The Project Gutenberg eBook ofSteam EnginesThis ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.Title: Steam EnginesAuthor: AnonymousRelease date: December 19, 2010 [eBook #34701]Language: EnglishCredits: E-text prepared by Chris Curnow, Harry Lamé, and the Online Distributed Proofreading Team (http://www.pgdp.net) from page images generously made available by Internet Archive (http://www.archive.org)*** START OF THE PROJECT GUTENBERG EBOOK STEAM ENGINES ***
This ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.
Title: Steam EnginesAuthor: AnonymousRelease date: December 19, 2010 [eBook #34701]Language: EnglishCredits: E-text prepared by Chris Curnow, Harry Lamé, and the Online Distributed Proofreading Team (http://www.pgdp.net) from page images generously made available by Internet Archive (http://www.archive.org)
Title: Steam Engines
Author: Anonymous
Author: Anonymous
Release date: December 19, 2010 [eBook #34701]
Language: English
Credits: E-text prepared by Chris Curnow, Harry Lamé, and the Online Distributed Proofreading Team (http://www.pgdp.net) from page images generously made available by Internet Archive (http://www.archive.org)
*** START OF THE PROJECT GUTENBERG EBOOK STEAM ENGINES ***
EACH NUMBER IS A UNIT IN A SERIES ON ELECTRICAL ANDSTEAM ENGINEERING DRAWING AND MACHINEDESIGN AND SHOP PRACTICE
Copyright, 1911, The Industrial Press, Publishers ofMachinery,49-55 Lafayette Street, New York City.
A steam engine is a device by means of whichheatis transformed intowork. Work may be defined as the result produced by a force acting through space, and is commonly measured in foot-pounds; a foot-pound represents the work done in raising 1 pound 1 foot in height. The rate of doing work is calledpower. It has been found by experiment that there is a definite relation between heat and work, in the ratio of 1 thermal unit to 778 foot-pounds of work. The number 778 is commonly called the heat equivalent of work or the mechanical equivalent of heat.
Heat may be transformed into mechanical work through the medium of steam, by confining a given amount in a closed chamber, and then allowing it to expand by means of a movable wall (piston) fitted into one side of the chamber. Heat is given up in the process of expansion, as shown by the lowered pressure and temperature of the steam, and work has been done in moving the wall (piston) of the closed chamber against a resisting force or pressure. When the expansion of steam takes place without the loss of heat by radiation or conduction, the relation between the pressure and volume is practically constant; that is, if a given quantity of steam expands to twice its volume in a closed chamber of the kind above described, its final pressure will be one-half that of the initial pressure before expansion took place. A pound of steam at an absolute pressure of 20 pounds per square inch has a volume of practically 20 cubic feet, and a temperature of 228 degrees. If now it be expanded so that its volume is doubled (40 cubic feet), the pressure will drop to approximately 10 pounds per square inch and the temperature will be only about 190 degrees. The drop in temperature is due to the loss of heat which has been transformed into work in the process of expansion and in moving the wall (piston) of the chamber against a resisting force, as already noted.
The steam engine makes use of a closed chamber with a movable wall in transforming the heat of steam into mechanical work in the manner just described.Fig. 1shows a longitudinal section through an engine of simple design, and illustrates the principal parts and their relation to one another.
Longitudinal Section through the Ames High-speed EngineFig. 1. Longitudinal Section through the Ames High-speed Engine
Fig. 1. Longitudinal Section through the Ames High-speed Engine
The cylinderAis the closed chamber in which expansion takes place, and the pistonB, the movable wall. The cylinder is of cast iron, accurately bored and finished to a circular cross-section. The piston is carefully fitted to slide easily in the cylinder, being made practically steam tight by means of packing rings. The work generated in moving the piston is transferred to the crank-pinHby meansof the piston-rodC, and the connecting-rodF. The piston-rod passes out of the cylinder through a stuffing box, which prevents the leakage of steam around it. The cross-headDserves to guide the piston-rod in a straight line, and also contains the wrist-pinEwhich joins the piston-rod and connecting-rod. The cross-head slides upon the guide-plateG, which causes it to move in an accurate line, and at the same time takes the downward thrust from the connecting-rod.
