TEMPERATURE OF WASTE GASES FROM
VARIOUS INDUSTRIAL PROCESSES
+-----------------------------------------------------+
|+-----------------------------------+---------------+|
||Waste Heat From |Temperature[50]||
|| | Degrees ||
|+-----------------------------------+---------------+|
||Brick Kilns | 2000-2300 ||
||Zinc Furnaces | 2000-2300 ||
||Copper Matte Reverberatory Furnaces| 2000-2200 ||
||Beehive Coke Ovens | 1800-2000 ||
||Cement Kilns | 1200-1600[51]||
||Nickel Refining Furnaces | 1500-1750 ||
||Open Hearth Steel Furnaces | 1100-1400 ||
|+-----------------------------------+---------------+|
+-----------------------------------------------------+
The power that can be obtained from waste gases depends upon their
temperature and weight, and both of these factors vary widely in
different commercial operations. Table 52 gives a list of certain
processes yielding waste gases the heat of which is available for the
generation of steam and the approximate temperature of such gases. It
should be understood that the temperatures in the table are the average
of the range of a complete cycle of the operation and that the minimum
and maximum temperatures may vary largely from the figures given.
The maximum available horse power that may be secured from such gases is
represented by the formula:
W(T-t)s
H. P. = ------- (23)
33,479
Where W = the weight of gases passing per hour,
T = temperature of gases entering heating surface,
t = temperature leaving heating surface,
s = specific heat of gases.
The initial temperature and the weight or volume of gas will depend, as
stated, upon the process involved. The exit temperature will depend, to
a certain extent, upon the temperature of the entering gases, but will
be governed mainly by the efficiency of the heating surfaces installed
for the absorption of the heat.
Where the temperature of the gas available is high, approaching that
found in direct fired boiler practice, the problem is simple and the
question of design of boiler becomes one of adapting the proper amount
of heating surface to the volume of gas to be handled. With such
temperatures, and a volume of gas available approximately in accordance
with that found in direct fired boiler practice, a standard boiler or
one but slightly modified from the standard will serve the purpose
satisfactorily. As the temperatures become lower, however, the problem
is more difficult and the departure from standard practice more radical.
With low temperature gases, to obtain a heat transfer rate at all
comparable with that found in ordinary boiler practice, the lack of
temperature must be offset by an added velocity of the gases in their
passage over the heating surfaces. In securing the velocity necessary to
give a heat transfer rate with low temperature gases sufficient to make
the installation of waste heat boilers show a reasonable return on the
investment, the frictional resistance to the gases through the boiler
becomes greatly in excess of what would be considered good practice in
direct fired boilers. Practically all operations yielding waste gases
require that nothing be done in the way of impairing the draft at the
furnace outlet, as this might interfere with the operation of the
primary furnace. The installation of a waste heat boiler, therefore,
very frequently necessitates providing sufficient mechanical draft to
overcome the frictional resistance of the gases through the heating
surfaces and still leave ample draft available to meet the maximum
requirements of the primary furnace.
Where the temperature and volume of the gases are in line with what are
found in ordinary direct fired practice, the area of the gas passages
may be practically standard. With the volume of gas known, the draft
loss through the heating surfaces may be obtained from experimental data
and this additional draft requirement met by the installation of a stack
sufficient to take care of this draft loss and still leave draft enough
for operating the furnace at its maximum capacity.
Where the temperatures are low, the added frictional resistance will
ordinarily be too great to allow the draft required to be secured by
additional stack height and the installation of a fan is necessary. Such
a fan should be capable of handling the maximum volume of gas that the
furnace may produce, and of maintaining a suction equivalent to the
maximum frictional resistance of such volume through the boiler plus the
maximum draft requirement at the furnace outlet. Stacks and fans for
this class of work should be figured on the safe side. Where a fan
installation is necessary, the loss of draft in the fan connections
should be considered, and in figuring conservatively it should be
remembered that a fan of ample size may be run as economically as a
smaller fan, whereas the smaller fan, if overloaded, is operated with a
large loss in efficiency. In practically any installation where low
temperature gas requires a fan to give the proper heat transfer from the
gases, the cost of the fan and of the energy to drive it will be more
than offset by the added power from the boiler secured by its use.
