Ball EngineFig. 42. The Ball Engine
Fig. 42. The Ball Engine
The Ball engine, as shown inFig. 42, is a typical horizontal single valve high-speed engine with a direct-connected dynamo. It is very rigid in design and especially compact for the power developed. The valve is of the double-ported type shown inFig. 2, having a cover plate for removing the steam pressure from the back of the valve. The piston is hollow with internal ribs similar to that shown inFig. 29, and is provided with spring packing rings carefully fitted in place. The governor is of the shaft type, having only one weight instead of two, as shown inFig. 37.
The Sturtevant Vertical EngineFig. 43. The Sturtevant Vertical Engine
Fig. 43. The Sturtevant Vertical Engine
The Sturtevant engine shown inFig. 43is a vertical high-speed engine of a form especially adapted to electrical work. Engines of this general design are made in a variety of sizes, and are often used on account of the small floor space required. In the matter of detail, such as valves, governors, etc., they do not differ materially from the high-speed horizontal engine.
Moderate Speed Engine of the Four-valve TypeFig. 44. Moderate Speed Engine of the Four-valve Type
Fig. 44. Moderate Speed Engine of the Four-valve Type
Fig. 44illustrates a moderate-speed engine of the four-valve type. These engines are built either with flat valves, or with positively driven rotary or Corliss valves, the latter being used in the engine shown. It will be noticed that the drop-lever and dash-pot arrangement is omitted, the valves being both opened and closed by means of the wrist-plate and its connecting rods. This arrangement is used on account of the higher speed at which the engine is run, the regular Corliss valve gear being limited to comparatively low speeds. All engines of this make are provided with an automatic system of lubrication.The oil is pumped through a filter to a central reservoir, seen above the center of the engine, and from here delivered to all bearings by gravity. The pump is attached to the rocker arm, and therefore easily accessible for repairs.
The Harris Corliss EngineFig. 45. The Harris Corliss Engine
Fig. 45. The Harris Corliss Engine
The standard Harris Corliss engine shown inFig. 45, is typical of its class. It is provided with the girder type of frame, and with an outboard bearing mounted upon a stone foundation. The valve gear is of the regular Corliss type, driven by a single eccentric and wrist-plate. The dash pots are mounted on cast-iron plates set in the floor at the side of the engine, where they may be easily inspected. The governor is similar in construction to the one already described, and shown inFig. 27. The four engines so far described are simple engines, the expansion taking place in a single cylinder.Figs. 46to48show three different types of the compound engine.
The Skinner Tandem EngineFig. 46. The Skinner Tandem Engine
Fig. 46. The Skinner Tandem Engine
The engine shown inFig. 46is of a type known as the tandem compound. In this design the cylinders are in line, the low-pressure cylinder in front of the high-pressure, as shown. There is only one piston rod, the high-pressure and low-pressure pistons being mounted on the same rod. The general appearance of an engine of this design is the same as a simple engine, except for the addition of the high-pressure cylinder. The governor is of the shaft type and operates by changing the cut-off in the high-pressure cylinder. The cut-off in the low pressure cylinder is adjusted by hand to divide the load equallybetween the two cylinders for the normal load which the engine is to carry.
American Ball Duplex Compound EngineFig. 47. American Ball Duplex Compound Engine
Fig. 47. American Ball Duplex Compound Engine
The engine shown inFig. 47is known as a duplex compound. In this design the high-pressure cylinder is placed directly below the low-pressure cylinder, as indicated, and both piston rods are attached to the same cross-head. The remainder of the engine is practically the same as a simple engine of the same type.
The Monarch Corliss EngineFig. 48. The Monarch Corliss Engine
Fig. 48. The Monarch Corliss Engine
Fig. 48shows a cross-compound engine of heavy design, built especially for rolling mill work. In this arrangement two complete engines are used, except for the main shaft and flywheel, which are common to both. The engine is so piped that the high-pressure cylinder exhausts into the low-pressure, through a receiver, the connection being under the floor and not shown in the illustration. One of the advantages of the cross-compound engine over other forms is that the cranks may be set 90 degrees apart, so that when one is on a dead center the other is approximately at its position of greatest effort.