The crank-pin is connected with the main shaftIby means of a crank arm, which in this case is made in the form of a disk in order to give a better balance. The balance wheel or flywheelJcarries the crank past the dead centers at the ends of the stroke, and gives a uniform motion to the shaft. The various parts of the engine are carried on a rigid bedK, usually of cast iron, which in turn is bolted to a foundation of brick or concrete. The power developed is taken off by means of a belted pulley attached to the main shaft, or, in certain cases, in the form of electrical energy from a direct-connected dynamo.
When in action, a certain amount of steam (1⁄4to1⁄3of the total cylinder volume in simple engines) is admitted to one end of the cylinder, while the other is open to the atmosphere. The steam forces the piston forward a certain distance by its direct action at the boiler pressure. After the supply is shut off, the forward movement of the piston is continued to the end of the stroke by the expansion of the steam. Steam is now admitted to the other end of the cylinder, and the operation repeated on the backward or return stroke.
Section of Cylinder, showing Slide ValveFig. 2. Section of Cylinder, showing Slide Valve
Fig. 2. Section of Cylinder, showing Slide Valve
An enlarged section of the cylinder showing the action of the valve for admitting and exhausting the steam is shown inFig. 2. In this case the piston is shown in its extreme backward position, ready for the forward stroke. The steam chestLis filled with steam at boiler pressure, which is being admitted to the narrow space back of the piston through the valveN, as indicated by the arrows. The exhaust portMis in communication with the other end of the cylinder andallows the piston to move forward without resistance, except that due to the piston-rod, which transfers the work done by the expanding steam to the crank-pin. The valveNis operated automatically by a crank or eccentric attached to the main shaft, and opens and closes the supply and exhaust ports at the proper time to secure the results described.
Having discussed briefly the general principle upon which an engine operates, the next step is to study more carefully the transformation of heat into work within the cylinder, and to become familiar with the graphical methods of representing it. Work has already been defined as the result of force acting through space, and the unit of work as the foot-pound, which is the work done in raising 1 pound 1 foot in height. For example, it requires 1 × 1 = 1 foot-pound to raise 1 pound 1 foot, or 1 × 10 = 10 foot-pounds to raise 1 pound 10 feet, or 10 × 1 = 10 foot-pounds to raise 10 pounds 1 foot, or 10 × 10 = 100 foot-pounds to raise 10 pounds 10 feet, etc. That is, the product of weight or force acting, times the distance moved through, represents work; and if the force is taken in pounds and the distance in feet, the result will be in foot-pounds. This result may be shown graphically by a figure called a work diagram.
A Simple Work DiagramFig. 3. A Simple Work Diagram
Fig. 3. A Simple Work Diagram
InFig. 3, let distances on the lineOYrepresent the force acting, and distances onOXrepresent the space moved through. Suppose the figure to be drawn to such a scale thatOYis 5 feet in height, andOX10 feet long. Let each division onOYrepresent 1 pound pressure, andeach division onOX1 foot of space moved through. If a pressure of 5 pounds acts through a distance of 10 feet, then an amount of 5 × 10 = 50 foot-pounds of work has been done. Referring toFig. 3, it is evident that the heightOY(the pressure acting), multiplied by the lengthOX(the distance moved through), gives 5 × 10 = 50 square feet, which is the area of the rectangleYCXO; that is, the area of a rectangle may represent work done, if the height represents a force acting, and the length the distance moved through. If the diagram were drawn to a smaller scale so that the divisions were 1 inch in length instead of 1 foot, the areaYCXOwould still represent the work done, except each square inch would equal 1 foot-pound instead of each square foot, as in the present illustration.
