1.3) According to type of operation:
According to type of operation furnaces are divided in to two groups. These two groups are again subdivided in to different types.
A) Batch furnace
1. Box furnace
2. Muffle furnace
3. Pit furnace
4. Lift off cover furnace
5. Bogie hearth furnace
B) Continuous furnace.
1. Conveyor furnace
2. Rotary hearth furnace
3. Tunnel furnace.
4. Rotary furnace
A) Batch furnace:
The name batch furnace denotes that in these type the heat treatment process is carried out in different batches. This type of furnace is used for batch production. Loading and unloading of the work is done manually, and hence considerable amount of manual labor is involved. They are suitable for the heat treatment of varying sizes of jobs, and also different processes can be performed for different batches. They have flexibility in operation and they are ideal for plant based on job orders.
1) Box type furnace:
It is a simple box shaped furnace, which has a door at one end. The loading and unloading of the charge is through this same single door and it is done manually. For heavy and bulky work mechanical means may be used for loading and unloading. A detachable bottom can be fitted in order to reduce the difficulty in moving heavy job. But proper fitting and sealing has to be ensured prior to the heat treatment. Box type furnaces can be used over wide range of operation conditions. They can be heated up electrically or by fuel
2) Muffle furnace:
They are very common in use. The most important part of this furnace is a muffle made up of a special refractory material. Muffle is a hollow cuboid in which the charge is loaded. Any fuel or electricity can be used to heat these furnaces.
Electrically heated muffle furnaces are most popular and are extensively used for the heat treatment of small parts. Muffle is surrounded by a heating element such as Nichrome or Kanthal wire. Fuel fired muffle furnaces are of two types, namely direct and indirect. The indirect one has the advantages of
Ó Less scaling and reduced contamination,
Ó Possibility of controlling the atmosphere within the muffle,
Ó Better uniformity of temperature distribution.
3) Pit furnace:
Pit type batch furnaces are used for heavy and large parts. They are generally heated up electrically. It is essentially a cylindrical refractory chamber. Most of its parts situate bellow the ground level and hence referred as pit furnace. At top it has an opening and the loading and unloading of are done from top. Cranes or other lifting devices are used for loading and unloading.
4) Lift off cover furnace:
It’s also called as bell type furnace. It is a fixed base furnace and consists of two containers. The base is generally made of heat resisting alloy steel. The work pieces are kept on this base. Once job is loaded, the inner container is placed on the base. Then another bell shaped container called the outer container is placed over the inner one. Then the furnace is heated up electrically. Inner shell is made up of heat resisting alloy steel and the outer one is a metal encased refractory. They are widely used in wire industries for annealing coils of steel wire, strips and nonferrous metals under controlled atmosphere.
5) Bogie hearth furnace:
This type of furnace is considered as an improved box type furnace. The furnace is specifically suitable for the heat treatment of bulky and heavy components. This can be used for small components also. The furnace consists of a refractory topped run out bogie acting as a hearth, which carries the parts for heat treatment.
B) Continuous furnaces:
In continuous furnaces, the charge is introduced from one end of the furnace. Then it is forced to move to the other end from where it is unloaded. The complete process is adjusted in such a way that the unloaded component is heat treated to the desired specifications. Only one specific operation is performed in such furnaces. By employing continuous furnaces, manual labor is minimized to a great extend. Such furnaces are well suited for mass production units where parts are subjected to well established heat treatment cycles. These furnaces are recommended for continuous processing and for reproducible quality of heat-treated parts.
1) Rotary hearth furnace:
It’s a continuous type furnace, which has a rotating hearth. The hearth rotates along its vertical axis. Te parts to be treated are charged through an opening. After the operation the parts are discharged through the same opening or an adjacent opening. The speed of rotation I adjusted in such a way that the heat treatment cycle is completed by the time the hearth undergoes one full rotation.
2) Conveyor furnace:
This consists of an endless conveyor belt, which moves at a very slow pace. He components to be treated are place on this belt. The components on the belt enter the furnace from one end and come out from the other end. During the movement the heat-treating cycle is completed. The components are collected in a box or dropped into he quenching tank. Theses furnaces are generally used for hardening and tempering treatment.
3) Tunnel furnace:
Tunnel furnace is commonly used for annealing. The charge is loaded in a number of cars and the cars are pushed slowly in to the furnace. The cars are passed slowly through the furnace by mechanical means. Since the moving path is circular, finally the cars come to the starting point. The continuous movement allows the unloading of the treated parts and the loading of another charged car simultaneously.
4) Rotary furnace:
Rotary furnace is used for heat treatment of small parts, for processes like hardening, tempering, gas carburizing etc. it consists of a steel drum with a refractory lining. It rotates at a slow speed around its horizontal axis. Generally this axis is kept at an inclination.
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