Patent Application
20130277022
This invention relates to internal combustion engines and, in particular, to methods and apparatus for reducing exhaust emissions.
It is well known in the art to use exhaust gas recirculation (EGR) as a means of controlling the emissions of nitrous oxides (NOx) from internal combustion engines. In a typical EGR system, a portion of the exhaust gases (typically from 5 to 15%) is reintroduced into the induction system along with the fresh charge of air and fuel. The exhaust gas, which is essentially inert, displaces the amount of combustible mixture in a gasoline (Otto cycle) engine. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture. Because NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature, the lower combustion temperatures caused by the reduction in combustible mixture or excess oxygen reduces the amount of NOx the combustion produces.
In 2002, United States environmental protection agency implemented regulations that required exhaust gas recirculation coolers to be implemented in passenger vehicles and light trucks equipped with diesel engines as a means of further reducing the NOx emissions from these vehicles. Such exhaust gas recirculation coolers are typically of the gas-to-liquid heat exchanger variety and are most often of a shell-and-tube heat exchanger design in which the exhaust gas passes through a plurality of tubes encased in a shell through which the engine coolant circulates. U.S. Pat. No. 8,079,409 and U.S. Pat. No. 7,213,639 are typical of such exhaust gas recirculation cooler designs
Difficulties associated with exhaust gas recirculation coolers in diesel engines include the fact that reducing the combustion temperature increases the amount of soot formed by the combustion process. This soot tends to deposit in the tubes of the exhaust gas recirculation cooler where it acts as an insulating layer that reduces the thermal efficiency of the exhaust gas recirculation cooler. Additionally, if the engine coolant runs low, the heat exchanger may be starved of coolant and may experience a so-called “thermal event” in which the cooler tubes, heated nearly to the temperature of the exhaust gas, thermally expand to a degree that exceeds the structural integrity of the heat exchanger.
Various methods have been suggested to improve the longevity of exhaust gas recirculation coolers, including use of expansion joints, forming the tubes in the shape of elongated bellows and/or manufacturing the exhaust gas recirculation cooler as a series of short modules, each of which has a relatively small overall thermal growth. For example, U.S. Pat. No. 6,460,502 issued to Challis, suggests construction of an EGR cooler in which the shell portion includes a plurality of 90 degree bends formed as corrugated bellows. According to Challis, the bellows sections have increased compliance over a straight-walled shell and, therefore, the bellows provide for better accommodation of thermal expansion or other movements. U.S. Pat. No. 7,213,639 issued to Danielsson et al. suggests an EGR cooler in which the flow of the exhaust gas enters through a central row of tubes and exits through a peripheral row of tubes. According to Danielsson, the reversing flow reduces the risk of local hot spots due to stagnation of coolant flow.
The present invention comprises a heat exchanger for transferring heat between two fluids, for example between a hot exhaust gas and a liquid coolant. In one embodiment, the heat exchanger comprises a shell surrounding at least two tube bundles attached at both ends to a tube header. Each of the tube bundles is constructed from a plurality of individual tubes that are twisted into identical helixes formed about a common helical axis. Because each individual tube is formed in the shape of a helix, rather than as a straight tube, the individual tubes behave in a manner similar to a spring, rather than a column. Consequently, thermal elongation of the individual tubes is resolved primarily as an increase in the helical diameter of the tubes rather than an elongated column. This results in a considerably reduced axial force on the tube attachments and tube header. Moreover, since each tube is free to expand or contract with temperature, a single tube that is subjected to a thermal event will expand to relieve its own thermal stress. Accordingly, a heat exchanger constructed in accordance with the teachings of the invention is more resistant to failures caused by a thermal event than prior art heat exchangers with moveable headers in which the entire header must move as a unit and which, therefore, cannot accommodate a single tube that is expanding at a greater rate than the adjacent tubes. Additionally, a heat exchanger constructed in accordance with the teachings of the invention inherently promotes more turbulent flow of the coolant passing over the tubes than a comparable straight-tube heat exchanger. Additionally, because the geometry of the tubes is not parallel to the coolant flow, use of helical tubes reduces or eliminates the necessity of installing baffles and therefore reduces or eliminates the problems associated with baffles causing formation of eddie currents in the coolant.
