Dust collection principles

Principles and structures of fabric filters

A fabric filter is a dust collection device using a woven or non-woven filter bag that filters and collects the dust in process gas. When the filter cloth is made into a cylindrical-shaped bag and suspended, it is referred to as a baghouse or a fabric filter.
The inside of a large fabric filter is divided into several compartments. Each compartment contains several filter bags in which filter cloth sewn into cylindrical shapes is installed. A hopper is provided in the bottom of the compartment to collect the dust removed from the filter bags. The fabric filter dust collection principle is that the dust layer that adheres to and is deposited on the surface of the filter bag and the interior of the filter cloth (the primary dust layer) filters and collects the dust contained in the process gas.
Normally the filtration velocity of the process gas passing through a filter cloth is about 0.3-2 m per minute, and the pressure loss is 1-2 kPa. As the dust layer collected on the surface of the filter cloth becomes thicker, the pressure loss of the filter cloth increases, so the collected dust is intermittently removed. Removing the dust layer is carried out by methods such as (1) Mechanical shaking, (2) Reverse pressure, or (3) Pulse jet. In most cases the dust collection efficiency of fabric filters is 99% or higher, and the dust concentration at the outlet is less than 10mg/m3N.

It is important to select the filter material of a filter bag to suit the gas temperature, humidity, gas composition, and dust characteristics. The life of a filter bag is several years, but varies greatly depending on the gas and dust characteristics.
The equipment cost of a fabric filter is lower than that of an electrostatic precipitator, but the maintenance cost of replacing the filter bag, etc., and the operating cost of the system fan to compensate for the pressure loss is somewhat larger. Fabric filters are widely used as the dust collection systems for industrial processes with a medium or low quantity of process gas.

Dust removal methods

A fabric filter consists of several filter bags made of filter cloth sewn into cylindrical shapes and support frames that support the filter cloth. Process gas is filtered on the surface of the filter cloth, and purified gas flows out.
The collected dust is removed from the filter cloth through cleaning methods such as (1) the pulse-jet method and (2) the pulsative reverse pressure method. The structure and characteristics of these methods are shown in the table. The process gas filtration speed of the pulse-jet method is 1.3 to 1.6 times that of the pulsative reverse pressure method (our figures), so it is possible to filter at high speeds, and the fabric filter size can be made smaller. The pulsative reverse pressure method uses a long filter cloth with a large-diameter opening, so it has the advantage of having a longer filter cloth life than the pulse-jet method.

Dust removal methods Pulsative reverse air method Pulse-jet method
  1. Filtration speed
  2. Filter cloth size
  3. Filter cloth life
  4. Pressure loss
Medium speed
Long filter cloth with large-diameter opening
3-5 years
About 1.5 kPa
High speed
Medium-length filter cloth with a medium-diameter opening
2-4 years
About 1.5 kPa

Pulse-jet type dust removal mechanism

Momentarily blowing compressed air from the top of the filter bag and cage assembly produces a pulse-jet gas flow, removing the dust that has deposited on the surface of the filter bag.