Tips for Selecting Highly Efficient Cyclones
The right mix of design and operating variables can dramatically boost separation efficiency
C
yclone dust collectors have been used – and miused – all over the world for more than 100 years. One reason for the misuse is a common perception among users that all cyclones are "created equal" – that is, as long as a cyclone resembles a cylinder with an attached cone, it will do its job.
However, to maximize separation efficiency in a specific application requires a precise cyclone design, engineered to exactly fit many possible variables. A well-designed cyclone, for instance, can achieve efficiencies as high as 99.9+% when operated properly within the envelope of its specifications. Nonetheless, cyclones are often used only as first-stage filters for performing crude separations, with final collections being carried out by more-costly baghouses and scrubbers.
Compared with baghouses and scrubbers, cyclones have two important considerations in their favor. One, they are almost always safer – in terms of the potential for generating fires and explosions – than fabric filters. In fact, cyclones are the most suitable dust collectors when dealing with process temperatures and pressures higher than 2000 deg. F and 500 atm. Second, cyclones have lower maintenance costs since there are no filter media to replace.
In practice, cyclones can be effectively incorporated as product collectors or final filters in most recovery and pollution-control applications. However, it should be noted that when the final filter requires an environmental permit and must qualify as a Best Available Control Technology (BACT), cyclones have historically not fared well against baghouses and scrubbers.
The finer points
Cyclones remove solids from gases, solids from liquids, and liquids from gases by maximizing the effect of several major and minor natural forces. The forces involved in these separations are:
- Mass rotational forces, which can be centrifugal, inertial, or gravitational.
- Drag forces, which opposes the mass forces to enable particulates to be separated from a flowing gas.
- Secondary forces, including those resulting from fluid resistance, dynamic pressure, centripetal effects and reactance.
The design and operation of a cyclone for a particular application depends on the proper interaction of these mass, drag and secondary forces. Many formulas using these forces for specifying generic cyclone designs are available.
In general, these formulas work well for bulk materials with large particle sizes of consistent aerodynamic shape. However, recent high-efficiency cyclones can achieve 99+% on particulates as small as five micrometers (Stokes equivalent diameter) and with specific gravities of less than 1.0. As a result, the design and operation of such cyclones depends strongly on correctly specifying the properties of the carrying gas, particulates, and shape of the cyclone. Also, one should ensure that the duct and material discharge system are correctly designed, and that any changes in the nature of the carrier gas and particulate materials are accounted for during collection. THe maintenance of the cyclone is another important consideration.
- Entering gas. The correct description of the gas conveying the particulates should be based on accurate measurements of gas properties as the gas enters the cyclone inlet. The gas conditions are defined as an actual flowrate at a defined gas density and viscosity. This is important since gas pressure and density can vary with process pressure, geographical location, humidity and temperature.
- Entering particulates. The correct definition of the entering particulate is a major problem because it, like the gas, must be defined as it enters the cyclone inlet. Often, engineers who want to replace a cyclone of marginal efficiency with a high-efficiency design define the particulates based on previous outlet-emissions tests or on material being discharged from the existing cyclone. THis is unacceptable.
The recommended procedure is to send a sample of the material as it enters the inlet to a qualified laboratory to determine its aerodynamic parrticle-size distribution. The results provide the Stokes equivalent diameters of the particulates, represented as percent finer than a specific size. This allows the particulates to be described in spherical-ball equivalents at a homogeneous density, and is the easiest way to predict separation efficiency.
The need for this can be best understood by a simple – if obvious – demonstration. Take any two identical pieces of paper and drop them from the same height at the same time. Observe how they twist and turn as they fall, landing in separate places on the floor. One paper may even take a longer time to land than the other. It is obvious that the reasons for the differing behavior of the pieces of the paper is due to aerodynamic effects and forces that are not obvious from the physical properties of the particles , which are identical.
These aerodynamic effects are further amplified when the particulates differ even slightly in their shapes. For example, if one of the pieces of paper is crumpled and dropped, it would fall down more quickly, and the drop path would be more predictable – that is, straight down to the floor. This, despite the fact that the paper is the same size and weight as the original, except for a more-aerodynamic shape.
Refining a model
The demonstration shows how critical it is to define the particulates' size distribution aerodynamically in order to correctly design cyclone collectors. In fact, while the above exercise assumes that the two "particles" have identical densities, many applications deal with mixed materials consisting of varying specific gravities and size distributions.
