Air-to-Cloth Ratio in Baghouses: Keeping Everything in Proportion

As industries continue to grow, they have a huge impact on the environment. With the increasing demand for clean air, new techniques are being developed to curb the effects of air pollutants both within and outside facilities, rendering using a baghouse a must. However, having a baghouse alone might not be sufficient to solve the problem. The air-to-cloth ratio in a baghouse plays a significant role in ensuring their proper operation.

Baghouse air-to-cloth ratio, in simple terms, is the amount of air that passes through the baghouse filter media compared to the total surface area of the filter media. The air-to-cloth ratio in a baghouse is calculated as cubic feet per min (CFM) per square foot of filter media. This ratio can determine the efficiency of a baghouse system in filtering out the pollutants.

Most industrial processes release dust, particulate matter, and other particles into the air. The primary purpose of the baghouse is to filter these pollutants from the air before releasing it into the atmosphere. The baghouse system consists of filter media that trap the pollutants while allowing clean air to pass through. Since the filter media eventually become clogged, it is critical to have the right air-cloth ratio in a baghouse to prevent the filter media from clogging up too quickly.

If there are too few filter bags, or the bags are too large, the particulate will collect more quickly, creating higher pressure and airflow resistance. When this happens, the baghouse filter air-to-cloth ratio becomes too low, and the baghouse's efficiency begins to decrease. Conversely, if there are too many bags, or the bags are too small, the air-to-cloth ratio is too high, and the onset of new build-up cannot be achieved. Achieving the right balance is critical for having an effective baghouse system.

This ratio represents the amount of air that flows through the filtration media per unit area of cloth, expressed in units of cubic feet per minute per square foot (cfm/ft2). To calculate the air-to-cloth ratio, divide the total volumetric airflow in cfm by the total filtration area in square feet. For example, if a baghouse has an airflow of 10,000 cfm and a total cloth area of 5,000 ft2, the air-to-cloth ratio is 2:1 (10,000/5,000).

What is the typical air-to-cloth ratio in baghouses? The optimal ratio is usually determined by considering the type of application, dust concentration, and other parameters. Dust collectors also have different self-cleaning systems. Sites featuring finer particles or chemicals typically use pulse jet baghouses, those with large particles tend to use reverse air baghouses, while shaker baghouses are frequently preferred when the budget and temperature are low. Here is the typical baghouse air-to-cloth ratio in different industrial environments and how they differ depending on the facility.

Example 1: Coal-fired power plant baghouse with fly ash
Coal-fired power plants typically have high dust concentrations that require a lower air-to-cloth ratio for effective filtration. The air-to-cloth ratio in a reverse air baghouse for fly ash is typically around 3:1-2:1. This means that for every 2 or 3 cubic feet of gas entering the baghouse, there is 1 square foot of filter cloth available. The low ratio ensures that the bags do not become saturated with dust and can effectively capture the particles.

Example 2: Woodworking baghouse
Woodworking facilities usually have lower dust concentrations than cement plants, so their baghouses tend to have a higher air-to-cloth ratio. A typical air-to-cloth ratio for a reverse air baghouse in a woodworking plant is around 8:1. With this ratio, the baghouse can handle a larger gas volume without overloading the filter bags. This higher ratio also means that the bags need to be cleaned less often, thereby reducing downtime and maintenance costs.

Example 3: Chemical processing baghouse
Chemical processing facilities often have very low dust concentrations and require a much higher air-to-cloth ratio than other industries. A chemical processing baghouse might have a shaker baghouse air-to-cloth ratio as high as 20:1, meaning that 20 cubic feet of gas can pass through each square foot of filter cloth. With such a high ratio, the bags will not clog very easily and even the smallest particles can be captured, ensuring that the process will be able to adhere to strict environmental regulations.

Example 4: Pharmaceutical baghouse
Pharmaceutical plants have stringent requirements for air quality and require a baghouse with a very high efficiency rating. The air-to-cloth ratio for a pharmaceutical baghouse, assuming jet pulse cleaning is used, might be as low as 2:1 to ensure that the bags can capture even the finest particles and prevent them from entering the environment. With such a low ratio, the bags can become easily saturated, so frequent cleaning is required to maintain proper performance.