Part 30

Stacks for Wood-fired Boilers--For boilers using wood as fuel, there is but little data upon which to base stack sizes. The loss of draft through the bed of fuel will vary over limits even wider than in the case of coal, for in this class of fuel the moisture may run from practically 0.0 per cent to over 60 per cent, and the methods of handling and firing are radically different for the different classes of wood (see chapter on Wood-burning Furnaces). As economy is ordinarily of little importance, high stack temperatures may be expected, and often unavoidably large quantities of excess air are supplied due to the method of firing. In general, it may be stated that for this class of fuel the diameter of stacks should be at least as great as for coal-fired boilers, while the height may be slightly decreased. It is far the best plan in designing a stack for boilers using wood fuel to consider each individual set of conditions that exist, rather than try to follow any general rule.

One factor not to be overlooked in stacks for wood burning is their location. The fine particles of this fuel are often carried unconsumed through the boiler, and where the stack is not on top of the boiler, these particles may accumulate in the base of the stack below the point at which the flue enters. Where there is any air leakage through the base of such a stack, this fuel may become ignited and the stack burned. Where there is a possibility of such action taking place, it is well to line the stack with fire brick for a portion of its height.

Draft Gauges--The ordinary form of draft gauge, Fig. 35, which consists of a U-tube, containing water, lacks sensitiveness in measuring such slight pressure differences as usually exist, and for that reason gauges which multiply the draft indications are more convenient and are much used.

[Illustration: Fig. 35. U-tube Draft Gauge]

[Illustration: Fig. 36. Barrus Draft Gauge]

An instrument which has given excellent results is one introduced by Mr. G. H. Barrus, which multiplies the ordinary indications as many times as desired. This is illustrated in Fig. 36, and consists of a U-tube made of one-half inch glass, surmounted by two larger tubes, or chambers, each having a diameter of 2½ inches. Two different liquids which will not mix, and which are of different color, are used, usually alcohol colored red and a certain grade of lubricating oil. The movement of the line of demarcation is proportional to the difference in the areas of the chambers and the U-tube connecting them. The instrument is calibrated by comparison with the ordinary U-tube gauge.

In the Ellison form of gauge the lower portion of the ordinary U-tube has been replaced by a tube slightly inclined to the horizontal, as shown in Fig. 37. By this arrangement any vertical motion in the right-hand upright tube causes a very much greater travel of the liquid in the inclined tube, thus permitting extremely small variation in the intensity of the draft to be read with facility.

[Illustration: Fig. 37. Ellison Draft Gauge]

The gauge is first leveled by means of the small level attached to it, both legs being open to the atmosphere. The liquid is then adjusted until its meniscus rests at the zero point on the left. The right-hand leg is then connected to the source of draft by means of a piece of rubber tubing. Under these circumstances, a rise of level of one inch in the right-hand vertical tube causes the meniscus in the inclined tube to pass from the point 0 to 1.0. The scale is divided into tenths of an inch, and the sub-divisions are hundredths of an inch.

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.

[Illustration: Fig. 38. Peabody Draft Gauge]

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 × 0.97 ------------------------------------------ Heat value per pound of dry coal × 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 × .97 = 16,975, and the evaporation per pound of dry fuel from and at 212 degrees will be 165,516 ÷ 16,975 = 9.75 pounds. The heat absorbed per pound of dry fuel will, therefore, be 9.75 × 970.4 = 9461 B. t. u. Hence, the efficiency by (31) will be 9461 ÷ 13,516 = 70.0 per cent. The total combustible burned will be 16,975 - 2396 = 14,579, and the evaporation from and at 212 degrees per pound of combustible will be 165,516 ÷ 14,579 = 11.35 pounds. Hence, the efficiency based on combustible from (32) will be (11.35 × 97.04) ÷ 15,359 = 71.79.[**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 212° 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.

[Illustration: Portion of 15,000 Horse-power Installation of Babcock & Wilcox Boilers, Equipped with Babcock & Wilcox Chain Grate Stokers at the Northumberland, Pa., Plant of the Atlas Portland Cement Co. This Company Operates a Total of 24,000 Horse Power of Babcock & Wilcox Boilers in its Various Plants]

(B) Loss due to heat carried away in the steam produced by the burning of the hydrogen component of the fuel. In burning, one pound of hydrogen unites with 8 pounds of oxygen to form 9 pounds of steam. Following the reasoning of item (A), therefore, this loss will be:

Loss in B. t. u. per pound = 9H((212-t)+970.4+.47(T-212)) (34)

where H = the percentage by weight of hydrogen.

This item is frequently considered as a part of the unaccounted for loss, where an ultimate analysis of the fuel is not given.

(C) Loss due to heat carried away by dry chimney gases. This is dependent upon the weight of gas per pound of coal which may be determined by formula (16), page 158.

Loss in B. t. u. per pound = (T-t)×.24×W.

Where T and t have values as in (33),

.24 = specific heat of chimney gases,

W = weight of dry chimney gas per pound of coal.

(D) Loss due to incomplete combustion of the carbon content of the fuel, that is, the burning of the carbon to CO instead of CO_{2}.

10,150 CO Loss in B. t. u. per pound = C×--------- (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 × per cent ash × 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:

(11.71×970.4)÷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:

= 9×.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(CO+N)) (-------------------)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,

(11×14.33+8×4.54+7(0.11+81.02)) (-----------------------------)×78.57 = 13.7 pounds. ( 3(14.33+0.11) )

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

13.7×0.24(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 ----------×.7857×10,150 = 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 10.00×.179 = 1.79 per cent of the total fuel represents the proportion of this total unconsumed in the ash and the loss due to this cause is

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

The heat absorbed by the boilers per pound of dry fuel is 11.71×970.4 = 11,363 B. t. u. This quantity plus losses (A), (B), (C), (D) and (E), or 11,363+22+618+1311+61+261 = 13,636 B. t. u. accounted for. The heat value of the coal, 14,225 B. t. u., less 13,636 B. t. u., leaves 589 B. t. u., unaccounted for losses, or 4.15 per cent.

The heat balance should be arranged in the form indicated by Table 58.

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.