Method for operating one or multiple compressors and corresponding system

Abstract

The invention relates to a method for operating one or multiple compressors (110, 120, 130, 140), wherein the multiple compressors are arranged in series for compressing a fluid (a) in successive manner, in particular in order to achieve a pre-defined suction pressure upstream of the one or the most upstream one (110) of the multiple compressors, wherein for the one or each of the multiple compressors, a safety operating correlation and an optimum operating correlation are provided, wherein each of the safety operating correlation and the optimum operating correlation provide a correlation between a rotational speed of the respective compressor and a volume flow of fluid through the respective compressor, wherein each of the safety operating correlations is defined with a respective safety margin to a respective surge limit of the respective compressor, wherein each of the optimum operating correlations is shifted, with respect to the respective safety operating correlation, towards higher volume flows and / or lower rotational speeds, the method comprising, for the one or each of the multiple compressors: controlling a pressure at an inlet of the compressor by adjusting a rotational speed of the respective compressor, such that an operating point of the compressor does not go beyond a current operating correlation towards the surge limit, wherein the current operating correlation is between the respective safety operating correlation and the respective optimum operating correlation or equal to one of them

Description

[0001] The present invention relates to a method for operating one or multiple compressors, wherein the multiple compressors are arranged in series for compressing a fluid in successive manner, in particular in order to achieve a pre-defined suction pressure upstream of the one or the most upstream one of the multiple compressors, and to a corresponding system comprising one or a plurality of compressors.Background

[0002] Compressors in series can be used to generate low system pressures (e.g., 15 to 30 mbara) to realize cooling of a fluid, typically, Helium, below a temperature of 2 K, e.g., in a connected cryo-system. Such compressors are also called cold compressors. Typically, turbo radial compressors are used. The individual compressors can also be called compressor stages. Using such refrigeration compressors makes it possible to significantly reduce the size of a required warm vacuum screw compressor. The cold compressor flow, which has, e.g., a temperature of 20K, can in addition be used to cool the high-pressure Helium flow in a respective cryo-system. Thus, cold compressors reduce costs on the one hand and increase the system efficiency of below 2 K systems on the other hand.

[0003] However, controlling the individual compressors is extremely complex, because on the one hand they have to be protected against instabilities such as surge, and on the other hand a change in a compressor located upstream has a strong influence on the compressor stages located downstream. Cold compressors can be controlled by means of or based on characteristic maps which are typically provided in advance by the manufacturer of the compressors.

[0004] Different control strategies can be used, e.g., using a combined characteristic map for all compressor stages or using individual characteristic maps for the compressor stages. This have, however, often low efficiency. Thus, an object of the present invention is to provide a way to provide a more efficient way of operating a plurality of compressors.Disclosure of the invention

[0005] This object is achieved by providing a method for operating one or multiple compressors and a corresponding system comprising multiple compressors with the features of the independent claims. Embodiments of the invention are the subject of the dependent claims and of the description that follows.

[0006] The invention relates to operating one or multiple compressors, preferably turbo radial compressors, wherein multiple compressors are arranged in series for compressing a fluid in successive manner. A preferred number of compressors is three, four or five, however, also only one or two, or more than five compressors can be used. Such compressors can comprise a drive, e.g., an electrical drive. Such compressors can also be provided with a magnet bearing for their drive. As mentioned, characteristic maps are typically provided in advance by the manufacturer of compressors. Typically, such characteristic maps are individual to a certain type or kind of compressor or even to an individual compressor. Different types of compressors can be provided with different characteristic maps.

[0007] Such a characteristic map (or characteristic diagram) provides, for each of a plurality of rotational speeds for the compressor, a correlation (speed correlation) between a pressure ratio (i.e., a ratio between the pressure at the outlet of the compressor and at the inlet of the compressor; this ratio can be referred to reference values) and a volume flow (or mass flow) of the fluid to be compressed, through the compressor; it is noted that the volume flow is often provided as so-called reduced volume flow, i.e., as normalized values without units. It is mentioned that only the use of the volume flow rate or the non-dimensionalized volume flow rate makes the characteristic maps generally valid, i.e. independent of the inlet conditions. Typically, such a correlation is a line or is provided as a line - a speed line - that defines the correlation between the pressure ratio and the volume flow. Such a characteristic map also comprises a surge limit. An operation of the compressor beyond this limit can harm the compressor and should be avoided. A best efficiency point for each speed line is, typically, approximately in the middle of such speed line; moving on the speed line towards the surge limit, efficiency typically decreases slightly, and moving on the speed line towards high volume flows, efficiency typically decreases sharply.

