Illustration showing Lead of Slide ValveFig. 9. Illustration showing Lead of Slide Valve
Fig. 9. Illustration showing Lead of Slide Valve
It will be seen by reference toFig. 8that the portionsOIandIOare wider than the ports which they cover. This extra width is called thelap,OCbeing the outside lap andDIthe inside or exhaust lap. The object of outside lap is that the steam may be shut off after the piston has moved forward a certain distance, and be expanded during the remainder of the stroke. If there were no outside lap, steam would be admitted throughout the entire stroke and there would be no expansion. If there were no inside lap, exhaust would take place throughout the whole stroke, and the advantages of premature release and compression would be lost. Hence, outside lap affects the cut-off, and inside lap affects release and compression. A valve hasleadwhen it begins to uncover the steam port before the end of the return stroke of the piston. This is shown inFig. 9, where the pistonPis just ready to start on its forward stroke as indicated by the arrow. The valve has already opened a distance equal to the lead, and the steam has had an opportunity to enter and fill the clearance space before the beginning of the stroke. The lead varies in different engines, being greater in high-speed than in low-speed types.
Diagrammatical View of EccentricFig. 10. Diagrammatical View of Eccentric
Fig. 10. Diagrammatical View of Eccentric
Relations of Crank and EccentricFig. 11. Relations of Crank and Eccentric
Fig. 11. Relations of Crank and Eccentric
The slide valve is usually driven by an eccentric attached to the main shaft. A diagram of an eccentric is shown inFig. 10. An eccentric is, in reality, a short crank with a crank-pin of such size that it surrounds the shaft. The arm of a crank is the distance between the center of the shaft, and the center of the crank-pin. The throw of an eccentric corresponds to this, and is the distance between the center of the shaft and the center of the eccentric disk, as shown atainFig. 10. The disk is keyed to the shaft, and as the shaft revolves, the center of the disk rotates about it as shown by the dotted line, and gives a forward and backward movement to the valve rod equal to twice the throwa.
InFig. 11letArepresent the center of the main shaft,Bthe crank-pin to which the connecting-rod is attached (seeH,1), and the dotted circle throughBthe path of the crank-pin around the shaft. For simplicity, let the eccentric be represented in a similar manner by the crankAb, and its path by the dotted circle throughb.Fig. 12shows a similar diagram with the pistonPand the valve in the positions corresponding to the positions of the crank and eccentric inFig. 11, and in the diagram at the right inFig. 12. The piston is at the extreme left, ready to start on its forward stroke toward the right. The crank-pinBis at its extreme inner position. When the valve is at its mid-position, as inFig. 8, the eccentric armAbwill coincide with the lineAC,Fig. 11. If the eccentric is turned on the shaft sufficiently to bring the left-hand edgeO,Fig. 8, of the valve in line with the edgeCof the port, the arm of the eccentric will have moved from its vertical position to that shown by the lineAb´inFig. 11. The angle through which the eccentric has been turned from the vertical to bring about this result is called theangular advance, and is shown by angleCAb´inFig. 11. The angular advance evidently depends upon the amount of lap.
If the valve is to be given a lead, as indicated inFig. 12, the eccentric must be turned still further on the shaft to open the valve slightly before the piston starts on its forward movement. This brings the eccentric to the positionAbshown inFig. 11. The angle through which the eccentric is turned to give the necessary lead opening to thevalve is called theangle of lead, and is shown by angleb´Ab. By reference toFig. 11, it is seen that the total angle between the crank and the eccentric is 90 degrees, plus the angular advance, plus the angle of lead. This is the total angle of advance.
The relative positions of the piston and valve at different periods of the stroke are illustrated inFigs. 12 to 16.Fig. 12shows the piston just beginning the forward stroke, the valve having uncovered the admission port an amount equal to the lead. The crank is in a horizontal position, and the eccentric has moved from the vertical an amount sufficient to move the valve toward the right a distance equal to the outside lap plus the lead. The arrows show that steam is entering the left-hand port and is being exhausted through the right-hand port.
