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Dust Collection Basics

Choosing an Industrial Air Filtration System

 

Choosing the right industrial air filtration equipment is a major decision for any business.  The health and safety of your employees are at stake, and the profitability of you operation depends on it. In many cases, the efficient operation of your plant and machinery depends on good dust collection and clean factory air. That is why you need a dust collector manufacturer that does more than just take your order.  You need a company that takes the time to understand your dust collection and clean air requirements and to recommend or build the right system for your needs.

 

DustCollectorSource is an industry-leading manufacturer of dust collectors, industrial vacuums, and HEPA air cleaning machines.  In the following paragraphs, we present an overview of information you may find useful in selecting the right equipment for your particular dust control or clean-air problem.

 

So Why Bother Purchasing and Installing a Dust Collection System?

 

Improved Product Quality  —  Because it can provide a nearly dust-free environment, a well-designed dust collection system can reduce product defects caused by dust and airborne particulate contamination.

 

Better Morale and Employee Retention  —  A cleaner workplace leads to better productivity, better employee morale and job satisfaction and can reduce hiring and training costs.

 

Increased Productivity  —  A cleaner, safer workplace contributes to increased productivity by reducing equipment downtime, worker’s sick time off, and on the job injuries.

 

Improved Health and Safety  —  Thanks to their high filtration efficiency and safe easy maintenance, dust collectors and HEPA air cleaning machines can provide the cleanest, and healthiest work environments.

 

OSHA/EPA Compliance  —  Both OSHA and EPA requirements for clean air in the workplace are increasing annually.  An effective dust collection and air cleaning system can meet and exceed these new requirements.

 

Lower Operating Costs  —  An effective dust control system can reduce maintenance expense, increase equipment useful life, and lead to reduced energy costs by recirculation of clean air.

 

Lower Insurance Premiums  —  By improving shop safety and air cleanliness, these systems can often produce savings in health insurance and workers’ compensation insurance premiums.

 

Elements of a Dust Collection System

 

Most dust collection systems can be viewed a collection of components all carefully engineered to perform together and accomplish its mission (debris pickup, air cleaning, fume extraction, etc.):

 

Pickups, Shrouds, Nozzles, Hoods

 

This is the “source” end of the dust collection problem.  It may consist of a hand-held nozzle,  a  shroud around a saw or grinder, a fume hood over a work station, or an entire painting booth.  The “pick-up” end of the system must be carefully designed to meet the particular needs of the problem at hand and to minimize the power, airflow, and pressure requirements to be imposed on the dust collector design.

 

Duct and Hose Design

 

The dust or other particulate materials from the pickups must be carried to the collector in hoses or ducts.  These can be as small as 1 ¼  inch in diameter (in the case of a hand-held tool) up to 12” in diameter in the case of a medium sized dust collection system of 4000 CFM capacity.  The ducts and hoses can be of many different material, from PVC to galvanized steel to stainless steel, depending upon the materials to be conveyed or the code requirements of the particular industry.

 

Cyclone Collectors and Pretanks

 

Most dust collection systems have some form of  pretank or cyclone separator as the first stage of filtration.  In selecting a good approach the type of material (particle sizes, wet/dry, and other considerations) must be taken into account as well as the amount of material to be collected per day.  The effectiveness of the pre-filtration will heavily determine the maintenance cycle (how often the main collection tanks will have to be emptied),  clogging of filters (also a maintenance issue) and the overall efficiency of the system.

 

Filtration

 

Standard filtration in most industrial dust collectors is by use of fabric bags.  We use a #10 polyester duck material for the filters in all of our dust collectors.  This material provides for 99% filtration efficiency for particle sizes of 10 micron and larger and is still approximately 90% efficient down to the 3-5 micron range.  Where the smaller particulates must be removed, HEPA (High Efficiency Particulate Air) filtration is added to the standard unit. HEPA filters are rated and tested to 99.97% efficiency at particle sizes of  0.3 micron and larger.  The range of coverage of the HEPA filtration (0.1 to 10 micron) covers such materials as smokes, pollens, small airborne particulates from sanding, grinding, polishing operations, powder residues from pharmaceutical, plastics, foods, and other manufacturing operations, and many other airborne contaminates.  These particulates in the one micron category are invisible, remain in the air, and also remain in the lungs when the contaminated air is breathed.  Some materials which were previously thought not to be hazardous (such as wood dust) have been recently classified as carcinogenic by the EPA and thus a larger and larger percentage of the dust collection systems currently being installed are employing HEPA filtration in addition to the standard fabric filtration.  This trend is likely to continue.  Within a few years, the vast majority of all dust collection and air cleaning systems for commercial and industrial application will employ HEPA filtration as standard.  Dust Collector Source offers all of its models of dust collectors, industrial vacuums, and air cleaning machines with HEPA filtration as an option.  Approximately 80% of our current sales is made up of HEPA units.