The selection of an engine depends upon a number of conditions which vary to a considerable extent in different cases. Among these may be mentioned first cost, size and character of plant, available space, steam economy, and utilization of the exhaust steam. The question of first cost is usually considered in connection with that of operation, and items such as interest and depreciation are compared with the saving made through the saving in steam with high priced engines.
The principal use of the stationary engine is confined to the driving of electric generators and the furnishing of motive power in shops and factories. For the first of these uses, in cases where floor space is limited, as in office buildings, and where the power does not exceed about 100 I. H. P., the simple non-condensing high-speed engine is probably employed more than any other type. For larger installations, a saving may usually be made by the substitution of the moderate-speed four-valve engine. The question of simple and compound engines in this class of work depends largely upon the use made of the exhaust steam. In winter time the exhaust is nearly always utilized in the heating system, hence steam economy is not of great importance, and the simple engine answers all purposes at a smaller first cost. In localities where the heating season is comparatively short and fuel high, there is a decided advantage in using compound engines on account of their greater steam economy when operated within their economical range as regards load.
In large central plants where low cost of operation is always of first importance, it is common practice to use the best class of compound condensing engines of moderate or low speed. Those equipped with some form of Corliss valve gear are frequently found in this class of work. In the generation of power for shops and factories, where there is plenty of floor space, low-speed engines of the Corliss type are most commonly used. When space is limited, very satisfactory results may be obtained by using the moderate-speed four-valve engine. In deciding upon an engine for any particular case, the problem must be studied from all sides, and one be chosen which best answers the greatest number of requirements.
The principal information sought in the usual test of a steam engine is:
1. The indicated horsepower developed under certain standard conditions.
2. The friction of the engine, from which is determined the mechanical efficiency.
3. The steam consumption per indicated horsepower.
4. The general action of the valves.
5. The pressure conditions in the cylinder at different periods of the stroke.
The ultimate object of an efficiency test is to determine the foot-pounds of work delivered by the engine per pound of coal burned in the boiler furnaces. The general method of finding the pounds of dry steam evaporated per pound of coal has been treated inMachinery’sReference Series No. 67, “Boilers,” under the head of “Boiler Testing.” In the present case it is, therefore, only necessary to carry the process a step further and determine the foot-pounds of work developed per pound of steam.
The apparatus used in engine testing, in addition to that used in boiler testing, consists of a steam engineindicatorand reducing device for taking diagrams, and aplanimeterfor measuring them afterwards. If the test is made independently of the boiler test, a calorimeter for measuring the amount of moisture in the steam should be added to the outfit.
It has already been shown how a diagram may be made to represent graphically the work done in a steam engine cylinder during one stroke of the piston. The diagrams shown thus far have been theoretical or ideal cards constructed from assumed relations of the pressure acting and the distance moved through by the piston. An indicator is a device for making a diagram of what actually takes place in an engine cylinder under working conditions. Such a diagram shows the points of admission, cut-off, and release, and indicates accurately the pressures acting upon both sides of the piston at all points of the stroke.
Steam Engine IndicatorFig. 49. Steam Engine Indicator
Fig. 49. Steam Engine Indicator
A common form of steam engine indicator is shown inFig. 49. It consists of a cylinderCwhich is placed in communication atEwith one end of the engine cylinder by a proper pipe connection, provided with a quick opening and closing cock or valve. The cylinderCcontains a piston, above which is placed a coil spring of such strength that a given pressure per square inch acting upon the lower side of the piston will compress the spring a definite and known amount. Extending through the cap or head of cylinderCis a stem attached to the piston below, and connected by suitable levers with a pencil pointP. The arrangement of the levers is such that a certain rise of the piston causes the pointPto move upward in a vertical line a proportional amount.
