Steam, Its Generation and Use Part 39

Capacity--Capacity, as already defined, is the ability of a definite amount of boiler-heating surface to generate steam. Boilers are ordinarily purchased under a manufacturer's specification, which rates a boiler at a nominal rated horse power, usually based on 10 square feet of heating surface per horse power. Such a builders' rating is absolutely arbitrary and implies nothing as to the limiting amount of water that this amount of heating surface will evaporate. It does not imply that the evaporation of 34.5 pounds of water from and at 212 degrees with 10 square feet of heating surface is the limit of the capacity of the boiler. Further, from a statement that a boiler is of a certain horse power on the manufacturer's basis, it is not to be understood that the boiler is in any state of strain when developing more than its rated capacity.

Broadly stated, the evaporative capacity of a certain amount of heating surface in a well-designed boiler, that is, the boiler horse power it is capable of producing, is limited only by the amount of fuel that can be burned under the boiler. While such a statement would imply that the question of capacity to be secured was simply one of making an arrangement by which sufficient fuel could be burned under a definite amount of heating surface to generate the required amount of steam, there are limiting features that must be weighed against the advantages of high capacity developed from small heating surfaces. Briefly stated, these factors are as follows:

1st. Efficiency. As the capacity increases, there will in general be a decrease in efficiency, this loss above a certain point making it inadvisable to try to secure more than a definite horse power from a given boiler. This loss of efficiency with increased capacity is treated below in detail, in considering the relation of efficiency to capacity.

2nd. Grate Ratio Possible or Practicable. All fuels have a maximum rate of combustion, beyond which satisfactory results cannot be obtained, regardless of draft available or which may be secured by mechanical means. Such being the case, it is evident that with this maximum combustion rate secured, the only method of obtaining added capacity will be through the addition of grate surface. There is obviously a point beyond which the grate surface for a given boiler cannot be increased. This is due to the impracticability of handling grates above a certain maximum size, to the enormous loss in draft pressure through a boiler resulting from an attempt to force an abnormal quant.i.ty of gas through the heating surface and to innumerable details of design and maintenance that would make such an arrangement wholly unfeasible.

3rd. Feed Water. The difficulties that may arise through the use of poor feed water or that are liable to happen through the use of practically any feed water have already been pointed out. This question of feed is frequently the limiting factor in the capacity obtainable, for with an increase in such capacity comes an added concentration of such ingredients in the feed water as will cause priming, foaming or rapid scale formation. Certain waters which will give no trouble that cannot be readily overcome with the boiler run at ordinary ratings will cause difficulties at higher ratings entirely out of proportion to any advantage secured by an increase in the power that a definite amount of heating surface may be made to produce.

Where capacity in the sense of overload is desired, the type of boiler selected will play a large part in the successful operation through such periods. A boiler must be selected with which there is possible a furnace arrangement that will give flexibility without undue loss in efficiency over the range of capacity desired. The heating surface must be so arranged that it will be possible to install in a practical manner, sufficient grate surface at or below the maximum combustion rate to develop the amount of power required. The design of boiler must be such that there will be no priming or foaming at high overloads and that any added scale formation due to such overloads may be easily removed.

Certain boilers which deliver commercially dry steam when operated at about their normal rated capacity will prime badly when run at overloads and this action may take place with a water that should be easily handled by a properly designed boiler at any reasonable load. Such action is ordinarily produced by the lack of a well defined, positive circulation.

Relation of Efficiency and Capacity--The statement has been made that in general the efficiency of a boiler will decrease as the capacity is increased. Considering the boiler alone, apart from the furnace, this statement may be readily explained.

Presupposing a constant furnace temperature, regardless of the capacity at which a given boiler is run; to a.s.sure equal efficiencies at low and high ratings, the exit temperature in the two instances would necessarily be the same. For this temperature at the high rating, to be identical with that at the low rating, the rate of heat transfer from the gases to the heating surfaces would have to vary directly as the weight or volume of such gases. Experiment has shown, however, that this is not true but that this rate of transfer varies as some power of the volume of gas less than one. As the heat transfer does not, therefore, increase proportionately with the volume of gases, the exit temperature for a given furnace temperature will be increased as the volume of gases increases. As this is the measure of the efficiency of the heating surface, the boiler efficiency will, therefore, decrease as the volume of gases increases or the capacity at which the boiler is operated increases.

Further, a certain portion of the heat absorbed by the heating surface is through direct radiation from the fire. Again, presupposing a constant furnace temperature; the heat absorbed through radiation is solely a function of the amount of surface exposed to such radiation.

Hence, for the conditions a.s.sumed, the amount of heat absorbed by radiation at the higher ratings will be the same as at the lower ratings but in proportion to the total absorption will be less. As the added volume of gas does not increase the rate of heat transfer, there are therefore two factors acting toward the decrease in the efficiency of a boiler with an increase in the capacity.

