The baghouse, an asphalt essential

Astec Australia explains the importance of a high-quality and well-maintained baghouse to the efficient operation of an asphalt plant.

As contractors produce the mixes needed to build, upgrade and improve our roads, asphalt plants with the highest production rates and efficiency will be crucial.

One of the key features in an asphalt plant that will need to be taken care of is the baghouse.

Installing a baghouse that not only works efficiently, but also operates properly at all times, is paramount to mix production success.

For a baghouse to function properly, there are a few things asphalt producers need to keep in mind.

Baghouse Maintenance Matters

Like any component at an asphalt plant, the baghouse needs regular maintenance in order to ensure proper operation.

An Astec Pulse Jet baghouse needs the following items checked for routine maintenance:

Check that all solenoids are functioning properly and replace if necessary.
Check pulse duration, interval and pressure. Monitor pressure drop across the bags.
Monitor the pressure in the manifold to make sure the bags are pulsing at an appropriate pressure.
Visually examine the bags monthly.
Make sure blowpipes are properly aligned to ensure pulse air enters the bag. Never walk on blowpipes as this can warp them and cause misalignment.
Examine venturi’s and be sure they are in good working order as these components are what induce the shockwave to go down through the bag.
Check for air leaks in the exhaust system, including aggregate and recycle inlets, connection bands on the ductwork, holes in equipment, access doors not tightened and missing or damaged seals.

If these items are not attended to, over time bag performance will decline and failure may occur as dust is pulled into the felt. As the resulting pressure drop increases, particulate penetration will escalate due to the need to maintain burner suction.

Catherine Sutton-Choate, Director of Environmental Compliance and Product Documentation at Astec says if plant owners are not performing at least some of the basic maintenance procedures on their baghouse they can run into problems.

“Anything that prevents baghouse bags from cleaning is going to increase the differential pressure across the bags. That in turn increases the overall differential pressure throughout the entire production facility from the drum through to the fan,” Sutton-Choate says.

She says exhaust fans are only designed to generate a certain amount of static pressure and so if the pressure drop through the equipment gets too high, then the fans simply cannot generate enough airflow to sustain the production process and production will start to suffer.

An increase in pressure differentiation across the baghouse, is called blinding. It means something is restricting the air from being able to pass through the felt of the bags.

One common way to blind the bag is by pulling dust into the felt, thereby embedding dust particles into the bag. Sutton-Choate says this is one of the main reasons bags can fail and must be replaced.

Blinding can also occur if the temperatures within the baghouse drop below the dew point. This can cause moisture to start condensing out of the flue gases which creates mud on the bags, interior walls and everywhere else.

“Leakage, which is the silent killer of production, uses up baghouse capacity. It can also lead to condensation inside the baghouse,” Sutton-Choate says.

“Mud cannot be removed by the cleaning system. It requires somebody to physically get inside the baghouse and scrape the mud off the bags.”

Bag Quality a key to prevent failure

Astec bags are made with a 14 ounce minimum felt, which provides a consistent density to prevent dust loading across the bag.

“The tighter the density range of the bag, the better,” Sutton-Choate says.

“This allows for more uniform density in the felt itself, which is going to result in more even loading along the length of the bag.”

Uniform loading across the bag is important because if there are areas along the felt that are not as thick, more air can get through the thinner areas which will lead to the bag wearing out faster.

Understanding the Significance of Hydrolysis

Another factor that leads to premature bag failure is hydrolysis or the weakening of the bags due to chemical breakdown.

Hydrolysis affects the ability of the bags to clean because it causes them to shrink significantly. This can lead to blinding as previously mentioned, even if the cleaning system is functioning properly.

Hydrolysis also causes bags to have weakened fibres, which may result in holes developing as the bags are more susceptible to abrasion by aggregate particles or cleaning at excessive pulse pressures.

The baghouse is prone to hydrolysis because the phenomenon is caused by exposure to moisture at elevated temperatures. This combination of moisture and heat is exactly what is produced during the aggregate drying process at an asphalt mixing plant.

For the last several years, Astec has been working to educate customers on what hydrolysis is and how they can minimise its effects.

“We minimise hydrolysis by offering solutions to control the maximum temperature of the air going into the baghouse because hydrolysis is so dependent on temperature,” Sutton-Choate says.

Astec recommends using the V-Pack temperature control system as one way to help mitigate hydrolysis.

The Astec V-pack system uses variable frequency drives to speed up and slow down the rotational speed of the drum in response to the temperature of the gases coming from the drum.

Through these drum speed changes, the system is able to effectively control baghouse temperature and maintain it at a level to reduce the incidence of bag hydrolysis.

“As a manufacturer, we are constantly working to improve the performance of our baghouses in order to achieve higher and higher collection efficiency,” Sutton-Choate says.

“Not only that, but we need to keep up with the ever-changing environmental standards around the world.”