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

Topics: dust collection equipment, dust collector replacement bags, bag filter cleaning

Posted on January 13, 2023
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, helping comply with OSHA regulations, and enhancing product quality. Or, in certain situations, it aids company profitability.  

Depending on the type of dust generated by your facility, either nuisance or process dust, the baghouse discharge can remove a health and safety hazard or aid with your revenue generation. Here are the top considerations for selecting a baghouse discharge system: 

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 plant production. In addition, the system enables compliance with the air quality standards or regulations overseen and enforced by various government bodies.

horizontal cartridge collector

This is a Horizontal Cartridge Collector with multiple discharge equipment (Rotary Airlocks feeding into a bulk bag) on the bottom of each pyramidal hopper.

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  

The baghouse collects two types of dust: nuisance dust or product/process dust. Nuisance dust is discarded after its collection as it has no commercial value. Product or process dust has value as part of the production stream or as a product available for resale. For example, lithium carbonate is collected as part of the production stream for battery production. Fly ash dust is sold to cement manufacturers.  

Once collected from the baghouse discharge, process dust is cycled back into the production stream. The dust’s future purpose, whether for discard or further processing, is one factor that dictates the type of discharge required. Overall, process dust has value to the original manufacturer and represents lost revenue if discarded.  

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, a mixture of diverse dust collected by a central vacuum unit or tied to a batch-type process. Some facilities 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 process dust instead of 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 is a safety feature when working with combustible dust and is always recommended in a hazardous environment. Coal dust, for example, is notorious for its combustibility. The OSHA site has a wealth of information about combustible dust, proper analysis, and housekeeping considerations. Or ask the experts at Sly Inc. if you are unsure about your dust’s combustion 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 dust collection system should have one initial point of suction and one point of discharge for the air. The hopper should remain closed except for discharge purposes. Otherwise, an open hopper creates a second suction point and interrupts the discharge. The dust will reverse course and get sucked back or re-entrained into the baghouse filters.  

There are a couple 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 comparably 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 be applied to a batch system with light loading, on a pyramid hopper, for example. Plant personnel can trigger the discharge manually or rely on an automatic operating 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, 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 conveyor or a truck for removal.  

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 

For proper discharge design, operators need to consider dust load, type of dust, and 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.  

The Danger Created by Bridging  

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 form a connection or “bridge” across a discharge outlet. Bridging also creates more significant risks if the dust is combustible.  

When this bridging blocks the discharge pathway, it traps dust in the hopper and can raise the pressure drop in the system to an unacceptable level. Design features that can address this can include: 

  • An enlarged discharge opening. 
  • Elimination of sharp corners or valleys within the hopper. 
  • Calculating an effective angle of repose. 

This angle of repose is 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 risk having to shut down operations to fix a problem that could be addressed earlier.  

Make your workplace a safer environment. Capture dust meant for disposal or recover dust as part of the process stream to improve profitability. Talk to the experts at Sly Inc. about the design of your next baghouse and the proper discharge components. Call us today.


Tags: dust collection equipment, dust collector replacement bags, bag filter cleaning

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