 

Blower Package and Motor

 

Following the filter package is the “Blower Package”.  In all high quality dust collectors, the air is sucked through the filter package, so that the blower package and motor remain clean.  The blower package is designed to suck a certain volume of air (measured in CFM or Cubic Feet per Mimute), with a certain pressure/airflow curve. Dust collectors operate in the medium pressure range (5-20 inches water gauge pressure).  Industrial vacuums, which are designed for smaller hoses/ducts operate in the high pressure range (80-130 inches water gauge pressure). We manufacture dust collection systems in the 500 CFM (3/4 HP) to 4000 CFM (10 HP) range.  This is the lower end of the sizing range, other manufacturers produce equipment up to 200 HP.  We also manufacture a complete line of high-pressure industrial vacuum systems in sizes from 80 CFM (2 HP) up to 450 CFM (12.5 HP).  These systems are designed for continuous duty applications.

 

Dust Collector System Cabinets

 

We manufacture our dust collector line of products in four cabinet sizes: a 100 series, 200 series, 300 series, and 400 series.  These models of dust collector range up to our largest HEPA dust collector model, which is approximately 36” square in footprint and approximately 86 inches tall.  All of these dust collector models are suitable for both portable and built-in applications.  We also manufacture some of our HEPA air cleaning equipment and industrial vacuum systems specifically for portable use.

 

Product Selection Guidelines

 

In selecting a dust collector or industrial vacuum we must first consider the nature of the pickup point and whether a high-pressure system is needed.  If the orifice at the pickup point (s) of the collection system is of the order of 1 ½ inches in diameter to 2 inches in diameter then we likely are in the high pressure region.  For picking up of heavy particulate with hose/orifice diameters of this size the required airflows are approximately:  60 CFM for 1 ¼ inch orifices, 90 CFM for 1 ½ inch orifices, and 125 CFM for 2 inch orifices.  These airflows maintain 100 feet per second of air velocity in the hose, which is necessary to carry heavy particulate vertically upwards.  These airflow/veolcity relationships require relatively high pressure levels from the vacuum system and determine the nature of the motor and fan design.

 

In our line of high-pressure industrial vacuum systems, we use two different types of vacuum motors.  First is the intermittent type (a brush-type multi-pole motor).  This is the same type of vacuum motor used in the “shop-vac” or home type of vacuum.  They are suitable for light duty or intermittent use, and if operated 24/7 or even for hours at a time will provide only a short useful life before the motor’s brushes must be replaced or the motor is burnt out.  The second type is the continuous-duty industrial vacuum motor.  Our vacuums use motors in sizes from 2 to 12.5 HP manufactured by Siemens of Germany, which are the finest industrial vacuum motors available.  They are efficient, quiet and can be operated 24/7 for years trouble-free.

 

When larger hose sizes are used, and larger amounts of airflow (500 CFM and up) must be provided, then we move into the realm of  the dust collector, which functionally is still a vacuum cleaner, but with much higher airflow and operating in the medium pressure region (5 to 20 inches water gauge pressure).  The industrial dust collectors that we manufacture employ TEFC motors in sizes from ¾ HP to 10 HP and custom designed fans to provide the desired pressure airflow curve.  The power required to suck a given quantity of air through the pickups and filters is a function of the product of the airflow and pressure drop, and thus it is mandatory that careful attention be paid to the design of the pickup points and ductwork to minimize these pressure losses.  An optimum system design will provide adequate suction at the pickup points to gather the particulate matter to be cleaned, adequate airflow to carry the material through the duct system, the proper filtration to remove the material from the air stream, while at the same time using an efficiently sized fan/motor combination to minimize power consumption.  We also like to provide these dust collection and vacuum systems at an affordable price!

