Abstract:
The invention relates to a compression apparatus (10) comprising a turbocompressor (1) with a plurality of compression stages (1a, 1b, 1c, 1d) as well as infeed flows (5b, 5c, 5d), where the compression apparatus (10) serves for operating a refrigerant circuit, especially with a hydrocarbon gas such as propane, ethylene, methane, or a mixture thereof, wherein a mixer device (8a, 8b, 8c) is arranged outside the turbocompressor (1) and wherein a preceding and a succeeding compression stage (1a, 1b, 1c, 1d) are connected to one another via the mixer device (8a, 8b, 8c) in such a manner that the infeed flow (5b, 5c, 5d) provided for the succeeding compression stage (1b, 1c, 1d) opens into the mixer device (8a, 8b, 8c) in order to mix the infeed flow (5b, 5c, 5d) with a main flow (6a, 6b, 6c) of the preceding compression stage (1a, 1b, 1c).

Description:
The invention relates to a compression apparatus discloses a multi-stage turbocompressor for a refrigerant circuit wherein infeed gas and compressed gas are thoroughly intermixed and introduced to successive turbocompressor stages. 
     BACKGROUND OF THE INVENTION 
     It is known that turbocompressors can be used for refrigerant circuits in ethylene and ammonia installations or for liquefaction of natural gas and petroleum gas. Such installations are designated as LNG (liquefied natural gas) installations or as LPG (liquefied petroleum gas) installations. Petroleum gases collected at the site of a find are fed in compressed form to an LPG installation. The LPG installation separates the components of this gas from one another by stepwise relaxation and cooling. In installations for liquefying natural gas (LNG installations) the natural gas delivered under pipeline pressure is very strongly cooled by a plurality of refrigerant circuits, which are operated with hydrocarbon gases, especially propane, ethylene or methane. The cooled natural gas is liquefied by a succeeding relaxation. The end product of both processes is a transportable liquefied gas. An LPG or LNG installation requires a considerable cryogenic cooling power. A propane circuit which is operated with a large turbocompressor is often used as a first preliminary cooling step. The propane circuit is generally executed in several cooling stages, with a multi-stage compressor with one or more intermediate infeeds being used. Such a cooling system with a propane circuit and intermediate infeeds is known from &#34;Refrigeration System Stability Linked to Compressor and Process Characteristics, Clifford E Lucas, Chemical Engineering Process, November 1989&#34;, with the intermediate infeed known therefrom being illustrated in FIG. 1. 
     A drawback of this known cooler device is to be seen in that it tends to instable behaviour under certain operating conditions. One reason for this is to be found in the intermediate infeeds, the infeed amount of which can be very large, sometimes larger than the actual main flow in the compression stage. The infeed amount has a lower temperature than the main flow. The mixture of main flow and infeed amount can have an inhomogeneous mixing ratio on entering the rotor, which can lead to instable behaviour in the compression stage. 
     SUMMARY OF THE INVENTION 
     A further disadvantage of this known cooling apparatus is that it requires a relatively thick shaft, since the majority of rotors are arranged on the shaft. 
     This object is satisfied in particular by the fact that a mixer apparatus is arranged outside the turbocompressor, that a compressor stage preceding the mixer device and one following it with reference to the direction of flow are connected to one another via the mixer device, and that the infeed flow provided for the following compression stage opens into the mixer device in order to mix the infeed flow with an outlet flow, or the main flow, of the preceding compression stage. 
     An advantage of the invention is to be seen in the fact that the mixer device effects a good mixing of the main flow and the infeed flow, so that a fluid with a homogeneous temperature distribution is fed into the succeeding compression stage. This leads to a more stable operating behaviour of the compression stage. A propane refrigeration circuit can have a volume of the intermediate infeeds which is partially larger than that of the main flow and has a lower temperature as well. By mixing these two volume flows outside the compressor it is possible to achieve the result that the machine characteristic can be determined completely independently of the admixed volume and its temperature. A thermodynamic design of a compressor can thus be performed in a completely conventional manner based on the averaged entry state of the fluid. 
