GB2344161A

GB2344161A - Exhaust gas cooler

Info

Publication number: GB2344161A
Application number: GB9928056A
Authority: GB
Inventors: Masayoshi Usui; Hideo Ryuu
Original assignee: Usui Kokusai Sangyo Kaisha Ltd
Current assignee: Usui Kokusai Sangyo Kaisha Ltd
Priority date: 1998-11-27
Filing date: 1999-11-26
Publication date: 2000-05-31
Also published as: DE19957379A1; GB9928056D0

Abstract

A shell and tube EGR cooler 1 has heat exchanger tubes 2 arranged in a fixed state between tube sheets 3 provided in the interior of a shell, end caps 4 fixed to both end portions of the shell and is provided with recirculated exhaust gas inflow and outflow ports 4-1, 4-2. The heat exchanger tubes are formed so as to extend downward or upward along the circumference of the shell (see figs 2a and 2b).

Description

TITLE OF THE INVENTION: EGR COOLER BACKGROUND OF THE INVENTION: Field of the Invention: This invention relates to an EGR cooler using a liquid coolant for engines as a refrigerant.

Description of the Prior Art: A method of taking out a part of an exhaust gas from an exhaust system, and returning this exhaust gas to an engine to add the same to a gaseous mixture is called EGR (Exhaust Gas Recirculation). The EGR serves to obtain many effects including the effects of suppressing the occurrence of Nox (nitrogen oxide), reducing a pumping loss and a loss of heat radiation to a liquid coolant ascribed to a temperature drop of an exhaust gas, increasing a ratio of specific heat owing to the variation of amount and composition of an operating gas, and improving a cycle efficiency owing to the increase in the ratio of specific heat. Accordingly, it is said that the EGR is an effective method of improving the thermal efficiency of an engine.

However, it has been ascertained that, when both an EGR temperature and an EGR amount increase, the deterioration of the durability of an EGR valve, which causes the valve to be early broken in some cases, due to the thermal action of EGR, the necessity of provision of a water cooled structure for the prevention of early breakage of the EGR valve, and a decrease in the fuel consumption due to a decrease in a charging efficiency ascribed to an increase in the intake air temperature arise. In order to avoid such inconveniences, an apparatus for cooling an EGR gas with a liquid coolant for an engine is used.

A shell and tube heat exchanger is generally utilized as this apparatus.

A shell and tube heat exchanger utilized in this case is formed so as to have, as shown in Figs. 7 and 8 which illustrate an example of such a heat exchanger, a shell 21 provided at both end portions thereof with a cooling medium inflow port 21-1 and a cooling medium outflow port 21-2; heat exchanger tubes 22 provided in the interior of the shell 21 and formed of straight tubes both end portions of which are fixed by soldering to tube sheets 23 comprising a sheet metal and disposed with the outer circumferential end portions thereof fixed by soldering to the shell 21; an end cap 24 provided with a recirculated exhaust gas inflow port 24-1 and fixed to one end portion of the shell 21; an end cap 24 provided with a recirculated exhaust gas outflow port 24-2 fixed to the other end portion of the shell 21; and fastening flanges 25 fitted around and fixed to outer opened end portions of the gas inflow port 24-1 and gas outflow port 24-2 of the end caps 24.

Cooling a recirculated exhaust gas by using cooling water, such as a liquid coolant for an engine as a refrigerant in this conventional shell and tube heat exchanger will be discussed.

When the shell 21 is filled with cooling water, problems do not arise. For example, when a liquid coolant for an engine decreases to cause the water level in the shell 21 to lower, so that some of the heat exchanger tubes 22 which are in the vicinity of an inner circumferential surface of the shell 21 show up above the water surface to be exposed to vapor, the temperature of outer surfaces of these heat exchanger tubes rises to the level of that of the recirculated exhaust gas passing through the interior thereof. As a result, high thermal stress occurs, and the heat exchanger tubes are destroyed from soldered joint portions or expanded at the portions thereof which are between the tube sheets 23 to be buckled and bent, to cause the lifetime of the EGR cooler to be shortened. When the heat exchanger tubes show up over the whole length thereof above the water surface, they are not substantially cooled, so that the heat exchanging performance decreases.

