Patent Publication Number: US-8123493-B2

Title: Screw compressor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a screw compressor. 
     2. Description of the Related Art 
     In a conventional screw compressor having a pair of male and female screw rotors with rotor shafts disposed in the horizontal direction, a timing gear and bearings are mounted on each of the rotor shafts and are lubricated and cooled with oil to prevent damage thereof. 
     In connection with the related art, an oil bath method is mentioned as a general lubricating method used in case of low- or medium-speed rotation. In the oil bath method it is desirable to install an oil gauge and thereby make it possible to check the oil level easily so that the oil level lies at the center of a bottom roller in principle. However, when it is intended to adopt the oil bath method for lubrication of bearings and timing gears in a positive-displacement compressors such as, for example, a screw compressor, it may not always be possible to install an oil gauge at such a position as permits easy visual checking of the oil level due to, for example, a unique external shape of a bearing casing for accommodating the bearings and timing gears. 
     In Japanese Patent No. 2580020, compressed air is taken out from a casing of an air compressor and is conducted to a throttle portion of an oil mist producer, then with a negative pressure generated in the throttle portion, lubricating oil is sucked up and made into oil mist, then oil mist is fed for lubrication to bearings and timing gears accommodated within a casing. According to this conventional method it is possible to avoid exhaustion of the lubricating oil, but there remains the problem that the position of oil level cannot be confirmed. 
     In Japanese Patent Laid-Open No. 2003-148370, an oil level sensor is mounted within a casing of a scroll compressor, and when the oil level becomes lower than the position of the oil level sensor, a valve opens in accordance with a signal provided from the oil level sensor, and lubricating oil is supplied from an oil container which stores the lubricating oil. In the scroll compressor, however, rotor shafts are disposed in the vertical direction, and if the lubricating method is applied to a screw compressor with rotor shafts disposed in the horizontal direction, the oil level varies greatly due to splash of oil by timing gears and it is difficult to effect an accurate oil level detection. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a screw compressor having rotor shafts disposed horizontally and bearings for supporting the rotor shafts, the bearings being lubricated by an oil bath method, the screw compressor permitting accurate confirmation of the level of oil present within a bearing casing and being not likely to cause oil shortage in the bearings. 
     According to the present invention, as means for achieving the above-mentioned object, there is provided a screw compressor comprising a pair of rotor shafts disposed horizontally; bearings for supporting the rotor shafts; a bearing casing for accommodating the bearings; an oil sump formed in a bottom of the bearing casing, the oil sump being structured so as to allow a lower portion of the bearings to be soaked into oil for lubrication; a chamber provided separately from the bearing casing; an oil line for communication between the oil sump and the chamber; and oil level detecting means disposed in the chamber. 
     When rotors are rotating in the screw compressor, oil level detecting means disposed in the interior of the screw compressor cannot be utilized because the oil level at the bottom of the bearing casing is oscillating due to splash of oil. However, according to the aforesaid means, since there is provided an oil line for communication between the bearing casing and the chamber separate from the bearing casing, the oil level can be detected accurately with the oil level detecting means disposed in the chamber which is provided separately from the bearing casing in the compressor, without being influenced by variations in oil level in the interior of the compressor. 
     Preferably, there is provided a communication line for communication between a space positioned above an oil level in the chamber and a space positioned above an oil level in the bearing casing. With this structure, the oil level can be detected accurately with the oil level detecting means disposed in the chamber which is provided separately from the bearing casing in the compressor, without being influenced by variations in oil level in the interior of the compressor. 
     Preferably, a pair of timing gears are accommodated within the bearing casing, the timing gears being mounted on one ends of the rotor shafts and meshing with each other, and the position of connection between the oil sump formed in the bearing casing with the timing gears accommodated therein and the oil line corresponds to a lower-limit height of oil level of the oil sump permitting lubrication of the bearings. With this structure, lubrication can be done while allowing a timing gear lower portion to be soaked into oil. 
     Preferably, there is provided an oil supply line for the supply of oil to the bearing casing, and when the oil level detecting means detects as a value of the oil level in the chamber a value lower than a preset first value indicative of a lower limit, oil is fed into the bearing casing through the oil supply line. With this structure, the oil level can be detected by the oil level detecting means disposed in the chamber separate from the bearing casing without being influenced by variations in oil level in the interior of the casing and it is possible to replenish a required amount of oil into the casing through the chamber and the oil supply line. 
     Preferably, there is provided an oil discharge line for the discharge of oil from the bearing casing, and when the oil level detecting means detects as a value of the oil level in the chamber a value higher than a preset second value indicative of an upper limit, oil is discharged from the bearing casing through the oil discharge line. With this structure, the oil level can be detected by the oil level detecting means disposed in the chamber separate from the bearing casing without being influenced by variations in oil level in the interior of the casing and it is possible to discharge a required amount of oil from the interior of the casing. 
     Preferably, there are provided an oil supply line for the supply of oil to the bearing casing, an oil discharge line for the discharge of oil from the bearing casing, and oil temperature detecting means disposed in the oil line, and when the oil level detecting means detects as a value of the oil level in the chamber a value lower than a preset first value indicative of a lower limit, oil is fed into the bearing casing through the oil supply line, while when the oil level detecting means detects as a value of the oil level in the chamber a value higher than a preset second value indicative of an upper limit, oil is discharged from the bearing casing through the oil discharge line, further, when the oil level detecting means detects as a value of the oil level in the chamber a value higher than a preset third value lying between the first value and the second value and when the oil temperature detecting means detects as a value of the oil temperature a value higher than a preset value, the oil is discharged from the bearing casing through the oil discharge line. With this structure, by further providing oil temperature detecting means in the oil supply line, it is possible to discharge oil on the basis of the value of oil level and that of oil temperature. 
     According to the present invention, when rotors disposed horizontally of the screw compressor are rotating, the oil level detecting means disposed in the interior of the compressor cannot be utilized because the oil level at the bottom of the bearing casing is oscillating, but since there are provided a chamber separate from the bearing casing in the compressor, as well as an oil line and a communication line both for communication between the bearing casing and the chamber, the level of oil present in the interior of the compressor can be detected and checked accurately by the oil level detecting means detecting the oil level in the chamber separate from the bearing casing in the compressor without being influenced by variations in oil level in the interior of the compressor. Consequently, it is possible to avoid the occurrence of oil shortage in the bearings. 
     Besides, since oil is replenished in combination with oil level control, it is possible to prevent damage of the bearings caused by lowering of the oil level. 
     The chamber separate from the compressor can be of a simple structure serving as both a structure for detecting the oil level in the oil sump formed in the bearing casing and a structure for the supply of oil. 
     If the amount of oil present in the interior of the compressor becomes too large, the oil temperature rises as a result of agitation of the oil and it is possible that there may occur damage of the bearings due to lowering of viscosity. However, such a possibility can be eliminated by making control based on the value of oil level and that of oil temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram (partially in section) of a screw compressor according to an embodiment of the present invention; and 
         FIG. 2  shows differences from the atmospheric pressure at various positions (heights) from the bottom of a discharge-side bearing casing during operation of the compressor in the case where an oil line and an oil sump formed in the bearing casing which accommodates timing gears are connected with each other at the height of a trip oil level. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention will be described below with reference to the accompanying drawings. 
