Can you have clear ideas when buying a vacuum furnace?
Of course yes and the subject, only apparently so complex, is addressed by reasoning
In the previous article we got to
know some of the elements that contribute to the realization and perfect functioning
of a vacuum oven, starting from the three which, over time, have
remained more constant. To them was added another trio, that become more popular
in recent years. Here they are in line:
- pumping and vacuum measurement systems
- leak detection.
- sealing elements
- components: flanges, valves, tightenings, etc.
Today I'm going to complete the review of the
10 elements to evaluate before buying a new vacuum furnace.
I'm talking about:
- heat transfer
The core of the activity of a high vacuum furnace is concentrated
in two parts: a double wall vacuum seal chamber, water-cooled,
and an insulated thermal chamber with low conductivity materials
(graphite wafer), in which the resistor and the charge to be treated are positioned.
The operation of the vacuum heat treatment furnace provides two possible levels
- without partial gas
- with partial gas.
In the first case the heat transfer, by the action of the molecules moving from
the hot part to the cold one, takes place under a free molecular conduction
regime: at more severe operating conditions, the exchanged energy has a value close
to 0.025 kW/m2 ( therefore truly negligible). The heat exchanged in the
transition regime is also negligible, as is the transmission heat in the viscous
field (0.08 kW/cm2). It can be deduced that the energy exchanged by thermal
conduction in a high vacuum furnace is, however, practically nil, irrespective of
the value of the pressure (vacuum level).
In case of thermal exchange due to irradiation by partial gas the values are
very different. I also consider the most difficult operating conditions here: values
of 3÷4 kW/m2 are achieved. The practice confirms the scientific data,
bringing to light also an important aspect for the purposes of the thermal uniformity
of the steady-state system: that of the heat leaks from the chamber.
Some constituent elements of the system work, against our will, against its perfect
functioning. I would like to mention the heat loss caused by the load supports and/or
drafts of radiation, but above all those generated by power feedthroughs.
The power feedthroughs are of large cross-section graphite bars connected to water-cooled
copper clamps: three or four in number, these are one of the coldest areas in the
chamber too. Also the load support points are potential energy escape vehicles,
which we tend to obviate by inserting refractory materials with low conductivity
(alumina pads) between the graphite part and the steel mantle. In some cases they are added by calculating heating zones that have higher specific energy, near to the load support base.
I’ll move now on another element of the oven based on vacuum technology
that has undergone major revisions in recent years. I'm talking about the
resistor and, in particular, the sizing of its supply chain (static
converter, transformer, bars, etc.). In fact, if this must be such as to quickly
reach the set temperature to the heating elements, the same - however - must be
large enough to reach the set temperature to the charge in the shortest time, compatibly
with the geometries and arrangement of the pieces. The current trend is to provide
a power supply to the resistor of about four times the power dissipated
at full power by the system at maximum temperature (1270°C).
The resistor surface also plays a fundamental role in accelerating
the process under conditions of low temperature treatments (for example, in the
case of the austenitization temperature, at "only" 800°C), in which the
power that can be supplied by irradiation is always low.
The position of the resistor is also extremely important: today
it is established with certainty that in
horizontal geometry furnaces, in
the presence of charges consisting of several pieces, the optimal heating is that
achieved with the heating elements placed in the upper and lower surface of the
furnace. This leads to a similar thermal cycle both for the pieces placed at the
periphery of the charge and for those at the center of the charge.
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The flow of gas is crucial in the process of turning off the
To obtain the same thermal history for all the pieces is one of the primary interests
of the vacuum furnace designer: for this reason, the procedure to start the charge
hardening phase in gas is carefully studied.
What does this procedure consist of?
At the beginning, pressurized gas is introduced into the vessel; when the set
pressure has been reached, the resistor is de-energized and the gas is recirculated
at low temperature through a large high-head impeller. The gas, circulating, takes
on heat and then transfer it along the path to a heat exchanger. The path of
the gas is important, because it must flow through the charge, so that even
the smallest and inner elements are cooled.
When designing a vacuum heat treatment system, solutions with geometries that allow
gas to bypass the charge or only touch the outer surfaces must be avoided: the fluid
threads of gas that pass through the charge must not have components of speed perpendicular
to the pieces.
What to pay attention to?
The gas flow section must have a dimension equal to that of the piece
basket. The best channeling of the gas occurs with paths from the bottom to the
top, since a flow distribution congruent with the dislocation of the pieces
is realized, which present themselves to the gaseous streams as on a single surface.
Otherwise, with a path from top to bottom, the flow regularity would be disturbed
by the pieces of greater height, which act causing the so-called "umbrella
effect" on the underlying parts.
In the furnace for vacuum heat treatment, in any case, gas paths with axial and
central geometry must be avoided inside the thermal chamber: in fact, the pieces
would tend to assume different temperatures, the process would be badly controlled
and would run the risk of important deformations on the pieces themselves.
As the only alternative I would suggest splitting the flow: a contemporary jet
from the top and from the bottom solves the problem of fast hardening on large pieces
or on overlapping stacked baskets.
I arrived at the vibrant heart of vacuum technology treatments: the modernity
of this system lies above all in the fact that the cycle is completely automatic.
Once designed and assembled, the high vacuum furnace becomes manageable as a work
center: the active presence of operators during the process is neither foreseen
Extremely accurate and safe, the microprocessor programming guarantees the repeatability
of the cycle and the start of automatic sequence checks.
These process controls diagnose the different states of activity of the furnace
for heat treating under vacuum. A series of indicators highlights
situations of major malfunction or disservice. The system is able to automatically
set itself in safety conditions in the face of anomalous events such as:
- lack of energy
- lack of water
- lack of vacuum
- lack of gas, etc.
Within a short time the operation of a vacuum oven required less and less supervision
and surveillance: a great advantage for the company, which finds itself a system
capable of working even at night, offering the maximum economic yield. In a few
years there has been a leap of generations in vacuum technology. Moreover, pressurization,
in addition to allowing the heat treatment of more "difficult" materials,
considerably reduces cooling times at lower temperatures.
This is how this presentation of the key elements of a vacuum heat treatment
furnace closes: I wanted to offer you a little review of their basic characteristics,
imagining what your questions might be and that this could give you a hand in the
approach with the designer and the price quotation. The best operating result is
obtained when every aspect has been adequately taken into account.
The specialist will find in you a prepared and aware interlocutor: the best conditions
to start a successful collaboration.
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