Abstract:
A method for operating a compressor and arrangement. The method includes: feeding an intake flow into an inlet of the compressor, compressing the intake flow by the compressor to give an outlet flow, introducing at least one first part flow of the outlet flow into a bypass station as first bypass flow, controlling the feeding-in of the first bypass flow from the bypass station into the inlet of the compressor, depending on operating parameters of the compressor, cooling at least one second part flow of the outlet flow, and controlling the feeding-in of the cooled second part flow as second bypass flow into the inlet of the compressor, depending on operating parameters of the compressor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2014/060660 filed May 23, 2014, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 102013210067.1 filed May 29, 2013. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a method for operating a compressor, having the following steps: 
         [0003]    feeding an intake flow into an inlet of the compressor, —compressing the intake flow by means of the compressor to give an outlet flow, —introducing at least one first part flow of the outlet flow into a bypass station as first bypass flow, —controlling the feeding-in of the first bypass flow from the bypass station into the inlet of the compressor, depending on operating parameters of the compressor, characterized by the following steps: —cooling at least one second part flow of the intake flow, —controlling the feeding-in of the cooled second part flow as second bypass flow into the inlet of the compressor, depending on operating parameters of the compressor. In addition, the invention also relates to an arrangement by means of which the method can be carried out. 
       BACKGROUND OF INVENTION 
       [0004]    Compressors, in particular turbocompressors, generally require a bypass line such that, during startup or in low-flow operating states, sufficient flow or mass flow or flow rate or volume flow can always be passed through the compressor, in order not to drop below the surge limit. In the surge state, particularly large vibrations arise in the compressor, which can result in destruction of the machine. For that reason, the surge limit—a limit line in the characteristic diagram of the compressor —should be approached no closer than a determined safety margin during operation of the compressor. In this context, a surge limiter establishes, where relevant, a critical proximity to the surge limit in the characteristic diagram of the compressor and, in the event of corresponding approximation, prompts the opening of a bypass valve in order to expand, in an outlet line of the compressor, the gas at increased pressure, such that it can be fed back in on the suction side of the compressor. In this context, the compressor generally refers to a compressor unit which delivers a process gas flow at an increased pressure. The compressor is also often termed a process stage. 
         [0005]    A fundamental difference is drawn between cooled bypass lines and un-cooled bypass lines. The gas is drawn either directly downstream of the last stage of the compressor—that is to say upstream of an after cooler which may be present—or downstream of a cooler located downstream of the last stage—such that the bypass is either un-cooled or cooled. In addition, there is also the possibility of providing a cooler only for the bypass line, such that the outlet of the compressor is un-cooled during closed-loop control with a closed bypass line. 
         [0006]    The un-cooled bypass is principally used for media with a pronounced Joule-Thomson effect. The throttling in the bypass valve causes the temperature of the medium to drop, such that, in the case of these gases, additional cooling of the diverted quantity is often not necessary. The magnitude is dependent on the end pressure. The suction temperature of the compressor changes in regular fashion with the opening of a bypass valve. This change has an influence on the characteristic diagram of the compressor, in particular on the first stage of the compressor. A colder and “heavier” intake flow increases the pressure ratio which can be achieved. Increasing the temperature and reducing the molecular weight have an opposite effect. As a consequence of the changed characteristic of the first stage, the calibration of the stages with respect to one another also changes. This gives rise to closed-loop control problems, such that partially guaranteed operating points are no longer possible under certain circumstances. A particularly critical problem arises from the fact that opening the bypass valve can cause the temperature of the intake flow to rise, as a consequence of which the operating point of the compressor moves closer to the surge limit, which can cause the surge limiter to further open the bypass valve. This self-reinforcing procedure leads to the entire flow of the compressor being guided into the bypass. Such positive feedback is undesirable. 
         [0007]    Turbocompressors with bypasses which, in the event of surge, can be used to leave the critical operating state are already known from JP 2003 287 299 A and US 2011/0048046 A1. 
         [0008]    Arrangements containing features of the preamble of the independent claims are already known from documents WO 2009/050175 A1, U.S. Pat. No. 4,921,399 A, WO 2012/007553 A1. 
       SUMMARY OF INVENTION 
       [0009]    The invention has the object of resolving the above-described problems with the surge limiter and the opening of the bypass valve, and thus of increasing the availability of the compressor. 
         [0010]    The inventive solution provides a method of the type mentioned in the introduction, with the additional characterizing features of the independent method claim. Also proposed is an arrangement as claimed in the independent device claim. 