Exhaust BeginsFig. 16. Exhaust Begins
Fig. 16. Exhaust Begins
InFig. 13it is seen that the valve has traveled forward sufficiently to open the steam port to its fullest extent, and the piston has moved to the point indicated. The exhaust port is still wide open, and the relative positions of the crank and eccentric are shown in the diagram at the right. InFig. 14the eccentric has passed the horizontal position and the valve has started on its backward stroke, while the piston is still moving forward. The admission port is closed, cut-off having taken place, and the steam is expanding. The exhaust port is still partially open.
InFig. 15both ports are closed and compression is taking place in front of the piston while expansion continues back of it. Release occurs inFig. 16just before the piston reaches the end of its stroke. The eccentric crank is now in a vertical position, pointing downward, and exhaust is just beginning to take place through the left-hand port.This completes the different stages of a single stroke, the same features being repeated upon the return of the piston to its original position. The conditions of lap, lead, angular advance, etc., pertain to practically all valves, whatever their design.
In the following are shown some of the valves in common use, being, with the exception of the Corliss, modifications of the plain slide valve, and similar in their action.
Double-Ported Balanced Valve.—A valve of this type has already been shown inFig. 2. This valve is flat in form, with two finished surfaces,and works between the valve-seat and a plate, the latter being prevented from pressing against the valve by special bearing surfaces which hold it about 0.002 inch away. The construction of the valve is such that when open the steam reaches the port through two openings as indicated by the arrows at the left. The object of this is to reduce the motion of the valve and quicken its action in admitting and cutting off steam.
Engine with Piston ValveFig. 17. Engine with Piston Valve
Fig. 17. Engine with Piston Valve
Piston Valve.—The piston valve shown inFig. 17is identical in its action with the plain slide valve shown inFig. 8, except that it is circular in section instead of being flat or rectangular. The advantage claimed for this type of valve is the greater ease in fitting cylindrical surfaces as compared with flat ones. The valve slides in specialbushings which may be renewed when worn. Piston valves are also made with double ports.
Section through Cylinder of Engine of the Four-valve TypeFig. 18. Section through Cylinder of Engine of the Four-valve Type
Fig. 18. Section through Cylinder of Engine of the Four-valve Type
Four-Valve Type.—Fig. 18shows a horizontal section through the cylinder and valves of an engine of the four-valve type. The admission valves are shown at the top of the illustration and the exhaust valves at the bottom, although, in reality, they are at the sides of the cylinder. The advantage of an arrangement of this kind is that the valves may be set independently of each other and the work done by the two endsof the cylinder equalized. The various events, such as cut-off, compression, etc., may be adjusted without regard to each other, and in such a manner as to give the best results, a condition which is not possible with a single valve.
Different Types of Corliss ValvesFig. 19. Different Types of Corliss Valves
Fig. 19. Different Types of Corliss Valves
Longitudinal Section through Corliss EngineFig. 20. Longitudinal Section through Corliss Engine
Fig. 20. Longitudinal Section through Corliss Engine
The Gridiron ValveFig. 21. The Gridiron Valve
Fig. 21. The Gridiron Valve
Gridiron Valve.—One of the principal objects sought in the design of a valve is quick action at the points of admission and cut-off. This requires the uncovering of a large port opening with a comparatively small travel of the valve. The gridiron valve shown inFig. 21is constructed especially for this purpose. This valve is of the four-valve type, one steam valve and one exhaust valve being shown in the section. Both the valve and its seat contain a number of narrow openings or ports, so that a short movement of the valve will open or close a comparatively large opening. For example, the steam valve in the illustration has 12 openings, so that if they are1⁄4inch in width each, a movement of1⁄4inch of the valve will open a space 12 ×1⁄4= 3 inches in length.
Corliss Valve.—A section through an engine cylinder equipped with Corliss valves is shown inFig. 20. There are four cylindrical valves in this type of engine, two steam valves at the top and two exhaust valves at the bottom. This arrangement is used to secure proper drainage. The action of the admission and exhaust valves is indicated by the arrows, the upper left-hand and the lower right-hand valve being open and the other two closed.