 

Another class of machine should be considered here.  When extensive ductwork is not required and when the primary dusts are low volume and airborne, then the HEPA air cleaner should be considered.  We manufacture two models of HEPA air cleaner which are suitable for such tasks as maintaining clean air at construction sites, asbestos abatement, lead paint abatement, removing contaminants such as pollens and odors from the air, etc.  Our air cleaning machines operate in a yet lower pressure range than the dust collectors (5 inches water gauge).  The air is sucked in and processed through three stages of filtration (Metal mesh, phenolic paper, and HEPA) and returned to the ambient environment very clean.  These machines are manufactured in two airflow sizes 1000 CFM  and 2000 CFM and are designed to be very portable  and to operate from standard 115 VAC single phase power lines.

 

After deciding on the class of machine required, we then proceed to the task of sizing the machine.  We develop a sizing requirement for the machine by looking at the particular airflow requirement of each pickup point (as an example a woodshop with four pieces of woodworking equipment).  These are added up and the effects of the ductwork taken into account to determine the amount of airflow needed.  It is often desirable to consider including “blast-gates” (air valves) in the lines for each piece of equipment to be serviced.  This approach allows a smaller system to be used if not all of the equipments are to be used simultaneously.  These blast-gates can be automated so that the turning on of a particular equipment will energize the dust collector and open the appropriate valves.  This style of dust collector system design is being used more and more to achieve a “hands-free” system operation.  We can provide this automated type of gating approach be incorporated into your dust collection system design.

 

Other types of equipment can be incorporated into the system design to meet individual needs.  Where large amount of storage is required (as might be the case in a production woodshop), pretanks or holding bins can be included in the system.  Sometimes fluid recovery is necessary and separation tanks are included in the system design.  We also have available pickup arms (sometimes called fume arms) for applications such as welding smoke.

 

 

Applications of  Dust Collection Systems

 

Dust collection and air cleaning equipment is applied in almost every conceivable industry worldwide.  We discuss below some particular industries and their application of our dust collector and industrial vacuum products.  This should provide examples to suggest applications of this technology to your particular problem.

 

Printing Industry

 

All printing presses have some form of dust collection incorporated at various stages of the printing and paper handling process.  Both our high-pressure vacuums and our medium pressure dust collection systems are used by printing press manufactures and users at the slitters and formers to remove the paper dust that is generated in the cutting and folding operations.  Without this dust removal, print quality (as well as air quality in the pressroom) degrades rapidly and press maintenance cost increases rapidly.

 

Foods Industry

 

Our high-pressure industrial vacuums are used in sugar cane mills in the Philippines for round the clock cleanup of process equipment.  Our HEPA air cleaning machines are used by a major domestic producer for maintenance of air quality in its candy making operations. Our dust collectors are used in several commercial bakery operations in the U. S. for removing residue from dry mixing of ingredients.

 

Pharmaceutical Industry

 

In the pharmaceutical industry, various products are handled, transported, packaged, stamped into pill form, mixed, and so forth.  In all stages of these operations, residue is produced, dust becomes airborne, and many requirements arise for cleaning of ambient air and process equipment. HEPA dust collection is used extensively in this industry both for protection of the workers’ lungs and for protection of the purity of the product from outside airborne contaminants.

 

Asbestos Abatement

 

In construction and remodeling operations both residential and commercial old air ducting, insulation and building materials are removed.  These materials (especially those installed before 1965) often include asbestos.  When these materials are disturbed dust is created and this dust is often asbestos laden and becomes a major hazard for the workers who breathe it.  Our HEPA air cleaning machines were designed especially for this problem.  A large suction hose is attached to the machine and taken into the workspace (such as an attic).  Large volumes of air are continually sucked from the workplace and cleansed (using HEPA filtration), reducing or eliminating the danger to the workers’ lungs from breathing of this air.

 

 

Construction Industry

 

As with asbestos abatement, most construction operations involve a lot of dust.  In the refinishing or sanding of hardwood floors, it is very important to keep control of the dust generated. First, in the case of refinishing operations one is usually working in an established home, and the dust generated by the sanding tends to get into everything.  It seems to go through walls.  The fine dust is a major annoyance to the homeowner who seems to be cleaning it up for months after the contractor is gone.  This problem can be addressed very effectively by attaching a high-pressure vacuum system to each of the active sanding equipments during use.  The vacuum system is usually mounted in a truck or trailer and a long hose run to the house for connection to the sanders.  This enables all of the dust to be collected at the source before it has a chance to get away and settle on everything in the house.  We manufacture industrial vacuums very well suited to this task.  They are usually powered on the truck or trailer by a gasoline generator.  The other problem in hardwood floor finishing is contamination of the finish by the residual dusts.  This is usually handled by vacuuming up the dust well prior to application of the stains and varnishes.  But the residual dusts hanging around in the air can also settle out into the wet finish materials causing problems.  This problem can be addressed by using one of our air cleaning machines during all of the floor sanding operations. It is also a good idea to run the air cleaning machine overnight prior the application of the stains and finishes if time permits as this will produce completely clean air in the room and nothing left to drop into the wet finish.

