of the pipes were made of No. 4 sheet copper ("Stubbs" gauge) and the fourth was made of No. 3 sheet. The following were the results, in lbs. per sq. in., of bursting-pressure: The theoretical bursting-pressure of the pipes was calculated by using the figures obtained in the tests for the strength of copper sheet with a brazed joint at 350° F. Pipes 1 and 2 are considered as having been annealed. The tests of specimens cut from the ruptured pipes show the injurious action of heat upon copper sheets; and that, while a white heat does not change the character of the metal, a heat of only slightly greater degree causes it to lose the fibrous nature that it has acquired in rolling, and a serious reduction in its tensile strength and ductility results. All the brazing was done by expert workmen, and their failure to make a pipe-joint without burning the metal at some point makes it probable that, with copper of this or greater thickness, it is seldom accomplished. That it is possible to make a joint without thus injuring the metal was proven in the cases of many of the specimens, both of those cut from the pipes and those made separately, which broke with a fibrous fracture. Rule for Thickness of Copper Steam-pipes. (U. S. Supervising Inspectors of Steam Vessels.)-Multiply the working steam-pressure in lbs. per sq. in. allowed the boiler by the diameter of the pipe in inches, then divide the product by the constant whole number 8000, and add .0625 to the quotient; the sum will give the thickness of material required. EXAMPLE.-Let 175 lbs. working steam-pressure per sq. in. allowed the 175 X 5 boiler, 5 in. = diameter of the pipe; then +.0625.1718+ inch, thickness required. 8000 Reinforcing Steam-pipes. (Eng., Aug. 11, 1893.)-In the Italian Navy copper pipes above 8 in. diam. are reinforced by wrapping them with a close spiral of copper or Delta-metal wire. Two or three independent spirals are used for safety in case one wire breaks. They are wound at a tension of about 11⁄2 tons per sq. in. Wire-wound Steam-pipes.-The system instituted by the British Admiralty of winding all steam-pipes over 8 in. in diameter with 3/16-in. copper wire, thereby about doubling the bursting-pressure, has within recent years been adopted on many merchant steamers using high-pressure steam, says the London Engineer. The results of some of the Admiralty tests showed that a wire pipe stood just about the pressure it ought to have stood when unwired, had the copper not been injured in the brazing. Riveted Steel Steam-pipes have recently been used for high pressures. See paper on A Method of Manufacture of Large Steam-pipes, by Chas. H. Manning, Trans. A. S. M. E., vol. xv. Valves in Steam-pipes.-Should a globe-valve on a steam-pipe have the steam-pressure on top or underneath the valve is a disputed question. With the steam-pressure on top, the stuffing-box around the valve-stem cannot be repacked without shutting off steam from the whole line of pipe; on the other hand, if the steam-pressure is on the bottom of the valve it all has to be sustained by the screw-thread on the valve-stem, and there is danger of stripping the thread. A correspondent of the American Machinist, 1892, says that it is a very uncommon thing in the ordinary globe-valve to have the thread give out, but by water-hammer and merciless screwing the seat will be crushed down quite frequently. Therefore with plants where only one boiler is used he advises placing the valve with the boiler-pressure underneath it. On plants where several boilers are connected to one main steam-pipe he would reverse the position of the valve, then when one of the valves needs repacking the valve can be closed and the pressure in the boiler whose pipe it controls can be reduced to atmospheric by lifting the safety-valve. The repacking can then be done without interfering with the operation of the other boilers of the plant. He proposes also the following other rules for locating valves: Place valves with the stems horizontal to avoid the formation of a water-pocket. Never put the junction-valve close to the boiler if the main pipe is above the boiler, but put it on the highest point of the junction-pipe. If the other plan is followed, the pipe fills with water whenever this boiler is stopped and the others are running, and breakage of the pipe may cause serious results. Never let a junction-pipe run into the bottom of the main pipe, but into the side or top. Always use an angle-valve where convenient, as there is more room in them. Never use a gate valve under high pressure unless a by-pass is used with it. Never open a blow-off valve on a boiler a little and then shut it; it is sure to catch the sediment and ruin the valve; throw it well open before closing. Never use a globe-valve on an indicator-pipe. For water, always use gate or angle valves or stop-cocks to obtain a clear pas sage. Buy if possible valves with renewable disks. Lastly, never let a man go inside a boiler to work, especially if he is to hammer on it, unless you break the joint between the boiler and the valve and put a plate of steel between the flanges. A Failure of a Brazed Copper Steam-pipe on the British steamer