FOOTNOTES
[52] Results secured by this method will be approximately correct.
[53] See “Chimneys for Crude Oil”, C. R. Weymouth, Trans. A. S. M. E., Dec. 1912.
EFFICIENCY AND CAPACITY OF BOILERS
Two of the most important operating factors entering into the consideration of what constitutes a satisfactory boiler are its efficiency and capacity. The relation of these factors to one another will be considered later under the selection of boilers with reference to the work they are to accomplish. The present chapter deals with the efficiency and capacity only with a view to making clear exactly what is meant by these terms as applied to steam generating apparatus, together with the methods of determining these factors by tests.
Efficiency—The term “efficiency”, specifically applied to a steam boiler, is the ratio of heat absorbed by the boiler in the generation of steam to the total amount of heat available in the medium utilized in securing such generation. When this medium is a solid fuel, such as coal, it is impossible to secure the complete combustion of the total amount fed to the boiler. A portion is bound to drop through the grates where it becomes mixed with the ash and, remaining unburned, produces no heat. Obviously, it is unfair to charge the boiler with the failure to absorb the portion of available heat in the fuel that is wasted in this way. On the other hand, the boiler user must pay for such waste and is justified in charging it against the combined boiler and furnace. Due to this fact, the efficiency of a boiler, as ordinarily stated, is in reality the combined efficiency of the boiler, furnace and grate, and
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The efficiency will be the same whether based on dry fuel or on fuel as fired, including its content of moisture. For example: If the coal contained 3 per cent of moisture, the efficiency would be
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where 0.97 cancels and the formula becomes ([31]).
The heat supplied to the boiler is due to the combustible portion of fuel which is actually burned, irrespective of what proportion of the total combustible fired may be.[54] This fact has led to the use of a second efficiency basis on combustible and which is called the efficiency of boiler and furnace[55], namely,
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The efficiency so determined is used in comparing the relative performance of boilers, irrespective of the type of grates used under them. If the loss of fuel through the grates could be entirely overcome, the efficiencies obtained by ([31]) and ([32]) would obviously be the same. Hence, in the case of liquid and gaseous fuels, where there is practically no waste, these efficiencies are almost identical.
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As a matter of fact, it is extremely difficult, if not impossible, to determine the actual efficiency of a boiler alone, as distinguished from the combined efficiency of boiler, grate and furnace. This is due to the fact that the losses due to excess air cannot be correctly attributed to either the boiler or the furnace, but only to a combination of the complete apparatus. Attempts have been made to devise methods for dividing the losses proportionately between the furnace and the boiler, but such attempts are unsatisfactory and it is impossible to determine the efficiency of a boiler apart from that of a furnace in such a way as to make such determination of any practical value or in a way that might not lead to endless dispute, were the question to arise in the case of a guaranteed efficiency. From the boiler manufacturer’s standpoint, the only way of establishing an efficiency that has any value when guarantees are to be met, is to require the grate or stoker manufacturer to make certain guarantees as to minimum CO2, maximum CO, and that the amount of combustible in the ash and blown away with the flue gases does not exceed a certain percentage. With such a guarantee, the efficiency should be based on the combined furnace and boiler.
General practice, however, has established the use of the efficiency based upon combustible as representing the efficiency of the boiler alone. When such an efficiency is used, its exact meaning, as pointed out on [opposite page], should be realized.
The computation of the efficiencies described on [opposite page] is best illustrated by example.
Assume the following data to be determined from an actual boiler trial.
| Steam pressure by gauge, 200 pounds. |
| Feed temperature, 180 degrees. |
| Total weight of coal fired, 17,500 pounds. |
| Percentage of moisture in coal, 3 per cent. |
| Total ash and refuse, 2396 pounds. |
| Total water evaporated, 153,543 pounds. |
| Per cent of moisture in steam, 0.5 per cent. |
| Heat value per pound of dry coal, 13,516. |
| Heat value per pound of combustible, 15,359. |
The factor of evaporation for such a set of conditions is 1.0834. The actual evaporation corrected for moisture in the steam is 152,775 and the equivalent evaporation from and at 212 degrees is, therefore, 165,516 pounds.
The total dry fuel will be 17,500 × .97 = 16,975, and the evaporation per pound of dry fuel from and at 212 degrees will be 165,516 ÷ 16,975 = 9.75 pounds. The heat absorbed per pound of dry fuel will, therefore, be 9.75 × 970.4 = 9461 B. t. u. Hence, the efficiency by ([31]) will be 9461 ÷ 13,516 = 70.0 per cent. The total combustible burned will be 16,975 - 2396 = 14,579, and the evaporation from and at 212 degrees per pound of combustible will be 165,516 ÷ 14,579 = 11.35 pounds. Hence, the efficiency based on combustible from ([32]) will be (11.35 × 97.04) ÷ 15,359 = 71.71.
For approximate results, a chart may be used to take the place of a computation of efficiency. Fig. 39 shows such a chart based on the evaporation per pound of dry fuel and the heat value per pound of dry fuel, from which efficiencies may be read directly to within one-half of one per cent. It is used as follows: From the intersection of the horizontal line, representing the evaporation per pound of fuel, with the vertical line, representing the heat value per pound, the efficiency is read directly from the diagonal scale of efficiencies. This chart may also be used for efficiency based upon combustible when the evaporation from and at 212 degrees and the heat values are both given in terms of combustible.
