Pressure drop (Δp), a very important baghouse design variable, describes the resistance to air flow across the baghouse: the higher the pressure drop, the higher the resistance to air flow. Pressure drop is usually expressed in millimeters of mercury or inches of water. The pressure drop of a system (fabric filter) is determined by measuring the difference in total pressure at two points, usually the inlet and outlet. The total system pressure drop can be related to the size of the fan that would be necessary to either push or pull the exhaust gas through the baghouse. A baghouse with a high pressure drop would need more energy or possibly a larger fan to move the exhaust gas through the baghouse.
Many different relationships have been used to estimate the pressure drop across a fabric filter. In a baghouse, the total pressure drop is a function of the pressure drop across both the filter and the deposited dust cake. Some pressure losses due to friction also occur as the gas stream moves through the baghouse.
The simplest equation used to predict pressure drop across a filter is derived from Darcy's law
governing the flow of fluids through porous materials and given as:
The term k1 is the fabric resistance (also called drag) and is a function of exhaust gas viscosity and filter characteristics such as thickness and porosity. Porosity describes the amount of void volume in the filter.
The pressure drop across the deposited dust cake can be estimated by using Equation 2 (Billings and Wilder 1970). This formula is also derived from Darcy's law and the simplified form is given as:
The term k2 is the dust-fabric filter resistance coefficient and is determined experimentally. This coefficient depends on gas viscosity, particle density and dust porosity. The dust porosity is the amount of void volume in the dust cake. The porosity is related to the permeability. Permeability for the fabric only is defined in American Society of Testing and Materials (ASTM) standard D737-69 as the volume of air which can be passed through one square foot of filter medium with a pressure drop of no more than 0.5 inches of water. The term k2 is dependent on the size of the particles in the gas stream. If the particles are very small (< 2μm) k2 is high. If k2 is high, then the pressure drop will tend to increase and the bags will have to be cleaned more frequently.
Filtration velocity also has an effect on k2. In more recent tests, conducted in the late 1980's under controlled conditions, the relationships of k2, particle size, and velocity have been shown more clearly. Researchers including Dennis, Cass, and Cooper (1977) and Davis and Kurzyske (1979) showed that both particle size and velocity have an effect on k2.
The total pressure drop equals the pressure drop across the filter plus the pressure drop across the cake and is given as:
Use equations 3 and 4 only as an estimate of pressure drop across shaker and reverse-air cleaning baghouses. In the industrial filtration process, complicated particle-fabric interactions are occurring just after the filtration cycle begins. In addition, the filter resistance factor k1 can take on two values; one value for the filter before it is brought on-line and another after the filter has been cleaned.When the dust cake builds up to a significant thickness, the pressure drop will become exceedingly high (> 10 in. H2O or 25 cm H2O). At this time the filter must be cleaned. Some dust will remain on the cloth even after cleaning; therefore, the filter resistance level will be higher than during original conditions. A baghouse is normally operated with a pressure drop across the unit of 4 to 10 in. H2O. But many units operate at less than 6 in. of H2O. Bag cleaning is usually initiated when the pressure drop approaches this point.
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