Abstract:
The invention concerns a plunger piston compressor ( 1 ) for refrigerants, particularly for CO 2 , with a case-like housing, having a compressor block ( 2 ) with a compression section comprising a cylinder ( 13 ) as first housing element and a bottom part ( 3 ) as second housing element, the compressor block ( 2 ) and the bottom part ( 3 ) being connected with each other, and with a motor ( 25 ) with a drive shaft ( 23 ) located in the inner chamber ( 17 ). It is endeavoured to provide a high-pressure refrigerant compressor with a simple design, which is cost effective in manufacturing. For this purpose, it is ensured that the drive shaft ( 23 ) is only supported on one side of the motor ( 25 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in International Patent Application No. PCT/DK2004/000591 filed on Sep. 9, 2004 and German Patent Application No. 103 42 421.0 filed Sep. 13, 2003.  
       FIELD OF THE INVENTION  
       [0002]     The invention concerns a plunger piston compressor for refrigerants, particularly for CO 2 , with a case-like housing, having a compressor block with a compression section comprising a cylinder as first housing element and a bottom part as second housing element, the compressor block and the bottom part being connected with each other and delimiting a closed inner chamber, and with a motor with a drive shaft located in the inner chamber.  
       BACKGROUND OF THE INVENTION  
       [0003]     Such a refrigerant compressor is known from U.S. Pat. No. 2,583,583. The motor is grip-held in the bottom part. The drive shaft of the motor is supported in both the bottom part and in the compressor block. On the compressor block is fixed a cylinder, in which a piston is arranged to be reciprocating. The piston is driven by the drive shaft via a connecting rod.  
         [0004]     Another refrigerant compressor is known from DE 195 16 811 C2. This compressor is enclosed by a relatively thin-walled case. The involved compressive strength merely permits the use of traditional refrigerants, for example, partially halo-genated fluorohydrocarbons, for example R134a, or hydrocarbons, like propane or isobutene, which have even less greenhouse potential. Such refrigerant compressors have relatively low working pressures, which, on the suction side, are for example lower than 5 bar. In order to decouple the vibrations, which occur because of the oscillating movement of the piston mass, from the housing and thus from the environment, the compressor itself is supported in the housing via a spring arrangement.  
         [0005]     Lately, carbon dioxide (CO 2 ) has become more and more used as refrigerant. CO 2  has the advantage of a better environmental compatibility, particularly with regard to ozone and greenhouse potential. Next to the incombustibility, an additional advantage is that also thermodynamically it is more favourable. Further, CO 2  has a substantially higher volumetric refrigerating capacity. This substantially reduces the refrigerant volume, which is required to produce a certain refrigerating capacity, in relation to refrigerants, which are based on hydrocarbons.  
         [0006]     However, with CO 2 -systems, much higher working pressures occur. On the suction side they can be up to 70 bar and on the pressure side up to 160 bar. Accordingly, a pressure-proof design of the compressor is necessary, which requires an accordingly stable housing. This requirement requires great efforts and involves relatively high costs.  
         [0007]     An oil-free, semi-hermetical piston compressor for small refrigerating capacities is known from the publication “Small Oil Free Piston Type Compressor for CO 2 ” by Heinz Baumann, published under “Proceedings of the International Purdue Compressor Technology Conference 2002, Purdue, USA, C25-3”. Further to the compressor block with a first shaft bearing, the compressor housing comprises a cylindrical tube element, in which the stator of the drive motor is fixed, a second bearing element for the drive shaft, and a motor-side housing cover. The individual parts of the housing bear on each other with flange surfaces and are fixed to each other by several screw bolts distributed on the circumference.  
         [0008]     A corresponding compressor, as known from EP 0 378 967, comprises a compressor block with four cylinders, each offset by 90°. The compressor has a so-called Scotch-Yoke drive, in which each two pistons facing each other are connected by means of a yoke, which again is connected with a crank pin of a drive shaft via a sliding piece. The movement directions of the two sliding pieces or yokes, respectively, are here perpendicular to each other. During the rotation of the drive shaft, each piston pair performs a common reciprocating movement.  
