Abstract:
A centripetal demister system and method for the removal of moisture from a moisture laden gas stream using both centrifugal and centripetal force to shear the moisture from a inlet gas, the centripetal force also directing the inlet gas radially inward for collecting the demisted gas. The present invention is particular useful for removal of environmentally harmful particulates contained within the moisture laden gas stream, the particulates and moisture being force against the interior of the housing by centrifugal force while the demisted gas is drawn radially inward by centripetal force.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for demisting a gaseous stream and more particularly to the separation of moisture and solid materials from gaseous streams through the use of a centripetal demister. 
     The conventional technique for demisting gaseous streams has typically involved the use of filtration. However, moisture collecting on the filter media drastically reduces the life of th filters. Replacement of the these filters, such as High Efficiency Particulate Air (HEPA), is time consuming, expensive and inherently impacts operating schedules and budget. Consequently, means for reducing the degradation of the filter media have been sought such as the use of pre-filters installed upstream of HEPA filters. This has been found to be of limited benefit, typically extending the life of the filter media by only a few days. Also, pre-filters and filter media tend to become clogged from solid materials that may be contained within the gaseous stream. 
     It is an object of the present invention to provide a demister apparatus for separating moisture and solid materials from a moisture laden gaseous stream by using centripetal force. 
     It is another object of the present invention to provide an economical centripetal demister apparatus for separating harmful moisture and solid materials from a gaseous stream in order to recover or vent the demisted gas. 
     It is still a further object of the present invention to provide a demister method and apparatus capable of removing or demisting and removing small size droplets from a gaseous stream. 
     It is still a further object of the present invention to provide a demister method and apparatus that uses much less liquid than a conventional water scrubber and further that the liquid used in the present invention can be recovered and used again. 
     It is still a further object of the present invention to provide a demister method and apparatus for removing moisture and environmentally harmful solid material prior to a secondary filtration such as a HEPA filter. 
     It is still another object of the present invention to provide demister system that is self cleaning and will not plug readily with any solutions, mists, or mists and solids. 
     Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing and other objects, a method and apparatus for demisting a gaseous stream is provided. The demister apparatus comprises a housing having at least one gas inlet through which moisture laden gas at a first pressure enters. The housing also contains at least a portion of a rotatable hollow shaft having vanes, stiff bristles or wire brushes, extending radially outward therefrom. The hollow shaft has multiple holes throughout the portion of the shaft that is contained within the housing. 
     The rotation of the shaft, and vanes attached thereto, creates a centrifugal force that separates the moisture, and any particles contained within the gaseous stream, from the gas. The moisture (fluid) and particles are then collected from the housing through a drain, such as a gravity, or pumped drain. 
     Upon separation of the moisture and/or particles from the gaseous stream, the demisted gas is then drawn radially inward by centripetal force through the holes in the shaft wall. The centripetal force is provided by maintaining the pressure within the hollow shaft at a lower level than the pressure of the gas at the inlet to the housing. The demisted gas can then be collected or, since the harmful particulates have been removed, simply exhausted into the ambient atmosphere. 
     In another preferred embodiment of the present invention, a moisturizing system is provided to the gas stream prior to its introduction into the housing. By providing sufficient moisture, particulates within the gas become entrained in the moisture, thereby facilitating the removal of the particulates from the gaseous stream. The water or other fluid can be introduced either in the gas inlet of injected into the hollow shaft core where it is flung radially outward and hence collected as waste. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated in the accompanying drawings where: 
     FIG. 1 is a schematic perspective view, partly in section, of an apparatus according to the invention including a drainage system and optional gas collection system. 
     FIG. 2 is a perspective view, in section, of an alternate construction of the rotating hollow shaft and vanes. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, similar components are referred to by the same reference numeral in order to simplify the understanding of the sequential aspect of the drawings. 
     Referring now to FIG. 1 a schematic view of a centripetal demister  10  according to the invention is shown. The demister  10  comprises a housing  12  having a first end  14  and a second end  16 . The housing  12  has at least one gas inlet  15  through which moisture laden gas  17  is introduced into the housing  12 . Apertures are provided in the first and second ends  14  and  16  respectively, through which a rotatable hollow shaft  22  is positioned. As shown in the section view of FIG. 1, the portion of the hollow shaft  22  that is contained within the housing  12  has a plurality of holes  24  through which gas may enter the interior portion of the hollow shaft  22 . 
