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MR. P. H. KEMBLE (Windsor Locks, Conn.): I ask Mr. Emmet, in reference to the lack of economy of the turbine in the smaller sizes, how small a turbine he would recommend for the use of a small station, as against the Corliss engines, where the capacity of unit would not be over 250 kilowatts.
MR. E. J. RICHARDS (Newburgh, N. Y.): The figures that have been given apply to large stations, of which there are comparatively few in this country. I ask what we may expect in the way
of economy in units of 500 and 750 kilowatts as compared with the large units.
MR. BIBBINS: The striking comparison between turbine and engine units shown in Figure 5 emphasizes the great necessity for making the proper corrections for pressure, vacuum and superheat in all comparisons of water rates for either engine or turbine machinery. Although it is true that the reciprocating engine does not benefit so greatly by increased vacuum as does the impulse type of turbine, yet the improvement is clearly defined in the tests at the Manhattan station quoted by Mr. Stott in his paper. Thus the steam consumption of the 5000-kw engine-type unit varied approximately 2.75 per cent per inch of vacuum as against the 7 to 10 per cent that Mr. Emmet applies to the turbine. On the other hand, the correction for superheat should, if anything, be higher with the reciprocating engine than with the turbine, owing to the existence of certain heat losses in the engine that are practically absent from the turbine.
I assume from the caption that the turbine curve in Figure 5 is obtained from the 5000-kw Yonkers turbine test under 175 pounds pressure, 150 degrees superheat and 28-inch vacuum; but I do not see that adequate correction has been made for superheat, as the engines were run on saturated steam. The 0.3 pound difference between the turbine curves, Figures 2 and 5, would correspond to only 2 per cent correction for 150 degrees superheat, which, it seems to me, should at least be from 12.5 to 15 per cent. A superheat correction of 12.5 per cent for 150 degrees superheat would bring the turbine water rate up to 16.9 pounds per kilowatt-hour, or very close to that of the engine. Furthermore, with respective vacua lower than 28 inches, the steam-turbine consumption would increase about 5 per cent faster than that of the engine.
It is thus apparent that comparisons of this kind are apt to produce very erroneous impressions unless the results are brought to exactly the same conditions. I should like to ask Mr. Emmet if this is the case in Figure 5, and if so, what percentage correction he has used for pressure, superheat and vacuum, in bringing the turbine results to a comparable basis.
Mr. F. W. BULLOCK (Jamestown, N. Y.): Mr. Emmet speaks of 29.31 inches of vacuum. I should like to ask what is the altitude at Chicago, also how near 29.31 is to absolute or perfect vacuum.
MR. B. S. JOSSELYN (Baltimore, Md.): May I ask Mr. Emmet to state the best results he has obtained in pounds of coal per kilowatt output with the Curtis turbine, using best grade of bituminous coal?
MR. SANDS: I ask Mr. Emmet, operating the turbine at 130 degrees superheat, what would be the effect on the turbine of a sudden increase of superheat, say 200 or 225 degrees.
Mr. LAWRENCE MANNING (Owosso, Mich.): I ask what has been the best efficiency actually obtained at full load of 300 and 500-kw turbine units as now manufactured; that is, at 150 pounds steam pressure, with and without 100 degrees superheat.
MR. EMMET: I do not think I can give very positive information as to the relative costs of auxiliaries of different degrees of vacuum. I will explain, however, that these units in Chicago are equipped with condensers in the base. Our latest designs of condensers in the base produce such vacuum as we are producing there, but are not exactly the same as those used in Chicago. They are put into bases of the same height as those ordinarily used for exhaust alone, so there is virtually no additional condenser; the condenser is part of the turbine, and the expense incident to the condenser is the cost of putting in 20,000 feet of 0.75-inch tubes under one of these units—which we have called 9000-kw-which produce these vacua. These tubes and heads and connections depend largely upon the conditions of the installation, but they are not very different in cost from condensers as ordinarily used for less degrees of vacuum. We believe that these condenser bases are decidedly better than exterior condensers. They give better access of steam to the tubes. The steam is blown into the tubes at higher velocity; it comes out of the wheel at about 600 feet per second and enters the mass of tubes, and we think we can by that means get along with less surface.