In another embodiment, the two tube bundles are formed with opposite helical twists, e.g., the first tube bundle has tubes wound in a helix having a right-hand helix and the second tube bundle has tubes wound in a left-hand helix. This enables the tube bundles to be positioned with their helical axes closer to each other than would be possible if all of the tube bundles had the same direction of twist. In another embodiment, the heat exchanger is formed of several tube bundles arranged in a rectangular array with each tube bundle having the opposite twist from each of the adjacent tube bundles.
The present invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying drawing figures in which like references designate like elements and, in which:
The drawing figures are intended to illustrate the general manner of construction and are not necessarily to scale. In the detailed description and in the drawing figures, specific illustrative examples are shown and herein described in detail. It should be understood, however, that the drawing figures and detailed description are not intended to limit the invention to the particular form disclosed, but are merely illustrative and intended to teach one of ordinary skill how to make and/or use the invention claimed herein and for setting forth the best mode for carrying out the invention.
With reference to the figures and in particular
In the illustrative embodiment of
A shell 28 extends between bulkhead 16 and bulkhead 26 and is mechanically coupled to bulkhead 16 and to bulkhead 26 (e.g. by welding, brazing or similar rigid attachment) to form a fluid-tight seal between the bulkheads and the shell. Shell 28 is provided with a coolant inlet passage 30 and a coolant outlet passage 32 to enable a flow of coolant to flow into shell 28 past the tubes contained within shell 28 and then out of shell 28 to an external radiator or other means of discharging the heat rejected from tubes 20-24. Although in the illustrative embodiment of
With additional reference to
As discussed hereinbefore, because each individual tube 20, 22, 24 is formed in the shape of a helix, rather than as a straight tube, thermal elongation of the individual tubes is resolved primarily as an increase in helical diameter of the tubes rather than as a column elongation. This results in a considerably reduced axial force exerted by the tubes on bulkheads 16 and 26. For example, if a straight stainless steel 5/16 inch diameter tube having a length of 16.5 inches, a cross-sectional area of 0.01922 in2 is subjected to a 400° F. temperature change, if unconstrained, the length of the stainless steel tube will increase by 0.0653 inches (400° F.×9.9E6 in/in ° F.—the approximate thermal coefficient of expansion of stainless steel). If the tube is constrained by the bulkheads, the force exerted by the tube on the bulkheads is in excess of 2100 pounds.
If on the other hand the tube is twisted into a helix having a helical diameter of 0.361 inch and a helical pitch of 4.83 inches per revolution, then according to Hooke's law the force exerted by the tube on the bulkheads for the same 400° F. temperature change is reduced to slightly over 52 pounds, which is reduction is stress of more than 40:1. Because the helically wound tubes behave as coil springs, it should be observed that increasing the helical diameter and/or decreasing the helical pitch angle will cause a corresponding further reduction in the spring rate and, therefore, further reduce the stress on the bulkheads, while increasing the diameter and/or thickness of the tubes will cause a corresponding increase in the spring rate. Accordingly, variations in helical pitch, helical diameter, tube diameter, and tube thickness to accommodate the heat transfer, thermal expansion and other design constraints of a particular application are considered within the scope of the invention.
With additional reference to
With additional reference to
L2=(t+d)√{square root over (3)}
With additional reference to
Although certain illustrative embodiments and methods have been disclosed herein, it will be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods may be made without departing from the invention. For example, although in the illustrative embodiment each tube bundle is made from three individual tubes, bundles consisting of two tubes, three tubes, four tubes or more are considered within the scope of the invention. A three tube bundle is merely preferred because of the efficiency in space utilization inherent in a three tube bundle. Additionally, although the tubes forming the tube bundles in the illustrative embodiment are circular in cross section, tubes having non-circular cross sections may be advantageously used in a heat exchanger incorporating features of the present invention and therefore are considered within the scope of the invention. Also, it should be observed that although the helical axis of the tube bundles extend from bulkhead-to-bulkhead, it is not necessary that the tube bundles be continuously helical from bukhead-to-bulkhead as long as they are helical about a common helical axis over some portion of their length. Accordingly, it is intended that the invention should be limited only to the extent required by the appended claims and the rules and principles of applicable law. Additionally, as used herein, references to direction such as “up” or “down” are intend to be exemplary and are not considered as limiting the invention and, unless otherwise specifically defined, the terms “generally,” “substantially,” or “approximately” when used with mathematical concepts or measurements mean within ±10 degrees of angle or within 10 percent of the measurement, whichever is greater.
| Number | Date | Country | |
|---|---|---|---|
| 61635007 | Apr 2012 | US |
