Gas turbine rotor blade platform

BACKGROUND OF THE INVENTION:

Field of the Invention:

The present invention relates to a gas turbine moving blade platform constructed so as to enhance a cooling performance thereof.

Description of the Prior Art:

Fig. 8 is a cross sectional view of a representative prior art gas turbine moving blade platform. In Fig. 8, numeral 80 designates a platform in its entire form and numeral 51 designates a moving blade of first stage. Numeral 52 designates a leading edge passage of the moving blade 51 and cooling passages 83, 84 are provided to this leading edge passage 52 communicating therewith and extending toward respective side portions of the platform 80. The cooling passages 83, 84 connect to cooling passages 85, 86, respectively, of both side portions and the cooling passages 85, 86 open at a rear end of the platform 80, respectively.

In a front portion of the platform 80, there are provided cooling passages 87 and 88, 89 and 90, respectively, on both sides thereof and these cooling passages 88 to 90 are bored inclinedly from a lower surface toward an upper surface of the platform 80 to open at the upper surface so that cooling air is blown therefrom. Also, in a rear portion of the platform 80, there are bored cooling passages 91, 92, 93 to extend likewise inclinedly from the lower surface toward the upper surface of the platform 80 and to open at the rear end thereof so that the cooling air is blown therefrom.

Further, in a central portion of the platform 80, there are provided cooling passages 94, 95, 96, 97, 98 and these cooling passages are also bored inclinedly from the lower surface toward the upper surface of the platform 80 so that the cooling air is blown from the upper surface, wherein an outlet end portion of each of the cooling passages 94 to 98 is worked to enlarge in a funnel-like shape so that the cooling air is diffused at the upper surface.

Fig. 9 is a contracted cross sectional view taken on line H-H of Fig. 8, wherein the cooling passages 85, 86 are provided in both side portions of the platform 80 and the cooling passage 97 is bored inclinedly from the lower surface toward the upper surface of the platform 80.

Fig. 10 is a contracted cross sectional view taken on line I-I of Fig. 8, wherein there are provided the cooling passage 85 extending from the front portion toward the rear portion of the plat form 80 to open at the rear end and the cooling passages 87, 94 to 98 extending inclinedly, so that the cooling air is blown therethrough rearwardly and upwardly, respectively.

In the platform 80 constructed as above, cooling air which has been supplied into the moving blade 51 through the leading edge passage 52 flows portionally into the cooling passages 85, 86 for cooling of both side portions of the platform 80 to then flow out of the rear end of the platform 80. Also, the cooling passages 87 to 90, 91 to 93, respectively, are provided inclinedly in the front and rear portions of the platform 80 so that cooling air is introduced thereinto from the lower surface of the platform 80 to flow out of the upper surface of the front and rear end portions of the platform 80. Further, the cooling passages 94 to 98 are provided inclinedly in the central portion and cooling air flows therethrough from the lower surface of the platform 80 to flow out of the upper surface thereof. Thus, the entire portion of the platform 80 is cooled by the cooling air flowing therein and flowing out thereof.

In the representative prior art gas turbine moving blade platform as described above, there are provided linearly extending main cooling passages of the cooling passages 85, 86, and in addition thereto, there are provided a multiplicity of cooling passages of the cooling passages 87 to 90, 91 to 93, etc. which pass through the platform 80 inclinedly and thus have a comparatively long inclined route. Hence, in the platform 80, there are provided many such cooling air supply passages and work process of the platform itself becomes complicated, and a cooling structure of platform which can be made simpler and still has an excellent cooling effect to cool an entire portion of the platform uniformly has been expected.

SUMMARY OF THE INVENTION:

It is therefore an object of the present invention to provide a gas turbine moving blade platform in which supply passages and flow passages of platform cooling air are simplified so that work process of the platform is facilitated as well as an entire portion of the platform is cooled uniformly with result that a cooling effect thereof is enhanced.

