Walking furnaces

In pusher furnaces, the product (work load) is pushed through the furnace in steps by a hydrauhc or electromechanical mechanism that pushes each load into the furnace, thus pushing all work in the furnace ahead one work space. The walking-beam furnace lifts the work load on a walking beam, advances the load a step, and returns the work to the hearth. The walking beam then returns to its original position (under the hearth) in preparation for the next step. 

At temperatures above 1150°C, alloys used for the hearth or material handling systems in low and medium temperature furnaces lose strength rapidly (2) and temperatures are reached where ceramic refractories are required to support the work. This results in less use of roUer-hearth and belt-type hearths and greater use of pushers or walking-beam designs for continuous furnaces. 

Beryllium is a toxic element, and the reduction operation is therefore carried out in a well-ventilated special double enclosure. The furnace controls are located outside the enclosure. The ball milling of the reduced mass is carried out in walk in-type fume hoods. 

April 30, 2000— A maintenance man just happens to walk past the Cyclone Ash Bin Enclosure of the Deactivation Furnace (DFS) and notices smoke, heat and a bulged out door. There is a fire going on that no one had detected. The fire ignites and decomposes the charcoal in the filter system of the Ash Bin and would have entered the filter banks of the MDB if it hadn t been discovered by a worker out for a walk. The fire was precipitated when the blind flange was installed in preparation for an entry into the DFS to clear a jam in the Heated Discharge Conveyor. 

Fig. 9.23 Longitudinal section through a walking beam sintering furnace [B.25].

Fig. 9.24 Sketch showing a cross section through the hot zone of a walking beam furnace (left vertical heating element right horizontal heating element) [B.25].

Cameras observe slabs or billets as they are set on "walking beams" to be moved through reheat furnaces. They also monitor the center of the furnaces for proper distance between slabs and to be sure slabs are held properly before they are lifted from the furnace and placed on the rolling or forge line. In these applications, a lens with a forward-oblique 45° direction of view often furnishes a useful perspective as it views the material within the furnace (see Figure 16.9). 

Typical furnace camera coverage in a walking beam furnace... 

Typical furnace camera coverage of walking beam furnace in steel industry. ("High-Temperature Furnace Camera Systems/ Lenox Application Solutions in the Steel Industry (reference sheets with diagrams for reheat furnaces, vacuum degas-sers, remelt/reverberatory furnaces).)... 

Furnace type Box shape, walking-beam type... 

In continuous furnaces, cast or wrought heat-resisting alloys are used for skids, hearth plates, walking beam smictures, roller, and chain conveyors. In most furnaces, the loads to be heated rest on the hearth, on piers to space them above the hearth, or on skids or a conveyor to enable movement through the furnace. To protect the foundation and to prevent softening of the hearth, open spaces are frequently provided under the hearth for air circulation—a ventilated hearth. ... 

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). 

Unlike most other conveyorized furnaces, walking beam furnaces accommodate top- and bottom-zone-firing. When used at lower temperatures (e.g., for annealing light sections such as pipe), the beam and supports may be of high-grade alloy without water cooling. 

The so-called accordion effect upsets the supposedly steady pattern of temperature progression as load pieces move through the zones of multizone reheat furnaces, whether rotary, pusher, walking beam, or walking hearth. (See chap. 6.)... 

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.)... 

Walking hearth reheat furnaces. These furnaces are mostly used for... 

Example 4.1 Determine the size needed for a three-zone 1200 C, top-fired-only walking hearth furnace with half the furnace using enhanced heating for 100 tph of 127 mm x 127 mm x 6.71 m (5" x 5" x 22 ) steel billets. 

With these two benefits, the effective use of the four long sides of the product for heat transfer can reach between 85 and 90% of two-side heating in a full walking beam furnace without the water losses and maintenance of the water-cooled support structure. Therefore, the need for two-side heating with a full walking beam furnace can be avoided, except for slab heating where spaces between product are not available. 

Higher furnace capacity is necessary to keep pace with other mill improvements. Recommendations 1 to 8 below suggest ways to match the furnace capacity to the production line equipment in series with it. Furnace types such as rotary hearth, walking beam, walking hearth, pushers, and some other high-temperature continuous furnaces can benefit from one or more of these recommendations. 

Slot Heat Losses from Rotary and Walking Hearth Furnaces (add this heat requirement to the available heat required in 2.1)... 

Example 4.6.9 Find the heat loss from the slots of a 20 ft long (6.1 m) furnace zone that has two walking beams with 1" (25 mm) wide slots on either side of each... 

---