Image Credit: Koger Air Corporation, Aget Manufacturing Company, Ecologix Environmental Systems

 

Cyclone separators utilize gravity and a vortex to remove particulates from gaseous streams. Industrial cyclones are used in pollution control applications most commonly as a first stage, lower cost method for removing larger particulate matter (PM) from effluent gas streams. Because cyclone separators do not incorporate filter media or moving parts, the pressure drop (therefore, operating costs) and maintenance requirements tend to be low. They can also be constructed to withstand harsh operating conditions, and since separation in cyclones is a dry process, the equipment is less prone to moisture corrosion.

 

Advantages

Disadvantages

  • Low capital cost.
  • High operating costs (due to pressure drop).
  • Ability to operate at high temperatures.
  • Low efficiencies (particularly for small particles).
  • Can handle liquid mists or dry materials.
  • Unable to process "sticky" materials.
  • Low maintenance requirements (no moving parts).

 

  • Small footprint - requires relatively small space.

 

 

Cyclone Separator Operation

Cyclone separators operate by incorporating centrifugal, gravitational, and inertial forces to remove fine particles suspended in air or gas. These types of separators use cyclonic action to separate particulates from a gas stream. Typically, PM enters the cyclone separator at an angle (perpendicular to the flow stream, tangentially, or from the side), and is then spun rapidly. A centrifugal force is created by the circular airflow that throws the particulate towards the wall of the cyclone. Once the PM hits the wall, it falls into a hopper below. “Clean” exhaust is then either blown through or recirculated to be filtered again.

 

 
General layout and function of a reverse flow cyclone, indicating the flow paths of the gas and particulate matter (labeled powder in this diagram). Image Credit: GEA Niro

 

It is important to keep in mind that the centrifugal force (Fc) a cyclone generates on a particle is related to the tangential air velocity (vt), particle mass (m), and the particle’s radial distance from the cyclone wall (r) by the function:

Fc = m • vt2 / r

 

Selection

For custom designs or manufacturer assisted selection, an engineer or industrial buyer will fill out a data sheet of specifications for the cyclone and its design environment.

 

 
Typical data sheet for custom or manufacturer assisted cyclone selection, including a large range of design specifications. Image Credit: Fisher-Klosterman, Inc.

 

When selecting a cyclone separator, industrial buyers must consider various performance specifications, process conditions, construction, and applications. This guide is designed to help with this selection process by pointing out the key considerations and design aspects of cyclones.

 

Cyclone Performance

Cyclones may be purchased from existing manufacturer’s stock or custom built to certain customer design specifications. In either case, it is important to know the required specifications and process conditions for operation.

 

Specifications

The primary means to selecting the right cyclone separator(s) is matching its performance specifications with the application requirements. These specifications include process airflow and minimum filtered particle size.

  • Airflow or volumetric flow rateis the air flow generated or handled by a cyclone. This is usually given in cubic feet per minute (cfm). Some manufacturers provide ranges in standard cubic feet per minute (scfm), as flow rates can vary greatly as environmental conditions stray for standard temperature and pressure. To convert a given actual flow rate (QA) in actual cubic feet per minute (acfm) to standard flow (QS) in scfm, utilize the combined ideal gas equation which includes volume (V), pressure (P), and temperature (T) at actual and standard conditions.

(PSVS)/TS = (PAVA)/TA

 

If we know the temperature and pressure at actual conditions, we can substitute volume (V) with flow rate (Q, equivalent to volume per unit time) and rearrange to solve for QS.

 

QS = QA • (PA/PS) • (TS/TA)

 

A cyclone with a higher airflow can accommodate larger gas streams for treatment. For effective operation, the airflow of the system at the point of installation should fall within the rated airflow range of the cyclone.

  • Minimum filtered particle size is the smallest particle size a cyclone can filter to any measurable efficiency. It is generally measured in microns or micrometers (µm). If a cyclone is being used as the primary or only means of particle separation, the smallest particle size needed to be removed should be above this minimum. Actual collection efficiencies of a cyclone vary greatly based on the design of the cyclone, operating flow rate, and various properties of the gas and PM.
  • Collection efficiency, capture rate, or recovery rate is the overall removal efficiency of PM from an air stream. Manufacturers sometimes provide estimated efficiencies upfront; however actual results will vary based on the properties of particles in the gas stream, and on certain environmental conditions.

