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General information

  • Type of system, process, device or machine
  • Mode of operation of the filter system (continuous or intermittent)
  • Gas properties (e.g., hazardous to health, flammable or corrosive)
  • Particle properties (e.g., hazardous to health, flammable, hygroscopic, sticky or agglomerating)
  • Gas/dust mixture properties (e.g. explosive)

Design checklist

  • Height above sea level in m
  • Data on gas being cleaned
    • Flow rate in m³/h
    • Temperature in °C
    • Composition (e.g., volume fractions)
    • Humidity in g/kg dry air
    • Water dew point (acid dew point, if necessary) in °C
    • Density in kg/m³
    • Gas pressure when entering separator in hPa
    • Desired clean-gas dust concentration in mg/m³

  • Data on particles
    • Average concentration in raw gas in g/m³
    • Maximum concentration in raw gas in g/m³
    • Particle size distribution
    • Density in g/cm³
    • Bulk density in g/cm³ or t/m³
    • Composition by material constituent in relation to dry substance in % by weight
    • Water content in relation to dry substance in % by weight

Volume flow (Q)

The basis for the design of filtering separators is knowledge of the flow rate being cleaned. This either depends on the process or on extraction conditions (e.g., dust protection at the workstation)

Important factors are:

  • Temperature
  • Pressure
  • Gas atmosphere
  • Properties of dust material being separated and its concentration

A starting point for determining air flow rates is the capture velocity at open surfaces, inlets, hoods and machine coverings

Q [m³/min] = A [m²] x v [m/s] x 60

Emissione. g.capture velocity [m/s]
staticdegreasing/galvanic bath, smock0,25 - 0,5
slowfilling, welding, slow conveyor transports, manual bag emptying0,5 - 1,0
fastcrusher, spraying booth, automatic sac/barrel filling1,5 - 2,5
turbulentgrinding, sawing, polishing, sandblasting, trommelbis 10,0

Another starting point for determining air volume is information on the design of pneumatic conveying equipment for bulk materials of all types.

Determining the necessary filter area

As an initial approximation, the size of the filter required can be determined using the following equation:

Q = volume flow of gas passing through
A = filter area
f = specific fair-to-cloth ratio

Particle characteristicsEffects on filtration performance
sticky-humidgood particle separation, bad dropping of filter cake
gut agglomerierend (großer Schüttwinkel), trockengood particle separation, good dropping of filter cake
frei fließend (kleiner Schüttwinkel), trockenbad particle separation, good dropping of filter cake

 

 

Air-to-cloth ratio

The air-to-cloth ratio should be generally between 0.5 m³/(m² min) and 2.5 m³/(m² min). Common pressure drops of the filter media are between 400 Pa and 1,500 Pa. These ranges are determined, among others, by the following aspects: 

  • dust characteristics
  • air-to-cloth ratio
  • type of the filter media
  • clean-gas dust content
  • lifetime

Parameters affecting the air-to-cloth ratio

Among others, the following aspects need to be considered to determine the air-to-cloth ratio: 

  • the raw-gas dust content
  • the target clean-gas dust content
  • the target system pressure drop
  • the target service life of filter medium
  • the gas composition (specifically the moisture content)
  • the design of the separator /space requirements
  • the cleaning type of the filter medium

Estimation of the air-to-cloth ratio according to Flatt

The theoretical air-to-cloth ration feff is calculated then: = f ₓ An ₓ B ₓ C ₓ D ₓ E ₓ F ₓ G ₓ H ₓ I

Since each single parameter can are within a range of 0.45 to 1.5, there can be substantial differences to the basic value f. The influence is reduced with for e. g. a pocket filter AJN or bag filter and having uncritical process conditions.

In the simplest case, the factor can even be 1.  The air-to-cloth ration feff can be halve compared to f in case of specific applicaitons with very fine dust and high temperature. therefore, please conatct a specialist in any case.

Source: Friedrich Löffler et al.: Staubabscheidung mit Schlauchfiltern und Taschenfiltern. Vieweg, 1984, S. 247.

Typical air-to-cloth ratio for filtration separators with compressed-air cleaning

dust kinds / applicationsair-to-cloth ratio m³/(m² min) with bag filter or pocket filter
iron oxides (kiln de-dusting in steel works)1 - 1,5
fly ash from coal burning, handling1 - 1,5
fly ash from coal burningcombustion depending on pre-separation and method of combustion0,5 - 1,5
gypsum dust (burning of gypsum)1 - 1,6
wood flour (grinding dust with content of glue)1,1 - 2,5
sand conditioning of foundry molding sand1 - 2
thermic spray process of aluminum0,5 - 0,6
wheat grinding2,5 - 4,1
zinc oxid dust (exhaust above zinc bath)1 - 1,5
cement dust (transportation, loading)1,1 - 2
sugar classification1,1 - 2

Specific air-to-cloth ratio

 The specific face velocity f [m³/m²*min] affects the effectiveness and separation efficiency of the dust separator. 

ParameterSpecific air-to-cloth ratio (f)
highlow
Filter areasmallerlarger
Pressure losslargersmaller
Separation efficiencyworsebetter
Abrasionlargersmaller
Approaching flowdisadvantageousadvantageous
Space requirementsmallerlarger
Investmentsmallerhigher
Operating / maintenanceexpensiveless expensive

 

 

 

Comparison of standards of filter classes

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