The makers furnish a non-drying oil for the liquid, usually a 300 degrees test refined petroleum.

A very convenient form of the ordinary U-tube gauge is known as the Peabody gauge, and it is shown in Fig. 38. This is a small modified U-tube with a sliding scale between the two legs of the U and with connections such that either a draft suction or a draft pressure may be taken. The tops of the sliding pieces extending across the tubes are placed at the bottom of the meniscus and accurate readings in hundredths of an inch are obtained by a vernier.



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

Efficiency of boiler,} Heat absorbed per pound of fuel furnace and grate } = ———————————————- (31) Heat value per pound of fuel

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

Heat absorbed per pound of dry coal x 0.97 ————————————————————— Heat value per pound of dry coal x 0.97

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,

Efficiency of boiler and furnace[55]

Heat absorbed per pound of combustible[56] = ——————————————————— (32) Heat value per pound of combustible

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.

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 CO_{2}, 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 x .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 x 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 x 97.04) / 15,359 = 71.79.[**should be 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.

[Graph: Evaporation from and at 212deg. per Pound of Dry Fuel against B.T.U. per Pound of Dry Fuel

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 CO_{2}. 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, T = temperature of the flue gases, .47 = the specific heat of superheated steam at the atmospheric pressure and at the flue gas temperature, (212-t) = B. t. u. necessary to heat one pound of water from the temperature of the boiler room to 212 degrees, 970.4 = B. t. u. necessary to evaporate one pound of water at 212 degrees to steam at atmospheric pressure, .47(T-212) = B. t. u. necessary to superheat one pound of steam at atmospheric pressure from 212 degrees to temperature T.



(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)x.24xW.

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 CO_{2}.

10,150 CO Loss in B. t. u. per pound = Cx————- (35) CO_{2}+CO

C = per cent of carbon in coal by ultimate analysis,

CO and CO{2} = per cent of CO and CO{2} by volume from flue gas analysis.

10,150 = the number of heat units generated by burning to CO_{2} 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).

Loss in B. t. u. per pound = per cent C x per cent ash x B. t. u. per pound of combustible in the ash (usually taken as 14,600 B. t. u.) (36)

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, 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.

TABLE 57

DATA FROM WHICH HEAT BALANCE (TABLE 58) IS COMPUTED

+ + + + Steam Pressure by Gauge, Pounds 192 Temperature of Feed, Degrees Fahrenheit 180 Degrees of Superheat, Degrees Fahrenheit 115.2 Temperature of Boiler Room, Degrees Fahrenheit 81 Temperature of Exit Gases, Degrees Fahrenheit 480 Weight of Coal Used per Hour, Pounds 5714 Moisture, Per Cent 1.83 Dry Coal Per Hour, Pounds 5609 Ash and Refuse per Hour, Pounds 561 Ash and Refuse (of Dry Coal), Per Cent 10.00 Actual Evaporation per Hour, Pounds 57036 .- C, Per Cent 78.57 H, Per Cent 5.60 Ultimate O, Per Cent 7.02 Analysis -+ N, Per Cent 1.11 Dry Coal Ash, Per Cent 6.52 '- Sulphur, Per Cent 1.18 Heat Value per Pound Dry Coal, B. t. u. 14225 Heat Value per Pound Combustible, B. t. u. 15217 Combustible in Ash by Analysis, Per Cent 17.9 .- CO_{2}, Per Cent 14.33 Flue Gas -+ O, Per Cent 4.54 Analysis CO, Per Cent 0.11 '- N, Per Cent 81.02 + + -+ + +

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:

(11.71x970.4)/14,225 = 79.88 per cent.

The heat losses are:

(A) Loss due to moisture in coal,

= .01831 ((212-81)970.4.47(480-212)) = 22. B. t. u., = 0.15 per cent.

(B) The loss due to the burning of hydrogen:

= 9x.0560((212-81)970.4.47(480-212)) = 618 B. t. u., = 4.34 per cent.

(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:

(11CO{2}+8O+7(CON)) (—————————-)C ( 3(CO{2}CO) )

where CO_{2}, 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,

(11x14.33+8x4.54+7(0.11+81.02)) (——————————————-)x78.57 = 13.7 pounds. ( 3(14.33+0.11) )

Therefore the loss of heat in the dry gases carried up the chimney =

13.7x0.24(480-81) = 1311 B. t. u., = 9.22 per cent.

