Abstract:
A relief valve for turbines of exhaust turbochargers. Hot exhaust gas flows against this type of relief valves and the relief valves heat up significantly. Sensitive components such as the springs or the membrane can be damaged as a result. The relief valves are normally designed so that the membrane and a radiation panel are adjacent to a first chamber. Air is continuously guided through the first chamber in order to specifically cool the membrane and protect the membrane from excessive heating.

Description:
The present application is a 371 of International application PCT/EP2012/001018, filed Mar. 8, 2012, which claims priority of DE 10 2011 013 429.8, filed Mar. 9, 2011, the priority of these applications is hereby claimed and these applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention pertains to a relief valve for turbines of exhaust gas turbochargers. 
     An exhaust gas turbocharger for an internal combustion engine usually contains a turbine and a compressor. The turbine is usually driven by the exhaust gas of the internal combustion engine, and the rotor of the turbine is connected in some way to the rotor of the compressor, as a result of which the rotation of the turbine rotor causes the compressor rotor to rotate also. The compressor then delivers combustion air under pressure to the associated internal combustion engine. A problem with turbochargers of this type is that the rpm&#39;s of the turbine rotor and thus of the compressor rotor increase with the rpm&#39;s and/or load of the internal combustion engine. At high operating rpm&#39;s or loads of the internal combustion engine, it is possible for the turbine and the compressor to be driven at excessive rpm&#39;s. It is also possible for the compressor to supply combustion air to the engine at pressures which are higher that the maximum allowable pressures for the machine. 
     Devices which go into effect when the rotor speed or load exceeds a certain value have already been installed in exhaust gas turbochargers. These devices usually have a relief valve, which allows at least some of the engine exhaust gas to bypass the turbine when the rpm&#39;s or load of the engine reaches a predetermined value. A relief valve of this type is usually designed as a poppet valve with a valve stem guided in a valve housing. The valve is actuated by the force of a spring and/or by a diaphragm, which forms the boundary of a pressure chamber and is actuated by compressed air. 
     A problem which occurs with the use of poppet valves is that the valve spring is exposed to the very high exhaust gas temperatures of the engine exhaust gas and thus becomes extremely hot. The heat flows along the valve stem and causes the valve stem and the valve housing to overheat. The direct conduction of heat via the valve stem and also the thermal radiation from the valve housing, for example, can cause the spring and the diaphragm to overheat. This overheating can cause the following: 
     the failure of the diaphragm and 
     a loss of stiffness of the spring, which leads to changes in the spring pretension and thus in the working point of the valve. 
     The prior art includes examples of how these problems are said to be prevented. Thus DE 30 09 453 C2 discloses a control device for the relief valves of exhaust gas turbochargers, in which the diaphragm is protected from thermal radiation by a radiation shield plate. This radiation shield plate is arranged in a space and divides this space into two chambers. A first chamber is bounded by the diaphragm and the shield plate, and a second chamber is bounded by the shield plate and the valve housing. Compressed air flows into the second chamber, flows around one side of the shield plate, and thus cools it. It then flows toward the valve disk along the valve stem through the gap formed between the valve stem and its guide in the valve housing, thus cooling the valve stem, and finally arrives in a space on the hot side, which is bounded by a hot-side radiation shield plate. From this hot-side space, the compressed air flows through a gap between the valve stem and the hot-side radiation shield plate and into the exhaust gas channel. The radiation shield plate protects the valve housing of the relief valve from the heat of the engine exhaust gas. 
     In this type of design for a control device for relief valves, the cooling effect on the diaphragm is not sufficient. The protection which the hot-side radiation shield plate provides from the hot engine exhaust gas in the exhaust gas channel is also low. 
     DE 35 09 019 C2 discloses a relief valve for a turbine of an exhaust gas turbocharger on an internal combustion engine. As also in the case of the above-cited DE 30 09 453 C2, a space in which a diaphragm is arranged is again provided, this space being divided by a radiation shield plate into a first chamber and a second chamber. Here, too, compressed air flows into the second chamber to cool the radiation shield plate from one side. The two chambers are connected to each other by an opening in the radiation shield plate. Here, too, the cooling effect leaves much to be desired. 
     SUMMARY OF THE INVENTION 
     The goal of the invention is to improve the cooling effect and thus to avoid the overheating of the diaphragm. 
     According to the invention, a relief valve is provided 
     with a valve disk cooperating with a valve seat and 
     with a valve stem guided in a valve housing, 
     wherein a space, in which a diaphragm connected to the valve stem is arranged, is provided in the valve housing; 
     wherein the diaphragm is connected around its outer periphery with a sealing effect to the valve housing; 
     wherein a radiation shield plate, which is located in the space on the side of the diaphragm facing the valve disk and a certain distance away from the diaphragm, extends around the valve stem, this shield plate dividing the space into two chambers, namely, a first chamber bounded by the diaphragm and the radiation shield plate, and a second chamber, bounded by the radiation shield plate and a wall of the valve housing; 
     wherein, in the area of the chambers, the valve housing is designed with at least one air inlet on an outer periphery for supplying air; 
     wherein at least some of the air entering through the air inlet flows through in the guide along the valve stem toward the valve disk; and 
     wherein the radiation shield plate comprises openings allowing the passage of air. 
     The air flows continuously through the first chamber formed by the diaphragm and the radiation shield plate. 
     The inventive relief valve advantageously allows the air to flow directly and continuously around the diaphragm, thus cooling it, so that heat which might damage the diaphragm is prevented from building up in the first chamber. 
     According to a preferred embodiment of the invention, the first chamber is designed with the air inlet. The advantage here is that cool air flows directly into the first chamber and that the air inlet or the air inlets can be designed in such a way that the air flows around the diaphragm in optimal fashion. 
     According to another preferred embodiment of the invention, the first chamber is designed with the air inlet and at least some of the air flowing out of the first chamber flows into the second chamber. The advantage here is that cool air flows directly into the first chamber and that the air inlet or the air inlets can be designed in such a way that the air flows around the diaphragm in optimal fashion. Some of the outflowing air flows through the second chamber and prevents heat from accumulating there. As a result, the diaphragm is cooled even more effectively. 
     According to another preferred embodiment of the invention, the second chamber is designed with the air inlet, and the air flowing out of the second chamber flows into the first chamber. The advantage here is that the air flowing into the first chamber can be directed straight at the diaphragm to ensure good cooling. In addition, the conventional design of a relief valve can be retained, which is favorable in terms of cost. 
     According to another preferred embodiment of the invention, the air inlet opens out into a ring-shaped channel, which is open to the second chamber. The advantage here is that the air can flow uniformly into the second chamber. 
     According to another preferred embodiment of the invention, the air inlet is shared equally between the first and second chambers, and the air flows continuously through at least the first chamber. The advantage here is that cool air flows into both chambers, flows around and cools the diaphragm on one side, and also prevents heat from accumulating in the second chamber on the other side. Overall, a good cooling effect is obtained for the diaphragm. 
     According to another preferred embodiment of the invention, the air flowing out of the first chamber proceeds by way of a ring-shaped gap between the valve stem and the radiation shield plate. The first advantage here is that the flow through the first chamber is oriented in a single direction, and the second advantage is that the hot valve stem is cooled by the air in concentrated fashion, which improves the overall cooling effect. 
     According to another preferred embodiment of the invention, the outer periphery of the radiation shield plate is connected with a sealing effect to the valve housing. The advantage here is that the relief valve of the conventional design can be retained. 
     According to another preferred embodiment of the invention, a ring-shaped gap is present between the outer periphery of the radiation shield plate and the valve housing. The advantage here is that the direction of the air flow around the diaphragm can be optimized; the air can, for example, flow from the outside toward the inside. 
     According to another preferred embodiment of the invention, several openings, distributed uniformly on a radius, are provided in the radiation shield plate. The advantage here is that the air flows into the first chamber with a high degree of uniformity. 
     According to another preferred embodiment of the invention, several openings are provided in the radiation shield plate to ensure a uniform air flow, these openings being arranged in such a way that the cross-sectional area of the openings on the side facing the air inlet is smaller than that of the openings on the side facing away from the air inlet. The advantage here is that the air flow can be optimally adjusted. 
     According to another preferred embodiment of the invention, the openings in the radiation shield plate are all of equal size. The advantage here is that, when the shield plate is installed, there is no need to make sure that the openings are in any special position. 