     Also, as can be seen in FIG. 1, vanes  26  are attached to the hollow shaft  22  and extend radially outward therefrom. In a preferred embodiment of the invention, the vanes  26  can be stiff resilient bristles, brushes, wire brushes, thin tubes, or rods. A system for rotating the hollow shaft  22  is shown in FIG. 1 as a motor  28  connected to the shaft. Sealed bearings  18  can also be provided to promote a longer operating life for the rotating shaft  22 . Generally, it is desired to provide a rotation speed from between 100 rpm to 10,000 rpm, depending upon the moisture content of the inlet gas, desired dryness of the demisted gas, and the size of the entrained particles. An optional baffle  27  may be provided near the housing gas inlet  15  to break the gas inlet stream so a jet of gas will not impinge directly on the vanes  26 . 
     In operation of the present invention a moisture laden gas stream  17  enters the housing though housing inlet  15 . Additionally, the moisture laden gas stream can further include solid matter, such as particulates. A moisturizing system can be provided to establish or maintain a desired moisture content of the gas entering the housing inlet  15 . For example, the moisturizing means could be the injection of a mist into the incoming gaseous stream prior to, or proximately to, the gas entering the housing. Alternatively, as shown in FIG. 2, the moisturizing system could include injecting moisture from a water injection tube  38  contained within the core of the hollow shaft  22 , thereby providing a moisture flow counter to the gas entering the housing. 
     The pressure of the gas entering the housing is greater than the pressure maintained within the interior of the hollow shaft  22 . An optional pressurization system can be employed upstream of the housing gas inlet  15  to maintain a desired inlet gas pressure. Multiple inlets can be provided in the present invention, thereby increasing the throughput of the gas to be process by the centripetal demister. Upon entering the housing the moisture laden gas, in the form of a fine or coarse mist, contacts the rotating vanes. The mist droplets, and solid particles contained therein, if any, are forced to the interior walls of the housing  12  by a centrifugal force that is created from the rotation of the shaft  22  and vanes  26 . Due to the relatively low mass of the gas molecules, the centrifugal force from the rotating vanes has little effect on the gas, so that the droplets and particulates are sheared from the gaseous stream. 
     By maintaining the pressure within the hollow shaft at a level lower than the pressure of the inlet gas, a centripetal force is created. Due to this centripetal force the gas enters the housing and is directed radially inward through the holes  24  in the shaft  22 . The demisted gas  30  is then vented from the shaft  20 . Also exhaust gas collection means  32  may be provided to the present invention to recover the demisted gas and enhance the removal of the demisted gas from the housing. The exhaust gas collection means can be sealed for handling exhaust gases still containing small amounts of harmful particulates or for handling radioactively contaminated gases. 
     Another feature of the centripetal demister include the use of a drain to remove collected fluid and particles from the housing. The drain can be a gravity drain  31  through the bottom of the housing. Alternatively, the drain can be a pump system, such as a sump pump or external pump, to remove volumes of collected fluid and solid materials. The sump or drain furthermore can be U-shaped to act as a water trap preventing purified gas from escaping the housing. 
     The present invention is not limited in size, shape or configuration. The demister of the present invention can be configured in a vertical or horizontal position, or any other position in between, thereby enhancing the number of applications that the centripetal demister can be used. The diameter of the brush wires may be fine or coarse, and the speed may be any value up to failure of the brush. Experimental studies have shown that a lower rotation speed may actually be preferred in some circumstances, depending upon the configuration of the brush wires. For example, at very high speed (e.g., 5500 rpm) the wire bristles tend to separate into bands, thereby permitting unobstructed flow of the inlet gas to the hollow shaft core. This is easily solved by providing spacers periodically along the length of the hollow shaft or by packing the brushes more compactly on the shaft. 
     Also a preferred method for securing the wire brushes (i.e., vanes  26 ) to the hollow shaft  22  consists of sliding the brush axially into a dove tail groove machined into the hollow shaft  22 . The fluid passage holes  24  are provided in the hollow shaft  22  along lines parallel to and between the dove tail grooves. This method for attaching the wire brush to the hollow shaft  22  eliminates any clear path of the inlet gas to the hollow shaft without contact with the wire brush and hence allows a high speed of rotation. 
     Referring now to FIG. 2, an alternate embodiment of the invention is shown. As shown in FIG. 2, the vanes  26  can be offset from the hollow shaft  22  by rigid supports (discs)  34  spaced along the length of the hollow shaft, the rigid supports  34  thereby connecting the vanes  26  to the hollow shaft  22 . An advantage to this embodiment is that it allows almost any rotation speed to be attained without the brush wires separating into bands. 
     The operating pressure can be any value from very low (e.g., atmospheric) to very high (e.g., hundreds of atmospheres) limited only by the strength of the housing and piping system. As long as the inlet pressure is higher that the pressure maintained within the core of the hollow shaft, the system will function. 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments described explain the principles of the invention and practical application and enable others skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.