 

Woodshop Dust Control

 

Almost all professional woodshops now use some form of dust control.  This is necessary for two reasons.  First is the simple issue of handling the sawdust.  The old fashioned way was to let the dust fall where it may and clean it up when necessary.  It has proven more cost effective to provide source capture and handling than to just let it happen.  A dust collection system with ducts to each piece of equipment is now the universal approach in most commercial operations.  Second, OSHA and EPA requirements for clean air in the workplace mandate that shops control the amount and quality or particulate in the air.  Workers and not permitted to breath dust contaminated air.  The EPA has also recently declared wood-dust to be carcinogenic and even more stringent requirements appear to be on the way.  More and more wood shops are now employing HEPA filtration to provide true laboratory quality air for their workers.

 

Welding Smoke

 

The smoky byproducts of welding operations are highly toxic smokes and particulates which must be removed or at least diverted from being breathed by the welding operator.  Currently, large-air-volume dust collectors are used to suck the air away from the worker and cleanse it.  HEPA filtration, activated carbon filtration, or both, are used to purge the toxic components and particulate matter from the air.  We manufacture both HEPA and activated carbon air cleaners well suited to this task.  In welding smoke applications, some form of fume arm is typically used to get the point of air suction right next to the source of the contamination.  These movable arms also can be easily and quickly placed by the operator to provide the best source pickup.

 

Sanding, Polishing, Grinding

 

Dust collection is usually employed on all grinding, sanding, and polishing operations in industry.  Small, hand-held parts are applied to sanders or grinders to remove rough edges from cutting, stamping, or casting, and dust and residue is created by the cutting wheel.  Usually fixed shrouds are employed around the cutting wheels, and the suction hose is attached to the shroud and ducted to the dust collection system.  We usually recommend HEPA filtration be used in these types of systems as the dust collector will also serve the purpose of providing general air cleansing functions as well as removing the material from the grinding operation. Overall air quality in the factory can be substantially improved with this approch.

 

Rubber and Plastics Deflashing

 

Our dust collectors have been successfully employed in cleanup or residue from rubber, plastics, and composites processing. In rubber manufacturing we have added special water separation tanks at the input side of the industrial vacuum system to provide for pumping out and reuse of the fluids used in the cleaning process.

 

Paint Booths and Fume Extraction

 

Most industrial paint operations now require that effective fume and vapor control be incorporated in the facility.  These control measures are needed to protect not only the workers who must be in the point facility, but also the general neighborhood around the facility and the global environment from contamination. Water-bath type of filtration is common for this application;  the contaminated air is sucked through a water bath wherein the vapors are removed.  Activated carbon is also commonly used as a filter media for this application.  Our HEPA dust collectors are designed so that the HEPA filter module can be replaced with an activated carbon cartridge.  The cartridge is refillable.  When the activated carbon material becomes saturated with contaminate, the cartridge is removed from the dust collector and refilled with fresh activated carbon granules.  The old material is then disposed of properly or in some cases can be reprocessed and reused.  Some of our new dust collector models will have provision for both HEPA and activated carbon filtration.

 

Oil Mist Extraction

 

On most numerically controlled machine tools oil mists are applied continuously around the cutting heads.  These mists must be removed from the workspace around the cutting heads to avoid build-up that interferes with the operation of the machine.  It is also necessary to prevent these oil mists from contaminating the general air of the shop.  Our firm does not currently manufacture equipment suitable for this oil mist extraction problem.

 

Dental Laboratories

 

In dental laboratories, operators are typically sitting at a desk or workbench using small grinding, sanding and polishing equipment.  They require a suction port right at the desk surface to pickup the residue from these operation.  The dust and residue produced is of various ceramic and metal compositions and requires that it be cleaned from the air with high quality filtration if the air is to be returned to the laboratory.  HEPA filtration is always required in these applications. Our firm has done complete systems for dental laboratories including HEPA dust collectors, ducting, and desktop suction ports.

