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HEARTH FURNACES USED IN MANUFACTURING

The heating may be by gas, oil or electricity. The electrically - heated hearth furnaces are usually similar in general appearance to fuel-fired furnaces. The electric resistance heating elements, which will convey heat directly to the charge by radiation, are supported in the furnace wall by the refractory lining. In low temperature furnaces, the heat is transmitted to the charge by the enforced circulation of electrically heated air. 

Hearth furnaces for oil or gas fuels are built with the heating chamber and combustion space arranged in the following ways.

STATIONARY FURNACES :

This consists of a rectangular structure of steel lined with fire bricks. One end of  this structure is provided with a movable door which is also lined with fire bricks. This construction is known as a box - type or batch type furnace. This may be classified as follows :

Direct  fuel-fired furnace : 

This is very popular in all temperature ranges. The metal is heated in direct contact with the products of  combustion from the fuel. This type of construction is more adaptable for annealing steel castings and for heating steel which is to be forged.

In-Direct Fuel-Fired Furnace : 

This is extensively used in heat - treating practices up to  a temperature of about 1100*C. It has a heating chamber which separates the combustion space from the work piece. Reduced scaling and contamination from the fuels are the advantages in its use.

Muffle Furnace : 

The muffle, made from suitable refractory , is a permanent part of  the surface and contains all of the work space. In this furnace, the hot gases surround the heating chamber but do not enter it. This is used principally in tool rooms for the heat - treatment of dies and special tools to avoid scaling, decarburization, and other compositional changes. 

Recirculation Furnace : 

This is used for tempering, toughening, and stress-relieving, in the temperature ranges below 723*C. In this furnace the fuel, gas or oil, is burned in a chamber and products of combustion are circulated through the work space by a suitably arranged centrifugal fan and baffling.  


Stationary Furnace



MOVABLE - HEARTH FURNACES :


There are two principal types of  movable hearth furnaces : 

(1) The car bottom, and 

(2) The rotary type .

These furnaces are strictly specialized and work on a mass production scale. 

Car bottom Furnace : 

In the car bottom furnace, excluding the bottom, the furnace is similar in shape to a fixed - hearth furnace. The charging and discharging of a large heat-treating furnace is done most conveniently by mounting the hearth on a car which can be moved from the furnace for loading and unloading.

Rotary-Hearth Furnace : 

The rotary - hearth furnace is used for hardening, tempering, and carburizing process. The furnace structure is a refractory-lined shell that encloses a rotating hearth. The materials are charged as individual pieces, or as small pieces in trays, through the door. They do not require much floor space, nor do they require boxes made of non-scaling steels.  


Rotary-Hearth Furnace




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