TABLE 59

TESTS OF BABc.o.c.k & WILc.o.x BOILERS WITH VARIOUS FUELS

______________________________________________________________________ |Number| | | | Rated | | of | Name and Location | Kind of Coal | Kind of | Horse | | Test | of Plant | | Furnace |Power of| | | | | | Boiler | | | | | | | |______|___________________________|________________|_________|________| | |Susquehanna Coal Co., |No. 1 Anthracite|Hand | | | 1 |Shenandoah, Pa. |Buckwheat |Fired | 300 | |______|___________________________|________________|_________|________| | |Balbach Smelting & |No. 2 Buckwheat |Wilkenson| | | 2 |Refining Co., Newark, N. J.|and Bird's-eye | Stoker | 218 | |______|___________________________|________________|_________|________| | |H. R. Worthington, |No. 2 Anthracite|Hand | | | 3 |Harrison N. J. |Buckwheat |Fired | 300 | |______|___________________________|________________|_________|________| | |Raymond Street Jail, |Anthracite Pea |Hand | | | 4 |Brooklyn, N. Y. | |Fired | 155 | |______|___________________________|________________|_________|________| | |R. H. Macy & Co., |No. 3 Anthracite|Hand | | | 5 |New York, N. Y. |Buckwheat |Fired | 293 | |______|___________________________|________________|_________|________| | |National Bureau of |Anthracite Egg |Hand | | | 6 |Standards, Was.h.i.+ngton, D.C.| |Fired | 119 | |______|___________________________|________________|_________|________| | |Fred. Loeser & Co., |No. 1 Anthracite|Hand | | | 7 |Brooklyn, N. Y. |Buckwheat |Fired | 300 | |______|___________________________|________________|_________|________| | |New York Edison Co., |No. 2 Anthracite|Hand | | | 8 |New York City |Buckwheat |Fired | 374 | |______|___________________________|________________|_________|________| | |Sewage Pumping Station, |Hocking Valley |Hand | | | 9 |Cleveland, O. |Lump, O. |Fired | 150 | |______|___________________________|________________|_________|________| | |Scioto River Pumping Sta., |Hocking Valley, |Hand | | | 10 |Cleveland, O. |O. |Fired | 300 | |______|___________________________|________________|_________|________| | |Consolidated Gas & Electric|Somerset, Pa. |Hand | | | 11 |Co., Baltimore, Md. | |Fired | 640 | |______|___________________________|________________|_________|________| | |Consolidated Gas & Electric|Somerset, Pa. |Hand | | | 12 |Co., Baltimore, Md. | |Fired | 640 | |______|___________________________|________________|_________|________| | |Merrimac Mfg. Co., |Georges Creek, |Hand | | | 13 |Lowell, Ma.s.s. |Md. |Fired | 321 | |______|___________________________|________________|_________|________| | |Great West'n Sugar Co., |Lafayette, Col.,|HandFired| | | 14 |Ft. Collins, Col. |Mine Run |Extension| 351 | |______|___________________________|________________|_________|________| | |Baltimore Sewage Pumping |New River |Hand | | | 15 | Station | |Fired | 266 | |______|___________________________|________________|_________|________| | |Tennessee State Prison, |Brushy Mountain,|Hand | | | 16 |Nashville, Tenn. |Tenn. |Fired | 300 | |______|___________________________|________________|_________|________| | |Pine Bluff Corporation, |Arkansas Slack |Hand | | | 17 |Pine Bluff, Ark. | |Fired | 298 | |______|___________________________|________________|_________|________| | |Pub. Serv. Corporation |Valley, Pa., |Roney | | | 18 |of N. J., Hoboken |Mine Run |Stoker | 520 | |______|___________________________|________________|_________|________| | |Pub. Serv. Corporation |Valley, Pa., |Roney | | | 19 |of N. J., Hoboken |Mine Run |Stoker | 520 | |______|___________________________|________________|_________|________| | |Frick Building, |Pittsburgh Nut |American | | | 20 |Pittsburgh, Pa. |and Slack |Stoker | 300 | |______|___________________________|________________|_________|________| | |New York Edison Co., |Loyal Hanna, Pa.|Taylor | | | 21 |New York City | |Stoker | 604 | |______|___________________________|________________|_________|________| | |City of Columbus, O., |Hocking Valley, |Detroit | | | 22 |Dept. Lighting |O. |Stoker | 300 | |______|___________________________|________________|_________|________| | |Edison Elec. Illum. Co., |New River |Murphy | | | 23 |Boston, Ma.s.s. | |Stoker | 508 | |______|___________________________|________________|_________|________| | |Colorado Springs & |Pike View, Col.,|Green Chn| | | 24 |Interurban Ry., Col. |Mine Run |Grate | 400 | |______|___________________________|________________|_________|________| | |Pub. Serv. Corporation |Lancas.h.i.+re, Pa. |B&W.Chain| | | 25 |of N. J., Marion | |Grate | 600 | |______|___________________________|________________|_________|________| | |Pub. Serv. Corporation |Lancas.h.i.+re, Pa. |B&W.Chain| | | 26 |of N. J., Marion | |Grate | 600 | |______|___________________________|________________|_________|________| | |Erie County Electric Co., |Mercer County, |B&W.Chain| | | 27 |Erie, Pa. |Pa. |Grate | 508 | |______|___________________________|________________|_________|________| | |Union Elec. Lt. & Pr. Co., |Mascouth, Ill. |B&W.Chain| | | 28 |St. Louis, Mo. | |Grate | 508 | |______|___________________________|________________|_________|________| | |Union Elec. Lt. & Pr. Co., |St. Clair |B&W.Chain| | | 29 |St. Louis, Mo. |County, Ill. |Grate | 508 | |______|___________________________|________________|_________|________| | |Commonwealth Edison Co., |Carterville, |B&W.Chain| | | 30 |Chicago, Ill. |Ill., Screenings|Grate | 508 | |______|___________________________|________________|_________|________|

