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Sanitary, Safe and Sensible—Designing the Best Baghouse Discharge System

Topics: dust collector replacement bags, dust filters, bag filter cleaning

Posted on November 11, 2022
5 minute read

Whether building a new facility or instituting renovation and updates, there are important considerations when selecting a baghouse discharge system. The baghouse discharge system does more than eliminate nuisance dust. It can help improve worker safety by eliminating hazards, help comply with OSHA regulations, improve product quality or, in certain situations, aid with company profitability. Find out the X top considerations when selecting a baghouse discharge system. Depending on the type of dust generated by your facility, the baghouse discharge can be a source of recycled revenue adding to company profitability.  

The baghouse constitutes part of a facility’s air filtration system. As part of the dust collection process, the baghouse captures and filters airborne particles resulting from manufacturing or agricultural practices before these particles can impact worker and workplace safety and/or impact plant production. In addition, the system enables compliance with the air quality standards or regulations overseen and enforced by various government bodies.  

The Necessity of a Proper Discharge 

Dust should not collect within the baghouse and must be discharged for disposal or repurposing. After the bags or filters entrap the dust, an automatic cycle knocks the dust loose from the bags to fall into the hopper. The dust can remain in the hopper temporarily but should be discharged at regular intervals.  

Dust buildup within the hopper can cause several problems. Depending on the abrasiveness of the dust, it could cause premature wear and tear on the filters or bags. Re-entrainment can increase the concentration of dust by the inlet, or contaminate air already cleared from dust. Worst of all, it could provide fuel for a combustible dust event such as a fire or explosion.  

Types of Dust Collected by the Baghouse  

There are two types of dust the baghouse collects: nuisance dust or product/process dust. Nuisance dust is discarded after its collection as it has no commercial value. Product dust is dust with value for resale, such as fly ash sold to cement manufacturers, or lithium carbonate, for example. Process dust has value to the original manufacturer and represents lost revenue if discarded. Once collected from the baghouse discharge, process dust is recycled back into the production stream at some point. The dust’s future purpose is one factor that dictates the type of discharge required.  

Manual or Automatic Discharge  

There can be a manual or an automatic discharge from the hopper. A manual discharge is recommended only when there is an exceptionally light dust load collected from a system. This can be the case with nuisance dust, which is a mixture of diverse types of dust collected by a central vacuum unit or tied to a batch type process. Some facilities might generate as little as a pound of dust per week. In this case, a continuous discharge would waste energy. Even a single 55-gallon drum’s worth of dust per week is still considered a light dust loading.  

Anything above this amount, or a process dust as opposed to nuisance dust would benefit from an automatic discharge. A semi-automatic system can depend on plant personnel to trigger the automated discharge once the hopper is filled or be fully automated on a regular schedule to empty the hopper.  

Continuous discharge supplies a safety feature when working with combustible dust and is always recommended in a hazardous dust environment. Coal dust for example, is notorious for its combustibility. The OSHA site has a wealth of information about combustible dusts, proper analysis, and housekeeping considerations. Or ask the experts at Sly Inc. if unsure about your dust’s combustible potential.  

Gated Hoppers Maintain Proper Baghouse Vacuum Points 

The hopper is gated for a few distinct reasons. One, it allows the operation to put the appropriate container underneath the hopper to collect the discharge. Secondly, a gated hopper helps maintain the proper vacuum in the baghouse.  

Typically, a baghouse operation should have one initial point of suction and one point of discharge for the air. The hopper should remain close except for discharge purposes, otherwise, an open hopper creates a second point of suction and interrupts the discharge. The dust will reverse course and get sucked back or re-entrain into the baghouse filters. There are a few options for the gated mechanism: 

Slide gate—A slide gate has two separate gates that operate one at a time. The first gate allows the dust to enter the dump valve or the discharge point. Once the first gate is closed, the second slide gate opens to allow the dust to discharge into the container. Operating one gate at a time helps preserve the baghouse system’s vacuum or pressure.  

Rotary Valve—This works in a comparable manner to the slide gate, to protect the vacuum or pressure differential within the baghouse. Rotary valves often are selected to discharge directly into a conveyor, typically a screw or auger conveyor that moves the captured and discharged dust back into the process stream. This type of valve is best with light dust loads and non-hazardous materials.  

A rotary valve can also apply to a batch system with light loading, on a pyramid hopper for example, with its automated discharge triggered manually by plant personnel, as an example of a manually operated automatic system.  

There is a pressure differential between the clean air plenum and the dirty plenum. A control panel can be programmed to automatically shut down the baghouse in case of a high-pressure differential beyond the scope of the system, as a safety feature.  

Discharge Height Can Vary to Suit Plant Requirements 

The typical discharge height for a baghouse from Sly, Inc. is five feet from the ground, into including any valves or accessories. The discharge has a one-foot airlock leaving four feet of clearance underneath the discharge valve. Customers can dictate or specify a different discharge height for six or seven feet of clearance, if for example, the company is discharging to a piece of equipment. 

Another option is a flexible hose attached to the end of the valve. The hose allows for different sized containers to be placed underneath the discharge valve. Flexible hoses are not used in applications with explosive or highly combustible materials.  

Dust Characteristics that Impact Discharge Design 

Operators need to consider dust load, type of dust and dust characteristics for proper discharge design. Characteristics such as abrasion, corrosion, and agglomeration or hygroscopicity all will have an impact.  

For example, there are two extremes for agglomeration. An example of a poor agglomerate would be dry beach sand, while wet snow would serve as an example of particularly good agglomeration. When a substance packs easily or agglomerates, the angle of the hopper for discharge is critically important to ensure the dust doesn’t accumulate in the hopper.  

Accumulation or bridging can occur with hygroscopic dust such as soda ash, lime, or limestone, which are all good at absorbing moisture. These can cake up and bridge across a discharge in the hopper in the valley angles. Bridging also creates greater risks if the dust is combustible.  

When the dust is hydroscopic, such as soda ash, lime, or limestone, it tends to cake up and bridge across a discharge. The bridging can block the discharge pathway, trap dust in the hopper and raise the pressure drop in the system to an unacceptable level. A few design points or features can involve an enlarged discharge opening, avoiding sharp corners or valleys within the hopper and calculating an effective angle of repose (the relation between dust’s flow characteristics and the maximum angle to facilitate its flow). Calculating this value helps determine the slope or steepness of the hopper.  

Dust characteristics such as these are best addressed during the design phase. While baghouses and discharges can be retrofitted or modified after installation, operators run the risk of having to shut down operations to fix a problem that could be addressed earlier on.  

Talk to the experts at Sly Inc. about the design of your next baghouse and the proper discharge components. Make your workplace a safer environment and capture dust for disposal or redistribution.

Call us today.  





Tags: dust collector replacement bags, dust filters, bag filter cleaning

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