Frequently Asked Questions
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Why are large amounts of dust generated in a converting or combining winder operation?
During the converting operations of high quality tissue, dust is generated each time the sheet is flexed. During the normal sheet travel through the various converting steps (unwind, printing, embossing or winding operation), dust and some fiber will be dislodged from the sheet. As current tissue products become dryer and softer, combined with the higher speeds of manufacturing equipment, dust is becoming a greater problem in the tissue industry. Dust which is produced on a piece of equipment will remain suspended within the confines of the operating level of the converting room until gravity and low air currents allow the dust to settle and accumulate on all surfaces areas. This is particularly true on overhead duct, beams and support girders.
How does Brunn Air Systems approach Dust Control?
The Brunn Air Systems approach to controlling dust is to capture and control dust at or near the point of dust generation. This theory includes the following key items and the associated reason that determine the overall system efficiency:
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Correct design and positioning of the hoods at the points of greatest dust generation
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Correct positioning of the hoods provide maximum control and capture of dust, while avoiding sheet affect during the manufacturing or converting process
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Brunn Air Systems will use strategically designed and positioned baffles to improve the performance of hoods at a given location.
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Air Ramps will be used at strategic locations to influence the air flow toward the dust hoods in the system
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Computer modeling is used for the development of lower energy, high efficiency dust control systems
How can a mill Avoid Fires as a result of dust accumulation?
The new NFP regulations mandate a limit to the amount of build up allowed on exposed duct. One of the steps was to issue a Combustible Dust National Emphasis Program on March 12, 2008 Directive Number: CPL 03-00-008. OSHA is also reviewing the modification of language in the housecleaning provision to clarify to employers the requirements to prevent accumulations of combustible dust. The National Emphasis Program issued refers to several codes published by The National Fire Protection Association (NFPA). OSHA compliance officers are instructed to refer to NFPA standards to determine whether an employer is in violation of the housekeeping standard. For this reason the NFPA code should be followed in regards to dust producing equipment, dust accumulations, and general housekeeping of your facility.
So what does this all mean for the tissue industry? First you need to obtain the OSHA NEP directive for combustible dusts and all applicable NFPA codes. OSHA also has documentation available that identifies the elements and conditions that cause fires and explosions.
They also have material available to help increase the awareness of the hazards of dust in your facility to you and your employees. In addition to the NFPA codes, Factory Mutual provides property loss prevention data sheets that have recommendations on how to handle combustible dusts and are available on their website. The codes and recommendations available should be reviewed carefully in order to understand how they should be applied for your particular facility. Many of the NFPA codes are retroactive which means a review of all existing dust systems and housekeeping policies should be performed. After review, all systems and policies should be updated to bring them in compliance with the applicable codes. The key for preventing dust explosions and fires starts with a properly designed and operating dust control system that captures dust at the source.
Brunn Air Systems solution is to capture the dust at the source of dust generation through a full dust extraction system equipped with well defined control zones. This will eliminate well over 90% of the generated dust and if pneumatic cleaning is performed, our systems high flow floor sweeps are a target for dust being pneumatically removed from within the confines of a tissue machine or converting system. The floor sweep hoods are strong enough that the dust directed to the target hood is easily captured and sent to the final filtration point. The capture and control of the pneumatically removed dust at the source before dispersion throughout the total building will prevent the material build up and the resulting fire danger within the mills ventilation system. High flow systems can be designed and added to most existing dust extraction systems.
What is the purpose of a Manual Paper Trap?
The purpose of the Manual Paper Trap is to remove paper that may be drawn into the dust extraction hoods, thereby, keeping paper out of the primary filter housing. Winders that are equipped with manual paper traps position them between the hoods and the bypass valve. Manual paper traps prevent large tails, streamers, or other scraps of fugitive paper from plugging the dust control filter. The paper trap is designed for easy inspection and cleaning by the operator. The paper trap is a series of bars that will trap any paper that may have been drawn up into a dust collection hood. The access door is equipped with a Lexan door for visual inspection of the paper trap. The door is equipped with a counterweight and a pull cord that the operator can pull during system bypass for manual cleaning.
The paper trap is located on the winder side of the bypass valve and can be cleaned when the system is in a bypass mode. The door can be opened by pulling the cord attached to the latch in the center of the door. A rake with an extended handle can be used to assist in the cleaning of the metal grill. This door should not be opened while air is being drawn through the hoods. The paper trap is under vacuum during the dust collection process and would be very hard to open.
How does the Brunn Air System Venturi Scrubber - Recirculation Water System work?
The recirculation system uses a recirculation pump that is fed through the return water from the scrubber and a make-up line for the water source. The level detector regulates the level and make-up flow into the scrubber tank. A magnetic flow meter and a manual valve will regulate the flow of water to the Venturi. The slurry is captured in a well that surrounds the standpipe for the recirculated water feed. The slurry is pumped from the well and is delivered to destinations such as a hydro pulper, felt pit, off machine silo or U drain. The recirculation system should only be used on non-wet strength materials. Wet strength materials will not breakdown fast enough and will plug the slurry well. The purpose for using this type of scrubber water system is if the water is not going to be returned to the system for fiber recovery. The water loss may become an issue in the operation of the scrubber.