(D) The loss due to incomplete combustion as evidenced by the presence of CO in the flue gas analysis is:

0.11 —————x.7857x10,150 = 61. B. t. u., 14.33+0.11 = .43 per cent.

(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 10.00x.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

1.79 per cent x 14,600 = 261 B. t. u., = 1.83 per cent.

The heat absorbed by the boilers per pound of dry fuel is 11.71x970.4 = 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.

TABLE 58

HEAT BALANCE

B. T. U. PER POUND DRY COAL 14,225

B. t. u. Per Cent Heat absorbed by Boiler 11,363 79.88 Loss due to Evaporation of Moisture in Fuel 22 0.15 Loss due to Moisture formed by Burning of Hydrogen 618 4.34 Loss due to Heat carried away in Dry Chimney Gases 1311 9.22 Loss due to Incomplete Combustion of Carbon 61 0.43 Loss due to Unconsumed Carbon in the Ash 261 1.83 Loss due to Radiation and Unaccounted Losses 589 4.15 Total 14,225 100.00

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 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 CO_{2} 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] 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 x 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.



1. OBJECT

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.



2. PREPARATIONS

(A) Dimensions:

Measure the dimensions of the principal parts of the apparatus to be tested, so far as they bear on the objects in view, or determine these from correct working drawings. Notice the general features of the same, both exterior and interior, and make sketches, if needed, to show unusual points of design.

The dimensions of the heating surfaces of boilers and superheaters to be found are those of surfaces in contact with the fire or hot gases. The submerged surfaces in boilers at the mean water level should be considered as water-heating surfaces, and other surfaces which are exposed to the gases as superheating surfaces.

(B) Examination of Plant:

Make a thorough examination of the physical condition of all parts of the plant or apparatus which concern the object in view, and record the conditions found, together with any points in the matter of operation which bear thereon.

In boilers, examine for leakage of tubes and riveted or other metal joints. Note the condition of brick furnaces, grates and baffles. Examine brick walls and cleaning doors for air leaks, either by shutting the damper and observing the escaping smoke or by candle-flame test. Determine the condition of heating surfaces with reference to exterior deposits of soot and interior deposits of mud or scale.

See that the steam main is so arranged that condensed and entrained water cannot flow back into the boiler.

If the object of the test is to determine the highest efficiency or capacity obtainable, any physical defects, or defects of operation, tending to make the result unfavorable should first be remedied; all foul parts being cleaned, and the whole put in first-class condition. If, on the other hand, the object is to ascertain the performance under existing conditions, no such preparation is either required or desired.

(C) General Precautions against Leakage:

In steam tests make sure that there is no leakage through blow-offs, drips, etc., or any steam or water connections of the plant or apparatus undergoing test, which would in any way affect the results. All such connections should be blanked off, or satisfactory assurance should be obtained that there is leakage neither out nor in. This is a most important matter, and no assurance should be considered satisfactory unless it is susceptible of absolute demonstration.



3. FUEL

Determine the character of fuel to be used.[62] For tests of maximum efficiency or capacity of the boiler to compare with other boilers, the coal should be of some kind which is commercially regarded as a standard for the locality where the test is made.

In the Eastern States the standards thus regarded for semi-bituminous coals are Pocahontas (Va. and W. Va.) and New River (W. Va.); for anthracite coals those of the No. 1 buckwheat size, fresh-mined, containing not over 13 per cent ash by analysis; and for bituminous coals, Youghiogheny and Pittsburgh coals. In some sections east of the Allegheny Mountains the semi-bituminous Clearfield (Pa.) and Cumberland (Md.) are also considered as standards. These coals when of good quality possess the essentials of excellence, adaptability to various kinds of furnaces, grates, boilers, and methods of firing required, besides being widely distributed and generally accessible in the Eastern market. There are no special grades of coal mined in the Western States which are widely and generally considered as standards for testing purposes; the best coal obtainable in any particular locality being regarded as the standard of comparison.

A coal selected for maximum efficiency and capacity tests, should be the best of its class, and especially free from slagging and unusual clinker-forming impurities.

For guarantee and other tests with a specified coal containing not more than a certain amount of ash and moisture, the coal selected should not be higher in ash and in moisture than the stated amounts, because any increase is liable to reduce the efficiency and capacity more than the equivalent proportion of such increase.

The size of the coal, especially where it is of the anthracite class, should be determined by screening a suitable sample.



