Dust collection systems require the proper airflow to operate correctly.  Without the correct air flow, dust will not be removed from collection points and the collection equipment (baghouse, cartridge collectors, cyclones, etc.) will not operate with maximum efficiency. However, if too much air is being pulled through the system, a variety of issues might ensue, such as product being lost, removal efficiency lowered, utility usage increased.

Conventional Way to Measure Airflow

The normal way to measure airflow is to insert a pitot tube in the ductwork to measure the air velocity in an airstream, and then calculate the velocity (and airflow).  This is a manual process that requires a trained individual.  It is time consuming and to a certain degree an art, since the velocity in a ductwork changes depending how close to the wall you are.

Effective Way to Measure Airflow

Obviously, you cannot walk next to a piece of equipment or meter and check the velocity through the ductwork.  However, if you have a cyclone dust collector you can.  Cyclone pressure drops are based on the airflow through them.  The more air you send through a cyclone, the higher the pressure drop.  Each cyclone design has its own equation to determine pressure drop based on the airflow.  With this equation, you can estimate the airflow based on the pressure drop in your cyclone.

The GPC Dust Collector is an efficient way to handle your dust collecting needs. Simply click below for direct access to the GPC spec.

Exhaust Fans: The Motive Force of a Dust Collection System

In this video, Aerodyne discusses how system changes and fan performance affect your dust collection system.

To learn more about which dust collector, please contact our experts at 440-543-7400 or visit our website: www.dustcollectorhq.com.

Baghouse filters are great for dust collection; however, if they develop a hole, then there is a clear path for the dust to take through the filter.  Over time, the hole will increase in size, allowing more and more dust through.  Filters with holes decrease the removal efficiency of the dust collection system and could violate your air permit.

The filters in a baghouse are the primary dust collecting component.  The filters build up an initial layer of dust, preventing dust from penetrating the filter while allowing air to pass through.  As the dust layer increases in thickness, it is harder for air to pass through.  This is measured by an increasing pressure drop.  Since most dust collection systems don’t use Variable Frequency Drives (VFDs) and airflow transmitters to control the airflow, the airflow through the system will decrease as the pressure drop across the bags increases.  Most baghouses use compressed air blown down the bag, to expand the bag off its cage to jar the most recent layers of dust off the filter. Other baghouses use a shaking cycle to remove the top layer of dust.

Over time, the constant expanding and contracting of the bag during the cleaning process could cause a hole to develop in the bag.  There are a few different ways for the bag filter to develop holes:

1. Gradual expansion of natural gaps in the filter during the cleaning process.
2. The bag being caught and ripped on the metal cage during the cleaning process.
3. High temperature causing localized failure of the filter.
4. Abrasive dust gradually wearing out filter materials.
5. High velocity of the air gradually wearing out the filters.

There are bag break sensors available on the market that will let you know when they detect a bag break.  Often, this is done by measuring the amount of dust particles after the baghouse and when the concentration suddenly jumps, an alarm is set.  Certain baghouse controllers will measure the compressed air used during the cleaning process.  From this, they can tell which row of bags have a hole so you can replace only a row of bags instead of all the bags in the baghouse.

A yearly maintenance inspection of the baghouse will help you avoid operational issues that could develop throughout the year.  This can prevent you from getting a fine or having to shut down the system in an emergency.

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When selecting the airlock for your dust collector, it is important to think about air leakage.  A little air leakage through the airlock isn’t a usually a big deal with many baghouse applications, but air leakage can cause major issues with some applications.  So it is important that you identify if your system will have issues with air leakage and then select the proper airlock valve.

Some dust collection system issues that could come with air leakage include:

1. Decrease airflow at pickup points – If an airlock leaks too much air into a system, it could decrease the airflow at the pickup point, thereby causing less dust to be collected.
2. Blowing dust out of the airlock – If too much air leaks out of the system through the airlock, the collected dust might be blown out of the airlock, causing the collected dust scatter on the ground around the dust collector.
3. Dust that reacts with water – Humidity from outside the process can cause dust to become sticky, bridge, or even explosive in special circumstances.
4. Systems that are oxygen deficient for reaction or explosive prevention – Outside air can introduce oxygen to the system which could feed a reaction or move the process into the explosive zone.
5. Systems that have a high pressure or low vacuum - Leaking airlocks can bleed pressure or allow outside air into a high vacuum, requiring the pressure control system to work harder.

If you have an issue like this, you might want to look at your airlock.  While rotary valves are the industry standard, and currently the only valve approved for explosion isolation in the NFPA, they do constantly leak air into or out of the system.  The reason is the rotor has a small clearance between it and the valves housing.  This allows a path for air to travel.  So while this path is small you will always get some leakage into or out of the system with a rotary valve.

Other valves that will minimize leakage include

• Double dump / knife valve – This valve uses what is essentially two valves in series. And while one of the valves is open, the other is closed.  This prevents air from leaking in or out since there is no direct opening through the valve.  The major drawback of this valve is that the capacity of the valve is severely restricted.  A rotary valve will have a higher flowrate through the same diameter valve.  The other advantages include that these are better with chunky or fibrous materials.
• Trickle valve – A trickle valve (like the Vacu-Valve®) uses the vacuum of the system to seal a sleeve. This prevents air from leaking into the system.  Gravity will then pull the dust through the sleeve once enough material is built up above the valve.  This valve can be used if: the system has negative pressure; the dust is free-flowing dust and won’t bridge; and the dust is not explosive.  The advantages of a trickle valve include no moving parts, low cost, low maintenance, and no power is needed.

In summary, when designing your dust collection system, don’t just pick a rotary valve, make sure that air leakage will not cause an issue.  And if it does, then look for an alternative that will minimize the leak leakage.

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