A Comprehensive Analysis of the Root Causes of Dust Collector Filter Bag Blinding
1. What is Bag Blinding?
“Bag blinding” is a phenomenon and common issue in the operation of dust collector filter bags, and one of the reasons leading to their failure. There is no unified industry definition. The author defines it as follows: Bag blinding occurs during the long-term operation or shutdown of filter bags under conditions involving high humidity or contact with oily substances. During this process, dust accumulates, adheres, or forms a crust on the filtration surface or within the filter media. This layer cannot be effectively removed by the online cleaning system, resulting in a significant increase in operational resistance.
After bag blinding occurs, dust densely covers the filter bag surface, drastically reducing the effective filtration area of the material. Air permeability drops sharply, and operational resistance becomes very high. This increases the load on the induced draft fan, leading to higher energy consumption. In severe cases, it can overload the fan, causing it to stop. Furthermore, the persistently high pressure drop across the filter bag forces the online cleaning system to operate frequently. This not only consumes large amounts of compressed air but also severely reduces the mechanical strength of the filter material, significantly shortening its service life. Consequently, if filter bags operate in a blinded state for extended periods, significantly higher operating costs must be incurred. If the problem is not resolved promptly and effectively, entire sets of filter bags may need replacement.
2. Causes of Bag Blinding
In practice, the factors causing bag blinding are complex. For specific cases, analysis must start from the process gas, the dust collector itself, the filter media, operational controls, and other related factors to identify the true root cause and facilitate problem prevention and resolution.
2.1. Moisture Condensation Blinding (No. 1)
Generally, the operating temperature of a baghouse dust collector must be at least 25°C above the acid dew point temperature for safe operation of the filter bags, primarily to prevent condensation. When the operating temperature falls below the dew point, liquid water condenses. This water mixes with dust and collects on the filter bag surface, causing bag blinding.
Higher moisture and sulfur trioxide (SO₃) content in the flue gas raises the dew point temperature. This means the dust collector’s continuous operating temperature must be set higher. However, due to the physical limitations of filter media materials and operational economy, the continuous operating temperature must be controlled within a suitable range. If these two aspects cannot be reconciled, condensation is inevitable. Therefore, water content, SO₃ content, and operating temperature are the three primary factors determining condensation.
- Example: One dust collector operated steadily at about 150°C. However, as it primarily incinerated complex solid, liquid, and gaseous wastes from chemical plants, the water and sulfide content in the flue gas varied significantly (water content sometimes exceeded 25%), causing large fluctuations in the dew point temperature. Condensation was unavoidable, and bag blinding was extremely severe. Concurrently, due to acidic substance penetration and prolonged operation, dust gradually formed a hard crust on the filter bag surface, leading to a complete loss of air permeability.
- Pulse Cleaning Air: The compressed air used for pulse cleaning can also contribute to condensation. Its temperature is typically much lower than the dust collector’s operating temperature. During the brief pulse, this cold air rapidly cools the upper part of the filter bag. If the temperature drops below the dew point, condensation occurs on the outer surface of the bag in that area. The effect of the cold air diminishes as it travels down the bag. Therefore, bag blinding is often more severe within approximately one meter of the bag opening.
- Air Leakage: The dust collector’s air leakage rate is another potential cause of condensation. Typically, the leakage rate is around 1–2%, but it can increase over time. This becomes a significant issue in cold winter climates, especially in northern regions. Higher leakage rates have a greater impact on local temperatures inside the collector, increasing the risk of condensation and bag blinding in the leak areas.
- Wall Effects: Although dust collector walls are insulated, they are in direct contact with the external environment and are therefore cooler than internal parts. Field experience often shows that the degree of bag blinding is much higher for the outer ring of filter bags near the walls.
2.2. Adhesive Blinding (No. 2)
This refers to blinding caused by highly adhesive dust. Even in the absence of condensation, this dust adheres strongly to the fiber surface and cannot be dislodged by the online cleaning system. Causes include:
1.Inherently Sticky Dust: Dust with high inherent viscosity forms strong molecular bonds with filter fibers. Examples include oily particles, ammonium sulfate from denitrification processes, and hydrated lime used in desulfurization.
2.Deliquescent Dust: Some non-sticky dusts are hygroscopic (absorb moisture from the air). After being captured on the fiber surface, they absorb moisture and form a solution, leading to adhesion. An example is sugar dust.
3.Dust Forming Hard Crusts: Some dusts may have low initial adhesion but can absorb moisture from the flue gas and undergo recrystallization or form new, hard compounds that create a “crust” on the filter media. An example is the desulfurization by-product calcium sulfate (gypsum).
4.Fine, Moist Dust: Even non-sticky dust can cause blinding if the particles are very fine and have high moisture content. Such dust can form a uniform, adherent layer of “floating dust” on the filter surface. This layer has weak adhesion but is difficult to remove. Over time, this layer thickens. This is common in coal-fired power plants, particularly with filter bags longer than 6 meters.
- Example: A power plant in northern China experienced a shutdown when the dust collector’s differential pressure rose rapidly from about 1500 Pa to 2200 Pa due to this issue. Baghouse differential pressure should ideally be controlled below 1000 Pa.
2.3. Structural & Operational Blinding (No. 3)
This category includes all bag blinding caused by dust collector design, component structure, or improper operational practices.
Filter Media Structure and Treatment:
◦ Low Density: Some filter media have insufficient needle-punch density, resulting in a loose fibrous surface. Even with singeing, fine dust can penetrate deeply into the media. Internal dust accumulation, combined with water vapor, leads to blockage from the inside out.
■ Example: A northern power plant had to replace entire bag sets after less than a year due to this problem.
◦ Untreated Fiber Ends: If filter media is not singed or calendered, exposed fiber ends remain. These ends act as nucleation sites for condensation, initiating dust accumulation that spreads and bridges across fibers, forming large blinded areas. In waste incineration plants using pure PTFE membrane filter bags, stubborn dust crusts on fiber ends are often observed if the membrane is not adequately protected.
◦ Lack of Hydrophobic Treatment: For applications with high humidity or risk of acid condensation, failing to use hydrophobically treated filter media makes bag blinding more likely.
2. Filter Bag Fit on Cage: Pulse cleaning relies on the rapid acceleration of the bag and its subsequent impact against the cage. This inertial shockwave creates an “avalanche effect” that dislodges the dust cake. The cleaning effectiveness depends heavily on the magnitude of this bag movement and impact.
◦ Problem: If the filter bag’s circumference is too small, or if it undergoes excessive thermal shrinkage during use, it can become tightly “strapped” to the cage. This restricts bag movement during pulsing, resulting in weak shocks and ineffective dust removal. The accumulated dust eventually hardens, leading to bag blinding.
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