4. APPARATUS AND INSTRUMENTS[63]

The apparatus and instruments required for boiler tests are:

(A) Platform scales for weighing coal and ashes.

(B) Graduated scales attached to the water glasses.

(C) Tanks and platform scales for weighing water (or water meters calibrated in place). Wherever practicable the feed water should be weighed, especially for guarantee tests. The most satisfactory and reliable apparatus for this purpose consists of one or more tanks each placed on platform scales, these being elevated a sufficient distance above the floor to empty into a receiving tank placed below, the latter being connected to the feed pump. Where only one weighing tank is used the receiving tank should be of larger size than the weighing tank, to afford sufficient reserve supply to the pump while the upper tank is filling. If a single weighing tank is used it should preferably be of such capacity as to require emptying not oftener than every 5 minutes. If two or more are used the intervals between successive emptyings should not be less than 3 minutes.

(D) Pressure gauges, thermometers, and draft gauges.

(E) Calorimeters for determining the calorific value of fuel and the quality of steam.

(F) Furnaces pyrometers.

(G) Gas analyzing apparatus.



5. OPERATING CONDITIONS

Determine what the operating conditions and method of firing should be to conform to the object in view, and see that they prevail throughout the trial, as nearly as possible.

Where uniformity in the rate of evaporation is required, arrangement can be usually made to dispose of the steam so that this result can be attained. In a single boiler it may be accomplished by discharging steam through a waste pipe and regulating the amount by means of a valve. In a battery of boilers, in which only one is tested, the draft may be regulated on the remaining boilers to meet the varying demands for steam, leaving the test boiler to work under a steady rate of evaporation.



6. DURATION

The duration of tests to determine the efficiency of a hand-fired boiler, should be 10 hours of continuous running, or such time as may be required to burn a total of 250 pounds of coal per square foot of grate.

In the case of a boiler using a mechanical stoker, the duration, where practicable, should be at least 24 hours. If the stoker is of a type that permits the quantity and condition of the fuel bed at beginning and end of the test to be accurately estimated, the duration may be reduced to 10 hours, or such time as may be required to burn the above noted total of 250 pounds per square foot.

In commercial tests where the service requires continuous operation night and day, with frequent shifts of firemen, the duration of the test, whether the boilers are hand fired or stoker fired, should be at least 24 hours. Likewise in commercial tests, either of a single boiler or of a plant of several boilers, which operate regularly a certain number of hours and during the balance of the day the fires are banked, the duration should not be less than 24 hours.

The duration of tests to determine the maximum evaporative capacity of a boiler, without determining the efficiency, should not be less than 3 hours.



7. STARTING AND STOPPING

The conditions regarding the temperature of the furnace and boiler, the quantity and quality of the live coal and ash on the grates, the water level, and the steam pressure, should be as nearly as possible the same at the end as at the beginning of the test.

To secure the desired equality of conditions with hand-fired boilers, the following method should be employed:

The furnace being well heated by a preliminary run, burn the fire low, and thoroughly clean it, leaving enough live coal spread evenly over the grate (say 2 to 4 inches),[64] to serve as a foundation for the new fire. Note quickly the thickness of the coal bed as nearly as it can be estimated or measured; also the water level,[65] the steam pressure, and the time, and record the latter as the starting time. Fresh coal should then be fired from that weighed for the test, the ashpit throughly cleaned, and the regular work of the test proceeded with. Before the end of the test the fire should again be burned low and cleaned in such a manner as to leave the same amount of live coal on the grate as at the start. When this condition is reached, observe quickly the water level,[65] the steam pressure, and the time, and record the latter as the stopping time. If the water level is not the same as at the beginning a correction should be made by computation, rather than by feeding additional water after the final readings are taken. Finally remove the ashes and refuse from the ashpit. In a plant containing several boilers where it is not practicable to clean them simultaneously, the fires should be cleaned one after the other as rapidly as may be, and each one after cleaning charged with enough coal to maintain a thin fire in good working condition. After the last fire is cleaned and in working condition, burn all the fires low (say 4 to 6 inches), note quickly the thickness of each, also the water levels, steam pressure, and time, which last is taken as the starting time. Likewise when the time arrives for closing the test, the fires should be quickly cleaned one by one, and when this work is completed they should all be burned low the same as the start, and the various observations made as noted. In the case of a large boiler having several furnace doors requiring the fire to be cleaned in sections one after the other, the above directions pertaining to starting and stopping in a plant of several boilers may be followed.