Another Form of Work DiagramFig. 4. Another Form of Work Diagram
Fig. 4. Another Form of Work Diagram
InFig. 4the diagram, instead of being rectangular in form, takes a different shape on account of different forces acting at different periods over the distance moved through. In the first case (Fig. 3), a uniform force of 5 pounds acts through a distance of 10 feet, and produces 5 × 10 = 50 foot-pounds of work. In the second case (Fig. 4), forces of 5 pounds, 4 pounds, 3 pounds, 2 pounds, and 1 pound, act through distances of 2 feet each, and produce (5 × 2) + (4 × 2) + (3 × 2) + (2 × 2) + (1 × 2) = 30 foot-pounds. This is also the area, in square feet, of the figureY54321XO, which is made up of the areas of the five small rectangles shown by the dotted lines. Another way of finding the total area of the figure shown inFig. 4, and determining the workdone, is to multiply the length by the average of the heights of the small rectangles. The average height is found by adding the several heights and dividing the sum by their number, as follows:
This, then, means that the average force acting throughout the stroke is 3 pounds, and the total work done is 3 × 10 = 30 foot-pounds.
Work Diagram when Pressure drops UniformlyFig. 5. Work Diagram when Pressure drops Uniformly
Fig. 5. Work Diagram when Pressure drops Uniformly
InFig. 5the pressure drops uniformly from 5 pounds at the beginning to 0 at the end of the stroke. In this case also the area and work done are found by multiplying the length of the diagram by the average height, as follows:
or 25 foot-pounds of work done.
The object ofFigs. 3,4and5is to show how foot-pounds of work may be represented graphically by the areas of diagrams, and also to make it clear that this remains true whatever the form of the diagram. It is also evident that knowing the area, the average height or pressure may be found by dividing by the length, andvice versa.
The Ideal Work Diagram of a Steam EngineFig. 6. The Ideal Work Diagram of a Steam Engine
Fig. 6. The Ideal Work Diagram of a Steam Engine
Fig. 6shows the form of work diagram which would be produced by the action of the steam in an engine cylinder, if no heat were lost by conduction and radiation. Starting with the piston in the position shown inFig. 2, steam is admitted at a pressure represented by the height of the lineOY. As the piston moves forward, sufficient steam is admitted to maintain the same pressure. At the pointBthe valve closes and steam is cut off. The work done up to this time is shown by the rectangleYBbO. From the pointBto the end of the strokeC, the piston is moved forward by the expansion of the steam, the pressure falling in proportion to the distance moved through, until at the end of the stroke it is represented by the vertical lineCX. At the pointCthe exhaust valve opens and the pressure drops to 0 (atmospheric pressure in this case).
As it is always desirable to find the work done by a complete stroke of the engine, it is necessary to find the average or mean pressureacting throughout the stroke. This can only be done by determining the area of the diagram and dividing by the length of the stroke. This gives what is called the mean ordinate, which multiplied by the scale of the drawing, will give the mean or average pressure. For example, if the area of the diagram is found to be 6 square inches, and its length is 3 inches, the mean ordinate will be 6 ÷ 3 = 2 inches. If the diagram is drawn to such a scale that 1 inch onOYrepresents 10 pounds, then the average or mean pressure will be 2 × 10 = 20 pounds, and this multiplied by the actual length of the piston stroke will give the work done in foot-pounds. The practical application of the above, together with the method of obtaining steam engine indicator diagrams and measuring the areas of the same, will be taken up in detail under the heading of Steam Engine Testing.
Before taking up the construction of an actual engine diagram, it is first necessary to become familiar with certain terms which are used in connection with it.
Cut-off.—The cut-off is the point in the stroke at which the admission valve closes and the expansion of steam begins.
Ratio of Expansion.—This is the reciprocal of the cut-off, that is, if the cut-off is1⁄4, the ratio of expansion is 4. In other words, it is the ratio of the final volume of the steam at the end of the stroke to its volume at the point of cut-off. For example, a cylinder takes steam at boiler pressure until the piston has moved one-fourth the length of its stroke; the valve now closes and expansion takes place until the stroke is completed. The one-fourth cylinderful of steam has become a cylinderful, that is, it has expanded to four times its original volume, and the ratio of expansion is said to be 4.
Point of Release.—This is the point in the stroke at which the exhaust valve opens and relieves the pressure acting on the piston. This takes place just before the end of the stroke in order to reduce the shock when the piston changes its direction of travel.