The Monarch Engine with Corliss Valve GearFig. 22. The Monarch Engine with Corliss Valve Gear.—A, Rod to Eccentric; B, Governor;C, Reach Rod; D, Radial Arm; E, Steam Valve; F, Bell-crank; G, Wrist Plate;H, Exhaust Valve; K, Dash-pot
Fig. 22. The Monarch Engine with Corliss Valve Gear.—A, Rod to Eccentric; B, Governor;C, Reach Rod; D, Radial Arm; E, Steam Valve; F, Bell-crank; G, Wrist Plate;H, Exhaust Valve; K, Dash-pot
Side and sectional views of different forms of this type of valve are shown inFig. 19. They are operated by means of short crank-arms which are attached to a wrist-plate by means of radial arms or rods, as shown inFig. 22. The wrist-plate, in turn, is given a partial backward and forward rotation by means of an eccentric attached to the main shaft and connected to the upper part of the wrist-plate by a rod as indicated. The exhaust valves are both opened and closed by the action of the wrist-plate and connecting rods. The steam valves are opened in this manner, but are closed by the suction of dash pots attached to the drop levers on the valve stems by means of vertical rods, as shown.
Figs. 23 to 26. Action of Corliss Valve Gear
The action of the steam or admission valves is best explained by reference toFigs. 23 to 26. Referring toFig. 23,Ais a bell-crank which turns loosely upon the valve stemV. The lower left-hand extension ofAcarries the grab hookH, while the upper extension is connected with the wrist-plate as indicated. Ordinarily the hookHis pressedinward by the springS, so that the longer arm of the hook is always pressed against the knock-off camC. The camCalso turns upon the valve stemVand is connected with the governor by means of a reach rod as indicated inFig. 23and shown inFig. 22. The drop leverBis keyed to the valve stemV, and is connected with the dash pot by a rod as indicated by the dotted line. This is also shown inFig. 22. The end of the drop lever carries a steel block (shown shaded inFig. 23), which engages with the grab hookH.
Governor for Corliss EngineFig. 27. Governor for Corliss Engine
Fig. 27. Governor for Corliss Engine
When in operation, the bell-crank is rotated in the direction of the arrow by the action of the wrist-plate and connecting-rod. As the bell-crank rotates, the grab hook engages the steel block at the end of the drop leverBand lifts it, thus causing the valve to open, and to remain so until the bell-crank has advanced so far that the longer arm of the grab hookHis pressed outward by the projection on the knock-off cam, as shown inFig. 24. The drop lever now being released, the valve is quickly closed by the suction of the dash pot, which pulls the lever down to its original position by means of the rod previously mentioned.
Dash-pot for Corliss EngineFig. 28. Dash-pot for Corliss Engine
Fig. 28. Dash-pot for Corliss Engine
The governor operates by changing the point of cut-off through the action of the camC. With the cam in the position shown inFig. 25, cut-off occurs earlier than inFig. 24. Should the cam be turned in the opposite direction (clockwise), cut-off would take place later. A detailed view of the complete valve mechanism described is shown assembled inFig. 26, with each part properly named. A detail of the governor is shown inFig. 27. An increase in speed causes the revolving ballsBBto swing outward, thus raising the weightWand the sleeveS. This in turn operates the leverLthrough rodRand a bell-crank attachment, as shown in the right-hand view. An upward and downward movement of the balls, due to a change in speed of the engine, swings the leverLbackward and forward as shown by thefull and dotted lines. The ends of this lever are attached by means of reach-rods to the knock-off cams, this being shown more clearly inFig. 22. The connections between the leverLand camCare such that a raising of the balls, due to increased speed, will reduce the cut-off and thus slow down the engine. On the other hand, a falling of the balls will lengthen the cut-off through the same mechanism.
Mention has already been made of the dash pot which is used to close the valve suddenly after being released from the grab hook. The dash-pot rod is shown inFig. 26, and indicated by dotted lines inFigs. 23to25. A detailed view of one form of dash pot is shown inFig. 28. When the valve is opened, the rod attached to leverB,Figs. 23and24, raises the pistonP,Fig. 28, and a partial vacuum is formed beneath it which draws the piston and connecting rod down by suction as soon as the leverBis released, and thus closes the valve suddenly andwithout shock. The strength of the suction and the air cushion for this piston are regulated by the inlet and outlet valves shown on the sides of the dash pot.
