Engineering task and calculation objective
This module calculates lamellar (vane-type) droplet separators according to Section L4.3.4 of the VDI-Wärmeatlas (VDI Heat Atlas, 12th edition, 2019). Vane-type separators consist of parallel, corrugated or zigzag-folded plate packs: the droplet-laden gas flows through the lanes between the vanes, is accelerated at each turn, and the droplets that cannot follow the changes of direction impinge on the vane surface, coalesce into a film and drain off. They are used in wet scrubbers, cooling towers, downstream of columns and evaporators, and in air conditioning.
For the design of a vane-type separator, the approach velocity, lane width and lane length, deflection angle (45°, 60° or 90°) as well as the inside, outside and bend radius of the turn are the decisive geometry parameters; on the property side, gas density, liquid density and the dynamic viscosity of the gas enter the calculation. From these quantities, the method determines the limiting droplet diameter and the fractional separation efficiency based on the inertial principle.
The module thus provides the basis for sizing the separation area for a required gas flow rate and for staying within the operating limit against re-entrainment of the separated liquid film.
Standard and calculation basis: VDI-Wärmeatlas, 12. Auflage 2019
Calculation workflow
- Define the geometry of the vane pack: Lane width and lane length, deflection angle (45°, 60° or 90°) as well as inside radius, outside radius and bend radius of the turns describe the flow channel; the number of lanes yields the separation area of the separator.
- Enter operating point and fluid properties: The approach velocity upstream of the pack, the density of the gas, the density of the liquid and the dynamic viscosity of the gas define the inertia and drag forces acting on the droplets.
- Evaluate droplet trajectories in the turn: In each turn, a centrifugal acceleration acts on the droplet, which depends on velocity and bend radius; the flow drag of the gas opposes it. From this force balance follows which droplets reach the lane wall.
- Determine limiting droplet diameter and separation efficiency: The limiting droplet diameter marks the droplet size that is just barely separated within the available lane length; for the incoming droplet size distribution, the fractional and the overall separation efficiency of the pack follow from it.
- Check operating limits: The approach velocity must remain below the limit at which the draining liquid film is re-atomized (re-entrainment); at too low a velocity, on the other hand, inertial separation deteriorates. This yields the permissible operating window of the separator.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Lane length | l | m |
| Outside radius | ra | m |
| Inside radius | ri | m |
| Deflection angle (45°; 60° or 90°) | α | ° |
| Separation area | A | m² |
| Flow velocity | vo | m/s |
| Lane width | s | m |
| Bend radius | r | m |
| Dynamic viscosity (gas) | ηg | mPa·s |
| Density (liquid) | ρl | kg/m³ |
| Density (gas) | ρg | kg/m³ |
Calculated results
| Quantity | Symbol | Unit |
|---|---|---|
| Volume flow rate | Gasvolumenstrom | m³/s |
| Setting velocity of cut size droplets | ws | m/s |
| Centrifugal acceleration | z | m/s² |
| Tangential velocity | u | m/s |
| Diameter cut size droplets (Stokes) | de | m |
| Reynolds number cut size droplets (Stokes) | Re | – |
| Diameter of cut size droplets (Eq.12) | de | m |
| Pressure drop | Δp | Pa |
| Cut size droplets | d* | m |
Frequently asked questions
What influence does the deflection angle have on separation?
The sharper the turn (90° versus 45°), the greater the lateral acceleration on the droplets and the finer the droplets that are separated – but the pressure drop increases correspondingly. In practice, the angle is chosen as a compromise between the required limiting droplet diameter, the permissible pressure drop and the fouling tendency of the sharper edges.
Why is there an optimal approach velocity?
Inertial separation improves with increasing velocity, because the droplets can follow the turns less well. Above a critical velocity, however, the gas flow tears open the film draining along the vanes and carries out secondary droplets – the separation efficiency collapses. The operating window typically lies at a few meters per second and depends on the material system and the mounting orientation.
How does the vertical mounting orientation differ from the horizontal one?
With horizontal flow through vertical vanes, the film drains downward transverse to the gas flow and can tolerate higher gas velocities. With vertical flow from bottom to top, the film must drain against the gas flow; the re-entrainment limit is considerably lower. The mounting orientation must therefore be taken into account when setting the permissible approach velocity.
For which droplet sizes is a vane-type separator suitable?
As an inertial separator, it reliably separates droplets above about 10 to 20 µm; finer droplets and aerosols largely follow the gas flow through the turns without resistance. For mists in the single-digit micrometer range, wire mesh, fiber bed or venturi separators are the more suitable designs, if necessary as a second stage downstream of the vane pack.