[0008] For multiple compressors arranged in series, operating or controlling them can be difficult, as already mentioned above. Operating a compressor typically comprises controlling a pressure at an inlet of the compressor by adjusting a rotational speed of the respective compressor, based on a current volume flow of the fluid through the respective compressor.

[0009] A possible control strategy or control scheme is to use a combined characteristic map for all compressor stages, i.e., all compressors in the line are considered as one compressor. A lot preparatory work is necessary for this, since the characteristic maps of the individual compressor have to be combined into one single characteristic map. On the one hand, this is very prone to error and, on the other hand, ongoing operation has shown that much readjustment of the control system is necessary. An advantage is that there is only one control variable due to the combination of the individual compressors. This scheme is, however, rather historically conditioned, since at the time of implementation of such control schemes controlling units were less powerful than they are today; modern powerful control units render the mentioned advantage moot.

[0010] Another control scheme is that each compressor is controlled individually via its characteristic map. Based on a characteristic map, an operating line - or, generally, an operating correlation - for a compressor can be determined or defined, which operating correlation or line defines or assigns one of the correlations and thus a rotational speed of the compressor that is required (or suggested) for a certain volume flow.

[0011] Thus, during operation of the compressor, for a current value of a volume flow (which can be measured) of fluid through the compressor, a speed line and thus a rotational speed can be determined based on the operating line. The operating lines or correlations for the plurality of compressors can be determined such that the overall efficiency is high. When an operating point of the compressor reaches or is at the operating line or correlation, the compressor is controlled such that the operating point only moves on the operating line or correlation, i.e., the operating point of the compressor should not go beyond the operating correlation towards the surge limit.

[0012] It has turned out that a problem with this scheme is that the compressors are typically forced to operate very close to the surge limit although they are designed for best efficiency point, whereby there is a risk of mechanical damage to the compressor and / or its drive if the surge limit is exceeded. Typically, compressors are operated for all operating points of the system at a high pressure ratio near the surge limit. Depending on the inlet temperature, there is a maximum rotational speed at which the compressor reaches its maximum permissible rotational speed. When this speed is reached, the compressor remains on the respective speed line assigned to this rotational speed and the operating point is automatically shifted towards higher flows, where a sharp drop in efficiency is expected.

[0013] Thus, on the one hand, the characteristic map (i.e. its possible areas of operation) is hardly utilized when operating each compressor close to the surge limit, and on the other hand, efficiency is reduced compared to the design point.

[0014] Furthermore, the compressor stages located downstream reach their maximum speed earlier due to the increased pressure ratio of the stages located upstream. This automatically shifts the operating point of the compressor stages located downstream close to the surge limit, at which, however, a sharp drop in efficiency is to be expected.

[0015] The increased pressure ratio, which results from the defined operating line or correlation close to the surge limit, causes the following problem: the downstream compressor stages (i.e. their operating points) are shifted close to the surge limit because they experience an increased inlet temperature due to the high pressure ratio of the upstream compressor stages. This significantly reduces compressor efficiency. The second compressor stage operates at a high pressure ratio on the operating line described and prevents the first compressor stage from reaching this operating line at all. As a result, the first compressor stage also operates at a low efficiency closer to the surge limit and the maximum achievable overall efficiency of the plurality of compressors is greatly reduced.

[0016] In view of that, an improved control scheme for operating multiple compressors, wherein the multiple compressors are arranged in series for compressing a fluid in successive manner, is provided. For each of the multiple compressors, a safety operating correlation (or line) and an optimum operating correlation (or line) are provided. The first safety correlation and the optimum operating correlation are defined based on the individual characteristic map assigned to the respective compressor, like the (single) operating correlation described above. As mentioned above, the method can also be used with just one compressor.

[0017] Each of the safety operating correlations is defined with a respective safety margin to the surge limit of the individual characteristic map of the respective compressor. Such first operating correlations can, for example, correspond to the (single) operating correlations described above.

[0018] Each of the optimum operating correlations is shifted, with respect to the respective safety operating correlation, towards higher volume flows. In an embodiment, the optimum operating correlation is defined based on (or is laid through) design operating points of the respective compressor. Such design operating points are typically provided by the manufacturer of the compressor and are operating points providing optimum or essentially optimum efficiency. The design operating points are typically provided on the speed correlations or speed lines of the respective characteristic map. Thus, the optimum operating correlation can also be called best efficiency points correlation or best efficiency points line.