________________________________________________________________ |Number|Grate |Dura-|Steam |Temper-|Degrees|Factor| Draft | | of |Surf. | tion|Pres. | ature | Super | of | In | At | | Test |Square|Test | By | Water | -heat |Evapo-|Furnace|Boiler| | | Feet |Hours|Gauge |Degrees|Degrees|ration|Inches |Damper| | | | |Pounds| Fahr. | Fahr. | |Upr/Lwr|Inches| |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 1 | 84 | 8 | 68 | 53.9 | |1.1965| +.41 | .21 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | +.65 | | | 2 | 51.6 | 7 | 136.3| 203 | 150 |1.1480| .47 | .56 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 3 | 67.6 | 8 | 139 | 139.6 | 139 |1.1984| .70 | .96 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 4 | 40 | 8 | 110.2| 137 | |1.1185| .33 | .43 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 5 | 59.5 | 10 | 133.2| 75.2 | |1.1849| .19 | .40 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 6 | 26.5 | 18 | 132.1| 70.5 | |1.1897| .33 | | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | +.51 | | | 7 | 48.9 | 7 | 101. | 121.3 | |1.1333| -.20 | .30 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 8 | 59.5 | 6 | 191.8| 88.3 | |1.1771| .50 | | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 9 | 27 | 24 | 156.3| 58 | |1.2051| .10 | .24 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 10 | | 24 | 145 | 75 | |1.1866| .26 | .46 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 11 | 118 | 8 | 170 | 186.1 | 66.7 |1.1162| .34 | .42 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 12 | 118 | 7.92| 173 | 180.2 | 75.2 |1.1276| .44 | .58 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 13 | 52 | 24 | 75 | 53.3 | |1.1987| .25 | .35 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 14 | 59.5 | 8 | 105 | 35.8 | |1.2219| .17 | .38 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 15 | 59.5 | 24 | 170.1| 133 | |1.1293| .12 | .43 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 16 | 51.3 | 10 | 105 | 75.1 | |1.1814| .21 | .42 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 17 | 59.5 | 8 | 149.2| 71 | |1.1910| .35 | .59 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 18 | 103.2| 10 | 133.2| 65.3 | 65.9 |1.2346| .05 | .49 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 19 | 103.2| 9 | 139 | 64 | 80.2 |1.2358| .18 | .57 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 20 | 53 | 9 | 125 | 76.6 | |1.1826| +1.64 | .64 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 21 | 75 | 8 | 198.5| 165.1 | 104 |1.1662| +3.05 | .60 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 22 | | 9 | 140 | 67 | 180 |1.2942| .22 | .35 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 23 | 90 |16.25| 199 | 48.4 | 136.5 |1.2996| .23 | 1.27 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 24 | 103 | 8 | 129 | 56 | |1.2002| .23 | .30 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | +.52 | | | 25 | 132 | 8 | 200 | 57.2 | 280.4 |1.3909| +.19 | .52 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | +.15 | | | 26 | 132 | 8 | 199 | 60.7 | 171.0 |1.3191| .04 | .52 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 27 | 90 | 8 | 120 | 69.9 | |1.1888| .31 | .58 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 28 | 103.5| 8 | 180 | 46 | 113 |1.2871| .62 | 1.24 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 29 | 103.5| 8 | 183 | 53.1 | 104 |1.2725| .60 | 1.26 | |______|______|_____|______|_______|_______|______|_______|______| | | | | | | | | | | | 30 | 90 | 7 | 184 | 127.1 | 180 |1.2393| .68 | 1.15 | |______|______|_____|______|_______|_______|______|_______|______| ______________________________________________________________ |Number|Temper-| Coal | | of | ature | Total | Moist-| Total |Ash and| Total |DryCoal| | Test |FlueGas|Weight:| ure | dry | Refuse|Combus-|/sq.ft.| | |Degrees| Fired | Per | Coal | Per | tible | Grate | | | Fahr. |Pounds | Cent | Pounds| Cent | Pounds|/Hr.Lb.| |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 1 | | 11670 | 4.45 | 11151 | 26.05 | 8248 | 16.6 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 2 | 487 | 8800 | 7.62 | 8129 | 29.82 | 5705 | 19.71 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 3 | 559 | 10799 | 6.42 | 10106 | 20.02 | 8081 | 21.77 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 4 | 427 | 5088 | 4.00 | 4884 | 19.35 | 3939 | 15.26 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 5 | 414 | 9440 | 2.14 | 9238 | 11.19 | 8204 | 15.52 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 6 | 410 | 8555 | 3.62 | 8245 | 15.73 | 6948 | 17.28 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 7 | 480 | 7130 | 7.38 | 6604 | 18.35 | 5392 | 19.29 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 8 | 449 | 7500 | 2.70 | 7298 | 27.94 | 5259 | 14.73 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 9 | 410 | 15087 | 7.50 | 13956 | 11.30 | 12379 | 21.5 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 10 | 503 | 29528 | 7.72 | 27248 | | | 24.7 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 11 | 487 | 20400 | 2.84 | 19821 | 7.83 | 18269 | 21.00 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 12 | 494 | 21332 | 2.29 | 20843 | 8.23 | 19127 | 22.31 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 13 | 516 | 24584 | 4.29 | 23529 | 7.63 | 21883 | 18.85 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 14 | 523 | 15540 | 18.64 | 12643 | | | 28.59 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 15 | 474 | 18330 | 2.03 | 17958 | 16.36 | 16096 | 12.57 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 16 | 536 | 12243 | 2.14 | 11981 | | | 23.40 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 17 | 534 | 10500 | 3.04 | 10181 | | | 21.40 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 18 | 458 | 18600 | 3.40 | 17968 | 18.38 | 14665 | 17.41 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 19 | 609 | 23400 | 2.56 | 22801 | 16.89 | 18951 | 24.55 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 20 | 518 | 10500 | 1.83 | 10308 | 12.22 | 9048 | 21.56 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 21 | 536 | 25296 | 2.20 | 24736 | | | 41.0 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 22 | 511 | 14263 | 8.63 | 13032 | | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 23 | 560 | 39670 | 4.22 | 37996 | 4.32 | 36355 | 25.98 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 24 | 538 | 23000 | 23.73 | 17542 | | | 21.36 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 25 | 590 | 32205 | 4.03 | 30907 | 15.65 | 26070 | 29.26 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 26 | 529 | 24243 | 4.09 | 23251 | 12.33 | 20385 | 22.01 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 27 | 533 | 22328 | 4.42 | 21341 | 16.88 | 17739 | 29.64 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 28 | 523 | 32163 | 13.74 | 27744 | | | 33.50 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 29 | 567 | 36150 | 14.62 | 30865 | | | 37.28 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 30 | | 30610 | 11.12 | 27206 | 14.70 | 23198 | 43.20 | |______|_______|_______|_______|_______|_______|_______|_______|

