Controlling the Furnace's Natural Atmosphere (Combustion Control)
Since most metals oxidize in a natural combustion atmosphere, the question arises: Why bother with combustion control? There are several possible answers. If the fuel-to-air ratio is close to the theoretical value, the burnout per unit weight of metal heated will be minimized.
The properties of the oxide scale vary with the fuel-to-air ratio. However, it should be emphasized that the thickness of the oxide scale, or the weight of the scale produced per unit area of the charge, depends largely on the furnace temperature and exposure time, and to a lesser extent on the composition of the combustion products. The idea that simply increasing the fuel quantity, creating a fuel surplus, regardless of furnace temperature or heating time, will significantly reduce oxidation is incorrect.
The oldest method of controlling the furnace's natural atmosphere is to manually adjust the air volume according to the fuel flow rate by visual observation (perhaps occasionally supplemented by exhaust gas analysis) to achieve the desired atmosphere. For certain fuels and certain types of burners, the operator can achieve a fairly close match to the desired setting. However, for fuels with high hydrogen content and premixed burners, accurate visual assessment is impossible unless careful observation of the thickness of the iron oxide scale on the workpiece after it leaves the furnace.
In industrial furnaces, automatic furnace pressure control is a prerequisite for automatic furnace atmosphere control. This aspect has been discussed at the beginning of this chapter.
Automatic combustion control (a synonym for natural furnace atmosphere control) can be achieved using three methods: ① Interlocking the valves regulating the fuel and air flow rates; ② Using the flow rate of one fuel/air mixture to control the other; and ③ Controlling exhaust gas composition (flue gas analysis).
Accurate combustion control is easiest to achieve with clean gas fuels. Using dirty coal gas and liquid fuels makes it more difficult to maintain accurate ratios. Using solid fuels offers the lowest accuracy. In any case, however, combustion control does not require extreme precision, as an excess or a fraction of an percent of air has minimal impact on fuel conservation and charge oxidation.
The purpose of combustion control is to maintain a constant fuel-to-air ratio under any heat load. Aside from the complete combustion of the fuel within the burner bricks, the furnace atmosphere constantly changes along the path of the flame.
- Interlock Valve Control: Interlock valves are shown in Figures 120 and 121. These valves are commercially available, and if certain measures are taken during use, a reasonable ratio can be maintained. Some variation in the ratio is inevitable, as even slight changes in the orifice shape alter the flow coefficient. The necessary measures are: ① Keep the fuel pressure upstream of the control valve constant.