[0019] Operating the multiple compressors comprises, for each of the multiple compressors: controlling a pressure at an inlet of the compressor by adjusting a rotational speed of the respective compressor, such that an operating point of the compressor does not go beyond a current operating correlation towards the surge limit; this can also be based on a current volume flow of the fluid through the respective compressor. In particular, if the operating point of the compressor reaches or is at a current operating correlation, the rotational speed of the respective compressor is determined based on the current operating correlation and a current volume flow of the fluid through the respective compressor. This means that the operating point is only moved along the current operating correlation. The current operating correlation is between the respective safety operating correlation and the respective optimum operating correlation or equal to one of them. In particular, the current operating correlation is chosen or determined, within the safety operating correlation and the optimum operating correlation, based on the current situation. The operating point can be defined by a current volume flow of the fluid through the respective compressor, the current rotational speed and, preferably, also the current pressure ratio of the compressor. Typically, however, either volume flow or pressure ration is used. For turbo compressors (in particular when close to the surge limit), the volume flow is preferably used because the respective curve is relatively flat and a small change of the pressure ratio will result a big change in the volume flow. Thus, using the pressure ration would result in a less exact measurement than the volume flow. It is noted that each compressor has inlet pressure and temperature, and the volume or mass flow can be measured, e.g. via a venturi, behind the last compressor. The rotational speeds can be read directly from the drives. The respective values of these parameters are then provided for the control.

[0020] In case of only one compressor, there is only one characteristic map and one current operating correlation. The pressure at the inlet of the one compressor is, thus, controlled so as to achieve the pre-defined suction pressure at the inlet or upstream of the one compressor; this pressure is, thus, the actual controlled variable.

[0021] In case of multiple compressors, in an embodiment, the pressure at the inlet of the most upstream one of the multiple compressors is controlled so as to achieve the pre-defined suction pressure at the inlet or upstream of the one compressor; this pressure is, thus, the actual controlled variable. For each the remaining ones of the multiple compressors, however, is controlled only insofar as the operating point of the respective compressor does not go beyond the current operating correlation towards the surge limit. The pressure at the inlet of each of these remaining compressors results from the pressure at the inlet of the first (most upstream) compressor and the flow of the fluid through the successive compressors.

[0022] Both the safety operating correlation and the optimum operating correlation are, in an embodiment, defined in such a way that they are shifted in parallel, considering various margins from the surge limit, with the shift being proportional to the rotational speed of the compressors. This provides an extended control option for the suction pressure by shifting the operating correlation that passes through the best efficiency points in the direction of the safety operating correlation in order to lower the suction pressure. This means that as soon as all compressors have reached their optimum operating correlation, the current operating correlation is shifted or moved towards the safety operating correlation, in order to further reduce the suction pressure at a given volume flow.

[0023] All compressors can reach their safety operating correlation at the same time, but these correlations are the maximum possible pressure ratio at a given volume flow; otherwise, the surge limit would be passed. If the suction pressure remains higher than the target value, a bypass valve can be opened, such that fluid from the outlet of the most downstream compressor is provided to the inlet of the most upstream compressor (in case of only one compressor, this applies to the outlet and inlet of the one compressor), increasing the flow through the compressors, thus achieving a higher pressure ratio, allowing the suction pressure to drop further. Opening the bypass valve transfers warm mass from the outlet of the most downstream compressor to the inlet of the most upstream compressor, increasing the volume flow to be pumped.

[0024] In addition, or alternatively, once the operating points of the one or all of the multiple compressors have reached their respective safety operating correlation, additional fluid to the inlet of the one or the most upstream compressor, in order to further reduce the suction pressure from another source, e.g., evaporating liquid fluid.

[0025] This improved control scheme allows for a higher efficiency than the previous schemes mentioned above and at the same time makes better use of the given width of the characteristic map of a compressor, which has a further positive effect on the overall efficiency and, in addition, avoids the proximity to unstable operating areas (surge) in the design points. Pressures below 100 mbara, or even below 60 mbara or 20 mbara or 15 mbara can be achieved in this way. Likewise, the implementation of this scheme in a control system is simple.

[0026] The overall efficiency of the compressors is significantly improved compared to previous control schemes. Since the compression heat of cold compressors also represents a load for the actual cryogenic system, the cryogenic system can also operate more efficiently with this control scheme, which reduces system operating costs.

