Steam, Its Generation and Use Part 33

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 a.s.sumed 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 a.s.sure 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.

[Ill.u.s.tration: 7800 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers, Equipped with Babc.o.c.k & Wilc.o.x Chain Grate Stokers at the Metropolitan West Side Elevated Ry. Co., Chicago, Ill.]

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 cla.s.s 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 quant.i.ty 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 cla.s.s 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 cla.s.s 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 cla.s.ses 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 quant.i.ties of excess air are supplied due to the method of firing. In general, it may be stated that for this cla.s.s 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 acc.u.mulate 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.

[Ill.u.s.tration: Fig. 35. U-tube Draft Gauge]

[Ill.u.s.tration: 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 ill.u.s.trated in Fig. 36, and consists of a U-tube made of one-half inch gla.s.s, surmounted by two larger tubes, or chambers, each having a diameter of 2 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.

[Ill.u.s.tration: 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 circ.u.mstances, a rise of level of one inch in the right-hand vertical tube causes the meniscus in the inclined tube to pa.s.s 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.

[Ill.u.s.tration: Fig. 38. Peabody Draft Gauge]

EFFICIENCY AND CAPACITY OF BOILERS

Two of the most important operating factors entering into the consideration of what const.i.tutes 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 establis.h.i.+ng 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 ill.u.s.trated by example.

a.s.sume 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.