Filtration system and method

Abstract: A filtration system and method. The filtration system comprises a manifold (11) having an inlet (14), a plurality of filter elements (12) mounted to the manifold (11), and a scheme for convectively cooling the filter elements (12) to limit the temperature of the filter elements (12) to a predetermined temperature. The filtration method comprises the steps of passing a fluid through a plurality of filter elements (12) and convectively cooling the filter elements (12) to limit the temperature of the filter elements (12) to a predetermined temperature.

Description: The present invention relates to the filtering of fluids and, more particularly, to the filtering of fluids by a filter element and the removal of heat from the filter element. In a nuclear power plant, the nuclear reactor and most of the components associated with the reactor are typically enclosed within a containment building. The containment building may be constructed of reinforced concrete, having walls several feet thick, and serves to trap radioactive debris and protect the outside environment in the event of an accident. Unfortunately, if the accident results in the generation of sufficiently high pressures, the containment building may rupture. Radioactive debris will then be scattered over a wide area and any radioactive components remaining in the ruptured containment building may be exposed to the outside environment. US-A-4,698,202 discloses a process for controlling the discharge from a reactor containment structure. In one discharge path, a valve or rupture disk opens when the pressure inside a building exceeds a predetermined value. The path includes a regenerative heat exchanger, a deposition course for solid fission products and a filter system for gaseous fission products and suspended particles. US-A-3796,024 discloses a filter assembly in a chimney in the exit path of smoke. A blower directs cool ambient air from an inlet towards the filter assembly to cool the smoke. The present invention provides a system comprising a containment building having a predetermined worst case total heat load and a filtration system coupled to the containment building and adapted to remove particulates which include sources of heat, the filtration system comprising a manifold, and a plurality of filter elements mounted to and communicating with the manifold and spaced from one another a distance sufficient to limit the temperature of the filter elements to a maximum desirable temperature when the filtration system is exposed to the predetermined worst case total heat load and cooled by natural convection. During normal operating conditions and during abnormal incidents when the pressure within the containment building does not exceed a predetermined value, the vent member keeps the vent closed, isolating the inside of the containment building from the outside. However, if an accident is sufficiently severe to raise the pressure inside the containment building above the predetermined value, the vent member will open the vent. The high pressure gas then escapes the containment building by passing through the vent, into the manifold, and through the filter cartridges. Releasing the gas reduces the pressure inside the containment building, thereby reducing the risk that the containment building will rupture. By passing the gas through the filter cartridges, any radioactive particulate matter borne by the gas is trapped in the filter cartridges and prevented from contaminating the environment. As the radioactive chemical elements in the trapped particulate matter decay, they generate heat which raises the temperature of the filter elements. To prevent particulate matter from melting or vaporizing and thus passing through the filter elements and to prevent the filter elements from melting, the cooling scheme convectively cools the filter elements by creating a flow of fluid such as air across the filter elements. Thus, the temperature of the filter elements may be maintained below a predetermined temperature, such as the melting point of the filter elements or the melting point or vaporization point of the radioactive debris trapped in the filter elements. One example of the cooling scheme would be an arrangement of the filter elements which spaces the filter elements from one another a distance sufficient to limit the temperature of the filter elements to the predetermined temperature by natural convection. Alternative cooling schemes which may be used separately or together with the arrangement of the filter elements include a structure such as a cooling tower which creates a draft by natural convection or an apparatus such as a blower or fan which creates a flow of air to cool the filter elements by forced convection. The present invention also provides a filtration method for removing particulates which include sources of heat comprising the steps of of passing a fluid through a plurality of filter elements and cooling the filter elements by spacing the filter elements from one another a distance sufficient to limit the temperature of each filter element to a maximum desirable temperature when exposed to a predetermined worst-case total heat load and while being cooled by natural convection. In the accompanying drawings: Figure 1 is a schematic diagram of a filtration system and a vent system embodying the present invention; Figure 2 is a cross-sectional view of a filter element of Figure 1; and Figure 3 is a side view of the manifold and filter elements of Figure 1. As shown in Figure 1, an exemplary filtration system 10 embodying the invention generally comprises a manifold 11, several filter elements 12, and a scheme for convectively cooling the filter elements 12. The manifold 11, which has an inlet 13 for admitting fluid into the manifold 11, serves to distribute the fluid among the filter elements 12. Particulate matter, which includes solid particles as well as liquid droplets if the fluid is a gas, is removed from the fluid as it flows through the filter elements 12. The cooling scheme serves to convectively cool the filter elements 12 by creating a flow of a cooling fluid such as air along the filter elements 12. A filtration system 10 embodying the invention has many applications. For example, it may be used as a filter in chemical processes where the components continue to react as they are being filtered, thereby generating heat in the filter elements. An example of such a chemical process is the collection of burning or oxidizing charcoal ash. However, the filtration system 10 is particularly useful in a vent system for a containment building of a nuclear power plant. As shown in Figure 1, the vent system includes, in addition to the manifold 11, filter elements 12, and cooling scheme, a vent 14 which communicates between the inside of the containment building 15 and the manifold 11. A vent member 16, such as a rupturable disc or a spring-actuated valve assembly, blocks the vent 14 if the pressure inside the containment building 15 is below a predetermined value and opens the vent 14 if the pressure inside the containment building 15 exceeds the predetermined value. During normal operating conditions or during abnormal incidents when the pressure within the containment building 15 remains below the predetermined value, the vent member 16 keeps the vent 14 closed, isolating the inside of the containment building 15 from the outside. In the event of an accident severe enough to raise the pressure inside the containment building 15 above the predetermined value, the vent member 16 opens the vent 14. The high pressure gas then escapes the containment building 15 by passing through the vent 14, into the manifold 11, and through the filter cartridges 12. Releasing the gas reduces the pressure inside the containment building 15, thereby reducing the risk that the containment building 15 will rupture. By passing the gas through the filter cartridges 12, any radioactive particulate matter borne by the gas is trapped in the filter cartridges 12. By trapping the radioactive debris in the filter elements 12, the environment remains free of radioactive particulate matter. Further, clean-up is facilitated because the detachably mounted filter elements which contain the radioactive debris may be relatively easily and safely removed and stored. However, as the radioactive chemical elements in the trapped particulate matter decay, they generate heat which is capable of damaging the filter elements 12 or melting or vaporizing the particulate matter. To effectively filter the gas and limit the temperature while heat is being generated by the trapped radioactive debris, the filter elements 12 may be variously configured. Preferably, each filter element 12 preferably has a generally hollow, cylindrical, pleated configuration and comprises a porous metal filter medium 17, as shown in Figure 2. Although other configurations, such as a flat pack or disc, may be suitable, the hollow, cylindrical, pleated configuration is preferable because it has a larger filtration surface area. Although other filter media, such as a polymeric filter media, may be suitable if the temperature can be maintained at a low enough level, the metal filter medium is preferable because it can withstand higher temperatures without damage. Further, because the metal filter media has a higher thermal conductivity, it more effectively radiates heat than other filter media. One example of a suitable porous metal filter medium is a fiber metal medium formed from sintered stainless steel. The filter element 12 may further include a core 18 disposed around the inside of the pleated filter medium 17 to provide additional structural support. A cage 19 may be disposed around the outside of the pleated filter medium 17 to protect the medium and to support the medium 17 against the force of any outwardly flowing gas. To minimize interference with the convective cooling of the filter medium 17, the cage 19 is preferably very open and may have, for example, a 70% open area. The filter elements 12 are preferably detachably mounted to the manifold 11. The filter elements may be mounted inside the manifold with the gas flowing outside-in through the filter elements. However, to more effectively allow convective cooling of the filter elements 12, the filter elements 12 preferably extend outside of the manifold 11, as shown in Figure 3, with the end of the pleated filter medium 17 being capped by an impermeable end cap 20. The gas thus flows inside-out through the filter elements 12. The filter elements are preferably oriented so that their axes are horizontal. Rather than cantilever the filter elements 12 from the manifold 11, the end caps 20 of the filter elements 12 may be attached to a support structure (not shown). Further, the exterior surfaces of the filter elements are preferably exposed to the atmosphere in order to most effectively cool the filter elements 12. The filter elements 12 may be contained within an enclosure (not shown), for example, if it is desirable to maintain the filter elements 12 in an inert atmosphere when the vent system is not in operation. However, when the vent system is in operation, it is most preferable that the support structure or enclosure do not provide any significant resistance to air flow over the filter elements 12. Consequently, the enclosure may comprise a rupturable shroud. To further protect the filter elements 12 and prevent melting or vaporization of the particulate matter from the heat generated by the trapped radioactive debris, the filter elements 12 are convectively cooled. Although convective cooling may be effected in any suitable manner, the preferred cooling scheme comprises an arrangement of the filter elements which spaces the filter elements from one another a distance sufficient to limit the temperature of the filter elements 12 to a predetermined temperature by natural convection. The arrangement may preferably be a regular array, such as that shown in Figures 1 and 2. The spacing between filter elements may be calculated according to conventional heat transfer methods by determining a maximum desirable temperature for the filter element and assuming a worst-case heat load for the filter elements 12. For example, a total heat load of 50 Kw may be assumed and a maximum desirable temperature of 300°C may be selected. For a filter element formed from sintered stainless steel fiber metal and having a 5.7 cm (2 1/4 inch) outside diameter with forty 12.7 mm (1/2 inch) deep pleats around the circumference, it is expected that approximately 85.3 linear meters (280 linear feet) of the filter element would be required. If each element were 1.83 meters (6 feet) long, then a regular array of about 47 filter elements with centerlines spaced approximately 11.4 cm (4 1/2 inch) from each other may be provided. As an alternative to or in addition to the arrangement of the filter elements 12, the cooling scheme may include a structure 21 such as a draft tower which creates a draft of air. The filter elements 12 may then be positioned in the draft to convectively cool them. The cooling scheme may also include an apparatus 22, such as a blower or fan, for creating a flow of air. The filter elements 12 may then be positioned in the air flow to cool them by forced convection. The arrangement of filter elements is preferable, however, because, in the event of an accident, electrical energy may not be available to power the blower.

