Furnace is a device use for heating or it is a equipment to heat materials to change their shapes like rolling and
forging or to change the properties of the material. Mostly furnace is used for melting metal for casting
.
Taking into account the system
for creating heat, furnaces are comprehensively grouped into two sorts specifically
combustion sort (utilizing energizes) and electric sort. In the event of
combustion sort furnace, contingent on the sort of combustion, it can be
comprehensively named oil terminated, coal let go or gas let go.
• Based on the method of charging
of material furnaces can be delegated
(i)
Irregular or Batch sort furnace or Periodical
furnace and
(ii)
Continuous furnace.
• Based on method of waste heat
recuperation as recuperative and regenerative furnaces.
• Another sort of furnace order
is made in light of method of heat exchange, mode of charging and method of
heat recovery .
Characteristics of an
Efficient Furnace:
Furnace
ought to be planned so that in a given time, however much of material as could
reasonably be expected can be heated to a uniform temperature as would be
prudent with the slightest conceivable fuel and work. To accomplish this end,
the accompanying parameters can be considered.
• Determination of the amount of
heat to be granted to the material or charge.
• Liberation of adequate heat
inside of the furnace to heat the stock and defeat all heat misfortunes.
• Transfer of accessible piece of
that heat from the furnace gasses to the surface of the heating stock.
• Equalization of the temperature
inside of the stock.
• Reduction of heat misfortunes
from the furnace to the base conceivable .
Furnace Energy Supply
Since the results of pipe gasses
straight forwardly contact the stock, kind of fuel picked is of significance.
For instance, a few materials won't endure sulfur in the fuel. Likewise
utilization of strong powers will create particulate matter, which will meddle
the stock spot inside the furnace. Thus, dominant part of the furnaces use
fluid fuel, vaporous fuel or electricity as vitality information. Melting
furnaces for steel, cast iron use electricity in induction and circular segment
furnaces. Non-ferrous melting uses oil as fuel.
Oil Fired Furnace
Furnace oil is the significant
fuel utilized as a part of oil terminated furnaces, particularly to reheat and
heat treatment of materials. LDO is utilized as a part of furnaces where
vicinity of sulfur is undesirable. The way to productive furnace operation lies
in complete combustion of fuel with least overabundance air. Furnaces work with
efficiencies as low as 7% as against upto 90% achievable in other combustion
hardware, for example, evaporator. This is a result of the high temperature at which
the furnaces need to work to take care of the required demand. For instance, a
furnace heating the stock to 1200oC will have its fumes gasses leaving atleast
at 1200oC coming about in a gigantic heat misfortune through the stack. In any
case, upgrades in efficiencies have been realized by systems, for example,
preheating of stock, preheating of combustion air and other waste heat
recuperation frameworks.
Regular Furnace System
i)
Forging
Furnaces
The manufacturing furnace is
utilized for preheating billets and ingots to accomplish a "produce" temperature.
The furnace temperature is kept up at around 1200 to 1250oC. Manufacturing furnaces,
utilize an open chimney framework and the vast majority of the heat is
transmitted by radiation.
The run of the mill stacking in a manufacturing
furnace is 5 to 6 tons with the furnace working for 16 to 18 hours every day.
The aggregate working cycle can be isolated into
a) heat-up time
b) drenching
time and
c) Manufacturing
time. Particular fuel utilization relies on the sort of material and number of "reheats" required.
ii) Rerolling Mill Furnace
a) Batch
sort
A case sort
furnace is utilized for clump sort rerolling factory. The furnace is
essentially utilized for heating up scrap, little ingots and billets measuring
2 to 20 kg. for rerolling. The charging and releasing of the
"material" is done physically and the last item is in the type of
bars, strips and so on. The working temperature is around 1200 oC. The
aggregate process duration can be further sorted into heat-up time and
rerolling time. Amid heat-up time the material gets heated upto the required
temperature and is evacuated physically for rerolling. The normal yield from
these furnaces changes from 10 to 15 tons/day and the particular fuel
utilization changes from 180 to 280 kg. of coal/ton of heated material.
b) Continuous Pusher Type:
The procedure stream and working
cycles of a ceaseless pusher sort is the same as that of the cluster furnace.
The working temperature is around 1250 oC. For the most part, these furnaces work
8 to 10 hours with a yield of 20 to 25 tons for every day. The material or
stock recoups a piece of the heat in pipe gasses as it moves down the length of
the furnace. Heat retention by the material in the furnace is moderate,
enduring and uniform all through the cross-area contrasted and group sort.
iii) Continuous Steel Reheating Furnaces
The fundamental capacity of a
reheating furnace is to raise the temperature of a bit of steel, regularly to
somewhere around 900°C and 1250oC, until it is sufficiently plastic to be
squeezed or rolled to the fancied area, size or shape, The furnace should
likewise meet particular necessities what's more, targets as far as stock
heating rates for metallurgical and profitability reasons. In nonstop
reheating, the steel stock structures a ceaseless stream of material and is
heated to the fancied temperature as it goes through the furnace.