         [0009]     A further semi-hermetical two-cylinder piston compressor with a similar design is shown in “CO 2 -Verdichter und-Ausrüstungen”, Die Kälte-und Klimatech-nik 10/2002, pages 116 to 123. Also here, the motor housing is flanged onto the compressor block. A cover is fixed on the compressor block by means of several screws distributed in the circumferential direction.  
         [0010]     The CO 2 -compressors known from the state of the art are characterised by the design characteristics of the traditional several-cylinder, semi-hermetical compressors, which are dimensioned for large refrigerating capacitiess and accordingly have a higher mechanical stability. The pistons of the individual cylinders are moved by a drive shaft, which is supported in at least two main bearings. These bearings, which are located in different housing parts, must be oriented accurately in relation to each other during mounting, which involves relatively large efforts.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     The invention is based on the task of providing a high-pressure refrigerant compressor with a simple design and cheap in manufacturing.  
         [0012]     With a plunger piston compressor as mentioned in the introduction, this task is solved in that the drive shaft is only supported on one side of the motor.  
         [0013]     An alignment of different bearings to the same drive shaft can thus be avoided. This simplifies the embodiment. A one-sided bearing has until now not been regarded as an option for high-pressure refrigerant compressors, as the high pressures result in a corresponding load of the shaft and thus of the bearing. Surprisingly, it has turned out, however, that also with high-pressure refrigerants a one sided bearing is sufficient.  
         [0014]     Preferably, the drive shaft is supported on the side of the motor, which is adjacent to the compression section. Thus, the drive shaft is, in a manner of speaking, “symmetrically” loaded on both sides of the bearing. On one side, the motor is engaging. On the other side, the crankshaft drive is engaging. In both cases, the length of the drive shaft between the force contact points and the bearing is relatively short, so that the load on the bearing remains small, also when the individual forces are not exactly the same.  
         [0015]     Preferably, the drive shaft has a counterweight, and a connecting rod bearing is located between the counterweight and the bearing. Also this measure contributes to reducing the load on the bearing. The contact point of the connecting rod is as closely adjacent to the bearing as possible.  
         [0016]     It is also advantageous that, in the area of their connection parallel to the drive shaft, the compressor block and the bottom part overlap axially on their whole circumference and that the connection is formed in the overlapping area. Thus, in principle, it is possible to transfer the basic design known from domestic refrigerant compressors, comprising a compressor block, a crankshaft drive and a drive motor, to the requirements of a high-pressure refrigerant compressor, thus achieving a simplified compressor embodiment. As CO 2  used as refrigerant has a higher volumetric refrigerating capacity than refrigerants on the basis of hydrocarbons, a predetermined performance will permit a reduction of the cylinder volume and the piston diameter. The reduced oscillating masses in connection with the increased total mass of the compressor caused by the correspondingly stably formed housing lead to a compressor design, in which the compression section no longer has to be mechanically decoupled from the housing by means of springs. To the outside, the whole compressor works with relatively little vibration. Accordingly, it is no longer required to enclose the compressor block in a casing, on the contrary, the compressor block can be used as a housing element, which surrounds the inner chamber together with the bottom part. This again causes that the bottom part must have a relatively stable design. It can, for example, be made as a thick-walled deep-drawn part by cold forming of a steel plate with a thickness of approximately 8 mm. However, such a design makes it difficult to make radial flanges, as known from the state of the art, for connecting the compressor block and the bottom part with each other by means of several clamp bolts distributed on the circumference. A hermetical and undetachable welding connection, which would satisfy the stability requirements in itself, would bring so much heat into the design that the high temperatures occurring would damage parts of the compressor. These problems are reduced in a simple manner in that one housing element is inserted into the other, the two housing elements being connected with each other in the resulting overlapping area. This connection is so stable that the pressure ruling in the inner chamber cannot push the housing elements apart in the axial direction. Further, it has to be so tight that the gas available in the inner chamber cannot flow off in an uncontrolled manner. These are in fact the only requirements for the connection in the overlapping area. As will be described below, such a connection can be made in a relatively simple manner, also when both the compressor block and the bottom part are made to be relatively stable, that is, have relatively large wall thicknesses.  
