CHAPTER XIV. COMPOUND OR STAGE EXPANSION ENGINES. We have referred to steam-jacketing, and in a lesser degree to superheating, as tending to prevent liquefaction in engine cylinders, but they are far from doing so entirely, in ordinary engines, and in order to realise the full benefits of a high ratio of expansion, the system of dividing the expansion into stages, carried out in two or more separate and successive cylinders, must be adopted. Engines of this description are generally called 'compound or stage expansion engines.' Invention and abandonment of compound engines. This system was invented as far back as 1781, but was, however, soon abandoned, for it is principally adapted for high pressures, which were not then in use; but when the fact was fully accepted that in order to make long voyages remunerative, the pressures and rates of expansion of steam must be increased so as to reduce the expenditure of coal, the question of the stresses brought on the framing and shafting of the engine by working the steam at a high rate of expansion in a single cylinder became one of great importance; as, in large engines especially, the variation of pressure during the stroke would be so great that the maximum stresses produced would probably be dangerous to the structure unless it were made excessively strong. Attention was again directed to the employment of the compound engine, in which the high-pressure steam acts on a small piston only, and a reduced pressure on the large piston, which reduces the maximum stresses on the framing, &c, and makes the turning moments more uniform. Re-introduction of system. Causes of advantages. With the increase of pressure the system was re-introduced, and the economy resulting was so decided that its application for marine purposes soon became universal. As the working pressures increased additional stages in the expansion became desirable, and this led to the triple and quadruple expansion engines now so extensively used. The principle is simply an extension of James Watt's idea of keeping the steam vessel or cylinder as warm as possible and the condenser as cool as possible. With simple condensing engines, the cylinder into which the boiler steam is admitted is also open to the condenser for nearly the whole period of the return stroke of the piston, so that its temperature, or at least that of a certain layer of thickness of the internal surface, together with any water remaining in the cylinder, may be supposed to be considerably cooled during this part of the stroke, to be again raised in temperature by the liquefaction of the entering steam, thereby causing a considerable loss due to the direct transfer of heat to the condenser, without the performance of any work, as previously explained. This loss by liquefaction is greater, the greater is the difference between the initial and final temperatures, so that with increased pressures and temperatures, the difference or range of temperature in the cylinder becomes greater, and the loss from this cause when expanding to the full extent in a single cylinder is proportionately increased. If, however, we divide the expansion into two or more stages, the cylinder into which the high-pressure steam is admitted is never open to the condenser, and its temperature is never reduced below that of the intermediate receiver; also, the steam condensed and evaporated in the first cylinder re-appears as working steam in the second cylinder, instead of passing straight to the condenser. The useful work done by it is one source of the economy of stage expansion engines. The loss from liquefaction in the second cylinder is also reduced, in consequence of the smaller range of temperature between admission and exhaust in that cylinder. Another way in which the adoption of stage expansion engines has increased the efficiency of the steam, is by reducing the clearance spaces into which the boiler steam is admitted. These clearance spaces are much smaller in the high-pressure cylinder of a compound engine than they would be had the whole expansion taken place in one large cylinder. At each stroke of the engine this space has to be filled, while no work is being done by the piston, so that the loss of efficiency due to the waste of steam by clearance spaces is much less in the compound engine than in the simple engine. On the other hand, the compound or stage expansion engine has losses of efficiency, due to sudden expansion and wire-drawing between the cylinders, which do not exist in the simple engine, but these losses are of much smaller magnitude than the gains just described, so that to obtain the highest economy from high-pressure steam the stage expansion engine is essential. We have only referred in this chapter to the effect on the efficiency of steam by the use of compound or stage expansion engines, but, as will appear later, this type of engine has other advantages. Trials of double compound versus simple engines with the same steam pressure and ratio of expansion. The experiments made by Mr. Emery on the engines of the 'Bache' gave valuable information as to the comparative efficiencies of the two systems, and the results tabulated in Chapter XIII should be carefully studied in this connection. On reference to the table it will be seen that the consumption of water per I.H.P. per hour was always considerably less in the compound than in the simple expansion engine when working at about the same rate of expansion. This is the case when the cylinders are jacketed, as well as when they are not jacketed. From Columns 1 and 8 with steam jacket in use, it is seen that with an expansion of between five and six times, which proved to be the most economical rate for each type of engine, the feed-water used per I.H.P. per hour was in the simple engine 23.15 lbs., whilst in the compound engine it was only 20.36 lbs., showing in this case a gain in economy at 80 lbs. steam pressure by the use of the compound engine of rather over 12 per cent. At the higher rates of expansion the percentage of economy due to the compound engines was still higher. The consumption of feed-water for the power developed was, both in the compound and in the simple engine, considerably greater at the high rates of expansion shown in Columns 3 and 11 than at the lower rates. In each case, therefore, the efficiency of the steam was considerably reduced when the rate of expansion was increased beyond a certain point. Tri-compound or triple expansion engines.