______________________________________________________________ |Number| Water | | Flue Gas a.n.a.lysis | | of |Actual | Equiv.|ditto /|% Rated|CO_{2} | O | CO | | Test |Evapor-|Evap. @|sq.ft. |Cap'ty.| Per | Per | Per | | | ation |>=212 |Heating|Develpd| Cent | Cent | Cent | | |/Hr.Lb.|/Hr.Lb.|Surface|PerCent| | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 1 | 10268 | 12286 | 4.10 | 118.7 | | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 2 | 8246 | 9466 | 4.34 | 125.7 | | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 3 | 9145 | 10959 | 3.65 | 105.9 | | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 4 | 5006 | 5599 | 3.61 | 104.7 | 12.26 | 7.88 | 0.0 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 5 | 7434 | 8809 | 3.06 | 87.2 | | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 6 | 2903 | 3454 | 2.91 | 84.4 | | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 7 | 7464 | 8459 | 2.82 | 81.7 | | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 8 | 9164 | 10787 | 2.88 | 83.5 | | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 9 | 4374 | 5271 | 3.51 | 101.8 | 11.7 | 7.3 | 0.07 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 10 | 8688 | 10309 | 3.44 | 99.6 | 12.9 | 5.0 | 0.2 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 11 | 24036 | 26829 | 4.19 | 121.5 | 12.5 | 6.4 | 0.5 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 12 | 25313 | 28544 | 4.46 | 129.3 | 13.3 | 5.1 | 0.5 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 13 | 9168 | 10990 | 3.42 | 99.3 | 9.6 | 8.8 | 0.4 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 14 | 11202 | 13689 | 3.91 | 113.5 | 9.1 | 9.9 | 0.0 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 15 | 7565 | 8543 | 3.21 | 93.1 | 10.71 | 9.10 | 0.0 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 16 | 9512 | 11237 | 3.74 | 108.6 | | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 17 | 9257 | 11025 | 3.70 | 107.2 | | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 18 | 15887 | 19614 | 3.77 | 108.7 | 11.7 | 7.7 | 0.0 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 19 | 21320 | 26347 | 5.06 | 146.7 | 11.9 | 7.8 | 0.0 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 20 | 9976 | 11978 | 3.93 | 112.0 | 11.3 | 7.5 | 0.0 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 21 | 28451 | 33066 | 5.47 | 158.6 | 12.3 | 6.4 | 0.7 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 22 | 10467 | 13526 | 4.51 | 130.7 | 11.9 | 7.2 | 0.04 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 23 | 20700 | 26902 | 5.30 | 153.5 | 11.1 | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 24 | 14650 | 17583 | 4.40 | 127.4 | | | | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 25 | 28906 | 40205 | 6.70 | 194.2 | 10.5 | 8.3 | 0.0 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 26 | 23074 | 30437 | 5.07 | 147.0 | 10.1 | 9.0 | 0.0 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 27 | 20759 | 24678 | 4.85 | 140.8 | 10.1 | 9.1 | 0.0 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 28 | 21998 | 28314 | 5.67 | 161.5 | 8.7 | 10.6 | 0.0 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 29 | 24386 | 31031 | 6.11 | 177.1 | 8.9 | 10.7 | 0.2 | |______|_______|_______|_______|_______|_______|_______|_______| | | | | | | | | | | 30 | 30505 | 37805 | 7.43 | 215.7 | 10.4 | 9.4 | 0.2 | |______|_______|_______|_______|_______|_______|_______|_______|

