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:
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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