[0027] The operating window is extended compared to previous control schemes because the new operating line (the optimum operating correlation) can be laid directly through the best efficiency points of the compressor stages. Consequently, the compressors operate further away from the unstable surge limit at the system design point than with existing control schemes. The risk of mechanical damage to a compressor due to process disturbances, causing it to operate briefly in the unstable range, is thus greatly reduced.

[0028] The new operating scheme is also easy to implement in the control system. For each compressor, the surge limit can be parameterized (for example, using a third-degree polynomial). The safety operating correlation and the optimum operating correlation can be determined for each compressor using a defined margin from the surge limit, e.g., a certain per-cent margin. These parameters are already sufficient to implement the compressor control.

[0029] A data processing system according to the invention, e.g., a computer, a control unit or other computing system, is configured, to perform the steps of a method according to the invention.

[0030] There is also provided a corresponding system comprising one or multiple compressors, wherein the multiple compressors are arranged in series for compressing a fluid in successive manner. The system further comprises a data processing system as mentioned above.

[0031] For further embodiments and advantages of the system it is referred to the description of the method and respective embodiments, which applies accordingly. In particular, the system implements the method according to any one of the mentioned embodiments. This may require the system comprising one or more additional components mentioned in the respective embodiment.

[0032] The implementation of a method according to the invention in the form of a computer program or computer program product with program code for performing all method steps is also advantageous, since this causes particularly low costs, especially if an executing control unit is still used for further tasks and is therefore present anyway. Finally, a machine-readable storage medium is provided with a computer program stored thereon as described above. Suitable storage media or data carriers for providing the computer program are, in particular, magnetic, optical and electrical memories, such as hard disks, flash memories, EEPROMs, DVDs and the like. It is also possible to download a program via computer net-works (Internet, intranet, etc.).

[0033] Such a download can take place wired or wirelessly (e.g. via a WLAN network, a 3G, 4G, 5G or 6G connection, etc.).

[0034] Further advantages and embodiments of the invention will be apparent from the description and the accompanying drawing. The invention is illustrated schematically by means of embodiments in the drawing and is described below with reference to the drawing.Short description of the figures

[0035] Fig. 1illustrates a system for explaining the invention; Figs. 2 to 6illustrate, in diagrams, characteristic maps of compressors for explaining an embodiment of the invention.

[0036] Fig. 1 schematically illustrates an exemplary system 100 for explaining the invention. The system comprises multiple, by means of example four, compressors 110, 120, 130, 140, which are arranged in series for compressing a fluid a in successive manner. It is noted that each of the compressors 110, 120, 130, 140 comprises a drive which is not explicitly shown here.

[0037] The multiple compressors can be used in connection with a cryo-system 102, e.g., a customer system, where the fluid, as stream a, is received from and where very low pressure is to be achieved. After compression, the fluid, as stream b, can be supplied to another cryo-system 106, e.g., a so-called cold-box. The fluid is supplied, e.g., via different components like valves and others to the first one, i.e., the most upstream compressor 110. After the last one, i.e., the most downstream compressor 140, the fluid is supplied, e.g., via a valve, to cryo-stem 106.

[0038] This is particular in order to achieve a pre-defined suction pressure upstream of the most upstream one of the multiple compressors, i.e., upstream of compressor 110. Such suction pressure can be, e.g., 60 mbara or even 20 mbara or below. This is required to achieve a temperature below 2 K in the cryo-system 102. A typical fluid to be used is Helium.

[0039] The system 100 further comprises different measurement units in order to measure parameters, e.g., volume flow (or mass flow), temperature and pressure, before and after each of the compressors 110, 120, 130, 140. It is noted that between two consecutive compressors, one set of measurement units is sufficient to measure the respective values after the upstream one and before the downstream one of the two compressors. Note that not all of these measurement units might be necessary; for example, a single measurement unit for flow measurement could be sufficient, e.g., at the outlet of the most downstream compressor.

[0040] Using the setup as shown in Fig. 1 can preferably be used if the operating points are determined via volume flow. Inlet pressure and temperature allow to determine the volume flow through the respective compressor. In addition, the inlet temperature allows to determine the reduced speed (similar to the mass flow, the absolute speed is only valid for a certain inlet temperature; that is why dimensionless units are used). If the operating points were defined by the pressure ratio, the volume flow measurement could be eliminated. However, as described above, the pressure ratio as a measured variable results in a larger error close to the surge limit, since the pressure ratio changes only slightly here.

