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suadcd by a '"Monday" hammer failed to make a passage. Our idea, had we succeeded, was to fit an ordinary tube stopper. (Fig. 2 shows the details of such a stopper, only the caps and stays are in different tubes.)
The tube had to be blanked off somehow, so in preference to cutting it out, we made the following repair quickly and in such a manner that we had no further trouble.
Two stays, 1 inch diameter, were put through the neighboring tubes, as shown in Fig. 2. Joints of well-made red lead putty and gauze wire were inserted between the ends of the tube and caps. Two plates, 1% inches thick, were in the meantime being prepared to fit over the ends of two stays, and of sufficient breadth to cover the cap. Two other joints of same material were put between the caps and plates.
When everything was ready the nuts were screwed up, care being taken to put an equal strain on both stays, so as to prevent the plates canting.
The boiler was filled up, and when water was past the tube level all was tight. When steam was raised a little more tension was put on the stays from the front end. As aforesaid, we had no further trouble with this tube.
When we reached port a good many of the tubes were renewed, to the great satisfaction of the ship's engineers.
Analyzing a Marine Steam Plant*
A good boiler will, under favorable conditions, absorb 80 to 85 percent of the heat given off by the fuel; but more usual results range from 60 to 70 percent, and some boilers of poor design, in poor condition, will run as low as 50 percent, particularly if crowded. To be efficient a boiler must have plenty of heating surface as compared to the grate surface and must not be crowded in respect to the load it carries. Practical conditions demand a compromise between efficiency and capacity. Also it must be remembered that it is not pounds of coal per horsepower that the owners care about, but the pounds of coal per mile traveled with passenger ships and pounds of coal per ton of freight per mile with cargo boats. The coal used per ton of freight is much more important than the coal per horsepower, and some boats make a better showing per ton mile when their steam plant shows a larger amount of coal per indicated horsepower. All steamship economics, whether expense or revenue, should be figured out on a basis of a ton mile, rather than per horsepower, because this gives a basis by which expenses and supplies can be compared with revenue.
The efficiency of the boiler depends upon the soot in the tubes and the scale within the boiler, among other things. The proper care of soot and scale is so wrell known to the practical engineer that nothing need be said, except to emphasize the importance of anything that will help these conditions, a? they greatly affect the economy of the boiler.
Many more boilers are improperly designed than generally supposed. They are strong enough, for the government sees to that; but the proportions are often not what they should be. The boiler has been built cheaply, and is small for the work expected or does not make proper allowance for reductions in pressure required by the government, or the smokestack is small, or large, or not high enough, or the tubes are not proportioned in area to the furnace, or are too short to bring the gases down to a proper stack temperature.
* For previous letters on this subject see April, May and June
In comparing boilers the most important characteristic is the ratio between the heating surface and the grate surface. In highly economical boilers this figure rises to forty—that is, the heating surface is forty times the grate surface. Some boilers on land, notably the German Fermenich, have as high as 64, but 40 is the average for a watertube marine boiler, while Scotch boilers vary from 25 t0 35- 0° tne other hand, the Scotch boiler must burn more coal per square foot of grate surface in order to compensate for its deficiency in heating surface.
The Scotch boiler weighs more and takes more space than any other type of steam boiler in use to-day for the same power and steam pressure, with the single exception of the locomotive type, which is the heaviest of all. The arguments for the Scotch boiler are its relatively low cost, its ease of access and upkeep, its ability to steam for long periods without attention and its capacity for absorbing variations of steam consumption and firing, because of the immense body of water which it holds.
A watertube boiler, whatever its advantages, is a constant worry to a water tender, because of the quickness with which it "loses its water," and feed lines cannot be kept going absolutely without stoppage, and while an auxiliary feed line may be in perfect condition theoretically, it is often found at the critical moment with frozen valves, missing valve wheels, or plugged with dirt, or with a leaky, dried-out gasket in some joint, or the auxiliary feed pump has been used as a fire pump and is full of mud, etc. All of these elements must be considered in those larger economies which are realized from well kept schedules and .reliable trips.
A Scotch boiler evaporates about ]A pound of water for each cubic foot of space occupied, and weighs about 2.3 pounds for each pound of water evaporated. A watertube boiler evaporates 1 pound to 1% pounds for each cubic foot of space occupied, and weighs 1 pound to 1^ pounds for each pound of water evaporated.
As this series of letters is intended to deal primarily with the analysis of existing plants, the foregoing explanations are made only that readers may "size up" an existing plant in which they may be interested, with a view to placing it with regard to what the plant ought to accomplish.