         [0011]    The subclaims which respectively refer back, contain advantageous refinements of the invention. A compressor according to the invention is also often termed a process stage, which process stage generally has multiple stages or impellers. While the invention is adapted for use in turbocompressors, use in piston engines is also possible in principle. The invention permits rapid closed-loop control of the thermodynamic parameters of the bypass flow—in particular of the temperature—so as to prevent a disadvantageous change in the corresponding thermodynamic parameters of the intake flow due to the feeding-in of the bypass flow, as is the case in the prior art. According to the invention, the first bypass flow, the second bypass flow and a mixture of the two bypass flows can be regulated such that it is possible to set a temperature which is advantageous for the intake condition of the compressor. It is thus also possible, in addition to avoiding surge, to optimize the efficiency of the overall installation with the aid of the surge limiter. Since an inventive bypass station generally sets, by means of valves, the magnitude of the first bypass flow, of the second bypass flow and of a mixture of the two bypass flows, a rapid reaction to thermal demands of the compression process is possible. 
         [0012]    An advantageous refinement of the invention provides that the operating parameters of the compressor for controlling the feeding-in of the first bypass flow and/or of the second bypass flow can be the temperature of the intake flow upstream or downstream of the feeding-in of the first bypass flow and of the second bypass flow, or the temperature of the first bypass flow, or the temperature of the second bypass flow, or the temperature of a mixture of the first bypass flow and the second bypass flow, or the mass flow of the first bypass flow, or the mass flow of the second bypass flow, or the chemical composition of the intake flow, or the chemical composition of the first bypass flow, or the chemical composition of the second bypass flow, or a speed of rotation of the compressor, or a pressure characteristic number or a pressure ratio of the compressor. 
         [0013]    Advantageous refinements of the invention provide that the operating parameters of the compressor for controlling the feeding-in of the first bypass flow and/or of the second bypass flow are: the temperature of the intake flow upstream of the feeding-in of the first bypass flow and of the second bypass flow and/or the temperature of the intake flow downstream of the feeding-in of the first bypass flow and of the second bypass flow and/or the temperature of the first bypass flow and/or the temperature of the second bypass flow and/or the temperature of a mixture of the first bypass flow and the second bypass flow and/or the mass flow of the first bypass flow and/or the mass flow of the second bypass flow and/or the composition of the intake flow and/or the composition of the first bypass flow and/or the composition of the second bypass flow and/or a speed of rotation (N) of the compressor and/or a pressure characteristic number or a pressure ratio of the compressor. 
         [0014]    It is in addition also possible that the feeding-in of the first bypass flow or of the second bypass flow is controlled on the basis of a combination of the above-mentioned parameters. The chemical composition of the bypass flows are in this case particularly expedient as the basis for a closed-loop control, because the outlet flow from the compressor, after cooling, is often thermodynamically in the two-phase range and, in the event of corresponding cooling, liquid components can precipitate out such that the outlet flow can have a different chemical composition to that of the intake flow of the compressor. This difference can have a significant influence on the characteristic diagram of the compressor when feeding the bypass flow into the intake flow. If the process medium or intake flow is for example moist air or moist carbon dioxide, the bypass flow can be significantly drier than the intake flow. 
         [0015]    Expediently, the control of the feeding-in of the first bypass flow and/or of the second bypass flow is configured such that the intake flow into the inlet of the compressor approaches a first setpoint temperature after feeding-in of the first bypass flow and/or the second bypass flow. 
         [0016]    Corresponding direct temperature measurements of the intake flow of the compressor in the inlet can be used as a basis for this. Other possibilities emerge through the temperature measurement of the intake flow upstream of the feeding-in of the bypass flow and the measurement of the first bypass flow or of the second bypass flow or of a mixture thereof, and a thermodynamic calculation, which takes place in the control unit, of the resulting temperature in the inlet of the compressor. It is further conceivable that the first bypass flow and the second bypass flow, or a mixture of the two bypass flows, is expanded by means of a valve or other throttle prior to entry into the intake flow, and the resulting Joule-Thomson effect is taken into account by the closed-loop control algorithm of the control unit as a temperature change, such that temperature measurements upstream of this expansion valve or of this expansion throttle are sufficient to determine the temperature of the feeding-in of the bypass flow into the intake flow with sufficient precision. To that end, a state equation of the process fluid is stored in the closed-loop control. 
         [0017]    In addition, it may be expedient to determine the mass flow of the first bypass flow and/or of the second bypass flow and/or of a mixture of the two bypass flows—for example by means of a differential pressure measurement across a throttle—and to feed this measurement value to the control unit of the bypass station, such that the sum of bypass flows fed into the intake flow leads, in dependence on their temperature and composition, to the desired thermodynamics, in particular to the desired temperature of the intake flow entering the compressor. 
         [0018]    In essence, the purpose of the control unit is to control bypass valves for setting the first bypass flow, the second bypass flow and/or a mixture of the two bypass flows on the basis of the measured thermodynamic parameters of the various bypass flows and/or of the intake flow. 