SUMMARY OF THE INVENTION: The present invention has been developed with a view to solving the problems encountered in the above-described conventional shell and tube heat exchanger, and aims at providing an EGR cooler capable of preventing heat exchanger tubes from being buckled and bent and prolonging the lifetime of the cooler, by arranging straight heat exchanger tubes diagonally or using longitudinally bent heat exchanger tubes so that at least a part of each heat exchanger tube is positioned in the liquid coolant without causing the heat exchanger tubes to show up over the whole length thereof above the water level even when the level of the liquid coolant in a shell lowers from a certain cause; and capable of cooling a recirculated exhaust gas sufficiently without lowering the heat exchanging performance.

The EGR cooler according to the present invention comprises a substantially horizontally disposed shell and tube EGR cooler, and is mostly formed by arranging in a fixed state heat exchanger tubes on tube sheets fixed to the portions of an inner surface of a shell which are in the vicinity of both end portions thereof, which shell is provided with a cooling medium inflow port and a cooling medium outflow port in the portions thereof which are in the vicinity of both end portions thereof; fixing end caps to both end portions of the shell; providing a recirculated exhaust gas inflow port and a recirculated exhaust gas outflow port in the end caps; and fitting and fixing fastening flanges around and to outer opened end portions of the gas inflow port and outflow port of the end caps, the characteristics of this EGR cooler residing in the construction of the heat exchanger tubes arranged in a fixed state between both end tube sheets, a first mode of embodiment being characterized in that at least heat exchanger tubes provided on the upper side of the interior of the shell and in the vicinity of the inner surface thereof are formed of straight tubes arranged so as to extend downward or upward along the circumference of the shell and between both end tube sheets, a second mode of embodiment being characterized in that the same heat exchanger tubes as in the first mode of embodiment which comprise straight tubes, and which are arranged so as to extend downward or upward along the circumference of the shell and between both end tube sheets, are formed of heat exchanger tubes having at both end sections thereof straight portions extending in parallel with the axis of the shell, a third mode of embodiment being characterized in that at least heat exchanger tubes arranged on the upper side of the interior of the shell and in the vicinity of the inner surface thereof are formed of heat exchanger tubes extending downward or upward in the circumferential direction of the shell between both end tube sheets, or a combination of heat exchange tubes extending downward and those extending upward, in the circumferential direction of the shell between both end tube sheets, a fourth mode of embodiment being characterized in that the same heat exchanger tubes as in the third embodiment which comprise heat exchanger tubes fixed to both end tube sheets so as to extend downward or upward in the circumferential direction of the shell, or which comprise a combination of heat excgabger tubes fixed thereto so as to extend downward and those fixed thereto so as to extend upward, in the circumferential direction of the shell, are formed of heat exchanger tubes having bent, curved or straight portions, a fifth mode of embodiment being characterized in that the same heat exchanger tubes as in the third embodiment which comprise heat exchanger tubes arranged between both end tube sheets so as to extend downward or upward, or a combination of heat exchanger tubes extending downward and those extending upward, in the circumferential direction of the shell, and which have bent or curved portions, are formed of heat exchanger tubes having straight portions at both end sections thereof.

The purpose in the present invention of forming at least the heat exchanger tubes provided on the upper side of the interior of the shell and in the vicinity of the inner surface thereof so that they comprise straight tubes arranged so as to extend downward or upward along the circumference of a shell, or heat exchanger tubes extending downward or upward or both downward and upward in the circumferential direction of a shell, or heat exchanger tubes extending downward or upward or both downward and upward in the circumferential direction of a shell and having bent or curved portions, is to always leave at least a part of each heat exchanger tube in the water even when the level of a liquid coolant in the shell lowers from a certain cause.

