Abstract:
A centrifugal impeller includes a body of antistatic plastic and a back plate made of metal, preferably aluminium. The back plate is fixed to the body is such a way that the back plate can not make accidental contact with a volute of a blower in which the impeller is deployed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional patent application claims priority under 35 U.S.C.§119(a) from Patent Application No. 201010103259.0 filed in The People&#39;s Republic of China on Jan. 27, 2010. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to a centrifugal impeller and in particular, to a centrifugal impeller having antistatic properties for use in a premix blower for a gas fired water heater. 
       BACKGROUND OF THE INVENTION 
       [0003]    A prior art centrifugal impeller for a premix blower for a gas fired water heater is made from metal and includes a back plate, a cover plate with an inlet opening and vanes connected to the back plate and the cover plate. The impeller is arranged inside a volute of the blower and is fixed to a shaft of a motor by way of the back plate. The inlet of the cover plate is located adjacent an inlet of the volute. In operation, a mixture of gas and air flows into the volute through the inlet, passes through the passages formed between the vanes, and flows out from an outlet of the volute. As the impeller is exposed to the mixture of gas and air, the impeller and the volute have antistatic properties to avoid a build up of static on the impeller, which otherwise may result in a static discharge igniting the gas and air mixture. The back plate, vanes and cover plate are made of the same material. 
         [0004]    Japanese published Patent Application No. JP9-126185 discloses an impeller whose back plate, cover plate and vanes are all made of aluminum wherein two side edges of the vanes are respectively fixed to the cover plate and back plate. The weight of the impeller is high and its assembly process is complex. Also, accidental contact between the impeller and the volute may produce sparks with the potential to ignite the gas air mixture. To solve these problems an impeller with a back plate, cover plate and vanes all made of antistatic plastic was developed. However its cost is many times the cost of the aluminum impeller. 
       SUMMARY OF THE INVENTION 
       [0005]    Hence there is a desire for an antistatic impeller which is light, will not generate sparks if it makes contact with the volute when in use and is cost effective. 
         [0006]    Accordingly, in one aspect thereof, the present invention provides a centrifugal impeller comprising a body and a back plate mounted to the body, wherein the body is made of antistatic plastic and the back plate is made of metal. 
         [0007]    Preferably, the body is a monolithic construction of antistatic plastic. 
         [0008]    Preferably, the back plate is made of aluminium. 
         [0009]    Preferably, the body comprises a cover plate and a plurality of vanes which extend between the cover plate and the back plate. 
         [0010]    Preferably, the body further comprises an outer ring which is connected to radially outer ends of the vanes; and the back plate is arranged inside the outer ring. 
         [0011]    Preferably, the outer ring extends axially beyond the back plate. 
         [0012]    Preferably, the back plate is fixedly connected with a radially inner surface of the outer ring by plastic deformation of the outer ring. 
         [0013]    Preferably, the back plate has a plurality of through holes and the vanes have protrusions which extend through the through holes. 
         [0014]    Preferably, the protrusions fixedly connect the back plate to the vanes. 
         [0015]    Preferably, the body further comprises an inner ring which is connected to radially inner ends of the vanes; and the back plate has a central hole having a periphery fixedly connected with the inner ring. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    A preferred embodiment of the invention will now be described, by way of example only, with reference to figures of the accompanying drawings. In the figures, identical structures, elements or parts that appear in more than one figure are generally labelled with a same reference numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
           [0017]      FIG. 1  is an exploded view from above of a centrifugal impeller according to the preferred embodiment of the present invention; 
           [0018]      FIG. 2  is an exploded view from below of the impeller of  FIG. 1 ; and 
           [0019]      FIG. 3  is a sectional view of the assembled impeller. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    The preferred centrifugal impeller comprises a body  10  and a back plate  60  mounted to the body  10 . One of the body  10  and the back plate  60  is made of plastic and the other is made of metal. 
         [0021]    Preferably, it is the body  10  that is made of antistatic plastic and the back plate  60  is made of metal. The body  10  comprises a cover plate  20  opposite to the back plate  60  and a plurality of vanes  30  which are connected to the cover plate  20  on one side. An air inlet  22  is formed at the center of the cover plate  20 . The outer radial ends of the vanes  30  extend beyond the outer periphery of the cover plate  20 . Preferably, the body is formed as a single piece item or monolithic construction by injection molding. 
         [0022]    The body  10  further comprises an outer ring  40  which is connected to the outer radial ends of the vanes  30 . The outer ring  40  is in an axial extended tubular structure. The back plate  60  is disposed inside the outer ring  40  and may be fixedly connected with the inner surface of the outer ring  40  by bonding, welding or preferably by plastic deformation of the outer ring. Preferably, the outer ring  40  is provided with an axial end (the end remote from the cover plate  20 ) which extends beyond the back plate  60  in the axial direction. This arrangement prevents the outer edge of the back plate from accidentally making contact with the volute. The inner radius of the outer ring  40  is larger than the outer radius of the cover plate  20  to facilitate removing the body  10  from the molding die. 
         [0023]    The vanes  30  are provided with protrusions  32  on the side remote from the cover plate  20 . The back plate  60  is provided with through holes  62  corresponding to the protrusions  32  for passage of the protrusions  32 . Preferably, the protrusions  32  extend through the holes  62 , beyond the back plate  60  and are fixedly connected with the back plate  60 . Preferably, the protrusions  32  are hot staked or otherwise plastically deformed to fix the back plate to the body. 
         [0024]    The body  10  further comprises a hub  50  which comprises a mounting post  52  with a mounting hole and a cone  54  extending from the mounting post  52 . An inner ring  56  is formed at the end of the cone  54 . The impeller is connected to the motor by fitting and fixing the mounting post  52  to the motor shaft, thus the impeller rotates with the shaft, in a manner generally known. The inner radial ends of the vanes  30  are connected to the outer surface of the cone  54 . The back plate  60  is provided with a central hole  64  whose periphery is fixedly connected with the inner ring  50 , preferably by plastic deformation of the inner ring. Thus the inner edge of the back plate is prevented from making accidental contact with the volute. 
         [0025]    Preferably, the back plate  60  is made of aluminum, ideally by stamping a sheet of aluminum. 
         [0026]    In the preferred embodiment of the present invention, the body  10  and the back plate  60  are respectively made of plastic and metal, so the weight of the impeller is lighter and the cost is lower than an impeller made entirely of antistatic plastic. The impeller may be used in a premix blower for a gas fired water heater, because the axial end of the outer ring  40  and/or the protrusions  32  extend beyond the back plate  60 , preventing direct contact between the metal back plate  50  and a metal volute of the blower. Such direct contact may generate sparks which would ignite the gas/air mixture with catastrophic results, should the impeller become dislodged during use. 
         [0027]    In the description and claims of the present application, each of the verbs “comprise”, “include”, “contain” and “have”, and variations thereof, are used in an inclusive sense, to specify the presence of the stated item but not to exclude the presence of additional items. 
         [0028]    Although the invention is described with reference to one or more preferred embodiments, it should be appreciated by those skilled in the art that various modifications are possible. Therefore, the scope of the invention is to be determined by reference to the claims that follow.