Powder and Bulk Engineering, March 1996 39

tic

W.J. Klh nczak and G.D. Applewhite Dustex Corp.

This arl icle explains how you can configure a dust collec- tor for j our pneumatic conveying system to provide effi- cient, cc 1st-effective dust collection. Three major sections cover tl lese topics for pneumatic conveying systems: common dust collection problems; ways to reduce dust collectic a complexity, headroom requirements, and cost; and chc osing a baghouse or cartridge dust collector for your pneumatic conveying system. Related information providc s tips on cleaning cartridge dust collectors and vacuun i pneumatic conveying systems.

n a PI ieumatic conveying system, a dust collector is located at tht: system’s terminal point and separates material from I the c mveying gas (typically air). The material drops into a

receiver bin and dust is captured in the dust collector’s filter ele- ments. [j Mitor’s note: While this article concentrates on apply- ing bagliouse dust collectors, many of the configurations discusse here also work with cartridge dust collectors. The use of cartric Ige collectors for pneumatic conveying applications is growing. See the section, “A final word: Choosing a baghouse or cartriclge dust collector,” for advice on selecting a collector for your system.]

Pneuma tic conveying is one of the most demanding applica- tions for a dust collector. A pneumatic conveying system can have a aide dust loading range per unit of airflow, handle dusts with wiclely varying characteristics, and operate at high pres- sure or low vacuum. The system is also subject to dust load surges ai id headroom restrictions. Regardless, many users still install ‘‘L niversal-design” dust collectors on pneumatic convey-

ing systems without paying attention to each system’s specific needs. The result? Less-than-optimal efficiency, as well as other problems.

Common dust collection problems in pneumatic conveying systems

In most dust collector applications, the largest dust load is from 10 to 15 grains per cubic foot. (A pound of dust contains 7,000 grains.) But in a pneumatic conveying system, the dust collec- tor’s typical dust load is 150 grains. For this reason, designing a dust collector for a pneumatic conveying application concen- trates on reducing the dust load to the collector.

In the past, a typical pneumatic conveying system blew dust- laden air into a cyclone, causing material to fall directly into a receiver bin. Material often discharged from the cyclone hop- per into the bin simply by centrifugal force and gravity, without requiring a rotary airlock feeder. This simple design required a relatively large diameter cyclone with a low pressure drop. It was successful for paper and wood chip conveying systems, which could achieve collection efficiencies over 96 percent be- cause of the large particle sizes. The remaining dust was con- sidered harmless to the environment.

But over time users realized the escaped dust had some value as an ingredient in other products or as boiler fuel and began to switch to a more efficient, smaller diameter cyclone with a higher pressure drop to capture more of the dust. The problem was that the pressure at the cyclone bottom was no longer close enough to atmospheric pressure and prevented the dust from falling into the receiver bin. So the users installed a rotary air- lock feeder at the cyclone bottom to promote dust flow into the bin, as shown in Figure la. In other applications, the percentage of dust escaping through the high-efficiency cyclone was too high to be discharged into the atmosphere, so users added a bag- house dust collector to the system, as shown in Figure lb.

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The resulting dust collection systems have become more com- plex and require more headroom. They also cost more to oper- ate than traditional systems because they have higher pressure

Powder and Bulk Engineering, March 1996

drops.

Ways to reduce dust collection complexity, headroom requirements, and cost in pneumatic conveying

There are many ways to reduce the complexity, headroom re- quirements, and cost of pneumatic conveying dust collection systems, including:

Eliminating the cyclone and discharging the dust-laden air di- rectly into a dust collector.

Adding acentrifugal inlet to a cylindrical dust collector.

Using other horizontal and vertical inlet configurations.

Discharging the dust-laden air directly into the receiver bin and venting the bin with a dust collector, thus eliminating the dust collector hopper.

