Air extraction and filtration

Demanding environmental, labor and productivity standards in today’s manufacturing industry, with special emphasis on those areas where raw materials in dust phase are transported and processed, require the implementation of suction and filtering systems that guarantee on the one hand the elimination of pollution sources, as well as the improvement in process performance through product recovery.

Of the various types of existing suction installations, and focusing on localized dust extraction systems, we can basically distinguish two highly widespread configurations in the industry:

1-Centralized dust collectors

This is a system aims to take the pollutant in the closest possible place from the point where it has been generated in one or more sources, driving it towards the collector device, four basic components are distinguished:

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a- Collector device, typically bells or nozzles through which the contaminants are captured, which must have the appropriate geometry that allows the dust to be carried away.

b- Laying of ducts, in charge of conducting the air loaded with the pollutant at the appropriate speed.

c- Purifying equipment, it fulfills the function of treating the pollutant by separating the transport air from the particulate, such as cyclone separators, bag filters, dust collectors, etc.

d- Impeller, generally a centrifugal fan that will provide the energy necessary for the charged air to circulate through hoods, ducts and scrubber, guaranteeing the flow and pressure necessary to overcome the restrictions of the conduction circuit (system pressure drop).

2- Compact or Insertable Dust Collectors:

Unlike the centralized system, these are mounted on the point to be controlled and in the same place of capture, the dust is sucked and retained by the scrubber and then discharged into the device where the pollutant was generated, recovering it for the process.

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Bag filters

Within the group of scrubbing equipment used in localized aspiration systems, bag filters are the most widespread and representative equipment for the separation of solids in a gaseous flow by means of filtering elements based on technical textiles.

Fabric filters separate particles by obstruction, impact, interception, diffusion and electrostatic attraction. The fabric is made of fibrous materials, natural or synthetic, and can be woven or non-woven (felts). The advances of the last decades, in the production and development of nonwoven textiles (needlepunched) managed to impose them by improving the mechanical resistance, chemical attack and high temperatures.

Needlepunched fabrics are identified by their thickness and weight per unit area, being a porous medium through which the air to be filtered circulates so that the particles are retained on the “dirty” side of the fabric and the clean gas passes through the filtering mass. Another parameter that identifies the textile is the permeability, which is defined as the volume of air that passes through a surface in a unit of time with a given pressure difference. The combination of both parameters determines the retention efficiency of the fabrics.

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The needlepunched fabrics used in filtration have a mesh that provides mechanical resistance to the textile, on the other hand, manufacturers apply a treatment on the side that will be in contact with the dust (usually flamed) thus presenting a smooth surface that will facilitate the detachment of the material retained superficially. The maximum retention efficiency is obtained progressively until the smallest particles are retained to generate the so-called filter cake, a common practice called preloading that speeds up the process by introducing into the circuit inert material of controlled particle size that generates this cake.

Given the successive impregnation of material, baghouse filters are equipped with systems that help to remove the retained dust, called cleaning system, so the filters are classified according to the method used for dedusting.

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a- Mechanical (shaking, vibration, etc.)

b- Counter current air injection at low pressure. (centrifugal fan in the order of 0.035 – 0.050 bar)

c- Medium pressure counter current air injection. (roots blower approximately 0.4 -0.5 bar)

d- Compressed air pulses or pulse jet (high pressure between 5 – 6 bar).

Focusing on the Pulse Jet system filters, the bag cleaning system is basically composed of:

  1. Compressed air buffer tank
  2. Diaphragm valves with pneumatic or electric pilot, in charge of allowing the cleaning pulses.
  3. Diaphragm valves with pneumatic or electric pilot, in charge of allowing the cleaning pulses.
  4. Blowing pipes or flutes, in charge of introducing the compressed air pulses inside each hose.
  5. Accessories for driving and connecting the compressed air circuit.
  6. Accelerator or Venturi tubes. (many times they are part of the baskets or sleeve holder retainers)
  7. Sequencer for the compressed air circuit.
  8. Programmable electronic sequencer, in charge of managing the sequence and duration of the cleaning cycles. Often associated with or including a differential pressure gauge that enables the enabling of cleaning cycles on demand, within a range of preset pressures.
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The cleaning cycle is on-line so there is no interruption to the filter operation, a usual configuration involves short duration pulses (typically 200 – 500 milliseconds) and a jet of compressed air at a pressure of between 5 – 6 bar, the pulse is transmitted to a line of bags (typically no more than 14 bags per row) and the usual sequence is in the order of 30 – 120 seconds between blows, these may be fixed in time or on demand depending on the pressure difference across the bags which is a direct function of the saturation state of the filters.

Equipment selection and sizing

Regarding equipment selection and sizing, the physical-chemical characteristics of the product, type and characteristics of the emission points, pollutant concentration, site conditions and/or installation environment, requirements and regulations, etc. must be taken into account.

This information allows the designer of localized aspiration installations to know the flow rate of the installation, therefore, the necessary filtering surface, characteristics of the filtering elements and other elements of the system.

Among the baghouse selection / sizing parameters, the filtering velocity, also known as air to cloth ratio, stands out. This is the flow rate of contaminated air that must pass through 1 unit of filtering surface, typically defined in m/min (m³/min/m²). It is set according to the product to be filtered, contaminant concentration, etc.

Formula de Aspiracion y filtrado 2 1 3

Where:
Vf= Filtration velocity (m/min)

Q= Polluted air flow rate (m³/min)

AfT= Total filtration area (m²)

Ø=Hose diameter (m)

H= Hose length (m) (m/min)

H= Hose length (m) (m)

n= Number of hoses (m)

n= Number of bags

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The filtration ratio takes values between 1 – 6 m/min. Although there are tables and empirical formulas that allow to approximate the ideal value for each pollutant, in general an analysis of samples of the material is resorted to, antecedents and previous experiences with the same material are not conclusive, but they contribute a better panorama at the time of solving an installation.

The other important parameter when sizing a baghouse is the can velocity, which is defined as the upward velocity of air through the open area between the filter bags inside a dust collector. If the interstitial velocity of the upward airflow is too high, the dust expelled from the bags during cleaning will not settle into the hopper. Instead, it will be reincorporated and carried back to the surface of the bag, causing high pressure drop, excessive use of compressed air and shorter bag life, a typical value being in the order of 0.5 – 1.2 m/sec.

Where:
Vi= Interstitial velocity (m/sec)

Q= Air flow rate (m³/sec).

AT= Filter cross section (m²).

Am= Cross sectional area of the bags. (m²)

Ø=Sleeve diameter (m)

H=Sleeve length (m) (m)

H= Hose length (m)

n= Number of sleev

Formula de Aspiracion y filtrado 3


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