In order to achieve said object, the present invention provides means of following (1) to (6): <ul id="ul0001" list-style="none" compact="compact"><li>(1) A gas turbine moving blade platform characterized in comprising a cavity formed in the platform around a base portion of the moving blade for introducing thereinto a cooling air; and a plurality of cooling holes communicating with said cavity and opening at a peripheral end surface of said platform.</li><li>(2) A gas turbine moving blade platform as mentioned in (1) above, characterized in that said plurality of cooling holes are provided inclinedly upwardly toward said peripheral end surface of the platform from said cavity.</li><li>(3) A gas turbine moving blade platform as mentioned in (1) above, characterized in that there is provided an impingement plate at a bottom portion of said cavity for introducing therethrough the cooling air into said cavity.</li><li>(4) A gas turbine moving blade platform as mentioned in (1) above, characterized in that there is provided a cooling hole passing through the platform inclinedly, communicating at its one end with said cavity and opening at its the other end at an upper surface of the platform.</li><li>(5) A gas turbine moving blade platform characterized in comprising two cooling passages, each being provided in the platform on each side of the moving blade, communicating at its one end with a leading edge passage of the moving blade and having at its the other end an opening at a side end surface of the platform; a cover for closing said opening of each of said two cooling passages; and at least three linearly formed cooling passages in the platform, each communicating at its one end with any one of said two cooling passages and having at its the other end an opening at a rear end surface of the platform.</li><li>(6) A gas turbine moving blade platform characterized in that the platform consists of an upper platform and a lower platform, there is formed a cavity between said upper platform and lower platform on each side of ventral and dorsal sides of the moving blade, and characterized in comprising a cooling passage, being bored in said upper platform along each of both side portions of said upper platform, communicating at its one end with said cavity at a front portion of the platform and having at its the other end an opening at a rear end surface of the platform; and a multiplicity of cooling holes, being bored in said lower platform and passing through upwardly into said cavity thereabove from a bottom surface of said lower platform.</li></ul>

In the platform of (1) above, the cooling air flows into the cavity formed around the moving blade and the platform around the moving blade forms almost the entire portion of the cavity, thereby substantially the entire platform is cooled uniformly by this cavity. Further, there are provided the plurality of cooling holes, communicating with the cavity, at the peripheral portions of the platform and the cooling air flows out thereof while cooling the peripheral portions. Thus, by the effect of the cavity and the cooling holes of the peripheral portions, the entire portion of the platform is cooled uniformly. Further, the complicated and lengthy cooling passages as seen in the prior art are eliminated and such a simple structure is realized as having only the cavity and the short cooling holes of the peripheral portions, wherein the supply source of the cooling air to the cooling holes is the cavity only, hence the work of the platform becomes facilitated.

In the platform of (2) above, the cooling holes of (1) above are provided inclinedly, thereby cooling effect in the thickness direction at the peripheral portions of the platform is increased. In the one of (3) above, the cooling air flows into the cavity through the impingement plate, thereby the cooling of the cavity is done efficiently by the effect of the impingement cooling. Also, in the one of (4) above, the cooling holes are provided not only at the peripheral portions but also in the upper surface of the central portion of the platform, thereby the cooling of the platform is done further effectively.

In the invention of (5) above, in order to simplify the platform cooling structure, number of the linearly formed cooling passages is increased to three or more, which is more than in the prior art, with the peripheral cooling holes or the lengthy cooling passages being omitted instead, so that the cooling function of the above-mentioned cavity or cooling holes is effected by the increase of the linear cooling passages. Further, the cooling passages communicating with the leading edge passage of the moving blade are constructed simply to pass through the platform to open at both side end surfaces thereof and the opening portions are closed by the covers, thus the workability thereof is enhanced. By such construction, the platform is made in a structure in which the work process is easy and still the cooling performance is ensured.

In the invention of (6) above, the cavity is formed between the upper and lower platforms and the cooling air is introduced into the cavity, thereby the entire plane portion of the platform is cooled and both of the side end portions of the platform are cooled by the cooling passages. The cooling air flows into the cavity from the inner side (rotor side) of the platform through the multiplicity of holes provided in the lower platform. The cooling air which has entered the cavity flows through the cavity toward the front portion of the platform to enter the cooling passages provided on both sides of the moving blade along both of the side portions of the upper platform and then flows out of the rear end surface of the upper platform.

The platform is constructed by the cavity formed between the upper and lower platforms, the cooling passages of both side portions of the upper platform and the multiplicity of holes of the lower platform, wherein the complicated and inclined passages as seen in the prior art platform cooling structure are eliminated, thereby a simple structure is realized, workability thereof is enhanced and the platform is cooled uniformly with enhanced cooling effect.

BRIEF DESCRIPTION OF THE DRAWINGS:

Fig. 1 shows a gas turbine moving blade platform of a first embodiment according to the present invention, wherein Fig. 1(a) is a plan view of the platform and Fig. 1(b) is a cross sectional view taken on line A-A of Fig. 1(a).

