Continuous reheating furnace is widely used in the steel industry for treating slabs and billets so that these can then undergo further mechanical work. The temperature of the steel is between 900°C —1250°C. The temperature is raise so that it is soft enough to be processed mechanically into desired shapes. Hence, the reheating of the slabs prior to further processing is crucial in the industry. It is also quite challenging because of the unpredictability of the process and the lack of overall control.
This is why industrial furnace manufacturers have to ensure that the furnace meets stock heating rates. The energy loss has to be kept at a minimum as the steel moves in the furnace. This is why the both the outlet and inlet doors of a continuous reheating furnace are small. The small size limits air infiltration to minimum. There are different continuous reheating furnaces for different purposes.
Industrial furnaces require substantial investment. Before you buy a continuous reheating furnace, it is important to understand the different types and their pros and cons. Here is a comparative analysis:
Typically a pusher furnace is used to charge and discharge the steel stock, moving it on skids with water-cooled supports. The hearth in the pusher has a slope that tilts towards the discharge end. The burners are located either at the top/ bottom or at the discharge end. The top/bottom burner furnaces heat stock at the respective ends. These furnaces also have a chimney at the discharge end. The chimney has a recuperator for heat recovery. Top-fired furnaces the hearth is divided into distinct zones.
The pusher furnace is comparatively economical than moving hearth furnaces in the long run. The installation and maintenance costs are quite low. Top/bottom burner furnaces have a few added advantages. The stock is heated quickly with low temperature differentials in the stock. This reduces the time taken for processing. The furnace lengths can be kept comparatively short.
Energy loss from the water cooling base on the skids, the stock size is limited because of friction build-up, the effect of the the skid on the quality and extra effort required in emptying the furnace.
Walking beam furnace
It gets its name from the walking beams that arise from the bottom to ensure that the stock is raised. These beams move forward with the stock, lowering at the end to place it on the stationary ridges. The beams retreat to the front of the furnace after the stock is removed. Today industrial furnace manufacturers are building walking beam furnaces that can reach up to 1100°C.
It removes the disadvantages of the pusher furnace, such as complications for the skid and stock pile-up. It allows bottom heating of the stock, which gives us better control over the heating, ensuring better uniformity and flexibility. We can shorten the length and time taken in the furnace.
Water cooling again means high energy loss. While bottom heating ensures uniformity, the furnace may at times require side heating. This is a less efficient method of heating and does not have the same uniformity.
Walking hearth furnace
These stock here rests on refractory blocks that are fixed.The blocks arecome out through openings in the hearth. The name comes from the motion of the stock transport where it moves by ‘walking the hearth’, as the phrase goes.
It is a simple design that is easy to construct and empty at the end of the process. The energy loss and damage to the stock are minimal here. It can also handle different sizes of stock.
The furnace suffers from non-uniformity of heating.
Continuous re-circulating bogie furnace
It is shaped like a long and narrow tunnel. The stock is placed on a cart with wheels, known as a bogie. The bogie has a refractory hearth. The bogies move in a chain, like a train, covering the furnace length. At the discharge end the stock is removed. The bogies then retreat to the charge end.
This is the best design when one os dealing with compact stock with varied sizes.
In this design, the stock has to be heated, then cooled and reheated. This involves a loss of heat. The narrow and long shape of the furnace also creates operational difficulties. However, some modern industrial furnace manufacturers are now working on more efficient design.