Compression.—This acts in connection with the premature release in order to reduce the shock at the end of the stroke. During the forward stroke of an engine the exhaust valve in front of the piston remains open as shown inFig. 2. Shortly before the end of the strokethis closes, leaving a certain amount of steam in the cylinder. The continuation of the stroke compresses this steam, and by raising its pressure forms a cushion, which, in connection with the removal of the pressure back of the piston by release, brings the piston to a stop and causes it to reverse its direction without shock. High-speed engines require a greater amount of compression than those running at low speed.
Clearance.—This is the space between the cylinder head and the piston when the latter is at the end of its stroke; it also includes that portion of the steam port between the valve and the cylinder. Clearance is usually expressed as a percentage of the piston-displacement of the cylinder, and varies in different types of engines. The following table gives approximate values for engines of different design.
A large clearance is evidently objectionable because it represents a space which must be filled with steam at boiler pressure at the beginning of each stroke, and from which but a comparatively small amount of work is obtained. As compression increases, the amount of steam required to fill the clearance space diminishes, but on the other hand, increasing the compression reduces the mean effective pressure.
Initial Pressure.—This is the pressure in the cylinder up to the point of cut-off. It is usually slightly less than boiler pressure owing to “wire-drawing” in the steam pipe and ports.
Terminal Pressure.—This is the pressure in the cylinder at the time release occurs, and depends upon the initial pressure, the ratio of expansion, and the amount of cylinder condensation.
Back Pressure.—This is the pressure in the cylinder when the exhaust port is open, and is that against which the piston is forced during the working stroke. For example, inFig. 2the small space at the left of the piston is filled with steam at initial pressure, while the space at the right of the piston is exposed to the back pressure. The working pressure varies throughout the stroke, due to the expansion of the steam, while the back pressure remains constant, except for the effect of compression at the end of the stroke. The theoretical back pressure in a non-condensing engine (one exhausting into the atmosphere) is that of the atmosphere or 14.7 pounds per square inch above a vacuum, but in actual practice it is about 2 pounds above atmospheric pressure, or 17 pounds absolute, due to the resistance of exhaust ports and connecting pipes. In the case of a condensing engine (one exhausting into a condenser) the back pressure depends upon the efficiency of the condenser, averaging about 3 pounds absolute pressure in the best practice.
Effective Pressure.—This is the difference between the pressure on the steam side of the piston and that on the exhaust side, or in other words, the difference between the working pressure and the backpressure. This value varies throughout the stroke with the expansion of the steam.
Mean Effective Pressure.—It has just been stated that the effective pressure varies throughout the stroke. The mean effective pressure (M. E. P.) is the average of all the effective pressures, and this average multiplied by the length of stroke, gives the work done per stroke.
Line of Absolute Vacuum.—In the diagram shown inFig. 6, the lineOXis the line of absolute vacuum; that is, it is assumed that there is no pressure on the exhaust side of the piston. In other words, the engine is exhausting into a perfect vacuum.
Atmospheric Line.—This is a line drawn parallel to the line of absolute vacuum at such a distance above it as to represent 14.7 pounds pressure per square inch, according to the scale used.
Constructing a Steam Engine Work DiagramFig. 7. Constructing a Steam Engine Work Diagram
Fig. 7. Constructing a Steam Engine Work Diagram
One of the first steps in the design of a steam engine is the construction of an ideal diagram, and the engine is planned to produce this as nearly as possible when in operation. First assume the initial pressure, the ratio of expansion, and the percentage of clearance, for the type of engine under consideration. Draw linesOXandOYat right angles as inFig. 7. MakeORthe same percentage of the stroke that the clearance is of the piston displacement; makeRXequal to the length of the stroke (on a reduced scale). Erect the perpendicularRAof such a height that it shall represent, to scale, an absolute pressure per square inch equal to 0.95 of the boiler pressure. Draw in the dotted linesAKandKX, and the atmospheric lineLHat a height aboveOXto represent 14.7 pounds per square inch. Locate the point of cut-off,B, according to the assumed ratio of expansion. Points on the expansion curveBCare found as follows: Divide the distanceBKinto anynumber of equal spaces, as shown bya,b,c,d, etc., and connect them with the pointO. Through the points of intersection withBP, asa´,b´,c´,d´, etc., draw horizontal lines, and througha,b,c,d, etc., draw vertical lines. The intersection of corresponding horizontal and vertical lines will be points on the theoretical expansion line. If the engine is to be non-condensing, the theoretical work, or indicator diagram, as it is called, will be bounded by the linesABCHG.