Figs. 29to37show various engine details, and illustrate in a simple way some of the more important principles involved in steam engine design.
A partial cross-section of an adjustable piston is shown inFig. 29, and a longitudinal section of the same piston inFig. 30. The principal feature to be emphasized is the method of automatic expansion employed to take up any wear and keep the piston tight. In setting up the piston a hand adjustment is made of the outer sleeve or ringRby means of the set-screwsAA. RingRis made in several sections, so that it may be expanded in the form of a true circle. Further tightness is secured without undue friction by means of the packing ringPwhich fits in a groove inRand is forced lightly against the walls of the cylinder by a number of coil springs, one of which is shown atS. As the cylinder and piston become worn, screwsAare adjusted from time to time, and the fine adjustment for tightness is cared for by the packing ringPand the coil springsS.
Typical Cross-HeadFig. 31. A Typical Cross-head
Fig. 31. A Typical Cross-head
The points to be brought out in connection with the cross-head are the methods of alignment and adjustment. A typical cross-head is shown in cross and longitudinal sections inFig. 31. Alignment in a straight line, longitudinally, is secured by the cylindrical form of the bearing surfaces or shoes, shown atS. These are sometimes made V-shaped in order to secure the same result. The wear on a cross-head comes on the surfacesS, and is taken up by the use of screw wedgesW, shown in the longitudinal section. As the sliding surfaces become worn, the wedges are forced in slightly by screwing in the set-screws and clamping them in place by means of the check-nuts.
Methods Commonly Used for Taking Up Wear in a Connecting-rodFigs. 32 and 33. Methods Commonly Used for Taking Up Wear in a Connecting-rod
Figs. 32 and 33. Methods Commonly Used for Taking Up Wear in a Connecting-rod
The method commonly employed in taking up the wear in a connecting-rod is shown inFigs. 32and33. The wear at the wrist-pin is taken by the so called brasses, shown atBin the illustrations. The inner brass, in both cases, fits in a suitable groove, and is held stationary when once in place. The outer brass is adjustable, being forced toward the wrist-pin by a sliding wedge which is operated by one or more set-screws. InFig. 32the wedge is held in a vertical position, and is adjusted by two screws as shown. The arrangement made use of inFig. 33has the wedge passing through the rod in a horizontal position, and adjusted by means of a single screw, as shown in thelower view. With the arrangements shown, tightening up the brasses shortens the length of the rod. In practice the wedges at each end of the rod are so placed that tightening one shortens the rod, and tightening the other lengthens it, the total effect being to keep the connecting-rod at its original length.
Common Form of Throttling GovernorFig. 36. Common Form of ThrottlingGovernor
Fig. 36. Common Form of ThrottlingGovernor
A common form of outboard bearing for an engine of the slow-speed or Corliss type is illustrated inFig. 34. The various adjustments for alignment and for taking up wear are the important points considered in this case. The plateBis fastened to the stone foundation by anchor bolts not shown. Sidewise movement is secured by loosening the boltsC, which pass through slots in the bearing, and adjusting by means of the screwsS. Vertical adjustment is obtained by use of the wedgeW, which is forced in by the screwA, as required. The inner bearing and bed piece of a heavy duty Corliss engine is shown inFig. 35. The bearing in this case is made up of four sections, so arranged that either horizontal or vertical adjustment may be secured by the use of adjusting screws and check-nuts.
Engines of the slide-valve type are usually provided either with a fly-ball throttling governor, or a shaft governor. A common form of throttling governor is shown inFig. 36. As the speed increases the ballsWare thrown outward by the action of the centrifugal force, and being attached to arms hinged above them, any outward movement causes them to rise. This operates the spindleS, which, in turn, partially closes the balanced valve in bodyB, thus cutting down the steam supply delivered to the engine. The action of a throttling governor upon the work diagram of an engine is shown inFig. 38. Let the full line represent the form of the diagram with the engine working at full load. Now, if a part of the load be thrown off, the engine will speed up slightly, causing the governor to act as described, thus bringing the admission and expansion lines into the lower positions, as shown in dotted lines.
