Tips for Improved Dust Capture

June 21, 2016

10 Min Read
Tips for Improved Dust Capture
Material build up in ducting

Manufacturing facilities have in common the need for effective dust capture to ensure the air quality within the facility is at a safe level for the employees and that accumulations of dust are minimized to reduce the risk of explosions and fires. The same issues keep popping up with dust collection systems that just do not function as expected. The following is a list of the most common design and operating issues that contribute to poor operation of the dust collection system.

1. Hood design. In order for a hood to be effective a couple of things have be put in play. First, the hood must be located in the correct spot. The hood has to be located as close as possible to the dust source, without interfering with the process. The further a hood is away from the dust source, the less effective it will be. For example, the suction efficiency of a 12-in-diam hood drops to 7.5% at a distance of 12 in. from the hood face. In suction systems, once a hood is located much more than a foot away from the source then it really does not have much effect. The diagram below (Figure 1) shows the percentage of the hood capture velocity from the distance of the hood face.
    
Once the hood is in the correct location it must be designed correctly for the application. If there is a large amount of material that needs to be removed by the hood, then dense phase pneumatic conveying calculations need to be made to make sure the hood will not plug with material. The face velocity of the hood needs to be considered. Is the velocity across the face of the hood fast enough to remove the material at the process’s desired speed? Once the hood is designed correctly then the static pressure loss of that hood needs to be calculated, which plays an important part in the next item that is over looked in good dust collection design.

2. Correct fan. If the system is designed perfectly but the wrong fan is installed on the system, then nothing about that system will work. There will not be enough suction at the hoods, even if they are designed correctly, the material will start collecting in the ducting and ends up being a mess.

When sizing the fan a few things have to be taken into consideration. First of all, the total system static pressure (TSP) must be calculated, which includes the static pressure of the hoods, system pressure line losses, filter media operational differential pressure, fan discharge silencer, and fan discharge ducting. There will be one line in the system, usually the longest but not always that sets the static pressure for the system. Based on the expected or measured velocity in the ducting, the length of straight runs, number of elbows, material of the ducting the system static pressure can then be calculated. This gives you the system static pressure, which is not the TSP. For the TSP you also have to add in the static pressure loss of the duct collector. If you are dealing with a wet scrubber, the manufacturer should have an expected pressure drop. With a cyclone the pressure drop can be calculated through various methods. With dust collectors it depends on the air-to-cloth ratio, velocity, and interstitial velocity. All of these parameters will change over the life of the system. A common problem with dust collection system design is to underestimate the filter differential pressure based on the clean bags and, more importantly, when the bags become dirty. The filter differential pressure is related to the air-to-cloth ratio and dust particle size. The air-to-cloth ratio is the velocity of the air through the filter media. This is calculated by dividing the cu ft/min (CFM) by the total area of the filter media (sq ft). The higher the air-to-cloth ratio, the higher the expected operational differential pressure.

The relationship between the expected operational differential pressure drop and air-to-cloth ratio is best determined by experience. Industries with similar dust concentrations and particle size will have similar operating differential pressures. If access is not available to existing operating differential pressures, general guideline is to allow the same operating differential pressure as the air-to-cloth ratio. For example, very fine dust should be collected in a dust collector having an air-to-cloth ratio of less than 2 FPM, and a corresponding operating differential pressure allowance when sizing the TSP would be 2 in. wg. As the dust collector is used, the operational differential static pressure will increase as the filters age. It also depends on what type of dust collector you have. Some dust collectors only clean when shutdown, so that static pressure builds during use. This whole range of static pressures from new bags, through the operating cycle to when it is time to change the bags, will give you a different static pressure. When this is added to your total system static pressure you must then choose a fan that will operate through the range, from new bags to old bags. Too often a fan is chosen that does not have any flexibility, once the static pressure increases as the bags get dirty and the flow of the fan decreases and the hoods will not work as designed. When ordering a new fan or assessing an old one, always look at the fan curve and see what volume of air you will get out of the fan based on the expected calculated static ranges. This will ensure that the fan can produce enough flow.