Namely, when the surface of a liquid coolant in a shell lowers from a certain cause to a level at which the heat exchanger tubes in the vicinity of the inner surface of a shell show up above the water level therein, heat exchanger tubes comprising such straight tubes as are used in a prior art EGC cooler which are arranged so as to extend in parallel with the axis and inner surface of the shell are exposed over the whole length thereof to the outside of the liquid coolant, but heat exchanger tubes comprising straight tubes arranged so as to extend downward or upward along the circumference of a shell, or tubes having downwardly extending portions and/or bent or curved portions show up only at the portions of a certain length thereof above the water level, with at least a part of each of these heat exchanger tubes left immersed in the cooling water.

Consequently, the temperature of the outer surfaces of the tubes does not increase over the whole length thereof to that of a recirculated exhaust gas, and the expansion of the heat exchanger tubes can be suppressed.

The heat exchanger tubes having downwardly extending portions and/or upwardly extending bent or curved portions have, in addition to these effects, an effect of absorbing at the bent or curved portions thereof the elongation thereof, if any, occurring when the heat exchanger tubes extend due to the heat of a recirculated exhaust gas.

The shape of the heat exchanger tubes having bent or curved portions is not specially limite. The heat exchanger tubes having downwardly or upwardly extending bent or curved portions include sloping heat exchanger tubes in which the heights of both end portions thereof are set different, or V-shaped heat exchanger tubes having downwardly bent intermediate portions.

The heat exchanger tubes having a combination of downwardly and upwardly extending bent or curved portions include, for example, heat exchanger tubes having a mountain-shaped (upwardly bent) portion at one end section of. each thereof and a downwardly bent portion at the other end section of each thereof. The difference in height between both end portions of the tubes or the depth of the V-shaped recessed portions or the height of the mountain-shaped portions is set on the basis of a water level predicted in accordance with the dimensions of a EGR cooler in use or the volume of a shell and the length and diameter of the heat exchanger tubes.

As is clear from Fig. 5 which will be referred to later, the fixing of the heat exchanger tubes to tube sheets is done by employing a method comprising passing the end portions of the heat exchanger tubes through the through holes made in advance in the tube sheets, and soldering or welding the same end portions. Therefore, in the case of the heat exchanger tubes, which comprise straight tubes arranged so as to extend downward or upward along the circumference of a shell, out of the heat exchanger tubes in the present invention, both end portions thereof are passed diagonally through the through holes made in the tube sheets, but the soldering or welding of these end portions is done without any trouble. However, in order to fix the heat exchanger tubes to the tube sheets more easily and reliably, it is preferable to provide both end sections of the heat exchange tubes, which comprise straight tubes or bent portion-carrying tubes, with straight portions extending in parallel with the axis of the shell for carrying out a soldering or welding operation, and this enables the soldering strength or the welding strength of these end portions to be further improved.

BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1 is a partially cutaway front view showing an embodiment of the apparatus according to the present invention; Fig. 2 is a reference drawing of heat exchanger tubes, which are developed into plans, in this apparatus, wherein: Fig. 2 (a) is a reference drawing showing examples of heat exchanger tubes comprising straight tubes having no inflection point; and Fig. 2 (b) is a reference drawing showing examples of heat exchanger tubes comprising straight tubes provided with straight portions at both end sections thereof; Fig. 3 is a partially cutaway front view showing another example of the apparatus according to the present invention; Fig. 4 is a side view of this apparatus; Fig. 5 is a sectional view taken along the line V-V in Fig.

