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HANDBOOKS FOR GENERAL REFERENCE AND DATA
1.2 HEATING SYSTEM CLASSIFICATION
Any heating system will have two main components, the heat generator G and the work or the object P that is to be heated.
heat transfer process T. Based on these three, a general clas-1. Figure clas-1.1(A) shows a system that has the generator and the work separated by a distance. Heat is trans-mitted through the medium in between, as shown by the marked arrows. This is obviously an inefficient system as a considerable portion of the generated heat will be wasted (as shown by the unmarked arrows).
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sification system can be proposed as shown in Figure 1.1.
Heat will be transferred from the generator to the object by a
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A pot or a piece of metal placed on an electric plate or gas burner belongs to this system. We call this an
“open” system.
The above system can be converted into a “closed”
system as shown. We now surround the generator and the work by providing an “insulated” enclosure E.
Now all the generated heat will reach the work either directly or via reflection from the enclosure. It is pre-sumed that the enclosure is 100% reflecting.
Most of the indirectly heated resistance furnaces (Figure 1.1(B)) belong to this class. Heat transfer from the generator to the enclosure to the work is an impor-tant design factor for this type. Fuel-fired furnaces also belong to this class. An important difference in this Figure 1.1 Systems in which the generator G and the work P are separate.
G P
t1 < t2
t1 < t3 < t2
t1 t2
P t2 P
t2
A. “Open System”
B. “Closed System”
Fuel
Combustion Gas E, t3 G, t1
G, t1 T
E T
T T T T T T
T T
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Introduction 5
type is the heat lost from combustion gases that leave the furnace enclosure. This heat loss is also quite con-siderable and reduces efficiency despite the presence of the enclosure.
2. Other types of heating systems do not have separate generator and work. By using certain techniques, heat is directly generated in the work as shown in the Figure 1.2. The heat is created by radiation R gener-ated in a source S. An enclosure may or may not be required. Appreciable amount of heat may be lost from the work if it is heated much above the sur-rounding. These processes are usually very fast and such heat loss can be minimized. It may appear that these systems are highly efficient. However, the gen-eration of radiation in the source S is very inefficient (∼5–10%) but there are several other advantages which will be discussed later.
Induction heating, direct resistance heating, and In induction heating, high frequency (103–105 Hz) electromagnetic oscillation is created in the work by Figure 1.2 Heating system in which “nonthermal” radiation is used to create heat in the work. Heat generator and work are not separate.
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microwave heating belong to this class (Figure 1.3).
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using an oscillator and an inductor. The work gets heated by the induced eddy current. In microwave heating the work is placed in an electromagnetic field of very high frequency (1011–1014 Hz). The molecules of the work vibrate and create heat. In direct electric resistance or capacitive heating the work is made a component of an electric circuit. The power I2R or dielectric losses in it due to current I produce heat.
Later we will discuss all these processes in detail to highlight their individual features.
Heat transfer in all furnaces of this type is directly related to the surfaces of the generator and to the work taking part in the transfer, and the ratio FP/FG becomes a decisive factor in the design.
The heat transfer from the generator to the work takes place over paths through the medium. Thus, the properties of the medium decide the mode and effi-ciency of transfer. The medium may be air or a special Figure 1.3 Special heating modes.
. .
. . .
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Introduction 7
gas or combustion gases, or vacuum and is called the
“atmosphere.” Thus a classification based on atmo-sphere arises, such as vacuum furnace, protective atmosphere furnace, salt bath furnace and the like.
3. Two modern heating processes that need to be spe-cially mentioned are laser and electron beam heating.
To some extent they belong to both systems mentioned above. A laser is a light beam having a single wave-length and excellent collimation. When focused on the work surface, it produces a spot with very high energy density (∼105–108 W/cm2) which melts or evaporates the material in the spot. Penetration below the surface is low An electron beam is a stream of high-energy elec-trons which can be focused on the work with similar power density. Heat is produced when the electrons lose their kinetic energy on impact. The penetration is deeper than that with laser beam heating.
Both techniques are useful for precision heating/
melting at a spot (∼ 0.1–2.0 mm diameter) that can be easily controlled. They are useful heat sources for cut-ting, welding, and drilling operations requiring preci-sion. Energywise, they are highly inefficient but offer many other advantages than any other process can.
These techniques are now in commercial use and are dis-cussed in detail.