[0041] The system 100 further comprises a bypass valve 150, by means of which fluid from the outlet of the most downstream compressor 140 can provided to the inlet of the most upstream compressor 110, in order to further reduce the suction pressure.

[0042] The system 100 further comprises a storage 160 for liquid fluid (e.g., also Helium). If required, some of the liquid fluid can be evaporated and provided, as additional fluid c, to the inlet of the most upstream compressor 140, in order to further reduce the suction pressure.

[0043] The system 100 further comprises a data processing system 104, e.g., a control unit, by means of which each of the multiple compressors 110, 120, 130, 140 can be operated or controlled. This typically includes receiving the measured values of the mentioned parameters (or from at least some of them), determining command values for the drive and providing them to the respective compressor or its drive.

[0044] Figs. 2 to 6 illustrate, in diagrams, characteristic maps of compressors for explaining an embodiment of the invention.

[0045] In Fig. 2, a characteristic map 200 for a compressor is shown. The diagram shows, on the vertical axis, a ratio π between the pressure at the outlet of the compressor and at the inlet of the compressor, and, on the horizontal axis, a volume flow (or mass flow) V (often also referred to as φ) of the fluid to be compressed, through the compressor, in normalized values without units.

[0046] The characteristic map 200 provides, for the compressor, for each of a plurality of rotational speeds, a correlation between the pressure ratio and the volume flow (or mass flow). Such correlations are shown as lines (speed lines) and one of them is referred to by 210. Note that these speed lines correspond to different rotational speeds of the compressor; the actual values of these rotational speeds for the speed lines are not of particular relevance for the present invention. Further, typical speed lines correspond to a certain percentage of a design rotational speed limit e.g., 50%, 60%, 70%, 80%, 90%, 95%, 100%. Also, e.g. 105% can be provided, as an absolute maximum rotational speed. While 105% can sufficient, e.g., for the method discussed herein, some compressors may also be operated at up 120% of a design rotational speed limit as an absolute maximum rotational speed.

[0047] The characteristic map 200 also comprises a surge limit 220. An operation of the compressor beyond this limit can harm the compressor and should be avoided. An efficiency optimum for each speed line is, typically, approximately in the middle of such speed line; moving on the speed line towards the surge limit 220 (to the left in Fig. 2), efficiency typically decreases slightly, and moving on the speed line towards high volume flows (to the right in Fig. 2), efficiency typically decreases sharply.

[0048] By means of example, a design operating point 232 is shown, which provides good or even best efficiency. A maximum load operating point 234 is also shown.

[0049] Further, two operating correlations, a safety operating correlation 240 and an optimum operating correlation 242, are shown as lines (operating lines). The safety operating correlation 240 is defined with a safety margin to the surge limit 220, e.g., with a safety margin of 2%. The optimum operating correlation 242 is shifted, with respect to the safety operating correlation 240, towards higher volume flows and / or lower rotational speeds, e.g. with a further margin with respect to the surge limit 220 or the safety operating correlation 240, respectively. The margin of the optimum operating correlation is preferably defined in such a way that it passes through the design points of the compressor.

[0050] Typically, such a characteristic map including the surge limit is provided for each compressor by the manufacturer. The two operating correlations, the safety operating correlation 240 and the optimum operating correlation 242, can be determined by the designer of the compressor when implementing the compressor in a system.

[0051] Note that typically, one of such characteristic maps is provided for each of the multiple compressors, and these characteristic maps can be different from each other. The characteristics maps of the remaining compressors will look more or less similar if no specific numbers are considered. However, the speeds (speed lines) will be different, these increase steadily from compressor to compressor, from upstream to downstream. The most upstream compressor can have the largest wheel diameter for this purpose. For example, the speed line 210 in Fig. 2 can correspond to about 150 rpms (90% of design rotational speed limit). The respective speed line 410 in Fig. 4 can correspond to about 295 rpms (90% of design rotational speed limit), the respective speed line 510 in Fig. 5 can correspond to about 515 rpms (90% of design rotational speed limit), and the respective speed line 610 in Fig. 6 can correspond to about 635 rpms (90% of design rotational speed limit).

[0052] For operating the multiple compressors, for each of the compressors, a pressure at an inlet of the compressor is controlled by adjusting a rotational speed of the respective compressor. This in particular means that for the most upstream compressor 110, the pressure at the inlet of the compressor is controlled to achieve the pre-defined suction pressure, e.g., the suction pressure required for cryo-system 102. For the remaining compressors the pressure at the inlet results from this most upstream suction pressure and the entire system as such and the flow of the fluid through it. Basically, the speed can be adjusted more or less deliberately within the correlation map. However, the surge limit should not be exceeded. Thus, a current operating correlation or operating line is used, such that an operating point of the compressor does not go beyond that current operating correlation towards the surge limit. The latter applies to all compressors within the control.

