Dust collection
Encyclopedia : D : DU : DUS : Dust collection
Dust control systems come in many different area-specific configurations (Figure 1). For example, a plant’s railcar-unloading system will usually have a ducted dust collection system, a passive enclosure chamber, or some form of control spray. Dust conveyed to the storage pile typically receives some form of conditioned water spray or surfactant. By contrast, dust from the crushing station contains a higher fraction of fine particulate matter, so it typically calls for a ducted dust collector.
A dust collector is a steel enclosure containing porous filter media that separate fine dust particles from a flowing stream of dirty air. The most common filter media used in collectors are filter bags and cartridges. Dust particles build up on the outside of the media and form a coating called “dust cake.” It is this layer that does the actual job of filtering fine particles. As the cake builds up, the pressure drop across the filter bag rises.
Adjusting the cleaning cycle of the collector so it periodically cleans most of the cake off the filter medium prevents bag blinding. Too-frequent cleaning not only contributes to bag wear but also can remove all of the dust cake, enabling dust to bleed through.
Products for improving flow are commonly used to help pre-coat new bags to provide a uniform coating that shortens the break-in time for efficient collector operation. The size of the dust collector depends on the system’s required airflow. The quantity of bags or cartridges determines the filtration area and is a function of both the air-to-cloth ratio and the type of dust being collected. For coal dust, the air-to-cloth ratio is typically between 4 and 6 fpm, depending on the concentration of dust loading. The geometry of the collector housing and the number of bags establish the operating parameters of the unit. The open space between all the bags or cartridges in the housing is the flow area, where the dirty air travels. The average velocity of the air stream through this space is called the “can velocity.” The upper limit of can velocity for coal dusts ranges between 200 and 300 fpm. If it is higher, fine dust purged from bags or cartridges during the cleaning cycle will tend to become re-entrained in the airflow and end up back in them. This phenomenon is most pronounced for collectors with long bags.
Because can velocity is directly related to air-to-cloth, choosing a low-cost dust collector with a high air-to-cloth often leads to operating problems. Bag-cleaning methods vary between models and manufacturers but typically involve reversing the flow of air through the bags. Bags should be visually inspected when bag-cleaning problems are suspected or there is evidence of continual bag blinding. Commercial services are available to evaluate failed filter bags. Analysis could determine whether there is a system problem or a need to switch to a different filter media to better match the dust characteristics.
Workshop Dust Collection
Design and Installation of an Effective Dust Collection System
Before you purchase a dust collector, you should first layout a system, determine how many and what kind of tools will be connected to the system, and where they will be in relation to the dust collector. Once you have done your layout, with a few simple calculations you can determine what size dust collector best suits your needs based on capacity in CFM (Cubic Feet per Minute of air flow) and Static Pressure Loss.
STEP 1 - How to Layout your Shop
The example above illustrates a simple 3 machine layout. In laying out your shop, try to follow these basic guidelines: Place machines requiring the highest CFM (air flow in cubic feet per minute) closest to the dust collector (see chart below)
Try to keep runs as short as possible Try to kep all duct work at the same height as the dust ports on the machines Try to keep directional change fittings (T's and Y's and Elbows) to a minimum Typical CFM ratings for woodworking equipment:
Machine CFM Required 12" Planers 350 13"+ Planers 400 Shapers 400 Bandsaws 400 Radial Arm Saws 350 Tablesaws 350 Disc Sanders 300 Belt Sanders 300 Floor Sweeps 350 Jointers 350 Drill Presses 300 Scroll Saws 300
NOTE: We recommend installing a blast gate for every tool to help direct all of the air flow to the tool being used.
STEP 2 - Determining the Duct Diameter Needed
Find the tools you will be using tht require the most CFM from the chart above. If you will be running more than one tool at a time, add the CFM numbers of those tools together. Using the chart below, determine the duct diameter that will be required for your system. We do not recommend using duct smaller than 4" in diameter.