         [0017]     Preferably, the connection is free of auxiliary joining parts. This simplifies the mounting.  
         [0018]     In a particularly advantageous embodiment, one of the two housing elements has an outer thread and the other housing element has an inner thread in the overlapping area, the outer thread and the inner thread engaging in each other. In order to make the connection between the compressor block and the bottom part, the two housing elements are put together and turned in relation to each other. The engagement between the outer thread and the inner thread is sufficient for preventing an axial disassembling of the two housing elements, also with high pressures in the inner chamber.  
         [0019]     In an alternative embodiment, it is ensured that in the overlapping area, the two housing elements are clamped to each other radially. Thus, the connection merely exists in the form of a frictional connection between the compressor block and the bottom part. When, however, a sufficient force causes this frictional connection, the resulting connection is sufficient to fix the bottom part reliably on the compressor block.  
         [0020]     It is preferred that the radially outer housing element in the overlapping area is shrunk-fit or pressed onto the radially inner housing element. Thus, sufficient tension force can be achieved, so that the outer housing element can exert sufficiently large pressure forces on the inner housing element.  
         [0021]     Both with a screwed connection and with a clamped connection, a generally auxiliary joining part free and unwelded connection of the housing of the compressor is achieved.  
         [0022]     Preferably, the radially inner housing element in the overlapping area has a circumferential, radial flange, which radially supports the outside of the housing element located radially outside in the overlapping area, at least in the area of its front end. In principle, an increased pressure in the inner chamber involves the risk that the housing expands radially. This expansion might cause that the radially outer housing element expands in the overlapping area, thus reducing the length of the overlapping area, which again would have negative consequences for the clamping forces in the overlapping area. Such an expansion is avoided in that the radially outer housing element is radially retained from the outside in the area of its front end.  
         [0023]     It is preferred that the flange has a conical recess and that the front end of the radially outer housing element in the overlapping area has a correspondingly conically chamfered front side. The outer housing element and the inner housing element thus get a suitable engagement with each other.  
         [0024]     It is preferred that the bottom part is located radially outside in the overlapping area. The bottom part is the housing element with the simplest design, compared with the compressor block. Therefore, it is simpler to provide an inner thread here, as there are no disturbing parts. When the connection is a clamped connection in the form of a shrink-fit, it is substantially simpler to heat the bottom part and to cool it during shrinking, as here there is no risk that elements of the compression section will be influenced.  
         [0025]     It is preferred that, at least in the overlapping area, the bottom part has an increased wall thickness. The increased wall thickness improves the stability of the connection.  
         [0026]     Preferably, the compressor block encloses the complete circumference of the cylinder. The compressor block in itself has a certain stability, which is required to resist the pressure in the inner chamber. This stability of the compressor block is utilised also for adopting the cylinder. During operation, the highest pressures exist in the cylinder. When the cylinder is not made as a separate component, which is connected with the compressor block, but as a bore in the compressor block, a high mechanical stability can be achieved in a simple manner. Of course, a liner can also be provided in such a bore, in order to improve the frictional behaviour between the piston and the inner cylinder wall.  
         [0027]     Preferably, the cylinder has a lateral opening, which is connected with a compression chamber delimited by the cylinder and a piston displaceable in the cylinder, when the piston is in its lower dead point. In certain applications, systems working with a high-pressure refrigerant, particularly CO 2 , may, for energetic reasons, require a supply of refrigerant to the compression chamber, whose pressure is between the suction pressure and the high pressure of the refrigeration system. This is particularly the case, when it is not required to reduce the pressure of the complete amount of previously compressed refrigerant gas in order to provide the required refrigerating capacity. In the embodiment shown, this “medium pressure gas” is led into the compression chamber at the end of a suction stroke, that is, in a piston position, which is close to the lower dead point. This way of realising a multi-stage compression is energetically advantageous in connection with CO 2 , as the pressure ratio of the compressor is reduced, which will increase the efficiency. In housing-fixed compressor designs, this is particularly easily realised. The medium pressure gas can namely be supplied direct from the outside to the compression chamber through a bore made in the compressor block, whose opening into the compression chamber is covered by the piston during almost the complete piston stroke. Merely in a piston position close to the lower dead point, the bore is not closed by the piston, but has a connection to the compression chamber.  
