Chapter 30
A convenient rule for large stacks, 200 feet high and over, is to provide 30 square feet of cross sectional area per 1000 rated horse power.
Stacks for Oil Fuel--The requirements of stacks connected to boilers under which oil fuel is burned are entirely different from those where coal is used. While more attention has been paid to the matter of stack sizes for oil fuel in recent years, there has not as yet been gathered the large amount of experimental data available for use in designing coal stacks.
In the case of oil-fired boilers the loss of draft through the fuel bed is partially eliminated. While there may be practically no loss through any checkerwork admitting air to the furnace when a boiler is new, the areas for the air passage in this checkerwork will in a short time be decreased, due to the silt which is present in practically all fuel oil. The loss in draft through the boiler proper at a given rating will be less than in the case of coal-fired boilers, this being due to a decrease in the volume of the gases. Further, the action of the oil burner itself is to a certain extent that of a forced draft. To offset this decrease in draft requirement, the temperature of the gases entering the stack will be somewhat lower where oil is used than where coal is used, and the draft that a stack of a given height would give, therefore, decreases. The factors as given above, affecting as they do the intensity of the draft, affect directly the height of the stack to be used.
As already stated, the volume of gases from oil-fired boilers being less than in the case of coal, makes it evident that the area of stacks for oil fuel will be less than for coal. It is assumed that these areas will vary directly as the volume of the gases to be handled, and this volume for oil may be taken as approximately 60 per cent of that for coal.
In designing stacks for oil fuel there are two features which must not be overlooked. In coal-firing practice there is rarely danger of too much draft. In the burning of oil, however, this may play an important part in the reduction of plant economy, the influence of excessive draft being more apparent where the load on the plant may be reduced at intervals. The reason for this is that, aside from a slight decrease in temperature at reduced loads, the tendency, due to careless firing, is toward a constant gas flow through the boiler regardless of the rate of operation, with the corresponding increase of excess air at light loads. With excessive stack height, economical operation at varying loads is almost impossible with hand control. With automatic control, however, where stacks are necessarily high to take care of known peaks, under lighter loads this economical operation becomes less difficult. For this reason the question of designing a stack for a plant where the load is known to be nearly a constant is easier than for a plant where the load will vary over a wide range. While great care must be taken to avoid excessive draft, still more care must be taken to assure a draft suction within all parts of the setting under any and all conditions of operation. It is very easily possible to more than offset the economy gained through low draft, by the losses due to setting deterioration, resulting from such lack of suction. Under conditions where the suction is not sufficient to carry off the products of combustion, the action of the heat on the setting brickwork will cause its rapid failure.
It becomes evident, therefore, that the question of stack height for oil-fired boilers is one which must be considered with the greatest of care. The designer, on the one hand, must guard against the evils of excessive draft with the view to plant economy, and, on the other, against the evils of lack of draft from the viewpoint of upkeep cost. Stacks for this work should be proportioned to give ample draft for the maximum overload that a plant will be called upon to carry, all conditions of overload carefully considered. At the same time, where this maximum overload is figured liberally enough to insure a draft suction within the setting under all conditions, care must be taken against the installation of a stack which would give more than this maximum draft.
TABLE 56
STACK SIZES FOR OIL FUEL
ADAPTED FROM C. R. WEYMOUTH'S TABLE (TRANS. A. S. M. E. VOL. 34)
+----------------------------------------------------+ |+--------+-----------------------------------------+| || | Height in Feet Above Boiler Room Floor || ||Diameter+------+------+------+-----+--------------+| || Inches | 80 | 90 | 100 | 120 | 140 | 160 || |+--------+------+------+------+------+------+------+| || 33 | 161 | 206 | 233 | 270 | 306 | 315 || || 36 | 208 | 253 | 295 | 331 | 363 | 387 || || 39 | 251 | 303 | 343 | 399 | 488 | 467 || || 42 | 295 | 359 | 403 | 474 | 521 | 557 || || 48 | 399 | 486 | 551 | 645 | 713 | 760 || || 54 | 519 | 634 | 720 | 847 | 933 | 1000 || || 60 | 657 | 800 | 913 | 1073 | 1193 | 1280 || || 66 | 813 | 993 | 1133 | 1333 | 1480 | 1593 || || 72 | 980 | 1206 | 1373 | 1620 | 1807 | 1940 || || 84 | 1373 | 1587 | 1933 | 2293 | 2560 | 2767 || || 96 | 1833 | 2260 | 2587 | 3087 | 3453 | 3740 || || 108 | 2367 | 2920 | 3347 | 4000 | 4483 | 4867 || || 120 | 3060 | 3660 | 4207 | 5040 | 5660 | 6160 || |+--------+------+------+------+------+------+------+| +----------------------------------------------------+
Figures represent nominal rated horse power. Sizes as given good for 50 per cent overloads.
Based on centrally located stacks, short direct flues and ordinary operating efficiencies.
Table 56 gives the sizes of stacks, and horse power which they will serve for oil fuel. This table is, in modified form, one calculated by Mr. C. R. Weymouth after an exhaustive study of data pertaining to the subject, and will ordinarily give satisfactory results.
Stacks for Blast Furnace Gas Work--For boilers burning blast furnace gas, as in the case of oil-fired boilers, stack sizes as suited for coal firing will have to be modified. The diameter of stacks for this work should be approximately the same as for coal-fired boilers. The volume of gases would be slightly greater than from a coal fire and would decrease the draft with a given stack, but such a decrease due to volume is about offset by an increase due to somewhat higher temperatures in the case of the blast furnace gases.
Records show that with this class of fuel 175 per cent of the rated capacity of a boiler can be developed with a draft at the boiler damper of from 0.75 inch to 1.0 inch, and it is well to limit the height of stacks to one which will give this draft as a maximum. A stack of proper diameter, 130 feet high above the ground, will produce such a draft and this height should ordinarily not be exceeded. Until recently the question of economy in boilers fired with blast furnace gas has not been considered, but, aside from the economical standpoint, excessive draft should be guarded against in order to lower the upkeep cost.
Stacks should be made of sufficient height to produce a draft that will develop the maximum capacity required, and this draft decreased proportionately for loads under the maximum by damper regulation. The amount of gas fed to a boiler for any given rating is a fixed quantity and if a draft in excess of that required for that particular rate of operation is supplied, economy is decreased and the wear and tear on the setting is materially increased. Excess air which is drawn in, either through or around the gas burners by an excessive draft, will decrease economy, as in any other class of work. Again, as in oil-fired practice, it is essential on the other hand that a suction be maintained within all parts of the setting, in this case not only to provide against setting deterioration but to protect the operators from leakage of gas which is disagreeable and may be dangerous. Aside from the intensity of the draft, a poor mixture of the gas and air or a "laneing" action may lead to secondary combustion with the possibility of dangerous explosions within the setting, may cause a pulsating action within the setting, may increase the exit temperatures to a point where there is danger of burning out damper boxes, and, in general, is hard on the setting. It is highly essential, therefore, that the furnace be properly constructed to meet the draft which will be available.
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.
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.
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.
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.
(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: