Abstract:
A compressor assembly includes compressor means having an inlet, an outlet and a means for drawing air through the inlet and exhausting compressed air through the outlet, rotary drive input means, and gear means for changing the speed of the rotary drive input means and having an input connected to the rotary drive input means and an output driving the compressor means, the gear means comprising planetary gear means.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of United Kingdom (UK) Patent Application No. 0919643.7, filed on Nov. 10, 2009, and entitled Compressor Assembly, which is incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to compressors, and more particularly to a compressor assembly. 
         [0003]    Tankers carrying dry bulk materials, such as cement, flour, plastic pellets, etc., are unloaded by pressurizing the tank with air, typically up to 3 bar absolute pressure. For a road tanker the preferred mounting of the compressor is normally within the chassis of a towing tractor unit. The drive for the compressor is provided by means of a power take off attached to the aperture of the engine gear box of the tractor unit and connected to the compressor via a prop-shaft. 
         [0004]    Typically, but not exclusively, screw-type compressors are used. In order to achieve the necessary volumetric efficiency, the screws of a screw compressor must run at relatively high speeds, of the order of 7000 to 9000 rpm. However, the maximum speed of the vehicle power take-off shafts is typically between 1600 and 1800 rpm. Consequently, the speed of the power take-up shaft must be increased by a factor of approximately in 4.5 in order to produce the required speed for the compressor. 
         [0005]    Existing screw compressors used on road tankers achieve the step-up by means of a very large gear wheel mounted on the power take-off shaft which engages with a very small pinion connected to the compressor. However, the large gear wheel takes up a great deal of space, making the compressor very large and heavy and in some cases making it impossible for the compressor to fit inside the chassis of the tractor unit. In some cases, it is necessary to mount the compressor outside the chassis and to use alternative drives such as hydraulic drives, belt drives, separate engine drives, electric motor drives and the like, which adds substantially to the cost, weight and maintenance of the installation. 
         [0006]    In addition, in recent years, the use of three-axle tractor units has increased, reducing still further the space available to mount the compressor. 
         [0007]    Moreover, many haulage operators like to fit larger fuel tanks on the chassis to allow fuel to be bought in the country where it is cheapest. This reduces still further the space available for mounting the compressor or, alternatively, reduces the size of fuel tanks which can be mounted on the chassis. 
         [0008]    What is needed is a compact compressor assembly. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention relates to compressors, and more particularly to a compressor assembly. 
         [0010]    In general, in one aspect, the invention features a compressor assembly including compressor means having an inlet, an outlet and a means for drawing air through the inlet and exhausting compressed air through the outlet, rotary drive input means, and gear means for changing the speed of the rotary drive input means and having an input connected to the rotary drive input means and an output driving the compressor means, the gear means comprising planetary gear means. 
         [0011]    In another aspect, the invention features a system having a flow path for fluid, the system including pump means for pressurizing fluid for flowing along the flow path, cooling means through which the flow path passes for dispersing heat from the flowing fluid, a fluid-operated motor actuated by the pressurized fluid, and a fan driven by the fluid-operated motor for generating an air current passing over the cooling means. 
         [0012]    Other features and advantages of the invention are apparent from the following description, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention will be more fully understood by reference to the detailed description, in conjunction with the following figures, wherein: 
           [0014]      FIGS. 1(   a ) and  1 ( b ) are a side view and end view respectively of an exemplary screw compressor in accordance with the present invention; 
           [0015]      FIG. 2  is a longitudinal cross-section through the exemplary screw compressor of  FIG. 1 ; 
           [0016]      FIG. 3  is a cross-section through the exemplary screw compressor of  FIG. 1 , looking in the direction of arrows X-X of  FIG. 2 ; 
           [0017]      FIG. 4  is a schematic representation showing an exemplary operation of the exemplary screw compressor of  FIG. 1 ; 
           [0018]      FIG. 5  is a schematic representation showing a first variant of the exemplary screw compressor of  FIG. 1 ; and 
           [0019]      FIG. 6  is a schematic representation showing a second variant of the exemplary screw compressor of  FIG. 1 . 
       
    
    
       [0020]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0021]    As shown in  FIGS. 1 to 4 , a screw compressor includes an elongate metal housing  10  having a base wall  12 , two generally parallel side walls  14 ,  16  extending perpendicularly from the base wall, two inclined shoulder portions  18 ,  20  at the upper end of each of the side walls and a top wall  22  extending parallel to the base wall  12 . Two parallel end walls  24 ,  26  extending perpendicularly to the base, side and top walls  10 ,  14 ,  16 ,  22  are located one at each end of the housing. 
         [0022]    The housing  10  encloses a conventional twin helical screw compressor  28 , having two helical screws, one female  28   a  and the other male  28   b  shown schematically in the drawings, which has an inlet  30  and outlet  32  in the housing  10 . The screw compressor is driven by means of a step-up gear box  34 , which is typically driven by a power take-off via prop shaft (not shown) of a lorry of other vehicle. 
         [0023]    The gear box  34  has two input shafts  36 ,  38 , extending through one of the end walls  24  and offset both vertically and horizontally with respect to each other. Each input shaft  36 ,  38  is mounted in bearings  42  in the housing (only the bearings for the upper shaft  36  are visible in  FIG. 2 ) and each input shaft  36 ,  38  carries one of two identical spur gears  44 ,  46  which meshes with the spur gear carried by the other input shaft, which ensures that both input shafts  36 ,  38  rotate, irrespective of which one is driven. 
         [0024]    It will be noted that the upper input shaft  36  and lower input shaft  38  rotate in opposite directions. This allows the compressor assembly to be used without modification, irrespective of the direction of rotation of the power take-off shaft. In practice, the shaft of the correct rotational sense is connected to the power take-off shaft and the other unused shaft is covered by a removable cap  40 . 
