In this article we will discuss about:- 1. Types of Resistance Ovens 2. Uses of Resistance Ovens 3. Temperature Control.
Types of Resistance Ovens:
1. Resistance Furnace:
This type of oven is used for cooling a heated job slowly for removing its brittleness is known as annealing. In this usual process of annealing the job is left covered with a scale of oxide which must be removed in order to have bright finish.
2. Air Circulation Furnaces:
The charge may not get uniformly heated in an ordinary furnace in which heat is transferred to the charge by radiation. In such furnaces air is passed over the heating elements and this heated air is circulated over the charge uniformly to all parts of the charge by conduction.
3. Salt-Bath Heating:
Salt bath furnaces are used for the purposes of tempering and hardening of steel tools. This type of furnace consists of a bath of some salt such as molten sodium chloride and two electrodes immersed in it. When the current between electrodes immersed in the salt is passed heat is developed and the temperature of the salt bath between 1000°C to 1500°C is used. The transfer of heat in this case is by conduction.
Uses of Resistance Ovens:
These are used for the followings:
1. Drying and baking pottery
2. Commercial and domestic cooking.
3. Heat treatment of metals like annealing, hardening etc.
4. Stoving of enamelled ware.
Temperature Control of Resistance Oven:
Temperature control is necessary in resistance ovens or furnaces. Temperature may be kept constant or varied according to requirements. Control may be manual or automatic.
Temperature can also be controlled by various combinations of groups of resistances used in oven or furnace in the following ways:
1. Use of Variable Number of Elements:
In this method, the number of heating element in working are changed, so total power input is changed.
2. Change of Connections:
In this method the elements are arranged to be connected either in series or in parallel or combination of both in star or in delta by means of switches at different instants according to the requirements.
3. Transformer Tappings:
The voltage across the oven can be controlled by changing the transformer tappings. This can be done by stepping down the supply voltage to the ovens. This is the economical method for voltage control. Moreover in this method automatic temperature control is done by thermostatic control.
4. External Series Resistance:
The voltage across the oven can be controlled by varying external resistance connected in series with the circuit. Its use is limited to small furnaces due continuously wastage of power in the controlling resistance.
Protective device such as overload relay is provided and set to trip the circuit at 10 or 15% above normal current against damage. In addition to above, fuses are also provided either in the main circuit of oven or in the hold on coil of the energizing contactor to provide necessary protection due to failure of automatic control system.