_______________________________________________________ |Number| Proximate a.n.a.lysis Dry Coal | Equiv.|Combnd.| | of |Volatl.| Fixed | Ash |B.t.u./|Evap. @|Efficy.| | Test |Matter |Carbon | Per | Pound |>=212/|Boiler | | | Per | Per | Cent | Dry | Pound |& Grate| | | Cent | Cent | | Coal |DryCoal|PerCent| |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 1 | | | 26.05 | 11913 | 8.81 | 71.8 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 2 | | | | 11104 | 8.15 | 72.1 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 3 | 5.55 | 80.60 | 13.87 | 12300 | 8.67 | 68.4 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 4 | 7.74 | 77.48 | 14.78 | 12851 | 9.17 | 69.2 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 5 | | | | 13138 | 9.53 | 69.6 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 6 | 6.13 | 84.86 | 9.01 | 13454 | 9.57 | 69.0 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 7 | | | | 12224 | 8.97 | 71.2 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 8 | 0.55 | 86.73 | 12.72 | 12642 | 8.87 | 68.1 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 9 | 39.01 | 48.08 | 12.91 | 12292 | 9.06 | 71.5 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 10 | 38.33 | 46.71 | 14.96 | 12284 | 9.08 | 71.7 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 11 | 19.86 | 73.02 | 7.12 | 14602 | 10.83 | 72.0 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 12 | 20.24 | 72.26 | 7.50 | 14381 | 10.84 | 73.2 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 13 | | | | 14955 | 11.21 | 72.7 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 14 | 39.60 | 54.46 | 5.94 | 11585 | 8.66 | 72.5 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 15 | 17.44 | 76.42 | 5.84 | 15379 | 11.42 | 72.1 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 16 | 33.40 | 54.73 | 11.87 | 12751 | 9.38 | 71.4 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 17 | 15.42 | 62.48 | 22.10 | 12060 | 8.66 | 69.6 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 18 | 14.99 | 75.13 | 9.88 | 14152 | 10.92 | 74.88 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 19 | 14.40 | 74.33 | 11.27 | 14022 | 10.40 | 71.97 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 20 | 32.44 | 56.71 | 10.85 | 13510 | 10.30 | 74.6 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 21 | 19.02 | 72.09 | 8.89 | 14105 | 10.69 | 73.5 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 22 | 32.11 | 53.93 | 13.96 | 12435 | 9.41 | 73.4 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 23 | 19.66 | 75.41 | 4.93 | 14910 | 11.51 | 74.9 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 24 | 43.57 | 46.22 | 10.21 | 11160 | 8.02 | 69.7 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 25 | 22.84 | 69.91 | 7.25 | 13840 | 10.41 | 72.6 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 26 | 32.36 | 60.67 | 6.97 | 14027 | 10.47 | 72.1 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 27 | 33.26 | 54.03 | 12.71 | 12742 | 9.25 | 70.4 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 28 | 28.96 | 46.88 | 24.16 | 10576 | 8.16 | 74.9 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 29 | 36.50 | 41.20 | 22.30 | 10849 | 8.04 | 71.9 | |______|_______|_______|_______|_______|_______|_______| | | | | | | | | | 30 | | | 10.24 | 13126 | 9.73 | 71.9 | |______|_______|_______|_______|_______|_______|_______|

[Ill.u.s.tration: 15400 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers and Superheaters, Equipped with Babc.o.c.k & Wilc.o.x Chain Grate Stokers at the Plant of the Twin City Rapid Transit Co., Minneapolis, Minn.]

This increase in the efficiency of the boiler alone with the decrease in the rate at which it is operated, will hold to a point where the radiation of heat from the boiler setting is proportionately large enough to be a governing factor in the total amount of heat absorbed.

The second reason given above for a decrease of boiler efficiency with increase of capacity, viz., the effect of radiant heat, is to a greater extent than the first reason dependent upon a constant furnace temperature. Any increase in this temperature will affect enormously the amount of heat absorbed by radiation, as this absorption will vary as the fourth power of the temperature of the radiating body. In this way it is seen that but a slight increase in furnace temperature will be necessary to bring the proportional part, due to absorption by radiation, of the total heat absorbed, up to its proper proportion at the higher ratings. This factor of furnace temperature more properly belongs to the consideration of furnace efficiency than of boiler efficiency. There is a point, however, in any furnace above which the combustion will be so poor as to actually reduce the furnace temperature and, therefore, the proportion of heat absorbed through radiation by a given amount of exposed heating surface.

Since it is thus true that the efficiency of the boiler considered alone will increase with a decreased capacity, it is evident that if the furnace conditions are constant regardless of the load, that the combined efficiency of boiler and furnace will also decrease with increasing loads. This fact was clearly proven in the tests of the boilers at the Detroit Edison Company.[74] The furnace arrangement of these boilers and the great care with which the tests were run made it possible to secure uniformly good furnace conditions irrespective of load, and here the maximum efficiency was obtained at a point somewhat less than the rated capacity of the boilers.

In some cases, however, and especially in the ordinary operation of the plant, the furnace efficiency will, up to a certain point, increase with an increase in power. This increase in furnace efficiency is ordinarily at a greater rate as the capacity increases than is the decrease in boiler efficiency, with the result that the combined efficiency of boiler and furnace will to a certain point increase with an increase in capacity. This makes the ordinary point of maximum combined efficiency somewhat above the rated capacity of the boiler and in many cases the combined efficiency will be practically a constant over a considerable range of ratings. The features limiting the establis.h.i.+ng of the point of maximum efficiency at a high rating are the same as those limiting the amount of grate surface that can be installed under a boiler. The relative efficiency of different combinations of boilers and furnaces at different ratings depends so largely upon the furnace conditions that what might hold for one combination would not for another.

In view of the above, it is impossible to make a statement of the efficiency at different capacities of a boiler and furnace which will hold for any and all conditions. Fig. 40 shows in a general form the relation of efficiency to capacity. This curve has been plotted from a great number of tests, all of which were corrected to bring them to approximately the same conditions. The curve represents test conditions.

The efficiencies represented are those which may be secured only under such conditions. The general direction of the curve, however, will be found to hold approximately correct for operating conditions when used only as a guide to what may be expected.

[Graph: Combined Efficiency of Boiler and Furnace Per Cent against Per Cent of Boiler's Rated Capacity Developed

Fig. 40. Approximate Variation of Efficiency with Capacity under Test Conditions]

Economical Loads--With the effect of capacity on economy in mind, the question arises as to what const.i.tutes the economical load to be carried. In figuring on the economical load for an individual plant, the broader economy is to be considered, that in which, against the boiler efficiency, there is to be weighed the plant first cost, returns on such investment, fuel cost, labor, capacity, etc., etc. This matter has been widely discussed, but unfortunately such discussion has been largely limited to central power station practice. The power generated in such stations, while representing an enormous total, is by no means the larger proportion of the total power generated throughout the country.

The factors determining the economic load for the small plant, however, are the same as in a large, and in general the statements made relative to the question are equally applicable.

The economical rating at which a boiler plant should be run is dependent solely upon the load to be carried by that individual plant and the nature of such load. The economical load for each individual plant can be determined only from the careful study of each individual set of conditions or by actual trial.

The controlling factor in the cost of the plant, regardless of the nature of the load, is the capacity to carry the maximum peak load that may be thrown on the plant under any conditions.

While load conditions, do, as stated, vary in every individual plant, in a broad sense all loads may be grouped in three cla.s.ses: 1st, the approximately constant 24-hour load; 2nd, the steady 10 or 12-hour load usually with a noonday period of no load; 3rd, the 24-hour variable load, found in central station practice. The economical load at which the boiler may be run will vary with these groups:

1st. For a constant load, 24 hours in the day, it will be found in most cases that, when all features are considered, the most economical load or that at which a given amount of steam can be produced the most cheaply will be considerably over the rated horse power of the boiler.

How much above the rated capacity this most economic load will be, is dependent largely upon the cost of coal at the plant, but under ordinary conditions, the point of maximum economy will probably be found to be somewhere between 25 and 50 per cent above the rated capacity of the boilers. The capital investment must be weighed against the coal saving through increased thermal efficiency and the labor account, which increases with the number of units, must be given proper consideration.

When the question is considered in connection with a plant already installed, the conditions are different from where a new plant is contemplated. In an old plant, where there are enough boilers to operate at low rates of capacity, the capital investment leads to a fixed charge, and it will be found that the most economical load at which boilers may be operated will be lower than where a new plant is under consideration.

2nd. For a load of 10 or 12 hours a day, either an approximately steady load or one in which there is a peak, where the boilers have been banked over night, the capacity at which they may be run with the best economy will be found to be higher than for uniform 24-hour load conditions.