An engineer is always faced with two aspects of economy which are more or less opposed; he wants to operate as economically as possible, but a far more important feature is to be able to make his trips reliably and maintain his schedules to the last word. Repairs and improvemerits must be so carefully planned that the earning power of the boat is not interfered with. When a boat is laid up, or coaling, or in drydock, it is not earning a revenue for the owners and the boat's ability to earn revenue is its only right to existence. When it cannot do that, it must be scrapped. It is well to keep the expenses down, but it is far more important to keep the earnings up. This side of the question must be considered in connection with the boilers more than any other part of the ship, as it is entirely a matter of judgment when they should be cleaned, repaired, etc. In the same manner must an engineer decide about the use of boiler compounds, zinc plates, feed water filters, purifiers, bottom blow, surface blow, and the extent to which the boiler is crowded. The deciding question in connection with each of these devices is, do they increase the earning power of the ship?
The writer has in mind a coastwise steamer which installed evaporators. They saved a small amount in fresh water bills, but took the services of a good man and helper for one day each trip to clean. The superintending engineer transferred them to a transpacific steamer and spent the money realized on devices for cooling off the fireroom, which resulted in such increased efficiency of the firemen that the earning power of the ship was perceptibly increased and the water necessary for the short coastwise trips was purchased outright.
Another chief found a boiler compound which removed practically all of the boiler scale, but it corroded the boiler and went over into the engine, coated all of the working surfaces and interfered with the lubrication, and the boat missed a trip while the engines were cleaned out and the corroded boilers thoroughly inspected.
The two important points of the operation of a steam boiler, relative to economy, are the condition of the surfaces of the metal transmitting the heat and the difference of temperature. If the surfaces are coated with soot on the outside and scale on the inside, the efficiency may be very low indeed. The temperature of ingoing feed water is something more than 120 degrees F. The boilers' temperatures run from 300 to 500 degrees where a superheater is used. The furnace temperature ranges from 3,000 degrees Fahr. to 1,000 or less in the stack. The difference between the temperatures of the furnace and boiler is a measure of the rate of flow from the furnace to the boiler. If the furnace temperature is very high, more heat will go to the boiler, but more will also go up the stack. If the fire is not crowded, the boiler gets more of the heat because the stack temperatures are not so high. It is doubtful if superheaters increase the earning power of a ship unless on very long voyages, when consideration is given to their upkeep and the difficulty of lubricating the high pressure cylinders and valves and the ever-present trouble with rings, especially where the high pressure cylinder is jacketed, as on the U. S. S. Tacoma.
Another phase of economy is the preserving of the life of a boiler. The policy governing this falls on the superintending engineer more than the ship's chief because, owing to the unfortunate condition of our merchant marine, the average chief does not look upon his position as sufficiently permanent to take a deep and abiding interest in the permanent preservation of his plant. Such care as this would include protecting the boiler from sudden changes of temperature, such as opening the connection doors or the furnace doors except when necessary, the careful use of boiler compounds and zinc plates, corrosion from various causes chemical and physical in character, and cleaning of the interior.
After the heat has once been transmitted to the water and steam, the problem becomes one of preventing its
loss through radiation. The water and steam are only vehicles for the heat and the economy of the steam engine is limited by the physical properties of water and steam. Gas engines use the products of combustion as the vehicle of heat and are limited by their physical peculiarities. Refrigerating machines use ammonia, or ether, or carbon dioxide, or sulphur dioxide, or air, because each of these vehicles has properties which make refrigeration possible. When a pound of water is raised from about 120 degrees to the temperature corresponding to 165 pounds pressure, about 250 B. T. U. are required; then when the water is converted into steam, 850 B. T. U. in addition are required. Therefore, any condensation represents losses at the rate of 850 B. T. U. for each pound of steam condensed. The steam, as it arrives at the engine, contains from 2 to 10 percent of moisture. Take, for example, a 1,000 horsepower engine using 12 pounds of steam per hour per horsepower (an average condition), making a total of 12,000 pounds of water per hour. Five percent of moisture would be 600 pounds. At 850 B. T. U. this would be 510,000 B. T. U. Assuming that the coal runs an average of 8,000 B. T. U. per pound and the efficiency of the boiler is an average of 65 percent, there is a net 5,200 B. T. U. delivered to the water for each pound of coal, therefore 510,000 divided by 5,200 gives about 980 pounds of coal going to waste per hour because of the moisture in the steam. A good steam separator will remove the most of the moisture; but this is not economy, because it does not restore the heat which has been lost by the condensation of the steam.