The shaft governor is used almost universally on high-speed engines, and is shown in one form inFig. 37. It consists, in this case, of two weightsW, hinged to the spokes of the wheel near the circumferenceby means of suitable arms. Attached to the arms, as shown, are coil springsC. The ends of the arms beyond the weights are connected by means of leversLto the eccentric disk. When the engine speeds up, the weights tend to swing outward toward the rim of the wheel, the amount of the movement being regulated by the tension of the springsC. As the arms move outward, the levers at the ends turn the eccentric disk on the shaft, the effect of which is to change the angle of advance and shorten the cut-off. When the speed falls below the normal, the weights move toward the center and the cut-off is lengthened. The effect of this form of governor on the diagram is shown inFig. 39. The full line represents the diagram at full load, and the dotted line when the engine is under-loaded.
Shaft Governor for High-speed EngineFig. 37. Shaft Governor for High-speed Engine
Fig. 37. Shaft Governor for High-speed Engine
Under the general heading of steam engine economy, such items as cylinder condensation, steam consumption, efficiency, ratio of expansion, under- and over-loading, condensing, etc., are treated.
The principal waste of steam in the operation of an engine is due to condensation during the first part of the stroke. This condensation is due to the fact that during expansion and exhaust the cylinder wallsand head and the piston are in contact with comparatively cool steam, and, therefore, give up a considerable amount of heat. When fresh steam is admitted at a high temperature, it immediately gives up sufficient heat to raise the cylinder walls to a temperature approximating that of the entering steam. This results in the condensation of a certain amount of steam, the quantity depending upon the time allowed for the transfer of heat, the area of exposed surface, and the temperature of the cylinder walls. During the period of expansion the temperature falls rapidly, and the steam being wet, absorbs a large amount of heat. After the exhaust valve opens, the drop in pressure allows the moisture that has collected on the cylinder walls to evaporate into steam, so that during the exhaust period but little heat is transferred. With the admission of fresh steam at boiler pressure, a mist is condensed on the cylinder walls, which greatly increases the rapidity with which heat is absorbed.
The amount of heat lost through cylinder condensation is best shown by a practical illustration. One horsepower is equal to 33,000 foot-pounds of work per minute, or 33,000 × 60 = 1,980,000 foot-pounds per hour. This is equivalent to 1,980,000 ÷ 778 = 2,550 heat units. The latent heat of steam at 90 pounds gage pressure is 881 heat units. Hence, 2,250 ÷ 881 = 2.9 pounds of steam at 90 pounds pressure is required per horsepower, provided there is no loss of steam, and all of the contained heat is changed into useful work. As a matter of fact, from 30 to 35 pounds of steam are required in the average simple non-condensing high-speed engine.
There are three remedies which are used to reduce the amount of cylinder condensation. The first to be used was called steam jacketing, and consisted in surrounding the cylinder with a layer of high-pressure steam, the idea being to keep the inner walls up to a temperature nearly equal to that of the incoming steam. This arrangement is but little used at the present time, owing both to the expense of operation and to its ineffectiveness as compared with other methods.
The second remedy is the use of superheated steam. It has been stated that the transfer of heat takes place much more rapidly when the interior surfaces are covered with a coating of moisture or mist. Superheated steam has a temperature considerably above the point of saturation at the given pressure; hence, it is possible to cool it a certain amount before condensation begins. This has the effect of reducing the transfer of heat for a short period following admission, and this is the time that condensation takes place most rapidly under ordinary conditions with saturated steam. This, in fact, is the principal advantage derived from the use of superheated steam, although it is also lighter for a given volume, and therefore, a less weight of steam is required, to fill the cylinder up to the point of cut-off. The economical degree of superheating is considered to be that which will prevent the condensation of any steam on the walls of the cylinder up to the point of cut-off, thus keeping them at all times free from moisture. The objections to superheated steam are its cutting effect in the passages through which it flows, and the difficulty experienced inlubricating the valves and cylinder at such a high temperature. The third and most effective remedy for condensation losses is that known as compounding, which will be treated under a separate heading in the following.
