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BLIZARD, FOSTER—SOME PARTICULARS OF DESIGN, OPERATION OF TWIN-FURNACE BOILERS 511
This boiler operates with an efficiency of about 86 to 88 per
cent. The C02 in the gas going to the air heater is normally over
15 per cent. In Table 2, some gas and water temperatures, taken
during normal operation, are tabulated.
Measurements of the impurities in the steam leaving the boiler
indicate that the total solids varies from 0.5 to 0.6 ppm with
total solids in the boiler water varying from 200 to 1400 ppm.
S e p a r a t e l y F ir e d S u p e r h e a t e r M a r i n e B o i l e r
Another type of separately fired superheater boiler is shown
in Fig. 7. Two of these boilers have been in use at sea since
1938, and were designed normally to generate 17,500 lb of steam
per hr at 400 psi, 725 F, and at an overload to give twice this
amount of steam at the same pressure and temperature. They
are installed on a tanker where at times approximately 30,000 lb
of saturated steam is required for pumping and 1500 lb of
superheated steam for other purposes. I t will be seen that there
are two furnaces. In the furnace with the single oil burner is a
superheater over which the gases from this burner pass and proceed
through a screen of water tubes to the main furnace where
there are three burners, and thence to the main-boiler heating surface.
The temperature of the superheated steam is thus under
complete control since no gases from the three main burners pass
over the superheater, and comparatively little radiation finds its
way to the superheater from the main furnace. The bulk of the
heat absorbed by the superheater is by radiation from the oil
flame which accounts for the comparatively small size of the
superheater.
In order to avoid overheating the superheater tubes, an automatic
device is provided which prevents the burner in the superheater
furnace from receiving oil, unless steam is passing through
the superheater. If at any time the temperature leaving the
superheater is too high, the supply of oil to the superheater burner
is automatically cut off.
No desuperheater is required with this boiler for supplying
saturated steam since, by varying the rate of firing in the superheater
furnace, the superheated steam can be held at the desired
temperature, regardless of the proportion of the total steam which
is being superheated. This is contrary to what occurs with a conventional
boiler and superheater since, with this latter arrangement,
if any steam is withdrawn from the drum the temperature
of the superheated steam rises and, if the quantity withdrawn increases
unduly, the steam in the superheater tubes may rise to too
high a temperature.
In a recent paper,8 a steam generator with two furnaces is
shown which produces steam over a range of 30,000 to 120,000
lb per hr at a final temperature of 850 F and 600 psi when fired
by oil or pulverized bituminous coal. It is required also to generate
from 30,000 to 90,000 lb of steam per hr at the same pressure
and temperature with pulverized lignite. Had the usual convection
superheater been used on this boiler, it would have been impossible
to obtain the same steam temperature with these widely
divergent fuels without arranging for by-passing the gases around
the superheater in quantities varying with the fuel used. However,
this boiler has two furnaces, in one of which is a superheater
so that by varying the relative rates of firing in the two
furnaces, the required steam temperature is obtained without
difficulty. The boiler is similar in principle to the marine boiler
previously described but differs from it slightly in that the products
of combustion from the superheater furnace, instead of
passing through the screen of tubes separating the two furnaces
for the full length of the furnace, pass through an aperture at the
rear of the superheater furnace to the saturated furnace. Al-
* “Superheat Control and Steam Purity in High-Pressure Boilers,"
by Martin Frisch, Trans. A.S.M.E., vol. 62, October, 1940, Fig. 4,
p. 607
though, in this type of boiler the gases from the superheater furnace
pass through the second furnace and thence to the boiler, no
trouble has been experienced in burning the fuel in both furnaces
with a slight amount of excess air.
A particular advantage with this type of boiler lies in the ease
with which it may be started. When starting, fires are lighted
only in that furnace in which there are waterwalls and the gases
do not pass over the superheater. Thus, there is no danger of
overheating the superheater when, in starting, no steam flows
through it.
Fig. 7 of a previous paper4 illustrates another type of boiler
with two furnaces which is designed to generate 100,000 lb of
steam per hr at 725 psi and 750 F final temperature when fired
with pulverized coal. It has been in operation at Oil City, Pa.,
since 1935. This boiler is equipped with a convection superheater
over which the gases from both furnaces pass. When
operating the boiler below one-half load, only one furnace is used
which causes the gases leaving the single furnace to be higher than
they would be were both furnaces in operation; this in turn raises
the rate of heat absorption of the convection superheater and so
raises the temperature of the steam at the lower rates. In fact,
with this mode of operation, there is but slight variation in the
temperature of the steam over a range of evaporation of 30,000
to 100,000 lb of steam per hr. The variation of the temperature
with the rating is shown in Fig. 8 of the paper4 referred to.
S u m m a r y
Three types of boilers each with two furnaces and separate
means of firing these furnaces have been described as follows:
1 A boiler with two furnaces, in one only of which is placed a
radiant superheater. In addition to the radiant superheater,
there is a convection superheater over which the gases from both
furnaces pass.
2 A boiler with two furnaces, in one of which is placed a superheater.
It differs from the first type in that the gases from the
superheater furnace pass through the furnace in which there is no
superheater. No convection superheater is provided. When, as
on a ship, a separate supply of saturated steam is required, the
steam may be taken directly from the steam drum.
3 A boiler with two furnaces from which the gases pass over a
convection superheater. No radiant superheater is provided.
With these boilers, which have two furnaces instead of one, it
is possible to increase the cooling surface surrounding the furnaces
and, by differential firing of the two furnaces, to control
the steam temperature over a wide range of steaming without
installing a large convection superheater which has to be bypassed
at the high loads if the temperature of the steam is to
be controlled.
Discussion
R. S. J u l s r u d . 6 Several interesting features of the design of
twin-boiler furnaces together with operating data of a large unit
have been given in this paper. The writer would like to ask some
questions concerning automatic control of steam output and
steam temperature with this unit. In the conventional singlefurnace
and convection-type superheater, automatic control of
steam output and steam temperature may be accomplished by
any one of several automatic-control systems now available.
However, with the twin-fumace boiler, a demand, say for in-
4 “What Are Logical Trends in Design for Steam Generation?” by
H. J. Kerr, John Van Brunt, and Martin Frisch, paper presented before
Power Division of Metropolitan Section, A.S.M.E., New York,
N. Y., Nov. 18, 1937
s Harmon-on-Hudson, N. Y. Mem. A.S.M.E