Department of Mechatronics Engineering

Materials and Manufacturing Processes

Assignment no. 02

Class BEMTS-IV-A

Submitted By Sadia Saeed

Roll No. 180842

Submission Date 15 – April – 2020

Submitted To Engr. Akhtar Khurshid

 Crucible Furnaces

Introduction:

Crucible furnaces are one of the oldest and simplest types of melting furnace unit used in
the foundry. The furnaces uses a refractory crucible which contains the metal charge. The
charge is heated via conduction of heat through the walls of the crucible. The heating fuel
is typically coke, oil, gas or electricity. Crucible melting is commonly used where small
batches of low melting point alloy are required. The capital outlay of these furnaces
makes them attractive to small non-ferrous foundries.
A crucible furnace is a simple and very old type of melting unit commonly used in
foundry. The crucible furnace typically uses a refectory crucible with contains a metal
charge. The actual crucible is a container that can withstand very high temperatures and
is therefore used to melt materials such as metals.
The charge is heated using conduction through the walls of the crucible, it’s usually
fueled by either coke, oil, gas or electricity.
Crucible furnace,metallurgical furnace consisting essentially of a pot
of refractory material that can be sealed. Crucibles of graphite or of high-grade fire clay
were formerly used in the steel industry, heated directly by fire; modern high-quality steel
is produced by refining in air-evacuated crucibles heated by induction. Metals such as
titanium, which must be protected from air while hot, are melted and annealed in
hermetically sealed crucibles.

What It’s Commonly Used For

It’s simple, any small batches of low melting point alloys. I would imagine that with the
soaring price of Gold, this would be a perfect material to use. The small furnace is perfect
for hobbyists, small businesses and vocational colleges and schools.

Construction:
The study carried out a design and construction of an oil-fired crucible furnace. The study
focused on ensuring a high efficiency in melting of aluminum, by effectively minimizing
heat losses, and maximizing heat generation. To achieve this, a composite refractory
material consisting of cement, asbestos, and clay in a ratio of 2:1:1 was used, and diesel
fuel was atomized at the rate of 6.31x10-5 m 3 /min, using an Air compressor of 50kPa,
volumetric air flow rate of 2 CFM, and power rating of 1hp. This generated a heat of 42.6
MJ at a working pressure of 0.4122 Mpa. 18.02% of the heat generated was lost due its
interaction with the environment. Changes in the furnace geometry were negligible
indicating a long service life potential. With a useful heat input of 34.92 MJ, the furnace
is able to melt 56 kg of aluminum at a pouring temperature of 720 0 C, leaving its
efficiency at 81.98%. The design is considered safe since the working pressure does not

exceed the working stress of its casing which is made of mild steel.
A 3D model of the crucible furnace sectioned right plane of the crucible furnace
 An assembly model of the crucible furnace

Components:
The crucible furnace is made up nine main components
 Crucible casing
 Crucible refractory layer
 Crucible pot
 Fuel atomizer
 Fuel delivery hose
 Air delivery hose
 Fuel tank
 Electric motor
 Air compressor

Component Description:
Crucible Casing:
This is the outermost part of the crucible furnace. It is made up of a 3660mm by 1525mm
by 2.5mm BS 1449- S1.2 (1991) Hot rolled mild steel sheets. Attached to it is a top
cover which prevents heat loss by convection. It is also designed to have a 200mm vent at
the upper end for the escape of flue gases and also another 200mm opening at the
lower end for introducing the atomized fuel needed for combustion. This lower end
leads to the furnace combustion chamber.

A 3D model of the Crucible Furnace casing

Crucible Refractory Brick wall:
This separates the crucible pot from the crucible casing. It functions to retain heat and
prevents heat loss from the furnace to the casing via conduction. It also helps maintain
high furnace temperature which enables complete fuel combustion. The refractory wall is
a composite made up of Portland cement, asbestos and clay in a ratio of 2:1:1.

A 3D model of the crucible furnace refractory wall internal structure of the furnace refractory wall

Crucible Furnace Pot:

The crucible pot contains the Aluminum to be melted. It is made up of a copper alloy
material because of the high thermal conductivity of copper and its melting temperature
being higher than that of aluminum. A spout is incorporated to aid pouring of the molten
aluminum.

A 3D model of the Crucible pot

Fuel Atomizer:
The fuel atomizer directs the spray into the combustion chamber while breaking up
the diesel oil into very fine particles thereby enhancing a fine spray. A return-flow
atomizer was used to achieve the needed fine spray. It works by mixing the air and fuel
which are introduced separately into it. The air and fuel are mixed in the whirling
chamber
so that a uniform swirl of air and fuel converge at the orifice plate causing a very fine
spray to be achieved.

