Chapter 33

The features entering into the plant efficiency are so numerous that it is impossible to make a statement as to a means of securing the highest efficiency which will apply to all cases. Such efficiency is to be secured by the proper relation of fuel, furnace and boiler heating surface, actual operating conditions, which allow the approaching of the potential efficiencies made possible by the refinement of design, and a systematic supervision of the operation assisted by a detailed record of performances and conditions. The question of supervision will be taken up later in the chapter on "Operation and Care of Boilers".

The efficiencies that may be expected from the combination of well-designed boilers and furnaces are indicated in Table 59 in which are given a number of tests with various fuels and under widely different operating conditions.

It is to be appreciated that the results obtained as given in this table are practically all under test conditions. The nearness with which practical operating conditions can approach these figures will depend upon the character of the supervision of the boiler room and the intelligence of the operating crew. The size of the plant will ordinarily govern the expense warranted in securing the right sort of supervision.

The bearing that the type of boiler has on the efficiency to be expected can only be realized from a study of the foregoing chapters.

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 quantity 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 assure 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 assumed, 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 BABCOCK & WILCOX 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, Washington, 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, Mass. |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, Mass. | |Stoker | 508 | |______|___________________________|________________|_________|________| | |Colorado Springs & |Pike View, Col.,|Green Chn| | | 24 |Interurban Ry., Col. |Mine Run |Grate | 400 | |______|___________________________|________________|_________|________| | |Pub. Serv. Corporation |Lancashire, Pa. |B&W.Chain| | | 25 |of N. J., Marion | |Grate | 600 | |______|___________________________|________________|_________|________| | |Pub. Serv. Corporation |Lancashire, 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 | |______|___________________________|________________|_________|________|