To obtain the desired equality of conditions of the fire when a mechanical stoker other than a chain grate is used, the procedure should be modified where practicable as follows:

Regulate the coal feed so as to burn the fire to the low condition required for cleaning. Shut off the coal-feeding mechanism and fill the hoppers level full. Clean the ash or dump plate, note quickly the depth and condition of the coal on the grate, the water level,[66] the steam pressure, and the time, and record the latter as the starting time. Then start the coal-feeding mechanism, clean the ashpit, and proceed with the regular work of the test.

When the time arrives for the close of the test, shut off the coal-feeding mechanism, fill the hoppers and burn the fire to the same low point as at the beginning. When this condition is reached, note the water level, the steam pressure, and the time, and record the latter as the stopping time. Finally clean the ashplate and haul the ashes.

In the case of chain grate stokers, the desired operating conditions should be maintained for half an hour before starting a test and for a like period before its close, the height of the throat plate and the speed of the grate being the same during both of these periods.



8. RECORDS

A log of the data should be entered in notebooks or on blank sheets suitably prepared in advance. This should be done in such manner that the test may be divided into hourly periods, or if necessary, periods of less duration, and the leading data obtained for any one or more periods as desired, thereby showing the degree of uniformity obtained.

Half-hourly readings of the instruments are usually sufficient. If there are sudden and wide fluctuations, the readings in such cases should be taken every 15 minutes, and in some instances oftener.

The coal should be weighed and delivered to the firemen in portions sufficient for one hour's run, thereby ascertaining the degree of uniformity of firing. An ample supply of coal should be maintained at all times, but the quantity on the floor at the end of each hour should be as small as practicable, so that the same may be readily estimated and deducted from the total weight.

The records should be such as to ascertain also the consumption of feed water each hour and thereby determine the degree of uniformity of evaporation.



9. QUALITY OF STEAM[67]

If the boiler does not produce superheated steam the percentage of moisture in the steam should be determined by the use of a throttling or separating calorimeter. If the boiler has superheating surface, the temperature of the steam should be determined by the use of a thermometer inserted in a thermometer well.

For saturated steam construct a sampling pipe or nozzle made of one-half inch iron pipe and insert it in the steam main at a point where the entrained moisture is likely to be most thoroughly mixed. The inner end of the pipe, which should extend nearly across to the opposite side of the main, should be closed and interior portion perforated with not less than twenty one-eighth inch holes equally distributed from end to end and preferably drilled in irregular or spiral rows, with the first hole not less than half an inch from the wall of the pipe.

The sampling pipe should not be placed near a point where water may pocket or where such water may effect the amount of moisture contained in the sample. Where non-return valves are used, or there are horizontal connections leading from the boiler to a vertical outlet, water may collect at the lower end of the uptake pipe and be blown upward in a spray which will not be carried away by the steam owing to a lack of velocity. A sample taken from the lower part of this pipe will show a greater amount of moisture than a true sample. With goose-neck connections a small amount of water may collect on the bottom of the pipe near the upper end where the inclination is such that the tendency to flow backward is ordinarily counterbalanced by the flow of steam forward over its surface; but when the velocity momentarily decreases the water flows back to the lower end of the goose-neck and increases the moisture at that point, making it an undesirable location for sampling. In any case it must be borne in mind that with low velocities the tendency is for drops of entrained water to settle to the bottom of the pipe, and to be temporarily broken up into spray whenever an abrupt bend or other disturbance is met.

If it is necessary to attach the sampling nozzle at a point near the end of a long horizontal run, a drip pipe should be provided a short distance in front of the nozzle, preferably at a pocket formed by some fitting and the water running along the bottom of the main drawn off, weighed, and added to the moisture shown by the calorimeter; or, better, a steam separator should be installed at the point noted.

In testing a stationary boiler the sampling pipe should be located as near as practicable to the boiler, and the same is true as regards the thermometer well when the steam is superheated. In an engine or turbine test these locations should be as near as practicable to throttle valve. In the test of a plant where it is desired to get complete information, especially where the steam main is unusually long, sampling nozzles or thermometer wells should be provided at both points, so as to obtain data at either point as may be required.



10. SAMPLING AND DRYING COAL

During the progress of test the coal should be regularly sampled for the purpose of analysis and determination of moisture.