         [0025]    The upper input shaft  32  is connected to the carrier  50  of a planetary gear set. Three equally-spaced identical planet gears  52  are rotatably mounted on the carrier  50  by means of bearings  54  and engage with an annulus  56  which is fixed with respect to the housing  10 . The planet gears  52  also mesh with a sun gear  58  which is connected to an output shaft  60  of the planetary gear set P which forms the input to the female helical screw  28   a  of the helical screw compressor  28 . The output shaft  60  is mounted in bearings  62  in the housing  10  and carries a spur gear  64  which meshes with a spur gear  66  of an input shaft  68  of the male helical screw  28   b  of the screw compressor  28 . The male input shaft  68  is mounted in bearings  70  in the housing and is aligned parallel to the input shaft  60  of the female helical screw  28   b . The opposite ends of the helical screw conveyors  28   a ,  28   b  are also mounted in bearings  72 ,  74  in the end wall  26  remote from the input shafts  36 ,  38 . 
         [0026]    The spur gear  64  of the input shaft  60  of the female helical conveyor  28   a  is larger than the spur gear  66  of the input shaft  68  of the male helical conveyor  28   b , whereby the male helical conveyor  28   a  runs faster than the upper helical conveyor. The step-up ratio in the embodiment described is 5:3, but a different step-up ratio, a step-down ratio or a 1:1 ratio may be used instead, depending on the circumstances and in particular depending on the model of helical screw compressor. 
         [0027]    In use, the screw compressor is located in the appropriate position, typically on the tractor unit of an articulated lorry, and inlet and output pipes (not shown) are connected to the inlet and outlet  30 ,  32  respectively. A prop-shaft (not shown) connects the power take-off to one of the two input shafts  36 ,  38  of the required direction of drive (clockwise or anticlockwise). The protective shaft extension cover is secured over the other, “unused shaft”. 
         [0028]    To start up the screw compressor, the power take-off is engaged. If the power take-off shaft is connected to the upper input shaft  36 , the carrier  50 , to which the input shaft is connected, is rotated at the same speed as the input shaft. The interaction of the carrier  50 , planet gears  52 , fixed ring gear  56  and sun gear  58  produce a step-up ratio, typically of the order of 4.5:1 (although this could be much larger or much smaller) and cause the output shaft  60  of the planetary gear set P to rotate much more quickly than the input shaft  36 . The female helical rotor  28   a  rotates at the same speed as the shaft  60  and, by virtue of the inter-engaging spur gears  64 ,  66  on the input shafts  60 ,  68  of the female and male rotors  28   a ,  28   b , the male rotor is also rotated. 
         [0029]    If the prop-shaft is connected to the lower input shaft  38  (with the protective shaft extension cover secured over the protruding end of the upper input shaft  36 ), the carrier  50  is still rotated at the same speed, by virtue of the meshing of the spur gears  44 ,  46  on the upper and lower input shafts  36 ,  38 . Operation of the screw compressor is therefore independent of which input shaft  36 ,  38  is connected to the prop-shaft. However, the rotation of the rotors  28   a ,  28   b  is fixed by design and the power take-off rotation can be clockwise or anticlockwise. It is therefore important that the input shafts  36 ,  38  facilitate rotation in both directions. 
         [0030]    The use of a planetary step-up gear set allows the overall size of the compressor assembly to be compact, which is particularly important if space for installation is limited, for example on the tractor unit of a lorry. 
         [0031]    A first variant of the embodiment of  FIGS. 1 to 3  is shown in  FIG. 4 . The screw compressor is identical to that of  FIGS. 1 to 3  and therefore has only been illustrated schematically. The differences from the first embodiment relate to the lubrication system. 
         [0032]    The lubrication system includes an oil pump  80  driven by the lower input shaft  38 . The pump  80  draws oil via a filter  81  from a sump  82  in the base of the housing  10  and pumps it out of the housing via a connecting hose  84  to a hydraulic motor  86 . A pressure relief valve  88  is also located between the pump  80  and the hydraulic motor  86  in order to protect the pump for excessively high pressures which can occur, for example, if the oil is viscous as a result of low ambient temperature at start-up of the compressor. 
         [0033]    The hydraulic motor  86  drives a fan  90  for an oil cooler  92  located downstream of the hydraulic motor (although it could be mounted upstream of the hydraulic motor instead). The cooled oil is then returned to the compressor housing  10  via a hose  94 , and is used to lubricate various components of the screw compressor in the normal way, via a number of flow restrictors R. 
         [0034]    By using a cooling fan which is driven by the pressurized oil, the cooling is not dependent on an external source of power, e.g. electricity, to drive the fan. Moreover, hydraulic systems generally have a high reliability, which minimizes the likelihood of breakdowns with the cooling system. 
         [0035]    A further variant is shown in  FIG. 6 . This variant includes all of the features of  FIG. 5  and corresponding components have been given the same reference numerals. However, the arrangement of  FIG. 6 . includes a further hydraulic motor  96 , between the first hydraulic motor  86  and the oil cooler  92 , which drives a second fan  98  for an air cooler  100  for cooling compressed air emerging from the air outlet  32  of the compressor. 
         [0036]    The invention is not restricted to the details of the foregoing embodiments. For example, although the embodiments refer to a screw compressor, other types of compressor could be used instead, for example a hook and claw compressor. Moreover, although the embodiments are particularly useful when used on a lorry, the invention is not restricted to such use and the compressor could be used in other locations, such as in general industrial use. 
         [0037]    It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the following claims.