This is obviously due to original investment, that is, a given amount of invested capital can be made to earn a larger return through the higher overload, and this will hold true to a point where the added return more than offsets the decrease in actual boiler efficiency. Here again the determining factors of what is the economical load are the fuel and labor cost balanced against the thermal efficiency. With a load of this character, there is another factor which may affect the economical plant operating load. This is from the viewpoint of spare boilers. That such added capacity in the way of spares is necessary is unquestionable.

Since they must be installed, therefore, their presence leads to a fixed charge and it is probable that for the plant, as a whole, the economical load will be somewhat lower than if the boilers were considered only as spares. That is, it may be found best to operate these spares as a part of the regular equipment at all times except when other boilers are off for cleaning and repairs, thus reducing the load on the individual boilers and increasing the efficiency. Under such conditions, the added boiler units can be considered as spares only during such time as some of the boilers are not in operation.

Due to the operating difficulties that may be encountered at the higher overloads, it will ordinarily be found that the most economical ratings at which to run boilers for such load conditions will be between 150 and 175 per cent of rating. Here again the maximum capacity at which the boilers may be run for the best plant economy is limited by the point at which the efficiency drops below what is warranted in view of the first cost of the apparatus.

3rd. The 24-hour variable load. This is a cla.s.s of load carried by the central power station, a load constant only in the sense that there are no periods of no load and which varies widely with different portions of the 24 hours. With such a load it is particularly difficult to make any a.s.sertion as to the point of maximum economy that will hold for any station, as this point is more than with any other cla.s.s of load dependent upon the factors entering into the operation of each individual plant.

The methods of handling a load of this description vary probably more than with any other kind of load, dependent upon fuel, labor, type of stoker, flexibility of combined furnace and boiler etc., etc.

In general, under ordinary conditions such as appear in city central power station work where the maximum peaks occur but a few times a year, the plant should be made of such size as to enable it to carry these peaks at the maximum possible overload on the boilers, sufficient margin of course being allowed for insurance against interruption of service.

With the boilers operating at this maximum overload through the peaks a large sacrifice in boiler efficiency is allowable, provided that by such sacrifice the overload expected is secured.

[Ill.u.s.tration: Portion of 4890 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers at the Billings Sugar Co., Billings, Mont. 694 Horse Power of these Boilers are Equipped with Babc.o.c.k and Wilc.o.x Chain Grate Stokers]

Some methods of handling a load of this nature are given below:

Certain plant operating conditions make it advisable, from the standpoint of plant economy, to carry whatever load is on the plant at any time on only such boilers as will furnish the power required when operating at ratings of, say, 150 to 200 per cent. That is, all boilers which are in service are operated at such ratings at all times, the variation in load being taken care of by the number of boilers on the line. Banked boilers are cut in to take care of increasing loads and peaks and placed again on bank when the peak periods have pa.s.sed. It is probable that this method of handling central station load is to-day the most generally used.

Other conditions of operation make it advisable to carry the load on a definite number of boiler units, operating these at slightly below their rated capacity during periods of light or low loads and securing the overload capacity during peaks by operating the same boilers at high ratings. In this method there are no boilers kept on banked fires, the spares being spares in every sense of the word.

A third method of handling widely varying loads which is coming somewhat into vogue is that of considering the plant as divided, one part to take care of what may be considered the constant plant load, the other to take care of the floating or variable load. With such a method that portion of the plant carrying the steady load is so proportioned that the boilers may be operated at the point of maximum efficiency, this point being raised to a maximum through the use of economizers and the general installation of any apparatus leading to such results. The variable load will be carried on the remaining boilers of the plant under either of the methods just given, that is, at the high ratings of all boilers in service and banking others, or a variable capacity from all boilers in service.

The opportunity is again taken to indicate the very general character of any statements made relative to the economical load for any plant and to emphasize the fact that each individual case must be considered independently, with the conditions of operations applicable thereto.

With a thorough understanding of the meaning of boiler efficiency and capacity and their relation to each other, it is possible to consider more specifically the selection of boilers.

The foremost consideration is, without question, the adaptability of the design selected to the nature of the work to be done. An installation which is only temporary in its nature would obviously not warrant the first cost that a permanent plant would. If boilers are to carry an intermittent and suddenly fluctuating load, such as a hoisting load or a reversing mill load, a design would have to be selected that would not tend to prime with the fluctuations and sudden demand for steam. A boiler that would give the highest possible efficiency with fuel of one description, would not of necessity give such efficiency with a different fuel. A boiler of a certain design which might be good for small plant practice would not, because of the limitations in practicable size of units, be suitable for large installations. A discussion of the relative value of designs can be carried on almost indefinitely but enough has been said to indicate that a given design will not serve satisfactorily under all conditions and that the adaptability to the service required will be dependent upon the fuel available, the cla.s.s of labor procurable, the feed water that must be used, the nature of the plant's load, the size of the plant and the first cost warranted by the service the boiler is to fulfill.

