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HomeInsufficient Airflow at the Hoods

Insufficient Airflow at the Hoods

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​by Bob Walters, Donaldson Torit Regional Service Engineer​

 
There are plenty of articles and resources regarding the proper design and installation of new dust collection systems, as well as checklists and protocols for preventative maintenance. By contrast, this article series seeks to address the topic of "My dust collector is not working. What should I do now?" The articles will identify and define the most prevalent failure modes in a dust collector. Then, they will elaborate on steps to diagnose and potentially troubleshoot the issues. For purposes of this article, a dust collector will be generally defined as a self-cleaning media-type unit – specifically a baghouse or a cartridge collector. (Cyclones, mist collectors, and other devices are not considered here.)

THIS SERIES OF MAINTENANCE TIP ARTICLES WILL FOCUS ON THE FOLLOWING FOUR PRIMARY FAILURE MODES:

1. Leaking dust collector
2. Insufficient airflow at the hoods (this article)
3. Short filter life
4. Electrical issues

A failure mode is a specific problem or shortcoming that is the result or symptom of a non-performing or under-performing system. Failure mode does not have to mean complete failure or non-operation. The term is used here to identify target areas that are not working as designed. In the more extreme cases, the failure mode causes the unit/system (or manufacturing process) to be non-operational. There are certainly many other failure modes, but the four listed above are the most prevalent. Stay tuned for the remaining two maintenance tip articles.

Note: This series intentionally excludes the failure modes of fire and explosion, which are more complex and deserving of a separate analysis. Additionally, extreme caution should be exercised when considering any troubleshooting or corrective action suggestions. Even seemingly-simple items should be done only by qualified and trained personnel with appropriate levels of personal protective equipment and safety precautions, such as lock-out tag-out, fall protection, confined space protocols, etc. Electric items should only be addressed by qualified electricians. When in doubt, call a professional to perform all work. A dust collector manufacturer will likely be able to provide a local referral.


MAINTENANCE TIP #2: INSUFFICIENT AIRFLOW AT THE HOODS​

​Many factors may influence the performance of a hood in a dust collection system including poor air flow at the hood. Factors which might reduce performance include:
• “Blown,” torn, or damaged filter media,
• A bent or damaged filter assembly,
• Incorrect installation of a filter element,
• A detached bag filter, or
• A mechanical failure of the tubesheet due to abrasion.

INITIAL SYSTEM DESIGN​
The owner of a dust collection system should recognize that poor hood performance may not be a consequence of only insufficient airflow. The issue may be related to inadequate or modified hood designs, which limit the capability of the hood to perform its function.

Always check the initial airflow requirements of the hood because that value required meeting a number of design criteria, including the determination of the optimal hood location. The variables determining required air volume include the degree to which a hood encloses or covers the process, as well as the shape and structure of the hood. Each of these variables will influence the total air required for the hood to perform effectively. The Industrial Ventilation: A Manual of Recommended Practice (published by the ACGIH) outlines the methods for estimating sufficient air volumes based on physical location and shape of hoods. Reconfirming assumptions used in the initial air volume estimates will often reveal differences between the initial design intent and the current reality on hood configuration.
MODIFICATIONS TO SYSTEM DESIGN​
​If the hood design looks good, but there still seems to be inadequate performance because of insufficient air volume, the next step is to confirm the duct layout allows proper distribution of air to each hood. A review of the initial design may reveal challenges or unintentional design modifications that adjusted air distribution in the system.

 
 
DUCT LAYOUT​
Duct layout and system design incorporate many design decisions to ensure design air volume is drawn from each hood location in a system. In some instances, standards or codes may involve requirements such as all hoods being open while in other situations provisions may allow some hoods to be closed when the associated equipment is not in use. In still other situations, a common designcomponent (blast gates) used to fine tune or balance the system can be adjusted by employees who might not understand the system design or by abrasion/wear. Both inadvertently create new airflow distribution patterns in the system. Reviewing the initial design against current conditions may help identify contributors to poor air distribution. If you have the good fortune of locating a summary document for a system when it was commissioned, you can compare initial values to what you have today. If you do not have access to the initial design documentation, you may need a design assessment to determine which design issues are influencing poor airflow distribution to hoods. This system assessment should include provisions for static pressure losses across collectors, as well as static pressure and flow capacity requirements for fans. During your design review, there are many factors you may want to check to ensure the assumptions made now match the system configuration. When considering system design conformance to your current system, think through the following:

• Does the design match the energy required by the hoods (hood static pressure)?
• Did the initial design account for any air acceleration in the system?
• Does the design match the actual length and size of the ducts?
• Does the design match the number and size of all elbows and branch entries?
• Did the design account for the existing dust collector inlet and exhaust duct shapes?
• How much energy did the design assume would be necessary for the dust collector? (Consider pressure drop across the filters when dirty, rather than clean.)
• Did the design account for the influence of duct on fan performance (system effects)?
Examine:
• Elbows at the inlet or outlet of the fan
• Duct size (Is it smaller than the fan inlet?)
• Damper on the inlet or outlet
• No duct on the exhaust of the fan
• Silencer on the discharge
• Were other components in the system accounted for in the static pressure requirements of the fan? Consider:
• After filters on the exhaust
• Return air ducts
• Back draft dampers
• Explosion isolation devices
• Exhaust stack​
INCREASED RESISTANCE IN DUCTS​
Many of the factors above may influence the performance of one or more hoods in a system by preventing adequate air volume from entering each hood. The build-up of material inside a duct, a consequence of poor transport velocity in the system, can also create a reduction in flow to one or more hoods in a system.

SYSTEM OPERATIONS​
In addition, some changes in the system can affect the performance of all the hoods by reducing the total air flow capacity of the system and fan. As an example, the system fan may not be able to deliver the design air volume. On a newly installed or recently serviced or modified system, check to confirm the fan rotation is correct. Turning fans backward draws air through the system at a rate significantly lower than design. Sometimes this is simply a wiring issue at the fan, but occasionally wiring changes elsewhere in the plant can reverse polarity of power to the fan causing a reversal in rotation. If fan rotation is good but flow is still below design, check the static resistance of system components which may be forcing the fan to operate at a lower volume delivery point on its performance curve. Evaluating components which produce static resistance over a range of values is a good starting place, and dust collectors are typically the first item in this list to check.

If filters in a collector are operating at elevated pressure drop, the increased resistance reduces the total air volume at each hood. If filters have been in service for a period of time, they may simply have reached the effective end of their service life and need to be replaced. If, however, relatively new filters exhibit an elevated differential pressure drop, another cause may need to be explored.

Has the collector experienced a recent upset condition which may have prematurely reduced filter performance? Or has the collector experienced a temporary failure in its cleaning system, resulting in filters not getting properly cleaned or reconditioned? Correcting the latter concern may allow filters to return to more expected pressure drop values, thus restoring air capacity of the system and increasing air flow and hood performance. Typical issues reducing the cleaning system performance for pulse-cleaned filters include inadequate pressure or volume of available compressed air. If the compressed air pressure is adequate, a check of pulse timing (both frequency and dwell) may reveal the issue. And a review of hardware conditions is always beneficial to ensure all components are in good working condition. Many pulse cleaning systems include items such as solenoid valves, which can wear and may require occasional replacement.

Hood performance defines the maximum system performance your collector can attain, so paying attention to how well hoods are working is important. Paying attention to the airflow the hoods require can ensure their continued performance. In addition, properly designed hoods can achieve the performance you need while conserving your plant air and energy for system operation.

Pay attention your hoods and your airflow. You have a lot riding on both!​

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