[0053] If the operating point of the compressor reaches or is at the current operating correlation, the rotational speed of the respective compressor is determined based on the current operating correlation and a current volume flow of the fluid through the respective compressor. In other words, the operating point is moved only on the current operating correlation or current operating line.

[0054] The current operating correlation is determined or chosen such that it is between the safety operating correlation 240 and the optimum operating correlation 242 or equal to one of them. In particular, there is no single operating correlation but the current operating correlation can be moved between the two operating correlations, the safety operating correlation 240 and the optimum operating correlation 242, based on the current situation, for example.

[0055] As mentioned, the controlled variable for the entire system is the suction pressure upstream of the most upstream one of the multiple compressors, i.e. upstream of compressor 110. This pressure shall achieve the pre-defined suction pressure. This pressure can, e.g., be controlled by means of a PI controller using the above-mentioned scheme. If all compressors are on their optimum operating correlation (efficiency best line), but the suction pressure is higher than the controller target value, the PI controller can start to regulate - between 0% and 100%. 0% corresponds to the efficiency best line and 100% to the safety operating correlation (safety line). Any value in between can then be calculated from the control. This means that if, for example, the controller outputs 50%, the parallel displacement of the operating lines allows this line to be calculated directly. The speed of the respective compressor can then be adjusted so that all compressors are kept on this 50% line, depending on the volume flow.

[0056] In Fig. 3, a diagram illustrating a way of how to implement such operating correlations is shown. The diagram shows, on the vertical axis, a rotational speed n (also frequency f could be used) of the compressor (here, a so-called reduced speed in 1 / s is used; note that typically a reduced unit is used with index "red" which is not applied here for sake of readability), and, on the horizontal axis, a volume flow (or mass flow) V of the fluid to be compressed, through the compressor, in normalized values without units (so-called reduced volume or mass flow or flow rate).

[0057] In the diagram, a surge limit 320, a safety operating correlation 340 and an optimum operating correlation 342 (as lines) are shown. The surge limit 320 corresponds to the surge limit 220 of the compressor. The safety operating correlation 340 and the optimum operating correlation 342 correspond to the safety operating correlation 240 and the optimum operating correlation 242 as shown in Fig. 2, however, in a different presentation.

[0058] The surge limit 320 can represented as polynomial, e.g. a three-degree polynomial. The safety operating correlation 340 and the optimum operating correlation 342 can be shifted with respect to the surge limit 320, in parallel to the surge limit, with the shift being proportional to the rotational speed of the compressor. In this way, it can be sufficient to provide the polynomial representing the surge limit 320 and the values of the shifts, e.g. 2% for the safety operating correlation 340 and 5% or 10% for the operating correlation 342, to the control unit. As mentioned, the optimum operating correlation can be defined based on design operating points of the respective compressor, wherein the design operating points are operating points providing at least essentially optimum efficiency. This is also shown in Fig. 2

[0059] In an embodiment, for starting up operation of the multiple compressors, the most downstream compressor 140 can be first started or operated. While the last compressor 140 regulates the suction pressure in front of the first compressor 110, the other compressors rotate just fast enough so that no pressure ratio is created. As the current operating correlation - this holds true for all of the multiple compressors - the optimum operating correlation can be used. When the operating point of compressor 140 has reached its optimum operating correlation (which is the current operating correlation), the next upstream compressor 130 is started and compressors 110 and 120 rotate just fast enough so that no pressure loss is created (pressure ratio equal to 1). This can be repeated until all four compressors are running, i.e. are operating on their efficiency best line.

[0060] Once the operating points of all of the multiple compressors have reached their optimum operating correlation, the respective current operating line for reach compressor are moved, from the respective optimum operating correlation (efficiency best line) towards the respective safety operating correlation (pump safety line). This allows greater usage of the correlation map, and thus, further reducing the suction pressure. As described above, this moving can be regulated (PI controller) and all lines between the efficiency best line and the pump safety line can be calculated by the control.