3, showing a case where the level of the cooling water in a shell of this apparatus lowers; Fig. 6 is a reference drawing of the same apparatus in which shapes to which the heat exchanger tubes in this apparatus are bent are developed into plans, wherein: Figs. 6 (a) and 6 (b) are reference drawings showing examples of heat exchanger tubes in which the heights of both end portions thereof are set different; Fig. 6 (c) is a reference drawing showing an example of a heat exchanger tube an intermediate portion of which is formed in the shape of the letter"V" ; and Fig. 6 (d) is a reference drawing showing an example of a heat exchanger tube having a processed portion between the upwardly and downwardly extending portions thereof; Fig. 7 is a longitudinally sectioned side view of an example of a conventional shell and tube EGR cooler, on which the present invention has been improved, with an intermediate portion thereof omitted; and Fig. 8 is a sectional view taken along the line VIII-VIII in Fig. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS: Referring to Figs. 1-6, reference numerals 1,11 denote a shell, 1-1,11-1 a cooling water inlet, 1-2,11-2 a cooling water outlet, 2,12 heat exchanger tubes, a reference numeral 2-1 heat exchanger tubes comprising straight tubes arranged so as to extend downward or upward along the circumference of the shell, reference numerals 12-1-12-5 heat exchanger tubes bent in the circumferential direction of the shell, 12-6 heat exchanger tubes comprising straight tubes, 3,13 tube sheets, 4,14 end caps, 4-1,14-1 a recirculated exhaust gas inflow port, 4-2,14-2 a recircualted exhaust gas outflow port, 5,15 fastening flanges, and a reference letter a a central angle of two straight crossing lines passing the axes of opposite heat exchanger tubes the uppermost portions of the outer circumferences thereof contact the water surface in the shell, and the axis of the shell.

Namely, the EGR cooler according to the present invention disposed substantially horizontally and shown in Figs. 1,2 (a) and 2 (b) has heat exchanger tubes arranged between and fixed to the tube sheets 3, and the heat exchanger tubes, which are positioned in the vicinity of the upper inner circumferential surface of the shell 1, out of the above-mentioned heat exchanger tubes are formed of heat exchange tubes 2-1 comprising straight tubes arranged so as to extend downward or upward between both end tube sheets and along the circumference of the inner surface of the shell 1, the other heat exchanger tubes being formed of straight tubes similar to those in a conventional EGR cooler.

The heat exchanger tubes 2-1 comprising the straight tubes shown in Figs. 2 (a) and 2 (b) are arranged with the heights of both tube end portions set different.

The heat exchanger tubes 2-1 comprising the straight tubes shown in Fig. 2 (a) are soldered or welded to the tube sheets 3 with both end portions thereof diagonally passed through the through holes made in the same tube sheets. When straight portions extending in parallel with the axis of the shell 1 are provided at both end sections of the heat exchanger tubes 2-1 as shown in Fig. 2 (b), the soldering or welding of these end portions to the tube sheets 3 is done more reliably, and the soldering or welding strength can be more improved.

In the EGR cooler having the construction shown in Figs.

1 and 2, a recirculated exhaust gas flowing from the inflow port 4-1 into the heat exchanger tubes 2 in the shell 1 is cooled with the cooling water supplied from the inlet 1-1 to the interior of the shell 1, and the cooled gas flows out from the outflow port 4-2 with the resultant cooling water discharged from the outlet 1-2. When the level of the cooling water in the shell 1 lowers due to a decrease in the quantity of the engine cooling water, the heat exchanger tubes 2-1 positioned in the vicinity of the upper inner surface of the shell 1 and comprising tubes arranged so as to extend downward or upward along the circumference of the inner surface of the shell 1 with the heights of both end portions of the tubes set different show up at the portions of the outer surfaces thereof which have a larger height above the lowered level of the cooling water in the shell 1 but the portions of the tubes which have smaller height remain in the water. Therefore, even when the portions of the tubes exposed to the upper side of the water level are heated with the recirculated exhaust gas, they are cooled by the portions of the tubes which remain in the water. Namely, the exposed portions are also cooled indirectly by heat transfer, and the temperature of these portions becomes considerably lower than that of the recirculated exhaust gas to enable the elongation of the heat exchanger tubes to be suppressed.

Consequently, the destruction, which is ascribed to the thermal expansion of the heat exchange tubes, of the soldered or welded portions of the heat exchanger tubes and tube sheets 3, and the bending and buckling of the heat exchanger tubes do not occur.

