24 Powder and Bulk Engineering, October 1991

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tem and estim John A. Constance The Engineers Collaborative, Inc.

Estimating costs is part of designing a dust control system. Careful planning during the design process will not only ensure that you design an effective dust control system, but help you develop an accurate cost estimate. This article discusses eight steps for design- ing a dust control system and accurately estimating the system’s cost.

ost dry solids processing operations generate dust, typ- ically at transfer points such as packaging stations M where drums, bags, and totes are filled or unloaded and

near belt and screw conveyor loading and unloading points. Dust is also generated by equipment such as crushers and ball mills, vi- brating screens, blenders, and scales.

You can use two methods to control the dust at these areas in your plant. You can seal the dust sources (for instance, by using a pneu- matic conveyor rather than a belt conveyor). Or, when sealing the dust sources isn’t possible or totally effective, you can install a dust control system (also called a dust control exhaust system). A dust control system typically consists of one or more capture hoods located at or near the dust sources and linked by ductwork to a dust collector and an exhaust fan. In operation, the exhaust fan creates a powerful airflow that pulls dust-laden air into the system’s capture hoods, through the ductwork, and through the dust collector, which removes the dust from the air before ex- hausting the air from the system. Sealing the dust sources or using a dust control system - or both - is good engineering practice that can reduce health risks and explosion hazards associated with the dust. A closed-loop dust control system can also recover high-value product by capturing the dust, collecting it, and then recycling it back into the process.

If you decide to install a dust control system (or upgrade an exist- ing dust control system), prepare for the design process by assem- bling a design team and planning to develop detailed design documents.

Work with a dust control or ventilation engineer (your plant’s en- gineer or, more typically, an independent consultant), an indus- trial hygienist (typically a consultant), and others from your plant, includingworkers who will be affected by the dust control system. The engineer can help you design the dust control system, specify

its components, and provide detailed design documents (includ- ing drawings and specifications) for use during system fabrica- tion and installation. The industrial hygienist can help you test workplace dust levels during system design and after the system is installed. The workers can help you determine how the system will affect the current process and the workers’ operating duties. The experience this team has with similar systems (or in the case ofa system upgrade, with the existing system) will help you design an effective dust control system and accurately estimate its cost. Later, you’ll work with an installation contractor, who typically works from the detailed design documents to fabricate the duct- work, solicit bids from equipment suppliers for other system components, order the components, and install the entire system. As a team, place a high priority on developing detailed design documents, which include engineering drawings (Figure I ) and specifications. Detailed design documents are formalized near the end of the design process based on your research and planning

during the process; they’ll help the system meet your objectives and permit you to see the system on paper so you can make changes before the system is fabricated or installed.

The detailed design documents are key to making an accurate cost estimate and staying within your budget. For instance, the documents’ degree ofdetail can make the installation contractor’s sheet metal fabrication bid for capture hoods and ductwork more accurate, which protects you and the contractor and reduces the final system’s cost. Detailed design documents can also help you avoid receiving an unusually low bid from the contractor, so you don’t have to contend with several construction extras once the contractor starts installing the system.

Detailed design documents will also ensure speedier installation and fewer communication problems with the installation con- tractor, as well as clarify the engineer’s and contractor’s responsi- bilities. For instance, if all specified system components are pur- chased or fabricated and installed according to the design documents but the system doesn’t operate properly, the engineer must redesign the system. However, if the contractor took short- cuts during fabrication or installation and the components don’t conform to the detailed design documents, the contractor is re- sponsible for reworking the system.

Once you’ve assembled your team and committed to preparing detailed design documents, you’re ready to design the dust con- trol system (or determine how to upgrade the existing system) and estimate its cost. This process can be broken into eight steps: determining objectives, analyzing your material’s characteristics, studying your process, designing capture hoods, designing duct- work, selecting the exhaust fan and dust collector, determining whether to exhaust or recirculate the cleaned air, and fine-tuning your cost estimate. Following the eight steps will provide a de- tailed system layout and a good idea of details such as the system’s effect on plant utilities, the system’s space requirements, and the additional load the system will place on your plant’s heating and air conhtioning systems. At the same time, the information will help you accurately estimate the system’s cost.

Step 1 Determining objectives To determine objectives for the dust control system, research your dust control needs and consult the workers who will operate or maintain the system or the affected process. This can help you determine the type of system that will work in your plant and es- timate the system’s cost.