[0061] Once the operating points of all of the multiple compressors have reached their respective safety operating correlation, the bypass valve 150 can be opened, such that fluid from the outlet of the most downstream compressor 140 is provided to the inlet of the most upstream compressor 110, in order to further reduce the suction pressure. Alternatively, or in addition, additional fluid c from storage 160 can be provided to the inlet of the most upstream compressor 140, in order to further reduce the suction pressure.

[0062] This control scheme or strategy results in different operating points for the different ones of the multiple compressors, which will be explained in the following based on the characteristic maps of the individual compressors.

[0063] While the characteristic map 200 of Fig. 2 can correspond to the characteristic map of the first compressor, Figs. 4, 5, 6 show characteristic maps 400, 500, 600 and corresponding operating correlations and operating points for the second, third and fourth compressor. The first to fourth compressors correspond to compressors 110 to 140 of Fig. 1. Note that the characteristic maps that are shown look similar, however, the maximum rotational speeds in the characteristic maps increase from the first to the fourth compressor, as mentioned above. Also, the operating points are different.

[0064] In each characteristic map, see Figs. 4, 5, 6, the surge limit 420, 520, 620 is shown. Further, in each characteristic map, the respective safety operating correlation 440, 540, 640 is shown and the respective optimum operating correlation 442, 542, 642 is shown. Also, one exemplary speed line 410, 510, 610 is shown.

[0065] In each characteristic map, two different operating points for a method in an embodiment, 250, 252; 450, 452; 550, 552; 650, 652 are shown, using a square and a triangle. For comparison, in each characteristic map, two different operating points 260, 262; 460, 462; 560, 562; 660, 662 for a method using only a single operating correlation, are shown, using a square and a triangle.

[0066] The operating points for the first compressor, Fig. 2, are coordinated such that the volume flow of the warm screw compressor (the warm screw compressor is part of the cryo-system) is precisely met. The points 250, 252 lie on the optimum operating correlation, so that a high efficiency is present. The points 260, 262 are shifted to higher flows, since the last three compressor stages already have such a high pressure ratio that the first compressor stage does not reach the operating line or correlation (line 240, which is the only operating line in this former control scheme). The first compressor is, thus, already operating at a higher efficiency with the new control scheme.

[0067] The operating points for the second compressor, Fig. 4, in the new control scheme (points 450, 452) lie on the optimum operating correlation, such that there is a high efficiency. The points 460, 462 (previous scheme) would actually lie on line 440, but here an additional limitation applies (largest reduced speed line given by the manufacturer), so that the operating points lie on this line and are maintained. The operating points 460, 462 are located slightly to the left of the best efficiency points, so that there is a slight drop in efficiency compared to these. The second compressor, thus, also operates with a higher efficiency when following the new control scheme. It is noted that if this limit did not apply under the previous scheme, this point would lie directly on the pump safety line; this made it even clearer that the efficiency was reduced.

[0068] The operating points for the third compressor, Fig. 5, in the new control scheme (points 550, 552) lie on the optimum operating correlation, such that there is a high efficiency. The points 560, 562 (previous scheme) would actually lie on line 540, but here an additional limitation applies (the maximum circumferential speed of the compressor, e.g. 240 m / s, is reached), so that the operating points lie on this line and are maintained. However, the operating points here almost coincide with the best efficiency line, so that (as with the points 550, 552 and the new control scheme) a high compressor efficiency is also present. The third compressor, thus, also operates with a higher efficiency when following the new control scheme, but a similar efficiency is achieved with the previous scheme (this is, however, only by chance). If the rotational speed limit did not apply, the point would also operate at a lower efficiency on the pump safety line. Likewise, compressor 110 would operate even closer to the blocking limit (i.e., at an even lower efficiency) if this limit and the one mentioned for Fig. 4 did not exist.

[0069] For the operating points for the fourth compressor, Fig. 6, in the new control scheme (points 650, 652) the limitation of the maximum circumferential speed (e.g. 240 m / s) takes effect and the operating points are maintained on these speed lines. This also applies for the points 660, 662 (previous scheme). For both schemes, the operating points are shifted towards higher flow rates, whereby both control schemes achieve a reduced efficiency. It is noted that compressor 140 could be redesigned to also operated at the best efficiency line. However, the overall situation shows that working with existing compressors or compressor designs allows efficiency improvements.

[0070] The fourth compressor, thus, operates at a limited speed at high flow rates for both control schemes, resulting in a reduced efficiency. Nevertheless, the new control scheme achieves a significantly higher overall efficiency, since the first three compressors operate on the optimum operating correlation, i.e., the efficiency best line. Further, the new scheme operates with significantly more distance to the surge limit, which provides additional security.