     According to another preferred embodiment of the invention, the openings in the radiation shield plate are of different sizes. The advantage here is that this makes it possible to adjust the air flow to ensure optimal cooling of the diaphragm. 
     According to another preferred embodiment of the invention, the openings are arranged near the outer periphery of the radiation shield plate. The advantage here is that the air is directed toward the entire surface of the diaphragm and thus cools it in optimal fashion. 
     According to another preferred embodiment of the invention, the radiation shield plate is designed with a web, which points toward the valve disk and rests against the opposite wall of the valve housing. The advantage here is that the radiation shield plate is positioned a certain distance away from the valve housing wall in a vibration-free manner. 
     According to another preferred embodiment of the invention, the web is designed in the form of a ring-shaped pleat, which extends around the valve stem and fits into and rests against a ring-shaped groove in the valve housing. The advantage here is that the pleat holds the radiation shield plate in position in the valve housing and also centers it. 
     According to another preferred embodiment of the invention, the web comprises at least one opening in its periphery. The advantage here is that air can flow out of the second chamber. 
     According to another preferred embodiment of the invention, the web comprises several openings uniformly distributed around its periphery. The advantage here is that the air can flow radially out of the second chamber in all directions. 
     According to another preferred embodiment of the invention, the web comprises several openings in its periphery which are at different distances from each other. The advantage here is that this makes it possible to configure the optimal air flow in the second chamber. 
     According to another preferred embodiment of the invention, the openings in the web are all of equal size. The advantage here is that these are easy to fabricate industrially. 
     According to another preferred embodiment of the invention, the openings in the web are of different sizes. The advantage here is that this makes it possible to configure the optimal air flow in the second chamber. 
     According to another preferred embodiment of the invention, the air flowing out of the gap between the guide and the valve stem flows into a space on the hot side, which is bounded by a valve housing wall lying on the downstream side and a hot-side radiation shield plate extending around the valve stem, wherein the outer periphery of the hot-side radiation shield plate is connected with a sealing effect to the valve housing wall, and that the hot-side radiation shield plate has air outlets in the area of its outer periphery. The advantage here is that air can flow radially through the hot-side space, and no heat will accumulate in the hot-side space, i.e., heat which could flow from there via the valve housing to the diaphragm and damage it. 
     According to another preferred embodiment of the invention, a gap is formed between the hot-side radiation shield plate and the valve stem, which gap represents another air outlet. The advantage here is that the air can also flow out along the valve stem and thus cool even the hot end of this shaft. 
     Exemplary embodiments of the invention are illustrated in the drawing and are described in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a cross section through the relief valve according to a first embodiment of the invention; 
         FIG. 2  shows a schematic cross-sectional view of the diaphragm area of a relief valve according to another embodiment; 
         FIG. 3  shows a schematic cross-sectional view of the diaphragm area of a relief valve according to another embodiment; 
         FIG. 4  shows a schematic cross-sectional view of the diaphragm area of a relief valve according to another embodiment; 
         FIG. 5  shows a schematic cross-sectional view of the diaphragm area of a relief valve according to another embodiment; 
         FIG. 6  shows a schematic cross-sectional view of the diaphragm area of a relief valve according to another embodiment; 
         FIG. 7  shows a top view of a radiation shield plate according to another embodiment; and 
         FIG. 8  shows a top view of a radiation shield plate according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows the essential components of a relief valve  1  according to a first embodiment for blowing off exhaust gas upstream of a turbine (not shown) of an exhaust gas turbocharger (not shown). A valve element  2 , consisting of a valve disk  3  and a valve stem  4 , is guided by its valve stem  4  in a valve housing  5 . The valve housing  5  is mounted on an exhaust gas housing  8 , in which an exhaust gas channel  9  and a blow-off channel  10  are integrated. The blow-off channel  9  and the relief channel  10  are connected to each other by an overflow opening  11 , into which a valve seat  7  is machined. The valve disk  3  cooperates with the valve seat  7  in the overflow opening  