Claims: A system comprising a containment building (15) having a predetermined worst case total heat load and a filtration system coupled to the containment building and adapted to remove particulates which include sources of heat, the filtration system comprising a manifold (11), and a plurality of filter elements (12) mounted to and communicating with the manifold (11) and spaced from one another a distance sufficient to limit the temperature of the filter elements (12) to a maximum desirable temperature when the filtration system is exposed to the predetermined worst case total heat load and cooled by natural convection. The system of claim 1 wherein the filter elements (12) extend outside the manifold (11). The system of claim 1 or 2 wherein the filter elements (12) are detachably mounted to the manifold (11). The system of any one of claims 1 to 3 wherein a structure (21) is provided to create a draft and wherein the filter elements (12) are disposed in the draft. The system of claim 4 wherein the structure (21) includes a tower. The system of any one of claims 1 to 3 further comprising additional cooling means consisting of an apparatus (22) cooperatively arranged with the filter elements (12) to cool the filter elements (12) by forced convection. The system of claim 6 wherein the apparatus (22) includes a blower which forces a flow of air past the filter elements (12). The system of any one of claims 1 to 7 wherein each filter element (12) includes a porous metal filter medium (17). The system of claim 8 wherein the metal filter medium (17) is pleated and formed in a generally hollow, cylindrical configuration. The system of any one of claims 1 to 9 wherein each filter element (12) is mounted to the manifold (11) to filter gas flowing inside-out through the filter element (12) thereby defining an upstream surface on the inside of the filter element (12) and a downstream surface on the outside of the filter element (12) and wherein for cooling a flow of gas is directed along the downstream surface of the filter elements (12). The system of any one of claims 1 to 10 wherein the manifold (11) has a vent (14), the vent (14) being coupled to the containment building (15) and the manifold (11), a member opening said vent (14) when the pressure inside said containment building (15) exceeds a predetermined value. The system of claim 11 wherein the member (16) comprises a rupturable disc. The system of any one of claims 1 to 12 wherein each filter element (12) has a generally hollow, cylindrical configuration and comprises a pleated, porous metal filter medium (17). The system of claim 13 wherein the filter elements (12) are mounted to the manifold (11) to filter a gas flowing inside-out through the filter elements (12) thereby defining an upstream surface inside the filter element (12) and a downstream surface outside the filter element (12) and wherein for cooling a flow of gas is directed along the downstream surfaces of the filter elements (12). The system of claim 13 or claim 14 wherein each filter element (12) has a generally hollow, cylindrical, pleated configuration and a blind end cap (20) attached to one end of the filter element (12) and wherein each filter element (12) is detachably mounted to the support assembly (11). A filtration method for removing particulates which include sources of heat comprising the steps of passing a fluid through a plurality of filter elements (12) and cooling the filter elements (12) by spacing the filter elements (12) from one another a distance sufficient to limit the temperature of each filter element (12) to a maximum desirable temperature when exposed to a predetermined worst-case total heat load and while being cooled by natural convection. The filtration method of claim 16 wherein convectively cooling the filter elments (12) includes an additional cooling of the filter elements (12) by forced convection. The filtration method of claim 16 or claim 17 wherein passing a fluid through a plurality of filter elements (12) comprises passing a gas from an upstream surface to a downstream surface of a hollow filter element (12) and wherein cooling the filter elements (12) includes directing a gas along the downstream surface of the filter elements (12).

Inventors: GEIBEL STEPHEN ARTHUR, HURWITZ MARK FRANCIS, GEIBEL, STEPHEN ARTHUR, HURWITZ, MARK FRANCIS
Applicants: PALL CORP, PALL CORPORATION
Application year: 1990
Main classification: B01D46
IPC codes: G21F9, B01D46, G21D3, G21C9