Furnace is an equipment or the instrument to melt metals for casting or heat materials for change of shape (rolling, forging etc) or change of properties (heat treatment).
Classification of Different Furnaces:
Based on the method of the generation of the heat, furnaces are broadly classified into 2 types namely combustion type and the electric type. In the case of combustion type of furnace, depending upon what kind of the combustion, it may be broadly classified as the oil fired, the coal fired or the gas fired.
• Based on mode of the charging of the material furnaces may be classified as
(i) Intermittent or the Batch type furnace or the Periodical furnace and
(ii) Continuous furnace.
• Based on mode of the waste heat recovery as the recuperative and the regenerative furnaces.
• Another type of the furnace classification is made based onthe mode of the heat transfer, mode of the charging and mode of the heat recovery .
Characteristics of the Efficient Furnace:
Furnace should be designed so that in between given time, as much of the material as possible can be
heated to an uniform temperature as possible with the least possible fuel & labour. To achieve the end, the following parameters can be considered.
• Determination of quantity of the heat to be imparted tomaterial or the charge.
• Liberation of the sufficient heat within furnace to heat stock and overcome all heat losses.
• Transfer of an available part of heat from the furnace gases to the surface of the heating stock.
• Equalisation of the temperature within the stock.
• Reduction of heat losses from the furnace to the minimum possible extent.
Oil Fired Furnace:
Furnace oil is the major fuel used in the oil fired furnaces especially for the reheating and heat treatment of the materials. The LDO is used in the furnaces where presence of sulphur is an undesirable. Key to efficient furnace operation lies in complete combustion of fuel with the minimum excess air. The Furnaces operate with efficiencies as low as 7% as against upto 90% achievable in other combustion equipment such as the boiler. This is because of high temperature at which furnaces have to operate to meet required demand example, a furnace heating the stock to 1200°C will have its exhaust gases leaving atleast at 1200°C resulting in a huge heat loss through the stack. However, improvements in efficiencies have been brought about by methods such as preheating of stock, preheating of combustion air and other waste heat recovery systems.
Q1: Explain about Heat Transfer in Furnaces?
Ans: The main ways in which heat is transferred to the steel in a reheating furnace are shown in the simple terms that is heat is transferred to the stock by:
• Radiation from flame,the hot combustion products and furnace walls and the roof.
• The Convection due to movement of hot gases over the stock surface
At the high temperatures employed in the reheating furnaces,dominant mode of the heat transfer is the wall radiation. Heat transfer by gas radiation is dependent on the gas composition (mainly the carbon dioxide and water vapour concentrations), the temperature and the geometry of the furnace.
Q2: What are the losses in Furnace?
Ans: Furnace losses include:
• Heat storage in the furnace structure
• Losses from the furnace outside walls or the structure
• Heat transported out of the furnace by load conveyors, the fixtures, the trays, etc.
• Radiation losses from the openings, hot exposed parts, etc.
• Heat carried by cold air infiltration into furnace
• Heat carried by excess air used in burners.
Q3: What is range of thermal efficiency in Batch forge?
Ans: 5–10 %
Q4: What is the value of Ambient temperature ?
Q5: What are the energy efficiency measures for an industry with furnace,give 2 examples ?
Ans : energy efficiency measures for an industry with furnace are :
1) Complete combustion with minimum excess air
2) Correct heat distribution