Controlling Furnace Pressure
In metal heating furnaces, uncontrolled free air ingress must be strictly prevented (the reason for this will be discussed below). Therefore, a slightly positive pressure must be maintained within the furnace.
This applies to furnaces with combustion products flowing through them and those filled with protective gas. The amount by which the pressure at different heights within the furnace (measured from the furnace bottom) exceeds atmospheric pressure varies; this is demonstrated in the chapter "Gas Movement in the Furnace" in the previous volume.
Therefore, the furnace pressure at the furnace bottom is generally used as the reference. A pressure of 0.6 mm H2O at this point is sufficient. In end-discharge, three-zone (point) heating slab heating furnaces, due to the effect of the hot gas column at the discharge end, a higher pressure (approximately 1.3 mm H2O) is required to maintain the heat in the equalizing zone.
While a pressure as low as 0.6 mm H2O is difficult to read on a water-filled U-tube manometer, such a low pressure is sufficient to generate a velocity of 3 to 6 m/s (depending on temperature) in the furnace gases. If there's a small hole in the furnace door, furnace gases will blast out, forming a "stream," the length of which indicates the furnace pressure. If the stream is transparent and barely visible, dust can be dropped onto it or smoke can be blown into it to reveal it.
Furnace pressure control
If the stream is not the right length, a damper can be used to adjust the flue outlet. Experienced furnace operators observe the relationship between the size of the stream and the amount of scale formed on the heated workpiece. These simple measurements were standard for all industrial furnaces at the beginning of this century and are still applicable today for furnaces without chimneys.
The situation is similar for furnaces filled with protective gas. The positive pressure in these furnaces is generally less than 0.6 mm of water column. If the protective gas is non-flammable, it will extinguish the lit wax at the point where it exits; if it is flammable, it will ignite the gas. If the finished product lacks a bright finish, it indicates that free air has entered.
Large furnaces, especially those with chimney exhaust, are equipped with pressure gauges. These instruments, installed in the furnace pressure automatic control system, require measuring the furnace pressure and atmospheric pressure at the same height.
bell-shaped instrument
Pressure differentials should be measured using instruments suitable for this purpose, such as a bell-shaped instrument (a bell-shaped instrument with two bellows at each end of a centrally supported lever, placed in the oil), a diaphragm-shaped instrument (with two bellows), or other similar instruments.
Using a diaphragm to measure pressure differentials is highly suitable, but the temperatures of the two media at the diaphragm must be the same. This condition is easily achieved when measuring furnace pressure, as the instrument does not need to be mounted on the furnace wall.
One design transmits the slight deflection of the diaphragm to a swiveling jet tube without frictional resistance. The operating principle is shown in Figure 114. Here, the spring adjusts the control pressure. The jet tube increases pressure on one side (or the other) of the power piston, thereby driving an actuator. In this case, the actuator is a flue gate.