The only true economy is to prevent, so far as possible, this loss of heat by lagging the boiler and steam pipes with the best lagging that can be procured. The lagging must be of such quality and quantity that the outside of the covering feels only slightly warm to the hand. Money is well spent on good lagging, as it is one of the truest and most profitable of economies.
The greatest value of superheated steam is that it practically eliminates the moisture. Saturated steam is at the same temperature as the water from which it came, but superheated steam is hotter than the water from which it came at that particular pressure and can, therefore, cool down a little before it condenses. Saturated steam,' on the other hand, condenses with the slightest cooling down. This elimination of moisture does not represent a real saving, though, because the heat is radiated slightly faster because of the higher temperature of superheated steam. Superheaters usually use waste heat from the stack, which represents a clear gain thermally, but they often cause practical troubles, as already noted.
All liquids have different boiling points at different pressures. They also have different pressures for various boiling points. For water at 212 degrees Fahr., the vapor pressure is 14.7 pounds per square inch (one atmosphere'). For pressures less than this, the temperatures, etc., are as shon-n in the table on page 388.
All of these data, as well as that for latent heat, may be secured from the steam tables which are in all technical engineer's pocket books, such as^Seaton's. Kent's, etc.
Since liquids have boiling points which vary with coinciding pressures, it follows that if the steam is not superheated, it is at the temperature due to the coinciding pressure. Conversely, if the pressure drops, the steam is left at the original temperature, which renders it superheated relative to the water, because it is at a higher temperature than the water. Dropping of the pressure and expansion are the same thing. If the expansion takes place while the steam is doing work, as in an engine cylinder, the temperature drops with the pressure, in accordance with the steam tables, hut if the expansion is made without the steam doing work, as in the receiver of an engine where the steam drops in pressure from the exhaust of the preceding cylinder, or through a reducing valve on the boiler, or through a half-closed throttle, the reduction in pressure leaves the steam at its original temperature, above the temperature due to the pressure, as given in the steam tables, and the steam is superheated. It has not been heated up. the pressure has simply dropped away from it. Not only is the steam left in a superheated state, but all of the moisture is immediately evaporated because of the reduced pressure. This is why an engine will stop knocking from water in the steam if the throttle be partially closed.
One of the least known and most prolific causes of a boiler priming is the improper placing of the feed water inlet to the boiler. For the water to be converted to steam requires a certain element of time. If the inlet be so placed that the water follows a normal route of circulation before coming to the surface, it will give no trouble: but if the inlet be placed at a point where normal circulation terminates and the water is converted to steam, much cold water will be carried away with the steam, giving the impression that the boilers are priming. In a Scotch boiler the water usually sinks on the sides and rises in the center. The writer has seen feed valves connected between the furnaces, and even over the furnaces, and in each case the boiler "primed" badly.
The next letter will take up the subject of the engine, treating it analytically from the standpoint maintained throughout this serial, of analyzing existing plants, with a view to increasing the earning power of the ship, rather than any attempt to write a text-book on marine engineering. After a consideration of the power plant in each of its departments and their economic relation to the whole, it is our hope (with the permission of the editor) to write a chapter or two upon auxiliary machinery and its relation to the earning power of a ship, cargo machinery, floating repair shops, and the construction of docks, from the standpoint of the efficiency and earning power of a steamship line.
E. N. Percy.
Losses To Merchant Shipping.—According to data compiled by the Journal of Commerce, the total losses to merchant shipping, arising from the war up to June 1 have been about 1,272 vessels of more than 2,585,362 gross
tons. Approximately 57 merchant vessels of all types, having an aggregate gross tonnage of about 116,724, were destroyed during the month of May.
Emergency Repair to Circulating Engine
During a trip in some heavy weather the nut on the top of the high pressure piston rod broke loose and backed off, due to the fact that there was no cotter pin through it. The nut backed off enough to punch a hole through the cast iron cylinder head, putting the engine out of commission. As there are usually no other means aboard most ships to circulate water through the condensers, except by the main circulating pumps, the temporary repair that we devised may be a handy piece of information for some other fellow in a similar fix.
This accident happened when we had some 1,400 miles to go. The weather was bad for two days and the barometer was still going down, so I considered that a temporary repair was urgent in order to keep the engines running at slow speed.
The cylinder head was cone shaped, as shown in Fig. 1, and the clearances between it and the piston small. When
Sketch or Repair* to Cylinder Head
the nut backed off, it punched the top out, as shown by the arrow in Fig. 1.
