Page 228 - Mechanical Engineers' Handbook (Volume 4)

5 Fuels and Combustion 217

Because of these and economic factors, cast or rammed refractories are replacing pre-
fired shapes for lining many types of large, high-temperature furnaces. Walls can be retained
by spaced refractory shapes anchored to the furnace casing, permitting reduced thickness as
compared to brick construction. Furnace roofs can be suspended by hanger tile at closer
spacing, allowing unlimited widths.
Cast or rammed refractories, fired in place, will develop discontinuities during initial
shrinkage that can provide for expansion from subsequent heating, to eliminate the need for
expansion joints.
As an alternative to cast or rammed construction, insulating refractory linings can be
gunned in place by jets of compressed air and retained by spaced metal anchors, a construc-
tion increasingly popular for stacks and flues.
Thermal expansion of steel furnace casings and bindings must also be considered. Where
the furnace casing is constructed in sections, with overlapping expansion joints, individual
sections can be separately anchored to building floors or foundations. For gas-tight casings,
as required for controlled atmosphere heating, the steel structure can be anchored at one
point and left free to expand elsewhere. In a continuous galvanizing line, for example, the
atmosphere furnace and cooling zone can be anchored to the foundation near the casting
pot, and allowed to expand toward the charge end.

5 FUELS AND COMBUSTION
Heat is supplied to industrial furnaces by combustion of fuels or by electrical power. Fuels
now used are principally fuel oil and fuel gas. Because possible savings through improved
design and operation are much greater for these fuels than for electric heating or solid fuel
firing, they are given primary consideration in this section.
Heat supply and demand may be expressed in units of Btu or kcal or as gallons or
barrels of fuel oil, tons of coal or kWh of electric power. For the large quantities considered
for national or world energy loads, a preferred unit is the ‘‘quad,’’ one quadrillion or 10 15
Btu. Conversion factors are
15
1 quad 10 Btu
6
172 10 barrels of fuel oil
6
44.34 10 tons of coal
12
10 cubic feet of natural gas
11
2.93 10 kWh electric power
At 30% generating efficiency, the fuel required to produce 1 quad of electrical energy
is 3.33 quads. One quad fuel is accordingly equivalent to 0.879 10 11 kWh net power.
Fuel demand, in the United States during recent years, has been about 75 quads per
year from the following sources:
Coal 15 quads
Fuel oil
Domestic 18 quads
Imported 16 quads
Natural gas 23 quads
Other, including nuclear 3 quads
Hydroelectric power contributes about 1 quad net additional. Combustion of waste prod-
ucts has not been included, but will be an increasing fraction of the total in the future.