 

Chemical Industry

 

Our dust collection systems are used in many areas of the chemical industry.  These applications include general cleanup of process machinery, air quality control in packaging operations, laboratory air quality control, and vacuum material handling.

 

Electronics and Printed Circuit Board Manufacture

 

In the manufacture of printed circuit boards for the electronics industry, dust collection is usually incorporated into the automated routing equipment which cuts apart individual boards, trims the boards and drills holes or vias for later plating through operations.  A small high-pressure vacuum system is attached to the small orifice (typically 3/8 inch in diameter) which is a part of the cutting head of the router.  The suction orifice takes away the residue from the routing operation.  We manufacture a small compact and quiet vacuum system which is perfect for this electronics application.  Depending on the materials employed in the printed circuit board manufacture, HEPA filtration is sometimes included in this design.

 

Duct Cleaning

 

Duct Cleaning is a growing industry.  Many options are now available for cleaning the air conditioning and heating ducts both in residential and commercial buildings.  Dust builds up in these ducts and harbors all sorts of biological and chemical contamination.  Firms that perform this duct cleaning service typically employ rotary brushes which are sent up through the duct work while at the same time a vacuum system is used to draw the dust and contaminated air out through the ducts to be cleaned.  Out HEPA air cleaning machines are perfect for this application.  A large hose is attached from the air cleaning machine to the duct and the air is drawn out from the ducts continuously during the cleaning process.

 

Glass Manufacture

 

Our dust collection equipment is used in this industry for keeping powders and residues from the glass manufacturing process from becoming a problem in the local factory environment.

 

Die Casting

 

In the die casting process, slag and other residue can accumulate in the die area and must be cleaned on a continuous basis.  Our dust collection equipment has been used to remove this slag material from the dies.

 

 

 

 

Duct System Design

 

In designing the ducting for a dust collection system, very careful attention must be paid to the selection and layout of the piping and interconnecting fittings.  Understanding of the following nomograph is key to the understanding of the basic principals of good duct design.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

For a given duct diameter, select the appropriate line labeled 1” to 16”. Then on the left hand scale identify a horizontal line corresponding to the airflow for the given duct.  One may then read the corresponding pressure loss and air velocity for the duct.  The pressure losses are given in inches water gauge for 100 feet of duct.  Since the pressure losses are linear with pipe length, simply adjust the pressure loss for pipe length (50 feet of pipe has one half the loss of 100 feet).  For conversion of inches mercury to inches water please refer to the following conversion chart.

 

For a system design to be effective the airflow around the pickup nozzle must be sufficient to carry the material into the duct system.  Additionally, the airflow must be adequate in each branch and trunk of the duct system to insure that the materials are carried to the dust collector and not left in the walls of the duct system.  Two standards are commonly used in industry. For light and most airborne materials, 3500 feet per minute velocity (approximately 60 feet per second) is adequate as a minimum system design parameter.  For other materials or heavier particulates 4500 feet per minute (approximately 75 feet per second) is the minimum standard.  In our practice, we tend toward the conservative and like to design all systems for a minimum duct velocity of 80-100 feet per second, which is sufficient to carry metal chips vertically upward in a duct.

 

Use of high quality materials and fittings in the duct system design is also important.  In medium-pressure dust collection systems, 22-gauge galvanized steel spiral pipe is the most common.  Fittings such as Y’s, T’s and couplers are standard catalog items.  These components are designed to slip together and are joined with tech screws, pop rivets, or spot welding.  Sealing of all joints and seams in the duct-work is critical to good system performance.  It is very easy to lose the bulk of your system pressure budget to leaks in the ducting system.
Pressure Conversions

 

The following chart may be used to convert the various pressure measurements
Design Study of a Nitro-Cellulose Dust Collection System

 

 

The following paragraphs present results of a design study performed on a dust collection system designed to remove explosive nitrocellulose dust from machining operations which produce shell casings for the U. S. government. The issues and designed considerations discussed may be useful to designers of  dust collection systems for other applications.

 

 

Materials

 

The material to be handled is a nitrocellulose-based felted fiber product residue from trimming and sanding manufacturing operations. Due to the explosive nature of the dusts produced extreme attention must be paid to fire and explosion safety at every point of the operation.  The material is handled dry at the pickup hoods, in the branch ducting, and in the main ducting.  At the point of entry into the cyclone collector, water injection is used to keep the material wet during the balance of the filtering and collection process.

