Page 285 - Mechanical Engineers' Handbook (Volume 4)

274 Furnaces

depressed at the entry end, have been estimated on a conservative basis as a function of
thickness heated from one side and final load temperature differential and are shown in Fig.
26. The ratios for heating time required for square steel billets, in various loading patterns,
are shown in Fig. 25. For other rectangular cross sections and loading patterns, heating times
can be calculated by the Newman method.
Examples of heating times required to reach final load temperatures of T 2300 F and
s
T 2350 F, with constant furnace wall temperatures, are
c
1. 12-in.-thick carbon steel slab on refractory hearth with open firing: 9 hr at 54.4 lb/
2
hr ft .
2
2. 4-in.-thick slab, same conditions as 1: 1.5 hr at 109 lb/hr ft .
3. 4 in. square carbon steel billets loaded at 8 in. centers on a refractory hearth: 0.79
2
hr at 103 lb/hr ft .
4. 4 in. square billets loaded as in 3, but heated to T 1650 F and T 1600 F for
s
c
2
normalizing: 0.875 hr at 93 lb/hr ft .
5. Thin steel strip, heated from both sides to 1350 F by radiant tubes with a wall
2
temperature of 1700 F, total heating rate for both sides: 70.4 lb/hr ft .
6. Long aluminum billets, 6 in. diameter, are to be heated to 1050 F. Billets will be
loaded in multiple layers separated by spacer bars, with wind flow parallel to their
length. With billets in lateral contact and with wind at a mean temperature of 1500 F,
estimated heating time is 0.55 hr.
7. Small aluminum castings are to be heated to 1000 F on a conveyor belt, by jet
impingement of heated air. Assuming that the load will have thick and thin sections,
wind temperature will be limited to 1100 F to avoid overheating thinner sections.
With suitable nozzle spacing and wind velocity, the convection heat-transfer coeffi-
2
2
cient can be H 15 Btu/hr ft and the heating rate 27 lb/hr ft .
c
16 SELECTING NUMBER OF FURNACE MODULES
For a given heating capacity and with no limits on furnace size, one large furnace will cost
less to build and operate than a number of smaller units with the same total hearth area.
However, furnace economy may be better with multiple units. For example, where reheating
furnaces are an integral part of a continuous hot strip mill, the time required for furnace
repairs can reduce mill capacity unless normal heating loads can be handled with one of
several furnaces down for repairs. For contemporary hot strip mills, the minimum number
of furnaces is usually three, with any two capable of supplying normal mill demand.
Rolling mills designed for operation 24 hr per day may be supplied by batch-type
furnaces. For example, soaking-pit-type furnaces are used to heat steel ingots for rolling into
slabs. The mill rolling rate is 10 slabs/hr. Heating time for ingots with residual heat from
casting averages 4 hr, and the time allowed for reloading an empty pit is 2 hr, requiring an
average turnover time of 6 hr. The required number of ingots in pits and spaces for loading
is accordingly 60, requiring six holes loaded 10 ingots per hole.
If ingots are poured after a continuous steelmaking process, such as open hearth furnaces
or oxygen retorts, and are rolled on a schedule of 18 turns per week, it may be economical
at present fuel costs to provide pit capacity for hot storage of ingots cast over weekends,
rather than reheating them from cold during the following week.
With over- and underfired slab reheating furnaces, with slabs carried on insulated, water-
cooled supports, normal practice has been to repair pipe insulation during the annual shut-