Abstract:
An electrostatic precipitator element comprises preferably a mesh made of at least two individually insulated conductors with defined openings therebetween. An electrical field in the openings electrostatically attracts suspended particulate to a respective conductors for collection.

Description:
TECHNICAL FIELD 
       [0001]    The present concept relates generally to an electrostatic precipitator for separating particles or droplets from aerosol flows, and more particularly, to an improved method and apparatus for separating oil from an oil/gas mixture. 
       BACKGROUND OF THE ART 
       [0002]    Electrostatic precipitation is known for removing suspended particulate matters from a gas (aerosol) flow for gas cleaning, air pollution control, oil/air separation, etc. The fundamental design of electrostatic precipitators has remained relatively unchanged since early applications of electrostatic precipitation in the nineteenth century. In its simplest form for a single stage precipitator, a high DC voltage is applied to a central electrode positioned in a grounded casing in order to cause a corona discharge to develop between the central electrode and the conductive interior surface of the casing. As the gas containing suspended particles flows between the electrode and the conductive interior surface of the casing, the particles are electrically charged by the corona ions. The charged particles are then precipitated electrostatically by the electric field onto the conductive interior surface of the casing where the charged particles neutralize. This normally involves very high voltages to achieve high electric field strengths, which causes a safety issue of arcing. This may be problematic for some applications where the fluids or gas/particle mixture may be ignited by a spark, such as in a fuel system or oil system of a gas turbine engine. 
         [0003]    Accordingly, there is a need to provide an improved electrostatic precipitator. 
       SUMMARY 
       [0004]    It is therefore one object to provide an electrostatic precipitator. 
         [0005]    In accordance with one aspect of the present concept, there is an electrostatic precipitator element which comprises a mesh made of at least first and second individually insulated conductors, sections of the first and second insulated conductors defining openings of the mesh, the first and second conductors being adapted to be connected to a source of electric voltage to thereby create an electric field in each of openings of the mesh; and means for directing an aerosol flow to pass through the openings of the mesh, resulting in particulate matters suspended in the aerosol flow being electrostatically attracted to and thus collected on an insulated surface of at least one of the first and second conductors to form liquid droplets. 
         [0006]    In accordance with another aspect of the present concept there is an electrostatic precipitator which comprises a container including an inlet for introducing a flow of an oil/air mixture thereinto, and a first outlet for discharging an air flow and a second outlet for discharging liquid oil; and a mesh supported within the container and located between the inlet and the first outlet, the mesh being formed with at least two conductors to define openings of the mesh to permit the oil/air mixture to flow therethrough, the two conductors being individually insulated and adapted to receive an electric voltage applied thereover to create an electric field in each of the openings, thereby attracting oil particles in the oil/air mixture to an insulated surface of at least one of the first and second conductors to form oil droplets to be discharged through the second outlet. 
         [0007]    In accordance with a further aspect of the present concept, there is a method provided for separating liquid from a liquid/gas mixture, which comprises 1) applying an electric voltage to a pair of individually insulated conductors in order to create an electric field in a vicinity of the individually insulated conductors; 2) directing a flow of the liquid/gas mixture to pass through the electric field in order to attract liquid particles suspended in the liquid/gas mixture to an insulated surface of at least one of the individually insulated conductors; 3) collecting liquid droplets formed on the insulated surface of at least one of the individually insulated conductors; and 4) discharging the flow which has passed through the electric field. 
