Ventilation – Having protecting air current – Including plural – layered currents
Reexamination Certificate
1998-10-06
2002-05-14
Dzierzynski, Paul (Department: 2881)
Ventilation
Having protecting air current
Including plural, layered currents
Reexamination Certificate
active
06386968
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for performing confinement. It relates more precisely to a method of confining pollution generated in the top volume and/or in the bottom volume of an enclosure filled with a fluid, and to apparatus associated with said method. Said method is industrial and it is original in that it is based on the natural phenomenon of thermal stratification. When it is implemented, it can surprisingly guarantee effective confinement in various contexts, and in particular in the most unfavorable context in which a hot pollution source is disposed at the bottom of an enclosure whose top portion is to be protected from said pollution source. The Applicant has developed the method and apparatus of the invention specially for the context of vitrifying fission products in the nuclear industry, with a view to protecting equipment of the hoist type disposed at the top of the vitrification cell from pollution given off by the melting pot and by the calciner. However, said method and apparatus of the invention, which method and apparatus are described in detail below, are in no way limited to this context.
BACKGROUND OF THE INVENTION
In the prior art, the phenomenon of fluid stratification is well known.
In Liquids
Gravity stratification is commonly observed in non-miscible liquids of different densities. It is very stable and requires high energy to cause the two phases to be mixed (emulsion). In the absence of an emulsion, the relatively small area of the interface per unit volume constitutes an effective barrier against the transfer of a solute or of particles in suspension from one phase to the other.
In Gases
A similar phenomenon can be observed, if the two phases are gases of different densities situated in an enclosure of large volume in which the ambient environment is disturbed little.
In this case, the stratification phenomenon is much less stable and the interface is less clearly defined than for the interface between liquid phases. The interface is replaced with a “mixing zone” due to Brownian motion and turbulent diffusion in which the mean concentration of one phase in the other varies continuously with a steep gradient when going up the vertical axis.
With reference more particularly to thermal stratification, it is known that differences in density exist in the same fluid because of differences in temperature; the fluid can then behave as two distinct phases, namely a cold phase and a hot phase. The two phases are not very miscible if the volumes involved are large, and they can thus present the same stratification phenomena as fluids of distinct compositions. This natural phenomenon of thermal stratification explains the following:
the ocean currents;
the meteorological phenomenon of temperature inversion and its effects on atmospheric pollution; and
the temperature profile (as a function of depth) of water in a mountain lake.
In the context of the present invention, it has been observed, surprisingly, that it is possible to control said natural phenomenon of thermal stratification so as to use it to create genuine confinement barriers artificially in horizontal planes both in liquids and in gases. As a person skilled in the art can easily understand, it is far from obvious that such control can be achieved. Indeed, it would seem unlikely to be possible for it to be achieved, in particular in a gaseous atmosphere which is very sensitive to convection currents and to turbulence. Preconceived opinion was therefore strongly unfavorable to basing an industrial method of confinement on the natural phenomenon of thermal stratification.
Known methods of confinement, implemented currently and while the invention was being developed, in particular to protect equipment from a polluting atmosphere charged with particles, are of the following types:
close protection with the use of a covering;
protection by curtains of air;
protection under laminar flow; and
protection based on thermophoresis.
After becoming acquainted with the present invention, the person skilled in the art can appreciate the advantages that the invention offers in various contexts over the above-mentioned prior art techniques. At this stage of the description, it is worth emphasizing that the method of the invention is effective, and that the accessory equipment required to implement it is compact.
OBJECTS AND SUMMARY OF THE INVENTION
It is proposed to describe below the two aspects of the invention, namely the method and the apparatus, in a general manner in a first part, and in a more detailed manner in a second part, with reference to a particular implementation.
The invention thus provides firstly a method of confining pollution generated in the top volume and/or in the bottom volume of an enclosure filled with a fluid, i.e. either a gas, which is in general air, or a liquid, which is in general water, the method confining the pollution by thermal stratification: the mean temperature of said top volume is maintained higher than the mean temperature of said bottom volume by an amount that is sufficient to ensure that said two volumes are separated by a turbulent intermediate zone of narrow width, referred to as the “mixing zone”, within which a steep temperature gradient is maintained; said intermediate zone constituting a virtual confinement barrier acting as a virtual partition in a horizontal plane.
Said method consists in creating artificially a confinement barrier between the top volume and the bottom volume of the enclosure by maintaining a sufficiently large temperature difference between said top and bottom volumes (the temperature of said top volume being obviously maintained higher than the temperature of said bottom volume). Said temperature difference must guarantee a sufficiently large density difference between the hot fluid in the top volume and the cold fluid in the bottom volume. Said density difference must be such that the effects of vertical forces, directed downwards for the cold fluid and upwards for the hot fluid (said forces, which are due Archimedes' thrust, being applied to the various volumes of the two phases, and tending to separate them by stratification) prevail over the effects of inertia forces due to the rate at which said volumes penetrate into the mixing zone. The inertia forces are due to the random speeds in the turbulence of the ambient environment, and they are responsible for diffusion mixing and for heat exchange between phases. Thus, by means of the invention, the enclosure is artificially subdivided into two distinct enclosures.
As specified above, the method of the invention may be implemented in an enclosure filled with gas (the enclosure is then more readily referred to as a “cell” or a “room”), or in an enclosure filled with liquid (such as a “pool”). In general, only one fluid, i.e. either a gas or a liquid, is involved. When the fluid is a gas it consists, in general, of air; when it is a liquid, it consists, in general, of water. Other gases, such as nitrogen, for example, and indeed other liquids are in no way excluded from the ambit of the invention. Similarly, the invention does not exclude the possibility of two different types of gas or of two different types of liquid being present in the enclosure. However, in such an event, the densities of the two fluids involved must be compatible with implementation of the method.
The method of the invention confines the two volumes of the enclosure, namely the top volume and the bottom volume, relative to each other, and any pollution being generated in one of said volumes is kept out of the other volume, or if pollution is being generated in both of said volumes, then each of said volumes is protected from the pollution being generated in the other volume.
The pollution may be of various types. Its source may, for example, consist of a mechanical source of dust, in particular radioactive dust (such as a sawing station, a shearing station, or a welding station, in general located at the bottom of an enclosure; such a station may equally well be
Mehlman Guillaume
Meline François
Cohen & Pontani, Lieberman & Pavane
Compagnie Generale des Matieres
Dzierzynski Paul
Smith II Johnnie L
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