Eliminating the cyclone Not using a cyclone and instead discharging the dust into the dust collector housing’s bottom, as shown in Figure 2, has obvi- ous advantages. Much dust will fall out by gravity into the col- lector’s hopper below the inlet. However, to ensure particles don’t directly impinge and wear the filter element bottoms, you need to install carefully designed baffles (typically like minia- ture louvers) or similar devices inside the collector. Besides preventing wear, the baffles ensure good airflow distribution to the filter elements. In an existing baghouse dust collector that exhibits bag wear, you can install baffles and add wear cuffs to

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the bag bottoms. The wear cuffs are typically a double thick- ness of the bag media or an abrasion-resistant nylon to prevent bag wear.

Powder and Bulk Engineering, March 1996

Adding a centrifugal inlet to a cylindrical dust collector A centrifugal inlet is rectangular and includes a transition sec- tion to the round conveying line. Material flowing through the centrifugal inlet swirls and begins to separate from the air, re- ducing the amount of dust reaching the filter elements. By adding a centrifugal inlet to a cylindrical collector below the fil- ter elements, as shown in Figure 3a, you can not only reduce the dust quantity reaching the filter elements but spread wear out over more media surface area. The cylindrical collector hous- ing also aids the swirling airflow and material and air separa- tion. While not applicable to a rectangular collector, this technique is quite effective for capturing dense dusts that ag- glomerate into heavy particles or contain many large particles.

But with particles that are hard to agglomerate or have low den- sities, or both, the air in the Figure 3a system continues to swirl as it flows upward through the filter element bottoms. If this swirling velocity exceeds the dust’s carrying velocity (the ve- locity at which buoyant forces prevent the particles from set-

tling), the dust won’t fall into the hopper and the collector will exhibit a rising pressure drop. In this case, you can use off-line cleaning (either shaking or pulse-jet) to restore the collector’s lower pressure drop.

In another variation, you can put the centrifugal inlet near the filter element tops in a cylindrical collector and add space be- tween the filter elements and the collector wall, as shown in Figure 3b. The space allows the swirling airflow to occur near the wall, away from the filter elements, thus preventing media wear. This technique is especially effective in a small-diameter collector. In a large-diameter collector, the airflow’s horizontal velocity will exceed the dust’s carrying velocity as the airflow enters the spaces between the bags. This could prevent the dust from falling into the hopper and instead sweep it into the adjoin- ing filter elements where it could abrade the media surface. In this case, it’s best to consider the configuration discussed in the next section.

Using other horizontal and vertical inlet configurations To avoid wear problems common with abrasive dusts such as fly ash, quartz, sand, and glass, which can wear even the high centrifugal inlet in the Figure 3b system, you can instead use a

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horizont 11 or vertical inlet. These inlets (high horizontal, high vertical ( entral, and high vertical), shown in Figure 4, direct the abrasive dust quickly and directly into the hopper discharge. This causes the high-speed particles to impact another dust layer in 1 he hopper rather than impact and wear the filter ele- ments and collector walls. The system can also include a minia- ture-lou ver baffle parallel to the filter elements. The baffle channel5 airflow downward into the hopper to prevent direct particle mpingement on the filter elements, yet allows air to distributl; sideways among them.

After inlpact, some of the particles slowly return upward, where they collect on the filter elements without abrading the media. Li rotary airlock feeder meters the dust from the dis- charge. ‘The collector can be equipped with a level control to keep the dust in the hopper at the minimum level required for particle impact.

Discharging the dust-laden air directly into the receiver bin

In the pp:vious systems, the receiver bins aren’t airtight, so the dust muid be fed from the dust collector discharge into the re- ceiver b n through a suitable feeder, such as a rotary airlock.

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The feeder has to seal the collector discharge against airflow upward through the feeder due to either positive or negative dif- ferential pressure across the feeder.

But you can greatly simplify such a system and reduce its head- room requirements by mounting the dust collector without its hopper directly on the receiver bin so dust discharges directly into the bin, as shown in Figure 5. To do this, you must ensure the bin is dust-tight and can withstand the system’s pressure rat- ing (although typically the bin’s structure is strong enough). [Editor’s note: For another solution, see the article “Engineer- ing study: Solving a bin vent problem in apneumatic unloading system” by Donald A. Siefers, elsewhere in this issue.]