         [0008]    Further details of these and other aspects of the present concept will be apparent from the detailed description and drawings included below. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]    Reference is now made to the accompanying figures depicting aspects of the present concept, in which: 
           [0010]      FIG. 1  is a schematic illustration of a mesh made of two insulated conductors to form an electrostatic precipitator element; 
           [0011]      FIG. 2  is a schematic illustration of a mesh made of three insulated conductors to form an electrostatic precipitator element; 
           [0012]      FIG. 3  is a schematic illustration in a side view of the mesh made of three insulated conductors of  FIG. 2 ; 
           [0013]      FIG. 4  is a schematic illustration of the mesh of  FIG. 1  rolled up to form a spiral shaped electrostatic precipitator element; 
           [0014]      FIG. 5  is a perspective illustration of the spiral-shaped electrostatic precipitator element of  FIG. 4 ; 
           [0015]      FIG. 6  is a schematic illustration in a cross-sectional view, of an electrostatic precipitator; and 
           [0016]      FIG. 7  is a schematic illustration in a side cross-sectional view, of another electrostatic precipitator. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    In  FIG. 1 , a piece of mesh indicated by numeral  10  is made of, for example, two interwoven insulated electric conductors  12  and  14 , such that sections of the respective conductors  12 ,  14  define openings  16  of the mesh  10 . The electric conductors  12  and  14  consist of a wire  18  or  20  wrapped by a layer of insulation (not indicated) such that no electric current flows between the two conductors  12 ,  14  when a steady DC voltage is applied across the wires  18 ,  20  of the respective conductors  12 ,  14 . 
         [0018]    However, an electric field is created in each of the openings  16  of the mesh  10  when the wires  18 ,  20  of the respective electric conductors  12 ,  14  are connected to a source of electric voltage, for example, an AC voltage with relatively high voltage magnitude thereof, as illustrated in  FIG. 1 . The mesh  10  with insulated electric conductors  12 ,  14  connected to the source of the AC electric voltage, adapted to function as an electrostatic precipitator element, is then disposed in an environment so as to allow an aerosol flow such as an oil/air mixture (not shown) to pass through the openings  16  of the mesh  10 . Because of the AC voltage applied to the wires  18 ,  20  of the respective electric conductors  12 ,  14 , alternating electric fields are created in the openings  16 , periodically changing the polarities of the electric fields. If the frequency of the AC voltage is expressed as 1/T, the polarity of the electric fields is reversed every T/2 seconds (or milliseconds, as the case may be). The particles (not shown) suspended in the aerosol flow, for example the oil particles suspended in air, are electrically charged by the displacement current when the oil/air mixture flows through the openings  16  of the mesh  10 . The electrically charged oil particles suspended in the air flow are attracted to one of the electric conductors  12 ,  14  which has a polarity opposite to the charges of the oil particles. The electrically charged oil particles reach the surface of the insulation of one of the electric conductors  12 ,  14 , and then are accumulated thereon due to the viscosity thereof. When the polarities of the electric field in each of the openings  16  is reversed after the time period T, the newly electrically charged oil particles suspended in a subsequent portion of the oil/air mixture flow are attracted to the other of the electric conductors  12 ,  14  which now has a polarity opposite to that of the charges of the oil particles. Therefore, the electrically charged oil particles are alternately attracted to the insulated surfaced of the respective electric conductors  12 ,  14 . 
         [0019]    In contrast to conventional electrostatic precipitators in which charged particles are neutralized on the conductive surface of an electrode having a polarity opposite to that of the charged particles, the charged oil particles of the present concept cannot be neutralized upon contact with one of the electric conductors  12 ,  14  because the electric conductors  12 ,  14  are insulated by the outer layer of insulation (this is a somewhat similar effect to the familiar experience of a charged birthday balloon sticking to another insulated surface, such as a wall). However, an alternating electric field created by the AC voltage, not only periodically converts its polarities but also periodically changes magnitude. In particular, the magnitude of the voltage of the electric field increases from zero to a maximum level and then decreases to zero in the first half of the time period T and then the polarities of the electric field reverse and the magnitude of the voltage thereof also increases from zero to the maximum level and then decreases to zero in the second half of the time period T. Thus, oil particles in the electric field are periodically electrically charged and neutralized. The oil droplets will stay on the surface of the insulation of the respective electric conductors  12 ,  14  by the oil&#39;s viscosity and or surface tension and thus will not be repelled by the electric conductor on which the oil particles are accumulated when that electric conductor reverses its polarity. The oil droplets are accumulated to a point at which the oil droplets drip from the mesh  10  under the force of gravity. 
