Engineering task and calculation objective
The FLD module sizes wire-mesh demisters and fiber filters for removing droplets and solid particles from gas streams. To calculate a demister you need, in addition to the geometry of the filter pad (cross-sectional area, height, wire diameter, packing density), above all the grade efficiency curve: it describes what fraction of the particles of a given diameter is captured in the mesh. FLD determines these grade efficiency curves using the methods established in the literature, in particular those of A. Bürkholz, "Droplet Separation" (VCH, 1989), the standard reference on droplet separation.
In practice, demisters are installed wherever entrained droplets would damage downstream equipment or cause product losses: above evaporators and column heads, upstream of compressors, in gas scrubbers and knockout drums. To describe the feed, the particle size distributions of DIN 66143 (power-law distribution after Gates-Gaudin-Schuhmann), DIN 66144 (logarithmic normal distribution) and DIN 66145 (RRSB distribution after Rosin, Rammler, Sperling and Bennett) are available for selection.
As a result, the calculation delivers not only the overall separation efficiency but also the pressure drop of the filter pad via drag and pressure-loss coefficients, together with graphical plots of the grade efficiency curve and the distribution function — the basis for choosing mesh type, face velocity and installed height economically.
Standard and calculation basis: A. Bürkholz, Droplet Separation, VCH. 1989
Calculation workflow
- Define the filter geometry: The filter pad is described first: filter cross-sectional area, filter height, filter mass or filter density, and wire diameter. From these, derived characteristics follow, such as filter volume, total wire length and the relative free-flow area (porosity) of the mesh.
- Enter operating conditions and fluid properties: Mean pressure, inlet temperature, density and dynamic viscosity of the gas, and the density of the particles or droplets are specified. From the gas mass flow and the filter cross-section, the flow velocity follows, and from it the Reynolds number of the flow around the wires.
- Describe the particle population: The loading of the feed is specified as particle loading, volume concentration or particle mass flow. For the size distribution, the user selects a distribution law to DIN 66143, 66144 or 66145 and parameterizes it via the particle size parameter and the spread parameter, or via the residue value at a reference particle size.
- Calculate the grade efficiency curve: For each particle diameter, the single-fiber collection efficiency is determined from the governing mechanisms; in demisters, inertial separation dominates, which is captured by the inertial separation parameter (Stokes number). Correction factors for flow direction and fluid properties are applied. Integration over the mesh yields the grade efficiency as a function of particle diameter.
- Evaluate overall separation efficiency and pressure drop: Convolving the grade efficiency curve with the selected particle size distribution gives the overall separation efficiency. In parallel, the drag coefficient, the pressure-loss coefficient and the pressure difference across the filter pad are calculated. The results are displayed graphically as grade efficiency and distribution curves.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Filter cross section area | F | m² |
| Filter mass | M | kg |
| Filter height | H | m |
| Filter volume | V | m³ |
| Filter density | G | kg/m³ |
| Wire diameter | D | m |
| Total wire length | L | m |
| Wire density | ρD | kg/m³ |
| Density | ρ | kg/m³ |
| Dynamic viscosity | η | mPa·s |
| Density | ρF | kg/m³ |
| Correction factor for flow direction | f | - |
| Relative flow cross section area | prel | - |
| Flow velocity | v | m/s |
| Reynolds number | Re | - |
| Drag coefficient | cw | - |
| Pressure drop coefficient | ζ | - |
| Pressure difference | ΔP | Pa |
| Fluid parameter | S | - |
| Correction factor | a* | - |
| Particle diameter | dp | m |
| Particle size | dp50 | m |
| Particle size | dp25 | m |
| Particle size | dp75 | m |
Frequently asked questions
How do the distribution laws of DIN 66143, 66144 and 66145 differ?
All three describe the particle size distribution of a particle population, but they differ in their mathematical approach: DIN 66143 uses a power-law grid (Gates-Gaudin-Schuhmann), DIN 66144 the logarithmic normal distribution, and DIN 66145 the RRSB distribution after Rosin, Rammler, Sperling and Bennett. Which distribution fits depends on the process that generated the droplets or particles; for mechanically generated spray mists, the RRSB or log-normal distribution usually matches reality best. When in doubt, plot a measured distribution on the corresponding distribution grid and choose the law that gives the best linearization.
Why must the face velocity of a demister be neither too low nor too high?
Inertial separation increases with gas velocity, because droplets then follow the streamlines around the wires less closely and impact on them. At too low a velocity, the collection efficiency for small droplets drops noticeably. At too high a velocity, re-entrainment sets in: the liquid draining through the mesh is broken up again by the gas and carried out, and the pressure drop also rises sharply. Demisters are therefore operated in a narrow velocity window, which is determined via load factors based on gas and liquid density.
For which droplet sizes is a wire-mesh demister suitable?
Wire-mesh demisters effectively remove droplets from about 3 to 5 µm upward; the grade efficiency rises quickly to nearly 100 % with increasing droplet diameter. For submicron aerosols and fine mists below 1 to 2 µm, inertial separation is not sufficient — fiber filters (candle filters) with much smaller fiber diameters are required there, in which diffusion and interception effects act in addition. The module covers both designs; the choice depends on the expected droplet size distribution.
What role does the pressure drop play in the design?
The pressure drop of a clean wire-mesh demister is small, typically a few millibar, but it enters directly into the energy balance of compressors or the available pressure difference of vacuum systems. It is calculated from the drag coefficient of the flow around the wires, the packing structure and the dynamic pressure. In operation it can rise considerably due to liquid loading or solids deposits; a margin over the calculated dry-state value should therefore be planned in.