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By Robert N. Britz,
President
Part II: Furnace Considerations
The growth in the use of forced convected heat in heat
treating processes, such as baking, tempering, normalizing, surface hardening,
annealing of ferrous and non-ferrous metals and alloys, precipitation hardening
of aluminum alloys, stress relieving, vacuum furnace cooling cycles and numerous
other applications, indicate the need for discussion of the major requirements
of successful convection furnace equipment.
While it is impossible to reduce every problem of convection
furnace or oven heating to a simple formula or chart, an attempt will be made to
explain the elementary principles of this heating method to approximate the
requirements of such equipment and appraise the factors which are of prime
importance in this method of furnace design.
Air Circulation Most Important
The success of a forced convected heat application
depends mainly upon two factors - the volume of air circulation and velocity
past the work required for a given heat transfer and temperature uniformity.
To a technician observing the progress of recirculative
convection heating of ovens and furnaces, it is quite apparent that the above
mentioned factors are the most abused ones, as most of the failures of
convection type furnaces to produce rapid enough heating and desired temperature
uniformity or cooling can be traced to the underestimation of the importance of
these factors and most of such installations are lacking in sufficient air
circulation and uniformity.
In the forced convection method of heating, air, products of
combustion and various protective atmospheres, serve as the heating media which
supply the heat for heating of the charges as well as for maintaining the
enclosures at a desired temperature with given limitations of temperature
fluctuations.
High Velocity Air
The ideal way of heating a metal object by convection
would be to immerse it in a stream of air of high velocity and constant
temperature. The air velocity would then govern the rate of heat transfer
between the heating medium and the object heated, the amount of fluctuation in
the temperature of the air stream governing the degree of uniformity to which
such heating could be held. A good forced convection type furnace design
will duplicate these ideal heating conditions, as far as practical and
economical limitations will permit, and the closer the approach to these ideal
conditions, the better will be the performance of the equipment.
Economy
Very close uniformities of heating, with permissible
variations of plus or minus 15° F or even plus or minus 5° F (Fig.1),
are sought for modern processes and as these can be attained, economically, only
by forced convection heating, it will be well to point out the requisites of a
successful furnace equipment of this type.
Equipment well designed for given conditions may approach
very closely the condition of absolute uniformities and no thermal heads of
heating regardless of temperatures of furnace loads. A number of
installations were observed in which attempts were made to obtain uniformity of
heating as close as plus or minus 2 or 5 degrees Fahrenheit with heated air 100
to 200° F higher in temperature than the desired maximum temperature of the
heated load blown directly against the furnace loads. While fairly uniform
temperature measurements may have been obtained at certain locations in the
furnace interiors, this was not the case throughout the furnace, unless large
volumes of forced convection and some type of heat input regulation was used.
What is the ideal maximum allowable temperature of the air or
any other heating medium circulated in a furnace which is to operate at a
given temperature - say at 1000° F, with a desired temperature uniformity of
plus or minus 15 degrees Fahrenheit? With the heating medium blown at a
high velocity directly at the furnace load, there is only one ideal answer to
this question - 1015° F. Thus, with a recirculative convection type
furnace, operating under the above desired condition, the maximum temperature of
the incoming heating medium will be 1015° F and when heat equilibrium has been
established the temperature of the recirculated heating medium will be not lower
than 985° F. Under such conditions, uniformity of plus or minus
15° F at 1000° F would be established throughout the furnace cycle.
How Much Air?
Consider an empty furnace of the box type which is to be
maintained by forced convection method of heating at 1200° F with a uniformity
of plus or minus 10° F throughout the interior. No matter how well
insulated, a certain amount of heat loss will occur through the furnace walls
and if a given volume of air at 1200 is introduced into the furnace and
withdrawn, the temperature of the outgoing air will be somewhat lower, due to
the heat which was abstracted from it and lost through the walls of the furnace,
Fig. 2. Thus the air
forced through the furnace will be lowered in temperature along its path from
the inlet to outlet, the reduction in temperature being directly proportional to
the heat loss through the furnace walls as well as to the weight of the air.
Conclusions
In practical applications, where furnace and oven loads
are not located close to the walls, the volumes of circulation may be somewhat
lower. This is especially true of furnaces and ovens with built in fans
and those with ducts, which surround the loads. With equipment of such
design the heat losses through the walls affect the interior and load
temperatures to a lesser extent, and consequently, the circulation volume
requirements are lower.
It is evident that the best fans for furnace and oven
applications are those having large inlets and outlets, and consequently, low
inlet and outlet velocities. It also follows that these fans operate at
the lowest speeds and have lower wheel stresses for the high temperatures
required.
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