The springs used above the piston vary in strength, and are designated as 20-pound, 40-pound, 60-pound, etc. A 20-pound spring is of such strength that a pressure of 20 pounds per square inch, acting beneath the piston in cylinderC, will raise the pencil point 1 inch. With a 40-pound spring, a pressure of 40 pounds per square inch will be required to raise the pencil 1 inch, and so on for the other strengths of spring.
The hollow drumDrotates back and forth upon a vertical stem at its center, its motion being produced by the stringH, which isattached by means of a suitable reducing motion to the cross-head of the engine. The return motion to the drum is obtained from a coil spring contained within it and not shown. The paper upon which the diagram is to be drawn is wound around the drumD, and held in place by the spring clipF.
In taking an indicator card, the length of stroke must be reduced to come within the limits of the drum, that is, it must be somewhat less than the circumference of drumD. In practice, the diagram is commonly made from 3 to 4 inches in length. There are a number of devices in use for reproducing the stroke of the engine on a smaller scale. The most accurate consists of a series of pulleys over which the cord passes on its way from the cross-head to the indicator drum.
The indicator is connected with the engine cylinder by means of special openings tapped close to the heads and either plugged or closed by means of stop-cocks when not in use. In some cases two indicators are used, one being connected to each end of the cylinder, while in others a single indicator is made to answer the purpose by being so piped that it can be connected with either end by means of a three-way cock. After the indicator is connected and the cord adjusted to give the proper motion to the drum, a card is attached, after which the three-way cock is opened and steam allowed to blow through the indicator to warm it up. The cock is now closed and the pencil pressed against the drum to get the so-called atmospheric line. The cock is again opened, and the pencil pressed lightly against the drum during one complete revolution of the engine. The cock is then thrown over to connect the indicator with the other end of the cylinder and the operation is repeated.
A Typical Indicator DiagramFig. 50. A Typical Indicator Diagram
Fig. 50. A Typical Indicator Diagram
The indicator card obtained in this way is shown inFig. 50. It is sometimes preferred to take the diagrams of the two ends on separate cards, but it is simpler to take them both on the same one, and also easier to compare the working of the two ends of the cylinder.
Diagram for Illustrating Method of ComputationFig. 51. Diagram for Illustrating Method of Computation
Fig. 51. Diagram for Illustrating Method of Computation
The analysis of a card for practical purposes is shown inFig. 51. Suppose, for example, that the length of the diagram measures 3.6 inches; the distance to the point of cut-off is 1.2 inch; and the distance to the point of release is 3.3 inches. Then, by dividing 1.2 by 3.6, the cut-off is found to occur at 1.2 ÷ 3.6 =1⁄3of the stroke. Releaseoccurs at 3.3 ÷ 3.6 = 0.92 of the stroke. Compression begins at (3.6 - 0.5) ÷ 3.6 = 0.86 of the stroke. The diagrams shown inFigs. 50and51are from non-condensing engines, and the back-pressure line is therefore above the atmospheric line, as indicated.
The indicator diagram gives a means of determining the mean effective pressure, from which the power of the engine can be found from the previously given equation
The method of determining the mean effective pressure is as follows: First measure the area of the card in square inches, by means of a planimeter (an instrument described later), and divide this area by the length in inches. This gives the mean ordinate; the mean ordinate, in turn, multiplied by the strength of spring used, will give the mean effective pressure in pounds per square inch. For example, suppose that the card shown inFig. 51is taken with a 60-pound spring, and that the area, as measured by a planimeter, is found to be 2.6 square inches. Dividing the area by the length gives 2.6 ÷ 3.6 = 0.722 inch as the mean ordinate, and this multiplied by the strength of spring gives a mean effective pressure of 0.722 × 60 = 43.3 pounds per square inch.