         [0019]    Particularly expediently, the feeding-in of the first bypass flow and/or of the second bypass flow is controlled in such a manner that the intake flow approaches a setpoint value of the product of the specific gas constant of the intake flow, the real gas factor of the intake flow and the temperature of the intake flow after feeding-in of the bypass flows. The specific gas constant of the intake flow is in this case dependent on the composition of the intake flow, while the real gas factor is dependent on the composition of the intake flow, the pressure of the intake flow and the temperature of the intake flow. The corresponding state equations for determining these thermodynamic parameters can be implemented in the control unit according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention is explained in more detail below with reference to special exemplary embodiments. 
           [0021]    In the drawings: 
           [0022]      FIG. 1  shows a strictly schematic representation of a flow chart of an arrangement according to the invention or of a method, 
           [0023]      FIGS. 2-5  each show a more particular exemplary embodiment of the invention as a schematic flow chart, 
           [0024]      FIG. 6  shows a simplified schematic representation of the logic of the closed-loop controller for controlling the bypass station of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0025]      FIGS. 1 to 5  each show a schematic flow chart of an inventive arrangement in order to illustrate the method according to the invention. Here,  FIG. 1  is somewhat more general in reproducing the invention.  FIG. 6  shows a logic diagram for the exemplary illustration of the inventive method for the example of an arrangement as shown in  FIG. 2 . 
         [0026]      FIG. 1  shows an inventive arrangement having a compressor CO which is shown here by way of example with one stage ST 1  and which compresses an intake flow MF to give an outlet flow VF, wherein the compressor CO is intercooled by means of an intercooler IC. On the outlet side of the compressor CO, there is a cooler COL which cools the compressed outlet flow VF or a part flow thereof.  FIG. 1  shows two different alternatives ALT 1 , ALT 2  of how the compressed fluid of the outlet flow is to be supplied to a possible subsequent consumer. In the first alternative ALT 1 , a possible consumer CON receives only cooled outlet flow VF, while the second alternative ALT 2  provides that the consumer CON receives un-cooled outlet flow VF. In each of the two alternatives ALT 1 , ALT 2 , a flap-type valve CV is provided on the outlet side, upstream of the connection to a consumer CON, such that the arrangement can be separated from the consumer—for example during shutdown. The patent application provides that functionally identical components are provided in part with identical reference signs. In the various figures, identical reference signs mean that the components are identical or have the same function. In the following, this will not be explained repeatedly in detail for each figure. 
         [0027]    The arrangements shown in  FIG. 1  and in  FIGS. 2-5  each have a bypass station BST which draws off part flows of the compressed outlet flow VF from the outlet EX of the compressor CO. One outlet line EXL of the compressor CO has the cooler COL. Upstream of the cooler COL, a first bypass flow BF 1  is supplied to the bypass station by means of a first bypass line BL 1 . Downstream of the cooler COL, a second bypass flow BF 2  is supplied to the bypass station BS by means of a second bypass line BL 2 . The bypass station BST controls the quantity of the bypass flows BF 1 , BF 2  in dependence on operating parameters of the compressor and feeds the first bypass flow BF 1  and the second bypass flow BF 2 —here as a mixture—through a third bypass line BL 3  as mixed third bypass flow BF 3  upstream of the inlet IN of the compressor CO to the intake flow MF. A control unit CU controls the bypass station BST such that, in dependence on operating parameters of the compressor CO, in each case a determined bypass flow or feeding-in of the first bypass flow BF 1  and of the second bypass flow BF 2  takes place. The principal objective of the control unit is in this case to prevent the compressor CO entering a state of surge. Optionally, the control unit can also serve for improving efficiency. 