It has been explained that cylinder condensation is due principally to the change in temperature of the interior surfaces of the cylinder, caused by the variation in temperature of the steam at initial and exhaust pressures. Therefore, if the temperature range be divided between two cylinders which are operated in series, the steam condensed in the first or high pressure cylinder will be re-evaporated and passed into the low-pressure cylinder as steam, where it will again be condensed and re-evaporated as it passes into the exhaust pipe. Theoretically, this should reduce the condensation loss by one-half, and if three cylinders are used, the loss should be only one-third of that in a simple engine. In actual practice the saving is not as great as this, but with the proper relation between the cylinders, these results are approximated.
Engines in which expansion takes place in two stages are called compound engines. When three stages are employed, they are called triple expansion engines. Compounding adds to the first cost of an engine, and also to the friction, so that in determining the most economical number of cylinders to employ, the actual relation between the condensation loss and the increased cost of the engine and the friction loss, must be considered. In the case of power plant work, it is now the practice to use compound engines for the large sizes, while triple expansion engines are more commonly employed in pumping stations. Many designs of multiple expansion engines are provided with chambers between the cylinders, called receivers. In engines of this type the exhaust is frequently reheated in the receivers by means of brass coils containing live steam. In the case of a cross-compound engine, a receiver is always used. In the tandem design it is often omitted, the piping between the two cylinders being made to answer the purpose.
The ratio of cylinder volumes in compound engines varies with different makers. The usual practice is to make the volume of the low-pressure cylinder from 2.5 to 3 times that of the high-pressure. The total ratio of expansion in a multiple expansion engine is the product of the ratios in each cylinder. For example, if the ratio of expansion is 4 in each cylinder in a compound engine, the total ratio will be 4 × 4 = 16. The effect of a triple-expansion engine is sometimes obtained in a measure by making the volume of the low-pressure cylinder of a compound engine 6 or 7 times that of the high-pressure. This arrangement produces a considerable drop in pressure at the end of the high-pressure stroke, with the result of throwing a considerable increase of work on the high-pressure cylinder without increasing its ratio of expansion, and at the same time securing a large total ratio of expansion in the engine.
In the case of vertical engines, the low-pressure cylinder is sometimesdivided into two parts in order to reduce the size of cylinder and piston. In this arrangement a receiver of larger size than usual is employed, and the low-pressure cranks are often set at an angle with each other.
Another advantage gained by compounding is the possibility to expand the steam to a greater extent than can be done in a single cylinder engine, thus utilizing, as useful work, a greater proportion of the heat contained in the steam. This also makes it possible to employ higher initial pressures, in which there is a still further saving, because of the comparatively small amount of fuel required to raise the pressure from that of the common practice of 80 or 90 pounds for simple engines, to 120 to 140 pounds, which is entirely practical in the case of compound engines. With triple expansion, initial pressures of 180 pounds or more may be used to advantage. The gain from compounding may amount to about 15 per cent over simple condensing engines, taking steam at the same initial pressure. When compound condensing engines are compared with simple non-condensing engines, the gain in economy may run from 30 to 40 per cent.
TABLE IV. STEAM CONSUMPTION OF ENGINES
The steam consumption is commonly called thewater rate, and is expressed in pounds of dry steam required per indicated horsepower per hour. This quantity varies widely in different types of engines, and also in engines of the same kind working under different conditions. The water rate depends upon the “cylinder losses,” which are due principally to condensation, although the effects of clearance, radiation from cylinder and steam chest, and leakage around valves and piston, form a part of the total loss.Table IVgives the average water rate of different types of engines working at full load.
The most economical ratio of expansion depends largely upon the type of the engine. In the case of simple engines, the ratio is limited to 4 or 5 on account of excessive cylinder condensation in case of larger ratios. This limits the initial pressure to an average of about 90 pounds for engines of this type. In the case of compound engines, a ratio of from 8 to 10 is commonly employed to advantage, while with triple-expansion engines, ratios of 12 to 15 are found to give good results.
