector reducing the downward velocity to less than 800 fpm assisting gravity and low velocity to help settle out the majority of the inlet dust loading. As with many projects there were some budget constraints.  Since the dryer only ran 10-12 hours per week we were asked to increase our A/C.  Our final quote was at a 5:1 A/C which we felt was still conservative considering the low drying hours. Sly received the order for a STJ-1415-10 with high side inlet to handle 16,860 ACFM at 187 deg.F.   The unit was configured with (1) hopper that would discharge into a screw conveyor to return the dust back into the building and the process.  We also insulated the entire collector (factory installed by Sly in Mississippi) to help prevent condensation.  The initial start-up did not go well.  One of the operators started the dryer by throwing in a lit newspaper and turned on the gas.  They had to replace the initial set of bags.  All the operators were given specific instructions on the proper start-up and shut down sequence to further avoid any condensation and fires.  Proper start-up requires pulling warm air into the collector before any material is feed into the dryer.  By pre-heating the collector condensation caused by hot moist air entering a cold baghouse is avoided.  Likewise on shut down it is proper to run warm air through the collector after the product feed has been stopped to pull any humidity out of the collector.  We also supplied a temperature controller and thermocouple to alarm high temperature since the collector was supplied with polyester felt bags.  The collector was started up in December of 1999 and has only had one (1) bag change.  The DNR was so pleased with the initial stack test that yearly testing was deleted from their permit requirements.

The Sly Tubejet Line

The Sly TubeJet collector was introduced to the market in 1990.  Over the years it has developed into our largest product line.  With (4) four distinct products, the TubeJet can handle almost any baghouse application.

 

Out of the four products, the STJ is by far the most popular.  It is simple in design and can be modified to fit a large number of applications.  The STJ has many features not found on other collectors.

 

-                  Hinged top access doors that are double gasketed to maximize sealing against the weather.

-                  Pulse pipes that are held in place by a stainless steel Morris coupling and requires no tools for removal.

-                  The fixed end of the pulse pipe is designed to be installed only one way which guarantees the pulse pipe is installed properly.  The end of the pipe is flattened with a pin welded on the top.  The back wall of the collector has a slotted bracket that guarantees the pulse pipe can only be installed so that the blow holes are always centered directly over the venturi opening.

 

The following features are part of any TubeJet collector:

 

-                  The complete cleaning system which includes the compressed air header, pulse valves, solenoid valves, timer, tubing and wiring are all factory installed.  Furthermore, the cleaning system is hooked up and tested before the collector leaves the plant.

-                  All interior surfaces are primed.

-                  All exterior surfaces receive a full prime and finish enamel.

-                  All steel for supports and filter access is blasted prior to shipping to Sly’s plant.

-                  Supports and walkways are pre-assembled and match marked.  All pieces have a tag that is attached to the piece with a wire tie.  The tag has the part number clearly marked and then the tag is sealed in plastic.

 

While selling standard units might be our preference, we all know that customizing based on application or customer preference is something that happens on a regular basis.  For this reason, you might want to be aware of some of the modifications we have made over the years.  Some of these are standard options while some have a high degree of customization.

 

-                  Walk-in clean air plenums.

-                  Special filter bag diameters such as 4-1/2” and 5-1/4”.  Smaller bag diameters may be used due to the need for wider bag to bag spacing.  Wider spacing is required if the material is fibrous with a tendency to bridge between the filter bags.  Normal bag to bag spacing with 5-3/4” diameter filter bags is 2-1/8”.  If we keep the row spacing standard (8”) and change to 4-1/2” dia. bags then the bag to bag spacing opens up to 3-1/2”.  This wider spacing can also be used to decrease interstitial velocities in low bulk density materials.

 

-                  Alloy construction for either product contact parts (tubesheet, filter housing and hopper) or gas contact, which includes the clean air plenum and pulse pipes.

 

-                  Many times with stainless steel construction  the application requires a special finish on the interior surfaces in contact with the product.  There are a couple of options that our shop is capable of doing.  Grinding all interior welds either smooth or flush.  Polishing these welds to either an 80 grit or 120 grit finish.

 

-                  Explosion protection.  This can include a copper or stainless steel grounding wire sewn into each filter bag.  Special NEMA 7/9 explosion proof electrical enclosures for the solenoid valves and timer.  Explosion relief vents designed to follow the most current NFPA 68 guidelines.  Internal sprinkler fittings.  In some cases we have worked with the customer to install fittings for a suppressant system.  Special filter bag material may also be required.  This would include fabric that is constructed either with some content of carbon or stainless steel fibers to dissipate static charges.

 

-                  In most high temperature applications or in the event of installation on a dryer or other process where moisture is present, the collector should be insulated.  This is typically done by installing a fiberglass mat or board on all exterior surfaces of the collector and then covering the insulation material with a metal cover.  We would normally use a 18 ga. galvanized sheet that is sheet metal screwed to lagging.  On occasion we have used aluminum and stainless steel sheeting.  All seams are overlapped and gasketed to weather proof the collector and protect the insulation.  Special attention is paid to the hinged top access doors and any hopper access.  All access doors are insulated by building a cavity on the inside of the door and then filling the cavity with insulation material.

 

-                  High side inlets of various configurations are available for applications requiring low interstitial velocities, material drop out to reduce the load on the filter bags.  Another reason we might use a high inlet would be to facilitate the inlet ductwork.  Some situations make it difficult to use a hopper inlet.

 

These are only some of the special adaptations we can use based on the application and/or the customers requirements.