Direct-Fired Furnace Equipment,Direct-Fired Furnace Equipment,Direct-Fired Furnace Equipment
With direct-fired furnace equipment, work being processed is directly exposed to the products of combustion, normally
referred to as flue products. To minimize the scaling (oxide) effect on the work, the flue products can be controlled or
varied by adjusting the fuel-air ratio of the combustion system. Although fuel-air adjustments can be made manually,
more precise control can be achieved automatically by a wide variety of fuel-air ratio control systems on the market
today. When direct-fired burner equipment is used in a heat-treating furnace, the parts being processed often are in some
primary or intermediate stage of manufacture. The oxide formed is not detrimental to the part because it will be removed
later in the manufacturing process. For example, forged parts are sometimes hardened in the as-forged condition, and
rough castings are annealed prior to machining.
Gas-Fired Equipment. Gaseous fuel used in heat-treating furnaces can be natural gas, straight propane, a propane-air
mix, or a relatively low-energy manufactured gas.
With the proper selection of burners, controls, orifices, and pipe sizes, a combustion system can be designed to operate on
2500 Btu (2635 kJ) propane gas, 1000 Btu (1055 kJ) natural gas, or 160 Btu (170 kJ) producer gas. The number refers to
the energy contained in a cubic foot (0.028 m3) of the gas. Manual adjustments are required for conversion from one gas
to another.
Oil-Fired Equipment. Almost any grade of oil that can be satisfactorily atomized can be burned in direct-fired
equipment. Lower-viscosity oils such as diesel fuel and No. 2 fuel oil can be easily atomized with pressurized (roomtemperature)
air. These are probably the fuel oils most commonly used for heat treating. Even with easily atomized oils,
caution should be employed in using them on flame-supervised furnaces operating below 760 °C (1400 °F) with
interrupted pilots. At low oil flows and excess air conditions, nuisance shutdowns can occur from the flame supervision
devices. In certain instances, as dictated by the National Fire Prevention Association, "constant pilots" may be used to
eliminate the shutdowns. Insurance carriers must approve the use of constant pilots for the particular application,
however. Heavier grades of oil must be atomized by a method other than low-pressure air. Normally, high-pressure air
and steam are used.
Burners that can be fired by either gas or oil are available. In most instances, oil is used as the standby fuel to be used in
peak periods when natural gas supplies are curtailed. Oil is considered desirable by some in the forging industry because
it creates a "softer" scale on the billet, which is more easily removed in forging. High vanadium, potassium, and sulfur
contents in fuel oil burned in the direct-fired process can reduce the useful life of various furnace components, especially
the nickel-chrome heat-resisting alloys.
Advantages of Fuel-Fired Furnaces. The following advantages are common to fuel-fired furnaces:
· Lower energy cost
· Easy to adjust or alter connected input
· Recuperator heat-saving devices can be added easily, and controlled cooling can be initiated easily with
proper design of combustion systems
· Faster heat-up times because inexpensive control factors can be added to accommodate the difference in
fuel burned during heat-up
Disadvantages of Fuel-Fired Systems. The following disadvantages are common to fuel-fired furnaces:
· Requires extensive ventilation systems
· Potential explosion or fire hazard
· Requires more manpower for start-up and shutdown
· Adjustment more difficult to maintain, resulting in excessive fuel use
· Only certain materials or types of products can be run in direct-fired furnaces due to the effect of high

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