I will answer one of the later questions in this connection by saying that the vacuum in Chicago is corrected to 30-inch barometer, and is absolute vacuum. There is only 0.7-inch absolute back-pressure—that is, 0.35 pound absolute back-pressure —that we are running with that heavy load in Chicago. Atmos-. pheric pressure has nothing to do with the absolute pressure or the economy of the machine. If the atmospheric pressure is low it makes it easier to pump out the air ; that is the only difference. The steam used by the auxiliaries in Chicago is very small when we allow for the heat returned to the boiler by heating the feed-water, and you must consider this in figuring auxiliary consumption. It pays to take live steam to heat feedwater in the interest of the circulation to boilers, and feed-water should always be heated. With the turbine in Chicago they take the steam from the auxiliaries and apply it to the feedwater. Applying that steam to the feed-water in this manner, the extra consumption of steam incident to the running of the auxiliaries is less than one per cent; probably 0.7 per cent. The first units installed in Chicago, which ran as high as 10,000 kilowatts, had a slightly different auxiliary from that used on the large machine, with which I am not so familiar. They developed about 70 indicated horse-power when running 10,000 kilowatts on the plant, but that steam, instead of being wasted, went right into the heaters and was used. The economy of this plant could be considerably increased, and ultimately will be, by taking steam from the middle of the turbine and adding it to the feed-water heaters so as to bring the temperature of feedwater higher; the auxiliary consumption being now so small that it does not sufficiently heat the feed-water. We can take the steam from the turbine after it has done more than half its work and use it to heat the feed-water; more than half can be taken out of the steam and its latent heat of condensation is still available for feed-water heat. This is being done in Boston.
As to how small a turbine can be advantageously used in central stations, that, of course, is a question of local conditions, or what we are comparing it with. I can say that with 250 kilowatts output on a condensing turbine we can get, with a fair vacuum,—28 inches of vacuum-23 pounds per kilowatt-hour. I have told you in my paper that there are three large engines in Schenectady that use 34 pounds per hour condensing. Since I wrote this paper I have been told of figures, given in a book published by Mr. Barrus, of a number of engine tests in commercial service that he has made, and from them a curve could be made of steam consumption under commercial conditions. From his figures, as reported to me, it would appear that our 34 pounds at Schenectady, which we considered preposterous, was not unusual, but that few of the engine plants tested had less than 20 pounds per kilowatt-hour and that on small sizes, in fact, on all sizes throughout the range, they were running 50 per cent above our best turbines, and our turbines are, as I have said, not dependent on adjustment, but are of a relatively fixed economy. If you have the turbine, and it is fit to run at all, it will give the results. The buckets may become clogged with dirt, or something of the kind, but if so they can be cleaned out. That is an unusual condition, and one that need not be considered.
I have been asked about 300-kw and 500-kw machines. I will say that in the 500-kw, four-stage machines we are now making, the steam consumption, with 28 inches of vacuum, saturated steam, is 19.5 pounds per kilowatt-hour at full load and a little better than that with overloads. I think that machines now coming out will be about a pound better than this, but I simply make this statement as something to base figures upon. In the case of the 300-kw machine I should say that while we could produce nearly as good results they would probably be two pounds higher. In our 300-kw turbine we use 1800 revolutions per minute on 60 cycles and 1500 revolutions on 25 cycles, and these speeds are low for such small turbines. But even under these disadvantageous conditions I think the turbines are much better than engines underload and overload, and almost as good at full load as the engine when in perfect adjustment; but the engine is seldom in perfect adjustment. In our own reciprocating engines—those that we make—the difference between a machined valve and an accurately ground valve will be about 25 per cent in steam consumption. When people use engines they do not get down to these refinements of fit, and the steam is consumed. There was a test made somewhere in the West, the results of which were sent us—a comparison between a reciprocating engine and one of our early 500-kw turbines; the turbine was a machine decidedly inferior in results to the machine just mentioned. They ran the machines with a fixed water-box load at first, and they found, both with the engine and with the turbine, about 21 pounds per kilowatt-hour as I remember it. Then they ran on their railway load, and running under the variable conditions of railway load the efficiency was away off--something like 25 per cent off—as compared with the turbine, which was about the same on variable load as on steady load.
I have been asked to state the effect of superheat and vacuum in these tests. I may say that in this engine and turbine comparison, which I have made in one of the curves, the engine was running with 28 inches vacuum in this particular case, there being good vacuum conditions in this station. The turbine curve shows a somewhat higher degree of vacuum, because the vacuum is obtainable and valuable with the turbine, while with the engine no increase of vacuum above 28 inches would improve the result in the slightest degree. The turbine curve is based on 28.5 inches of vacuum, which I consider a conservative figure, and there is no question but they would run as low as that in one of these Manhattan stations, and the engines in the test had 28 inches of vacuum. The engine has already got to the top of its range at 27 inches vacuum, and nothing is gained by increasing the vacuum. The question of pounds of coal per kilowatt-hour in station operation depends, of course, upon station management of boilers, the load-factor, and a lot of other conditions. The results actually being produced in Chicago are the equivalent of about 1.8 pounds of good coal per kilowatt-hour, that is with all the stand-by losses, all the waste and imperfection of combustion in boilers of rather low average efficiency. The indications are that with such a load-factor as
we have in Chicago, with perfect conditions as to combustion facilities and firing and distribution of load, they could get down with good coal to below 1.5 per kilowatt-hour, which is equivalent to