         [0028]      FIGS. 2 to 5  each show the arrangement and the method in somewhat more detail than  FIG. 1 . The compressor CO shown there has an inlet guide device IGV in the region of the inlet IN. The inlet guide device IGV makes it possible to adjust inlet guide vanes so as to produce a determined inflow angle α1 of the intake flow MF into the first stage ST 1  of the compressor CO. The compressor CO has two intercoolers IC 1 , IC 2  which are arranged between the first stage ST 1  and a second stage ST 2  or between the second stage ST 2  and a third stage ST 3 . Downstream of the third stage ST 3  is the outlet EX of the compressor CO, where the compressed outlet flow VF is guided into an outlet line EXL. Upstream of the subsequent cooler COL, a first part flow is, where relevant, supplied as first bypass flow BF 1  via the first bypass line BL 1  to a hot gas valve HGV, the first bypass valve BV 1  of the bypass valves BV. Downstream of the cooler COL, a second bypass flow BF 2  is supplied via a second bypass line BL 2  to a surge limiter valve PGV or the second bypass valve BV 2  of the bypass valves BV, which valve controls the supply of this cold bypass flow to a mixer MX in which the two bypass flows BF 1 , BF 2  are mixed together. As a consequence of the expansion of the bypass flows BF 1 , BF 2  by means of the bypass valves BV, the temperature of the bypass flow or of the third bypass flow BF 3 , which mixes with the intake flow MF upstream of the inlet IN and enters the compressor CO at the then resulting temperature, is set in the mixer MX in dependence on the Joule-Thomson effect of this fluid. A surge limiter ASC of a control unit CU of the bypass station BST signals to a ratio calculation unit PCU the requirements for controlling the feeding-in of the bypass flows BF 1 , BF 2 , which ratio calculation unit PCU actuates the bypass valves BV accordingly. In general terms, the feeding-in is controlled in dependence on operating parameters of the compressor CO. Specifically, the temperatures are measured by means of temperature measurement points, wherein the temperature of the intake flow is measured by means of a first temperature measurement point TT 1 , the temperature of the outlet flow VF is measured by means of a second temperature measurement point TT 2 , the temperature of the third bypass flow BF 3  is measured by means of a third temperature measurement point TT 3 , and optionally the temperature downstream of the cooler COL is measured by means of a fourth temperature measurement point TT 4 . In addition, the pressure of the intake flow is determined by means of a first pressure measurement point PT 1  and the pressure of the outlet flow VF is determined by means of a second pressure measurement point PT 2 . These measurements are evaluated by the bypass station BST or the control unit CU and result—as explained—in a corresponding valve setting of the bypass valves BV. 
         [0029]      FIG. 3  shows that the first bypass flow BF 1  and the second bypass flow BF 2  are supplied directly to a first bypass valve BV 1  in the form of a 3-way proportional valve which is actuated directly by the ratio calculation unit PCU. The total quantity of the resulting third bypass flow BF 3  is set by the second bypass valve BV 2  which is actuated by the surge limiter ASC. In this arrangement, the 3-way proportional valve, or also mixing valve, produces no notable pressure loss and can therefore cost-effectively take the form of a flap-type construction. A third check valve CV 3  is provided downstream of the first bypass valve BV 1  in the second bypass line BL 2 , such that the outlet flow VF flows through the first bypass valve BV 1  to the pressure-side process without circumventing the cooler COL. 
         [0030]    A further alternative is indicated in  FIG. 4 , in which, instead of a mixing valve in the second bypass line BL 2 , a first bypass valve BV 1  in the form of a regulating flap is provided in the first bypass line BL 1  and mixes the first bypass flow BF 1 , in a proportion controlled by the ratio calculation unit PCU, into the second bypass flow BF 2  before the mixture is fed to the second bypass valve BV 2  which expands the resulting third bypass flow in the third bypass line BL 3 , controlled by the surge limiter ASC. Since there is a pressure drop at the cooler COL, the outlet flow VF upstream of the cooler COL is at a somewhat higher pressure than downstream of the cooler, such that when the first bypass valve BV 1  is partially open, some of the hot first bypass flow still enters the first bypass line BL 1 . This effect can be additionally supported in that, for that part of the second bypass line BL 2  between the takeoff downstream of the cooler COL and the junction with the hotter first bypass flow BF 1 , a smaller cross section is selected or a diaphragm is integrated. 
         [0031]    A further modification to the system is shown in  FIG. 5 , in which a first diaphragm TH 1  causes a certain stagnation pressure in the second bypass line BL 2 . The measurement of a pressure differential PDT across the first diaphragm TH 1  allows the ratio calculation unit PCU to set the first bypass valve BV 1 , here too in the form of a mixing valve, so as to set the desired temperature at the third temperature measurement point TT 3  in the third bypass line BL 3  downstream of the second bypass valve BV 2 . 
         [0032]      FIG. 6  shows the operation of the control unit CU with the ratio calculation unit PCU and the surge limiter ASC. The diagram shown there relates to the circuit setup shown in  FIG. 2 . The second temperature measurement TT 2 , the second pressure measurement PT 2  and the first pressure measurement PT 1  are used by a first module IZHGV to calculate the resulting first temperature TH 1  of the isenthalpic state change in the first bypass valve BV 1 . A second module IZPGV of the ratio calculation unit calculates, from the fourth temperature TT 4 , the result of the second pressure measurement PT 2  and the result of the first pressure measurement PT 1 , a second temperature TP as the result of the expansion of the second bypass flow as a consequence of an isenthalpic state change in the second bypass valve BV 2 . Using the first temperature, the second temperature and the measured intake temperature from the first temperature measurement TT 1 , a third module MA determines the fraction a of the cold diversion according to the formula a=(T H -T s ) (T H -T P ). In order to set the second bypass valve BV 2 , the fraction a is multiplied with the signal X of the surge limiter ASC. The difference between the fraction a and  1  is multiplied with the signal X of the surge limiter ASC and is used as a setpoint value for opening the first bypass valve BV 1 .