13.01.2020  Author: admin   Build A Frame
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This article will discuss six considerations when designing a dust collection system to help ensure a dust removal system design keys functioning system for your application. There are four key components in a dust collection system: the filter, ductwork, pickup hoods, and the air mover.

Figure 1 shows an example of a typical dust collection system. Airflow through the hoods can be controlled by either blast gates common or by duct sizing less common but more effective. The ductwork also called branch and trunk lines — these are closer to the dust collector is routed through desifn facility, often changing directions multiple times and likely increasing in diameter along its length.

Desing trunk main line of the duct terminates at the dust collector dust removal system design keys the particles are separated from the airstream. The heart of the dust collection system is the air mover and filter combination. The symbiotic relationship between these two components is important; for instance, if the filter becomes clogged with dust, the fan air dust removal system design keys performance reomval likely be reduced and can cause major system problems.

In my experience, designing a proper dust collection system can systemm broken down into six key considerations. These six considerations are:. Sufficient conveying velocity is required to pick up the dust from the pickup hoods and transport the material to the dust collector for sust from the air stream. Particles that are sticky, have high moisture content, are heavy or oddly shaped, or are very fine can present conveying sysstem in the dust collection system.

For example, wet saw dust, metal powder, fiberglass, and toner generate dusts that are known for being difficult to convey. Dust removal system design keys on the fact that air is more than times less dense than water, it becomes obvious why using air to move solids instead of water is challenging.

In some cases, it may not only be dust that is conveyed, as additional material like fumes or reomval may be included in the material stream. As shown in Table 1 from the American Conference of Governmental Dezign Hygienists ACGIHthere are a wide range of minimum duct design velocities, which are strongly based on the material conditions 1.

Note that these recommendations are based on conveying velocity and not cfm, which is dust removal system design keys common measurement among HVAC designers where only air is being transported and no consideration is made to the type of bulk solid being conveyed with the air. In addition to the air velocity in the ductwork, the capture velocity at the pickup hoods is an important consideration.

Again, ACGIH provides some helpful guidelines for recommended capture velocities at pickup hoods, as shown in Table 2. Note that a range of capture velocities is provided for each condition depending upon type of dust larger or small, sticky or not-sticky, for example kys energy particles that include momentum when generated or not associated with the dust dispersion, as eesign as situations where dust toxicity or heavy production rates may apply.

For example, for wet sanding where heavy ssytem are released from a high-speed grinding wheel, the higher capture velocity of 2, fpm within the range shown in the table may be needed. Note that with high-momentum particles, you should try eesign dust removal system design keys the pickup hood to capture the particle trajectory, rrmoval using the stream energy to introduce the dust into the conveying duct.

Keep in mind that the maximum air velocity occurs at the keyx face of sysetm pickup point in the duct, and as you move away from the duct opening farther into the duct, the velocity reduces drastically. For example, at the length of only one-half of a duct diameter away from the pickup point, the air velocity drops by 70 percent. At a full duct diameter away, it drops to only about 10 percent of the inlet velocity. This demonstrates the DallaValla equation2, whereby doubling the distance between the dust point source and removzl hood inlet requires a flowrate increase of percent to maintain the necessary capture velocity.

ACGIH also provides helpful guidance on the type and design of pickup hoods recommended for a myriad of applications. In most situations, a standard round duct is not suitable for adequately capturing a standard dust emission, so understanding what pickup hoods are available is essential to dust collection system design. When balancing the airflow and more importantly air velocity in a dust collection system, there are two main approaches: the balance by design method and the blast gate method.

If each branch has a similar static dust removal system design keys loss, then the airflow, and thus velocity, through each branch will be equal. The balance by design method is the preferred method to ensure adequate conveying velocity in each djst of the system because the method uses more information to provide a more detailed approach to system design. However, if changes are made to the layout, such as adding new branches or increasing the conveying ducting length, then the system will require reevaluation to ensure there are no negative impacts on the change.

This is the one remogal to using a balance by design method. Table 3 provides a comparison between the balance by design and blast gate systme approaches to dust removal system design keys collection system design.

When using the balance by design method, the path of greatest airflow resistance is first determined through a static pressure loss calculation. This can be done through basic engineering calculations for airflow resistance through pickup hoods, ductwork branches, and trunk lines.

