Catenary furnaces

Furnaces for vertical strip or strand (wire) do not have a conveyor, per se, because the strip or wire can be pulled over a series of rollers after it has been threaded through the furnace. A catenary furnace is a continuous horizontal furnace most often used for annealing stainless-steel strip. A long, thin load is supported by rollers at the entrance and exit, and therefore hangs in the shape of a catenary curve within the furnace. (See box on page 132 and fig. 4.10.)... 

Fig. 4.10. Catenary furnace for heat treating metal strip. Careful strip tension control is needed to prevent strip sag to prevent strip contact with the flame. Better control can be achieved with the exit supporting roll water cooled and just within the exit end of the furnace and with a T-sensor near that roll and under the strip.

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. 

Catenary furnaces are excellent candidates for fiber linings to reduce the refractory heat storage (flywheel) effects. With a lightweight lining, line stops are generally less of a problem. 

Refer to figure 4.10 of a catenary furnace. The inside length between hot refractory surfaces at left and at right is L, and the mean inside height between hot refractory faces at top and bottom is H. Use the mathematical formula for a catenary curve to... 

Flue gas temperatures of waste heat boilers are only 100 to 150 F lower than from regenerative systems thus, fuel savings may be marginal. Waste heat boilers have proved effective with stainless-steel annealing catenary furnaces. They have adjacent steam requirements all year for cleaning their product after annealing. Their firing rates, flue gas temperatures, and heat stored in refractory are moderate, so water problem shutdowns are fewer. 

Robert A. Shannon has more than 50 years experience with engineering work. He has been North American Mfg. Co. s authority on steel reheat furnaces, soaking pits, and forging furnaces. He continues private consulting relative to his extensive experience with steel reheat, pelletizing, forging, heat treating, catenary furnaces, and industrial boilers. 

A2. By using a rotary hearth, a roller hearth, overhead trolleys suspending the load pieces, a pusher mechanism, a walking mechanism, or by suspending continuous strip or strands between rollers external to the furnace (catenary). 

Fig. 4.11. Normal left) and recommended (right) temperature sensor locations for catenary strip. The hollow shaft through the center of the added roll should be water cooled because the furnace temperature may be 2300 F (1260 C).

Burners should be about 2.5 ft (0.87 m) apart, above and below the strip. The burners above the strip should be on one side of the furnace and those below the strip on the other side, enhancing circulation velocity. The burners should have a near-flat heat-release pattern (preferably adjustable), providing a temperature profile across the furnace that is practically level. It is important to check the design and the actual operation to make sure that no bottom-row-burner flames impinge on the lowest part of the strip s catenary loop. 

Heat transfer rate is a function of the gas blanket thickness, which should be 3 ft above and below the strip. For the strip hanging in the natural shape of a catenary curve with, for example, the low point of the strip 1.5 ft (0.5 m) below the top surface of the supporting rolls, the furnace bottom should be 4.5 ft (1.4 m) below the strip s highest level. 

A low shrinkage value signifies a high fiber tension. If the tension is too high, then the fiber will break, and if it is too low, then the increased fiber catenary will permit the fiber to drag along the furnace floors, degrading the fiber cosmetics. 

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