Question

For continuous reverse air cleaning in a fabric filter, gas-to-fabric ratio normally varies from

• A. 8 to 15 cfm/sq ft
• B. 0.2 to 0.5 cfm/sq ft
• C. 50 to 60 cfm/sq ft
• D. 0.01 to 0.1 cfm/sq ft

Hint:

Air-to-Cloth (\(A/C\)) Ratio Summary Guideline: - Low-energy cleaning (Shaker Standard Reverse Air) \(\implies\) \(2 \text{ to } 5 \text{ cfm/ft}^2\). - High-energy continuous cleaning (Pulse Jet Continuous Reverse Jet) \(\implies\) \(8 \text{ to } 15 \text{ cfm/ft}^2\).

Answer 1

The Correct Option is A

Concept: A fabric filter (commonly known as a baghouse) is an air pollution control device designed to separate particulate matter or dust from industrial gas streams using fabric filter bags. A key design and operational parameter for these systems is the Gas-to-Fabric Ratio (also called the Air-to-Cloth ratio, \(A/C\)). The Gas-to-Fabric ratio is defined as the volumetric flow rate of gas passing through the filter divided by the total active filtering surface area of the fabric: \[ \text{Gas-to-Fabric Ratio} = \frac{\text{Volumetric Gas Flow Rate (ft}^3\text{/min)}}{\text{Total Active Cloth Area (ft}^2\text{)}} = \frac{\text{cfm}}{\text{ft}^2} \] This parameter represents the superficial velocity of the gas moving through the filter medium. Its optimal value depends heavily on the cleaning method used to remove accumulated dust cakes from the filter bags.

Step 1: Analyzing cleaning methods and their corresponding ratios.
Industrial baghouses generally use one of three main cleaning mechanisms, each operating within a specific gas-to-fabric ratio range:
• Mechanical Shaking: The tops of the bags are shaken mechanically. This requires low velocities to avoid embedding dust deep into the fabric fabric fibers: \( 2 \text{ to } 5 \text{ cfm/ft}^2 \).
• Reverse-Air Cleaning: A low-pressure plume of air flows in reverse, causing the bags to collapse slightly and release the dust cake: \( 2 \text{ to } 5 \text{ cfm/ft}^2 \).
• Continuous Pulse-Jet Reverse-Air Jet Cleaning: High-pressure pulses of compressed air continuously clean rows of bags while the system remains online. Because cleaning is frequent and intensive, it can handle much higher superficial velocities, typically ranging from 8 to 15 cfm/ft\(^2\).

Step 2: Evaluating the specific selection criteria for reverse-jet cleaning.
For industrial fabric filtration installations using continuous reverse air jet or pulse cleaning, a ratio of 8 to 15 cfm/sq ft balances filtration efficiency with manageable pressure drops across the system. Lower ranges (like 0.2 to 0.5) are too restrictive for large industrial volumes, while higher ranges (like 50 to 60) would blind the filters almost instantly. This confirms Option (A) as the correct range.