In practice, diagrams taken from the two ends of the cylinder usually vary more or less, due to inequalities in the valve action. Again, the effective area of the piston on the crank end is less than that on the head end, by an amount equal to the area of the piston rod. For these reasons it is customary to compute the mean effective pressure of all the cards separately, and take, for use in the formula, the average of the various computations. The corrected value of the piston area is, as already stated, equal to(2A-a)⁄2, in whichAis the area of the piston, andathe area of the piston rod. Substituting these values forAandPin the formula, together with the length of stroke and average number of revolutions per minute, the indicated horsepower is easily computed.
In making an ordinary test, diagrams are taken from both ends of the cylinder at 10-minute intervals for several hours, depending upon the accuracy required. The revolutions of the engine are counted for two or three-minute periods each time a pair of cards are taken, or still better, an automatic counter is used for the run, from which the average number of revolutions per minute may be determined.
The friction of the engine is determined by taking a pair of cards while “running light,” that is, with the belt thrown off, or the engine uncoupled, from the dynamo, if part of a direct-connected outfit. The friction load is then computed in horsepower from the indicator cards, and subtracted from the indicated horsepower when loaded. Thus we obtain the delivered or brake horsepower. The delivered horsepower divided by the indicated horsepower gives the mechanical efficiency. This may be expressed in the form of an equation as follows:
General Construction of PlanimeterFig. 52. General Construction of Planimeter
Fig. 52. General Construction of Planimeter
The planimeter is an instrument for measuring areas in general, and especially for measuring the areas of indicator cards. Some forms give the mean effective pressure directly, without computations, by changing the scale to correspond with the spring used in the indicator. A planimeter of this type is shown inFig. 52. The method of manipulating this instrument is as follows. Set the armBDequal to the length of the cardEF, by means of the thumb screwS, and set the wheel at zero on the scale, which must correspond to the spring used in the indicator. Next, place the pointDat about the middle of the area to be measured, and set pointCso that the armCBshall be approximately at right angles withBD. Then moveDto the upper left-hand corner of the diagram, and with the left hand moveCeither to the right or left until the wheel comes back exactly to the zero point on the scale; then press the point firmly into the paper. Now, goaround the outline of the diagram with pointDfrom left to right, finishing exactly at the starting point. The mean effective pressure may now be read from the scale opposite the edge of the wheel.
When very accurate results are required, the tracer pointDmay be passed over the diagram several times, and the reading divided by the number of times it is thus passed around. With short cards, 3 inches and under in length, it is best to make the armBDtwice the length of the card, and go around the diagram twice, taking the reading directly from the scale as in the first case.
When it is desired to determine accurately the water rate of an engine, a boiler test should be carried on simultaneously with the test upon the engine, from which the pounds of dry steam supplied may be determined as described inMachinery’sReference Series No. 67, “Boilers.” Knowing the average weight of steam supplied per hour for the run, and the average indicated horsepower developed during the same period, the water rate of the engine is easily computed. Sometimes the average cylinder condensation for a given type and make is known for certain standard conditions. In this case an approximation may be made from an indicator diagram which represents the average operation of the engine during the test.
Diagram for Calculating Steam ConsumptionFig. 53. Diagram for Calculating Steam Consumption
Fig. 53. Diagram for Calculating Steam Consumption
A diagram shows by direct measurement the pressure and volume at any point of the stroke, and the weight of steam per cubic foot for any given pressure may be taken directly from a steam table. The method, then, of finding the weight of steam at any point in the stroke is to find the volume in cubic feet, including the clearance and piston displacement to the given point, which must be taken at cut-off or later, and to multiply this by the weight per cubic foot corresponding to the pressure at the given point measured on the diagram. As this includes the steam used for compression, it must be corrected, as follows, to obtain the actual weight used per stroke. Take some convenient point on the compression curve, asQ, inFig. 53; measure its absolute pressure from the vacuum lineOXand compute the weight of steam to this point. Subtract this weight from that computed above for the given point on the expansion line, and the result will be the weight of steam used per stroke. The best point on the expansion line to use for this purpose is just before release, both because the maximum amount of leakage has taken place, and also because of the re-evaporation of a portion of the steam condensed during admission. The actual computation of the steam consumption from an indicator diagram is best shown by a practical illustration.