Theory of Operation of the Tank Section
The Venturi discharge into the scrubber tank generates a strong centrifugal force, while the fan’s exhaust develops a high-energy internal vortex that drives the conditioned particles against the tank's wet walls. The particles are now heavier and are forced to the wet outer wall due to centrifugal force. The washing action of the water against the wall moves the conditioned particle to the tank bottom. The upper and lower baffles that are located at the base of the scrubber will act as an anti swirl device, allowing the water to exit the stem pipe to be recirculated. The wet conditioned particles will combine to form slurry to be returned to the process.
How does the Pass Thru Venturi Scrubber work?
The pass through system is used when the recovered fiber and dust will be returned to the system. The system can be equipped with a positioning valve in the event a white water pump is large enough to feed the designed flow rate and the pressure required. Most mills elect to remove the total flow required at the Venturi form the off machine silo through a dedicated pump. The water is used for a few seconds, then the full flow of slurried white water is returned to the off machine silo by gravity.
In the Pass Through designs a slurry pump should not be used. A barometric leg is used in the system to provide the cone section with a reservoir of water that provides the hydraulic head to aid in the
transporting the slurry back to the off machine silo by gravity. This system design will process wet strength material without plugging, when the proper material-handling fan is used as the scrubber air source. This design provides the maximum water savings to the mill while providing very little maintenance over the long term. The level detector is used as an indicator of a plug condition.
How do you handle towel grades with a Venturi Scrubber?
Towel needs a little more attention to its required breakdown requirements. Warm to hot water is used at the venturi and in the scrubber reservoir to assist in the breakdown of the wet strength in towel. Another aid in the system is an in line de-lumper that will reduce any large pieces of towel slurry not yet fully reduced.
Section of the scrubber.
Internal baffle plates within the lower section of the scrubber are used as a mechanical shear to aid in the reduction of the tissue into slurry. The rotating action of the water/air flow will apply the force in the mechanical shear. We have experienced little or no wear on the fans on towel grades there are a number of fans in operation over 25 years.
What does a typical pulper exhaust system from Brunn Air System consist of?
Our typical pulper ventilation system would consist of a cross machine hood normally located under the scanner (when there is not a calender), or under the calender, equipped with access doors for operator inspection and cleaning if required.
We also equip the hood with baffles that are connected to the pulper side walls and are fabricated in stainless. At the end of each baffle near the water level is a shower that will assist in preventing tails or pieces of tissue from being drawn up into the vent hood, causing contamination.
Due to the high percentage of dust on the newer tissue products, we no longer use a mist eliminator due to the high maintenance requirements, but elect instead to use a venturi scrubber with little to no maintenance requirement. In order to modify an existing pulper vent, we would require a P&ID or layout of the existing system. The following is our typical Pulper Vent Systems with a Venturi Scrubber.
How do you address coating buildup on the collectors and inside the ductwork?
All our hoods are designed with a high plenum velocity as well as the duct runs. This does require a little more horsepower, but some of our older (20 years in operation) systems in Canada have been inspected every two years and have seen no major problems or requirements for cleaning
What would you do with the rich cloudy water leaving the Venturi Scrubber to recover the fiber content?
Our typical system will pull white water from the off machine silo and then put the scrubber discharge back into the same location. Over 90% of our systems have this type of water system. The scrubber discharge fiber that is recovered is returned to the system and does not become another burden to the effluent system.
Our systems have an advantage in that we can take large pieces of material into our hoods due to a fan design that will reduce the size of the tissue before the material enters into the venturi section of the scrubber. In addition the movement of the larger pieces of tissue through the duct and the high velocity of the system tends to assist in keeping the duct clean.
As for the scrubber, it is a self-cleaning design. It operates with a clear stack and the tank walls are constantly washed by the swirling water. In fact, the recommended interval for tank inspection is every 5 years and we have a number that have been in service for over 20 years without ever needing to have the tank walls cleaned.
How would you describe your chopper fan?
The chopper style fan is a typical heavy duty fan used on all Brunnschweiler and Kleissler dust system for over 30 years. The wheel, shaft and bearings are an extra heavy duty to take the punishment of paper being drawn in into the hoods during a sheet break. We have used Twin City and New York Blower as sources of fans for this application.
What is the purpose of the bypass valve?
The by-pass valve will shut off the air to the hoods once the winder speed is below 300 f/m and will open to allow air on the hood at speeds above 300 f/m. Because we do not want to start and stop the fan motors every time the winder is down for a sheet break or roll change, we use the by-pass valve to control the air on the hoods or floor sweep to ensure we do not break the sheet during a winder stop.
What are “air ramps”?
Air ramps are compressed air headers equipped with regulators that direct air flow to the cross members within the winder to clean the dust that builds up and drops into the product. We also install them on the floor under the unwind stand to blow the dust that accumulates on the floor toward the dust hood so the operator does not have a big cleanup job in this area. The air ramps within the machine use a solenoid to operate using a short burst of air during normal production runs that direct the dust towards a dust hood for capture and removal. Air ramps located on the floor of a machine will only operate during the floor sweep cycle.