Furnaces

Jun 3 • Notes • 4784 Views • 3 Comments on Furnaces

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.

 

Question answers:

 

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 ?

Ans:  40°C.

 

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

GATE Syllabus-

1. Gate Syllabus for Production and Industrial Engineering 2014

2. Gate syllabus Mining Engineering 2014

3. Gate Syllabus for Engineering Science 2014

4. Gate Syllabus for Metallurgical Engineering 2014

IES Syllabus-

1. IES Syllabus for General Ability

 

Classification of Different FurnacesFurnacesHeat Transfer in Furnacematerial furnacesOil Fired Furnace

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