         [0020]    The mesh  10  to be used as an electrostatic precipitator of the present concept, functions similarly to a capacitor which forms a closed circuit in order to allow a displacement current to flow through when connected to a source of AC voltage. In order to improve the performance of the electrostatic precipitator element, it is preferable to choose a highly dielectric insulator material for use as the insulation of the electrical conductors  12 ,  14 . In further consideration of the working environment of the electrostatic precipitator element of the present concept, particularly relating to gas turbine engines, Teflon™ is preferable as the dielectric insulation of the electric conductors  12 ,  14 . 
         [0021]    It is not recommended to use a source of AC voltage having a very high frequency such that most of the electrically charged oil particles attracted towards the electric conductors  12 ,  14 , will be repelled by the same one of the electric conductors because of a polarity reversal of that electric conductor before they reach their destination. Therefore, it is preferable that a time interval (travel t) needed for the electrically charged oil particles to travel through the alternating electric field to one of the electric conductors  12 ,  14  having an instant polarity which attracts the charged oil particles, is less than half the period T of the AC voltage (polarity reversal 0.5T) needed to complete one reversal of the polarities of the AC voltage. However, the travel t is determined by a plurality of factors such as the magnitude of the AC voltage and the size of an effective space of the electric field. The effective space of the alternating electric field is further determined by the physical geometry, configuration and size of the openings of the mesh. Therefore, the frequency and the magnitude of the AC voltage to be applied to the electrostatic precipitator element of the present concept, is determined depending on the particular configuration of the electrostatic precipitator element. 
         [0022]    Although a source of AC voltage is preferred to be connected to the electric conductors  12 ,  14  of the mesh  10 , a source of DC voltage may also be applicable for the mesh  10  used as an electrostatic precipitator element. For example, the wire  18  of electric conductor  12  may be connected to a positive end of a controllable DC voltage source (not shown) and the wire  20  of the electric conductor  14  is grounded or connected to a negative end of the controllable DC voltage source. In order to use the mesh  10  as an electrostatic precipitator element of the present concept, the controllable DC voltage is controlled to change the voltage magnitude periodically between zero and the maximum level in a predetermined frequency such that the electric field created in the respective openings  16  of the mesh  10  periodically changes strength between zero and a maximum level but does not change the polarities thereof. In such an application, the oil particles which are suspended in the oil/air mixture and charged in the electrical field, are attracted to the insulated surface of only one of the electric conductors  12 ,  14 . 
         [0023]    Referring to  FIGS. 2 and 3 , an electrostatic precipitator element indicated by numeral  10 ′ is described according to another embodiment. The electrostatic precipitator element  10 ′ is a modification of the mesh  10  of  FIG. 1  and similarly includes a mesh (not indicated) made of interwoven insulated electric conductors  12 ,  14  defining a plurality of openings  16  of the mesh. A third insulated electric conductor  22  extends interstitially through the openings  16  of the mesh in a woven or knitted manner as shown in  FIG. 3 . The insulated conductors  12 ,  14  and  22  have wires  18 ,  20  and  24  respectively. The respective wires  18 ,  20  and  24  are connected to a source of voltage, for example a 3-phase AC voltage, in which the respective phases  1 ,  2  and  3  of the AC voltage have the same frequency and magnitude but with a time differential (phase difference) relative one to another. In an alternative arrangement of an AC voltage connection (not shown), electric conductors  12 ,  14  can be connected to a source of AC voltage (one phase) similar to that of  FIG. 1  and the electric conductor  22  is always grounded. The added electric conductor  22  will improve the performance of the electrostatic precipitator element  10 ′ by improving the strength distribution of the electric field in each opening  16  of the mesh  10 ′. The added electric conductor  22  also increases the particle attaching surface area of the electrostatic precipitator element  10 ′, which results in an improvement in precipitation efficiency. It should be noted that the openings  16  defined by the sections of the respective electric conductors  12 ,  14  are sized large enough to not only allow the electric conductor  22  to extend therethrough but also to leave enough space around the electric conductor  22  to form adequate passages for an aerosol flow such as an oil/air mixture flow to pass through the electrostatic precipitator element  10 ′. 
