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.
[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]