Part of researching your dust control needs is measuring the dust in the workplace air. For instance, if the dust poses a health risk for workers, have the industrial hygienist monitor the ambient air near the process and analyze the air’s dust concentration (typi- cally by weighing the dust in a sample of air); this will help you de- velop guidelines for designing the dust control system. If, for in- stance, the analysis shows that dust emissions at one dust source don’t exceed the Occupational Safety and Health Administration (OSHA) permissible exposure limit, the source may not require dust control; determining this can save the cost of installing a cap- ture hood in that area. [Editor’s note: For more information on OSHA regulations concerning permissible exposure limits, see “OSHA regulations: Dust and fume inhalation hazards” by Thomas Godbey in Powder and Bulk Engineering, October 1989, page 41.1

Consult workers by explaining what the proposed dust control system will do and what benefits it will provide, the approximate size and location of components (especially the capture hoods), and how the system might affect the workers’ operating duties.

Powder and Bulk Engineering, October 1991 25

Also ask for the workers’ input (for instance, where capture hoods will fit best to ensure the hoods don’t hinder the process or the workers’ access to the equipment for service and repairs). This will not only help you formulate objectives, but improve the workers’ acceptance of the system once it’s installed so they help to operate and maintain it.

If your objective is to upgrade an existing dust control system and improve its performance without making major changes, re- search your dust control needs and consult workers in the same way. The information you find can help you continue to use most of the system’s existing parts - for instance, rather than replace the entire system, only replace the system’s capture hoods - and thus minimize the upgraded system’s cost.

Step 2: Analyzing your material’s characteristics Analyzing your material’s characteristics includes measuring its bulk density, moisture content, hygroscopicity, stickiness, abra- siveness, corrosiveness, flammability, explosiveness, and toxicity This analysis provides the information you need to decide whether the process requires a dust control system, where you need to locate capture hoods, and how to match the system’s components and operation to your material. For instance, if your material is moist during some part in the process, you probably won’t need dust control at that point. If your material is dry, per- haps you can wet it to control dusting without causing other prob- lems (although in such a case the material can still become dry and dusty later in the process). If your material is corrosive, you’ll need to use system components constructed of special materials. How specific material characteristics like these affect the design process and cost estimate is discussed under later design steps.

Step 3 Studying your process Studying your process means considering what equipment it in- cludes and observing how the equipment, the workers’ use of the equipment, and the process affect your dust control needs. These factors will help you determine the type and location of the sys- tem’s capture hoods and each hood’s dust capture velocity, all of which have a major effect on the cost estimate.

Considering the different amounts of dust produced by each type of equipment in the process will help you design capture hoods and select dust capture velocities. Have the industrial hygienist analyze air samples taken near the equipment to find each dust source and determine the amount of dust each source creates. (In a very dusty area, you may have to shut down the equipment, clean the area, and then start the equipment to pinpoint a dust source.) If, for instance, the analysis shows that the air near a large dump-station is very dusty, you’ll need a large capture hood that pulls the air at a high velocity.

Observing the process in operation will also provide information. For instance, you may observe that a vibrating conveyor creates more dust and requires more clearance under capture hoods than other equipment. Or you may observe that workers operate the same process in different ways, depending on production require- ments; they may move bulk powder continuously on a belt con- veyor and move small batches of the same powder in small totes. The transfer point for each handling method requires different amounts of dust control.

Step 4: Designing capture hoods Design your system’s capture hoods based on your analysis of the material characteristics in step 2 and your study of the process in step 3. The information will help you design and locate each cap- ture hood, as well as determine its construction material, what

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dust capture velocity each hood must provide, what air volume the system must supply to each hood, and how these factors affect your cost estimate.

Capture hood design and location are critical to the system’s per- formance, yet are probably the least understood parts of design- ing a dust control system. For best results, design each capture hood to fit as close as possible to the dust source without interfer- ing with the process. This also minimizes the air volume required to control the dust and thus helps prevent capturing the product. If you need to periodically clean the hoods (for instance, if the dust is sticky), design hoods that are easy to access. If you’re u p grading an existing system, just adding a properly designed and located capture hood - in many cases without replacing any of the system’s major components - can improve the system’s ef- ficiency. Though the upgraded system may not be as effective as a new system, as long as the upgrades satisfy your objectives, you can improve the system’s performance at relatively low cost and with minimum production downtime.

