Ductwork (dust highway)
The ductwork is the highway of the dust collection system. Proper duct velocity for most cement applications should be between 3500 and 4000 fpm (18 – 20 m/sec.) Speeds lower than 3500 fpm will allow dust in the airstream to fall out, causing dust buildup in the duct. Speeds higher than 4000 fpm could result in duct abrasion, especially in duct branches and elbows. If the ductwork is sized too small, the result is high velocity in the duct. This is evident in the large percentage of ductwork with holes and patches in almost all plants. The average plants spend a lot of time and money patching holes rather than correcting the design of the duct work. An increase in grain loading mixed with high velocities in ductwork only speeds up the wear. If the ductwork is sized too large, the velocities will be too low and result is dust fallout and build-up in the duct. Dust build-up in the duct creates a maintenance headache. In addition if enough dust build-up occurs it reduces the cross sectional area of the duct which affects overall air flow and air velocities at the collection hoods.
Adding additional vent hoods and ductwork to an existing system must be evaluated. Can the system handle the additional air requirement? Do I have to resize the ductwork in order to maintain the proper velocity based on the air flow increase? If not done properly the add-on collection hoods and ductwork can create an imbalance in the system.
Ductwork into the dust collector should have a straight run of at least eight diameters. This ensures the air and dust loading to the dust collector is evenly distributed over the whole dust collector. Uneven loading can cause abrasion of the filters and in extreme cases wear through the dust collector hopper walls. This is a very common mistake on small and large dust collector systems. In some instances the duct cannot be changed, resulting in the use of turning vanes in order to evenly distribute the dust and air. The turning vanes can correct design flaws, but unfortunately they will wear and require maintenance and replacement eventually. It is much easier to design the ductwork correctly, avoiding adding to the work load of the maintenance department.
If we have corrected the collection hood design and location and we are transporting the dust and air to the dust collector at the right speed our dust collector has a better chance of working properly. With that said we can’t just throw any dust collector into the system and expect it to function properly.Dust collector sizing is critical to a proper operating system. The air-to-cloth ratio must be right for the application. The air-to-cloth ratio is the relationship between the amount of air flowing through the dust collector and the total amount of filter media in the dust collector. The correct sizing (air-to-cloth ratio) of a dust collector is based on dust loading, dust particle size, temperature, moisture and dust weight.
Velocities are also important inside the dust collector. The velocity at the bottom of the filter bags, can velocity, has a bearing on how well filter bags clean. The recommended velocity is less than 300 fpm (1.5 m/sec). When a row of filters are cleaned, the dust on the filter surface is blown off and moves down the filter, some falling into the hopper. If the can velocity is too high the dust will not effectively fall from the filter bags, creating re-entrainment of dust on the filter were it was before cleaning. High can velocity results in higher than desired differential pressure and cleaning cycles. If the can velocities are excessively high it will also lead to filter bag abrasion. The high can velocity is caused by too much airflow through the dust collector housing. The solution to excessively high can velocity is to increase the housing size of the dust collector or reduce the airflow. Slightly high can velocity dust collectors can see an improvement of bag cleaning by replacing the filter bags and cages with shorter pleated filters. The shorter filter creates a lower velocity below the bags and aids the dust to fall in the hopper before it reaches the filters. This should be addressed on a case by case basis.
Dust collector cleaning controls play a critical role in dust collector operation. All dust collectors in a cement plant should be cleaned based on differential pressure. It is the only way to maintain consistent differential pressure in a dust collector. Consistent differential pressure yields consistent air flow and velocities through out the system. Cleaning on demand ensures the filter bags are only cleaned when necessary to maintain set differential pressure, thus prolonging filter life. The start and stop cleaning differential set points should never be more than ½ in. WC (13 mm) apart. Each dust collector will be different, but the average dust collector will operate very well between 5 in. and 6 in. WC (127 and 132 mm). Changing the cleaning controls to differential pressure is simple and inexpensive.
It is recommended to monitor all aspects of the system, including mechanical operations, differential pressure, emissions, hopper levels, number of cleaning cycles, and temperature. Example: Installing a sound monitoring system on a dust collector will alert personnel when a problem occurs, instead of waiting for a disaster.
This is part two of a three-part article written for World Cement’s August issue and abridged for the website. Subscribers can read the full August issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the August 2016 issue here.
Read the article online at: https://www.worldcement.com/special-reports/03082016/common-sense-dust-collector-system-maintenance-and-troubleshooting-part-two-39/