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Dust Collection Q&A

  •   Why does NFPA 484 require a dedicated dust collector for an aluminum polishing operation? Have you ever experienced having a stainless steel part polished causing an aluminum dust collector explosion? I read an old “ask the experts” question “(…We had a community college install a dust collection system inside a room next to their welding area. There are 32 welding stations and three grinding stations (with aluminum, carbon, and stainless steel dust)…” and you never said “don’t do that”.

    Aluminum dust is very reactive, not only with water but with other dusts.  This is why a dedicated dust collector is required.  Depending on the metals it is reacting with depends on the resultant reaction.  For instance, when  aluminum dust reacts with certain oxides (rusts) a thermite reaction can occur, this can create great amounts of hydrogen gas and heat.  In extreme cases combustion can occur, which is very dangerous in the presence of hydrogen gas.  For this reason aluminum is to have a dedicated dust collector.
    I personally have not experienced or heard of a dust collector exploding with an aluminum/stainless steel combination, but that does not mean that it cannot happen.  But I have experienced a aluminum/copper combination dust collector having reactions in it, that lead to excessive gas formation and small fires.  This was all caused by chemical reactions in the dust collector, and was solved when dedicated dust collectors for each product were used.
    The dust collectors that are used for welding applications are used to collect fuse and smoke, not dust.  Therefore it does not matter what type of welding you are doing.  Grinder or other hot work application are not to be connected to dust collector and are to follow proper hot work permitting systems in accordance with NFPA 51B.  If you are doing hot work and using the dust collector to collect the spark you run the risk of setting the bags on fire.

    I’m not familiar with the other question it is you are making reference too, who wrote it or who answered it.

  •   Currently we are collecting dust and fumes from a process that laser cuts adhesive paper. We are using an Amano PIE30 and are clogging filters within a weeks time. We believe that the adhesive from the paper is sticking to the filters and not coming off when the system cycles a purge. I am currently at a loos on how to maintain filter life while collecting what may be sticky dust particles that render the filter purging useless.

    There could be many different things going on here.

    One – as stated it could be the adhesive on the paper.  This could be helped/solved with a particular coating on the bags such as an membrane finish or an oleo-phobic finish.  Oleo-phobic finishes are typically for oily materials, but I have used them in different applications, some sticky, and have had good success.

    Two – It may not be the stickiness of the paper that is holding it to the filter.  It may be static electricity.  If everything is not bonded and grounded on the dust collector and ducting system it should be.

    Three – Filter material may just not be designed for this application and something along the lines of a wet scrubber may be more appropriate.

  •   We have been asked to perform dry ice cleaning in a cement silo. The only gas present from our side would co2. Is it possible that this could cause a dust explosion with the cement dust. There will be lots of ventilation. Your comments would be highly appreciated.

    First off, most types of cement have a Kst equal to 0 bar m/sec, which means that it cannot explode.  Check with the manufacturer and see if they know the Kst value of the dust.

    If it does have a Kst rating then there are procedures to follow to minimize the changes of an event.

    In order to have an explosion you must have five things present:
    1.    Containment
    2.    Oxidant (ie Oxygen)
    3.    Fuel
    4.    Ignition source
    5.    Dispersion of dust

    If any one of those five are not present then a dust explosion cannot occur.  By reducing or eliminating any one of the items you reduce the risk or eliminate it completely.

    Containment – You cannot change the containment, as it is your surroundings.

    Oxidant – If the concentration of oxygen is too low than an explosion cannot occur.  The addition of CO2 actually reduces the risk, because CO2 is actually used to suppress fires and explosions.

    Fuel – First off the fuel has to be explosible, which means it has to have a measurable Kst.  It also has to be in the correct concentration.  If it is too lean (LEL – lower explosive limit) there is not enough fuel for an explosion, at or below 25% of the LEL is considered safe.  The LEL is a tested value.  If there is too much (UEL upper explosive limit) fuel and the fuel to air ratios is above the UEL then there is not enough air for combustion to occur.  When cleaning, when the dust cloud starts to get thick (for example – so you cannot see a flash light through it).  Stop.  Wait for the dust to settle, remove the settled dust if possible.  Continue and repeat.  Ventilation also helps with this.

    Ignition sources – Take away all possible sources of ignition.  Static is a big one.  Use not sparking equipment.  Remove smokers from the area.  If your equipment that will be in the storage area has moving parts make sure they are not ceased and move freely and do not become hot spots.  Anything you can think of that could cause an ignition source.  Remove it.

    Dispersion of dust – this you cannot control, because you are going to be putting dust in suppression as is the nature of the task.

    I hope that helps, but again none of the above is required if there is not a measurable Kst.

  •   Is there any specific cyclone design guidelines that you recommend to use?

    There are many theories out there on how to design cyclones, and there are still many people and labs working on new theories. The first design theories that were developed are now referred to as Classical Cyclone Design, and although many people have tried to refute that this design theory is not correct, it is the closest approximation to actual results. There is no design theory or guidelines that have been developed to match actual outcomes. The closest are classical design theory.

  •   We have a dilatometer that we use to measure CTE (coeff of thermal expansion) of various materials. One of the components can be V and vanadium in +5 oxidation state can sublime at temperatures between 500 and 600 C. Currently the instrument is not in a hood and we limit experiments to 500 C max for such materials. Is there a way to design a trap/capture system, to make this operation safer in case of a system failure and potential V sublimation?