         [0024]    Referring to  FIGS. 4-6 , an electrostatic precipitator generally indicated by numeral  30 , is described according to one embodiment of the present concept. The electrostatic precipitator  30  includes a housing or container  32  having an inlet  34 , disposed on one end of the container  32 , for introducing a flow of an oil/air mixture into the container  32 . The container  32  further includes an outlet, preferably a plurality of outlets  36 , disposed preferably on the other end of the container  32  spaced apart one from another along the circumference of the container  32 , for discharging an air flow, and another outlet  38  preferably disposed at a lower part of the container  32  for discharging liquid oil. 
         [0025]    The container  32  is preferably cylindrical for receiving a spiral roll of a mesh  10  of  FIG. 1 . The mesh  10  of  FIG. 1  is formed in a rectangular sheet having a width smaller than the length of the cylindrical container  32 , and is rolled along a length of the mesh sheet to form the spiral roll as shown in  FIGS. 4 and 5 . The spiral roll of mesh  10  is supported within the container  32  and is located between the inlets  34  and the first outlet  36  to permit the air/oil mixture introduced from the inlets  34 , to flow therethrough. 
         [0026]    Referring now to  FIG. 6 , in a preferred arrangement, an inner cylindrical casing  40  is provided for supporting the spiral roll of mesh  10  and for directing the oil/air mixture flow within the container  32 . The inner cylindrical casing  40  has a diameter smaller than that of the container  32  and is affixed to the container  32  by for example, support elements  42  such that the cylindrical wall of the inner casing  40  is radially spaced apart from the cylindrical wall of the container  32  to form an annular passage  43  therebetween which is in communication with the outlets  36 . Casing  40  houses electrostatic spiral roll of mesh  10 . An inlet tube  46  is preferably disposed in the center of the spiral roll of mesh  10 , supported, for example by the inner casing  40  and preferably extending axially all the way through the spiral roll of mesh  10 . The tube  46  is in communication with or is integrated with the inlet  34 , and has a plurality of preferably holes  45  extending therethrough so as to allow air-oil flow within the tube  46  to be directed and discharged through the tube  46  into mesh  10 . Similarly to the inlet tune  46 , a plurality of holes  44  are preferably defined through the cylindrical wall of the inner casing  40  and are preferably distributed over the entire area of the cylindrical wall of the inner casing  40 . The inner casing  40  may be used to assist directing the oil/air mixture flow into the container  32  from the mesh  10 , and to flow axially along the annular passage  43  to outlets  36 . A generally radial outward flow is desired because the velocity of the air-oil mixture flow will tend to reduce as it moves from the inner diameter to the outer diameter, which assists the separation effect. The lower the velocity of the air near the exit, the lower the aerodynamic forces will be on the oil causing oil to exit with the air (undesired). Positioning the device as shown in  FIG. 6 , such that the oil outlet  38  and mixture inlet  34  are both lower than the air outlet(s)  36  also allows gravity to assist the separation process. 
         [0027]    A source of AC voltage  48 , for example, is also provided and the electric conductors  12 ,  14  of the spiral roll of mesh  10  are connected to the source of the AC voltage  48 . When the oil/air mixture flow is directed through the spiral roll of mesh  10 , oil particles suspended therein will be charged by the alternating electric field created in the vicinity of the electric conductors  12 ,  14 , particularly in the individual openings  16  of the spiral roll of mesh  10  and will be attracted to the insulated surfaces of the electric conductors  12 ,  14 . The remaining portion of the flow which is a relatively pure air flow, enters the annular passage  43  and is discharged from the outlets  36  to the atmosphere, or to a predetermined location, if desired. The oil particles attracted to the surfaces of the insulated electric conductors  12 ,  14  are eventually accumulated to form larger oil droplets. These large oil droplets under gravity drip to a lower portion of the cylindrical wall of the container  32  and are collected as liquid oil to drain out of the container  32  through the outlet  38 . 
