Balancing your dust collection system's airflow
Q: Our manufacturing process results in combustible dust. We have a good dust collection system, but we’re concerned about meeting OSHA requirements for balancing the dust collection system. What's the best method to do this?
Regardless of what you may hear on the street, the right amount of airflow doesn’t find its way through the dust collection system’s ductwork without your help. You must balance the airflow through all the branch ducts to maintain the design airflows through the system’s dust capture hoods.
Dust collection systems are balanced using one of two methods — static pressure balance or damper adjustment. The static pressure balance method
calculates the static pressure at each branch junction in the dust collection system, starting at the hood and branch duct farthest from the fan and moving branch by branch to the fan: At each branch junction, the static pressure in one branch must equal the static pressure in the joining branch to achieve the desired airflow quantity in each branch. You must be able to produce documented calculations for this method or an OSHA inspector may issue a citation claiming an unbalanced system. Even if this airflow balancing method has been used, you still need to check the system at startup to ensure that design exhaust air quantities are flowing through the system.
Most plant dust collection systems are designed using the damper adjustment method, in which dampers at various system locations are manually adjusted to achieve the correct airflow quantity. This method allows for some flexibility if you need to make changes to the hoods and ductwork in the future. But keep in mind that anytime you make changes, you must balance the system at startup.
Here are the steps for balancing a typical dust collection system:
- Review design drawings and specifications for the desired duct system airflow quantity.
- Precoat the dust collector filters and set the filter-cleaning system to the pressure-drop conditions recommended by the manufacturer.
- Prior to fan startup, set all balancing dampers to a 50 percent closed position.
- Start the exhaust fan and the supply air (makeup air) system fan.
- Check and adjust the supply air system fan to obtain design airflow conditions.
- Check the exhaust fan's speed, brake horsepower, and total airflow with a pitot traverse instrument (also called a pitot tube), the standard instrument for measuring velocity pressure. Insert the tube into the duct airstream to measure velocity pressure, then convert this to velocity, and, based on the duct's cross-sectional area, convert this to air quantity (in cubic feet per minute). Compare the data with the system design and specification documents.
- Adjust the exhaust fan speed to obtain total airflow design conditions.
- Use a pitot tube to measure duct airflows and adjust dampers as needed to obtain the desired airflow in each exhaust duct.
- Make any final exhaust fan adjustments to obtain the proper air balance for each branch duct, and record the final fan operating conditions (total airflow, speed, static pressure, and brake horsepower).
- Record the test data and compare it to routine airflow checks.
- After balance, lock all dampers in place. If your dust is combustible, NFPA 654 requires that the dampers be fixed so they can't be tampered with.
Balancing the airflow quantity in your dust collection system will ensure that it's running at its optimal levels, and following these steps will also assist you in producing the required OSHA documentation.
Think you’ve got what it takes to be the Expert?