Metallurgical apparatus – Process – Cooling
Reexamination Certificate
2000-12-12
2002-12-10
Kastler, Scott (Department: 1742)
Metallurgical apparatus
Process
Cooling
C266S265000, C075S709000
Reexamination Certificate
active
06491863
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Brief Description of the Invention
This invention generally addresses needs in the inerting of molten or solid metals, and in particular methods and apparatus to improve efficiency of use of cryogens such as argon in inerting molten or solid metals.
2. Related Art
In the metal casting industry, metals (ferrous or non-ferrous) are melted in a furnace, then poured into molds to solidify into castings. In the foundry melting operations, metals are commonly melted in electric induction furnaces. It is often advantageous to melt the metals under cover of inert gas (usually Ar or N
2
), rather than expose the metal to atmospheric air. The inert gas cover minimizes oxidation of the metal (including its alloying components), which increases yield and alloy recovery efficiency, and also reduces formation of metallic oxides which can cause casting defects (inclusions). The inert gas cover also reduces the tendency of the molten metal to absorb gases (chiefly O
2
and H
2
) from the atmosphere, which in turn reduces gas-related casting defects such as porosity. Other benefits of melt surface inerting include reduced slag formation, improved metal fluidity, increased furnace refractory life, and reduced need for de-oxidizers.
As the electric induction furnace is generally an open-top, batch melter, the inert gas (N
2
or Ar) is usually applied from above the furnace. Inert gas is usually applied throughout the entire melting cycle.
There are many types of furnace inerting techniques in practice today, but they can generally be classified into two major categories: Gas inerting, in which gaseous N
2
or Ar is (gently) blown into the top of the furnace; and liquid inerting, in which liquid N
2
or Ar is dripped or poured into the top of the furnace. In gas inerting, there are many different configurations of pipes and manifolds or distribution “rings” employed to blow the inert gas into the top of the furnace. These make use of varying gas pressures, velocities, discharge locations and angles of injection. Some try to minimize turbulence by creating gentle laminar flow. Some utilize a “swirling” pattern. Some techniques may employ a collar, shroud or cone-like assembly mounted on top of the furnace. However, with any gas inerting technique, it is difficult to produce and maintain a true inert (0% O
2
) atmosphere directly at the metal surface, because hot thermal updrafts from within the hot furnace are continually pushing the incoming cold inert gas up and away from the metal surface. As the hot air and gases rise, the induced draft is continually pulling fresh cold air toward the furnace. The injected inert gas will also entrain ambient air along with it as it is injected into the furnace. Because of these effects, it is difficult, if not impossible, for gas inerting techniques to provide a true inert (0% O
2
) atmosphere directly at the surface of the metal.
With liquid inerting (such as taught in U.S. Pat. No. 4,806,156), the liquid cryogen (typically N
2
or Ar) has higher density than its gas phase and air, and is much less likely to be pushed up and away from the melt surface by the thermal updrafts. The liquid drops or stream are much better able to fall all the way down to the actual metal surface (hot solid metal or molten metal). After contacting the metal surface, within a short time, the liquid vaporizes into a gas. (The appearance is similar to drops of water “dancing” on a hot pancake griddle). As the N
2
or Ar boils from liquid to gas, it expands volumetrically by a factor of 600-800 times as it rises. This expansion pushes ambient air away from the surface of the metal. In this manner, liquid inerting provides a more effective, true inert (0% O
2
) atmosphere directly at the metal surface, as compared to gas inerting. With liquid inerting, inert gas usage efficiency is generally increased; i.e. it requires a lower quantity of inert gas to achieve the same performance as gas inerting.
One drawback of liquid inerting is the difficulty of efficiently delivering the liquid N
2
or Ar to the furnace interior in a liquid state. The liquefied gas (preferably N
2
or Ar) is extremely cold (approximately −184° C.). In the storage tank and distribution piping, the liquid inert gas is continually absorbing heat from the surroundings. This ambient heat pickup manifests itself by boiling some of the liquid to vapor inside the storage tank and distribution piping. The tank and piping is insulated as much as practically possible (typically 7 to 11 cm foam, or vacuum-jacket). The tank-to-furnace piping distance is kept as short as possible (in practice, usually about 15 to 50 m). In spite of these efforts, there is always some amount of liquid that will unavoidably boil to vapor, due to this ambient heat pickup. In addition, some liquid will always “flash” boil to vapor by virtue of pressure reduction alone. The liquid is stored at elevated pressure (typically 2 to 7 bar) in the storage tank, in equilibrium with its vapor phase. Elevated pressure is necessary to provide the driving force to “push” the liquid out of the tank, through the distribution piping. As a matter of practicality, there is usually a vertical elevation rise in the piping which needs to be overcome, and there is some pressure drop through the final liquid discharge device (diffuser). So, as the liquid N
2
or Ar travels through the piping, pressure decreases (eventually to atmospheric pressure at the discharge point), and more and more of the liquid boils to vapor. Due to these combined effects (ambient heat pickup and pressure drop), by the time the liquid N
2
or Ar reaches the discharge point at the furnace, it is estimated that roughly 0.5% to as much as 30% has boiled to vapor, depending on the parameters of the particular system.
Due to volumetric expansion, the vapor (gas phase) occupies much more space than the liquid. In the piping, this expanding gas restricts, or “chokes” the flow of liquid by occupying a greater and greater portion of the volume available in the pipe. Hence the N
2
or Ar in the pipe can be mostly liquid by mass, but mostly vapor by volume.
The result is that “sputtering” or “surging” flow is observed at the discharge end of the pipe. “Sputtering” flow is a combination of gas and “spraying” liquid, often unsteady in appearance with time, with respect to the observed amount of liquid flow. “Surging” flow is a more extreme condition, in that there is observed alternating time periods of “gas only” discharge, and “gas plus liquid sputtering” discharge. Sputtering and surging flow is caused by the generated vapor “bubbles” working their way out of the system piping. The greater the percentage of vaporization, the more extreme the observed sputtering and/or surging will be.
Sputtering or surging flow will reduce the furnace inerting effectiveness, for liquid inerting processes. Compared to a compact, well organized and steady (small) liquid stream, or compared to relatively large droplets, a spray or mist of fine liquid droplets will have much greater surface area, and will therefore absorb heat from the furnace environment much more quickly, vaporizing more quickly, and therefore be less likely to fall all the way down to the metal surface in the liquid state, therefore providing a less effective inert atmosphere at the metal surface. The most effective liquid inerting is provided by a compact, well-organized and steady liquid stream, or by a steady succession of relatively large liquid droplets (minimum liquid surface area).
It is common to use a diffuser, or tight mesh screen (typically sintered metal filter, approximately 40 micron size), at the discharge of the liquid pipe, in order to minimize sputtering flow. The diffuser “catches” the sputtering spray of gas and small liquid drops, reducing the liquid velocity and re-organizing the drops into larger liquid droplets or a steady liquid stream, which generally drips out the bottom portion of the diffuser, while the gas generally seeps out the top. This diffuser is surrounded by an outer shroud, or cone,
Kastler Scott
L'Air Liquide-Societe' Anonyme a' Directoire et C
Russell Linda K.
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