       FIG. 1  illustrates a screw compressor  1  according to an embodiment of the present invention. The screw compressor  1  is an oil lubrication type screw compressor wherein screw rotors  2  and  3  are cooled and lubricated with oil. In the screw compressor  1 , a pair of male rotor  2  and female rotor  3  meshing with each other are accommodated within a rotor casing  4 . Rotor shafts  5  and  6  of the rotors  2  and  3  are disposed horizontally. One end of the rotor casing  4  is closed with a cover  7 , while to an opposite end of the rotor casing  4  is mounted a motor casing  11  which accommodates a motor  10  composed of a rotor  8  and a stator  9 . An end portion of the motor casing  11  is also closed with a cover  12 . The rotor shaft  5  of the male rotor  2  and a motor shaft  13  of the motor  10  share an integrally-formed shaft (separate rotor shaft  5  and motor shaft  13  may be coupled together using a coupling (not shown) or the like). A screw rotor-side end portion of the rotor shaft  5  is supported by the rotor casing  4  through a rotor-side rolling bearing  14 , an intermediate portion of the rotor shaft  5  located between the male rotor  2  and the motor  10  is supported by the rotor casing  4  through an intermediate rolling bearing  15 , and a motor-side end portion of the rotor shaft  5  is supported by the motor casing  11  through a motor-side rolling bearing  16 . Both ends of the rotor shaft  6  of the female rotor  3  are supported by the rotor casing  4  through rolling bearings  17  and  18 . The rotor casing  4  has a discharge-side bearing casing  21  for accommodating the bearings  14  and  17  and a suction-side bearing casing  22  for accommodating the bearings  15  and  18 . Oil sumps  25  and  26  are formed in the bearing casings  21  and  22 , respectively, to lubricate the bearings  14 ,  15 ,  17  and  18  in accordance with an oil bath method (or an oil bath splash lubrication method). 
     Spaces positioned above oil levels  27  and  28  in the suction-side bearing casing  22  and the discharge-side bearing casing  21  are in communication with each other through a communication line  29  and further communicate with the atmosphere. Lip seals  30  are provided on the rotor shafts  5  and  6  for partitioning between the interior and the exterior of the rotor casing (a compressing space) in which the rotors  2  and  3  are accommodated. Timing gears  31  and  32  are fixed to an end of the male rotor  2  and an end of the female rotor  3 , respectively, so as to mesh with each other. An air suction port  33  and an air discharge port  34  are formed in the rotor casing  4 . 
     A chamber  40  is provided in the exterior of the screw compressor  1  separately from the suction-side bearing casing  22  and the discharge-side bearing casing  21 . Oil is poured into the chamber  40  as will be described later. A space positioned above an oil level  41  in the chamber  40  is in communication through the communication line  29  with the spaces positioned above the oil levels  27  and  28  in the compressor bearing casings  21  and  22 . The communication line  29  is also in communication with the atmosphere. The space in the chamber  40  may be opened to the atmosphere without communication with the communication line  29 . An oil level sensor (oil level detecting means)  42  is provided in the chamber  40 . 
     The bottoms (oil sumps)  25  and  26  of the suction-side bearing casing  22  and the discharge-side bearing casing  21  in the screw compressor  1  and the bottom of the chamber  40  are in communication with each other through an oil line  43 . The oil sump  25  in the bearing casing  21  in which the timing gears  31  and  32  are accommodated and the oil line  43  are connected with each other at the height of a lower limit (here designated a trip oil level  48 ) of the oil level  27  in the oil sump  25  which permits lubrication of the bearings  14  and  17  as shown in  FIG. 2 . According to an experiential knowledge, this oil level lower limit (i.e., the trip oil level) is usually set to a position which passes approximately the center of a lower roller  49  (see  FIG. 2 ) of the bearing  17 . Although the oil line  43  also serves as an oil supply line, an oil supply line  47  separate from the oil line  43  may be connected to the bearing casings  21  and  22 . An oil temperature sensor  44  capable of detecting the temperature of the oil temperature To is provided in the oil line  43 . 
     An oil discharge line  45  branches from the oil line  43  and an opening/closing valve  46  for the discharge of oil is provided in the oil discharge line  45 . 
     Oil is fed from an oil tank  50  for storing oil, to the chamber  40  through an oil pump  51  and an oil cooler  52 . 
     In the oil lubrication type screw compressor  1  there is provided a control unit  60  which controls the oil pump  51  and the opening/closing valve  46  for the discharge of oil in accordance with detection signals provided from the oil level sensor  42  and the temperature sensor  44 . 
     In the screw compressor  1  structured as above, upon turning ON of the motor  10 , the male rotor  2  rotates via the rotor shaft  5 , and further the female rotor  3  rotates via the timing gears  31  and  32 . With rotation of the male and female rotors  2 ,  3 , the air which has been sucked from the suction port  33  into the rotor casing (compressing space) with the rotors  2  and  3  accommodated therein is compressed and discharged from the discharge port  34 . 
     Since the bottom of the chamber  40  and the bottoms (oil sumps)  25 ,  26  of the suction-side bearing casing  22  and the discharge-side bearing casing  21  are in communication with each other through the oil line  43 , and the oil level  41  in the chamber  40  and the oil levels  27 ,  28  in the bearing casings  21 ,  22  are subjected to the same pressure through the communication line  29 , the oil levels  27 ,  28  and  41  are at the same height in accordance with Pascal&#39;s principle. Actually, however, it has been known that in the case where the oil sump  25  in the discharge-side bearing casing  21  and the oil line  43  are connected with each other at the bottom, there occurs a discrepancy in height among the oil levels  27 ,  28  and  41 , if the oil is agitated with the timing gears  31  and  32 . 
     According to an experimental result obtained in a proof test (using a screw compressor  1  according to the present invention not provided with the oil line  43 ) conducted by the present inventors, under the operating conditions of 5000 rpm, 70° C., or 1000 rpm, 4° C., the pressure near the bottom of the discharge-side bearing casing  21 , especially the pressure near the bottom and on the side more distant from the screw rotors  2  and  3  in the discharge-side bearing casing  21 , was found to be a negative pressure of about −150 mmAq in comparison with the atmospheric pressure. 
     Judging from this experimental result, oil moves from the chamber  40  toward the discharge-side bearing casing  21  through the oil line  43  during operation due to a pressure difference developed between the chamber  40  held at the atmospheric pressure and the vicinity of the bottom of the discharge-side bearing casing  21  locally held at a negative pressure, with the result that a difference occurs between the height of the oil level  41  in the chamber  40  and that of the oil level  27  in the discharge-side bearing casing  21 . The differences in height among the oil levels  27 ,  28  and  41  vary depending on various conditions (e.g., the viscosity of oil and the number of revolutions of the timing gears  31  and  32 ). 
     On the other hand, in the proof test conducted by the present inventors, it was confirmed that if the position of connection between the oil sump  25  in the bearing casing  21  with the timing gears  31  and  32  accommodated therein and the oil line  43  was made equal to the height of the trip oil level  48 , there occurred little difference between the height of the oil level  41  in the chamber  40  and that of the oil level  27  in the discharge-side bearing casing  21  in a state in which the oil levels  27 ,  28  and  41  approached the trip oil level  48 . This is presumed to be for the following reason. The oil sump  25  in the bearing casing  21  is agitated with rotation of the timing gears  31  and  32 , resulting in generation of a negative pressure. Consequently, there occurs movement of oil from the chamber  40  to the discharge-side bearing casing  21  through the line  43 , so that there occurs a difference in height between the oil level  41  in the chamber  40  and the oil level  27  in the discharge-side bearing casing  21 . However, the closer the position of connection to the bottom of the discharge-side bearing casing  21 , the greater the influence of the negative pressure and the larger the amount of oil moving to the discharge-side bearing casing  21  and hence the more significant the difference in the height of oil level. Conversely, the closer the position of connection to the trip oil level, the smaller the influence of the negative pressure and the smaller the amount of oil moving to the discharge-side bearing casing  21  and hence the less significant the height of oil level. 
       FIG. 2  shows differences from the atmospheric pressure at various positions (heights) from the bottom of the discharge-side bearing casing  21  during operation of the screw compressor. The difference between the pressure at the height of the trip oil level  48  and the atmospheric pressure is Δp. However, the value of Δp is a very small value and the difference between the height of the oil level  41  in the chamber  40  and that of the oil level  27  in the discharge-side bearing casing  21  is of a substantially ignorable degree. 
     As noted above, if the oil sump  25  in the bearing casing  21  in which the timing gears  31  and  32  are accommodated and the oil line  43  are connected with each other at the height of the trip oil level  48 , it is possible to check positively whether the oil level  27  has reached the height of the trip oil level  48  while avoiding the occurrence of a difference in height between the oil levels  27  and  41 . By maintaining the height of the oil level  27  in the discharge-side bearing casing  21  so as not to become lower than the trip oil level  48 , the bearings  14 ,  15 ,  17  and  18  can be prevented from undergoing oil shortage. 