Claims

1. A method for operating one or multiple compressors (110, 120, 130, 140), wherein the multiple compressors are arranged in series for compressing a fluid (a) in successive manner, in particular in order to achieve a pre-defined suction pressure upstream of the one or the most upstream one (110) of the multiple compressors, wherein for the one or each of the multiple compressors, a safety operating correlation and an optimum operating correlation are provided, wherein each of the safety operating correlation and the optimum operating correlation provide a correlation between a rotational speed of the respective compressor and a volume flow of fluid through the respective compressor, wherein each of the safety operating correlations is defined with a respective safety margin to a respective surge limit of the respective compressor, wherein each of the optimum operating correlations is shifted, with respect to the respective safety operating correlation, towards higher volume flows and / or lower rotational speeds, the method comprising, for the one or each of the multiple compressors: controlling a pressure at an inlet of the compressor by adjusting a rotational speed of the respective compressor, such that an operating point of the compressor does not go beyond a current operating correlation towards the surge limit, wherein the current operating correlation is between the respective safety operating correlation and the respective optimum operating correlation or equal to one of them.

2. The method of claim 1, for operating multiple compressors, wherein, for the most upstream one (110) of the multiple compressors, the pressure at the inlet of the compressor is controlled to achieve the pre-defined suction pressure upstream of the most upstream one (110) of the multiple compressors, and wherein, for each of the remaining ones (120, 130, 140) of the multiple compressors, the pressure at the inlet of the compressor results from the pressure at the inlet of the most upstream one (110) of the multiple compressors and a flow of the fluid.

3. The method of claim 1 or 2, wherein, if the operating point of the respective compressor reaches or is at the current operating correlation, the rotational speed of the respective compressor is determined based on the current operating correlation and a current volume flow of the fluid through the respective compressor.

4. The method of any one of the preceding claims, wherein each of the optimum operating correlations is defined based on design operating points of the respective compressor, wherein the design operating points are operating points providing at least essentially optimum efficiency.

5. The method of any one of the preceding claims, wherein for the one or each compressor, the safety operating correlation and the optimum operating correlation are defined such that they are shifted in parallel from the surge limit, with the shift being proportional to the rotational speed of the compressor.

6. The method of any one of the preceding claims, for operating multiple compressors, further comprising, for starting up operation of the multiple compressors: Starting operation of the most downstream compressor; and Starting operation of a next upstream compressor, once the operating point of the next downstream compressor has reached its respective optimum operating correlation.

7. The method of any one of the preceding claims, further comprising, once the operating points of the one or all of the multiple compressors have reached their optimum operating correlation: Moving, for the one or each of the multiple compressors, the respective current operating correlation from the respective optimum operating correlation towards the respective safety operating correlation, in order to reduce the suction pressure.

8. The method of any one of the preceding claims, further comprising, once the operating points of the one or all of the multiple compressors have reached their respective safety operating correlation: Opening a bypass valve (150), such that fluid from the outlet of the one or the most downstream compressor is provided to the inlet of the one or the most upstream compressor, in order to further reduce the suction pressure.

9. The method of any one of the preceding claims, further comprising, once the operating points of the one or all of the multiple compressors have reached their respective safety operating correlation: Providing additional fluid (c) to the inlet of the one or the most upstream compressor, in order to further reduce the suction pressure.

10. The method of any one of the preceding claims, wherein the safety operating correlation and the optimum operating correlation are defined based on an individual characteristic map assigned to the respective compressor, wherein each of the individual characteristic map provides, for each of a plurality of rotational speeds for the respective compressor, a speed correlation between a pressure ratio and a volume flow, and wherein each of the individual characteristic maps comprises a surge limit.

11. The method of any one of the preceding claims, wherein the one or the multiple compressors are controlled, such that the pre-defined suction pressure at the inlet of the one or the most upstream one of the multiple compressors is achieved, wherein the pre-defined suction pressure is a pressure below 100 mbara, more preferably below 60 mbara, even more preferably below 20 mbara.

12. The method of any one of the preceding claims, wherein the fluid is Helium.

13. A data processing system (104) comprising means to perform the steps of the method of any one of the preceding claims.

14. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to perform the steps of the method of any one of claims 1 to 12.

15. A system (100) comprising one or multiple compressors (110, 120, 130, 140), wherein the multiple compressors are arranged in series for compressing a fluid in successive manner, the system further comprising a data processing system (10) according to claim 13.

Images