Page 286 - Mechanical Engineers' Handbook (Volume 4)

17 Furnace Economics 275

down for furnace maintenance, by which time some 50% of insulation may have been lost.
By more frequent repair, for example, after 10% loss of insulation, the added cost of lost
furnace time, material, and labor may be more than offset by fuel savings, even though total
furnace capacity may be increased to offset idle time.

17 FURNACE ECONOMICS
The furnace engineer may be called on to make decisions, or submit recommendations for
the design of new furnace equipment or the improvement of existing furnaces. New furnaces
may be required for new plant capacity or addition to existing capacity, in which case the
return on investment will not determine the decision to proceed. Projected furnace efficiency
will, however, influence the choice of design.
If new furnace equipment is being considered to replace obsolete facilities, or if the
improvement of existing furnaces is being considered to save fuel or power, or to reduce
maintenance costs, return on investment will be the determining factor. Estimating that return
will require evaluation of these factors:
Projected service life of equipment to be improved
Future costs of fuel, power, labor for maintenance, or operating supervision and repairs,
for the period assumed
Cost of production lost during operating interruptions for furnace improvement or strikes
by construction trades
Cost of money during the improvement program and interest available from alternative
investments
Cost of retraining operating personnel to take full advantage of furnace improvements

17.1 Operating Schedule
For a planned annual capacity, furnace size will depend on the planned hours per year of
operation, and fuel demand will increase with the ratio of idle time to operating time, par-
ticularly in furnaces with water-cooled load supports. If furnace operation will require only
a two- or three-man crew, and if furnace operation need not be coordinated with other
manufacturing functions, operating costs may be reduced by operating a smaller furnace two
or three turns per day, with the cost of overtime labor offset by fuel savings.
On the other hand, where furnace treatment is an integral part of a continuous manu-
facturing process, the provision of standby furnace capacity to avoid plant shutdown for
furnace maintenance or repairs may be indicated.
If furnace efficiency deteriorates rapidly between repairs, as with loss of insulation from
water-cooled load supports, the provision of enough standby capacity to allow more frequent
repairs may reduce overall costs.

17.2 Investment in Fuel-Saving Improvements
At present and projected future costs of gas and oil fuels, the added cost of building more
efficient furnaces or modifying existing furnaces to improve efficiency can usually be jus-
tified. Possible improvements include better insulation of the furnace structure, modified
firing arrangements to reduce flue gas temperatures or provide better control of fuel/air ratios,
programmed temperature control to anticipate load changes, more durable insulation of