In the EGR cooler shown in Fig. 3, the heat exchanger tubes, which are out of heat exchanger tubes arranged between and fixed to tube sheets 13, and which are in the vicinity of an upper inner circumferential surface of a shell 11, are formed of heat exchanger tubes 12-1 bent in the same direction, i. e., downward at the portions thereof which are between both end tube sheets 13 in the circumferential direction along the inner surface of the shell 11 so as to set the heights of both tube end portions different, and having short straight portions 12-ls at both tube end sections, while the other heat exchanger tubes are formed of heat exchanger tubes 12-6 comprising the same straight tubes as are used in a conventional EGR cooler.

The above heat exchanger tubes 12-1 are manufactured by carrying out a mechanical bending process by a bending apparatus.

A bending angle of the heat exchanger tubes 12-1 formed by this bending process is preferably set to a level not lower than the central angle a, which is shown in Fig. 5, of two straight crossing lines passing the axes of the heat exchanger tubes the uppermost portions of the outer circumferential surfaces of which contact the water surface in the shell, and the axis of the shell. When the heat exchanger tubes in the vicinity of the inner circumferential surface of the shell 11 are formed of tubes bent at an angle which is, for example, two times this central angle a, the heat exchanger tubes most exposed to the outside of the level of the cooling water show up over at least a half of the axial length thereof above the level-lowered water surface in the shell 11 but the remaining half portions of the tubes remain in the water and do not show up above the water surface. As referred to above, the bending angle of the heat exchanger tubes is set on the basis of a water level predicted in accordance with the dimensions of the EGR cooler or the volume of the shell 11. Concretely speaking, the bending angle may be not less than a, and around 3a is preferable.

The heat exchanger tubes 12 formed of the heat exchanger tubes 12-6 comprising heat exchanger tubes 12-1, bent downward along the inner surface of the shell 11 in the circumferential direction thereof with the heights of both tube end portions set different and having the small straight tube portions 12-ls at both end portions, and straight tubes are fixed at both tube end portions thereof in the same manner as those shown in Fig.

1 to tube sheets 13 of sheet metal by soldering or welding in the interior of the shell 11 provided with cooling water inlet and outlet 11-1,11-2, while the tube sheets 13 are fixed at their outer circumferential end portions to the outer surface of the shell by soldering or welding. An end cap 14 provided with a recirculated exhaust gas inflow port 14-1 is fixed to one end portion of the shell 11, and an end cap 14 provided with a recirculated exhaust gas outflow port 14-2 the other end portion thereof, fastening flanges 5 being fitted around and fixed to outer opened end portions of the recirculated exhaust gas inflow and outflow ports 14-1,14-2 of the end caps 14.

In the case of the EGR cooler having the construction shown in Figs. 3-5, a recirculated exhaust gas flowing from the inflow port 14-1 into the interior of the shell 11 is cooled in the same manner as that in the EGR cooler shown in Fig. 1 with the cooling water supplie from the inlet 11-1 into the interior of the shell 11, and discharged from the outflow port 14-2 with the cooling water discharged from the outlet 11-2. When the level of the cooling water in the shell 11 in the cooler of Figs.

3-5 lowers due to a decrease in the quantity of the engine cooling water, since the heat exchanger tubes 12-1 positioned in the vicinity of the upper inner surface of the shell 11 are bent in the circumferential direction of the shell 11 so as to set the heights of both tube end portions different, the axial part of each of the heat exchanger tubes 12-1 which has a larger height also shows up above the lowered cooling water level in the shell 11 but the axial part of each thereof having a smaller height remains in the water. Accordingly, even when the parts of the heat exchanger tubes which show up above the water level are heated with the recirculated exhaust gas, they are cooled by the parts thereof which are immersed in the water, so that the exposed parts are cooled indirectly by the heat transfer to a temperature considerably lower than that of the recirculated exhaust gas to enable the elongation of the heat exchanger tubes to be suppressed. As a result, the destruction of the portions of the heat exchanger tubes which are soldered or welded to the tube sheets 13 and the bending and buckling of the heat exchanger tubes which are ascribed to the thermal expansion of these tubes do not occur.

