Fig. 39. Efficiency Chart. Calculated from Marks and Davis Tables
Diagonal Lines Represent Per Cent Efficiency
Boiler efficiencies will vary over a wide range, depending on a great variety of factors and conditions. The highest efficiencies that have been secured with coal are in the neighborhood of 82 per cent and from that point efficiencies are found all the way down to below 50 per cent. [Table 59][57] of tests of Babcock & Wilcox boilers under varying conditions of fuel and operation will give an idea of what may be obtained with proper operating conditions.
The difference between the efficiency secured in any boiler trial and the perfect efficiency, 100 per cent, includes the losses, some of which are unavoidable in the present state of the art, arising in the conversion of the heat energy of the coal to the heat energy in the steam. These losses may be classified as follows:
1st. Loss due to fuel dropped through the grate.
2nd. Loss due to unburned fuel which is carried by the draft, as small particles, beyond the bridge wall into the setting or up the stack.
3rd. Loss due to the utilization of a portion of the heat in heating the moisture contained in the fuel from the temperature of the atmosphere to 212 degrees; to evaporate it at that temperature and to superheat the steam thus formed to the temperature of the flue gases. This steam, of course, is first heated to the temperature of the furnace but as it gives up a portion of this heat in passing through the boiler, the superheating to the temperature of the exit gases is the correct degree to be considered.
4th. Loss due to the water formed and by the burning of the hydrogen in the fuel which must be evaporated and superheated as in item 3.
5th. Loss due to the superheating of the moisture in the air supplied from the atmospheric temperature to the temperature of the flue gases.
6th. Loss due to the heating of the dry products of combustion to the temperature of the flue gases.
7th. Loss due to the incomplete combustion of the fuel when the carbon is not completely consumed but burns to CO instead of CO2. The CO passes out of the stack unburned as a volatile gas capable of further combustion.
8th. Loss due to radiation of heat from the boiler and furnace settings.
Obviously a very elaborate test would have to be made were all of the above items to be determined accurately. In ordinary practice it has become customary to summarize these losses as follows, the methods of computing the losses being given in each instance by a typical example:
(A) Loss due to the heating of moisture in the fuel from the atmospheric temperature to 212 degrees, evaporate it at that temperature and superheat it to the temperature of the flue gases. This in reality is the total heat above the temperature of the air in the boiler room, in one pound of superheated steam at atmospheric pressure at the temperature of the flue gases, multiplied by the percentage of moisture in the fuel. As the total heat above the temperature of the air would have to be computed in each instance, this loss is best expressed by:
| Loss in B. t. u. per pound | = | W | ( | 212 | - | t | + | 970.4 | + | .47 | (T - 212) | ) | (33) |
| Where | W | = | per cent of moisture in coal, |
| t | = | the temperature of air in the boiler room, [Pg 260] | |
| T [Pg 261] | = | temperature of the flue gases, | |
| .47 | = | the specific heat of superheated steam at the atmosphericpressure and at the flue gas temperature, | |
| (212-t) | = | B. t. u. necessary to heat one pound of water from thetemperature of the boiler room to 212 degrees, | |
| 970.4 | = | B. t. u. necessary to evaporate one pound of water at 212degrees to steam at atmospheric pressure, | |
| .47(T-212) | = | B. t. u. necessary to superheat one pound of steam atatmospheric pressure from 212 degrees to temperature T. | |
Portion of 15,000 Horse-power Installation of Babcock & Wilcox Boilers, Equipped with Babcock & Wilcox Chain Grate Stokers at the Northumberland, Pa., Plant of the Atlas Portland Cement Co. This Company Operates a Total of 24,000 Horse Power of Babcock & Wilcox Boilers in its Various Plants
(B) Loss due to heat carried away in the steam produced by the burning of the hydrogen component of the fuel. In burning, one pound of hydrogen unites with 8 pounds of oxygen to form 9 pounds of steam. Following the reasoning of item (A), therefore, this loss will be:
| Loss in B. t. u. per pound | = | 9H | ( | (212 - t) | + | 970.4 | + | .47 | (T - 212) | ) | (34) |
| Where | H | = | the percentage by weight of hydrogen. |
This item is frequently considered as a part of the unaccounted for loss, where an ultimate analysis of the fuel is not given.
(C) Loss due to heat carried away by dry chimney gases. This is dependent upon the weight of gas per pound of coal which may be determined by formula ([16]), page [158].
Loss in B. t. u. per pound = (T - t) × .24 × W.
Where T and t have values as in ([33]),
.24 = specific heat of chimney gases,
W = weight of dry chimney gas per pound of coal.
(D) Loss due to incomplete combustion of the carbon content of the fuel, that is, the burning of the carbon to CO instead of CO2.
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C = per cent of carbon in coal by ultimate analysis,
CO and CO2 = per cent of CO and CO2 by volume from flue gas analysis,
10,150 = the number of heat units generated by burning to CO2 one pound of carbon contained in carbon monoxide.