         [0028]     Preferably, the cylinder is delimited on one front side by a cylinder head, which comprises a suction chamber that is connected with the inner chamber. This connection ensures that refrigerant gas, which gathers in the inner chamber, can be sucked in during a suction stroke. Thus, an overfilling of the inner chamber with refrigerant gas is thus avoided. This causes that the pressure in the inner chamber can be kept at the level of the suction pressure.  
         [0029]     Preferably, the compressor block and the bottom part are connected by welding or soldering, the welding or soldering seam being limited to a radial outer area of the housing. The connection via a thread engagement or a radial tension, respectively, can in principle be made gas-tight, for example by using an anaerobically hardening sealing material, like Loctite 577, or by inserting a metallic sealing. However, such a connection is still detachable, so that only a semi-hermetical compressor is concerned. When, however, a welding or soldering seam is added, a hermetically enclosed compressor can be achieved. The welding or soldering seam is not supposed to have a large mechanical strength. It can therefore be limited to a relatively thin layer on the outside of the housing. When making such welding or soldering seams, there is no risk that the temperature inside the housing will increase significantly, so that a damaging of components inside the housing shall not be anticipated.  
         [0030]     Preferably, the side of the compressor block opposite the motor has a mounting opening closed by a closing plate, through which opening the drive shaft, the counterweights and the connecting rod can be inserted in the inner chamber. Then, the bearing can be made in one piece with the compressor block, that is, no additional connection measures are required between the outer wall of the compressor block and a component adopting the bearing. Still, the bearing is accessible from two sides. On the side facing the motor, there are no limitations anyway. On the opposite side the accessibility exists through the mounting opening. During operation, the mounting opening is of course closed by the closing plate, which can also be screwed into the compressor block. Also here, an aneorobically hardening sealing material can additionally be used. Also the use of an additional welding or soldering seam is possible.  
         [0031]     Preferably, a glass feed-through is screwed into the compressor block, which is arranged coaxially to the cylinder. This glass feed-through permits the connection of electrical lines of the motor with electrical supply lines from the outside, without abandoning the tightness of the housing. When arranging the glass feed-through coaxially to the cylinder, the bores, which are required for the cylinder and the glass feed-through can be made without requiring a placing or directional change of the compressor block in the corresponding production machine.  
         [0032]     Preferably, the connecting rod has an adjustable length. Then the dead volume in the compression chamber can be set in a relatively simple manner. The piston is taken to its upper dead point. The drive shaft is turned to a position, which corresponds to the upper dead point of the piston. The connecting rod is then adjusted exactly to the resulting length. An adaptation of the dead chamber caused by the use of differently thick sealings between the cylinder and the cylinder head is then no longer required. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     In the following, the invention is explained in detail on the basis of preferred embodiments in connection with the drawings, showing:  
         [0034]      FIG. 1  is a perspective, overall view of the compressor,  
         [0035]      FIG. 2  is a vertical section through a first embodiment of the compressor, and  
         [0036]      FIG. 3  is a vertical section through a part of a modified embodiment of the compressor. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]      FIG. 1  shows a one-cylinder, semi-hermetic CO 2  compressor  1  with a central compressor block  2 , which forms part of the compressor housing. Further components are connected with the compressor block  2  and delimit the inside of the compressor. These are, firstly, a cup or bell-shaped bottom part  3  and a closing plate  4 , secondly a cylinder head  5 , which comprises a valve plate  6  with integrated suction and pressure chambers and a cylinder head cover  7 . The compressor block  2  and the bottom part  3  are two housing elements, which, together with the closing plate  4  delimit an inner chamber  17 . Further shown are a suction connection  8 , through which the gaseous refrigerant is sucked in, and a pressure connection  9  for discharging the compressed refrigerant. Both connections are connected with the openings of the valve plate in a pressure tight manner. In the area of the bottom of the cup-shaped housing part  3  fixing devices  10  are located, which permit a safe mounting of the compressor  1  on a base by insertion of rubber block supports  11 .  
