Part 32

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.

[Illustration: 3460 Horse-power Installation of Babcock & Wilcox Boilers at the Chicago, Ill., Shops of the Chicago and Northwestern Ry. Co.]

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lot of coal has been reduced by quartering to, say, 100 pounds, a portion weighing, say, 15 to 20 pounds should be withdrawn for the purpose of immediate moisture determination.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÷34½).............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 34×970.4) 38 Efficiency of boiler, furnace and grate (100 × -------------)....per cent ( Item 36 )

( Item 35×970.4) 39 Efficiency of boiler and furnace (100 × -------------)...........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

[Illustration: Portion of 3600 Horse-power Installation of Babcock & Wilcox Boilers, Equipped with Babcock & Wilcox Chain Grate Stokers at the Loomis Street Plant of the Peoples Gas Light & Coke Co., Chicago, Ill. This Company has Installed 7780 Horse Power of Babcock & Wilcox Boilers]

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