Thethermal efficiencyof an engine is the ratio of the heat transformed into work to the total heat supplied to the engine. In order to determine this, theabsolutetemperature of the steam at admission and exhaust pressures must be known. These pressures can be measured by a gage, and the corresponding temperatures taken from a steam table, or better, the temperatures can be measured direct by a thermometer. The absolute temperature is obtained by adding 461 to the reading in degrees Fahrenheit (F.). The formula for thermal efficiency is:
in which
Example:—The temperature of the steam admitted to the cylinder of an engine is 340 degrees F., and that of the exhaust steam 220 degrees F. What is the thermal efficiency of the engine?
Themechanical efficiencyis the ratio of the delivered or brake horsepower to the indicated horsepower, and is represented by the equation:
All engines are designed to give the best economy at a certain developed indicated horsepower called full load. There must, of course, be more or less fluctuation in the load under practical working conditions, especially in certain cases, such as electric railway and rolling mill work. The losses, however, within a certain range on either side of the normal load, are not great in a well designed engine. The effect of increasing the load is to raise the initial pressure or lengthen the cut-off, depending upon the type of governor. This, in turn, raises the terminal pressure at the end of expansion, and allows the exhaust to escape at a higher temperature than before, thus lowering the thermal efficiency.
The effect of reducing the load is to lower the mean effective pressure. (SeeFigs. 38and39.) This, in throttling engines, is due to a reduction of initial pressure, and in the automatic engine to a shortening of the cut-off. The result in each case is an increase in cylinder condensation, and as the load becomes lighter, the friction of the engine itself becomes a more important part of the total indicated horsepower; that is, as the load becomes lighter, the mechanical efficiency is reduced.
So far as the design of the engine itself it concerned, there is no difference between a condensing and a non-condensing engine. Theonly difference is that in the first case the exhaust pipe from the engine is connected with a condenser instead of discharging into the atmosphere.
A condenser is a device for condensing the exhaust steam as fast as it comes from the engine, thus forming a partial vacuum and reducing the back pressure. The attaching of a condenser to an engine may be made to produce two results, as shown by the work diagrams illustrated in Figs. 40 and 41. In the first case the full line represents the diagram of the engine when running non-condensing, and the area of the diagram gives a measure of the work done. The effect of adding a condenser is to reduce the back pressure on an average of 10 to 12 pounds per square inch, which is equivalent to adding the same amount to the mean effective pressure. The effect of this on the diagram, when the cut-off remains the same, is shown by the dotted line in Fig. 40. The power of the engine per stroke is increased by an amount represented by the area enclosed by the dotted line and the bottom of the original diagram. Assuming the reduction in back pressure to be 10 pounds, which is often exceeded in the best practice, the gain in power by running condensing will be proportional to the increase in mean effective pressure under these conditions. For example, if the mean effective pressure is 40 pounds when running non-condensing, it will be increased to 40 + 10 = 50 pounds when running condensing, that is, it is50⁄40= 1.25 times as great as before. Therefore, if the engine develops 100 I. H. P. under the first condition, its final power will be increased to 100 × 1.25 = 125 I. H. P. under the second condition.
Fig. 41shows the effect of adding a condenser and shortening the cut-off to keep the area of the diagram the same as before. The result in this case is a reduction in the quantity of steam required to develop the same indicated horsepower. The theoretical gain in economy under these conditions will run from about 28 to 30 per cent for simple, and from 20 to 22 per cent for compound engines. The actual gain will depend upon the cost and operation of the condenser which varies greatly in different localities.
There are various ways of classifying steam engines according to their construction, the most common, perhaps, being according to speed. If this classification is employed, they may be grouped under three general headings: High-speed, from 300 to 400 revolutions per minute; moderate-speed, from 100 to 200 revolutions; and slow-speed, from 60 to 90 revolutions; all depending, however, upon the length of stroke. This classification is again sub-divided according to valve mechanism,horizontal and vertical, simple and compound, etc. The different forms of engines shown in the following illustrations show representative types in common use for different purposes.