         [0038]     The sectional view according to  FIG. 2  shows the design of the compressor according to  FIG. 1 . The compressor block  2  forms the upper part of the compressor housing. It comprises a projection  12 , in which a cylinder  13  is formed. Together with a piston  14  and the valve plate  6 , this cylinder delimits the compression chamber of the compressor  1 , which cannot be seen here, as the piston  14  is in its upper dead point. Further to a suction chamber  15 , the valve plate comprises a pressure chamber (not shown) and suction and pressure openings, which connect the compression chamber with the suction chamber and the pressure chamber (not to be seen in the section). A pressure equalising bore  16  connects the suction chamber  15  with the inner chamber  17  inside the compressor housing. The cylinder head cover  7  closes the suction chamber  15  and the pressure chamber towards the environment. For this purpose, the cover  7  and the valve plate  6  are fitted on the compressor block  2  by means of screw bolts  18 . Via a ball joint  19 , the piston  14  is connected with a connecting rod  20 , whose crank-side connecting rod eye  21  is rotatably supported around an eccentrically arranged crank pin  22  of the drive shaft  23 . The drive shaft  23  again is rotatably supported in a radial bearing  24  formed in the compressor block  2 , and is driven by a motor  25 . In the area of its upper end, the drive shaft  23  has a diameter expansion, with which it is axially supported on the housing of the radial bearing  24 .  
         [0039]     The connecting rod  20  is divided in two, a cylindrical end  60  of the piston-side part of the connecting rod being arranged to be displaceable in a bore  61  of the crank-side part  62  of the connecting rod  20 . After the dead volume setting (with the piston  14  in the upper dead point as shown), these two parts of the connecting rod  20  are then retained in their final position, for example by clamping the crank-side connecting rod part  62  on the cylinder-side end  60 . A traditionally used method for adjusting the dead volume by means of various thicknesses of a sealing located between the compressor block  2  and the valve plate  6  is not required.  
         [0040]     The motor  25  comprises a rotor  26 , which is fixed on the drive shaft  23 , and a stator, which is fixed on the compressor block  2  by means of fixing elements (not shown). An oil pump  28  is located in the bottom part of a through bore  29  of the rotor  26 , and immerses with its inlet opening  30  into the oil sump  31  at the bottom of the compressor housing. The oil pump  28  is a centrifugal pump, and, in a manner known per se, it supplies lubricating oil into a blind hole bore  32  of the drive shaft  23  and from there to openings  33  in the bearing areas of the compressor to be lubricated. The required oil quantity in the compressor is substantially reduced in relation to known, housing-fixed compressors, but also in relation to spring-decoupled designs, as the inlet opening  30  of the oil pump can be located very close to the bottom of the cup-shaped bottom part  3 , and the inner volume of the compressor on a whole is minimised.  
         [0041]     At its upper end, the compressor block  2  has a mounting opening  34 , through which the drive shaft  23 , the connecting rod  20  and counterweights  35  can be inserted during assembly. The inner wall of the opening  34  is provided with a thread  36 . Thus, the closing plate  4  can be screwed into and close the opening  34 . Using a suitable, anaerobically hardening thread-sealing material, for example Loctite 577, can for example, ensure a sealing. A tight welding by means of a circumferential welding seam, which, however, does not have to have a large mechanical strength, or a tight soldering, can also be used.  
         [0042]     Further, the compressor block  2  has a projection  37 , in which is formed an opening  38  with an inner thread  39 . The opening is made for adopting a glass feed-through element  40  with metallic pins  50  isolated from the housing, and ends in a chamber  51 , which is a part of the inner chamber  17 .  
         [0043]     Through this chamber  51 , the electrical supply wires for the drive motor are led to the stator  27 . The thread  39  can also be tightened with an anaerobically hardening material or by means of a welding or soldering.  