An unmanageable chamber built of
protecting materials for holding heat at the high working temperatures.
• A hearth to bolster or convey
the steel. This can comprise of unmanageable materials or an plan of metallic
backings that may be water-cooled.
• Burners that utilization fluid
or vaporous fills to raise and keep up the temperature in the chamber. Coal or
electricity can be utilized for reheating. A technique for evacuating the combustion
debilitate gasses from the chamber
• A strategy for presenting and
expelling the steel from the chamber.
• These offices rely on upon the
size and sort of furnace, the shape and size of the steel being prepared, and
the general design of the moving plant.
• Common frameworks incorporate
roller tables, transports, charging machines and furnace pushers.
Heat Transfer in
Furnaces :
The fundamental routes in which
heat is exchanged to the steel in a reheating furnace. In basic terms, heat is
exchanged to the stock by: 9 Radiation from the fire, hot combustion items and
the furnace dividers and rooftop; 9 Convection because of the development of hot
gasses over the stock surface. At the high temperatures utilized in reheating
furnaces, the predominant method of heat exchange is divider radiation. Heat exchange
by gas radiation is reliant on the gas piece (mostly the carbon dioxide and
water vapor fixations), the temperature and the geometry of the furnace.
Types of Continuous
Reheating Furnace:
Ceaseless reheating furnaces are essentially
sorted by the system by which stock is transported through the furnace. There
are two essential systems:
• Stock is butted together to
frame a flood of material that is pushed through the furnace. Such furnaces are
called pusher sort furnaces.
• Stock is put on a moving hearth
or supporting structure which transports the steel through the furnace. Such
sorts incorporate strolling shaft, strolling hearth, revolving hearth what's
more, nonstop recycling bogie furnaces.
The significant thought as for
furnace vitality use is that the bay and outlet openings ought to be negligible
in size and intended to maintain a strategic distance from air penetration.
i) Pusher Type Furnaces
The pusher sort furnace is famous
in steel industry. It has moderately low establishment and upkeep expenses
contrasted with moving hearth furnaces. The furnace may have a strong hearth,
yet it is likewise conceivable to push the stock along slips with water-cooled
underpins that permit both the top and base countenances of the stock to heated.
- ü Pusher
sort furnaces, be that as it may, do have a few weaknesses, including:
- ü Frequent
harm of headstrong hearth and slide blemishes on material
- ü Water
cooling vitality misfortunes from the slips and stock supporting structure in
top furthermore, base let go furnaces detrimentally affect vitality use;
- ü Discharge
must be joined by charge:
- ü Stock
sizes and weights and furnace length are constrained by contact and the probability
of stock heap ups.
- ü All
round heating of the stock is unrealistic.
ii) Walking Hearth Furnace
The strolling hearth furnace permits the stock to be transported through
the furnace in discrete steps. Such furnaces have a few alluring elements,
including: effortlessness of outline, simplicity of development, capacity to
cook for distinctive stock sizes (inside limits), irrelevant water cooling
vitality misfortunes and negligible physical stamping of the stock.
The fundamental impediment of
strolling hearth furnaces is that the base face of the stock can't be heated.
This would he be able to mitigated to some degree by keeping up expansive
spaces between bits of stock. Little spaces between the individual stock pieces
restricts the heating of the side confronts and expands the potential for
unsuitable temperature contrasts inside of the stock at release. Hence, the
stock living arrangement time may be long, conceivably a few hours; this may adverse
affect furnace adaptability and the yield may be influenced by scaling.
iii) Rotary hearth furnace
The rotary hearth furnace has had a tendency to supersede the recycling
bogie sort. The heating what's more, cooling impacts presented by the intruders
are dispensed with, so heat stockpiling misfortunes are less. The rotational
hearth has, however a more mind boggling outline with an annular shape and
rotating hearth.
iv) Continuous Recirculating Bogie type Furnaces
These sorts of moving hearth sort furnaces
have a tendency to be utilized for minimized supply of variable size and
geometry. In bogie furnaces , the stock is put on a bogie with a unmanageable
hearth, which goes through the furnace with others as a train. The whole
furnace length is constantly involved by intruders. Bogie furnaces have a
tendency to be long and limited and to experience the ill effects of issues
emerging from insufficient fixing of the hole between the intruders and furnace
shell, challenges in uprooting scale, and troubles in shooting over a limited
hearth width.
v) Walking Beam Furnaces:
The strolling bar furnace overcomes
numerous of the issues of pusher furnaces and licenses heating of the base face
of the stock. This permits shorter stock heating times and furnace lengths and
along these lines better control of heating rates, uniform stock release
temperatures and operational adaptability. In regular with top and base
terminated pusher furnaces, be that as it may, a significant part of the
furnace is beneath the level of the factory : this may be a constraint in some
applications.
