Environmental Science & Engineering - www.esemag.com - May 2004
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Computer simulation saves time and expense

By Sergei Burlatsky and
Richard F. Abrams, LSR Technologies Inc.

Computer simulation helped a manufacturer of an emissions control system to overcome a challenging problem, making it possible to get the design right the first time.

LSR Technologies, Inc. produces a centrifugal particulate emission control device that can often allow companies to meet US federal and local emissions requirements without the cost and problems associated with using fabric filters and wet scrubbers.

In scaling up their device, LSR engineers were concerned that the extra length of the separator might cause a reverse flow condition. To avoid the expense and lead-time involved in building and testing a prototype, they used computational fluid dynamics (CFD) to visualize the flow inside the separator. The simulation showed that simply scaling up their existing design would indeed create reverse flow, so they analyzed a number of alternative designs. The final design, which added a second cylinder to the main cylinder, worked well at the larger size.

The LSR Core Separator is a highperformance particulate control system designed to remove both micron and submicron particles entrained in gas streams. The technology overcomes the performance limitations of some cyclones by performing the tasks of separation and collection in two separate components.

Each unit has a single inlet for the stream to be treated and two outlets, one for the cleaned gas stream and the other containing a concentrated particulate recirculation stream. In operation, gas containing fine particulate matter is introduced through a tangential rectangular inlet. The particleladen gas develops a rotating flow pattern through the main annular chamber. Because of the centrifugal force on the rotating particles, they tend to move toward the outer wall of the cylinder. The particles then leave the cylinder through a tangential exit on the side opposite the inlet along with a small amount of the gas, which becomes the recirculation stream. The recirculation stream is directed to a conventional collector for removal of the particles from the system while the gas flows to the separator inlet. The dust particles recirculate repeatedly through the separator until they are collected. Clean gas leaves the cylinder through the "vortex finder" annular outlets at both ends of the cylinder.

The performance of the LSR Core Separator is comparable to that of more complicated and expensive technologies. This is because particle entrainment is reduced by the fact that the device functions only as a separator, avoiding the need for a change in flow direction. In addition, its smooth, cylindrical shape helps to prevent vortex formation.

The Core Separator is claimed to provide an unusually high price to performance ratio with a typical 94% to 99% overall collection efficiency and a cost of less than $3 (US) per cubic metre per hour. On the other hand, cyclones have a lower cost but less requirements. Baghouses, another primary alternative, offers only a slightly higher efficiency than the Core Separator, but at a higher cost. As a result, over sixty Core Separator systems have been placed in operation around the world.

E-mail: sfburlat@sjuphil.sju.edu

Computational Fluid Dynamics (CFD)
Figure 1: The geometry of the ElectroCore separator. Gas containing fine particulate matter (fly ash or dust) is introduced through a tangential rectangular inlet, shown in blue.
Figure 2: Velocity vectors throughout the device. The flow enters through the narrow rectangular inlet and swirls around the solid electrode before entering the smaller cylinder and moving axially toward the exit.
Figure 3: Crosssectional view. The in-plane velocity components are displayed. The main gas stream flows around the electrode in the middle, and part of the flow passes into the second chamber. The chambers are in a region where high fly ash/dust loading is expected.
Figure 4: Pressure distribution on the ElectroCore walls. Contours of static pressure on the walls of the separator show the axial pressure gradient that develops in the smaller cylinder as the particle-laden gas turns and flows towards the exit.
Figure 5: The efficiency of the particle tracks in the device colored by pressure. These illustrate both the swirling pattern in the main chamber as well as the axial pressure gradient in the smaller cylinder.
The illustrations show the detailed design information that was obtained from the CFD analysis. The flow enters through the narrow rectangular inlet and swirls around the solid central electrode before entering the smaller cylinder and moving axially toward the exit. The main gas stream flows around the electrode in the middle, and part of the flow passes into the second chamber. The flow that enters the smaller chamber has a high particle concentration, due in part to the centrifugal forces acting on the particles and in part to the radial electric field that results from the cylindrical anode at the core of the main cylinder and grounded cylinder walls. The passage between the two chambers is in a region where high fly ash/dust loading is expected. The effect of swirl on the flow field turbulence is included in the model.


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