Soak zones

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

At a continuous roller hearth type annealing furnace 24 pairs of regenerative radiant tube burners were applied to the heating and soaking zone shown in Figure 24.22. The material to be heat treated is cast iron joints and design net capacity of the furnace is... 

Cooling zone — 4— Heating zone, soaking zone ... 

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. 

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). 

In average practice, the aforementioned rigid specifications do not apply. In conformity with varying requirements, the length of the soaking zone ranges between one-fifth and one-third of the furnace length. 

Rotary Hearth Furnaces Rotary hearth furnaces have no water-cooled skid pipes, so the soak zone can be less than one-fifth of the total furnace length. Very rapid heating results in a short heating zone, but requires a long soak zone for thick material. Rotary hearth furnaces have problems, such as ... 

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. 

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. 

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. 

Disadvantages of walking beams relative to pushers are that walking beams have nearly twice as much skid-mark area and heat loss to water as pusher furnaces because of the walkers of the walking beams. However, these can be eliminated by a short soak zone at the discharge end of the furnace. (See reference 3.)... 

Soak Zone and Discharge (Dropout) Losses (see also sec. 4.6.2., add this heat requirement to the available heat required in 2.1)... 

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. 

Hg. 5.10. Continuous steel pusher reheat furnace side fired with regenerative burners in the top and bottom heat and preheat zones, and root tired in the soak zone. Preheat zones often have been designed as unfired preheat zones, which are good for fuel economy. However, also firing the preheat zones with regenerative burners would add capacity while retaining high fuel efficiency. (For a discussion of controls for this furnace, see sec. 6.11.1.)... 

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. 

If the heating zones from the charge door to the soak zone were shorter and more numerous, for example, seven instead of three top and bottom zones (and if firing were added in the charge zone), the furnace program would enter the correct action at the second or third piece extracted. Instability of the firing rates would be avoided, fuel rates reduced, and product quality improved. 

T/S5 T/S6 Soak zone Load temp (Same as 1 and 2 above)... 

Down-firing screen burners In 3 zones across the soak zone... 

Ten min before the delay, reset soak zone setpoints to 2250 F (1204 C) ... 

Ten min before the mill is to resume production, raise soak zone setpoints to normal ... 

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