11 . At least one spring  12 , installed in a spring chamber  13  of the relief valve  1 , is tensioned between a cover  14  of the relief valve  1  and a holder  15  for a diaphragm  17 . Because the holder  15  is connected to the valve stem  4 , the force of the spring is transmitted to the valve element  2 . In the nonactuated state of the relief valve  1 , the spring force causes the valve disk  3  to remain in a closed position in the valve seat  7  and thus separates the exhaust gas channel  9  from the relief channel  10 . As a result, all of the exhaust gas flows into the turbine of the exhaust gas turbocharger. Only when the valve element  2  is opened does some of the exhaust gas flow via the overflow opening  11  into the relief channel  10 , thus bypassing the turbine. In the valve housing  5 , a space  16  is provided, in which the diaphragm  17  is arranged; the diaphragm is connected to the valve stem  4 , and its outer periphery  18  is connected with a sealing effect to the valve housing  5 . The space  16  is divided by a radiation shield plate  19  into a first chamber  23  and a second chamber  24 . The radiation shield plate  19  is arranged opposite one of the walls  6  of the valve housing, extends around the valve stem  4 , and is connected by its outer periphery  20  with a sealing effect to the valve housing  5 . In addition, the radiation shield plate  19  has a ring-shaped pleat  25 , which points toward the valve disk  3  and extends around the valve stem  4 . A pleat wall  26  of the pleat  25  pointing toward the valve stem  4  projects into a ring-shaped groove  27  in the valve housing wall  6  and thus rests against one of the walls  28  of the ring-shaped groove  27  in such a way that the radiation shield plate  19  is centered and held in position. Openings  21  arranged near the outer periphery  20  of the radiation shield plate  19  allow air to pass through, as the ring-shaped groove  22  located between the radiation shield plate  19  and the valve stem  4  also does. The radiation shield plate  19  serves to protect the diaphragm  17  from damaging thermal radiation emanating from the hot valve housing  5  and the valve stem  4 . The thermal protection provided by the radiation shield plate  19  alone, however, is not sufficient to protect the diaphragm  17  from damage. For further thermal protection, a supplemental measure in the form of the air cooling of the diaphragm  17  is necessary. For this purpose, air flows via an air inlet  29  in the valve housing  5  into a ring-shaped channel  30  and from there into the second chamber  24 . The second chamber  24  is bounded on one side by the radiation shield plate  19 . The air flowing into the second chamber  24  flows through the openings  21  in the radiation shield plate  19  and then into the first chamber  23 . There the air flows around and cools the diaphragm  17  directly from one side, before leaving the first chamber  23  through the ring-shaped gap  22  between the radiation shield plate  19  and the valve stem  4  and entering a gap  31  between the valve stem  4  and the valve housing  5 . From this gap  31 , the air flows into a hot-side space  32 , which is bounded by the valve housing  5  and a hot-side radiation shield plate  33 , which rests on the valve housing  5 . The hot-side radiation shield plate  33  protects the relief valve  1  from direct contact with the hot exhaust gas in the relief channel  10 . As the air flows through the hot-side space  32 , it thus cools the hot-side radiation shield plate  33  and the valve housing  5 . After leaving the hot-side space  32 , the air enters the relief channel  10  through a ring-shaped gap  35  between the hot-side radiation shield plate  33  and the valve stem  5  and also through the air outlets  36  in the outer periphery  34  of the hot-side radiation shield plate  33 . 
       FIG. 2  shows a schematic cross-sectional view of the diaphragm area of a relief valve according to a second embodiment. The air inlet  229  opens out here directly into a first chamber  223 , which is formed by the diaphragm  217  and the radiation shield plate  219 . The radiation shield plate  219  is connected around its outer periphery  220  with a sealing effect to the valve housing  205  and is designed with a web  237 , which surrounds the valve stem  204  of the valve element  202  in ring-like fashion and rests against the valve housing wall  206  opposite the radiation shield plate  219 . As a result, the second chamber  224  is formed. The air flowing through the air inlet  229  into the first chamber  223  flows around and cools the diaphragm  217  and the radiation shield plate  219  on one side, before flowing out of the first chamber  223  through the ring-shaped gap  222  between the radiation shield plate  219  and the valve stem  204 . After entering the ring-shaped gap  222 , the air flows toward the valve disk  203  along the gap  239  between the web  237  and the valve stem  204  and through the gap  231  between the valve stem  204  and the valve housing  205 . The further course of the air flow is the same as that described in association with  FIG. 1 . 