Prodano was investigated by Prof. J. O. Arnold. He found that the brazing was originally sound, but that it had deteriorated by oxidation of the zinc in the brazing alloy by electrolysis, which was due to the presence of fatty acids produced by decomposition of the oil used in the engines. A full account of the investigation is given in The Engineer, April 15, 1898. The "Steam Loop" is a system of piping by which water of condensation in steam-pipes is automatically returned to the boiler. In its simplest form it consists of three pipes, which are called the riser, the horizontal, and the drop-leg. When the steam-loop is used for returning to the boiler the water of condensation and entrainment from the steam-pipe through which the steam flows to the cylinder of an engine, the riser is gen. erally attached to a separator; this riser empties at a suitable height into the horizontal, and from thence the water of condensation is led into the drop-leg, which is connected to the boiler, into which the water of condensa tion is fed as soon as the hydrostatic pressure in drop-leg in connection with the steam-pressure in the pipes is sufficient to overcome the boiler-pressure. The action of the device depends on the following principles: Difference of pressure may be balanced by a water-column; vapors or liquids tend to flow to the point of lowest pressure; rate of flow depends on difference of pressure and mass; decrease of static pressure in a steam-pipe or chamber is proportional to rate of condensation; in a steam-current water will be carried or swept along rapidly by friction. (Illustrated in Modern Mechanism, p. 807.) Loss from an Uncovered Steam-pipe. (Bjorling on Pumpingengines.) The amount of loss by condensation in a steam-pipe carried down a deep mine-shaft has been ascertained by actual practice at the Clay Cross Colliery, near Chesterfield, where there is a pipe 71⁄2 in. internal diam., 1100 ft. long. The loss of steam by condensation was ascertained by direct measurement of the water deposited in a receiver, and was found to be equivalent to about 1 lb. of coal per I.H.P. per hour for every 100 ft. of steam-pipe; but there is no doubt that if the pipes had been in the upcast shaft, and well covered with a good non-conducting material, the loss would have been less. (For Steam-pipe Coverings, see p. 469, ante.) (THE STEAM-BOILER. The Horse-power of a Steam-boiler.-The term horse power has two meanings in engineering: First, an absolute unit or measure of the rate of work, that is, of the work done in a certain definite period of time, by a source of energy, as a steam-boiler, a waterfall, a current of air or water, or by a prime mover, as a steam-engine, a water-wheel, or a windmill. The value of this unit, whenever it can be expressed in foot-pounds of energy, as in the case of steam-engines, water-wheels, and waterfalls, is 33,000 foot-pounds per minute. In the case of boilers, where the work done, the conversion of water into steam, cannot be expressed in foot-pounds of available energy, the usual value given to the term horse-power is the evap. oration of 30 lbs. of water of a temperature of 100° F. into steam at 70 lbs. pressure above the atmosphere. Both of these units are arbitrary; the first, 33,000 foot-pounds per minute, first adopted by James Watt, being considered equivalent to the power exerted by a good London draught-horse, and the 30 lbs. of water evaporated per hour being considered to be the steam requirement per indicated horse-power of an average engine. The second definition of the term horse-power is an approximate measure of the size, capacity, value, or "rating" of a boiler, engine, water-wheel, or other source or conveyer of energy, by which measure it may be described, bought and sold, advertised, etc. No definite value can be given to this measure, which varies largely with local custom or individual opinion of makers and users of machinery. The nearest approach to uniformity which can be arrived at in the term "horse-power," used in this sense, is to say that a boiler, engine, water-wheel, or other machine, "rated" at a certain horse-power, should be capable of steadily developing that horse-power for a long period of time under ordinary conditions of use and practice, leaving to local custom, to the judgment of the buyer and seller, to written contracts of purchase and sale, or to legal decisions upon such contracts, the interpretation of what is meant by the term "ordinary conditions of use and practice." (Trans. A. S. M. E., vol. vii. p. 226.) The committee of the A. S. M. E. on Trials of Steam-boilers in 1884 (Trans., vol. vi. p. 265) discussed the question of the horse-power of boilers as follows: The Committee of Judges of the Centennial Exhibition, to whom the trials of competing boilers at that exhibition were intrusted, met with this same problem, and finally agreed to solve it, at least so far as the work of that committee was concerned, by the adoption of the unit, 30 lbs. of water evap. orated into dry steam per hour from feed-water at 100° F., and under a pressure of 70 lbs. per square inch above the atmosphere, these conditions being considered by them to represent