Patent Publication Number: US-6991440-B2

Title: Two-stage screw compressor

Description:
RELATED APPLICATIONS 
   This application is a continuation of International Application Ser. No. PCT/EP02/06853, filed Jun. 20, 2002, the entire contents of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates to a two-stage rotary helical screw-type compressor. 
   BACKGROUND OF THE INVENTION 
   This invention pertains to a two-stage screw compressor. A screw compressor of this type is known from DE 299 22 878 U1, among other documents. Reference is made to the disclosure of this document in its entirety. 
   In this prior art screw compressor, each of the two compressor stages, which jut out from the gear housing parallel to one another, is enclosed by its own coolant housing that is connected to the coolant circuit or coolant sump through its own connection. Because of the space requirements of the coolant housings of the two compressor stages mentioned, the design of this prior art screw compressor is not particularly compact. 
   A prior art screw compressor is known from U.S. Pat. No. 4,174,196 that has two compressor stages whose compressor housings are arranged parallel to one another and are enclosed by a common inner housing. The inner housing is enclosed by a concentric outer housing and the annulus between the inner and outer housing is subdivided into chambers through which a coolant flows in order to cool tube bundles located within the chambers through which the compressed gas flows. A first coolant stream cools the compressed gas from the first compressor stage, and a second coolant cools the compressed gas from the second compressor stage. The coolant has essentially no direct cooling effect on the rotor housings of the two compressor stages. 
   SUMMARY OF THE INVENTION 
   An objective of the invention is to provide a two-stage screw compressor of the above type having a particularly compact design and whose manufacture is simplified and herein the utilization of the cooling effect of the coolant circuit is improved. 
   According to the invention, the rotor housings of both compressor stages are located in a common cooling housing that encloses them at a distance, and are preferred to be manufactured in one piece together with the cooling housing. The cooling housing has only one coolant inlet and one coolant outlet and is designed such that the coolant is forced to follow a flow path that passes around and cools the two rotor housings of the compressor stages one at a time. This results in an especially compact design and at the same time a simplification in the manufacture of the screw compressor, in addition to an improved utilization of the coolant. 
   Another way to design the screw compressor to be particularly compact is to place the four connections of the two compressor stages (inlet and outlet of the low-pressure stage, inlet and outlet of the high-pressure stage) one at each of the four sides of the square coolant housing. The available space at each of the lateral surfaces of the coolant housing can then be optimally used for the connection located there without being hindered by another connection on the same side. 
   Other features and advantages of the present invention will become apparent to those skilled in the art upon review of the following detailed description and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a screw compressor according to an embodiment of the invention. 
       FIG. 2  is a side view of a screw compressor according to an embodiment of the invention as seen in the direction of arrow A in  FIG. 3 . 
       FIG. 3  is a rear view of the screw compressor as seen in the direction of arrow B of  FIG. 2 . 
       FIG. 4  is a side view as seen in the direction of arrow C in  FIG. 3 . 
       FIG. 5  is a plan view of the screw compressor according to  FIG. 2 . 
       FIG. 6  is a perspective representation of the coolant housing of the screw compressor as seen from above left. 
       FIG. 7  is a perspective representation of the coolant housing according to  FIG. 6 , but as seen from below right. 
       FIG. 8  is a cross section through the coolant housing and the compressor stages perpendicular to their axes, according to the section line and the view direction of arrow D—D in  FIG. 4 . 
   