 

The collection and transport system deals with dry dusts and light chips.  This type of material generally requires transport air velocities of 3500 fpm minimum for good system performance.  Air velocities at the pickup points are highly dependent upon the type of operations being performed and generally require that the hood or shroud design be done empirically.  For the subject material, the hoods in place appear to be acceptable, with some exceptions as noted later in this report.  The two areas of desired improvement are that less of the material fall to the bottom of the workspace, and that less of the fine material be permitted to escape the collection system and become fine residue settling in other areas of the shop.  Both of these areas will be addressed later in the report.

 

 

Pickup-Points

 

The pickup hoods for the both the deflash station and the trimmers appear to be of good design.  Both are located toward the bottom of the work area and provide efficient airflow for pickup around the work piece.  Better pickup efficiency can be achieved by having more of an enclosure around the work-piece. It is also needed that the inlet air space around the hood be as small as possible to maintain higher velocity as air enters the hood.  For the hoods observed, it  does not appear to be practical to have a larger or more confining enclosure that is fixed  without unduly restricting the workers’ access to the work-piece.  The hood could be potentially moved into place automatically when each piece is started, but this would require a more complex mechanical design of each station.

 

 

Pneumatic Valves

 

At the points of pickup suction is gated on and off with the particular operation being performed.  The gating serves two functions.  First, it provides a suitable means of prevention of fire/explosion propagation throughout the ductwork in case of an incident. Second, it allows the collection system to operate efficiently, with only the ports in use to be open.

 

In the upgraded system, three additional pickup points are to be added.  There is also a desire to provide somewhat better pickup efficiency to eliminate the small amounts of dust which escape the primary pickups currently and form a film on adjacent factory surfaces.  It is also desired to improve the overall shop air quality without having to add additional air cleaning equipment.

 

Blast gates are currently opened and closed simultaneously with the work-piece motion.  At the beginning of the operation this appears to permit some material to escape the pickup.  At the end of the operation, the airflow is cut off simultaneously with the rotational power.  As the part is spinning down there appears to be a substantial opportunity for dust to escape the workspace.

 

It is recommended that a sequencer be added to the air gate control to provide an advance opening and a delayed closing of the air valve. A one second advance on startup and a 2-3 second delay on shutdown would seem to be an appropriate timing for the valve operation. This will not only improve the pickup efficiency, but will also remove a certain volume of air around the work-piece after shutdown and prevent some of the fine airborne dusts from drifting away from the operation.

 

It is recommended that the gating be maintained in the upgraded system, both from the safety standpoint and the collection efficiency standpoint, as outlined above.  There was discussion with the staff concerning the potential of leaving all of the pickup points open all the time, and adding a shutoff capability (triggered by UV fire detection).  Considering the on-off duty cycle of the operations, the three additional stations to be added, and the power/pressure/airflow of the current power unit, this could lead to somewhat substandard performance in terms of pickup airflow/velocity.  Duty cycles appear to be 30-50%, and this permits very good performance at each station with the power unit available, as only half of the stations are on at a time, on average.  Maintaining the existing approach, even with the addition of the sequencing, should allow good performance with the existing motor/fan.  Sizing of the branch transport ducts (2 and 3 inches) is suitable for the airflows and pressures available.

 

 

Branch Ducting

 

The entire transport system is constructed from welded stainless steel, which provides the abrasive/corrosion resistance needed as well as a suitable containment for fire/explosion. The branch ducts are constructed from welded two and three inch stainless steel tubing, with careful attention to welding quality and elimination of voids and sharp points which could become responsible for static discharge.

 

Sound design considerations appear to have been applied in the design of the branch ducting. Elbows are designed for R/D of greater than 3.0, and tapers designed for loss coefficients of less than 0.1.  Branch entry angles are all low, to reduce branch entry losses.

 

 

Main Duct

 

The main transport duct is constructed of welded stainless steel tubing, stepped down from 8″ diameter at the filter to 4″ at the “make-up air” end.  These ducts are adequately small to maintain minimum transport velocities for the light, dry material involved and yet adequately large to prevent undue pressure drop in the system.  There is sufficient size in these ducts to accommodate the additional stations and to accommodate additional airflow or motor power if needed at a later date.