Fig. 2 shows a gas turbine moving blade platform of a second embodiment according to the present invention, wherein Fig. 2(a) is a plan view of the platform and Fig. 2(b) is a cross sectional view taken on line B-B of Fig. 2(a).

Fig. 3 shows a gas turbine moving blade platform of a third embodiment according to the present invention, wherein Fig. 3(a) is a plan view of the platform, Fig. 3(b) is a cross sectional view taken on line C-C of Fig. 3(a) and Fig. 3(c) is a cross sectional view taken on line D-D of Fig. 3(a).

Fig. 4 shows a gas turbine moving blade platform of a fourth embodiment according to the present invention, wherein Fig. 4(a) is a plan view of the platform and Fig. 4(b) is a cross sectional view taken on line E-E of Fig. 4(a).

Fig. 5 shows a gas turbine moving blade platform of a fifth embodiment according to the present invention, wherein Fig. 5(a) is a plan view of the platform and Fig. 5(b) is a cross sectional view taken on line F-F of Fig. 5(a).

Fig. 6 is a plan view of a lower platform of the platform of Fig. 5.

Fig. 7 is a contracted cross sectional view taken on line G-G of Fig. 5(a).

Fig. 8 is a cross sectional view of a representative prior art gas turbine moving blade platform.

Fig. 9 is a contracted cross sectional view taken on line H-H of Fig. 8.

Fig. 10 is a contracted cross sectional view taken on line I-I of Fig. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS:

Herebelow, embodiments according to the present invention will be described concretely with reference to figures. Fig. 1 shows a gas turbine moving blade platform of a first embodiment according to the present invention, wherein Fig. 1(a) is a plan view of the platform and Fig. 1(b) is a cross sectional view taken on line A-A of Fig. 1(a).

In Fig. 1(a), numeral 1 designates a platform and numeral 51 designates a moving blade. Numeral 2 designates a cavity, which is formed in the platform 1 on one side portion thereof. Numerals 3, 4 designate also cavities, which are formed in the platform 1 on the other side portion thereof. Numerals 5, 6, 7, 8 designate a plurality of rows of cooling holes, respectively. The cooling holes 5 are bored in a periphery of said one side portion of the platform 1 inclinedly in communication with the cavity 2 so that cooling air is blown therethrough inclinedly upwardly, as described later. The cooling holes 6 are provided in communication with the cavity 3 so that the cooling air is blown therethrough likewise inclinedly upwardly in said the other side portion of the platform 1 and the cooling holes 7, 8 are provided in communication with the cavity 4 so that the cooling air is blown therethrough inclinedly upwardly in said the other side portion and a rear end portion, respectively, of the platform 1.

Numeral 9, 10 also designate cooling holes, which are provided on both sides of a ventral side and a dorsal side of the moving blade 51 in a central portion of the platform 1 so that the cooling air is blown therethrough likewise inclinedly upwardly. In an upper end portion of each of the cooling holes 9, 10, there is formed an enlarged portion in a funnel-like shape, as shown by numerals 9a, 10a, so that the cooling air diffuses therefrom on an upper surface of the platform 1.

In Fig. 1(b) which is a cross sectional view taken on line A-A of Fig. 1(a), there are formed the cavities 2, 4 in the platform 1, and therebelow fitted is an impingement plate 11 for closing the cavities 2, 4. Cooling air 70 is introduced through multiplicity of holes 12 provided in the impingement plate 11 so that the cavities 2, 4 are cooled by an impingement cooling. The cavity 3 is also fitted with the impingement plate 11 to be cooled by the impingement cooling.

On one side of the platform 1, there are provided the cooling holes 5 communicating with the cavity 2 and extending inclinedly upwardly to open at a side end of said one side of the platform 1 for blowing the cooling air inclinedly upwardly and the cooling holes 9 for blowing the cooling air likewise inclinedly upwardly at the central portion of the platform 1.

Also, on the other side of the platform 1, there are provided the cooling holes 7 extending inclinedly upwardly to open at a side end of said the other side of the platform 1 for blowing the cooling air inclinedly upwardly and the cooling holes 10 for blowing the cooling air likewise inclinedly upwardly at the central portion of the platform 1.