Internal structure a return-flow atomizer 3D model of a return-flow atomizer

Air and Fuel Delivery Hoses:

These functions to deliver air from the air compressor to the fuel atomizer and from the
fuel tank to the fuel atomizer. For the air and fuel delivery lines, a ¼’’ rubber hose was
used.
Fuel Tank:
This component is used to store the diesel fuel used in the combustion process for the
generation of the needed heat for melting the aluminum scraps. It is made up and
aluminum because aluminum is resistant to corrosion and favours long storage of diesel
fuel.
Electric Motor:
It serves as the prime mover of the air compressor. It converts the electrical energy which
is its source to mechanical energy used to drive the air compressor.
Air Compressor:
It utilizes the mechanical energy from the electric motor to suck in successive volumes of
atmospheric air, compresses the sucked-in air and then causes the compressed air to exit
at very high pressure. This high pressured air impacts on the diesel oil in the fuel
atomizer and with the aid of the return-flow atomizer, a fine spray is obtained.

Types:

There are many types of crucible furnaces in foundries which are classified according to
different principles. Some of the types are given below:
 Aluminum melting crucible furnaces
 Hydraulic tilting crucible furnaces 
  Metal melting crucible furnaces
 Gas fired crucible furnaces
 Electric crucible furnaces
 Brass melting furnaces
 Smelting furnaces
 Mining furnaces 
We commonly classify a furnace by the method used for removal of the metal from the
crucible.
 Tilting – you would mechanically tilt the crucible to the mould
 Lift Out – lifting it out then pouring it into the mould
 Bale Out/stationary pot – ladled the metal out to the mould

Tilting Type Furnace:

The tilting type furnace is raised above the ground level. It is mounted on two pedestals
and rotated by means of a rotating wheel. The furnace may be oil, coke or gas fired, and
forced draught is used. An ash-pan is also provided in case of coke fired furnace.

A drum of oil is placed at a height of 5-6 m above floor level, in case of oil fired furnace
crucibles with long pouring lips are used in tilting furnaces. The tilting is one with the
help of hydraulic devices or by hand rotating wheel. Tilting furnace is used for melting
large quantities of metal, up to 500 kg. 

Tilting-pot furnace

Lift-out crucible:
The lift-out furnace is made below the ground level, so that the crucible can easily be
lifted, operating from the floor of the shop. The metal to be melted is put into the
crucibles. The crucibles are placed in the heating chamber and sufficient coke being
packed around and above the crucible. A tall chimney attached, serves as a natural
draught. The chimney draught is controlled by means of a damper. A removable fire gate
is provided at the bottom of the furnace. After sufficient melting, the crucibles are lifted
by means of long handle tongs.

Lift-out crucible

Bale-Out Type (Stationary) Furnace:

The shape of the furnace is cylindrical and it is built above the ground level. The
cylindrical shape helps flames to sweep uniformly around the crucible. A fan is provided
for forced draught and waste gases are taken out from the opening at the top. Bale-out
type furnaces are generally oil or gases fired, and are used for melting nonferrous alloys
like aluminum, magnesium, zinc, lead etc. 

stationary pot, from which molten metal must be ladled

Working:
A crucible is a ceramic or metal container in which metals or other substances may be
melted or subjected to very high temperatures. While crucibles historically were usually
made from clay, they can be made from any material that withstands temperatures high
enough to melt or otherwise alter its contents.

For practical melting operation, the time required for melting of a complete crucible
charge is also highly important in addition to the values for energy consumption in
continuous melting. For example: the melting time in a gas-fired crucible with a volume
capacity of 350 kg, which was pre-heated and filled with approx. 20 % of molten metal
(liquid heel), is approx. 85 minutes. With an 800 kg crucible the melting time is 130
minutes, which is insignificantly more than for the small amount. If a cold crucible is
used, the time required for melting may increase by up to50 %. Electrically fired furnaces
roughly require double the melting time compared to gas-fired furnaces.
Average heat flux plot

Uses of crucible furnaces:

Crucible furnaces are one of the oldest and simplest types of melting unit used in the
foundry. The furnaces uses a refractory crucible which contains the metal charge. The
charge is heated via conduction of heat through the walls of the crucible. The heating fuel
is typically coke, oil, gas or electricity. Crucible melting is commonly used where small
batches of low melting point alloy are required. The capital outlay of these furnaces
makes them attractive to small non-ferrous foundries. Crucible furnaces are used for
melting and holding small batches of non-ferrous alloys. Crucible furnaces are the oldest
type of melting furnaces. A refractory crucible filled with the metal is heated through the
crucible wall.

Advantages and Disadvantages of crucible furnaces: