Dust Collector Sizing: What Size Do You Need?
A dust collection system is essential to any manufacturing process that creates nuisance or harmful contaminants in a confined environment. A dust collector filters particulates from factory air, collects the dust for disposal, and returns clean air to the environment. Alternatively, the dust collector can be an integral part of a powder processing system.
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If a dust collector is improperly sized, the escaped particulate matter can be harmful to the employee&#;s breathing and overall health. The dust can clog or lower the efficiency of factory equipment, and increase maintenance costs. In a process system, an improperly sized dust collector can restrict or limit production rates.
When the correct dust collector size is chosen, it will operate at peak efficiency, improving safety, compliance and overall productivity. Multiple factors must be considered when selecting a dust collector system. With an understanding of these basic parameters, you will see why their calculation and application to your baghouse will benefit your process.
Determining the correct size dust collector for your factory provides a budget-wise return on investment. The three primary calculations to determine the dust collector size you need are air volume, air-to-cloth ratio and interstitial velocity.
Air Volume
Air volume is the amount of air that will pass through the dust collector for cleaning. This is measured in cubic feet per minute (CFM).
Determining CFM at Hoods
C F M = fpm x area
In this calculation, fpm is feet per minute of air velocity. Depending on the dust characteristics, the correct CFM value should be approximately 100&#;200 feet per minute of air velocity multiplied by the cross-sectional area of the hood in square feet. Accurate measurement of the volume of air going through the baghouse is vital for adequate ventilation. Inadequate ventilation can clog equipment, create an unsafe working environment and increase problematic environmental emissions. If there is any question about the correct CFM for your process, contact your professional baghouse supplier.
Air-to-Cloth Ratio
Within the baghouse, cloth filter bags are the workhorses in the dust collection process. These filter bags (also known as filter socks or filter tubes) are suspended from a tube sheet in the baghouse. Dirty air passes over or through them (depending on the baghouse technology used), and the dust adheres to the filter media. In a pulse-jet collector, the bags are cleaned automatically without interruption of filtration (on-line cleaning).
The air-to-cloth ratio is the volume of air passing through one square foot of filter media. A conservative air-to-cloth ratio can extend filter life, while a higher air-to-cloth ratio could shorten the filter life. This ratio should be customized to the specific application conditions for maximum efficiency and filter life.
To calculate the air-to-cloth ratio, divide the air volume flowing though the dust collector&#;s inlet ducts by the total cloth area.
The importance of the correct air-to-cloth ratio becomes even more evident in the interstitial velocity.
Interstitial Velocity
Interstitial velocity refers to the upward movement of air in the space between the filter bags in the dust collector. If the interstitial velocity is too high, the dust pulsed off the bag will be re-entrained back into the bag instead of falling into the hopper for removal. This can cause issues, including:
- Shortened bag life
- Incomplete cleaning of the bag
- High pressure drops in the baghouse
- Excessive use of expensive compressed air
To design a dust collector for proper interstitial velocity, several types of adjustments can be made to add more space and more collection area.
Benefits of a Correctly Sized Baghouse
Dust collection is an essential and often-regulated component of any process that produces dust. Working with a reputable professional supplier or OEM will ensure that your air quality goals and dust recovery requirements are met.
Your productivity advantages and cost savings with a properly sized and calibrated baghouse include:
- Filter socks or dust bags that will clean while the system is online
- Longer process runs, without unscheduled shutdowns for cleaning
- Maintaining a correct pressure drop for uninterrupted airflow
- Economizing on compressed air usage and volume
- Eliminating recontamination of your factory air space
- Maximum recovery of valuable dust
A baghouse that is too large for the facility is a waste of resources in terms of cost, footprint and energy. Go too small with your baghouse, and it may appear you&#;ve saved on your initial investment, but the inefficiencies in collection could harm your staff, equipment and output. Sly is happy to work with you to get just the right size baghouse. You can contact us or request a quote. We look forward to helping you.
Figure Dust-collection Needs By The Numbers
Is it finally time to tackle the dust problem in your shop? Don't gamble by guessing on duct sizes and airflow. These basic calculations will tell you what flow capacity you need, what size ductwork that calls for, and how much static pressure loss your dust collector must overcome to work effectively.
First, you'll need to know the amount of air flowing in your system
Start by determining what the maximum airflow through the system will be. To do this, list the tools that you'll connect to the system. Beside each one, jot down the dust-collection airflow it requires in cubic feet per minute (CFM). You can come up with this figure several ways:
- Look it up in the tool manual. (Not all manuals specify it.)
- Use the typical airflow values shown in Table 1. (Download and print a PDF with the tables and worksheets shown in this article.)
- Figure the flow based on the size (thus, the flow capacity) of the tool's built-in dust-collection port. You can do this using one of these methods:
- For a round port, measure the diameter. Then, select the corresponding CFM value from Table 2, or
- For a rectangular port, calculate the area (multiply length times width, in inches). Then, multiply that area times 28 to find the approximate flow in CFM @ 4,000 feet per minute (FPM).
The single largest CFM figure on your list represents the maximum airflow your dust-collection system will have to support. (This assumes that airflow from each machine can be shut off with a blast gate. If you will have more than one machine operating at once or if a single blast gate serves more than one machine, add together the figures for those machines to find the maximum flow.)
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Enter this CFM figure on Worksheet 1.
Next, find the diameter for your system's main and branch ducts
The speed of air movement through a dust-collection system is critical. For systems carrying woodshop dust and chips, engineers recommend minimum air velocity of 4,000 FPM in branch lines (that's about a 45 mph breeze) and 3,500 FPM in the main duct. The speed of the air moving in the system may exceed these figures, but shouldn't fall below them. Maintaining the velocity at or above the minimum value ensures that dust and chips will remain in suspension as the air flows through the system.
Velocity of an airflow depends on duct size. Here's how to find the right main duct diameter for your system:
- Find the value on Table 2 under CFM @ 4,000 FPM that's nearest to&#;but less than&#;your system's maximum flow, which is the CFM figure you entered on Worksheet 1. (We're using 4,000 FPM for main and branch ducts for simplicity.)
- Read to the left on the table to find the duct diameter that corresponds to that flow.
Say, for example, your largest airflow is 440 CFM for an 8" jointer. The nearest lower figure in the CFM @ 4,000 FPM column of the chart is 350, which indicates a 4" duct.
Resist the temptation to step up to a larger duct in hopes of improving flow. At the same flow, a larger duct will reduce air velocity, perhaps enough to diminish performance. For example, 440 CFM of air flows through a 4" duct at around 5,000 FPM. In a 5" pipe, velocity for the same flow is only 3,200 FPM&#;lower than recommended. If the velocity drops low enough, the result will be a system that won't transport dust and chips at all.
Determine duct diameters for the system's branch lines in the same way. Treat each one separately.
Determine the static pressure loss in the system's ductwork
The final step in setting up your system is to calculate static pressure loss (SP loss). This figure represents the friction between the duct wall and air moving in the ductwork&#;friction that the blower must overcome to make air move through the system.
Figure each branch separately. Start by measuring the length of the branch duct in feet. Count the number of 90° and 45° bends in it. Where a branch enters the main duct through a 45° wye, count the wye as a 45° bend for the branch. Then, prepare a Duct SP Loss worksheet like Worksheet 2 shown below for each branch. Find values for the equivalent length of bends in Table 3.
Now, taking each branch duct separately, figure the static pressure loss for the portion of the main duct that runs from the point where that branch enters it to the dust collector, using the Worksheet 2 format. Add this figure to the branch duct's SP loss to find the total SP loss from the tool to the dust collector, and enter the values in Worksheet 3.
The largest value you calculate for your system then represents the static pressure loss your dust collector must be able to overcome. Enter this figure on Worksheet 1.
Worksheet 1 now shows the maximum CFM flow and static pressure loss for your system. To power your system, you'll need a dust collector that meets or exceeds both figures.
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