Walking hearth furnaces

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. 

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

Problem 1. Size a 3-zone, 2200 F top-fired-only walking hearth furnace... 

An effective and practical control is described next for a three-zone walking hearth furnace. The preheat zone should have a control T-sensor about 6 feet from the zone, with entry either through the roof or preferably high in the sidewall, in the exhaust gas flow. At that location, the T-sensor will be very sensitive to productivity and will prevent the waste gas temperature at low production from being hotter than it is during high production. 

Walking Hearth Furnace Control. The design of steel reheat furnaces has developed to such an extent that many early problems have been solved or at least remedied. However, the following are some difficulties that still cannot be estimated accurately enough to prevent concerns in final designs. 

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. 

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.)... 

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

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. 

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)... 

Slot losses in walking hearth and rotary furnaces due to infiltrated air and refractory condition... 

Preliminary Decisions "Walking hearth foiu-zone reheat furnace, with all zones longitudinally or side fired. Zone 1 (charge end) is to be unfired. Zones 2 and 3 are to be side fired, and zone 4 (soak) is to be fired longitudinally, using ambiet air in all burners. Fuel = natural gas. Hearth width should include 2 ft clearance on each end of 30 ft long billets = 34 ft. 

Furnace walking hearth Zones = A top, 0. bottom. Piece weight 2068 pounds... 

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. 

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). 

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