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368 Fbrging-Stamping - Heat Treating
should be given to the refractory linings for high temperature
work.
e. Finally working conditions must be considered
for these influence both choice of furnace and the results
obtainable. The selection of a furnace resolves
into a compromise. The better furnaces are high in
first cost, repairs arc less frequent but mure expensive,
floor space pccupied is large; on the other hand their
operation is satisfactory and efficient, the quality of
the product is good and better shop conditions prcbail.
f. In all of the discussion of fuels and furnaces,
the importance of skilled operatives must be carefullyconsidered.
Given the best of equipment, the most
efficient fuel, and as much automatic apparatus as
hitman ingenuity has been able to devise, the human
element must still be dealt with and its effect upon the
nature of our product, an effect which, in many cases.
is allowed to be most detrimental, merely because the
shortsightedness and false economy of the management
has prevented the employment of a man who
knows his tools — the furnace and the fuel. It is a
waste of money to install highly efficient furnaces
equipped with pyrometers and heat salvage devices.
all of which are obviously expensive, without providing
adequate intelligent supervision, capable of observing
and maintaining the quality of the product.
Figs. 1 and 2 which summarize the more important
factors governing the selection of fuels and furnaces
are adapted from similar charts furnished through the
courtesy of the W. S. Rockwell Companv of New
York.
Relative Thermal Efficiency.
It is a well known fact that most industrial heating
processes are carried on at fuel efficiencies far below
the values obtainable and insisted upon in other operations
of equal importance; particularly is this so, because,
in this country at least, but little attention has
been given to the advantages of heat saving devices.
The solid fuels require considerable excess air for
combustion, limiting the maximum possible efficiencies
right from the start. The liquid fuels too, require
excess air. although not as much as the solid ones.
Only gas can be operated on the theoretical quantities
required. If the flue products leave at a sufficiently
low temperature, the heat in the flue products can
be recovered, but with solid fuels stack temperatures
must be maintained in order to secure the necessarydraft.
Thus, even at low temperatures only comparatively
low efficiencies are obtainable. At higher temperatures
the effects become more and more marked
because not only do the flue losses increase but also
the radiation, leakage and other less important losses.
So many factors influence the efficiency of a furnace,
the total useful heat available and the temperatures
attained, that the prediction of the results with
accuracy is a very difficult problem especially- when
dealing with recuperative systems.
In the above discussion electricity as a heating
medium has been omitted for which extravagantlyhigh
efficiencies are often claimed. Efficiency is not
a function of the fuel so much as of its method of
utilization. Electricity, for example, can be used in
heat treating furnaces at very high efficiencies if the
furnace is perfectly insulated, the work continuous
and of such a nature that the time consumed for each
batch i^ comparatively long and if the doors and other
October. 1925
openings fit tightly. On the other hand not much
better than 50 per cent of the results under the above
conditions could be expected if the work is intermittent,
if the doors must be opened frequently, thus
producing the equivalent of an "excess air" loss in
combustible fuels, if insulation is not perfect or if any
detail of construction and operation has not been most
carefully considered.
And so with other heating methods whether coal.
coke, oil or gas is the fuel. Operation and construction
often count for more than the fuel. However,
considering successively various major heating operations
such as : baking at 450 deg. F.; heat treating at
1,600 deg. F.; forging at 2,300 deg. F.; and metal melting
at 2,800 deg. F.; it will be found that the maximum
possible efficiencies obtainable with different fuels
rapidly diverges. Therein lies a big advantage to a
fuel whose cost on a Btu. content basis is as high as
gas but whose usefulness increases the higher the
operating temperature. Considering the advantages
of recuperation in increasing flame temperatures and
efficiency the opportunities that are still open for the
further extension of gas as the ideal fuel can be realized.
It has been noticed that the value of the fuels
diverges as the temperature rises. The theoretical
thermal efficiency of gas decreases slowly enough to
offset the initial price advantage of coal or oil, particularly
if in the future better equipment becomes
available and heat saving devices arc adopted. The
applications of electricity, too. are being extended very
rapidly; and if the cost of electricity- decreases sufficiently,
a point may be reached where the comparative
cost even on a Btu. basis will not be unfavorable
to electric heat. As a matter of fact for the highest
temperatures such a comparison even at present prices
may not be entirely unfavorable. However, here lies
the opportunity of the progressive gas man to firmly
establish gas as the industrial fuel, now—to induce
manufacturers to heat by gas, not by selling them the
cheapest equipment, but by selling the best and most
efficient.
Gas, as the agent for all industrial heating, is wait-
;ng ready to be eageny used as fast as trained men ire
available to solve the problems that still are hindering
development. Every manufacturer wants for his
shops modern, efficient methods, speeding up his production
and relieving his workmen of as much drudgery
as possible. For this no greater or better medium
than gas is available.
Cost Curves and Their Use.
Determining what the cost of a particular operation
would be, using some other fuel and at some other
efficiency value is only a matter of elementary arithmetic,
but for greater convenience Chart I has been
plotted. This comparative fuel cost chart gives the
cost of 100,000 effective Btu. derived from coal, oil.
gas, or electricity for any efficiency value. Coal of
12,500 Btu. per lb.; oil of 20,000 per lb.; gas of 550
Btu. per cu. ft. and electricity with 3,412 Btu. per
kwh., have been taken as a basis for these curves.
The several curves for each fuel are designed to cover
a wide range in price; coal from $5 to $20 per ton of
2'°2o^S'; dl fr°m 5c t0 20c Per &all°n; gas from 50c
to $2.00 per M. cu. ft.; electricity from lc to 10c per
kwh. If the particular fuel costs in question fall between
the plotted values it is an easy matter to interpolate,
the curves being in direct arithmetical ratio to
one another.