The actual diagram will vary somewhat from the theoretical, as shown by the shaded lines. The admission line betweenAandBwill slant downward slightly, and the point of cut-off will be rounded, owing to the slow closing of the valve. The first half of the expansion line will fall below the theoretical, owing to a drop in pressure caused by cylinder condensation, but the actual line will rise above the theoretical in the latter part of the stroke on account of re-evaporation, due to heat given out by the hot cylinder walls to the low-pressure steam. Instead of the pressure dropping abruptly atC, release takes place just before the end of the stroke, and the diagram is rounded atCFinstead of having sharp corners. The back pressure lineFDis drawn slightly above the atmospheric line, a distance to represent about 2 pounds per square inch. AtDthe exhaust valve closes and compression begins, rounding the bottom of the diagram up toE.
The area of the actual diagram, as shown by the shaded lines inFig. 7, will be smaller than the theoretical, in about the following ratio:
Large medium-speed engines, 0.90 of theoretical area.Small medium-speed engines, 0.85 of theoretical area.High-speed engines, 0.75 of theoretical area.
The capacity or power of a steam engine is rated in horsepower, one horsepower (H. P.) being the equivalent of 33,000 foot-pounds of work done per minute. The horsepower of a given engine may be computed by the formula:
in which
The derivation of the above formula is easily explained, as follows: The area of the piston, in square inches, multiplied by the meaneffective pressure, in pounds per square inch, gives the total force acting on the piston, in pounds. The length of stroke, in feet, times the number of strokes per minute gives the distance the piston moves through, in feet per minute. It has already been shown that the pressure in pounds multiplied by the distance moved through in feet, gives the foot-pounds of work done. Hence,A×P×L×Ngives the foot-pounds of work done per minute by a steam engine. If one horsepower is represented by 33,000 foot-pounds per minute, the power or rating of the engine will be obtained by dividing the total foot-pounds of work done per minute by 33,000. For ease in remembering the formula given, it is commonly written
in which the symbols in the numerator of the second member spell the word “Plan.”
Example:—Find the horsepower of the following engine, working under the conditions stated below:
In this problem, then,A= 113 square inches;P= 40 pounds;L= 1.5 feet; andN= 600 strokes.
Substituting in the formula,
The mean effective pressure may be found, approximately, for different conditions by means of the factors in the followingtable of ratios, covering ordinary practice. The rule used is as follows: Multiply the absolute initial pressure by the factor corresponding to the clearance and cut-off as found fromTable II, and subtract the absolute back pressure from the result, assuming this to be 17 pounds for non-condensing engines, and 3 pounds for condensing.
Example 1:—A non-condensing engine having 3 per cent clearance, cuts off at1⁄3stroke; the initial pressure is 90 pounds gage. What is the M. E. P.?
The absolute initial pressure is 90 + 15 = 105 pounds. The factor for 3 per cent clearance and1⁄3cut-off, fromTable II, is 0.71. Applying the rule we have: (105 × 0.71) - 17 = 57.5 pounds per square inch.
Example 2:—A condensing engine has a clearance of 5 per cent. It is supplied with steam at 140 pounds gage pressure, and has a ratio of expansion of 6. What is the M. E. P.?
The absolute initial pressure is 140 + 15 = 155. The factor for a ratio of expansion of 6 (1⁄6cut-off) and 5 per cent clearance is 0.5, which gives (155 × 0.5) - 3 = 74.5 pounds per square inch.