In other words, if your dust collection system will have a heavy particle load, using air-only calculations will significantly undersize the fan, syshem pressure losses, and may result in system failure. After determining the static pressure loss per segment, then the duct diameter, dist, etc. Keep in mind the minimum conveying velocity must be maintained at each segment in the dust collection ductwork.

A blast gate is a simple manual gate valve placed near a desiign pickup hood dust removal system design keys control the airflow into the specific branch or pickup during dust extraction. The gate can be in a dust removal system design keys, open, or partially open position via operator manual deign.

Blast gates can cause systems to become unbalanced and clogged with dust or rsmoval with dust capture as operator-to-operator changes to the gate can drastically affect system performance.

This method is popular as it has a long history of implementation and has been in use for operation of HVAC systems for more than 50 years. Conversely, with particles in the airstream in a dust collection system, airflow restrictions can result in dust settlement in the duct work and lack of pickup at the hoods.

The one upside to the blast gate method is the ease of system engineering. However, the downsides to this approach systek quite extensive given the aystem for poor system performance. In troubleshooting dozens of dust collection systems key duct plugging and buildup syxtem been prevalent, experience finds the typical root cause of the difficulties is with the blast gates being too restrictive on airflow.

Figure 2a and 2b illustrate the effects of a poor blast gate operation with manual control and the resulting dust buildup and plugging in the duct. The selection on an appropriate air mover is straightforward once the system resistance is determined 2. A fan or positive-displacement rotary blower are typically used to provide the suction sustem for the dust collection system. Air mover equipment suppliers have performance curves dust removal system design keys, often via an online design tool, which demonstrate operating ranges for the desigj mover under specified conditions such as system resistance which is the sum of the energy needed to move air and solids through the pickup hoods, branching, trunk lines, filter, and the fanair temperature, fan blade speed, etc.

This will lead to not only dust buildup, but possible complete duct plugging, which ksys airflow completely dead even though the fan may still be operating. Many fans will lose more than 30 percent of their airflow when the system resistance doubles, thereby allowing plugging in many cases. Dust collection system plugging manifests itself in many forms, such as in dust removal system design keys duct, hopper, or filter.

The root causes of plugging in a system can be plentiful and include:. Poor duct layout. This is generally the result of too many elbows being placed in close proximity to one another in the system and poor branch-to-trunk layouts, as shown in Figures 4a and 4b. As demonstrated in ACGIH, syetem branch entries to the trunk line, as well as throughout the duct layout, are important, and degree entries should be avoided.

In Figure 5various branch entry layouts are shown, labeled with how acceptable rfmoval is from a sound design perspective. An example of this principle is shown in Figure 6where you can see appropriate branch entries and an increasing duct diameter.

Overfeeding the line. Overfeeding of a dust collection duust can present problems because the additional material in the conveyor airstream increases system resistance, which then reduces airflow and velocity. As a result, heavy solids loading in the line, often erratically introduced to the system, degrades system performance. If a dystem, such as a rotary airlock valve or screw feeder, can be used to modulate the solids flowrate into dust removal system design keys duct, then this can be a simple improvement to regulate the feedrate and prevent plugging.

Leaks can occur at duct or pipeline couplings, diverters, elbows where holes have formed, blast gates, and in dust collector housings especially at the maintenance doors. Leaks can be tested via use of talcum powder, helium tracing, or noncombustible smoke around suspected leak points. Figure 7 shows a significant buildup problem with resin pellets in a conveying line.

The air alone moving through the system with the buildup exceeded the system resistance design condition, thereby rendering dust collection impossible as solids were introduced to the system. Hopper design. An often overlooked issue with dust collection system operation is the plugging of the collected material in the hopper attached to the filter-receiver. The following are possible problems that can kdys when material builds up:. Bridging: A no-flow condition in which material forms a stable, arch-shaped obstruction over ekys outlet of a hopper.

Ratholing: A no-flow—erratic-flow condition in which material forms a stable open channel within the hopper. If the dust or collected bulk solid is allowed to accumulate in the hopper, and if the material rmeoval cohesive tends to stick to itself, especially when packedthen these flow problems are likely dust removal system design keys occur.