In the platform 1 of the first embodiment constructed as above, the cooling air 70 flows into the cavities 2, 3, 4 from a blade root portion of the moving blade 51 through the holes 12 of the impingement plate 11 for effecting the impingement cooling of these portions of the cavities, thereby the main portions around the moving blade 51 of the platform 1 is cooled uniformly. The cooling air further flows inclinedly through the cooling holes 5, 6, 7, 8 from said cavities 2, 3, 4 to flow out inclinedly upwardly of both side portions and rear portion of the platform 1 while cooling respective peripheral portions of the platform 1 from lower portions to upper portions thereof.

Thus, according to the platform 1 as mentioned above, the complicated passages as have been seen in the prior art are eliminated and the construction of the platform 1 is made such that main portions of the platform 1 are cooled entirely uniformly by the cavities 2, 3, 4 and the impingement plate 11 and the peripheral portions are cooled by the cooling air being flown out of the cavities 2, 3, 4, respectively, through the multiplicity of cooling holes 5 to 10 extending inclinedly upwardly in a comparatively short length, thereby the work process of the platform 1 becomes simplified and the entire portions including the peripheral portions of the platform 1 can be cooled uniformly without employing complicated and lengthy cooling passages.

Fig. 2 shows a gas turbine moving blade platform of a second embodiment according to the present invention, wherein Fig. 2(a) is a plan view of the platform and Fig. 2(b) is a cross sectional view taken on line B-B of Fig. 2(a). In Fig. 2(a), numeral 21 designates a platform, numerals 22, 23, 24 designate cavities formed in the platform 21 and numeral 25 designates cooling holes, which are formed on one side portion of the platform 21 in communication with the cavity 22, so that cooling air is blown therethrough inclinedly upwardly at a side end of said one side portion of the platform 21, as described later. Numerals 26, 27 also designate cooling holes, which communicate with the cavities 23, 24, respectively, on the other side portion of the platform 21 so that the cooling air is blown therethrough likewise inclinedly upwardly.

Numeral 28 designates also a cooling hole, which is formed in a single piece in communication with the cavity 22 so that the cooling air is blown therethrough inclinedly upwardly at a rear portion of the platform 21. In this rear portion of the platform 1, there is provided no other cooling hole in consideration of ease of work process.

In Fig. 2(b) which is a cross sectional view taken on line B-B of Fig. 2(a), there are formed the cavities 22, 24 in the platform 21 and the cooling holes 25, 27 are bored in both side end portions of the platform 1 communicating with the cavities 22, 24, respectively, and extending inclinedly upwardly to open at both side ends thereof, so that the cooling air is blown therefrom inclinedly upwardly.

In the platform 21 of the second embodiment constructed as above, there is provided no such impingement plate 11 as in the first embodiment and further the cooling hole 28 in the rear portion of the platform 21 is provided in single piece only, thereby the work process of the platform 21 is simplified greatly. The cooling air 70 flows directly into the cavities 22, 23, 24, respectively, to fill therein for cooling these portions of the cavities uniformly and then flows inclinedly upwardly through the cooling holes 25, 26, 27 of both side portions of the platform 21 and through the single cooling hole 28 of the rear portion thereof for cooling of the respective portions therearound to then flow out thereof.

The platform 21 of the second embodiment is effective for the case where a main flow gas of gas turbine is of a comparatively low temperature, wherein the cooling of the rear portion of the platform is done mainly by the cavity 24 so that the cooling hole in the rear portion thereof is made in a necessary minimum number for enhancement of the workability and still the cooling effect of the cavities 22, 23, 24 is sufficient for effecting the same uniform cooling of the platform as that effected by the first embodiment.

Fig. 3 shows a gas turbine moving blade platform of a third embodiment according to the present invention, wherein Fig. 3(a) is a plan view of the platform, Fig. 3(b) is a cross sectional view taken on line C-C of Fig. 3(a) and Fig. 3(c) is a cross sectional view taken on line D-D of Fig. 3(a). In Fig. 3(a), numeral 31 designates a platform, numeral 51 designates a moving blade and numerals 32, 33, 34 designate cavities formed in the platform 31. Numeral 38 designates cooling holes, which are bored in a rear portion of the platform 31 communicating with the cavity 34 and extending inclinedly upwardly from a lower surface of the platform 31 to open at a rear end thereof, like the cooling holes 8 of the first embodiment and the cooling hole 28 of the second embodiment, and numeral 39 also designates a cooling hole bored in the rear portion of the platform 31 communicating with the cavity 32 and extending inclinedly upwardly.

