Cyclone Separator Theory
What is a Cyclone Separator?
A cyclone separator is a mechanical device that uses centrifugal force to separate solid particles or liquid droplets from a gas stream. The contaminated gas enters tangentially near the top of a cylindrical vessel, creating a spinning vortex. Particles are thrown outward to the wall by centrifugal force, slide down to a collection hopper, while clean gas exits through a central outlet.
Cyclones are widely used in industry because they are simple, have no moving parts, low capital and maintenance costs, and can operate at high temperatures and pressures.
Principles of Centrifugal Separation
The separation mechanism in cyclones is based on centrifugal force. When a particle moves in a circular path at velocity V and radius r, it experiences a centrifugal force:
Fc = m × V² / r
where m = particle mass, V = tangential velocity, r = radius
The separation factor compares centrifugal acceleration to gravitational acceleration:
S = V² / (r × g)
Typical cyclones: S = 50-2500 (much stronger than gravity)
Stokes Law and Terminal Velocity
For small particles (Re < 1), the drag force follows Stokes Law:
Fdrag = 3π × μ × dp × v
where μ = gas viscosity, dp = particle diameter, v = relative velocity
At terminal velocity, drag force equals centrifugal force. The radial velocity toward the wall is:
vr = dp² × (ρp - ρg) × V² / (18 × μ × r)
Larger and denser particles move faster toward the wall and are collected more efficiently.
Cut Diameter (d₅₀) - Barth Equation
The cut diameter (d₅₀) is the particle size collected at 50% efficiency. It characterizes cyclone performance - smaller d₅₀ means better separation. The Barth equation provides:
d₅₀ = √[9μb / (πNeVi(ρp - ρg))]
where:
- μ = gas viscosity (Pa·s)
- b = inlet width (m)
- Ne = effective number of turns
- Vi = inlet velocity (m/s)
- ρp = particle density (kg/m³)
- ρg = gas density (kg/m³)
To reduce d₅₀ (improve efficiency):
- Increase inlet velocity Vi
- Reduce inlet width b (narrower inlet)
- Reduce cyclone diameter Dc (increases Ne)
- All these changes increase pressure drop
Collection Efficiency
The collection efficiency for a given particle size dp can be estimated using the Lapple model:
η = (dp / d₅₀)² / [1 + (dp / d₅₀)²]
This shows:
- For dp = d₅₀: η = 50%
- For dp = 2×d₅₀: η = 80%
- For dp = 5×d₅₀: η = 96%
- Particles much smaller than d₅₀ are poorly collected
Pressure Drop
The pressure drop across a cyclone is primarily due to inlet and outlet losses and the kinetic energy of the spinning gas. The Shepherd-Lapple correlation:
ΔP = K × ρg × Vi² / 2
where K = 16 × a × b (pressure drop coefficient)
Typical pressure drops:
- Standard efficiency cyclones: 500-1500 Pa (2-6 inH₂O)
- High efficiency cyclones: 1000-2500 Pa (4-10 inH₂O)
- Higher efficiency always means higher pressure drop
Standard Cyclone Geometries
Cyclone dimensions are typically expressed as ratios of the body diameter Dc:
| Dimension | Lapple (Standard) | Stairmand (High Eff.) |
|---|---|---|
| Inlet height (a/Dc) | 0.5 | 0.5 |
| Inlet width (b/Dc) | 0.25 | 0.2 |
| Exit diameter (De/Dc) | 0.5 | 0.5 |
| Cylinder height (h/Dc) | 2.0 | 1.5 |
| Total height (H/Dc) | 4.0 | 4.0 |
| Typical d₅₀ | 10-20 μm | 3-8 μm |
| Pressure drop | Lower | Higher |
Cyclone Types
Reverse Flow (Common)
Gas enters tangentially, spirals down the outer wall, then reverses and exits upward through a central tube. Most common type, good for general applications.
Uniflow (Axial)
Gas and particles exit in the same direction. Lower pressure drop but less efficient. Used when pressure drop is critical.
Multi-Clone
Multiple small diameter cyclones in parallel. Higher efficiency than single large cyclone for same total flow. Common in power plants.
Hydrocyclone
Uses liquid instead of gas. Separates solid particles or immiscible liquids from liquid streams. Common in mining and wastewater treatment.
Design Guidelines
Operating Conditions
- Inlet velocity: 12-20 m/s (higher = better efficiency, higher ΔP)
- Gas temperature: Cyclones can handle up to 1000°C
- Dust loading: 1-500 g/m³ (very flexible)
- Particle size: Best for >5 μm (can handle down to 2 μm with high-eff)
Important Considerations
- Smaller cyclone diameter = better efficiency but limited capacity
- Use multiple small cyclones instead of one large for high flow + high efficiency
- Prevent dust re-entrainment with proper hopper design and sealing
- Erosion at inlet can be severe - use wear-resistant materials
- Efficiency drops sharply below d₅₀ - not suitable as sole control for fine particles
Common Applications
- Cement plants: Pre-separator before bag filters
- Power plants: Fly ash collection from boilers
- Spray dryers: Product recovery
- Catalytic crackers: Catalyst recovery in refineries
- Wood processing: Sawdust and chip collection
- Grain handling: Dust control
- Air pollution control: Primary particulate removal
Advantages and Limitations
Advantages
- Simple construction, no moving parts
- Low capital and maintenance costs
- Can handle high temperatures
- Wide range of dust loadings
- Small footprint
- Dry collection (no water needed)
Limitations
- Poor efficiency for fine particles (<5 μm)
- Moderate to high pressure drop
- Inlet erosion with abrasive materials
- Sensitive to variations in flow rate
- Cannot handle sticky particles well
- Requires good hopper sealing
References
- Lapple, C.E., "Processes Use Many Collector Types", Chemical Engineering, 58(5), pp. 144-151 (1951)
- Stairmand, C.J., "The Design and Performance of Cyclone Separators", Trans. Inst. Chem. Eng., 29, pp. 356-383 (1951)
- Barth, W., "Berechnung und Auslegung von Zyklonabscheidern", Brennstoff-Wärme-Kraft, 8, pp. 1-9 (1956)
- Cooper, C.D., Alley, F.C., "Air Pollution Control: A Design Approach", 4th Edition, Waveland Press (2011)
- Hoffman, A.C., Stein, L.E., "Gas Cyclones and Swirl Tubes", 2nd Edition, Springer (2008)