250 - 400 CFM 4" DUCT 400 - 650 CFM 5" DUCT 650 - 800 CFM 6" DUCT
If you determine that your system will require greater than 800 CFM, you can calculate the appropriate duct diameter usinf the following equation below:
Duct Diameter = (CFM * 183) / 350
STEP 3 - Determining Static Pressure Loss
Determine the length of straight duct work in your layout from the duct collector to each tool. Count the number of each type of directional change fittings to each tool: Y-fittings, T-fittings, and Elbow Fittings. Each type of fitting is equivalent to a specific lenth of straight duct in the amount of Status Pressure Loss it causes. Refer to the following chart for reference:
Duct Dia. 90° Elbow 30° Y-Fitting 90° T-Fitting 4" 6 feet 3 feet 7 feet 5" 9 feet 4 feet 10 feet 6" 12 feet 5 feet 13 feet 7" 13 feet 6 feet 14 feet
Determine and add up the equivalent number of duct feet for all of the directional change fittings for each tool. Add this number to the total length of straight line duct from the dust collector to each tool. Once you have determined the total length of duct to each tool, (straight line duct plus fittings), you can then calculate the Statis Pressure Loss for each tool based on the chart below. Static Pressure Loss is measured in inches.
Duct Diameter Static Pressure Loss per foot of duct 4" Duct .055 in./ft. 5" Duct .042 in./ft. 6" Duct .035 in./ft. 7" Duct .026 in./ft. 8" Duct .022 in./ft.
Example In our sample layout, the system consists of 3 tools: a jointer, a planer and a tablesaw. The calculations for this system looks like:
Jointer Planer Tablesaw4" Dia. Duct 15 ft. 11.5 ft. 22 ft. 90° Elbow 1=6 ft. 0 1=6 ft. 30° Y-Fitting 0 1=3 ft. 0 90° T-Fitting 1=7 ft. 1=7 ft. 1=7 ft. TOTAL FEET 28 ft. 21.5 ft. 35 ft.
To determine the Statis Pressure Loss for each tool, multiply your total feet by .055" (4" diamter duct)
Jointer Planer TablesawStatis Pressure Loss 1.54 1.182 1.925
We recommend adding 1" of Static Pressure Loss to compensate for a dirty filter bag. The highest Static Pressure Loss for this system would be 1.925" for the tablesaw plus 1" for a dirty filter bag resulting in a total Static Pressure loss of 2.925".
STEP 4 - Determining which Duct Collector is Needed
Use the tool with the greatest Static Pressure Loss and the tool requiring the greatest CFM to determine the size of dust collector you needed for your system. If you will be running more than one tool at a time, you must add the CFM's and Static Pressure Loss of these tools. In our example, the tool with the greatest CFM is the planer at 400 CFM. Therefore, we must purchase a dust collector with a Static Pressure Loss rating of 8.5" and a CFM rating of at least 650. It would be wise in this circumstance to consider a larger Dust Collector such as the JET DC-1200CK.
STEP 5 - Preventing Static Electricity Build-up
There is always the danger of static electricity building up in the duct system and causing a flash explosion of fire. To prevent this, run ground wires through both the inside and around the outside of all the piping. Use stranded aluminum or copper, or solid copper wire for the inside and aluminum antenna guy wire for the outside. The JET Dust Collection Grounding Kit contains all of the necessary components to safely ground your system.
At directional change fittings, you can solder the wires on the inside of the fitting. Wires in Y-Joint fittings must be soldered facing away from the aire flow, or you can drill a small hole in each branch of the fitting—bring the wires out of the fitting and connect them with a wire nut. You must do this for all Blast Gates so you can completely close the gate. Seal the holes with caulk after the system is put together to prevent air leaks.
Wrap a wire around the outside of the hose and fittings to remove any static electricity that might build up on the outside of the ducts. Ground both the inside and outside wires to a metal part on each woodworking machine—including the Dust Collector. You may need to use a screw or bolt with a lock washer to ensure a complete ground connection.
If you have any questions about setting up your Dust Collection system or any other woodworking tool, please feel free to contact us at info@etoolclub.com. We are always glad to answer any question you may have and provide you with the best equipment, accessories and replacement parts for any of your woodworking equipment.
Good Luck! The Support Team [ETOOLCLUB]
External links
- [Dust Solutions]
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