In practice, it may not be possible to obtain an accurate sample of the material at the inlet. Here, it is best to develop a computer model of an existing cyclone, based on accurate measurements of its inlet, body, cone, discharge, vortex-breaking receiving hopper. This model – at defined inlet gas flows and particulate conditions – can then be used to refine the design to achieve higher efficiencies.
Some other factors that must be considered during design are space requirements and allowable pressure drop across a cyclone. For instance, space and efficiency requirements may dictate the use of cyclones in parallel (dual, quad or more), or an increase in pressure drop, or a combination of both, to create higher centrifugal forces for collection.
- Cyclone shape. The shape of a cyclone is a major factor in separation effeciency – for example, two cyclones with identical diameters but different geometric configurations for the inlet, cone, vortex finder, and material-discharge sections can have efficiencies that differ considerably.
In Figure 1, cyclone A's design may cause a number of problems because:
- The inlet may not provide an adequate inlet velocity and profile.
- The tangential inlet may encourage vortex-finder abrasion and improper inlet flow, due to vortex-finder interference. There is also potential for short-circuiting, allowing particulates to go up the vortex finder prior to reaching the maximum centrifugal force needed to get the material to the cyclone wall, from where it will fall to the receiver.
- A bad internal design can maximize turbulence that can redirect into the reverse flow of the of the inner vortex particulates that are normally caught.
- There is potential for high wear at the body and cone joint due to an abrupt transition, preventing material from smoothly transferring from the body to cone.
- A low-profile cone (i.e., one that is too short) promotes cyclone wear and particulate re-entrainment.
- There is no vortex-breaking hopper between the cyclone and the airlock.
- Duct Design. Improper duct design is one of the most common reasons for inadequate air volume to a cyclone. In fact, it is quite common to see engineers design system for a given air volume, and then purchase a fan that cannot deliver that volume because the static pressure of the system exceeds the static pressure capacity of the fan ordered.
In addition, there is a common tendency to put an elbow connection at the inlet of most cyclones
(Figure 2). In fact, for optimum efficiency, it is necessary to allow 6-8 diameters of straight duct before the cone inlet, to prevent "weighing" the air to the outside of the elbow and into the cyclone. - Material discharge. The poor design of a cyclone's material discharge section can cause re-entrainment. FOr instance, many users believe that a cyclone with an upstream fan does not require a vortex-breaker hopper or an airlock
(Figure 3). This is not true.
The truth is that the internal vortex, whether it is introduced positively or negatively by a fan or blower, has a suction that can reentrain particulates. Therefore, a vortex breaker and an airlock should always be a system consideration, irrespective of whether the fan is located upstream or downstream.
- Changes in Properties. In practice, changes in gas properties and of particulates during collection can cause many serious problems. For instance, a primary concern is when the dew point is reached in an uninsulated cyclone, transferring enough heat to cause internal condensation; thus, material collected dry may subsequently become wet and cake on a cyclone's walls.
Another potential problem is the buildup of electrostatic charges, leading to material bridging and, in some cases, ignition of the dust, resulting in fires or explosions. Thus, proper grounding is a requisite of all dust-collector applications. Sticky materials may require special treatment of interior surfaces with easy-release or low-friction coatings, to prevent buildup.
Troubleshooting
As with all types of process equipment, the efficiency of cyclones can change during operation. Some clues of incipient problems are:
Discharge of wetted materials that should be dry. Check:
- If the inlet versus outlet temperature is causing a dew-point problem
- If the cyclone's wall need insulation
Material plugs the discharge section. Check:
- If the material is different than expected – i.e., Is it wet, hot or sticky?
- If there is air retrainment, which can fluidize material above the airlock
- If the size of the airlock is adequate, or if it needs servicing
Operation of the cyclone has deteriorated over time. Check:
- If there are air leaks at doors and airlocks, and for abrasions and holes
- If there are dents in the walls
- If the fan has deteriorated
- If the system has been changed, such as by adding equipment or by changing the operating conditions.
Contact Fluid Engineering today at 800-841-9944 with all your needs for all for Fisher Klosterman cyclones!
Source: Amrein, David. "Tips for selecting highly efficienct cyclones." Chemical Engineering, May 1995.