I 



TRANSACTIONS OF SECTION G. 755 



2. On the most Economical Temperat^ire for Steam-engine Cylinders ; or, 

 Hot V. Cold Walls. By Brvan Donkin, M.Inst.C.E. 



The author calls attention to the important question of the most suitable tem- 

 perature of cylinder walls to obtain the maximum economy by reducing condeusa- 

 tion in steam cylinders to a minimum. To diminish condensation his experiments 

 prove that it is essential to reduce the difference of temperature between the 

 incoming steam and the cylinder walls. In most cases the cylinder walls are much 

 colder than the steam, and often one-half the weight of steam is condensed during 

 admission. Many experiments show that the nearer the temperature of the cylinder 

 wall is to that of the entering steam the greater is the economy. With walls 40^^ 

 to 60° Fahr. colder than the steam, as is olten the case, the consumption is greatly 

 increased, owing to the large amount of condensation. On the other hand, the 

 cylinder metal can be made too hot and heat wasted at exhaust. This has als(> 

 been experimentally proved by the author. The best results in steam economy 

 have been obtained when the temperature of the internal surfaces is a little higher 

 than that of the entering steam. 



Cylinder walls can be heated in various ways. The most usual is to raise their 

 temperature and that of the covers, by means of boiler steam introduced into the 

 jacket spaces. Another method, and one used a good deal on the Continent, is to 

 work with superheated steam. This steam has been employed in some cases with 

 advantage in the jackets as well as in the cylinders. Smoke jackets have generally 

 proved a failure. When the jacket is connected to a condenser having a good 

 vacuum, economy of steam is also obtained, but not so much as with steam in the 

 jackets. To secure the maximum economy also, it is important not only to 

 diminish the volume of the clearance spaces, but especially to reduce as much as 

 possible the area of the clearance boundary surfaces. In this way the weight of 

 iron heated and cooled per stroke so many degrees can be materially diminished. 

 The character of the internal surfaces, whether rough from the sand or turned, 

 and their position, horizontal or vertical, have also some influence on the trans- 

 mission of heat and condensation of steam, as verified by recent trials. 



Experiments have also shown that the cylinder wall in any working steam 

 engine is divided thermally into two parts ; the outer portion remains at a constant 

 temperature, and the inner or periodic portion is heated with each steam, and 

 cooled with each exhaust stroke. The relative proportion of these two parts, or 

 the depth to which the heat penetrates into the metal, depends largely upon the 

 speed of the engine, and on the temperature of the cylinder relatively to that of 

 the entering steam. In a non-jacketed engine, with one-inch cylinder walls, 

 working at thirty-five revolutions per minute, the depth to which the heat pene- 

 trates and fluctuates per stroke is at least about eight to nine millimetres from the 

 internal surface. The depth of heat-penetration for the same speed is much less 

 with hot than with cold Avails ; a less weight of metal is heated per stroke ; and 

 condensation is found to be much reduced. 



The author gives some of the results of eighty experiments on a small ver- 

 tical engine, with cylinder 6 in. diameter and 8 in. stroke, made expressly for 

 experimental research. The details are to be published in ' Proceedings of the 

 Institution of Mechanical Engineers.' The engine was worked with very diHerent 

 temperatures of walls, and many tests were made, condensing and non-condensing, 

 jacketed and non-jacketed, single and double acting, at ditiereut expansions, and 

 with both saturated and superheated steam in the cylinder and jackets. Care was 

 taken in these experiments only to vary one set of conditions at a time. Results 

 of two experiments are added, one with hot and the other with cold walls, or 

 one with steam and the other with air in the jackets. Both tests were made con- 

 densing, double-acting, with 50 lb. steam pressure, ^^ cut-off, and at a speed of 

 220 revolutions per minute. The walls were some iiO° Fahr. hotter with than 

 without steam in the jackets ; the steam consumption per I.H.P. per hour was 

 reduced from 41 j lb. to 28^ lb., or about 31 per cent. ; the thermal efficiency was 

 increased from 5'7 to S'l per cent., or 40 per cent., and the rate of initial condensa- 

 tion was reduced from 460 lb. to 217 lb per square foot per hour, or by more than 



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