 

At the rated 1200cfm and 5.1 Inches HG (70 Inches H20), the 8 inch main duct maintains 3500 cfm transport velocity

 

 

Cyclone Collector

 

Filtration for the  system is provided by a water-injected cyclone followed by HEPA filtration. This cyclone appears to operate efficiently and would have sufficient additional capacity for over 2000 CFM system performance, if needed.  There is a large discharge bin at the bottom of the cyclone, and this bin is emptied several times a day, as needed.

 

 

HEPA Filtration

 

Following the cyclone separator is a HEPA filtration system utilizing a standard HEPA filter cartridge.  This type of filtration is normally rated to remove 99.97% of particulates down to 0.4 micron size, which should be more than adequate to provide completely clean air return from the filtration system.  However, maintenance of this filtration system is needed to assure that it is operating within manufacturer’s ratings.

 

It is recommended that the HEPA filter cartridge be monitored regularly by installation of a 0-5 or 0-8 In H20 differential pressure gauge across the filter cartridge.  This type of filter typically exhibits 1 in H20 pressure drop when virgin at 1500 CFM and up to 3-4 in H20 pressure drop when ready for changing.  Life of this filter cartridge could be anywhere from a month to a few years, depending upon its environment.  The high moisture content present in the system could also affect filter life.  The manufacturer’s data for the filter cartridge should be consulted for the pressure/airflow curve and recommended change point.  It may be appropriate to install a new filter at the same time as the gauge and to monitor and record the pressure degradation a few times per week to gain knowledge of its performance in your particular environment. The manufacturer of the filter should also be consulted regarding the moisture- laden environment in which this filter is required to perform and its potential effect on the filter performance and lifetime. A regular change interval may be recommended.

 

 

Fan/Motor

 

The power unit for the System is a 30 hp turbine blower with rated performance at 1200 CFM at 5.1 inches Hg pressure.  With the gating and sequencing recommendations implemented as above, this power unit will be completely adequate to handle the three additional stations.  Due to the existing problems (and excessive noise generated by all three of the power units) it is recommended that the motors, bearings, and fans be serviced to assure that they are operating optimally.

 

 

Return Air

 

Return air from this system is currently discharged into the atmosphere. Air discharge from this system should be of very high quality and could be routed back to the factory space if air conditioning costs are of concern.

 

 

Design Methodologies

 

Several methodologies can be used to evaluate existing and proposed system designs.  The staff engineers utilized a Velocity Pressure Method as described in Section 5 of the Industrial Ventilation Manual, 23rd Edition.  The candidate system for Building 9 is shown in Figure 1.  It consists of the existing fan/motor/filter unit, one deflash station, and one trimmer station.  It is proposed to add one additional deflash station and two additional trimmer stations, with the main duct extended with 5-inch and 4-inch tubing and branches.  The existing stations are indicated by the letters J and H on the figure, and the proposed additions with the letters G, E, and T.  A “makeup” air valve is located at the end of the main duct and is indicated as C in the figure.

 

For each segment of the system a “Target volumetric flow rate” and a minimum transport velocity are given.  Then losses are calculated given the flow rate and various components of the system.  The calculations performed by the staff are shown in the following pages. Finally, a “corrected volumetric flow rate” is obtained.  This same approach is carried across all five of the proposed stations.  With all five of the stations in operation simultaneously (all valves open) and the specified airflow/pressure point of the fan, this analysis does yield an approximation of the system operation.  What has not been considered is the actual performance of each pickup hood with the “target” airflow.

 

For this type of system we must look to empirical data to establish the sufficiency of airflow for each type of pickup, considering the type of pickup hood in use.  The evaluation of the system as it is currently being operated can yield that information

 

With the pickup hoods for one deflasher and one trimmer in intermittent operation the efficiency of the collection system was observed.  The airflow at the two stations appears to be well in excess of the “targets” as stated in the VP Design Worksheet developed by the staff.  The two stations observed do appear to have good pickup efficiency, but this may be due partly to the higher than expected airflow at each point.

 

The airflow at the stations may be estimated by beginning with the specified performance point of the fan (1200 CFM at 5.1 inches HG, or 70 inches WG);  all estimates and calculations will be done in inches H20.  We begin by assuming both stations to be operating (valves open).  At 1000 CFM, the estimated losses in the cyclone, HEPA filter and main duct are 20″ WG.  This implies that up to an additional 50″ WG of pressure drop are available at the points where the branch ducts exit the main duct.