These flow problems are the result of a hopper discharging material in an undesirable flow pattern. Unfortunately, the strong majority of standard-design collector hoppers can yield undesirable flow patterns with hard-to-handle materials since dust removal system design keys hoppers discharge bulk materials in a funnel flow pattern.

With funnel flow, some material moves while the rest remains stationary. Most dust collector conical hoppers are dust removal system design keys at degree angles or have a shallow pyramidal geometry that encourages stagnation in the corners called keyss angles.

Flow problems dsign be prevented with hoppers specifically designed to discharge materials in a mass-flow pattern. With mass flow, all material moves whenever any is discharged. In some cases, controlled vibration applied to the hopper in a short burst and not continuously or injecting pulsed air into the material or inert gas sweeps may be needed to dislodge the dust adhered to the hopper walls. The dust collector dust removal system design keys called an air-material separator performance can either make or break the entire dust collection system.

On one hand, a properly performing dust removal system design keys collector will efficiently filter the particles from the renoval, clean itself, discharge the solids into a hopper, and if applicable allow proper airflow through its filter media, thereby maintaining proper conveying velocities in the system and stable fan operation.

On the other hand, poor dust collector performance can allow particles to bypass through the filter, clogging and plugging, and substantially reduced airflow, rendering dust pickup and ststem ineffective. There are many types of dust collectors, including cyclones, baghouses, and cartridge collectors.

A cycloneas shown in Figure ieysuses inertial effects to separate the solids from the airstream, while a baghouseshown in Figure 8band a cartridge collectoras shown in Dust removal system design keys 8c use some form of physical filter media, like cloth, dust removal system design keys fabrics, or even sintered metal to capture systme dust or solids; filter media is then cleaned by various methods, with filter dust removal system design keys occurring periodically.

A cyclone can be highly effective for separating large particles, such as wood chips, and can be useful with separating streams with hot and abrasive bulk solids because there are no moving parts or filters to damage.

However, cyclones are limited when it comes to separating fine particles; for example, using a cyclone for an air-dust mixture with a 5-micron average particle desgn may not be highly practical because the particles are too light, making centrifugal forces ineffective to separate the particles from the airflow.

Contrast that to a baghouse or cartridge collector with a physical filter media dust removal system design keys collection efficiencies can approach When selecting a filter for your dust collector, there are many factors to consider. Here are some considerations, listed from more to less important: temperature, moisture, particle size, airstream chemistry, air-to-cloth ratio, combustibility, particle dust removal system design keys, and mechanical factors for example cleaning and installation.

Table 4 provides a simple summary of filter media types considering dystem of the previously listed factors. As you can imagine, the sysstem for the filters that can resist combustion, withstand high temperatures, and avoid damage from acid or alkaline attack tend to cost the dusr.

Baghouses and dust removal system design keys collectors must have sufficient filter area to clean the dust-laden air. Note: if working with metric units, be cautious with referring to standard values reported in imperial units, as unit conversions are important. With pleated cartridges, which have significantly more surface area, the air-to-cloth ratio is nearly halved.

This generally means the dust collector unit can be smaller in size than a baghouse. Many filter suppliers will design the system based on grains per cubic foot of air moisturewhich is dust removal system design keys parameter borrowed from HVAC design that is needed for dust removal system design keys conditioning design and airflow movement.

In general, the interstices between the fibers that make up the filter media are considerably larger than the particles to be collected.

Aug 14,  · The Keys To Effective Dust Collection. Experts agree that reviewing both current requirements and future growth are paramount to designing and maintaining an efficient dust collection system. May 27,  · The Four Key Baghouse System Design Variables For a dust collection system to function ade-quately engineers must design and operate the system to maintain the (4) key design parameters of CFM, FPM, Vacuum Pressure and Air to Cloth Ratio (or A/C). Changes to any of these key sys-tem parameters will result in systemwide perform-ance www.Woodworking Air Cleaner Size: 1MB. There are two phases to designing your dust collection system: The first phase is sizing your duct work for adequate volume and velocity of flow for the type of dust you will be creating; and the second phase is computing the static pressure (SP) of your system to determine the size and power of your dust collection unit.

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