         [0028]      FIG. 7  illustrates an electrostatic precipitator  50  according to another embodiment. The electrostatic precipitator  50  includes a container  52  having an inlet  54  defined in one end thereof for introducing an oil/air mixture flow therein. The container  52  further includes a first outlet  56  on the other end thereof for discharging an air flow separated from the oil/air mixture flow entering the container  52 , and a second outlet  58  defined in a lower portion of the container  52  for discharging liquid oil separated from the oil/air mixture flow entering the container  52 . The container  52  is preferably in a rectangular prism configuration having substantially parallel side walls (not shown) and top and bottom walls (not indicated) to define a prismatic space for accommodating an electrostatic precipitator element  10 ′ of  FIG. 2 , which is multi-folded to form a plurality of layers L 1 , L 2 , . . . Ln. The three insulated electric conductors  12 ,  14 ,  22  of the electrostatic precipitator element  10 ′ (see  FIG. 2 ) are connected to a source of 3-phase AC voltage to create the desired electric fields within the container  52  such that when the oil/air mixture flow entering the container  52  through the inlet  54  passes through the plurality of layers L 1 , L 2 , . . . Ln of the electrostatic precipitator element  10 ′, the oil particles suspended in the flow are electrically charged by the electric fields and are attracted to the electrostatic precipitator element  10 ′ to form larger oil droplets thereon. Under force of gravity, the larger oil droplets drip onto the bottom wall of the container  52  and accumulate to form liquid oil which is directed to the second outlet  58  and drained out through the second outlet  58 . The remaining portion of the oil/air mixture flow which is a relatively pure air flow, is discharged through the first outlet  56 . 
         [0029]    It is preferable to position a partition plate  60  within the container  52  at the end where the inlet  54  is defined. The partition plate  60  is spaced apart from that end wall (not indicated) of the container  52  and has a plurality of holes  62  therethrough. The partition plate  60  is used to redistribute the oil/air mixture flow entering the container  52  through the inlet  54 , to the entire cross-section of the container  52 , before passing through the multiple layers of the electrostatic precipitator element  10 ′ in order to improve the performance of the electrostatic precipitator element  10 ′. 
         [0030]    The electrostatic precipitator  50  may be positioned either in a vertical position or a horizontal position which is shown in  FIG. 7  as a choice of description only. If the electrostatic precipitator  50  is in a vertical position, the outlet  58  and inlet  54  both are located in a lower end of the container  52  while the outlet  56  is located in an upper end of the container  52 . 
         [0031]    The electrostatic precipitator of the present concept uses woven insulated conductors such that significantly lower voltages are applied to the insulated electrodes in close proximity, in contrast to very high voltages applied to electrodes without insulation layers in conventional electrostatic precipitators, and can result in equal or higher strengths of electric fields. In addition, the insulated layers of the electrodes substantially eliminate the risk of arcing which may be problematic in some applications of conventional electrostatic precipitators where the fluids or gas-particle mixture may be ignited by a spark. 
         [0032]    This present concept permits oil/air separation in a gas turbine engine where a compact and arc-free system is preferable. 
         [0033]    The above description is meant to be exemplary only and one skilled in the art will recognize that changes may be made to the embodiments described without departure from the scope of the concept disclosed. For example, alternative to a mesh woven by insulated conductors, the electrostatic precipitator element may be otherwise made of two or more insulated conductors in a preferably close relationship of any configuration (i.e. not necessarily an organized mesh) to define air passages therebetween and to create a preferably substantially continuous region of electric field having sufficient strength to achieve the separation function described herein in the vicinity of the insulated conductors. Furthermore, the electrostatic precipitator element may be configured in any desirable form, such as to increase the surface areas of the insulated surfaces of the respective conductors in order to attract the electrically charged oil particles. The electrostatic precipitator can also be positioned in any orientation and not just those described. Although the electrostatic precipitator is described in an application of separating oil from an oil/air mixture flow, it should be understood that the system is applicable in general for separation of suitable liquids or solid particles from gases. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure and such modifications are intended to fall within the appended claims.