The broken head was taken off, and the piston removed from the rod. The crank brass of this engine was taken off and the piston rod, connecting rod and crosshead were then blocked up hard and secure. Then a piece of floor plate of ribbed steel 9/16 inch thick was cut with an air hammer to a rough circle of the diameter of the head, and stud holes drilled, and also a hole in the center to fit over the piston rod, as shown. Gaskets were made and grummets of asbestos packing were put around the studs and piston rod, and with washers on top of the grummets these were all set up tight. As the steam was turned in slowly, each nut was set up again and the engine was jacked so that the low pressure was just over the center.
After looking things over, steam was admitted to the engine and she started off very nicely, running satisfactorily until we were able to repair the broken head sufficiently to enable it being replaced. This was accomplished by forming a patch in the forge to fit over the break and securing this patch on the cylinder head with screws and iron cement. C. H. W.
Sale Of Chilean Ships To Be Prohibited.—Dispatches from Valparaiso indicate that the Chilean Congress, recently assembled, will pass an act prohibiting the sale of Chilean merchant ships to foreigners without the consent of the Chilean government. During the past sixteen months similar legislation has been enacted in the following countries: Great Britain, Belgium, Italy, AustriaHungary, Denmark, Germany, Russia, France, Greece, Norway, Brazil, Spain, Holland and Sweden.
Inquiries of General Interest Regarding Marine Engineering and Shipbuilding will be Answered in this Department
CONDUCTED BY H. A. EVERETT *
This department is maintained for the service of practical marine engineers, draftsmen and shipbuilders. All inquiries should bear the name and address of the writer. Anonymous communications will not be considered. The identity of the writer, however, will not be disclosed unless the editor is given permission to do so. Indicator cards taken from marine engines will be carefully analyzed, the defects pointed out, and the horsepoiver calculated, provided complete data are sent with the cards.
Size of Equalizer Leads
Q.—Our present plant consists of a 10-kilowatt. 125-voIt compound generator. A second unit, the exact duplicate of this is to be installed to operate in parallel with the first. Can you tell me how large an equalizer bus I should use? E. E.
A.—The equalizer leads may well be of the same size as the main generator leads. Practice varies somewhat in this matter, the range of size being from 0.5 to 1.0 times the sectional area of the leads to the generator.
Size of Propeller for Motor Boat
Q.—Will you please tell mc what size propeller I should use for a motor boat of the following dimensions: Length on watcrlinc, 34 feet 4 inches: breadth, 6 feet; displacement, fi.200 pounds: engine, six-cylinder, 6^-inch by 6J4-inch Niagain, 100 brake horsepower at 950 revolutions per minute. The hull is of the speed boat type and I hope to get 20 miles per hour out of her. M. B.
A.—For the horsepower given the speed is a reasonable one and a proper propeller would be 24 inches diameter by 26 inches pitch, three blades.
Direct and Indirect Valves
Q.—Isn't the use of the terms "direct" and "indirect" a recent innovation in describing engine valves? I do not remember having heard of it when I studied valves in school. Will you please tell me what they mean?
A.—A valve in which the admission of steam is controlled by the outside lap is called "direct," one taking steam on the inside is called "indirect." These terms have been in use for some time, though many of the earlier books on valve gears do not mention them.
Variation of Turning Moment in Propeller Shaft
Q.~Can you tell mc, in the case of a reciprocating engine, where it is well known that the turning moment acting on the propeller shaft is not uniform, if the amount of the variation is well known and what it would be for a compound engine of about 800 indicated horsepower which runs at 78 revolutions per minute? I can figure the mea-n turning moment due to the indicated horsepower, but what I want is. what force to use in the calculation for line shafting, etc.? S. H. P.
A.—About 40 percent for a compound engine with the cranks at 90 degrees is a reasonable figure and somewhat less, say 35 percent for a triple with cranks at 120 degrees. These figures are fairly well determined and are in common use for design. The figures quoted are the excess of the maximum over the mean torque or rotative effect.
Classification of Terry Turbine
Q.—In what classification of turbines would you place the Terry, and would its velocity diagram be similar to any of the other commonly known makes? T. T.
A.—The Terry turbine is an impulse turbine of the socalled velocity compounded type, that is, a high initial velocity is given to the steam by one expansion or pressure drop and this velocity is partially abstracted in the
* Professor of Marine Engineering, Post Graduate Department, United States Naval Academy, Annapolis, Md,
adjacent moving vanes with additional velocity abstractions by passing through additional moving vanes or as in the case of the Terry a repeated passing through the same vanes. The velocity diagram simulates very closely that of the Curtis, velocity compounded, single pressure stage, turbine.
