Almost everyone in this industry is familiar with using a cyclone for dust collection. Cyclones work great as pre-filters, decreasing loading and minimizing contamination before a baghouse or cartridge collector. However, many times you don’t have the head room to install them.  Or you have an existing dust collector and no room to install a cyclone pre-filter. 

Traditional cyclones are tall, vertically oriented structures. Indoors, they take up a lot of room. If they are outdoors, there can be maintenance issues.  There are even installations where a hole in the roof is required to install the cyclone. That is obviously very expensive.

All of these challenges can be overcome by selecting a horizontal cyclone.  Because they are horizontal, they are usually ⅓ the height of a traditional cyclone.  This means they can fit in low headroom areas.  The other benefit is that both the inlet and outlet of the cyclone are on the horizontal plane.  That means the cyclone can replace a 90° turn of ductwork (vertical or horizontal), further saving cost

What’s more, some types of horizontal cyclones have a design that increases the efficiency of dust collection, so that a smaller cyclone can match or exceed the efficiency of the tall cyclone. This saves space and cost, because material costs make up a significant portion of the cost of a cyclone

The short version of the story is this. The next time you are thinking about replacing your baghouse or cartridge collector because you can’t install a cyclone pre-filter, investigate a horizontal cyclone.

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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.

Evaluate your current dust collection system for operational efficiency. Simply click the button to get direct access to our evaluation guide.

Airlocks are very important for proper dust collector operation.  The airlock prevents air from entering or exiting the bottom of the dust collector.  The opening at the bottom of the dust collector is meant for discharging the dust from the system.  When air is entering or leaving from this area, it will cause issues. 

If the dust collector is under negative pressure (fan after the dust collector), then air will leak into the system from the opening.  This will re-entrain dust that has already been removed from the process air.  This will also decrease the air being captured at the pickup points (hoods), thereby making the dust collection system less effective.

If the dust collector is under positive pressure (fan before the dust collector), then air will blow out of the opening.  This will cause the dust being captured to fly out of the dust collector and scatter on the floor and re-enter the facility air.  Installing an airlock will prevent the above conditions.

Some applications have limitations on humidity, temperature, oxygen levels, etc.  If an airlock is not installed, there will be no way to control the conditions, as the outside environment will leak into the system.

Evaluate your current dust collection system for operational efficiency. Simply click the button to get direct access to our evaluation guide.

Baghouses and cartridge collectors are usually designed to handle a maximum of 6” Water Column (W.C.) of pressure drop.  This means that when the baghouse pressure drop is 6” W.C., the airflow through the system is enough for proper operation.  When the pressure drop across the bags is greater than 6” W.C., the airflow through the system is less than it should be. The baghouse uses a control system to keep the pressure drop through the system less than 6” W.C.  Most baghouses use compressed air to clean the bags, but shaking and sonic cleaners are also used.