         [0044]     For the assembly of the compressor  1 , the drive shaft  23  is inserted through the opening  34  in the compressor block  2  together with the connecting rod  20  and the counterweights  35 . After inserting the piston  14  in the cylinder  13  and adjusting the dead volume, the cylinder head  5  is attached. The openings  34  and  38  are now closed by means of the closing cover  4  and the glass feed-through  40 . The rotor  26  is pushed onto the shaft  23  and the oil pump  28  is inserted in the rotor. After fixing the stator  27  on the block  2 , the compressor merely has to be closed by means of the cup-shaped bottom part  3 , which is mounted over the motor  25  so as to axially overlap with the compressor block on part of its length. The resulting overlapping area  47  is closed in the circumferential direction.  
         [0045]     Due to the occurring high pressures, the bottom part  3  requires a sufficient wall thickness. Typically, it is made as a deep-drawn component from an 8 mm steel plate. It serves as cover for the motor  25 , the adoption of the oil sump  31 , and as carrier for the fixing arrangements  10 , which can be fixed by welding near a bottom  41  of the bottom part  3 . Beside the bottom  41 , the bottom part  3  has a cylinder-shaped side part  42 , on whose inner side is formed an inner thread  43  near the open end of the bottom part  3 . In order to maintain the required stability, also in the area of the inner thread  43 , the wall thickness of the upper section  44  of the sidewall  42  is increased. The inner thread  43  interacts with an outer thread  45  formed on the compressor block  2  and is also tightened with an anaerobically hardening sealing material, for example Loctite 577.  
         [0046]     The upper front face of the sidewall  42  is preferably conically chamfered towards the outside and, after finished assembly; it is supported on a correspondingly conical recess  46  on a circumferential, radial flange  53  of the block  2 . Thus, the sidewall  42  cannot expand with higher pressures, as it is retained in the recess from the radial outside. Thus, the risk of a radial movement of the cup-shaped bottom part  3  under the influence of the high pressures inside the housing can be reduced. Test pressures of more than 350 bar will therefore not harm the housing.  
         [0047]     Due to the detachable and dismountable housing parts  3 ,  4 ,  40  and the cylinder head  5 , the compressor described until now represents a semi-hermetical design. However, it would be possible to make the design hermetical by welding the gap along the screwed connections or the flanges of the cylinder head, respectively. For example, a circumferential welding seam along the contact line between the housing part  3  and the block  2  can achieve a hermetical sealing of the cup-shaped housing part  3 . The resulting pressure forces will be adopted by the screwed connection. The welding seam merely has a sealing function, and can be made with a small heat energy input.  
         [0048]      FIG. 3  shows a modified embodiment of a compressor without cylinder head, bottom part and mounting cover. The same parts have the same reference numbers as described in  FIG. 2 .  
         [0049]     Inside the cylinder  13 , the piston  14  is in its lower dead point. Accordingly, the compression chamber  48  can be seen in its full size. It can also be seen that the cylinder is located in the compressor block  2 , or, more precisely, in the projection  12 , and is accordingly supported on its entire circumference by the compressor block  2 .  
         [0050]     A bore  49  with a connection enlargement  52  is led through the projection  12 . The bore  49  ends in the compression chamber  48  in such a manner that the piston  14  releases it, when the piston is in the area of its lower dead point.  
         [0051]     With such a lateral bore it is possible to supply refrigerant gas to the compression chamber  48  with a pressure, which lies between the suction pressure and the high pressure of the system connected with the compressor. During the following compression movement, the piston  14  then compresses refrigerant gas, which has already been pre-compressed. This way of realising a multi-stage compression is energetically advantageous. As the pressure ratio of the compressor is lower, the efficiency increases. It is particularly easy to realise here, as the bore  49  can be led directly to the outside through the compressor block  2 . As soon as a compression stroke begins, the bore  49  is closed. Thus, a pressure increase outside the compression chamber  48  is practically impossible.  
         [0052]     While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present invention.