Powder and Bulk Engineering, October 1991

Figures 2a and b show two capture hoods that effectively capture dust from different dust sources without interfering with the process. Figure 2a shows a capture hood at the side of a drum being filled on a floor scale; the hood pulls air across the drum to capture rising dust and also adjusts to different drum heights and swings away for scale maintenance. Figure 2b shows a capture hood over a chute used to fill a blender; the hood pulls air across the chute’s top to capture the rising dust, and the hood’s position permits a worker to empty a drum of material into the chute with- out contacting the hood.

Determining the capture hoods’ construction material depends on several factors, including your material’s characteristics, the workplace environment, and your final product’s required qual- ity. If your material is abrasive or sticky, use heavy-gauge metal capture hoods, and in an abusive environment, make sure the capture hoods are strong enough to withstand daily use. If your material is corrosive, use coated steel, stainless steel, or plastic capture hoods. Ifyour final product’s quality must meet strict pu- rity or sanitary standards, as with food, pharmaceutical, and cos- metic products, use capture hoods constructed of stainless steel with smoothly ground welds to minimize any infestation of mi- croorganisms (but be aware that such measures can add 33 per- cent to a hood’s fabrication cost). If your dust is toxic, use more capture hoods and design them to closely fit the dust sources. In- clude any ofthese special construction costs in your cost estimate.

Next, determine what dust capture velocity each hood must pro- vide at each dust source. Consider the dust particles’ velocity and the ambient air’s velocity. The proper dust capture velocity is crit- ical for a hood handling a high-value product; too high a velocity will control the dust but can capture some product. Too high a ve- locity can also waste energy.

Determining each capture hood‘s dust capture velocity and dis- tance from the dust source will reveal the air volume your dust control system requires. If you can design small capture hoods very close to the dust sources, a low-volume, high-velocity system typically performs best and costs less than a high-volume, low-ve- locity system. A low-volume, high-velocity system provides just a few hundred cubic feet of air per minute for each capture hood and thus has compact ductwork and requires a smaller dust col- lector. However, if you can’t locate a capture hood close to the dust source because of space constraints or other factors or if the dust is generated from a source 2 or 3 square feet or larger, you’ll need a high-volume system with larger ductwork and a larger dust collector, which will increase the system’s cost.

Step 5 Designing ductwork Use the information you gathered in step 4 to design the duct- work and estimate its cost. Draw the ductwork‘s layout to show duct sizes (depending on air velocity through the duct, called duct air transport velocitv, and air volume), duct positions, and lengths of duct runs from each capture hood to the exhaust fan. Deter- mine what duct components - such as dampers, cleanouts, fit- tings, and connections - are required so you can estimate their cost.

Use your analysis of material characteristics in step 2 to select the ductwork‘s construction material and thickness. For instance, if your material is corrosive, construct the ductwork of coated steel, stainless steel, or plastic. If your material is abrasive or sticky, use heavy-gauge metal ductwork. Include the cost of special construc- tion reauirements like these in your system estimate.

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Also use your analysis of material characteristics to determine the duct air transport velocity. In general, if the captured dust has a high bulk density, use a duct air transport velocity higher than 4,000 ft3/min average. To prevent dust from settling out as it’s transported through the ductwork, you’ll need a high duct air transport velocity, which typically requires a larger horsepower exhaust fan.

Powder and Bulk Engineering, October 1991

Step 6: Selecting the exhaust fan and dust collector Use the information you determined in step 2 about your mate- rial‘s characteristics, in step 4 about dust capture velocity and air volume, and in step 5 about the ductwork configuration to select the exhaust fan and dust collector and estimate their costs. Typi- cally, a high dust capture velocity or high air volume requires a larger horsepower exhaust fan. The more energy lost through the system’s components, the more energy the exhaust fan will re- quire and, thus, the larger the fan. Other factors that can raise the exhaust fan’s cost are your material’s requirements for special coatings on fan surfaces or spark-resistant fan construction, or your plant’s requirements for vibration isolation equipment.

Select a dust collector based on the dust’s characteristics. As you select the dust collector, keep your cost estimate accurate by not- ing which dust characteristics, if any, require features that will raise the unit’s cost. For instance, if your dust is moist, sticky, or hygroscopic, you may need a dust collector equipped with Teflon- coated fabric filters to ensure dust slides easily off the filters, or you may need a wet collector, such as a high-energy venturi scrub- ber, to avoid plugging a dry collector’s fabric filters with moist dust. However, the wet collector is more expensive because it in- cludes a water supply system and a liquid waste disposal system and usually requires significantly more energy than a dry collector.