The power of an engine computed by the method just explained iscalled the indicated horsepower (I. H. P.), and gives the total power developed, including that required to overcome the friction of the engine itself. The delivered or brake horsepower (B. H. P.) is that delivered by the engine after deducting from the indicated horsepower the power required to operate the moving parts. The brake horsepower commonly varies from 80 to 90 per cent of the indicated horsepower at full load, depending upon the type and size of engine.
In proportioning an engine cylinder for any given horsepower, the designer usually has the following data, either given or assumed, for the special type of engine under consideration: Initial pressure, back pressure, clearance, cut-off, and piston speed.
These quantities vary in different types of engines, but in the absence of more specific data the values inTable IIIwill be found useful. The back pressure may be taken as 17 pounds per square inch, absolute, for non-condensing engines, and as 3 pounds for condensing engines as previously stated.
The first step in proportioning the cylinder is to compute the approximate mean effective pressure from the assumed initial pressure, clearance, and cut-off, by the method already explained. Next assume the piston speed for the type of engine to be designed, and determine the piston area by the following formula:
This formula usually gives the diameter of the piston in inches and fractions of an inch, while it is desirable to make this dimension an even number of inches. This may be done by taking as the diameter the nearest whole number, and changing the piston speed to correspond. This is done by the use of the following equation.
In calculating the effective piston area, the area of the piston rod upon one side must be allowed for. The effective or average piston area will then be(2A-a)⁄2, in whichA= area of piston,a= area of piston rod. This latter area must be assumed. After assuming a new pistondiameter of even inches, its effective or average area must be used in determining the new piston speed. The length of stroke is commonly proportioned to the diameter of cylinder, and the piston speed divided by this will give the number of strokes per minute.
Example:—Find the diameter of cylinder, length of stroke, and revolutions per minute for a simple high-speed non-condensing engine of 200 I. H. P., with the following assumptions: Initial pressure, 90 pounds gage; clearance, 7 per cent; cut-off,1⁄4; piston speed, 700 feet per minute; length of stroke, 1.5 times cylinder diameter.
By using the rules and formulas in the foregoing, we have:
M. E. P. = (90 + 15) × 0.63 - 17 = 49 pounds.
The nearest piston diameter of even inches is 16, which corresponds to an area of 201 square inches. Assume a piston rod diameter of 21⁄2inches, corresponding to an area of 4.9 square inches, from which the average or effective piston area is found to be(2 × 201) - 4.9⁄2= 198.5 square inches.
Determining now the new piston speed, we have:
Assuming the length of stroke to be 1.5 times the diameter of the cylinder, it will be 24 inches, or 2 feet.
This will call for 678.5 ÷ 2 = 340 strokes per minute, approximately, or 340 ÷ 2 = 170 revolutions per minute.
Some of the most important details of a steam engine are those of its valve gear. The simplest form of valve is that known as the plain slide valve, and as nearly all others are a modification of this, it is essential that the designer should first familiarize himself with this particular type of valve in all its details of operation. After this has been done, a study of other forms of valves will be found a comparatively easy matter. The so called Corliss valve differs radically from the slide valve, but the results to be obtained and the terms used in its design are practically the same. The valve gear of a steam engine is made up of the valve or valves which admit steam to and exhaust it from the cylinder, and of the mechanism which governs the valve movements, the latter usually consisting of one or more eccentrics attached to the main shaft.
Longitudinal Section of Slide Valve with PortsFig. 8. Longitudinal Section of Slide Valve with Ports
Fig. 8. Longitudinal Section of Slide Valve with Ports
Fig. 8shows a longitudinal section of a slide valve with the ports, bridges, etc. The valve is shown in mid-position in order that certain points relating to it may be more easily understood. The valve,V, consists of a hollow casting, with ends projecting beyond the ports as shown; the lower face is smoothly finished and fitted to the valve seatAB. In operation it slides back and forth, opening and closing the ports which connect the steam chest with the cylinder. Steam is admitted to the cylinder when either portCDorDCis opened, and is released when the ports are brought into communication with the exhaust portMN. This is accomplished by the movement of the valve, which brings one of the cylinder ports and the exhaust port both under the hollow archK. The portionsDMandNDof the valve seat are called the bridges.