FURNACE IN CHEMICAL INDUSTRY

A furnace is a device in which chemical energy of fuel or electrical energy is converted into heat which is then used to raise the temperature of material, called the burden or stock, placed with in the furnace. Furnace that is operating at temperature below 1200 oF (650 oC) are commonly called ‘oven’. In ceramic industry furnaces are called ‘kilns’. In the petrochemical and CPI (Chemical Process Industries) furnaces may be termed as ‘heaters’, ‘kilns’, ‘after burners’, ‘incinerators’ or ‘destructors’ .

 Classification of Furnaces
Furnaces are classified from different points of view in order to have an idea of representative types of various industrial furnaces which mainly comprise of three main portions:

1.      The fire place where combustion of fuel takes place.
2.      The working chamber or furnace proper where heat is transferred from products of combustion to the material under heating.
3.      The application for removal of flue gases.

Classifications of furnaces based on various factors are given below.

Based on the heat source:
(a)  Combustion / flame furnaces: These are furnaces where heat is developed due to combustion of fuels.
(b)  Thermo-electric furnaces: in these furnaces, heat is generated by the electricity.

Based on the type of fuel used:
(a)  Solid fuel fired furnaces
(b)  Liquid fuel fired furnaces
(c)  Gaseous fuel fired furnaces
(d)  Mixed / multi fuel fired furnaces

Based on method of heat transfer from fuel to charge under heating:
Furnaces in which the fuel is direct in contact with the material under heating.
(a)  Furnaces in which charge is heated by the products of combustion.
(b)  Furnaces in which material is heated by the way of heat transfer are through a solid wall.
(c)  Furnaces in which heat transfer takes place through a liquid medium surrounding the submerged material under heating.

Based on use of heat saving application:
(a)  Recuperative furnace: Here, flue gases and air circulates along alternate paths and the heat absorbed by the walls of the passages from the hot flue gas is taken away by cold water.
(b)  Regenerative furnaces: Here, flue gases are allowed to pass through alternate checker brickwork chambers after a definite interval of time. The cold air passing over the checker brick work takes away a portion of the heat accumulated by the checker brick and the latter again starts absorbing heat from the flue gases.

Based on charging system:
(a)  Manual charging furnace
(b)  Mechanical charging furnace 

Based on mode of operation:
(a)  Batch or continuous
(b)  Periodic furnaces
(c)  Continuous furnaces

Based on draught:
(a)  Natural / self draught furnace: operates with chimney or with open doors.
(b)  Forced draught furnaces: Here, forced draught fans are used both for supply of combustion air as well as removal of flue gas. This furnace normally operates at positive pressure.
(c)  Induced draught furnace: Here induced draft fan (I.D. fan) located at the bottom of the chimney sucks out the flue gas from the furnace such that it is at negative (vacuum) pressure.
(d)  Balanced draught furnace: Here, both forced draft fan for supply of combustion air and induced draught fan for sucking out flue gas are provided. They are so operated that the furnace pressure is almost atmospheric.

Based on shape of furnace:
(a)  Crucible furnace
(b)  Shaft furnace
(c)  Hearth furnace

Based on the way in which charge is handled:
(a)  Melting furnace
(b)  Roasting furnace
(c)  Reheating furnace
(d)  Pusher type furnace
(e)  Batch furnace

Based on the industries which use the furnaces:
(a)  Steel industry furnaces
(b)  Petroleum industry furnaces

8.2. Design and Operation:
                        A furnace or direct fired heater is equipment used to provide heat for a process or can serve as reactor which provides heats of reaction. Furnace designs vary as to its function, heating duty, type of fuel and method of introducing combustion air. However, all furnaces have some common features. Basically, fuel flows into the burner and is burnt with air provided from an air blower. There can be more than one burner in a particular furnace which can be arranged in cells which heat a particular set of tubes. Burners can also be floor mounted as in the picture above, wall mounted or roof mounted depending on design. The flames heat up the tubes, which in turn heat the fluid inside in the first part of the furnace known as the radiant section. In the chamber where combustion takes place, known as the firebox, the heat is transferred mainly by radiation to tubes around the fire in the chamber. The heating fluid passes through the tubes and is thus heated to the desired temperature. The gases from the combustion are known as flue gas. After the flue gas leaves the firebox, most furnace designs include a convection section where more heat is recovered before venting to the atmosphere through the stack.

Radiation Section:
                        It is the mode of heat transfer in which heat is transferred by the electromagnetic waves. Here heat is released by combustion of fuel into an open space and transferred by direct radiation from flame and by the radiation reflected back from refractory walls lining the chamber. The rate of heat transfer is
Q = fA (T14-T24)
where             Q = Heat flow by radiation alone to A (kJ/hour)   
                        T1= Temperature of source (°C)
                        T2 = Temperature of sink (°C)
                        f = Dimensionless factor to allow for both the geometry of the system of the system and the non-black emissivities of the hot and cold bodies
                        A = Effective heat transfer area of source or sink or cold body (m2)              

Convection Section:
                   It is the mode of heat transfer between a solid surface and the adjacent liquid or gas that is in motion and it involves the combined effects of conduction and fluid motion. Here the heat is recovered from the flue gases by convection mechanism. Combustion products pass through the stack of tubes where heat transfer takes place by the following relation
Qc= A (LMTD) Uc
Where  
 Qc = Heat duty for convective section KJ/hr
 A = Heat transfer area of convection   m2
 LMTD = Log mean temperature difference K
 Uc = Convective heat transfer coefficient KJ/hr m2.K

Combustion
Radiation in radiant section is arising due to the combustion of gaseous fuel. Combustion is the process in which the rapid chemical reaction of oxygen with the combustible portion of the fuel results in heat releases.” Here the following reactions are taking place:

Significance in the Process
                        According to requirements of reaction, temperature of 400 oC and pressure of 1775 pisa is required at the inlet of reactor. Multistage centrifugal pumps are used to achieve the desired pressure. For temperature, feed mixture (VGO& Hydrogen) is available at almost 100 oC, first of all we heat it up to 380 oC in a battery of heat exchangers by the reactor effluent which is at 410 oC. But 20 oC rise in temperature is still needed. By preliminary analysis of the flow sheet we see that there is no stream available at temperature higher than 380 °C so that heat may be recovered from it. Therefore we have to use direct heating method. For this purpose furnace has been used here to increase the temperature of the feed mixture up to 400 oC.

Selection Criteria
      The selection of a furnace is based upon the following points.

(a)  Kind of product to be fired.
(b)  Quantity to be produced.
(c)  Firing temperature.
(d)  Atmosphere of flame.
(e)  Kind of fuel.
(f)   Location and infrastructure.
(g)  Condition of load.
        
Ideal Furnace
 The ideal furnace will be one in which the rate of heat absorption is the maximum (at all points) that can be transferred to oil without causing, coaking, discoloration or decomposition.


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