One drawback to the Figure 5 system is that air and dust must flow upward to reach the filter elements, moving counter to dust falling into the receiver bin during the cleaning cycle. By understanding can velocity (airflow velocity to the dust collec- tor), you can overcome problems associated with this counter- current flow.

You can calculate the can velocity by subtracting the cross-sec- tional filter element area from the cross-sectional housing area

* High vertical inlet

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ith bridged pleats can be

filtration through the ate under the cleaning

uum, a 4-inch pres- the filter elements

cle is actuated. A better so- s to cycle the cleaning system at

G.D. Applewhite.

in a horizc ntal plane at the bottom of the filter elements, then di- viding that net area into the total airflow. While a coarse dust falling frcm the filter elements during the cleaning cycle isn’t likely to be carried by the can velocity and thus re-entrained in the airflop: fine dust - particularly submicron dust - and dust that’s harc to agglomerate can be carried by the can velocity.

In fact, evm if submicron dust makes up only a fraction of the total dust load in a pneumatic conveying application, most of the coarse dust will drop directly into the receiver and almost all of the submicron dust will be carried upward to the filter ele- ments. Tk is increases the pressure drop across the filter ele- ments an( I their required cleaning frequency. Sometimes the cleaning system can’t keep up when the dust load exceeds the cleaning system’s ability to remove dust faster than the dust ac- cumulates on the media surface. This makes it difficult to

achieve stable dust collector operation during conveying. If your conveying cycle is short, you can restore the pressure drop by.running several cleaning cycles after the airflow through the system stops. [Editor’s note: For more cleaning information on special applications, see the above sidebar, “Some cleaning tips for cartridge dust collectors and vacuum conveying systems.” ]

Thus if you handle fine dusts, make sure your system’s can ve- locity is as low as possible to prevent dust re-entrainment and a high pressure drop. You can reduce the can velocity to suitable levels in one of several ways:

Mount the filter elements with wider spacing.

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46 Powder and Bulk Engineering, March 1996

Mounting dust collector without hopper directly on receiver bin

Use shorter and more filter elements.

Allow the filter elements to protrude partially or completely into the receiver bin, as shown in Figure 6.

Use a cone-shaped dust collector housing, as shown in Figure 7.

On an existing baghouse dust collector and with low-density dust, install alternate rows of long and short bags, as shown in Figure 8.

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While these steps can increase the filter ratio (cubic feet of air per squa’e feet of filter media), which can increase the pressure drop, thi,; effect can be more than offset by the dust’s improved ability to fall into the receiver bin.

You can also use other dust collector configurations to help the dust fall ; nto the receiver bin. You can remove the dust collector hopper aid install the collector directly on the receiver bin, then add a hii,h or a vertical inlet, as shown in Figure 9. These inlet configurations prevent airflow from agitating the material in the recei fer bin and re-entraining particles. If you use this con- figuration, make sure to install baffles in the collector to prevent filter elei nent abrasion.

Another variation is to locate the inlet on the top center of a cy1indric:al dust collector that’s mounted on a cylindrical re- ceiver. 7 he dust in this system flows directly to the receiver’s center, a1 so preventing particle re-entrainment.

differences for this application. A cartridge collector costs less than a baghouse and also consumes less headroom and floor space. But it can’t handle low-density dusts or heavy dust loads from a pneumatic conveying system as efficiently as a baghouse can. A baghouse is also more forgiving: It can better handle the dust loading variations and moisture and condensation common in pneumatic conveying applications.

PBE

A find word: Choosing a baghouse or cartridge dust collector

If you’re choosing a dust collector for your pneumatic con- veying system, it’s helpful to know about some basic collector

W.J. Klimczak is sales and marketing manager at Dustex Corp., Box 7368, Charlotte, NC 28241; 704/588-2030. G.D. Applewhite is the company’s president. This article is adapted from an article the authors presented at the 1993 Powder & Bulk Solids Conference in Rosemont, Ill.