In Fig. 3(b) which is a cross sectional view taken on line C-C of Fig. 3(a), there are formed the cavities 32, 34 in the platform 31. Also, in Fig. 3(c) which is a cross sectional view taken on line D-D of Fig. 3(a), there are bored the cooling holes 38 and the cooling hole 39 in the rear portion of the platform 31.

In the platform 31 of the third embodiment described above, in further consideration of the workability than in the second embodiment, all the cooling holes of both side portions of the platform are omitted and only the cooling holes 38, 39 are provided in the rear portion only.

In the platform 31, cooling air 70 flows into the cavities 32, 33, 34, respectively, and thereby approximately the entire portion of the platform 31 is cooled uniformly. That is, the platform 31 of the third embodiment is appropriate for the case where requirement of the cooling of the platform is almost satisfied by the cavities 32, 33, 34 and especially the cooling of the rear portion and ambient portion thereof of the platform is aimed to be strangthened. Thus, the platform 31 is used effectively for said case, so that uniform cooling of the platform 31 is attained as well as there is obtained a further advantage in the workability than in the second embodiment.

Fig. 4 shows a gas turbine moving blade platform of a fourth embodiment according to the present invention, wherein Fig. 4(a) is a plan view of the platform and Fig. 4(b) is a cross sectional view taken on line E-E of Fig. 4(a). In Fig. 4(a), numeral 41 designates a platform and numeral 51 designates a moving blade. Numerals 42, 43 designate cooling passages, which are provided in communication with a leading edge passage 52 of the moving blade 51. The cooling passages 42, 43 are bored from respective side ends of the platform 41 to pass through the respective side portions for ease of the work process and covers 42a, 43a are attached to opening portions thereof, respectively, so as to close the respective side ends.

There are provided two cooling passages 45, 46 in one side portion of the platform 41 and the cooling passage 42 communicates with the cooling passages 45, 46. Also, there is provided a cooling passage 44 in the other side portion of the platform and the cooling passage 43 communicates with the cooling passage 44. The cooling passages 44, 45, 46 are constructed to open at a rear end surface of the platform 41 so that cooling air flows out thereof. In Fig. 4(b), arrangement of the cooling passages 44, 45, 46 is shown and cooling of the platform 41 is effected by the cooling passages 44, 45, 46, not by the cavities as employed in the first to third embodiments.

In the platform 41 as mentioned above, cooling air for cooling the moving blade 51 is led portionally into the cooling passages 42, 43 from the leading edge passage 52 of the moving blade 51 to flow through the linearly formed cooling passages 44, 45, 46 so that entire portion of the platform 41 is cooled, and there is provided no such cooling passage as provided inclinedly in the prior art nor there is provided such cooling holes in the peripheral portions as those employed in the first to third embodiments with result that the workability thereof is optimized.

According to the platform 41 of the fourth embodiment, both of the side end portions of the platform 41 are cooled by the cooling passages 44, 45 and the central portion thereof is cooled by the cooling passage 46. Although the platform 41 is inferior to the first to third embodiments in the cooling performance, if the workability is considered, it is the best embodiment. It is to be noted that although the cooling passage 46 has been described with respect to the example of the single passage at the central portion, two or more passages thereof are more preferable if such is allowable in terms of the design of the platform.

A fifth embodiment according to the present invention will be described with reference to Figs. 5 to 7. Fig. 5 shows a gas turbine moving blade platform of the fifth embodiment, wherein Fig. 5(a) is a plan view thereof and Fig. 5(b) is across sectional view taken on line F-F of Fig. 5(a).

In Figs. 5(a) and (b), numeral 61a designates an upper platform and numeral 61b designates a lower platform. The platform consists of the upper platform 61a and the lower platform 61b as shown in Fig. 5(b). Numerals 62, 63 designate cavities, which are formed between the upper and lower platforms 61a, 61b on both sides of a moving blade 51. Numerals 64, 65 designate cooling passages, which are bored in the upper platform 61a along both side portions thereof and connect at one end thereof to holes 64a, 65a, respectively, at a front portion of the platform and open at the other end thereof at a rear end surface of the platform. The holes 64a, 65a extend vertically in the front portion of the platform to pass through a portion of the upper platform 61a to communicate with the cavities 62, 63.