Insulating Materials for Cold Storage Compartments
Q.—We are planning to insulate a small compartment for the carriage of cold-storage products, and I wondered if you could tell me whether there have been engineering tests to determine the best substances for thermal insulation. What do you consider as the best taking account of the severe wear and tear that is inevitable in an installation of this sort? I should greatly appreciate any help that you could give me in making a decision. R. E.
A.—Of the commercially available products for insulation of refrigerated spaces cork and balsa wood rank highest from the thermal point of view, and these have been selected from the others by extensive tests reported from time to time in the engineering journals. Cork is used both in the ground form and in blocks and the balsa wood usually in boards or blocks.
Increasing Voltage of Generator Set
Q.—We have a 10 kilowatt generator direct-connected to a small two-cylinder steam engine which operates at constant speed. When the set is running under full load the lights which are at the ends of long leads are dim. I attribute this to a reduction in voltage. Will you kindly tell me if this is so, and if so what I can do to raise it, and will there be any likelihood of getting into other trouble by raising the voltage? V. T.
A.—The trouble is due to a reduction in voltage in the long leads, which may he remedied as follows with no danger of other trouble arising as a consequence of the changes:
(a) If your generator is compound wound and at present equipped with a series shunt, the addition of resistance to this series shunt will increase the voltage of the generator under load.
(b) A similar result can be obtained by adjusting the governor of the engine to give a slightly higher speed.
(c) If it is a shunt generator a few turns of series winding per pole will probably help boost the voltage, but this may be difficult to do if limited facilities only are available.
Trouble with Feed Pump
Q.—Can you suggest anything that would be likely to make a feed pump, which had worked satisfactorily for years, suddenly start jumping and persist in this despite all coaxing and care. The pump is a vertical, simplex, Worthington, used for boiler feed and discharging at about 190 pounds. It is eight years old, is in good mechanical adjustment and operates well except for this recent and persistent jumping forward on the discharge stroke. Do you know of any good book which describes the common commercial pumps from the operator's point of view? P. M.
A.—There are many possible explanations; one that suggests itself is that the erratic action may be due to air leaking in during the suction stroke which is compressed during the first part of the discharge stroke. Air may leak in through the stuffing-box, blow-holes in castings, gaskets, drains, etc. If the pump will not take a suction when starting, the valves may have been leaking from the feed line and the pump may be vapor bound, in which case turn a hose on it to cool it off. Descriptive booklets issued by the makers probably give the most reliable descriptions and the makers are glad to send them free on request. For a splendid description of com390
MARINE ENGINEERING August, 1916
mon operating troubles and their remedies see an article that appeared in the Journal of the American Society of Naval Engineers, November, 1915, entitled "Notes on Tumps," by S. M. Robinson, which will probably give you the information desired.
Ratio of Turbine Reduction Gearing
Q.—What is the ratio of the main turbine shaft to the driven shaft revolutions in a De Laval single-stage turbine-driven electric generating unit? The name plate gives the revolutions per minute of the electric generator as 2,000. T. E. G.
A.—Probably 10 to I, as this a common ratio for De Laval units of small size.
Q.—Will you please tell me whether the Ljungstrom turbine is of the impulse or reaction type? I have been unable to find a definite statement concerning this point and would greatly appreciate an answer.
A.—The Ljungstrom turbine is a radial flow turbine and embodies both principles, impulse and reaction, as there is a drop in pressure accompanied by an increase of velocity through each successive set of vanes, and the velocity so generated is used in the following set as an impulse. It probably falls most nearly into the "reaction" 'class if we accept the loose use of that term which classes the Parsons type as a "reaction" unit.
Rate of Evaporation for Scotch and Watertube Boilers
Q.—What is reasonably good actual evaporation for a Scotch boiler generating steam at 180 pounds and a watertube boiler generating steam at 250 pounds? Can a feed pump be relied upon to measure the water fed to a boiler if we count strokes and know the pump dimensions?
S. O. L.
A.—Actual evaporation of from 9 to 10 pounds per pound of coal of about 14,000 B. T. U. calorific value and natural draft is good for the Scotch boiler, and 9 to 10 pounds per pound of steam is good for general operation of the watertube boiler with moderate drafts. The feed pump is an erratic and inaccurate meter for the feed water, as the length of stroke varies and the slip varies in a given pump. Moreover, estimates of slip depend usually upon tests upon other pumps than the one in use and mechanical adjustment affects this factor enormously.
