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Aviation History
1919
1919 - 1190.PDF
The Moss supercharger : Above is shown the nozzle box and in duction system, and below the turbine wheel and water-cooled rear bearing gases are sent to the nozzle box of the turbine by means of substantial specially-designed exhaust headers which connect at their front ends to the flanges on either side of the nozzle box. The very large size of the exhaust headers and the nozzle box is evident from the illustrations. By this design the exhaust gases are offered as unrestricted a passage in as direct a path as possible to the turbine buckets, so that the discharged gases from the engine should possess a large percentage of their outlet energy on reaching the point of expansion. This free passage feature should also have an effect upon the cooling of the device. Unlike the Sherbondy machine, which has a circular nozzle plate, with nozzles all around the turbine wheel, the Moss nozzles are arranged in a semi-cicular plate set into the front side of the nozzle box, as shown in the illustration. This should be a factor in keeping down the temperature of the rotor, since the intense exhaust gas heat is not being directed to the entire rotor all the time, but to only about one-half of its circumference. In further comparison between this machine and the Sherbondy final design, it is also to be noted that due to the turbine and compressor being practically separate so far as their housings are concerned, the Moss device is of greater fore and aft width than the Sherbondy. This was no doubt partially due to a desire to permit as much air circula tion between the two parts as possible for cooling reasons. The construction has the disadvantage, however, that the machine cannot be installed on the Liberty engine with the propeller in its normal position. To accommodate the depth of the Moss machine there has to be a special propeller flange and hub construction in order to bring the screw forward a small distance. In building his machine this way, it is evi dent that the designer considered the advantage of greater cooling sufficient to outweigh any disadvantage due to the i necessity for alternating the propeller position to make room. Instead of the air being drawn into the compressor through an opening in the front centre, as with the Sherbondy machine, the Moss air intake is an integrally cast mouth that leads from the side of the housing to the compressor centre at the rear and just ahead of the turbine, where the air is caught up by the blades of the impeller. This part is therefore in the reversed position from that of the Sherbondy impellers, the entrance side of the blades being toward the *ear instead of to the front. A feature of the Moss machine is the water-cooled rear bearing which shows clearly in one of the illustrations. This is nothing more or less than a special water jacket that sur rounds the bearing proper, and through which water is cir culated from the regular water circulation system of the engine. The machine also is provided with self-aligning bear ings to take care of any slight inaccuracy of the position of the rotative parts due to the high speeds and high tempera tures encountered in operation. There is also a thrust bearing to handle the thrust created by the action of the exhaust gas on the turbine blades. In confirmation of the precautions taken in the design and the special features incorporated against overheating, the Moss apparatus has never given any appreciable overheating trouble in any operative tests yet conducted. The nozzles, which are subjected to the most severe heating conditions, have shown a slight red colour after considerable running, but none of the other parts has become hot enough to reach a visible red. Undoubtedly the successful results obtained in this respect are due largely to the provision for free air circulation around the turbine and equally free method of exhaust. In the writer's opinion Dr. Moss has been entirely justified in claiming slightly more room at the front of his machine than is allowed by the normal position of the pro peller of the Liberty engine, for this permits the design of the apparatus in such a way as to get about as good cooling effect as is possible in the necessarily limited space available. The matter of the heat dissipation is of premier importance in any development work of this nature. In order to prevent the nozzle box and exhaust manifolds from getting out of shape due to the temperature and the pressure of the gas within, these chambers are internally braced by means of cross stays which are welded to the casings. This was found necessary due to the fact that the nozzle box 50 2» 26 2« 22 20 It 16 . 12 10 BAROMf-TRIC PRiJSuet- - IKCHts MHJCUBY Bureau of Standards curve between pressure and altitude at a temperature of 50° F. ->
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