Select a representative shovelful from each barrow-load as it is drawn from the coal pile or other source of supply, and store the samples in a cool place in a covered metal receptacle. When all the coal has thus been sampled, break up the lumps, thoroughly mix the whole quantity, and finally reduce it by the process of repeated quartering and crushing to a sample weighing about 5 pounds, the largest pieces being about the size of a pea. From this sample two one-quart air-tight glass fruit jars, or other air-tight vessels, are to be promptly filled and preserved for subsequent determinations of moisture, calorific value, and chemical composition. These operations should be conducted where the air is cool and free from drafts.



When the sample lot of coal has been reduced by quartering to, say, 100 pounds, a portion weighing, say, 15 to 20 pounds should be withdrawn for the purpose of immediate moisture determination. This is placed in a shallow iron pan and dried on the hot iron boiler flue for at least 12 hours, being weighed before and after drying on scales reading to quarter ounces.

The moisture thus determined is approximately reliable for anthracite and semi-bituminous coals, but not for coals containing much inherent moisture. For such coals, and for all absolutely reliable determinations the method to be pursued is as follows:

Take one of the samples contained in the glass jars, and subject it to a thorough air drying, by spreading it in a thin layer and exposing it for several hours to the atmosphere of a warm room, weighing it before and after, thereby determining the quantity of surface moisture it contains.[68] Then crush the whole of it by running it through an ordinary coffee mill or other suitable crusher adjusted so as to produce somewhat coarse grains (less than 1/16 inch), thoroughly mix the crushed sample, select from it a portion of from 10 to 50 grams,[69] weigh it in a balance which will easily show a variation as small as 1 part in 1000, and dry it for one hour in an air or sand bath at a temperature between 240 and 280 degrees Fahrenheit. Weigh it and record the loss, then heat and weigh again until the minimum weight has been reached. The difference between the original and the minimum weight is the moisture in the air-dried coal. The sum of the moisture thus found and that of the surface moisture is the total moisture.



11. ASHES AND REFUSE

The ashes and refuse withdrawn from the furnace and ashpit during the progress of the test and at its close should be weighed so far as possible in a dry state. If wet the amount of moisture should be ascertained and allowed for, a sample being taken and dried for this purpose. This sample may serve also for analysis and the determination of unburned carbon and fusing temperature.

The method above described for sampling coal may also be followed for obtaining a sample of the ashes and refuse.



12. CALORIFIC TESTS AND ANALYSES OF COAL

The quality of the fuel should be determined by calorific tests and analysis of the coal sample above referred to.[70]



13. ANALYSES OF FLUE GASES

For approximate determinations of the composition of the flue gases, the Orsat apparatus, or some modification thereof, should be employed. If momentary samples are obtained the analyses should be made as frequently as possible, say, every 15 to 30 minutes, depending on the skill of the operator, noting at the time the sample is drawn the furnace and firing conditions. If the sample drawn is a continuous one, the intervals may be made longer.



14. SMOKE OBSERVATIONS[71]

In tests of bituminous coals requiring a determination of the amount of smoke produced, observations should be made regularly throughout the trial at intervals of 5 minutes (or if necessary every minute), noting at the same time the furnace and firing conditions.



15. CALCULATION OF RESULTS

The methods to be followed in expressing and calculating those results which are not self-evident are explained as follows:

(A) Efficiency. The "efficiency of boiler, furnace and grate" is the relation between the heat absorbed per pound of coal fired, and the calorific value of one pound of coal.

The "efficiency of boiler and furnace" is the relation between the heat absorbed per pound of combustible burned, and the calorific value of one pound of combustible. This expression of efficiency furnishes a means for comparing one boiler and furnace with another, when the losses of unburned coal due to grates, cleanings, etc., are eliminated.

The "combustible burned" is determined by subtracting from the weight of coal supplied to the boiler, the moisture in the coal, the weight of ash and unburned coal withdrawn from the furnace and ashpit, and the weight of dust, soot, and refuse, if any, withdrawn from the tubes, flues, and combustion chambers, including ash carried away in the gases, if any, determined from the analysis of coal and ash. The "combustible" used for determining the calorific value is the weight of coal less the moisture and ash found by analysis.

The "heat absorbed" per pound of coal, or combustible, is calculated by multiplying the equivalent evaporation from and at 212 degrees per pound of coal or combustible by 970.4.

Other items in this section which have been treated elsewhere are:

(B) Corrections for moisture in steam.

(C) Correction for live steam used.

(D) Equivalent evaporation.

(E) Heat balance.

(F) Total heat of combustion of coal.

