Soak zone temperature

A control method variation uses the output signal from a temperature control in a downstream zone as process variable for energy input in the next upstream zone, for example, soak zone temperature controls main heating zone input and/or heat zone temperature controls preheat zone temperature. Note that zones may sometimes be a series of closely spaced, separate catenary furnaces. If a very low setpoint for the output signal of the soak and/or heat zones is used to control the upstream zone, the soak time will be extended to allow the chrome carbides to dissolve into the strip and thereby produce a quality product. 

Front-end-fired furnaces should have soak zones to allow equalization independently of the heating zones. Otherwise, (see fig. 4.18) the heating zones must be limited to maximum soak-zone temperatures when the heating zone temperature could be higher for maximum productivity. 

Thirty min before the mill is to start, raise the soak zones temperature setpoints to 2250 F (1232 C) ... 

Soaking zone, where the reacted charge is superheated or soaked at temperature or, if desired, cooled before discharge... 

Control the furnace bottom temperatures with many small, high-velocity burners firing with constant air to hold the same temperatures below the load as above it. Install fuel meters on each zone. When the fuel flows reach minimum in all zones, hold for several hours, then remove the load from the furnace for processing. The benefits will accrue from shorter cycles, many times by 25% because uniformity of zone temperatures is held from charge-to-draw requiring minimum soak time. 

Ideally, there should be no transfer of heat in soak zones, except the temperature equalization within the pieces. In fact, a slight loss of heat from the top speeds equalization. Temperature equalization between surface and interior is considered to be of less importance than elimination of dark spots. The soaking zone eliminates or reduces dark spots, but does not necessarily eliminate cold centers, which show up as greater thickness in the finished product (rejects). 

Many believe that for greatest uniformity of temperature in top- and bottom-fired continuous furnaces, it is desirable to favor almost constant temperature from furnace end to end plus a soak zone for the ultimate heat flow rate per unit of time. This is not true if reflecting scale forms in the charge or preheat zone at temperatures above 2320 F (1270 C). Such scale will reduce heat transfer so that the product will be colder and productivity will be lower than if the charge zone had been limited to between 2250 F and 2300 F (1232 C and 1260 C). Reflecting scale develops when scale softens and becomes very smooth and the steel temperature under the scale has relatively low conductivity, preventing the steel from absorbing heat from the scale. 

The soak zone should be divided into three zones across the furnace width to permit profiling of the temperature of the product. With small to medium sized bars in a straight ahead mill, the head ends should be approximately 50 F above the body temperature and the tail should be about 60 F above the body temperature. The reason for the higher temperatures for the head and tail is overfill and underfill of the roll passes when the head and tail of the billets are not being stretched between mill stands, which is a problem even with loopers between roll stands. 

The required available heat for the soak zone will be the sum of (a) the remaining heat needed into the loads to heat them to good quality (b) heat losses to and from refractory, hearth materials, openings, and water-cooled devices and (c) heat absorbed by infiltrated air in warming to zone temperature. 

Figure 4.22 (top and bottom drawings) shows soak zone side-sectional views with T-sensor and burner locations (original and recommended). The two middle drawings show temperature profiles at three soak zone firing rates, plus heat consumption rates for losses, for cold air infiltration, and for heating the loads. The sum of these is the heat flux, which corresponds to available heat. 

Longitudinal firing of steel reheat furnaces in top and bottom heat and soak zones, including sawtooth-roof rotary furnaces, is used to reduce the number of burners and to develop a uniform temperature across the hearth. Otherwise, most of these furnaces would be side fired to hold the heat transfer temperature higher and longer (many times for as long as 40 ft, perhaps 25 ft, for longitudinally fired zones). 

Roof firing can provide uniform temperature across a hearth, especially in soaking zones. An almost-standard practice for soaking zones has been to use roof burners in three zones across the width of the furnace. Attempts to cut costs with only two zones have given very poor results. 

Ten min before mill restart, raise the soak zones to their normal temperature setpoints ... 

Procedure—phase D2—Heat Zone. For this example, assume a radiation shield curtain wall between the soak and heat zones. The design steel rolling temperature is 2150 F (1177 C), so it is reasonable to plan for a heat zone temperature of 2350 F certainly no higher than 2400 F. 

With a heat zone longitudinally top fired, the burner wall temperature would be 100°F (56°C) above the product discharge temperature and 100°F below the peak temperature of the zone at high fire. With side firing, the heat zone curve raises the zone entering temperature quickly to a peak of 2340 F (1280 C). The heat zone temperature then falls with greater slope than the soak zone to 2180 F (1193 C) just before the preheat zone starts to rise to a maximum of 2180 F (1193 C). 

The second question asked what would be an excessive temperature for this process. It was recommended that process hot spots (i.e., zones higher than 100°C) should be avoided. This requirement was met by keeping the heating lines, the walls of the melting pot, and the spray head thermally jacketed to maintain the appropriate internal soak temperature. As a result, the model presented a potential for hot spots at the skin surfaces of the lines and equipment walls. This needed to be investigated for its decomposition potential, and in fact, after several batches were processed, the flexible heat-traced lines had to be discarded because of a buildup of a blacked residue on the inner tubing walls. The kinetic model predicted how many batches could be run before this necessary replacement maintenance was required. 

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