Walking beams

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. 

Fig. 4.42 Hydraulic servomechanism with walking beam feedback linkage.

Operation of walking beam causing a bending action on wires at clamp and resulting in fatigue and cracking of wires, frequently before rope goes down into hole. 

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. ... 

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.)... 

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. 

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... 

Losses Through Open Doors, Cracks, Slots, and Dropouts, plus Gap Losses from Walking Hearth, Walking Beam, Rotary, and Car-Hearth Furnaces (see also sec. 4.6.9)... 

Refractory Heat Loss Sample Problem 8.1—Required Fuel Inputs. An added aspect of sample problem 8.1 (the same continuous walking beam steel reheat furnace) calculate the required gross heat input to each zone. (See worksheet tables 8.14 to 8.17.). 

Change 4. Increasing the heat transfer area of the steel to reduce heating time will reduce decarburization. A full walking beam furnace where the piece spacing can be increased to 2 1 or 2.5 1 provides for maximum heat transfer area on the billets therefore, the resulting minimized heating time can result in minimized decarburization. 

In furnaces with bottom zones, such as pusher or walking beam steel reheat furnaces, each skid rail, on which the loads rest or slide, consists of a schedule 160 pipe, 6.625" (0.1683 m) OD with 0.718" (18.24 mm) wall thickness, through which cooling water is circulated. A solid skid wear bar is securely welded onto the top surface of the pipe. The skid wear bars are often small diameter bars of heat-resisting, wear-resisting material. Their small diameter allows less contact area with the load pieces, thereby minimizing heat loss from the loads. 

Top-quality welding is crucial for all water-cooling-system parts. A weld without full penetration is a crack, a failure. All welds must be sound tested. The welding of skids is critical and should have full penetration welds to succeed. A very successful way to reduee expansion problems is to have the skids be short bar pieces with bevels on the ends and about in. (3.2 mm) spaces endwise between them. To reduce heat transfer to the skids, it is advisable to use a high-temperature, low-conductivity (such as cobalt) wear bar on the skids in walking beam stmctures. 

Structural quality shapes and plate (ASTM 36) usually provide satisfactory service for external furnace supports, shells, and external conveyor and walking beam components (see figure 9.13.)... 

Heavy wall water-cooled and insulated carbon steel pipe (ASTM 53) is used for rails, walking beams, and their supports. Effects of thermal expansion must be considered. 

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