       FIG. 3  shows a schematic cross-sectional view of the diaphragm area of a relief valve according to a third embodiment. The air inlet  329  opens out here directly into the first chamber  323 , which is formed by the diaphragm  317  and the radiation shield plate  319 . The radiation shield plate  319  is connected around its outer periphery  320  with a sealing effect to the valve housing  305  and is designed with a web  337 , which surrounds the valve stem  304  of the valve element  302  in ring-like fashion and rests against the valve housing wall  306  opposite the radiation shield plate  319 . As a result, the second chamber  324  is formed. The air flowing into the first chamber  323  through the air inlet  329  flows around and cools the diaphragm  317  and the radiation shield plate  319  on one side, before flowing out of the first chamber  323  through the ring-shaped gap  322  between the radiation shield plate  319  and the valve stem  304  and through the openings  321  in the radiation shield plate  319  close to its outer periphery  320 . Some of the air therefore flows into the second chamber  324 , cools the surrounding components, and flows through openings  338  in the web  337  into the gap  339  between the web  337  and the valve stem  304 . At this point, this part of the air flow combines with the part which flows out of the first chamber  323  through the ring-shaped gap  322  between the radiation shield plate  319  and the valve stem  304 . The combined air flow now flows toward the valve disk  303  through the gap  331  between the valve stem  304  and the valve housing  305 . The further course of the air flow is the same as that described in association with  FIG. 1 . 
       FIG. 4  shows a schematic cross-sectional view of the diaphragm area of a relief valve according to a fourth embodiment. The diaphragm  417  and the radiation shield plate  419  together form the first chamber  423 . The radiation shield plate  419  is connected around its outer periphery  420  with a sealing effect to the valve housing  405  and is designed with the web  437 , which surrounds the valve stem  404  of the valve element  402  in ring-like fashion and rests against the valve housing wall  406  opposite the radiation shield plate  419 . As a result, the second chamber  424  is formed. The air inlet  429  opens out into this second chamber  424 . The air flowing through this air inlet  429  cools the surrounding components before flowing through the openings  421  in the radiation shield plate  419  close its outer periphery  420  and into the first chamber  423 . There the air flows around and cools the diaphragm  417  and the radiation shield plate  419 , before flowing out of the first chamber  423  via the ring-shaped gap  422  between the radiation shield plate  419  and the valve stem  404 . After entering the ring-shaped gap  422 , the air flows toward the valve disk  403  through the gap  439  between the web  437  and the valve stem  404  and through the gap  431  between the valve stem  404  and the valve housing  405 . The further course of the air flow is the same as that described in association with  FIG. 1 . 
       FIG. 5  shows a schematic cross-sectional view of the diaphragm area of a relief valve according to a fifth embodiment. The diaphragm  517  and the radiation shield plate  519  together form the first chamber  523 . The radiation shield plate  519  again cooperates with the opposite valve housing wall  506  to form the second chamber  524 . The radiation shield plate  519  is connected around its outer periphery  520  with a sealing effect to the valve housing  505  except for an area which serves as an air inlet  529  for the two chambers  523  and  524 . In addition, the radiation shield plate  519  is designed with the web  537 , which surrounds the valve stem  504  of the valve element  502  in ring-like fashion and rests against the valve housing wall  506  opposite the radiation shield plate  519 . The air flowing through the air inlet  529  flows around and cools the diaphragm  517  and the radiation shield plate  519  on one side in the first chamber  523  before flowing out of the first chamber  523  via the ring-shaped gap  522  between the radiation shield plate  519  and the valve stem  504 . Although there is a continuous flow of air through the second chamber  524 , the air flowing through the air inlet  529  into the first chamber  523  causes vortices and turbulence to form in the second chamber  524 . These air movements ensure the cooling of the surrounding components of the second chamber  524 . After the air flows out of the first chamber  523  through the ring-shaped gap  522 , it flows toward the valve disk  503  through the gap  539  between the web  537  and the valve stem  504  and through the gap  531  between the valve stem  504  and the valve housing  505 . The further course of the air flow is the same as that described in association with  FIG. 1 . 