     A further advantage of the invention is to be seen in the fact that the compression apparatus can be divided into several individual compressors. Especially advantageous is the use of compressors with two rotors arranged opposite or counter to one another. The entry of the fluid into the compressor can thereby be arranged to be at the free end. In this way the diameter of the shaft and thus also the Mach number at the inlet to the rotor can be kept small. As a consequence of this the characteristic of the compression stage becomes stable over a broad range. In this way a smaller diameter of the rotor cover plate and thus a smaller inlet Mach number can be achieved for a given speed of rotation and required rotor diameter. 
     A further advantage of a compression apparatus consisting of a plurality of compressors is to be seen in the fact that the compressor no longer has a long shaft. This would lead to poor mechanical properties and to an instable behaviour especially when the long shaft also has narrow places for receiving the rotor. The compression apparatus in accordance with the invention permits the use of short and also thin shafts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in more detail with reference to the exemplary embodiments. Shown are: 
     FIG. 1 is a longitudinal section through a compressor with a known admixing arrangement; 
     FIG. 2a is a schematic representation of the compression apparatus in accordance with the invention; 
     FIG. 2b is a schematic representation of a further compression apparatus in accordance with the invention; 
     FIG. 3 is a schematic representation of a compressor with known admixing; 
     FIG. 4 is a further embodiment of a compression apparatus in accordance with the invention; 
     FIG. 5 is a partial view of a longitudinal section through a compressor; and, 
     FIG. 6 is a further exemplary embodiment of an arrangement of compression stages. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a compressor known from the state of the art with rotors 11a, 11b arranged on a shaft 2 which serve for the compression of a refrigerant. In the first compression stage the base flow 5a is compressed by the rotor 11a and emerges again as main flow 6a. An infeed flow 5b is fed to the compressor 1 via an inlet opening 1f and opens within the compressor housing 1e into the main flow 6a which has already been compressed by the rotor 11a. For this purpose correspondingly formed internal channels 60a, 60b, 61a are arranged in the compressor housing 1e. The two flows 6a, 5b are mixed and further compressed by the following rotor 11b to a main flow 6b. A disadvantage of this arrangement is seen in the fact that the two flows 6a, 5b do not mix homogeneously, which can lead to instable behaviour of the flow in the rotor 11b. 
     FIG. 3 shows a schematic representation of a known multi-stage refrigerant circuit with propane as used for large cooling circuits in LPG installations or LNG installations. The compressor 4 has compression stages 1a, 1b, 1c, 1d arranged in series and on a common shaft 2. The compressed refrigerant arrives via the end flow 6d at a condenser 3 and then further at the process 4. The process 4, represented only schematically, feeds feeder currents 5b, 5c, 5d to the individual compression stages 1a, 1b, 1c, 1d. 
     FIG. 2a shows a schematic representation of an embodiment of the compression apparatus in accordance with the invention. This has compression stages 1a, 1b, 1c, 1d arranged successively in series on a common shaft 2. The end flow 6d, or the final output, then discharges via a compressor 3 into a process 4 not illustrated in further detail, From the process 4 the refrigerant is fed via the basic flow 5a as well as the infeed flows 5b, 5c, 5d back to the individual compression stages 1a, 1b, 1c, 1d. Here the individual compression stages 1a, 1b, 1c, 1d are executed in such a manner that the refrigerant is led back out of the compressor housing 1e via an outlet line 6a, 6b, 6c, through which the main flow flows. Outside the compressor housing 1e there are arranged mixer devices 8a, 8b, 8c into which both the infeed flows 5b, 5c, 5d and the main flow 6a, 6b, 6c are introduced and, after a mixing of the two flows these are fed back via the infeed lines 7a, 7b, 7c to the compression stages 1b, 1c, 1d. The two flows 5b, 6a are mixed in the mixer device 8a in such a manner that the flow leaves the mixer device 8a with a homogeneous temperature distribution as well as a homogeneous velocity distribution and is fed to the compression stage 1b. Many embodiments are suitable as the mixer device, including in particular a static mixer, which as is known has within it only statically arranged inserts for homogenising the fluid. 