(G) Air for combustion and the methods recommended for calculating these results are in accordance with those described in different portions of this book.



16. DATA AND RESULTS

The data and results should be reported in accordance with either the short form or the complete form, adding lines for data not provided for, or omitting those not required, as may conform to the object in view.



17. CHART

In trials having for an object the determination and exposition of the complete boiler performance, the entire log of readings and data should be plotted on a chart and represented graphically.



18. TESTS WITH OIL AND GAS FUELS

Tests of boilers using oil or gas for fuel should accord with the rules here given, excepting as they are varied to conform to the particular characteristics of the fuel. The duration in such cases may be reduced, and the "flying" method of starting and stopping employed.

The table of data and results should contain items stating character of furnace and burner, quality and composition of oil or gas, temperature of oil, pressure of steam used for vaporizing and quantity of steam used for both vaporizing and for heating.

TABLE DATA AND RESULTS OF EVAPORATIVE TEST SHORT FORM, CODE OF 1912

1 Test of.................boiler located at................................ to determine...............conducted by.............................. 2 Kind of furnace.......................................................... 3 Grate surface.................................................square feet 4 Water-heating surface.........................................square feet 5 Superheating surface..........................................square feet 6 Date..................................................................... 7 Duration............................................................hours 8 Kind and size of coal....................................................

AVERAGE PRESSURES, TEMPERATURES, ETC.

9 Steam pressure by gauge............................................pounds 10 Temperature of feed water entering boiler.........................degrees 11 Temperature of escaping gases leaving boiler......................degrees 12 Force of draft between damper and boiler...........................inches 13 Percentage of moisture in steam, or number degrees of superheating..................per cent or degrees

TOTAL QUANTITIES

14 Weight of coal as fired[72]........................................pounds 15 Percentage of moisture in coal...................................per cent 16 Total weight of dry coal consumed..................................pounds 17 Total ash and refuse...............................................pounds 18 Percentage of ash and refuse in dry coal.........................per cent 19 Total weight of water fed to the boiler[73]........................pounds 20 Total water evaporated, corrected for moisture in steam............pounds 21 Total equivalent evaporation from and at 212 degrees...............pounds

HOURLY QUANTITIES AND RATES

22 Dry coal consumed per hour.........................................pounds 23 Dry coal per square feet of grate surface per hour.................pounds 24 Water evaporated per hour corrected for quality of steam...........pounds 25 Equivalent evaporation per hour from and at 212 degrees............pounds 26 Equivalent evaporation per hour from and at 212 degrees per square foot of water-heating surface........................pounds

CAPACITY

27 Evaporation per hour from and at 212 degrees (same as Line 25).....pounds 28 Boiler horse power developed (Item 27/34-1/2)...........boiler horse power 29 Rated capacity, in evaporation from and at 212 degrees per hour....pounds 30 Rated boiler horse power...............................boiler horse power 31 Percentage of rated capacity developed...........................per cent

ECONOMY RESULTS

32 Water fed per pound of coal fired (Item 19/Item 14)................pounds 33 Water evaporated per pound of dry coal (Item 20/Item 16)...........pounds 34 Equivalent evaporation from and at 212 degrees per pound of dry coal (Item 21/Item 16)...................................pounds 35 Equivalent evaporation from and at 212 degrees per pound of combustible [Item 21/(Item 16-Item 17)]......................pounds

EFFICIENCY

36 Calorific value of one pound of dry coal.........................B. t. u. 37 Calorific value of one pound of combustible......................B. t. u.

( Item 34x970.4) 38 Efficiency of boiler, furnace and grate (100 x ——————-)....per cent ( Item 36 )

( Item 35x970.4) 39 Efficiency of boiler and furnace (100 x ——————-)...........per cent ( Item 37 )

COST OF EVAPORATION

40 Cost of coal per ton of......pounds delivered in boiler room......dollars 41 Cost of coal required for evaporating 1000 pounds of water from and at 212 degrees........................................dollars



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 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 _ _____ ___ __ __

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 Analysis of Actual Equiv. ditto / % Rated CO_{2} O CO Test Evapor- Evap. @ sq.ft. Cap'ty. Per Per Per ation >212deg 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 Analysis Dry Coal Equiv. Combnd. of Volatl. Fixed Ash B.t.u./ Evap. @ Efficy. Test Matter Carbon Per Pound >212deg 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 _ __ __ __ __ __ __



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.