3. Balancing Dampers. People like to add onto system, stick a new branch here or there, and then wonder why it does not work. The most common design method for dust collection systems is to use balancing dampers on each line connected to a hood that can be adjusted to provide the required air flow. This is known as the “Blast Gate Method.” Once the hoods have been sized correctly, the volume of air at each collection point is then known. The size of the ducting is then determined by dividing the volume of air by the velocity to give you a cross sectional area, which then translates into a diameter. As 4.37-in. ducting is not commercially available; increase the diameter to 5 in. or reduce to 4 in. is required. Either decision will have an effect on the system static pressure. The 4-in. line will increase the pressure loss, and the 5-in. will lower the pressure drop but a slower velocity. During design, these changes must be considered to ensure the proper flow is achieved at each hood and in the ducting. With the design method using standard diameter ducting, each branch will have a different static pressure, and the static pressure will dictate where the air, and how much of it, will flow down the line. The system then has to be balanced with balancing gates installed at every drop. The balancing gates induce static pressure on the lines which drives the air required at each hood. Through balancing air can be forced to travel down the small branch lines that are further down the system achieving a balanced system. Once balanced, mark and, if necessary, lock the location of the balancing dampers. Any unauthorized change in the location of the balancing damper can have a dramatic drop in hood dust capture.

4. Over time, upset conditions that are not monitored can cause a significant loss of air flow, even with a properly designed and sized dust collection system. In some cases, these can be easily fixed. Common issues found include:

a. If using filter media, check if the filter media has become wet for any reason. Wet filter media will not work. The dust particles will not release during the cleaning cycle and the wet dust particles can permanently plug between the filter fabric fibers. If your operating filter differential pressure exceeds 10 in. wg, you likely have a filter media problem and a bag change would likely restore the flow.

b. Significant air flow loss can occur if an access door is accidently left open. You may not think this could happen in your facility, but many times an access door on the roof or backside of the dust collector is left open, resulting in significantly reduced flow at the hoods. A large hole or split in the ducting can have the same effect.

c. In some cases, systems are operating with the motor running missing or slipping belts. This is obviously an inexpensive and easy fix. You would be surprised how many times I have been asked to go to a site and review a poorly operating dust collector, only to find the drive belts from the motor to the fan are missing. The PLC shows that the motor is running, but it is often not enough to rely on the PLC when the dust collection system is performing poorly. There is no substitution for inspecting the fan and motor, which in some cases can be in inaccessible locations.

d. During upset conditions or poorly designed and operated systems, it is common to see build-up in the ducting. This build-up causes significant pressure loss and resulting poor hood capture.
    
Even if the system will eventually plug again, it is important to clean the ducting as required to minimize the time between excessive plugging and poor hood capture. Making routine duct inspection and cleaning part of a preventative maintenance program will provide significant dust hood capture until the root problem can be fixed and or the system is upgraded.

e. If the cleaning system is not working as per the original installation, the bags will plug and result in significant flow loss and poor capture at the hood. For shaker-style dust collectors, the shaker mechanism is generally interlocked to the fan such that when the fan is shut off, the shaker motor usually installed with a timer, will shake the bags for cleaning. It is common to require an increase in the shaker cleaning cycle to ensure the bags are sufficiently cleaned prior to restart. This is more prevalent when systems run continuously for 8 to 10 hours. During this time the fabric differential pressure will increase proportional to the operating time. If poor hood capture is observed, it may be better to shut down the dust collector to allow a cleaning cycle during a lunch or scheduled break. For larger dust collectors the fabric cleaning is done during operation with either a pulse jet or reverse air system. Continuous cleaning during operations ensures that the fabric differential pressure remains within an acceptable range during operating hours. Common problems with pulse jet cleaning system include insufficient pulse pressure and volume and broken solenoid or diaphragm valves. The pulse-cleaning equipment is an essential part of proper dust collector performance and must be part of regular scheduled maintenance.
    
Through the design and placement of hoods, selection of a fan based on the range of system static pressures, and the balancing of the system your dust collection systems may not be perfect but they will function a lot better. Properly designed, operated, and maintained dust collection systems can provide effective dust capture for decades. Improperly designed, operated, and maintained dust collection system can develop problems in the first month. It is essential to ensure that during the sizing phase of a new dust collection system that the owner operator fully understands what the expectations and performance of the dust collection system needs to be. It is becoming more common that a three-month performance guarantee be applied to new system installs to ensure that the system performs to the required specifications before the final project hold-back is released.
    
Diane Cave is a registered professional engineer in Nova Scotia and Ontario, Canada. She is currently the engineering manager at EPM Consulting in Halifax, Canada. She has worked extensively in the fields of dust collection, combustible dust protection, NFPA compliance, and material handling systems. Cave has spent the majority of her career as a design engineer exclusively in the area of dust collection systems, which involved extensive site troubleshooting and system analysis. She specializes in retrofitting existing dust collection systems to bring them up to current standards and codes, as well as improving system performance. For more information, visit www.epmconsulting.ca.

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