     Next, a description will be given about the supply and discharge of oil which are controlled by the control unit  60 . When the oil level sensor  42  in the chamber  40  detects that the oil level  41  has become lower than a preset first value “height LL” of lower limit, the oil pump  51  is activated by the control unit  60  and cooled oil is fed to the chamber  40  (and hence to the bearing casings  21  and  22 ) through the oil cooler  52  from the oil tank  50 . Upon lapse of a predetermined time after activation of the oil pump  51  or when the oil level  41  reaches a preset third value “height M” (higher than the first value “height LL” and lower than a second value “height H” to be described later), the oil pump  51  is turned OFF. As the oil level  41  in the chamber  40  is raised, the oil levels  27  and  28  in the oil sumps  25  and  26  also rise interlockedly to the same height as the oil level  41  in accordance with Pascal&#39;s principle. As a result of the rise of the oil level  41  in the chamber  40 , oil is fed to the bearing casings  21  and  22  and thus damage of the bearings  14 ,  15 ,  17  and  18  caused by lowering of the oil levels in those bearing casings can be prevented. It suffices for the “height LL” to be set to a value equal to or higher than the trip oil level  48 . 
     When the oil level  41  becomes higher than a preset second value “height H” which indicates an upper limit, the opening/closing valve  46  for the discharge of oil is opened, and oil is discharged from the bearing casings  21  and  22 , then upon lapse of a predetermined time after opening of the valve  46  or when the oil level  41  reaches the third value “height M,” the valve  46  is closed to terminate the discharge of oil from the bearing casings  21  and  22 . If too much oil is supplied, the amount of oil agitated by the bearings  14 ,  15 ,  17 ,  18  and the timing gears  31 ,  32  increases, resulting in increase of the oil temperature and lowering of viscosity, whereby there is a possibility of damage of the bearings  14 ,  15 ,  17  and  18 . However, such a possibility can be eliminated (excessive supply of oil can be prevented) by the structure described above. 
     Even if the oil level  41  is not higher than the preset second value “height H” indicative of an upper limit, if it is higher than the third value “height M” and the oil temperature To is higher than a preset upper-limit temperature, the opening/closing valve  46  for the discharge of oil may be opened and oil may be discharged from the bearing casings  21  and  22 . In this connection, there may be adopted a structure wherein when a predetermined time has elapsed after opening of the opening/closing valve  46 , or when the oil level  41  has reached a preset “height L” (lower than the third value “height M” and higher than the first value “height LL”), or when the oil temperature To drops to a preset temperature (a temperature lower than the above upper-limit temperature), the opening/closing valve  46  is closed to terminate the discharge of oil from the bearing casings  21  and  22 . Even with this structure, it is possible to eliminate the possibility of damage of the bearings  14 ,  15 ,  17  and  18  because of a rise of the oil temperature To and the resultant lowering of viscosity. Even if the oil temperature To is higher than the upper-limit temperature, if the oil level is lower than the “height M,” priority is given to ensuring the required amount of oil and the discharge of oil is not performed as described above. 
     Since the structure adopted in the above embodiment wherein the oil stored in the oil tank  50  is fed to the chamber  40  with the oil pump  51  combines the structure for detecting the oil levels  27  and  28  in the oil sumps  25  and  26  formed in the bearing casings  21  and  22  with the structure for the supply of oil to the bearing casings  21  and  22 , there also accrues a merit that the structure becomes simpler. 
     The above description is merely illustrative of the technical concept of the present invention, and technical concept of the present invention is not limited by the above embodiment. Modifications and changes may be made within the scope of the present invention. For example, the space which overlies the oil level  41  in the chamber  40  communicates through the communication line  29  with the space which overlies the oil levels  27  and  28  in the compressor bearing casings  21  and  22 , and it need not always be open to the atmosphere. 
     Further, the space which overlies the oil level  41  in the chamber  40  communicates through the communication line  29  with the space which overlies the oil levels  27  and  28  in the compressor bearing casings  21  and  22 , and it may be structured so as to permit the injection of inert gas through the communication line  29 .