-For steam pressures above 120 lbs. to 130 lbs. per square inch, which are now generally used, it has been found desirable to extend the compound system by dividing the expansion into three stages, so as to reduce the range of temperature in each cylinder, and still further limit the effects of liquefaction. The triple expansion type is now the most common one for modern marine engines, and the gain in economy by its use over the previous double compound engines fitted is well established. The saving of fuel with a triple expansion engine of 150 lbs. to 160 lbs. pressure may be taken as about 20 per cent. compared with the compound engine of about 90 lbs. pressure. The consumption of good fuel with the most successful triple expansion engines, where the engines are designed principally with a view to economy only, as in many vessels of the mercantile marine, is reported to be from 13 lbs. to 13 lbs. per I.H.P. per hour. The average of 28 steamers collected by Mr. Blechynden in 1891 gave 1.52 lbs. as the average consumption per I.H.P. with steam pressure averaging 158 lbs. per square inch at 64 revolutions per minute. Experiments on double versus triple compound engines.-A valuable series of experiments was made on six steamships by a committee of the Institute of Mechanical Engineers which reported in 1892. A few particulars of these trials, which included double and triple compound engines, are shown on the last page. The double compound engines were not steam-jacketed, and the reader should compare the results of these trials given in Columns 1 and 2, with 71 and 95 lbs. of steam respectively, with the corresponding trials without jackets in the 'Bache,' with 80 lbs. pressure, given in Columns 12, 13, and 14 of the table in Chapter XIII. It will be seen that the feed-water used in these recent trials was rather less than in the 'Bache.' The great efficiency of the 'Iona 'should be noticed, due to her high steam pressure of 180 lbs. and expansion of 19 times. Her consumption of 13.35 lbs. of water per I.H.P. per hour is about the lowest well authenticated result with a large marine engine. The conclusion to be drawn from these interesting experiments is that a substantial gain in economy is obtained by the triple expansion engines with higher steam pressures, which is shown by the consumptions of feed-water for the various vessels indicated on line 7. General conclusions.--It will appear from the observations made in the preceding chapters: 1. That economy is increased by the use of higher steam pressures, and expanding the steam, provided the expansion is not excessive. 2. That the amount of expansion required with high-pressure steam can be carried out most efficiently and economically in stage expansion engines, so that the variations of pressure and temperature in each cylinder are comparatively small. 3. That it is desirable with high pressures and ratios of expansion to surround the cylinder with a jacket filled with steam of high temperature to add to the efficiency of the expansion. These jackets have an important effect in small and slow-moving engines, but become less effective as size and speed are increased. 4. That additional efficiency of the steam would result from the use of superheaters, so that renewed efforts to overcome the practical difficulties attending their use appear desirable. CHAPTER XV. REGULATING AND EXPANSION VALVES AND GEAR. Regulating valve. The steam, after leaving the separator, when this is fitted, arrives at the regulating valve for the engines. The object of this valve is to regulate the supply of steam to the engines, so that the speed may be varied as required. The regulating valve, as originally fitted, consisted simply of a flat plate or disc in the pipe, having a central spindle passing through to the outside of the pipe, by means of which it could be turned so as to either close or open the passage as required. This is called a 'throttle valve,' but it is now seldom fitted, as it is difficult to keep even approximately tight with high-pressure steam, and it does not admit of sufficiently exact regulation of the speed of the engines. Flat valves, called 'gridiron valves,' were next used. They consisted of a number of bars with open spaces between them, sliding on a corresponding seating. As pressures increased, however, their friction became too great, and they were superseded by the 'double beat' or equilibrium valve, now used for regulating purposes. Equilibrium or double-beat valve.-A sketch of the double-beat regulating valve is shown in Fig. 126. It consists of two valves on the same spindle; the steam pressure acts on the top of one valve and on the bottom of the other, so that the valve is nearly in equilibrium, and little force is required to move it from its seat. The larger diameter of the lower valve in the arrangement shown must obviously be somewhat less than the smaller diameter of the upper valve, to enable the valve to be put in its place. The amount of opening for a certain height of lift is practically double that for the same lift of an ordinary single conical valve of the same diameter. Manoeuvring valve.-Owing to the frequent small changes of speed of engines required in war vessels, when steaming in the company of other vessels, due to the necessity of 'keeping station,' it has been found necessary in these ships to fit a special small valve. This valve is shown at A, in Fig. 126, and it admits steam from one side to the other of the regulating valve. It is found that the latter is so large that a small change in its opening makes a considerable alteration in the speed of the engine when steaming slowly, and small changes of speed are very difficult to obtain by its means. By means of the small manoeuvring valve, however, these necessarily small changes in the speed of the engine to meet the requirements of station keeping are secured. The valve fitted is generally an ordinary screw-down valve. |