Furthermore, the installation of such a fan will frequently increase the
capacity of the industrial furnace, in connection with which the waste
heat boilers are installed.
In proportioning heating surfaces and gas passages for waste heat work
there are so many factors bearing directly on what constitutes the
proper installation that it is impossible to set any fixed rules. Each
individual installation must be considered by itself as well as the
particular characteristics of the gases available, such as their
temperature and volume, and the presence of dust or tar-like substances,
and all must be given the proper weight in the determination of the
design of the heating surfaces and gas passages for the specific set of
conditions.
[Graph: Per Cent of Water Heating Surface passed over by Gases/Per Cent
of the Total Amount of Steam Generated in the Boiler
against Temperature in Degrees Fahrenheit of Hot Gases Sweeping Heating
Surface
Fig. 31. Curve Showing Relation Between Gas Temperature, Heating Surface
passed over, and Amount of Steam Generated. Ten Square Feet of Heating
Surface are Assumed as Equivalent to One Boiler Horse Power]
Fig. 31 shows the relation of gas temperatures, heating surface passed
over and work done by such surface for use in cases where the
temperatures approach those found in direct fired practice and where the
volume of gas available is approximately that with which one horse power
may be developed on 10 square feet of heating surface. The curve assumes
what may be considered standard gas passage areas, and further, that
there is no heat absorbed by direct radiation from the fire.
Experiments have shown that this curve is very nearly correct for the
conditions assumed. Such being the case, its application in waste heat
work is clear. Decreasing or increasing the velocity of the gases over
the heating surfaces from what might be considered normal direct fired
practice, that is, decreasing or increasing the frictional loss through
the boiler will increase or decrease the amount of heating surface
necessary to develop one boiler horse power. The application of Fig. 31
to such use may best be seen by an example:
Assume the entering gas temperatures to be 1470 degrees and that the
gases are cooled to 570 degrees. From the curve, under what are assumed
to be standard conditions, the gases have passed over 19 per cent of
the heating surface by the time they have been cooled 1470 degrees.
When cooled to 570 degrees, 78 per cent of the heating surface has been
passed over. The work done in relation to the standard of the curve is
represented by (1470 - 570) ÷ (2500 - 500) = 45 per cent. (These
figures may also be read from the curve in terms of the per cent of the
work done by different parts of the heating surfaces.) That is, 78 per
cent - 19 per cent = 59 per cent of the standard heating surface has
done 45 per cent of the standard amount of work. 59 ÷ 45 = 1.31, which
is the ratio of surface of the assumed case to the standard case of the
curve. Expressed differently, there will be required 13.1 square feet
of heating surface in the assumed case to develop a horse power as
against 10 square feet in the standard case.
The gases available for this class of work are almost invariably very
dirty. It is essential for the successful operation of waste-heat
boilers that ample provision be made for cleaning by the installation of
access doors through which all parts of the setting may be reached. In
many instances, such as waste-heat boilers set in connection with cement
kilns, settling chambers are provided for the dust before the gases
reach the boiler.
By-passes for the gases should in all cases be provided to enable the
boiler to be shut down for cleaning and repairs without interfering with
the operation of the primary furnace. All connections from furnace to
boilers should be kept tight to prevent the infiltration of air, with
the consequent lowering of gas temperatures.
Auxiliary gas or coal fired grates must be installed to insure
continuity in the operation of the boiler where the operation of the
furnace is intermittent or where it may be desired to run the boiler
with the primary furnace not in operation. Such grates are sometimes
used continuously where the gases available are not sufficient to
develop the required horse power from a given amount of heating surface.
Fear has at times been expressed that certain waste gases, such as those
containing sulphur fumes, will have a deleterious action on the heating
surface of the boiler. This feature has been carefully watched, however,
and from plants in operation it would appear that in the absence of
water or steam leaks within the setting, there is no such harmful
action.
[Illustration: Fig. 32. Babcock & Wilcox Boiler Arranged for Utilizing
Waste Heat from Open Hearth Furnace. This Setting may be Modified to
Take Care of Practically any Kind of Waste Gas]