     An advantage of the invention is to be seen in the fact that the compression stages 1a, 1b, 1c, 1d can be arranged in differing manners on the common shaft 2, or can be arranged on a plurality of separate shafts 2. FIG. 2b shows a schematically represented exemplary embodiment which is distinguished in comparison with FIG. 2a by a differing arrangement of the compression stages 1a, 1b, 1c, 1d. Compression stages 1a, 1b, 1c, 1d arranged adjacently on the shaft 2 are arranged counter to one another, i.e. the fluid of adjacently placed compression stages 1a, 1b, 1c, 1d flows axially in opposite directions. Thus the compression stages la and 1b, or 1b and 1c, or 1c and 1d are arranged counter to one another. This arrangement has the advantage that the forces of the compression stages 1a, 1b, 1c, 1d acting on the shaft 2 in the direction of the shaft better compensate one another. Otherwise the flow of refrigerant is laid out analogously to that of the exemplary embodiment in accordance with FIG. 2a however, only one mixer device 8a with the corresponding input and output lines 5b, 6a, 7a is shown for the sake of clarity. 
     FIG. 4 shows the exemplary embodiment shown in FIG. 2b in a more detailed layout. The shaft 2 shown in FIG. 2b is divided into two separate shafts in the exemplary embodiment in accordance with FIG. 4 which are connected to one another via a connector shaft 2a. The compression apparatus 10 comprises two compressors 1, which are connected to one another via the connector shaft 2aas well as the mixer devices 8a, 8b, 8c and the connection lines 5a, 5b, 5c, 5d, 6a, 6b, 6c, 6d carrying fluid flows. Each of the compressors 1 has two compression stages 1a, 1b, 1c, 1d which, as shown in FIG. 5, are arranged counter to one another on the shaft 2. A drive means 12, for example an electric motor, a gas turbine or a steam turbine, drives the first shaft 2, with this first shaft 2 being connected via the connector shaft 2a directly or via a gearbox to the shaft 2 of the second compressor 1 and driving this second shaft 2. Inlet and outlet openings of the compression stages 1a, 1b, 1c, 1d are led outwardly so that the mixer devices 8a, 8b, 8c can be arranged outside the compressor 1 and correspondingly connected by tubes in order to appropriately conduct the base flow 5a, the infeed flows 5b, 5c, 5d as well as the main flows 6a, 6b, 6c and the end flow 6d. 
     FIG. 5 shows the upper part of a longitudinal section through a compressor 1 as was used in FIG. 4. The compressor housing 1e has correspondingly formed channels so that the refrigerant flow 7a, 7b enters the compressor 1, is compressed by the rotors 11a, 11b, and leaves the compressor 1 as an output flow or main flow 6b, 6c respectively. Thus the compressor 1 shown has two compression stages 1b, 1c. Since the flow entering into the compressor 1 is arranged at the free end of the shaft 2, the shaft 2 and thus the entry diameter of the rotor can be made relatively small. The arrangement in accordance with FIG. 5 allows the use of a relatively thin shaft 2 and rotors 11a, 1b of small diameter. Such rotors 11a, 11b have a lower Mach number, which brings about a higher flow stability of the fluid in the compressor 1, especially in the rotor 11a, 11b. 
     FIG. 6 shows a further exemplary embodiment of a compressor 1 which on the right side has a compression stage 1a with one compressor stage, and on the left side a compression stage 2a with two compressor stages 1e, 1f connected in series, so that the refrigerant flow 5a leaves again only at the main flow 6a. A compressor stage 1e, if is understood to mean a compressor stage which has only a single rotor for performing the compression. A compression stage 1a, 1b can have a single compressor stage 1e, 1f or a plurality of compressor stages 1e, if connected in series.