Test for Steam Consumption of Peed Pump
Q.—I want to test out the steam consumption of a feed pump here without going too elaborately into it and without any more expense than necessary. I have thought that I might break the exhaust line and discharge directly into a barrel of cold water, which I could weigh before and alter. Do you think this would give me a reasonable estimate of it? Assist. Eng.
A.—If the feed pump is of moderate size, it is probable that the barrel of cold water will have insufficient condensing capacity to give reasonable accuracy. A large barrel will hold 55 gallons, which would weigh about 460 pounds of fresh water and 470 pounds of salt. If this were drawn at a temperature of 50 degrees Fahrenheit and we would condense a temperature of 180 degrees Fahrenheit, it would absorb approximately 180 — 50 = 130 B. T. U. per pound of water, or a total of 470 X 130 = 61,000 B. T. U. The heat given up by one pound of steam at atmospheric pressure (with a quality assumed of .90) is approximately the heat contents of the steam from the steam charts, minus the mean heat of the condensate, or 1,052 — 83 = 969 B. T. U., so that the barrel of water would condense only 61,000 ~ 969 = 63 pounds of steam, which would be a very short run for an average pump, and the result would be open to question to possibly 10 percent with ordinary weighing. A feed pump 7 inches steam by 4 inches water by 10 inches stroke uses about 500 pounds per hour, and if a run as short as five minutes can be considered typical, and the accuracy above quoted is sufficient, the method will do.
Comparison of Engine Performance
Q.—If we have two triple expansion engines of about the same indicated horsepower and revolutions per minute, but one working on a boiler pressure of 260 pounds absolute and the other on a boiler pressure of 180 pounds absolute, is it fair to compare steam consumption, dry steam, or even B. T. U.'s per indicated horsepower per hour, as measures of efficiency of design and' performance? It doesn't seem _ to me that it is, yet I do not know how to throw them on a basis which permits an equitable comparison. I should be greatly obliged if you would take up this point and give suggestions for doing it—provided, of course, that I am correct in my attitude as to the unfairness of comparison when the boiler pressures are so widely divergent. T. E.
A.—A comparison of engines on the basis of actual steam consumption given in pounds of steam used per indicated horsepower per hour is of no value unless qualified by a description of the "pound of steam," as the heat contents (which is really the energy available) differs enormously for a pound of steam at any given pressure dependent upon whether it is wet, dry, or superheated. The quotation of performance in terms of pounds of dry steam or B. T. U. per indicated horsepower per hour avoids this objection, and is correct as a thermal comparison or as a comparison of actual energy used to produce one indicated horsepower.
None of these, however, takes any cognizance of variations in external conditions or of the nearness to which the given unit approaches perfection in its field. Nearly every conception in engineering is relative, and a true comparison is properly one which takes account of the limitations which external conditions impose, therefore one of the best comparisons of engines is the ratio of the consumption in thermal units of the ideal, a Rankine engine, to that of the actual engine. This is variously called commercial efficiency, efficiency of performance, etc., and simply means the percentage of the ideal steam engine performance that the one under discussion has achieved. The Rankine, or ideal, engine would transform all the heat available by an adiabatic drop from the upper to the lower pressure into work and the expression for its efficiency is
Hi — Ht
where Hi = Heat contents at upper pressure. H2 = heat contents at lower pressure. (71= heat contents of the liquid at lower pressure. To illustrate by the case you cite of two engines of same general characteristics, but one operating on a cycle from 260 pounds absolute to, say, 2 pounds absolute, and the other from 180 pounds absolute to 2 pounds, using dry saturated steam at the upper pressure, the Rankine efficiency of the first is
1202 — 886
= 28.6 percent
1202 — 94 and of the second is
1195 — 904
= 26.4 percent.
1195 — 94
Therefore, for a fair comparison of performance, the lower pressure engine should be credited with this difference. That is, the consumption of the higher pressure engine should be augmented by
28.6 — 26.4
= 7.7 percent
to compare equitably the achievements of the designer, builder and operator in the final performance. Stated differently, a steam consumption of 14 pounds of dry steam per indicated horsepower per hour for the engine using steam at 260 pounds pressure, and a consumption of 14 X r-°77 = IS pounds of dry steam per indicated horsepower per hour for the engine using steam at 180 pounds are exactly equal attainments to the ideal. Therefore it is desirable to reduce engine tests to a common cyclical efficiency before analyzing for commercial efficiency.