   Before the constructions of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
   DETAILED DESCRIPTION 
   The two-stage screw compressor shown in  FIGS. 1–5  has a housing  1  that is provided with feet  3  to fasten to a base. The upper section  5  of the housing  1  has essentially the shape of a disc-shaped upright with two vertical end walls. Attached to the left end wall in  FIG. 2  is a coolant housing  7  that extends out freely. The coolant housing  7  encloses two compressor stages located in its interior, each of which consists of a rotor housing inside of which two screw rotors are located that mesh with one another, as will be explained hereafter with reference to  FIG. 8 . An oil supply basin  9  extends out from the bottom section of the housing  1  below the coolant housing  7  at a distance from it. The oil basin also can have feet  3  to support it on the base. On the side of the upper housing section  5  opposite the coolant housing  7  is a drive unit  13  with a preferably RPM-regulated motor that drives the screw rotors of the compressor stages via a gear housed in housing section  5 . The drive motor  13  can also run an oil pump that is placed in housing section  14  between the drive unit  13  and the branching gear in housing section  5 . 
   The screw compressor illustrated is preferred to be a dry-running screw compressor, i.e. the actual compression space containing the screw rotors is kept free of oil. The oil kept in circulation by the oil pump is used on one hand to lubricate the gear and the roller bearings of the screw rotors, and on the other hand as a coolant to externally cool the rotor housings of the two compressor stages. 
   With respect to the details of the drive system using the RPM-regulated drive motor, the branching gear and the integration of the oil pump into the housing, an example is shown in previously mentioned DE 299 22 878 U1, the entire contents of which are incorporated herein by reference. This also applies to other advantageous features disclosed in this document that can also be applied to the screw compressor according to this invention, such as the internally integrated oil pump, which alleviates the need for a seal, the direct coupling of the motor to the drive journal of the gear without the usual coupling, which requires considerable space, as well as the design of the motor-driven gear such that an optimum effectiveness is achieved within a prescribed RPM range of the motor, for example between 2500 and 5500 RPM. 
   The part of the screw compressor to which aspects of this invention mainly refer, namely the coolant housing  7  containing the compressor stages within, is illustrated in  FIGS. 6–7  in perspective views from two different directions of view and in  FIG. 8  in a cross section. 
   According to  FIG. 8 , a first compressor stage  15  is located inside of the coolant housing  7  (low pressure stage) with two meshing screw rotors  17 ,  19  that are held in a rotor housing  21  that has a cross section in the shape of a  FIG. 8 , and a second compressor stage  23  (high pressure stage) is placed with its axis parallel to the first stage with a pair of screw rotors  25 ,  27  that are held in a  FIG. 8  shaped rotor housing  29 . 
   The coolant housing  7  is in the shape of a box with an essentially square cross section so that it has two side walls  31 ,  35  parallel to the rotors  17 ,  19 ,  25 ,  27 , a top  37 , and a bottom  39 . 
   The coolant housing  7  has openings on the top and bottom that are closed off by bolted-on plates  41 ,  45 , and  47 . 
   At one of its ends, the coolant housing  7  transitions into a large surface flange  49  that is used to fasten the coolant housing  7  to the gear housing  5  and which has a matching exterior contour. At the other end, the coolant housing  7  is closed off by an end bearing cover  51  that is bolted to the coolant housing  7 . At the bottom of the bearing cover  51  is an oil drain fitting  53  that is connected to the oil supply basin  9  via a return line  55  ( FIGS. 1–5 ). 
   At the top of the coolant housing  7  is the inlet opening  57  to intake the gas to be compressed in the compression space inside the rotor housing  21  of the first compressor stage  15 . The outlet  59  for the gas compressed in the first stage is located in the right side wall  31  in  FIG. 7  (to the left in  FIG. 8 ). At the bottom  39  of the coolant housing  7  is the inlet opening  61  for the second compressor stage  23  (high pressure stage). The outlet opening  63  of the second compressor stage  23  is located in the left side wall  35  in  FIG. 6  (to the right in  FIG. 8 ). The outlet opening  59  of the low-pressure stage is connected to the inlet opening  61  of the high-pressure stage in general via an intermediate cooler (not shown in the drawings). Also not shown are the filter and/or muffler installed ahead of the intake opening  57  of the low-pressure stage as well as the muffler, cooler and/or filter installed after the outlet  63  of the high-pressure stage. This equipment can be of any desired design available to one trained in the art. 
   In the embodiment described, the total of four connections of the two compressor stages (entrance and exit ports  57 ,  59  of the low pressure stage  15 , entrance and exit ports  61 ,  63  of the high pressure stage  23 ) are placed one at each of the four sides  31 ,  35 ,  37 ,  39  of the coolant housing  7 , resulting in a compact design making good use of the available space at the four sides of the housing  7 . 
   As can be seen in  FIG. 8 , the rotor housings  21 ,  29  of the two compressor stages  15 ,  23  are placed at a sufficient distance from the walls of the coolant housing  7 . In the intermediate space formed thereby, a cooling medium circulates to effect common cooling of the two compressor stages  23 ,  15 . To this end, the coolant housing  7  has an inlet opening  65  at the bottom  39  for the cooling medium and an outlet opening  67  near the top of side wall  35  for the cooling medium. The coolant fed in at opening  65  flows through the coolant housing  7  and makes its way to the exits opening  67  in an S-shaped flow path that is indicated by the dot-dashed line  69 . This flow path is forced to occur via guide walls  71 ,  73  that extend at suitable points though the intermediate space between the rotor housing  21 ,  29  and the coolant housing  7 . The flow path  69  first flows around rotor housing  21  of the low pressure stage  15  and then around rotor housing  29  of the high pressure stage  23 , each time by a “wrap-around angle” of more than 180°, preferably nearly 360°. 
   It is preferred that the rotor housings  21 ,  29  of the compressor stages, the guide walls  71 ,  73  and the coolant housing  7  are produced as a one-piece housing block, as indicated by the uniform hatching in  FIG. 8 . 
   The foregoing detailed description describes only a few of the many forms that the present invention can take, and should therefore be taken as illustrative rather than limiting. It is only the claims, including all equivalents that are intended to define the scope of the invention.