(E) Loss due to unconsumed carbon in the ash (it being usually assumed that all the combustible in the ash is carbon).
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The loss incurred in this way is, directly, the carbon in the ash in percentage terms of the total dry coal fired, multiplied by the heat value of carbon.
To compute this item, which is of great importance in comparing the relative performances of different designs of grates, an analysis of the ash must be available.
The other losses, namely, items 2, 5 and 8 of the first classification, are ordinarily grouped under one item, as unaccounted for losses, and are obviously the difference between 100 per cent and the sum of the heat utilized and the losses accounted for as given above. Item 5, or the loss due to the moisture in the air, may be readily computed, the moisture being determined from wet and dry bulb thermometer readings, but it is usually disregarded as it is relatively small, averaging, [Pg 262] say, one-fifth to one-half of one per cent. Lack of data may, of course, make it necessary to include certain items of the second and ordinary classification in this unaccounted for group.
A schedule of the losses as outlined, requires an evaporative test of the boiler, an analysis of the flue gases, an ultimate analysis of the fuel, and either an ultimate or proximate analysis of the ash. As the amount of unaccounted for losses forms a basis on which to judge the accuracy of a test, such a schedule is called a “heat balance”.
A heat balance is best illustrated by an example: Assume the data as given in [Table 57] to be secured in an actual boiler test.
From this data the factor of evaporation is 1.1514 and the evaporation per hour from and at 212 degrees is 65,671 pounds. Hence the evaporation from and at 212 degrees per pound of dry coal is 65,671 ÷ 5609 = 11.71 pounds. The efficiency of boiler, furnace and grate is:
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The heat losses are:
(A) Loss due to moisture in coal,
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(B) The loss due to the burning of hydrogen:
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(C) To compute the loss in the heat carried away by dry chimney gases per pound of coal the weight of such gases must be first determined. This weight per pound of coal is:
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where CO2, O, CO and H are the percentage by volume as determined by the flue gas analysis and C is the percentage by weight of carbon in the dry fuel. Hence the weight of gas per pound of coal will be,
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Therefore the loss of heat in the dry gases carried up the chimney =
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(D) The loss due to incomplete combustion as evidenced by the presence of CO in the flue gas analysis is:
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(E) The loss due to unconsumed carbon in the ash:
The analysis of the ash showed 17.9 per cent to be combustible matter, all of which is assumed to be carbon. The test showed 10.00 of the total dry fuel fired to be ash. Hence 10.00×.179 = 1.79 per cent of the total fuel represents the proportion of this total unconsumed in the ash and the loss due to this cause is
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The heat absorbed by the boilers per pound of dry fuel is 11.71×970.4 = 11,363 B. t. u. This quantity plus losses (A), (B), (C), (D) and (E), or 11,363+22+618+1311+61+261 = 13,636 B. t. u. accounted for. The heat value of the coal, 14,225 B. t. u., less 13,636 B. t. u., leaves 589 B. t. u., unaccounted for losses, or 4.15 per cent.
The heat balance should be arranged in the form indicated by [Table 58].
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Application of Heat Balance—A heat balance should be made in connection with any boiler trial on which sufficient data for its computation has been obtained. This is particularly true where the boiler performance has been considered unsatisfactory. The distribution of the heat is thus determined and any extraordinary loss may be detected. Where accurate data for computing such a heat balance is not [Pg 264] available, such a calculation based on certain assumptions is sometimes sufficient to indicate unusual losses.
The largest loss is ordinarily due to the chimney gases, which depends directly upon the weight of the gas and its temperature leaving the boiler. As pointed out in the chapter on flue gas analysis, the lower limit of the weight of gas is fixed by the minimum air supplied with which complete combustion may be obtained. As shown, where this supply is unduly small, the loss caused by burning the carbon to CO instead of to CO2 more than offsets the gain in decreasing the weight of gas.
The lower limit of the stack temperature, as has been shown in the chapter on draft, is more or less fixed by the temperature necessary to create sufficient draft suction for good combustion. With natural draft, this lower limit is probably between 400 and 450 degrees.
Capacity—Before the capacity of a boiler is considered, it is necessary to define the basis to which such a term may be referred. Such a basis is the so-called boiler horse power.
The unit of motive power in general use among steam engineers is the “horse power” which is equivalent to 33,000 foot pounds per minute. Stationary boilers are at the present time rated in horse power, though such a basis of rating may lead and has often led to a misunderstanding. Work, as the term is used in mechanics, is the overcoming of resistance through space, while power is the rate of work or the amount done per unit of time. As the operation of a boiler in service implies no motion, it can produce no power in the sense of the term as understood in mechanics. Its operation is the generation of steam, which acts as a medium to convey the energy of the fuel which is in the form of heat to a prime mover in which that heat energy is converted into energy of motion or work, and power is developed.