As shown in Fig. 5(b), the platform consisting of the upper platform 61a and the lower platform 61b is disposed such that respective side ends of the upper platform 61a and the lower platform 61b stand closely to respective side ends of an upper platform 61a' and a lower platform 61b' of a moving blade, which is adjacent to the moving blade 51 in a blade rotational direction, with a seal pin 60 being disposed therebetween. In the lower platform 61b, there are bored a multiplicity of holes 66a, 66b passing through into the cavities 62, 63 from an inner side thereof (rotor side).

Fig. 6 is a plan view of the lower platform 61b of the above-mentioned platform. As shown there, in an entire plane portion of the lower platform 61b, there are bored arrayedly the multiplicity of holes 66a, 66b passing through into the cavities 62, 63, respectively.

Fig. 7 is a contracted cross sectional view taken on line G-G of Fig. 5(a). In Fig. 7, as already described in Figs. 5 and 6, there are bored in the upper platform 61a the cooling passage 64 extending in the front and rear direction and the hole 64a extending vertically for connecting the cooling passage 64 and the cavity 62 in the front portion of the upper platform 61a. In the lower platform 61b, there are provided arrayedly the multiplicity of holes 66a passing through into the cavity 62 from the inner side (rotor side). Numerals 67, 68 designate seal plates provided at the front and rear portions of the platform for sealing the interior thereof.

In the platform constructed as mentioned above, as shown in Fig. 5(b), cooling air 70 flows into the cavities 62, 63 from the inner side (rotor side) of the moving blade via the multiplicity of holes 66a, 66b of the lower platform 61b to flow toward the front portion of the platform while cooling inner wall surfaces of the cavities 62, 63 uniformly and then flows into the cooling passages 64, 65 provided in the side end portions of the upper platform 61a via the holes 64a, 65a provided in the upper platform 61a.

According to the platform of the fifth embodiment as described above, the platform is constructed by the upper and lower platforms 61a, 61b, the cavities 62, 63 are formed therebetween and there are provided the cooling passages 64, 65 in the upper platform 61a on both side portions thereof as well as the multiplicity of holes 66a, 66b arrayedly in the entire plane portion of the lower platform 61b passing through into the cavities 62, 63 from the inner side (rotor side). The cooling air 70 flows into the cavities 62, 63 from the inner side of the lower platform 61b through the holes 66a, 66b and then enters the cooling passages 64, 65 of the upper platform 61a through the holes 64a, 65a to flow out of the rear end surface thereof. By use of such construction, the entire platform can be made in a simple structure comprising the upper and lower large platforms 61a, 61b, the linearly formed cooling passages 64, 65, the short holes 64aand 65a, 66a and 66b, etc. and there are eliminated such complicated and inclined cooling passages as used in the prior art resulting in easiness of the work process.

Further, the construction is made such that the cavities 62, 63 are formed and the cooling air 70 is introduced into the cavities 62, 63 through the multiplicity of holes 66a, 66b, thereby the entire planes of the upper and lower platforms 61a, 61b can be cooled uniformly and both of the side end portions of the upper platform 61a, which is exposed to a high temperature combustion gas, are cooled effectively by the cooling passages 64, 65. Hence, the cooling effect of the entire platform is increased.

It is to be noted that although the multiplicity of holes 66a, 66b, described above, are disposed arrayedly in linear rows in Fig. 6, the present invention is not limited thereto but, naturally, the arrangement thereof may be made in a zigzag form or even irregularly if a uniform cooling of entire plane of the lower platform 61b is ensured.

In the first to third embodiments described above, there are formed the cavities in the platform and provided the cooling holes communicating with the cavities at the peripheral portions of the cavities, thereby the entire portion of the platform can be cooled uniformly and the cooling air passages and cooling air supply lines in the platform can be simplified with result that the work process of the platform becomes facilitated. Also, in the fourth embodiment, there are eliminated such complicated and inclined cooling passages as used in the prior art and the linearly formed cooling passages are provided instead, thereby the workability is enhanced further.

In the fifth embodiment, there are provided the cavities between the upper and lower platforms, the cooling passages of both side portions of the upper platform and the multiplicity of holes of the lower platform. By this construction, there are eliminated such complicated and inclined passages of the platform cooling lines as used in the prior art resulting in a simple structure and enhanced workability as well as a uniform cooling of the platform with a high cooling effect.

The invention has been described with respect to the embodiments as illustrated but the present invention is not limited thereto but may be naturally added with various modifications in the concrete structure within the scope of the following claims.