TABLE 60

ACTUAL EVAPORATION FOR DIFFERENT PRESSURES AND TEMPERATURES OF FEED WATER CORRESPONDING TO ONE HORSE POWER (34 POUNDS PER HOUR FROM AND AT 212 DEGREES FAHRENHEIT)

----------------------------------------------------------------------------------------------------------------------------------------- Temperature| | of | Pressure by Gauge--Pounds per Square Inch | Feed | | Degrees | | Fahrenheit | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 | 140 | 150 | 160 | 170 | 180 | 190 | 200 | 210 | 220 | 230 | 240 | 250 | -----------+-----------------------------------------------------------------------------------------------------------------------------| 32 |28.41|28.36|28.29|28.24|28.20|28.16|28.13|28.09|28.07|28.04|28.02|27.99|27.97|27.95|27.94|27.92|27.90|27.89|27.87|27.86|27.83| 40 |28.61|28.54|28.49|28.44|28.40|28.35|28.32|28.29|28.26|28.23|28.21|28.18|28.16|28.14|28.12|28.11|28.09|28.07|28.06|28.05|28.03| 50 |28.85|28.79|28.73|28.68|28.64|28.60|28.56|28.53|28.50|28.47|28.45|28.43|28.40|28.38|28.36|28.35|28.33|28.31|28.30|28.28|28.27| 60 |29.10|29.04|28.98|28.93|28.88|28.84|28.81|28.77|28.74|28.72|28.69|28.67|28.65|28.62|28.60|28.59|28.57|28.55|28.54|28.52|28.51| 70 |29.36|29.29|29.23|29.18|29.14|29.09|29.06|29.02|28.99|28.96|28.94|28.92|28.89|28.87|28.85|28.83|28.82|28.80|28.78|28.77|28.76| 80 |29.62|29.55|29.49|29.44|29.39|29.35|29.31|29.27|29.24|29.22|29.19|29.17|29.14|29.12|29.10|29.08|29.07|29.05|29.03|29.02|29.00| 90 |29.88|29.81|29.75|29.70|29.65|29.61|29.57|29.53|29.50|29.47|29.45|29.42|29.40|29.38|29.36|29.34|29.32|29.30|29.29|29.27|29.25| 100 |30.15|30.08|30.02|29.96|29.91|29.87|29.83|29.80|29.76|29.73|29.71|29.68|29.66|29.63|29.61|29.60|29.58|29.56|29.54|29.53|29.51| 110 |30.42|30.35|30.29|30.23|30.18|30.14|30.10|30.06|30.03|30.00|29.97|29.95|29.92|29.90|29.88|29.86|29.84|29.82|29.81|29.79|29.77| 120 |30.70|30.63|30.56|30.51|30.46|30.41|30.37|30.33|30.30|30.27|30.24|30.22|30.19|30.17|30.15|30.13|30.11|30.09|30.07|30.06|30.04| 130 |30.99|30.91|30.84|30.79|30.73|30.69|30.65|30.61|30.57|30.54|30.52|30.49|30.47|30.44|30.42|30.40|30.38|30.36|30.35|30.33|30.31| 140 |31.28|31.20|31.13|31.07|31.02|30.97|30.93|30.89|30.86|30.83|30.80|30.77|30.75|30.72|30.70|30.68|30.66|30.64|30.62|30.61|30.59| 150 |31.58|31.49|31.42|31.36|31.31|31.26|31.22|31.18|31.14|31.11|31.08|31.06|31.03|31.01|30.98|30.96|30.94|30.92|30.91|30.89|30.87| 160 |31.87|31.79|31.72|31.66|31.61|31.56|31.51|31.47|31.44|31.40|31.37|31.35|31.32|31.29|31.27|31.25|31.23|31.21|31.19|31.18|31.16| 170 |32.18|32.10|32.02|31.96|31.91|31.86|31.81|31.77|31.73|31.70|31.67|31.64|31.62|31.59|31.57|31.54|31.52|31.50|31.49|31.47|31.46| 180 |32.49|32.41|32.33|32.27|32.22|32.16|32.12|32.08|32.04|32.00|31.97|31.95|31.92|31.89|31.87|31.84|31.82|31.80|31.79|31.77|31.75| 190 |32.81|32.72|32.65|32.59|32.53|32.47|32.43|32.38|32.35|32.32|32.29|32.26|32.23|32.20|32.17|32.15|32.13|32.11|32.09|32.07|32.05| 200 |33.13|33.05|32.97|32.91|32.85|32.79|32.75|32.70|32.66|32.63|32.60|32.57|32.54|32.51|32.49|32.46|32.44|32.42|32.40|32.38|32.36| 210 |33.47|33.38|33.30|33.24|33.18|33.13|33.08|33.03|32.99|32.95|32.92|32.89|32.86|32.83|32.81|32.79|32.76|32.74|32.72|32.70|32.68| -----------------------------------------------------------------------------------------------------------------------------------------

The proper consideration can be given to the adaptability of any boiler for the service in view only after a thorough understanding of the requirements of a good steam boiler, with the application of what has been said on the proper operation to the special requirements of each case. Of almost equal importance to the factors mentioned are the experience, the skill and responsibility of the manufacturer.

With the design of boiler selected that is best adapted to the service required, the next step is the determination of the boiler power requirements.