Steam Engines.
by Anonymous.
CHAPTER I
ACTION OF STEAM ENGINES
A steam engine is a device by means of which _heat_ is transformed into _work_. Work may be defined as the result produced by a force acting through s.p.a.ce, 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 called _power_. 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 quant.i.ty 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.
Principle of the Steam Engine
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. 1 shows a longitudinal section through an engine of simple design, and ill.u.s.trates the princ.i.p.al parts and their relation to one another.
The cylinder _A_ is the closed chamber in which expansion takes place, and the piston _B_, 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-pin _H_ by means of the piston-rod _C_, and the connecting-rod _F_. The piston-rod pa.s.ses out of the cylinder through a stuffing box, which prevents the leakage of steam around it. The cross-head _D_ serves to guide the piston-rod in a straight line, and also contains the wrist-pin _E_ which joins the piston-rod and connecting-rod. The cross-head slides upon the guide-plate _G_, 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 shaft _I_ by 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 flywheel _J_ carries 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 bed _K_, 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.
[Ill.u.s.tration: Fig. 1. Longitudinal Section through the Ames High-speed Engine]
When in action, a certain amount of steam (1/4 to 1/3 of 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.
An enlarged section of the cylinder showing the action of the valve for admitting and exhausting the steam is shown in Fig. 2. In this case the piston is shown in its extreme backward position, ready for the forward stroke. The steam chest _L_ is filled with steam at boiler pressure, which is being admitted to the narrow s.p.a.ce back of the piston through the valve _N_, as indicated by the arrows. The exhaust port _M_ is in communication with the other end of the cylinder and allows 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 valve _N_ is 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.
Work Diagram
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 s.p.a.ce, 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.
[Ill.u.s.tration: Fig. 2. Section of Cylinder, showing Slide Valve]
In Fig. 3, let distances on the line _OY_ represent the force acting, and distances on _OX_ represent the s.p.a.ce moved through. Suppose the figure to be drawn to such a scale that _OY_ is 5 feet in height, and _OX_ 10 feet long. Let each division on _OY_ represent 1 pound pressure, and each division on _OX_ 1 foot of s.p.a.ce 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 to Fig. 3, it is evident that the height _OY_ (the pressure acting), multiplied by the length _OX_ (the distance moved through), gives 5 10 = 50 square feet, which is the area of the rectangle _YCXO_; 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 area _YCXO_ would still represent the work done, except each square inch would equal 1 foot-pound instead of each square foot, as in the present ill.u.s.tration.
[Ill.u.s.tration: Fig. 3. A Simple Work Diagram]
In Fig. 4 the 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 figure _Y54321XO_, 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 in Fig. 4, and determining the work done, 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:
5 + 4 + 3 + 2 + 1 ----------------- = 3, and 3 10 = 30 square feet, as before.
5
[Ill.u.s.tration: Fig. 4. Another Form of Work Diagram]
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.
In Fig. 5 the 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:
5 + 0 ----- 10 = 25 square feet, 2
or 25 foot-pounds of work done.
[Ill.u.s.tration: Fig. 5. Work Diagram when Pressure drops Uniformly]
The object of Figs. 3, 4 and 5 is 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, and _vice versa_.
Fig. 6 shows 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 in Fig. 2, steam is admitted at a pressure represented by the height of the line _OY_. As the piston moves forward, sufficient steam is admitted to maintain the same pressure. At the point _B_ the valve closes and steam is cut off. The work done up to this time is shown by the rectangle _YBbO_. From the point _B_ to the end of the stroke _C_, 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 line _CX_. At the point _C_ the 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 pressure acting 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 on _OY_ represents 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.
Definitions Relating to Engine Diagrams
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.
[Ill.u.s.tration: Fig. 6. The Ideal Work Diagram of a Steam Engine]
_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 is 1/4, 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 in Fig. 2. Shortly before the end of the stroke this 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 s.p.a.ce 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.
TABLE I. CLEARANCE OF STEAM ENGINES
Type of Engine Per Cent Clearance
Corliss 1.5 to 3.5 Moderate-speed 3 to 8 High-speed 4 to 10
A large clearance is evidently objectionable because it represents a s.p.a.ce 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 s.p.a.ce 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, in Fig. 2 the small s.p.a.ce at the left of the piston is filled with steam at initial pressure, while the s.p.a.ce 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 back pressure. 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.