FOOTNOTES
[54] To determine the portion of the fuel which is actually burned, the weight of ashes should be computed from the total weight of coal burned and the coal and ash analyses in order to allow for any ash that may be blown away with the flue gases. In many cases the ash so computed is considerably higher than that found in the test.
[55] As distinguished from the efficiency of boiler, furnace and grate.
[56] To obtain the efficiency of the boiler as an absorber of the heat contained in the hot gases, this should be the heat generated per pound of combustible corrected so that any heat lost through incomplete combustion will not be charged to the boiler. This, however, does not eliminate the furnace as the presence of excess air in the gases lowers the efficiency and the ability to run without excess air depends on the design and operation of the furnace. The efficiency based on the total heat value per pound of combustible is, however, ordinarily taken as the efficiency of the boiler notwithstanding the fact that it necessarily involves the furnace.
[57] See pages [280] and [281].
[58] Where the horse power of marine boilers is stated, it generally refers to and is synonymous with the horse power developed by the engines which they serve.
[59] In other countries, boilers are ordinarily rated not in horse power but by specifying the quantity of water they are capable of evaporating from and at 212 degrees or under other conditions.
[60] See equivalent evaporation from and at 212 degrees, page [116].
[61] The recommendations are those made in the preliminary report of the Committee on Power Tests and at the time of going to press have not been finally accepted by the Society as a whole.
[62] This code relates primarily to tests made with coal.
[63] The necessary apparatus and instruments are described elsewhere. No definite rules can be given for location of instruments. For suggestions on location, see A. S. M. E. Code of 1912, Appendix 24. For calibration of instruments, see Code, Vol. XXXIV, Trans., A. S. M. E., pages 1691-1702 and 1713-14.
[64] One to two inches for small anthracite coals.
[65] Do not blow down the water-glass column for at least one hour before these readings are taken. An erroneous indication may otherwise be caused by a change of temperature and density of the water within the column and connecting pipe.
[66] Do not blow down the water-glass column for at least one hour before these readings are taken. An erroneous indication may otherwise be caused by a change of temperature and density of the water within the column and connecting pipe.
[67] For calculations relating to quality of steam, see page [129].
[68] Where the coal is very moist, a portion of the moisture will cling to the walls of the jar, and in such case the jar and fuel together should be dried out in determining the total moisture.
[69] Say ½ ounce to 2 ounces.
[70] For methods of analysis, see page [176].
[71] For suggestions relative to Smoke Observations, see A. S. M. E. Code of 1912, Appendix 16 and 17.
[72] The term “as fired” means actual condition including moisture, corrected for estimated difference in weight of coal on the grate at beginning and end.
[73] Corrected for inequality of water level and steam pressure at beginning and end.
THE SELECTION OF BOILERS WITH A CONSIDERATION OF THE FACTORS DETERMINING SUCH SELECTION
The selection of steam boilers is a matter to which the most careful thought and attention may be well given. Within the last twenty years, radical changes have taken place in the methods and appliances for the generation and distribution of power. These changes have been made largely in the prime movers, both as to type and size, and are best illustrated by the changes in central station power-plant practice. It is hardly within the scope of this work to treat of power-plant design and the discussion will be limited to a consideration of the boiler end of the power plant.
As stated, the changes have been largely in prime movers, the steam generating equipment having been considered more or less of a standard piece of apparatus whose sole function is the transfer of the heat liberated from the fuel by combustion to the steam stored or circulated in such apparatus. When the fact is considered that the cost of steam generation is roughly from 65 to 80 per cent of the total cost of power production, it may be readily understood that the most fruitful field for improvement exists in the boiler end of the power plant. The efficiency of the plant as a whole will vary with the load it carries and it is in the boiler room where such variation is largest and most subject to control.
The improvements to be secured in the boiler room results are not simply a matter of dictation of operating methods. The securing of perfect combustion, with the accompanying efficiency of heat transfer, while comparatively simple in theory, is difficult to obtain in practical operation. This fact is perhaps best exemplified by the difference between test results and those obtained in daily operation even under the most careful supervision. This difference makes it necessary to establish a standard by which operating results may be judged, a standard not necessarily that which might be possible under test conditions but one which experiment shows can be secured under the very best operating conditions.
The study of the theory of combustion, draft, etc., as already given, will indicate that the question of efficiency is largely a matter of proper relation between fuel, furnace and generator. While the possibility of a substantial saving through added efficiency cannot be overlooked, the boiler design of the future must, even more than in the past, be considered particularly from the aspect of reliability and simplicity. A flexibility of operation is necessary as a guarantee of continuity of service.
In view of the above, before the question of the selection of boilers can be taken up intelligently, it is necessary to consider the subjects of boiler efficiency and boiler capacity, together with their relation to each other.
The criterion by which the efficiency of a boiler plant is to be judged is the cost of the production of a definite amount of steam. Considered in this sense, there must be included in the efficiency of a boiler plant the simplicity of operation, flexibility and reliability of the boiler used. The items of repair and upkeep cost are often high because of the nature of the service. The governing factor in these items is unquestionably the type of boiler selected.
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 [Pg 278] 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 [Pg 279] 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.
[Pg 280]
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
15400 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters, Equipped with Babcock & Wilcox 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 establishing 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.
