The pre-coating (or seasoning) process for baghouse filter bags is a critical step following the installation of new bags, ensuring optimal filtration efficiency and extending bag life. Below is a detailed explanation of the pre-coating process and the subsequent steps to bring the baghouse back online.
1. Select Pre-Coat Material:
- Seasoning Agent Selection: Choose a conditioning agent compatible with the baghouse system, such as dry fly ash or hydrated lime. Avoid materials like un-hydrated lime, pebble, quicklime, fly ash, or other hygroscopic, non-agglomerating, or spherical particles, as they can damage the filter media or cause blinding.
- Seasoning Agent Quantity: Determine the total square footage of filter media in the baghouse. For PreKote, use approximately 0.042 pounds per square foot of media. For other materials, such as diatomaceous earth or limestone, about 1 pound per 40 square feet is recommended.
Scenario:
- A baghouse with 1680 bags
- Bag dimensions: 6” diameter, 169” length)
Calculation:
- Surface area/bag = (6 x 3.14 x 169) ÷ 144 ≈ 22.2 sq ft
- Total surface area = 22.2 x 1680 ≈ 37,162 sq ft
- PreKote needed = 37,162 x 0.042 ≈ 1,560 lbs
Other materials may require around 1,500–2,000 lbs for the same baghouse.
2. Pre-Coating Process
- Isolate Process Gas
- Close all inlet dampers or valves to the process gas stream to prevent entry of process gas, which could damage new bags or disrupt dust cake formation.
- If available, open an auxiliary inlet or bypass to draw clean ambient air into the baghouse for pre-coating.
- Turn Off Cleaning System: Disable the pulse-jet cleaning system to prevent premature cleaning during pre-coating.
- Start Exhaust Fan
- Operate the exhaust fan at 60% of the actual operating airflow using clean ambient air only, drawn through the auxiliary inlet or pre-coat injection system.
- Ensure the fan does not pull process gas by verifying that all process gas inlets remain closed. This protects new bags from fine particulates, moisture, or sticky compounds that could cause blinding or damage.
- Apply Pre-Coat Material
- Feed the pre-coat material (e.g., PreKote, diatomaceous earth, or limestone) through a dedicated injection system (e.g., rotary feeder or venturi) at a rate of about 1/3 pound per minute per 1,000 ACFM of reduced airflow. For example, a 25,000 ACFM system at 50% airflow requires approximately 7.5 lbs/minute.
- Continue feeding until a uniform dust cake (about 1-2mm thick) forms on the filter bags. Inspect each compartment to ensure adequate coating and check the hopper for minimal pre-coat dropout.
Continue feeding until a uniform dust cake forms. Inspect compartments to ensure adequate coating and check hoppers for minimal pre-coat dropout.
3. Inspection and Adjustment
- Visual Inspection: After pre-coating, inspect the bags to confirm a consistent dust cake has formed. Ensure the pre-coat material is not excessively collecting in the hopper, indicating proper adhesion to the bags.
- Adjust Airflow: Gradually increase the airflow to normal design levels once the pre-coat is adequately applied.