The heat exchanger tubes 12-2 shown in Fig. 6 (a) which illustrates an example of a bent shape of heat exchanger tubes have both tube end sections of different heights. These heat exchanger tubes have straight tube portions 12-2s at both tube end sections thereof, and are bent so as to form linear slopes from larger height tube end sections to smaller height tube end sections. Therefore, even when the level of the cooling water in the shell 11 lowers to cause, for example, the larger height tube end sections of these heat exchanger tubes 12-2 to show up above the water level, the smaller height tube end sections sink under the water.

The heat exchanger tubes 12-3 shown in Fig. 6 (b) are modifie examples of the heat exchanger tubes of Fig. 6 (a), and have at both tube end sections thereof straight tube portions 12-3s longer than the straight tube portions 12-2s, these heat exchanger tubes 12-3 being bent so as to form linear or gently curved slopes extending from the higher straight tube portions 12-3s toward the lower straight tube portions 12-3s.

Accordingly, even when the level of the cooling water in the shell 11 lowers to cause, for example, the higher straight tube portions 12-3s of these heat exchanger tubes 12-3 to show up above the water level, the lower straight tube portions 123s sink under the water in the same manner as in the case of the heat exchanger tubes 12-2 shown in Fig. 6 (a).

The heat exchanger tubes 12-4 shown in Fig. 6 (c) are examples bent downward at intermediate portions thereof and having straight tube portions 12-4s at both tube end sections thereof. These heat exchanger tubes are bent at the intermediate portions thereof in the shape of the letter"V" in the axial direction thereof. when both tube end sections of these heat exchanger tubes 12-4 are provided in high positions, the intermediate portions thereof become low. Therefore, when the level of the cooling water in the shell 11 becomes lower than the positions of the straight tube portions 12-4s at both tube end sections, the V-shaped downwardly bent intermediate portions of the tubes sink under the water.

The heat exchanger tubes 12-5 shown in Fig. 6 (d) are examples of heat exchanger tubes having upwardly and downwardly bent portions, and straight tube portions 12-5s at both tube end sections thereof. These heat exchange tubes are bent so as to form mountain-shaped portions 12-5m at the higher tube end sections, and linear slopes extending toward tube end sections lower than the mountain-shaped portions 12-5m. When the level of the cooling water in the shell 1 lowers, the higher tube end sections of these heat exchanger tubes 12-5 show up above the water level, and the lower tube end sections thereof sink under the water. If the heat exchanger tubes should extend due to the heat of the recirculated exhaust gas, the elongation can be absorbed by the portions of large and small bending angles, i. e. three bent portions of each tube including the mountain-shaped portion 12-5m.

As described above, in the horizontally disposed EGR cooler according to the present invention, at least the heat exchanger tubes in the vicinity of the upper inner surface of the shell are formed of heat exchanger tubes comprising straight tubes arranged between tube sheets at both ends of the shell so as to extend downward or upward along the circumference of the shell, or heat exchanger tubes extending downward or upward or a combination of heat exchanger tubes extending downward and those extending upward, or heat exchanger tubes having circumferentially downwardly or upwardly extending bent or curved portions or a combination of circumferentially downwardly extending bent or curved portions and circumferentially upwardly bent or curved portions. Therefore, even when the level of the cooling water in the shell lowers from a certain cause to make the recirculated exhaust gas heat the portions of the heat exchanger tubes which show up above the water level, they are cooled by the portions of the same tubes which are under the water. Consequently, the exposed portions are also cooled indirectly, and the temperature thereof becomes considerably lower than that of the recirculated exhaust gas, so that the elongation of the heat exchanger tubes can be minimized. If the heat exchange tubes should extend due to the heat of the recirculated exhaust gas, the elongation can be absorbed by the bent or curved portions thereof. As a result, the destruction of the portions of the heat exchanger tubes which are soldered or welded to the tube sheets and the bending and buckling of these tubes which are ascribed to the thermal expansion of the same tubes can be prevented. This enables excellent effects to be displayed, i. e., a decrease in the heat exchanging performance of the tubes can be prevented even when the level of the cooling water in the shell lowers, and the lifetime of the cooler can be prolonged.