fairly average practice. The quan. tity of heat demanded to evaporate a pound of water under these conditions is 1110.2 British thermal units, or 1.1496 units of evaporation. The unit of power proposed is thus equivalent to the development of 33,305 heat-units per hour, or 34.488 units of evaporation. Your committee, after due consideration, has determined to accept the Centennial Standard, the first above mentioned, and to recommend that in all standard trials the commercial horse-power be taken as an evaporation of 30 lbs. of water per hour from a feed-water temperature of 100° F. into steam at 70 lbs. gauge pressure, which shall be considered to be equal to 34% units of evaporation, that is, to 341⁄2 lbs. of water evaporated from a feedwater temperature of 212° F. into steam at the same temperature. This standard is equal to 33,305 thermal units per hour. It is the opinion of this committee that a boiler rated at any stated number of horse-powers should be capable of developing that power with easy firing, moderate draught, and ordinary fuel, while exhibiting good economy; and further, that the boiler should be capable of developing at least one third more than its rated power to meet emergencies at times when maximum economy is not the most important object to be attained. Unit of Evaporation. It is the custom to reduce results of boilertests to the common standard of weight of water evaporated by the unit weight of the combustible portion of the fuel, the evaporation being consid. ered to have taken place at mean atmospheric pressure, and at the temperature due that pressure, the feed-water being also assumed to have been supplied at that temperature. This is, in technical language, said to be the equivalent evaporation from and at the boiling point at atmospheric pressurc, or "from and at 212° F." This unit of evaporation, or one pound of water evaporated from and at 212°, is equivalent to 965.7 British thermal units. Measures for Comparing the Duty of Boilers.-The measure of the efficiency of a boiler is the number of pounds of water evaporated per pound of combustible, the evaporation being reduced to the standard of "from and at 212°;" that is, the equivalent evaporation from feed-water at a temperature of 212° F. into steam at the same temperature. The measure of the capacity of a boiler is the amount of "hoiler horsepower" developed, a horse-power being defined as the evaporation of 30 lbs. of water per hour from 100° F. into steam at 70 lbs. pressure, or 341⁄2 lbs. per hour from and at 212°. The measure of relative rapidity of steaming of boilers is the number of pounds of water evaporated per hour per square foot of water-heating surface. The measure of relative rapidity of combustion of fuel in boiler-furnaces is the number of pounds of coal burned per hour per square foot of gratesurface. STEAM-BOILER PROPORTIONS. Proportions of Grate and Heating Surface required for a given Horse-power.-The term horse-power here means capacity to evaporate 30 lbs. of water from 100° F., temperature of feed-water, to steam of 70 lbs., gauge-pressure = 34.5 lbs. from and at 212° F. Average proportions for maximum economy for land boilers fired with good anthracite coal: Heating surface per horse-power.. 66 Ratio of heating to grate surface... 11.5 sq. ft. Water evap'd from and at 212° per sq. ft. H.S. per hour 3 lbs. Combustible burned per H.P. per hour. 3 66 3.6" 66 Coal with 1/6 refuse, lbs. per H.P. per hour. 66 66 66 66 66 66 66 The rate of evaporation is most conveniently expressed in pounds evaporated from and at 212° per sq. ft. of water-heating surface per hour, and the rate of combustion in pounds of coal per sq. ft. of grate-surface per hour. Heating-surface.-For maximum economy with any kind of fuel a boiler should be proportioned so that at least one square foot of heatingsurface should be given for every 3 lbs. of water to be evaporated from and at 212° F. per hour. Still more liberal proportions are required if a portion of the heating-surface has its efficiency reduced by: 1. Tendency of the heated gases to short-circuit, that is, to select passages of least resistance and flow through them with high velocity, to the neglect of other passages. 2. Deposition of soot from smoky fuel. 3. Incrustation. If the heating-surfaces are clean, and the heated gases pass over it uniformly, little if any increase in economy can be obtained by increasing the heating-surface be yond the proportion of 1 sq. ft. to every 3 lbs. of water to be evaporated, and with all conditions favorable but little decrease of economy will take place if the proportion is 1 sq. ft. to every 4 lbs. evaporated; but in order to provide for driving of the boiler beyond its rated capacity, and for possible decrease of efficiency due to the causes above named, it is better to adopt 1 sq. ft. to 3 lbs. evaporation per hour as the minimum standard proportion. Where economy may be sacrificed to capacity, as where fuel is very cheap, it is customary to proportion the heating-surface much less liberally. The following table shows approximately the relative results that may be expected with different rates of evaporation, with anthracite