Dust separators with filter bags

Dust separators with bags use mechanical means to ensure separation. Settling takes place due to gravity. A bag is placed in the dust chamber. In contrast to dust separation cyclones where particles are separated from the tangential influx of air by centrifugal forces, here the dusty air flows through at a speed of 0.4-0.8 m/s. Guide blades and louvers are installed to increase efficiency.

The dust-laden gases enter the contaminated gas section of the filter through ducts. Here they collide with an impact plate to separate out the larger particles. Now containing only fine dust, the gas enters the dust chamber and is evenly distributed along the tubes. The dust is on the outside of the tubes attached to the supporting frames. Due to the suction mode, the tube is pulled onto the frame and the dust is deposited on its outer surface. Thus cleaned, the gas heads through the tubes into the clean gas chamber and then through the ducts to the fan. The tubes are cleaned automatically by short bursts of clean air. An electronic control mechanism uses electromagnetic valves to operate the membrane valves for impulses lasting 0.1-1.5 seconds in accordance with the required settings. During each impulse, air at a pressure of 5-7 bars flows through a distributor tube to the attached injector and then into the tube. The bag, which has thus far been pressed against the frame in a star shape, is suddenly filled with air and under inertia the dust particles fall into the collection unit. This is one of the most widespread methods of cleaning the tubes, but it is not the only way. The following are some other solutions:

♦ Lateral swinging movements
♦ Longitudinal shaking
♦ Transverse shaking
♦ Counterflow bursts of air
♦ Counterflow blowing with air
♦ Pre-programmed longitudinal shaking
♦ Reverse-jet procedure

Filter flow resistivity

The two main components of pressure drop in filtration equipment are the clean filter material and the resistance of the deposited dust layer. The flow resistivity of lower density filters is less than that of higher density filters; since the flow rate generally remains in the laminar range, the resistance depends on the amount of gas and its value is directly proportional to filtration speed.

Factors determining the joint resistance of the separated dust layer and the filter:

♦ Surface load of filter layer
♦ Free volume of filter material
♦ Properties of the carrier gas (density, viscosity, moisture content, etc.)
♦ The concentration of dust in the gas, or the extent of dust load
♦ The properties of the dust (particle size, size distribution, morphology, physicochemical properties)

The pressure drop resulting from the resistance is determinative in terms of economical operation.

The movement of particles in a gravitational field

Two opposing forces affect the particles: gravity and drag force. The flow resistivity around the particles consists of dynamic resistance stemming from changes in the speed of the medium and friction resistance due to viscosity. The sink rate depends on the flow properties of the medium (Reynolds number), which is calculated differently in the laminar (Re<0.1), transitional (0.1<Re<1,000), and turbulent (1,000<Re) ranges.

♦ laminar (Re<0.1): friction is definitive, with dynamic resistance negligible
♦ turbulent (1,000<Re): dynamic resistance is definitive, with friction negligible
♦ transitional (0.1<Re<1,000): both effects must be taken into consideration

The motions of particles smaller than 0.1 μm are chaotic (Brownian) and these remain suspended in the air.

Dry mechanical

 Advantages:

♦ Suitable for separating dry dusts during continuous operation
♦ May be used over a wide range of temperatures
♦ Not sensitive to varying dust concentrations in the airflow

Disadvantages:

♦ Not suitable for separating particles smaller than 5 μm
♦ Unable to remove gaseous contaminants
♦ Separation efficiency strongly influenced by gas flow-rate