    With little details I guess the only way this can be answered is that yes a trap/capture hood can be designed. When the capture device is designed and put in place there are only a few ways that the collected material can be dealt with. At this point I assume we are dealing with a gas, as stated the vanadium has sublimated. A bag house could be used with ceramic filters or an electrostatic precipitator.

  •   Why does air to cloth ratio matter?

    Air to cloth ratio (ACR) is the relationship between the air flow (CFM or m3/s) of air going through the filter media to the surface area (ft2 or m2) of the filter media. Different dusts have different recommended air to cloth ratios. For fine dust, such as flour, the ACR can be as low as 2 FPM (or lower) and for heavier dust, such as wood chips, the ACR can be 6 FPM or higher.

    The ACR affects three major things: filter life, foot print of the dust collection unit and power usage of the fan.

    A higher ACR means more air is flowing through the filter media. This puts more pressure on the filter media and stresses the fibres, reducing filter life. This means more frequent filter changes.

    The size of a dust collector is determined by the ACR. The more filter media required, the more bags, envelopes, cartridges etc. are required, which increases the footprint of the unit. This is why pleated filters are so appealing; because of the pleats there is increased filter media per filter reducing the number of filters required and the overall foot print of the unit. Often filters will be placed too close together to reduce unit size, but this causes the interstitial velocity between the filters to be too high.  When the interstitial velocity is too high the filters cannot clean properly and leads to a whole gambit of other issues.

    A higher ACR causes a higher standard operating static pressure differential across the dust collector.  More work for the fan is required, if the fan has the ability to operate in that range. More work equals more power, which means a higher operating cost.

    When assessing the ACR of a unit it is usually better to go with a lower ACR, if the area permits it. The upfront costs of the large unit are quickly paid for by the frequent filter changes, higher operating costs and other unexpected headaches that can come with a high ACR.

  •   When do I need to change my filters and what is the point in changing it?

    There are many reasons to change the filters. The three main reasons are air quality requirements, static pressure increase, or water damage.

    When filters are first installed the fibers in the material are tight and close together. As they are used, whether they are bags, envelopes or cartridges the fibers in the material stretch. In different types of dust collectors, shaker style versus pulse air for example, the fibers may stretch faster as they are under more constant stress because of the dust collector pulsing.  As the fibers stretch the material weave becomes looser and eventually starts letting larger and fibers larger particles through the filters and more of them. The larger particles can get stuck in the weave and not allow the material to release off the filter media. Eventually the air emitted by the dust collector will exceed the air permit or, if the air is recycled, indoor air quality requirements. This may take a year or many years, it depends on frequency of use, bag loading, air to cloth ratio and frequency of cleaning. One aid for this is to install a particle sensor, with set points, on the clean side ducting of the dust collector. When the particulate count reaches it set point, too much material is getting through the bags, and they should be changed.

    The other main reason to change the filter media is static pressure. In dust collection system design static pressure is the driver of the whole system. The static pressure comes from the hood losses, ducting losses, static pressure difference from the clean side to the dirty side of the dust collector, and any inline devices or alternate filters that may be installed. All the static pressure values are dependent on speed of the air flow and can be calculated with the exception of the pressure differential across the dust collector. When filter media is used and becomes dirty the standard operating static pressure differential across the dust collector will increase and will keep increasing until the bags are changed. When the fan was sized it had a maximum system pressure that it was designed for, if that is exceeded then the flow in the system will decrease causing leakage will start at the hoods, material will settle in the ducting and the overall system functionality to decrease. To know when to change the bags, you need to know the static pressure rating on the fan and the system requirements. From that you can figure out how high the pressure differential across the dust collector can be before the fan flow starts to diminish. The static pressure across the tube sheet of the dust collector can be monitored with a magnehelic guage.

    If the filters get wet, even if they are dried afterwards, their performance is greatly reduced. Therefore if filter media gets wet, it is best just to change it.

  •   How do I know my dust collector is working properly?

    I get asked this question all the time. There are some easy ways to tell if your system is working as designed.

    The first thing to do is look at the operation of your hoods. If your system in not a new installation then you should have a baseline of how your hoods should be functioning. If you have a more fugitive dust than normal escaping than this is an indication that the flow in that hood or your system is lower than normal and the system not working correctly. A good marker on whether or not there is more fugitive dust is if there is dust piling up on adjacent flat surfaces. If there is, then the hoods are leaking.

    The next thing to check is inside your ducting. The system should have been designed so that material travels at the optimal speed and the dust does not settle out in the ducting. If you have dust settling in your ducting then this is also a good indication that your system is not running as it should.

    The last easy check is to see what is coming out of your dust collector into the collection device or into the recoup system. If the system is not discharging anything, than one of two things is happening. The dust collector is either not collecting any dust (which is why there is so much fugitive dust at the hoods) or the material is not emptying off the filter media. To tell which is happening check the dust collector differential pressure gauge. This is an indicator of how clean or dusty the bags are the dust collector. If the pressure is much higher than the normal operating pressure than likely the material is building up inside the dust collector.

    These are a few fast ways of checking to see if your dust collection system is functioning as it should.  There are many reasons for the above to be happening, but they all give lead back to the same place: issues with the dust collection system. These issues will require further investigation to get to the bottom of it.