       FIG. 6  shows a schematic cross-sectional view of the diaphragm area of a relief valve according to a sixth embodiment. The diaphragm  617  and the radiation shield plate  619  together form the first chamber  623 . The radiation shield plate  619  again cooperates with the opposite valve housing wall  606  to form the second chamber  624 . A ring-shaped gap  640  is present between the outer periphery  620  of the radiation shield plate  619  and the valve housing  605 . The ring-shaped gap  622  between the radiation shield plate  619  and the valve stem  604  of the valve element  602  is also present, around which the radiation shield plate  619  extends. In addition, the radiation shield plate  619  is designed with the web  637 , which surrounds the valve stem  604  in ring-like fashion and rests against the valve housing wall  606  opposite the radiation shield plate  619 . The air entering the first chamber  623  via the air inlet  629  flows around and cools the diaphragm  617  and the radiation shield plate  619  on one side; some of this air then flows through the ring-shaped gap  640  between the outer periphery  620  of the radiation shield plate  619  and the valve housing  605  and thus into the second chamber  624 . The rest of the air flows out of the first chamber  623  via the ring-shaped gap  622  between the radiation shield plate  619  and the valve stem  604  and thus enters the gap  639  between the web  637  and the valve stem  604 . The air flowing through the air inlet  629  and into the second chamber  624  cools the surrounding components and then flows through the openings  638  in the web  637  and thus into the gap  639  between the web  637  and the valve stem  604 . There this part of the air flow combines with the part flowing out of the first chamber  623  via the ring-shaped gap  622  between the radiation shield plate  619  and the valve stem  604 . The combined air flow now flows toward the valve disk  603  through the gap  631  between the valve stem  604  and the valve housing  605 . The further course of the air flow is the same as that described in association with  FIG. 1 . 
       FIG. 7  shows a top view of the radiation shield plate  719  according to another embodiment. In the assembled state of the relief valve (not shown), the valve stem (not shown) projects through the opening  721  in the center. The outer, uniformly distributed openings  721  allow the air to pass through. 
       FIG. 8  shows a top view of the radiation shield plate  819  according to another embodiment. In the assembled state of the relief valve (not shown), the valve stem (not shown) projects through the opening  821  in the center. The outer openings  821  allow the air to pass through. As in the figures described above, in the assembled state of the relief valve the air enters the chambers from one side and therefore nonuniformly. To make the air flow through the radiation shield plate  819  more uniformly and thus to improve the cooling of the diaphragm, the total cross-sectional area of the openings  821  arranged on the side facing the air inlet is smaller than that of the openings on the side facing away from the air inlet. 
     LIST OF REFERENCE NUMBERS 
     
         
           1  relief valve 
           2 ,  202 ,  302 ,  402 ,  502 ,  602  valve element 
           3 ,  203 ,  303 ,  403 ,  503 ,  603  valve disk 
           4 ,  204 ,  304 ,  404 ,  504 ,  604  valve stem 
           5 ,  205 ,  305 ,  405 ,  505 ,  605  valve housing 
           6 ,  206 ,  306 ,  406 ,  506 ,  606  valve housing wall 
           7  valve seat 
           8  exhaust gas housing 
           9  exhaust gas channel 
           10  relief channel 
           11  overflow opening 
           12  spring 
           13  spring chamber 
           14  cover 
           15  holder 
           16  space 
           17 ,  217 ,  317 ,  417 ,  517 ,  617  diaphragm 
           18  outer periphery 
           19 ,  219 ,  319 ,  419 ,  519 ,  619 ,  719 ,  819  radiation shield plate 
           20 ,  220 ,  320 ,  420 ,  520 ,  620  outer periphery 
           21 ,  321 ,  421 ,  721 ,  821  openings 
           22 ,  222 ,  322 ,  422 ,  522 ,  622  ring-shaped gap 
           23 ,  223 ,  323 ,  423 ,  523 ,  623  first chamber 
           24 ,  224 ,  324 ,  424 ,  524 ,  624  second chamber 
           25  pleat 
           26  pleat wall 
           27  ring-shaped groove 
           28  wall 
           29 ,  229 ,  329 ,  429 ,  529 ,  629  air inlet 
           30  ring-shaped channel 
           31 ,  231 ,  331 ,  431 ,  531 ,  631  gap 
           32  hot-side space 
           33  hot-side radiation shield plate 
           34  outer periphery 
           35  ring-shaped gap 
           36  air outlet 
           237 ,  337 ,  437 ,  537 ,  637  web 
           338 ,  638  openings 
           239 ,  339 ,  439 ,  539 ,  639  gap 
           640  ring-shaped gap