coal. Lbs. water evapor'd from and at 212° per sq. ft. heating-surface per hour: 3 3.5 4 6 7 9 2 2.5 5 Sq. ft. heating-surface required per horse-power: 17.3 13.8 11.5 6.8 5.8 9.8 8.6 Ratio of heating to grate surface if 1/3 sq. ft. of G. S. is required per H.P.: 52 41.4 34.5 29.4 25.8 20.4 17.4 13.7 12.9 11.4 Probable relative economy: Probable temperature of chimney gases, degrees F.: 450 450 450 518 585 652 720 787 The relative economy will vary not only with the amount of heating-surface per horse-power, but with the efficiency of that heating-surface as regards its capacity for transfer of heat from the heated gases to the water, which will depend on its freedom from soot and incrustation, and upon the circulation of the water and the heated gases. With bituminous coal the efficiency will largely depend upon the thorough ness with which the combustion is effected in the furnace. The efficiency with any kind of fuel will greatly depend upon the amount. of air supplied to the furnace in excess of that required to support combustion. With strong draught and thin fires this excess may be very great, causing a serious loss of economy. Measurement of Heating-surface.-Authorities are not agreed as to the methods of measuring the heating-surface of steam-boilers. The usual rule is to consider as heating-surface all the surfaces that are surrounded by water on one side and by flame or heated gases on the other, but there is a difference of opinion as to whether tubular heating-surface should be figured from the inside or from the outside diameter. Some writers say, measure the heating-surface always on the smaller side-the fire side of the tube in a horizontal return tubular boiler and the water side in a water-tube boiler. Others would deduct from the heating-surface thus measured an allowance for portions supposed to be ineffective on account of being cov. ered by dust, or being out of the direct current of the gases. It has hitherto been the common practice of boiler-makers to consider all surfaces as heating-surfaces which transmit heat from the flame or gases to the water, making no allowance for different degrees of effectiveness; also, to use the external instead of the internal diameter of tubes, for greater convenience in calculation, the external diameter of boiler-tubes usually being made in even inches or half inches. This method, however, is inaccurate, for the true heating-surface of a tube is the side exposed to the hot gases, the inner surface in a fire-tube boiler and the outer surface in a water-tube boiler. The resistance to the passage of heat from the hot gases on one side of a tube or plate to the water on the other consists almost entirely of the resistance to the passage of the heat from the gases into the metal, the resistance of the metal itself and that of the wetted surface being practically nothing. See paper by C. W. Baker, Trans. A. S. M. E., vol. xix. RULE for finding the heating-surface of vertical tubular boilers: Multiply the circumference of the fire-box (in inches) by its height above the grate; multiply the combined circumference of all the tubes by their length, and to these two products add the area of the lower tube-sheet; from this sum subtract the area of all the tubes, and divide by 144: the quotient is the number of square feet of heating-surface. RULE for finding the heating-surface of horizontal tubular boilers: Take the dimensions in inches. Multiply two thirds of the circumference of the shell by its length; multiply the sum of the circumferences of all the tubes by their common length; to the sum of these products add two thirds of the area of both tube-sheets; from this sum subtract twice the combined area of all the tubes; divide the remainder by 144 to obtain the result in square feet. RULE for finding the square feet of heating-surface in tubes: Multiply the number of tubes by the diameter of a tube in inches, by its length in feet, and by .2618. Horse-power, Builder's Rating. Heating-surface per Horse-power. It is a general practice among builders to furnish about 12 square feet of heating-surface per horse-power, but as the practice is not uniform, bids and contracts should always specify the amount of heatingsurface to be furnished. Not less than one third square foot of grate-surface should be furnished per horse-power. Engineering News, July 5, 1894, gives the following rough-and-ready rule for finding approximately the commercial horse-power of tubular or watertube boilers: Number of tubes X their length in feet X their nominal diameter in inches 50 nLd÷ 50. The number of square feet of surface nrd L n Ld in the tubes is 12 3.82' = and the horse-power at 12 square feet of surface of tubes per horse-power, not counting the shell, nLd÷ 45.8. If 15 square feet of surface of tubes be taken, it is nLd57.3. Making allowance for the heating-surface in the shell will reduce the divisor to about 50. Horse-power of Marine and Locomotive Boilers.-The term horse-power is not generally used in connection with boilers in marine practice, or with locomotives. The boilers are designed to suit the engines, and are rated by extent of grate and heating-surface only. |