 

We then view each pickup point and its associated branch ducting as a “load” to the main duct and estimate the airflow to be generated through this “load” by the available 50″ WG pressure.  For the two-inch diameter system, we are dealing with elbows, smooth tubing, gate valves, flexible hose, and the pickup hood.  The minimum cross sectional area of the hoods observed were all in excess of the minimum cross sectional area of the ducting.  For the typical trimmer station, we estimate the “load” of the transport system in terms of equivalent lengths of 2″ smooth tubing:

 

Conservatively estimated, the trimmer branch is equal in loading to approximately 50 ft of 2-inch smooth tubing, and with the pressure available would experience airflow of approximately of 250 CFM.

 

With three-inch diameter components, as used in all of the staff calculations except for the trimmer body vacuum, airflow will be approximately 600 CFM, with a single port operating alone.

 

We conclude that airflows in the system as it exists today with two operating stations are higher than anticipated, as well as providing satisfactory performance.  We believe that it is advisable to maintain these higher levels of airflow (than used in the staff calculations) in the upgraded system.  Since the “on” duty cycle of each station is in the range of 30-50%, this can be accomplished for the upgraded system by maintaining the gate-valve system.  The safety issue of having unused ports closed and the 1200 CFM limit of the existing power unit will both permit the addition of the three stations while maintaining good performance from each station.

 

 

Make-up Air

 

With most or all of the gate valves closed, there is a need to provide additional airflow to maintain normal operation of the fan and motor.  In the upgraded design, this is accomplished at the end of the main duct with a gate valve.  The precise method of control for this gate valve was not specified by the staff interviewed, but it is presumed that it would be opened with a logic control when two or less of the operational ports are open to provide for moderation of the static pressure at the joints and gate valves and to maintain transport velocity in the main duct.

 

 

Conclusions and Recommendations

 

As a result of the on-site review and subsequent analysis of the proposed design, we recommend the following:

 

 

Pickup-up Points, Hoods, Vents

 

The pickup hoods for the both the deflash station and the trimmers appear to be of good design.  Both are located toward the bottom of the work area and provide efficient airflow for pickup around the work piece.  Better pickup efficiency can be achieved by having more of an enclosure around the workpiece, but it is also needed that the inlet air space around the hood be as small as possible to maintain higher velocities as air enters the hood.  This does not appear to be practical without unduly restricting the workers’ access to the work-piece.

 

 

Sequencing of Gate Valves.

 

It is recommended that a sequencer be added to the air gate control to provide an advance opening and a delayed closing of the air valve. A one second advance on startup and a 2-3 second delay on shutdown would seem to be an appropriate timing for the valve operation. This will not only improve the pickup efficiency, but will also remove a certain volume of air around the work-piece after shutdown and prevent some of the fine airborne dusts from drifting away from the operation.

 

It is recommended that the valves be maintained in the upgraded system, both from the safety standpoint and the collection efficiency standpoint, as outlined above.

 

 

Pick-up Hoods, Building 6

 

At the eight sanding and de-flashing stations observed in Building 6, there appears to be a considerable amount of material which falls to the bottom of the workspace around the casing.  It was also observed that there is tape placed across the openings of the hoods.  It is recommended that the suction hose for these hoods be brought in from the bottom instead of the top of the hood.

 

At present, there is substantially more airflow from the top of the hood, and farther down the slot progressively less and less. Since the material tends to fall from gravity, the higher velocity air needs to be at the bottom of the hood to achieve maximum pickup efficiency.

 

 

Branch and Main Ducts

 

Both the branch and main ducting appear to be of good design and no changes are recommended.  In a high pressure system such as this, the losses associated with entry points of shallow angle (less than 30 degrees) and smooth elbows (with R/D of 3.0 or

 

higher) are virtually negligible, and high precision calculation of pressure losses is not usually required.

 

Cyclone Filtration

 

No changes are recommended for the basic water-injected cyclone filtration system.

 

 

HEPA Filtration

 

A regular monitoring and maintenance strategy is recommended for the HEPA filtration system, as outlined above.

 

 

Fan and Motor

 

A regular maintenance and service schedule need to me implemented for the fan and motor assembly.

 

 

Conclusions

 

With implementation of the above recommendations, the additional three stations can be adequately accommodated with the existing power unit, filtration, and ducting.  Some additional performance in pickup and air quality can be achieved with the sequencing as recommended above.  Further improvements in overall air quality (if needed) would require the addition of a separate low-pressure air cleaning system or modifications to the existing HVAC system.

 

 

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