Claims

What is claimed is:

1. A substantially horizontally disposed shell and tube EGR gas cooler having heat exchanger tubes arranged in a fixed state on tube sheets fixed to the portions of an inner surface of a shell which are in the vicinity of both end portions thereof, which shell is provided with a cooling medium inflow port and a cooling medium outflow port in the portions thereof which are in the vicinity of both end portions thereof; end caps fixed to both end portions of said shell; a recirculated exhaust gas inflow port and a recirculated exhaust gas outflow port provided in said end caps; and fastening flanges fitted around and fixed to outer opened end portions of said gas inflow port and said gas outflow port of said end caps, wherein at least the heat exchanger tubes out of said heat exchanger tubes which are arranged on the upper side of the interior of said shell and in the vicinity of an inner surface thereof are formed of straight tubes arranged between said both end tube sheets so as to extend downward or upward along a circumference of said shell.

2. An EGR cooler according to Claim 1, wherein said heat exchanger tubes formed of heat exchanger tubes comprising straight tubes arranged between said both end tube sheets so as to extend downward or upward along the circumference of said shell have at both end sections thereof straight tube portions extending in parallel with the axis of said shell.

3. An EGR cooler according to Claim 1, wherein the fixing of said heat exchanger tubes to said tube sheets is done by soldering or welding.

4. A substantially horizontally disposed shell and tube EGR gas cooler having heat exchanger tubes arranged in a fixed state on tube sheets fixed to the portions of an inner surface of a shell which are in the vicinity of the end portions thereof, which shell is provided with a cooling medium inflow port and a cooling medium outflow port in the portions thereof which are in the vicinity of both end portions thereof; end caps fixed to both end portions of said shell; a recirculated exhaust gas inflow port and a recirculated exhaust gas outflow port provided in said end caps; and fastening flanges fitted around and fixed to outer opened end portions of said gas inflow port and said gas outflow port of said end caps, wherein at least the heat exchanger tubes out of said heat exchanger tubes which are arranged on the upper side of the interior of said shell and in the vicinity of an inner surface thereof are formed out of heat exchanger tubes arranged between said both end tube sheets so as to extend downward or upward in the circumferential direction of said shell, or a combination of heat exchanger tubes arranged therebetween so as to extend downward in said direction and those arranged therebetween so as to extend upward in said direction.

5. An EGR cooler according to Claim 4, wherein said heat exchanger tubes arranged in a fixed state between said both end tube sheets and comprising heat exchanger tubes extending downward or upward in the circumferential direction of said shell, or a combination of heat exchanger tubes extending downward in said direction and heat exchanger tubes extending upward in said direction are formed of heat exchanger tubes having bent, curved or straight tube portions.

6. An EGR cooler according to Claim 4, wherein said heat exchanger tubes formed of heat exchanger tubes arranged between said both end tube sheets, extending downward or upward and having bent or curved portions, or a combination of heat exchanger tubes arranged therebetween, extending downward and having bent or curved portions and heat exchanger tubes arranged therebetween, extending upward and having bent or curved portions, have straight tube portions at both end sections thereof.

7. An EGR cooler according to Claim 5 or 6, wherein said heat exchanger tubes extending downward or upward and having bent or curved portions are formed so as to slope with the heights of both end portions thereof set different, or so as to be bent downward at the intermediate portions thereof in the shape of the letter"V".

8. An EGR cooler according to Claim 5 or 6, wherein said heat exchanger tubes having a combination of heat exchanger tubes extending downward and having bent or curved portions and heat exchanger tubes extending upward and having bent or curved portions are bent in the shape of a mountain (upward) at one end portion of each thereof, and downward at the other end portion of each thereof.

9. An EGR cooler according to Claim 4, wherein the fixing of said heat exchanger tubes to said tube sheets is done by soldering or welding.

10. An EGR cooler according to Claim 5 or 6, wherein a bending angle of said heat exchanger tubes is set not smaller than a central angle made by two straight crossing lines passing the axes of heat exchanger tubes the uppermost portions of the outer circumferential surfaces of which contact the water surface in said shell, and the axis of said shell.