If your dust is flammable or explosive, equip the dust collector with a sprinkler system and an explosion protection system and select the appropriate components (Figure 3). Ifyou use an explo-

sion vent, reinforce the dust collector’s housing to withstand an explosion’s pressure rise before the explosion vent ruptures. Use explosion check valves in inlet and outlet ducts to minimize the passage of flame and debris through the ductwork and out the capture hoods. Use fabric filters with electrically bonded filter media that provide a grounding path for electrostatic charges. In- clude the cost of such special features in your estimate.

Step 7: Determining whether to exhaust or recirculate the cleaned air

Also determine whether the dust control system will exhaust or recirculate the cleaned air, and reflect the chosen method’s cost in your cost estimate. If your dust is carcinogenic or highly toxic, cleaning and exhausting the air outside the plant is best. However, exhausting the air can require a makeup air system, which sup plies air to the workplace to ensure enough air is supplied to the dust control system during operation; in such a case, selecting a high-velocity, low-volume dust control system that requires less makeup air may be most effective.

If your plant is heated or air-conditioned much of the year, it may be best to recirculate the air, which requires a return air system from the dust collector, because heating or cooling the air sup- plied by a makeup air system raises energy costs. However, while recirculating the cleaned air saves energy, it can return contami- nants to the workplace that can adversely affect workers or the process. One way to minimize the risk of returning contaminated air to the workplace is to add a safety-monitoring filter to the re- circulating system: If the dust collector’s main filter fails, the safety-monitoring filter protects the system by plugging with dust, which reduces both the return airflow and the dust control sys- tem’s airflow. Monitoring the pressure drop across the safety- monitoring filter can signal this reduced airflow condition so a maintenance worker can correct the problem.

A recirculating dust control system’s cost depends on the return air system’s ductwork sue and options like the safety-monitoring filter, an alarm system (which triggers, for instance, when a fabric filter has broken), and ductwork insulation (required, for in- stance, when ductwork carries heated or cooled air outside the plant).

Step 8: Fine-tuning your cost estimate After performing steps 1 through 7 and completing your dust con- trol system design, you can formalize your detailed design docu- ments and fine-tune your cost estimate. The more research you’ve done and the more detailed your design documents are, the more accurate your cost estimate will be.

Now, to ensure your cost estimate is sound, build some contingen- cies into it. For instance, size ductwork to provide the lowest ac- ceptable air transport velocity, which increases the ductwork‘s size and estimated cost. You will still have final design flexibility to increase the velocity and reduce the ductwork to its most eco- nomical size. Choose a dust collector and an exhaust fan (and, if required, components for the recirculating return air system) that provide at least 10 percent more capacity than you need. Add an- other 15 percent of the system’s total estimated cost to cover the costs of installation-related items such as freight charges, unload- ing and moving components at the job site, rigging, contractor project management, renting equipment, and air system balanc- ing (which involves balancing the airflow to the system’s various capture hoods after installation). If you intend to measure the dust control system’s performance at startup, estimate the cost of sampling the workplace air both when the system is operating and

30 Powder and Bulk Engineering, October 1991

when it’s shut down. Also add an estimated inflation factor to ao count for inflation between system design and installation. Figure 4 is an example of a completed cost estimate for a dust control system.

Conclusion Now that you’ve completed the cost estimate, the job isn’t neces- sarily finished. As the dust control system is developed and in- stalled and as your plant is constructed, expanded, or renovated, your production process, production equipment layout, and pro- duction procedures can change. Such changes can compromise the standards you’ve set for the dust control system and require you to reevaluate each system component to ensure the system performs properly. In some cases, the changes can affect the sys- tem’s cost despite the contingencies you’ve built into the estimate. Being prepared to make such changes, though they raise the sys- tem’s cost above your estimate, is better than ignoring the need for such changes and installing a system that can’t control the dust it’s designed to handle. PBE

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John A. Constance, PE, is president of The Engineers Collaborative, Inc., PO Box503, Newtown, PA 18940; (215) 968-0762. Thefirm en- gineers and designs dust control systems for the chemical process in- dustries. Constance holds a BS in mechanical engineering and an MS in management science, both )om Stevens Institute ofTechno1- om, Hoboken, N.J.