Example—LetFig. 53represent a diagram taken from the head end of a 16 × 30-inch non-condensing engine, running at a speed of 150 revolutions per minute; the card is taken with a 60-pound spring; the clearance of the engine is 6 per cent; the average cylinder condensation is 20 per cent of the total steam consumption; the diameter of the piston rod is 3 inches.
Measuring the card with a planimeter shows the mean effective pressure to be 48.2 pounds. The area of the piston is 201 square inches; the area of the piston rod is 7 square inches; hence, the average piston area =(2 × 201) - 7⁄2= 198 square inches, approximately. Then
InFig. 53,GHis the atmospheric line;OXis the line of vacuum or zero pressure, drawn so thatGO= 14.7 pounds on the scale; andOYis the clearance line, so drawn thatON= 0.06NX. The linePQis drawn fromOXto some point on the compression line, as atQ. FromC, a point on the expansion line, just before release, the lineCFis drawn perpendicular toOX. The following dimensions are now carefully measured from the actual diagram (not the one shown in the illustration), with the results given:
On the indicator diagram, being taken with a 60-pound spring, all vertical distances represent pounds per square inch, in the ratio of 60 pounds per inch of height. The stroke of the engine is 30 inches or 2.5 feet. The length of the diagramNXis 3.5 inches; hence, each inch in length represents2.5⁄3.5= 0.71 feet. From the above it is evident that vertical distances inFig. 53must be multiplied by 60 to reduce them to pounds pressure per square inch, and that horizontal distances must be multiplied by 0.71 to reduce them to feet. Making these reductions gives:
As a card from the head end of the cylinder is taken to avoid corrections for the piston rod, the area is 201 square inches or 1.4 square foot. With the above data the volume and weight of the steam in the cylinder can be computed at any point in the stroke. When the piston is atC, the volume is 1.4 × 2.27 = 3.18 cubic feet. When the piston is atQ, the volume is 1.4 × 0.30 = 0.42 cubic foot. From a steam table the weight of a cubic foot of steam at 48.6 pounds absolute pressure is found to be 0.116 pounds. Therefore, the weight of steam present when the piston is atCis 3.18 × 0.116 = 0.369 pounds. The weight of steam present when the piston is atQis 0.42 × 0.116 = 0.049 pound. That is the weight of steam in the cylinder at release is 0.369 pound, and the weight kept at exhaust closure for compression is 0.049 pound.
The weight exhausted per stroke is therefore 0.369 - 0.049 = 0.32 pound. The number of strokes per hour is 150 × 2 × 60 = 18,000, from which the steam accounted for by the diagram is found to be 18,000 × 0.32 = 5760 pounds, or 5760 ÷ 217 = 26.5 pounds per indicated horsepower per hour. If the cylinder condensation for this type of engine is 20 per cent of the total steam consumption, the water rate will be 26.5 ÷ 0.8 = 33.1 pounds per indicated horsepower per hour.
In the present case it has been assumed, for simplicity, that the head- and crank-end diagrams were exactly alike, except for the piston rod. Ordinarily, the above process should be carried out for both head and crank ends, and the results averaged.
Transcriber's note:Inconsistencies have not been corrected (hyphenated vs non-hyphenated or spaced words), except horse-power (changed to horsepower) and cut off (changed to cut-off) as elsewhere.Minor typographical errors have been corrected.In-line multi-line formulas have been changed to single-line formulas, where necessary with the addition of brackets to prevent ambiguity.Most tables and illustrations have been moved to the paragraph where they are first discussed or mentioned.
Transcriber's note: