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Home , Slide valve, Superheating

JUNE 1921. 649

SUPERHEATING. - BY SIRHENRY FOWLER,- X.B.E., Member of Council.

Chairman : Sir JOHNDEWRANCE, K.B.E., Vice-President.

[dbridged.]

The Author introduced his subject with a summary of the well - known advantages of superheating. The development of superheating many years ago had been arrested by the difficulty of hibricaiion, which has been solved by the developments in mineral oils. Pure mineral oils have not been found satisfactory, and the best results have been obtained with blended oils, composed mainly of mineral oil with small quantities of fatty oil. One trouble which has not yet been perfectly overcome on locomotives is the deposits which accumulate in the cylinders and ports, and which have to be removed periodically. The amount of superheat which can be given to the has gradually increased owing to lubrication improvements. Ten years ago 150" I?. was quite a usual amount to give to steam used in turbines, but in locomotives it frequently rose to over 300" 3'. for short periods. Now, although the latter figure is rarely exceeded for any length of time in locomotives, it is worked up to in turbine practice. Roughly, within certain limits, the practical saving in steam with turbines is a little above 1 per cent for every 10" P. of superheat. With locomotives the saving varies between 15 per cent and 25 per cent, depending largely on circumstances. In order to do away to a large extent with the fluctuation of firing up, etc., it is often advisable to cover the tubes of the of stationary with some substance which will store the heat somewhat, so that the degree of superheat may be fairly constant. The chief change in locomotive practice is the abandonment of every type of damper without any detrimental effect. The cast-iron header has been in use for teh years with perfectly satisfactory results.

[TBE I.MEcH.E.]

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Discussion.

Professor D. A. LOWsaid that all engineers were agreed that superheating was a very great advantage. He pointed out that the saving due to superheating was greater the lower the ; and that the theoretical saving was less than the actual saving. He explained that the actual saving over the theoretical was due to the greater heat content of water over that of an equal volume of steam. This caused a greater heat transfer to the walls and a greater loss of heat through leakage when wet steam was used instead of superheated steam which had no water suspended in it.

Mr. THOMASSUGDEN agreed with Professor Low that a leakage of hot water was a much more serious than a leakage of the same volume of steam. With a drop- engine one might go to 650" F. ; he had worked up to 700" in that case. With a Corliss engine it was not safe to go to 500". As to lubrication, the ordinary lubricator was of no use ; the lubricant must be forced to the right place, otherwise the and seatings would be ruined. Of course, one did not get the same uniformity of in some processes of working the superheated steam as in other cases. For instance, in drying processes where it was needful to work within certain limited , superheat was too jumpy. In the case of a paper mill, for example, the paper might be scorched. Then there was the loss due to the transmission of steam which had been rightly referred to in the Paper. He had transmitted steam from 500 feet to 1,000 feet through pipes without losing all the superheat. There should be a difference of about 400" F. between the temperature of the which heated the superheater, and that of the superheated steam, otherwise one might have a very large superheater with little effect. He had recently applied a thermometer which, when the temperature was reached to which it was not desirable to go, rang a little alarum bell, and he had found it very useful.

Mr, F. TREVITHICKsaid that, although the separator was all right, yet it was necessary to add a water-heater to it and get all the heat one could out of the waste products. It was possible to get a feed-water temperature of 280" B. with waste gases at a temperature of anything in the neighbourhood of 800" F.

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He emphasized the importance of so drawing the gases through the -tubes that they should be equally distributed through all the tubes. At present he believed that only about two-thirds of the tubes in a locomotive boiler were really effective. That he had proved by running locomotives for months with 100 tubes plugged. He did not believe in reducing the number of tubes, but one ought to make sure that the gases passed properly through them all.

Mr. D. EARLEMARSH Baid that with steam temperatures of about 600" F. to 650" F., he had found that unless pure mineral oil was taken direct to the point where it was to be used with forced lubrication, one might as well give up superheating altogether ; and qua the question of using high superheated steam with ordinary flat-valves, it was impossible. A moderate amount of superheating was possible with flat valves, but -valves were essential for reasonably high temperatures. In the ease of locomotives the benefit due to superheating was enhanced because normally the steam was very wet. It was not what was generally known as a saturated steam, but it was a saturated steam of a very low degree carrying a good deal of water with it. He had found no difficulty with cast-iron as far as the locomotive was concerned, probably because the castings were small ; but he had had difficulty with cast-iron carcases for turbines. Kith high superheat they were quite fatal, and one must have steel. With regard t0 the quality of metal necessary to withstand superheat, Mr. Marsh instanced the case of a German-built turbine in which many rows of blades had to be completely renewed after running continuously for five years. He thought that the Germans did not understand how to make the high-class steel necessary for that class of work.

Mr. F. H. LIVENS(%ember of Council) asked whether, when were installed, it was considered an advantage to reduce the heating surface of the boiler and allow wetter steam to go to the superheater. He knew that in German practice in certain classes of engines it was quite common to reduce to a considerable extent the heating surface for a given size of grate if superheaters were added.

Sir HENRYFOWLER, in reply, regretted $hat the discussion had been confined almost entirely to the locomotive side of the question. He agreed with Mr. Marsh that much of the benefit which a

Downloaded from pme.sagepub.com at NANYANG TECH UNIV LIBRARY on June 5, 2016 662 SUPERHEATING. JUNE 1921. locomotive derived from superheating might be due to the fact that ordinary steam in practice was very wet. It was quite practicable to use slide-valves or even Corliss valves with superheated steam, provided that a suitable lubricating oil was forced to the working surfaces. He had run a slide-valve locomotive with a superheater and no trouble had occurred. Mr. Trevithick had raised the question of feed-water heaters on locomotives ; they added considerably to the weight. For the bearings a pure mineral oil was necessary, but a fatty oil or a fixed oil put into the steam itself was required. With regard to the temperature, he had measured steam temperatures up to 770" F. on locomotives. As regards the fluidity of superheated steam, it was an established fact that leakage of such steam past the piston at the beginning of the was greater than that of saturated steam. With regard to the point raised by Mr. Livens, he did not agree with the idea that the heating surface of a boiler should be reduced when a superheater was fitted, When he said that superheat had reduced the erosion of turbine blades, he had the low-pressure blades in mind. With initial superheat these blades would be working in drier steam, and it was Bhe wetness of the steam which caused erosion. He did not think that nowadays anyone would put down a turbine without adding a superheater to it.

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