This invention relates generally to smoothing gas velocity distribution in ducts and pipes, and more specifically relates to the use of perforated plates to achieve a more even velocity profile.
Gas flowing in ducts and pipes frequently exhibits uneven velocity distribution across the duct or pipe, with eddy currents and backflow often being observed. In power plants and chemical processing equipment it is often desirable to have an even gas velocity profile such as when the gas stream is subsequently being processed in a contacting device or a mist eliminator or when corrosive liquids exist downstream and backflow in the duct or pipe can result in excessive corrosion of the duct or pipe.
A common approach to smoothing the velocity profile and preventing backflow is to position a perforated plate across the flow in the duct or pipe, typically consisting of a flat plate with circular holes. The effect of the perforated plate is to distribute the gas more evenly across the plate since the pressure drop across an individual hole is proportional to the square of the velocity of the gas flowing through the hole so the gas molecules will tend to flow to holes with less flow until all the holes have approximately the same flow. As a result, the gas on the downstream side of the perforated plate will have a more uniform velocity profile.
Generally, the degree of velocity profile smoothing is a function of the pressure drop through the perforated plate with higher pressure drops resulting in more even velocity profiles downstream of the perforated plate. However, there are two general areas where the common approach of inserting a perforated plate with round holes does not result in a satisfactory outcome. The first general area where perforated plates with round holes are not satisfactory occurs in cases where the available pressure drop in the system for the perforated plate is relatively low, for example, less than two inches of water gauge, and the average gas velocity in the duct or pipe is greater than approximately ten feet per second. By “available pressure drop” is meant the pressure drop across the perforated plate that is tolerable when one takes into consideration the remaining pressure drops which are incurred in the system in order to perform its essential functions. In this case using perforated plates with round holes can often not produce an acceptable degree of velocity profile smoothing since the round hole size or diameter must be so large to avoid a pressure drop of two inches of water gauge or greater that there are not enough holes over a given plate area to adequately smooth the velocity profile.
The second general area where perforated plates with round holes are not satisfactory occurs in cases where there is adequate available pressure drop in the system for a perforated plate with round holes to adequately smooth the gas velocity profile and the gas velocity is greater than approximately ten feet per second. In this case the amount of pressure drop introduced by a perforated plate with round holes is so great that a substantial economic penalty results from the increased operating costs arising from the energy lost by the system as a result of the pressure drop across the perforated plate, and potentially also because of the increased capital costs due to the need for larger or incremental equipment to raise the system pressure back up to where there is sufficient pressure remaining downstream of the perforated plate for the equipment located there to perform adequately, such as to have adequate pressure to push the gas through sieve trays in gas treatment towers.
In the present invention, a perforated plate with a different hole type is used, which results in smaller holes for a given total open area, enabling acceptable gas velocity smoothing to be achieved at a lower pressure drop than would be achievable with round holes. Alternately, use of this different hole type results in a lower pressure drop than would be incurred with round holes in cases where the round holes can achieve adequate velocity profile smoothing. In a typical example the present invention can be used to smooth the velocity profile of a sulfur-containing flue gas entering a scrubbing tower in a flue gas desulfurization (“FGD”) unit where the flue gas is contacted with calcium carbonate slurry and the total pressure drop available for the system is relatively small. However the present invention is not limited to use with any specific type of gas treatment tower or other processing equipment; thus it can be used anywhere gas is flowing through a duct or pipe with an uneven velocity profile and an even velocity profile is desired, but the amount of pressure drop available for such smoothing is limited and use of perforated plates with round holes is uneconomical or infeasible. This applicability of this invention is particularly important where average gas velocities greater than ten feet per second occur in the duct or pipe.
The invention is diagrammatically illustrated, by way of example, in the drawings appended hereto, in which:
In accordance with the present invention a method and system is disclosed in which a perforated plate is used for smoothing the gas velocity profile in a duct or pipe, wherein the plate openings are hexagonal in shape, enabling a smaller hole size in the plate for a given total open area and pressure drop across the plate, thereby enabling use of the perforated plate in ducts and pipes with higher gas velocities. The openings are arranged in rows and columns on the plate and each hexagonal opening is oriented with respect to each of its neighboring hexagonal openings so that opposed flat sides of neighboring openings are parallel. Consequently the closed area of the plate defined between the flat sides of the neighboring openings is a continuous strip of constant width corresponding to the distance between the adjacent flat sides of neighboring openings.
While the present invention has been particularly set forth in terms of specific embodiments thereof, it will be understood in view of the present disclosure, that numerous variations on the invention are now enabled to those skilled in the art, which variations yet reside within the scope of the present teaching. For example, an enhancement of the preferred embodiment is to round the intersections of the flat sides of the hexagonal holes thereby increasing the structural strength and integrity of the plate without appreciably degrading its gas velocity profile smoothing characteristics. Such rounding helps avoid stress cracking at the juncture of the flat sides and need only involve a few percent of the length of a given flat side, for example 5%, to achieve this benefit.
Accordingly, the invention is to be broadly construed and limited only by the scope and spirit of the disclosure and of the claims now appended hereto.
This application claims priority from U.S. provisional patent application Ser. No. 60/993,698, filed on Sep. 12, 2007.
Number | Date | Country | |
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60993698 | Sep 2007 | US |