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Dust Collection System Design Study — Example

1.0        Introduction and summary

 

1.1       Statement of Work

 

Ryvac Engineering Company was contracted on 3 May 00 to provide consulting services to Armtec regarding dust collection systems at their manufacturing facilities in Coachella, California.  The Statement of Work is as follows:

 

  1. The contractor shall review proposed design changes for the present dust collection system located in Building 9 of the Armtec vacility.  This review shall include the following:

 

1.1              Working with the Armtec technical staff, examine the existing dust collection in building 9.

 

1.2              Working with the Armtec technical staff, review design criteria for the type of material and amount that must be transported outside the operation area.  This will include transport velocities, pressure and velocity drops in the system, fan sizing, ducting sizing, and hood design.

 

1.3              Working with the Armtec technical staff, make recommendations on design changes that may improve efficiencies and recommend manufacuring techniques.

 

  1. The contractor will evaluate the design methodology used by the Armtec technical staff in developing these design changes and ensure for sound engineering principles, based on their (the contractor’s) expertise in the design of dust collection systems.

 

  1. The contractor shall prepare a final report on their findings (in contractor format) within one week of the completion of these tasks.

 

 

1.2              Building 9 System Review

 

The on-site review of the Building 9 Dust Collection System was conducted on Monday, May 15, 2000. Also reviewed were the two dust collection systems in the other building

 

The system evaluation was approached in the following order:

 

1.         Material, safety, and clean air considerations

 

2.         Pick-up points, hoods, vents, and branch transport ducts

 

3.         Main duct transport

 

4.         Filtration

 

5.         Fan and Motor

 

The review was completed and the overall design found to be generally in accord with industry standards.  There appears to be adequate reserve capability in the blower package to accomplish the desired upgrades to the system without adding additional power.  Safety and clean-air considerations appear to have been adequately addressed in the design of the current system.  Several recommendations will be made for improvements in the pickup efficiency and filtration;  these will enable the upgraded system to provide for an overall cleaner workplace and reduced residual dust.

 

 

1.3              Design Criteria Review

 

Pickup and transport velocities

 

1.4              Recommendations

 

Summarize

 

1.5              Methodology Evaluation

 

Theirs used,

 

simplified recommended

 

 

2.0        Building 9 System Review

 

The on-site review of your Building 9 Dust Collection System was conducted on Monday, May 15, 2000.  The two dust collection systems in the other building were also reviewed for potential improvements.

 

The system evaluation was approached in the following order:

 

1.         Material, safety, and clean air considerations

 

2.         Pick-up points, hoods, vents, and branch transport ducts

 

3.         Main duct transport

 

4.         Filtration

 

5.         Fan and Motor

 

 

 

2.1       Material

 

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.

 

 

 

 

 

2.2       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 workpiece. 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 workpiece.  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.

 

 

 

2.3              Pneumatic Valving

 

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 workpiece 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 workpiece 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.

 

 

 

2.4              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.

 

 

2.5              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

 

 

 

2.6              Cyclone Collector

 

Filtration for the Building 9 System is provided by a water-injected cyclone followed by HEPA filtration. This cyclone appears to operate efficiently and would hve 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.

 

 

2.7              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.

 

 

2.8              Fan/Motor

 

The power unit for the Building 9 System is a 30 hp Spencer 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.

 

 

 

2.9              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.

 

3.0       Design Criteria Review

 

3.1       Material

 

3.2       Pick-up and transport Velocities

 

3.3       Pressure and Velocity Drops

 

3.4       Fan and Duct Sizing

 

3.5       Hood  Design

 

3.6       Filtration

 

4.0       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 intermitent 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 1000cfm, 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 picup point and its associated braqnch ducting as a “load” to the main duct and estimate the airflow to be generated through this “load” by the avaliable 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 minumum 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:

 

Elbow:                                     3 ft

 

Branch duct                            10 ft

 

Elbow                                      3 ft

 

Gate valve                               10 ft

 

Flexible hose                           15 ft

 

Hood                                       5 ft

 

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 vac, 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.

 

 

5.0 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.

 

Figure 1

 

 

 

 

 

 

 

 

 

 

 

 

 





5.0       Conclusions and Recommendations

 

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

 

5.1       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 workpiece.

 

5.2       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 workpiece after shutdown and prevent some of the fine airborne dusts from drifting away from the operation.

 

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

 

5.3       Pick-up Hoods, Building 6

 

At the eight sanding/deflashing 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.

 

5.4       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.

 

5.5       Cyclone Filtration

 

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

 

5.6       HEPA Filtration

 

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

 

5.7       Fan and Motor

 

Maintenance

 

 

 

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.