If all engines developed the same amount of power from an equal amount of heat, a boiler might be designated as one having a definite horse power, dependent upon the amount of engine horse power its steam would develop. Such a statement of the rating of boilers, though it would still be inaccurate, if the term is considered in its mechanical sense, could, through custom, be interpreted to indicate that a boiler was of the exact capacity required to generate the steam necessary to develop a definite amount of horse power in an engine. Such a basis of rating, however, is obviously impossible when the fact is considered that the amount of steam necessary to produce the same power in prime movers of different types and sizes varies over very wide limits.
To do away with the confusion resulting from an indefinite meaning of the term boiler horse power, the Committee of Judges in charge of the boiler trials at the Centennial Exposition, 1876, at Philadelphia, ascertained that a good engine of the type prevailing at the time required approximately 30 pounds of steam per hour per horse power developed. In order to establish a relation between the engine power and the size of a boiler required to develop that power, they recommended that an evaporation of 30 pounds of water from an initial temperature of 100 degrees Fahrenheit to steam at 70 pounds gauge pressure be considered as one boiler horse power. This recommendation has been generally accepted by American engineers as a standard, and when the term boiler horse power is used in connection with stationary boilers[58] [Pg 265] throughout this country,[59] without special definition, it is understood to have this meaning.
Inasmuch as an equivalent evaporation from and at 212 degrees Fahrenheit is the generally accepted basis of comparison[60], it is now customary to consider the standard boiler horse power as recommended by the Centennial Exposition Committee, in terms of equivalent evaporation from and at 212 degrees. This will be 30 pounds multiplied by the factor of evaporation for 70 pounds gauge pressure and 100 degrees feed temperature, or 1.1494. 30 × 1.1494 = 34.482, or approximately 34.5 pounds. Hence, one boiler horse power is equal to an evaporation of 34.5 pounds of water per hour from and at 212 degrees Fahrenheit. The term boiler horse power, therefore, is clearly a measure of evaporation and not of power.
A method of basing the horse power rating of a boiler adopted by boiler manufacturers is that of heating surfaces. Such a method is absolutely arbitrary and changes in no way the definition of a boiler horse power just given. It is simply a statement by the manufacturer that his product, under ordinary operating conditions or conditions which may be specified, will evaporate 34.5 pounds of water from and at 212 degrees per definite amount of heating surface provided. The amount of heating surface that has been considered by manufacturers capable of evaporating 34.5 pounds from and at 212 degrees per hour has changed from time to time as the art has progressed. At the present time 10 square feet of heating surface is ordinarily considered the equivalent of one boiler horse power among manufacturers of stationary boilers. In view of the arbitrary nature of such rating and of the widely varying rates of evaporation possible per square foot of heating surface with different boilers and different operating conditions, such a basis of rating has in reality no particular bearing on the question of horse power and should be considered merely as a convenience.
The whole question of a unit of boiler capacity has been widely discussed with a view to the adoption of a standard to which there would appear to be a more rational and definite basis. Many suggestions have been offered as to such a basis but up to the present time there has been none which has met with universal approval or which would appear likely to be generally adopted.
With the meaning of boiler horse power as given above, that is, a measure of evaporation, it is evident that the capacity of a boiler is a measure of the power it can develop expressed in boiler horse power. Since it is necessary, as stated, for boiler manufacturers to adopt a standard for reasons of convenience in selling, the horse power for which a boiler is sold is known as its normal rated capacity.
The efficiency of a boiler and the maximum capacity it will develop can be determined accurately only by a boiler test. The standard methods of conducting such tests are given on the following pages, these methods being the recommendations of the Power Test Committee of the American Society of Mechanical Engineers brought out in 1913.[61] Certain changes have been made to incorporate in the boiler code such portions of the “Instructions Regarding Tests in General” as apply to boiler testing. Methods of calculation and such matter as are treated in other portions of the book have been omitted from the code as noted.
[Pg 266]
Portion of 2600 Horse-power Installation of Babcock & Wilcox Boilers, Equipped with Babcock & Wilcox Chain Grate Stokers at the Peter Schoenhofen Brewing Co., Chicago, Ill.
1. OBJECT [Pg 267]
Ascertain the specific object of the test, and keep this in view not only in the work of preparation, but also during the progress of the test, and do not let it be obscured by devoting too close attention to matters of minor importance. Whatever the object of the test may be, accuracy and reliability must underlie the work from beginning to end.
If questions of fulfillment of contract are involved, there should be a clear understanding between all the parties, preferably in writing, as to the operating conditions which should obtain during the trial, and as to the methods of testing to be followed, unless these are already expressed in the contract itself.
Among the many objects of performance tests, the following may be noted:
Determination of capacity and efficiency, and how these compare with standard or guaranteed results.
Comparison of different conditions or methods of operation.
Determination of the cause of either inferior or superior results.
Comparison of different kinds of fuel.
Determination of the effect of changes of design or proportion upon capacity or efficiency, etc.