Parameter

When parameter increases, cyclone efficiency...

Particle Size

Increases

Particle density

Increases

Dust loading

Increases*

Inlet gas velocity

Increases*

Cyclone body diameter

Decreases

Ratio of cyclone body length to diameter

Increases

Smoothness of cyclone inner wall

Increases

Gas viscosity

Decreases

Gas density

Decreases

Gas inlet duct area

Decreases

Gas exit pipe diameter

Decreases

*Only increases to a point before efficiency drops off.
Relation of various parameters to cyclone design efficiency.

  • Pressure drop is the amount of flow resistance the cyclone will create in the system. Pressure drop is usually measured in units of inches of water column (in WC) or Pascals (Pa). Pressure drop is a function of flow rate, gas density, and cyclone design.

Design Tip: Correctly designed cyclones should have a recommended pressure drop range of operation, which for most designs is between 2 and 6 in WC (About 500 to 1500 Pa) at ambient conditions. Above this range, costs for flow losses begin to outweigh increases in efficiencies. Below the limits, particle recovery becomes ineffective.

 

Process Conditions

Process conditions are the characteristics surrounding a cyclone’s operation. These include the air conditions and the particulate/dust conditions that are essential to finding or designing an appropriate cyclone for a given system. When having a cyclone custom built, these conditions are requested from the cyclone manufacturer.

  • Air conditions include the pressure (psig or psia), temperature (°F or °C), and moisture content (weight or volume percent) at the inlet of the cyclone. Solids collection becomes more difficult as humidity rises because the moisture tends to cause PM to adhere to the cyclone walls.

  • Dust conditions at the inlet of the cyclone can include material type, specific gravity, bulk density (lb/ft3), dust load (lb/hr or grains/ft3 air), and particle size distribution. Sizing, densities, and loads are needed to determine the required capacity and design specifications for sufficient capture. It is also important to specify any unique characteristics of the particulate matter that may lead to corrosion or degradation of certain construction materials.

Construction

A cyclone must be constructed properly in order to meet the sizing and performance requirements of its operation. The biggest variables in cyclone design are its openings (inlets and outlets) and its body size.

 

Classifications

Cyclones can be classified into two types based on the construction and orientation of the inlet and outlet. These types are reverse flow and uniflow. In reverse flow (seen in Figure 1 above), the gas enters through a tangential inlet at the top of the cyclone body, shaped to create a confined vortex gas flow. The clean gas exits through a central pipe also at the top of the body. In uniflow or “straight through” cyclones, the gas enters at one end of the body and leaves at the other end; this type is less frequently used in industry because it is a much less practical design in most cases.

 

Cyclones can also be classified into two types based on the body size. High efficiency designs are characterized by long bodies in addition to small openings. This construction allows for high recovery rates at higher pressure drops. High rate designs are characterized by shorter bodies in addition to larger openings, which allow for a larger volume with lower capture rates or pressure drops.

 

Sizing and Configuration

Sizing a cyclone involves many factors, but the most important dimensions to consider are those of the inlet and the main body chamber.

 

The sizing of inlets affects the airflow capacity of a cyclone, and also influences its pressure drop at a given flow velocity. Larger openings will tend to accept greater volumes of air and also will have lower pressure drops, but at the cost of efficiency. Inlet configurations can also vary based on the type of cyclone. The four types include tangential, axial, helical, or spiral.

 

Body size is important because, as mentioned above, the length of the cyclone body is an important factor in collection efficiency. Body size is also directly proportional to a cyclone’s overall capital cost, and determines the space it occupies and its height.

 

Application Considerations

Some cyclone models are predesigned for specific applications, while others are built for specialized environments. Among the most important application considerations is the dust and gas composition. Specific types of particulate may be corrosive, abrasive, sticky, explosive, or toxic, thus requiring special construction materials or designs in order to function properly.

 

Cyclones are used for filtration in a wide variety of applications: abrasives, coolant mists, explosive media, fine powders, metalworking chips, toxic media, and various production plant exhausts.

 

GEA Niro - Cyclone, Figure 1

 

Fisher-Klosterman - How to select and maintain a cyclone for maximum efficiency, Figure 2

 

Dr. L. Svarovsky - Gas Cyclones

 

Youtube - Modeled Cyclone Separator, Video

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