EP3167197B1 - Method for pressure and temperature control of a fluid in a series of cryogenic compressors - Google Patents

Description

The invention relates to a method for controlling the pressure and temperature of a fluid, in particular helium, in particular when starting up a cryogenic cooling system, or during the cooling process (cool-down) in a series of cryogenic compressors according to claim 1.

Radial or turbo compressors (hereafter referred to as compressors) in series are used to overcome or create large pressure differences (of the order of 1 bar).

Methods for controlling the pressure and temperature of a fluid in a series of cryogenic compressors are known, for example, from the following patent applications: US2005178134 . US2013232999 and US2006101836 ,

Such compressors are known from the prior art and usually have a shaft with at least one impeller (compressor wheel) or directly connected to the shaft blades, with which the fluid is compressed during rotation of the shaft. In the context of the present invention, the speed of the compressor means the number of complete rotations (360 °) of the shaft about the shaft axis per unit of time. Compressor, such as Turbo compressors, are divided in particular in radial and axial compressors. In the radial compressor, the fluid flows axially to the shaft and is deflected in the radial direction to the outside. In the case of the axial compressor, on the other hand, the fluid to be compressed flows through the compressor in a direction parallel to the shaft.

By adjusting the speeds of the compressor, the inlet pressure of the fluid to a first compressor, that is, the pressure at an input of the most upstream compressor of the series, regulated. As a result, in particular, the entry states are also established at the respective inlet of the other compressor arranged downstream of the first compressor. An entry condition is determined by the pressure and the temperature at the inlet of the respective compressor. In this case, the respective entry state on a compressor corresponds in each case to the state of Fluids at the output of the previous compressor. As a result, a change in the speed of one compressor always also influences the entry states of the fluid of the other compressors of the series.

In cryogenic systems, ie cooling systems that are designed for very low temperatures (1.5K-100K), especially for temperatures between 1.5K and 2.2K, the desired saturation temperature for the cold liquid is controlled by regulating the inlet pressure the suction side, that is the side from which the compressors suck the gas phase (vapor). In the compression process in the series (but also in a single compressor), the pressure at the outlet of the series and the temperature of the fluid flowing through the compressor is increased (polytropic compression process). To smooth the influence of operating point variations, so-called reduced variables are used, such as the reduced mass flow through the compressor or the reduced speed for the compressor in the control. To calculate these reduced sizes, you need the size per se (for example, the mass flow or speed of the compressor), the temperature, pressure, and the design values (or design points) of the compressors. The design values are the operating conditions of a compressor where the compressor operates with the greatest efficiency (most economically). Compressors have design values, for example, with regard to the speed, the temperature and the pressure above the respective compressor. The goal is to operate the compressors of the series close to their design points.

Usually, when starting up such a cryogenic refrigeration system, first the fluid on the suction side of the compressor series is cooled down very far (for example, from 300K to 4K). This can be done at atmospheric pressure, ie 1 bar. Lower temperatures are then realized with suppression. This process is also called cool-down. The pressure reduction on the suction side of the system is achieved by commissioning the compressor series. In particular, it serves to further lower the temperature above the fluid (pump-down). The temperature increase of the fluid due to the compression process when flowing through the compressor series, for example, three to four compressors is in the range of about 4K to 23K.

If the compressors of the series are not in operation, ie no compression takes place, the temperature of the mass flow at 4K is at the outlet of the compressor series, which, as explained below, can be problematic. A heat exchanger arranged downstream of the compressor series, which is used for cooling a parallel mass flow, is designed for example at 23K. However, if this heat exchanger is flowed through by the 4K cold mass flow from the compressor series for a long time, the parallel mass flow in the heat exchanger is cooled very far. However, since this parallel mass flow downstream is still being expanded by a turbine, condensation of the parallel mass flow within the turbine could occur. To avoid this condensation, the turbine is switched off, whereby the cooling process is temporarily interrupted. These operating conditions are to be avoided and are referred to as trip the system. On the other hand, if the compressors are started simultaneously with the system, ie, compress the fluid, warm fluid flows from the suction side through the compressors since the system is still warm. At these temperatures, the gas density of the fluid is very low. Therefore, the compressor will have very high speeds due to a predetermined target pressure, for example 20mbar on the suction side. However, the high gas temperature causes the compressors to quickly reach their maximum speeds. The cause of the high speeds is on the one hand the low preset target pressure and on the other hand the comparatively high temperatures at the compressors. In this area, it comes in the worst case to overspeed. Overspeed speeds are speeds for which the compressors are not designed and should therefore be avoided. Therefore, in the parallel cool-down and pump-down, the compression of the fluid in the compressor series would have to be interrupted again and again, so that the temperature in the compressors is not too high. As mentioned above, the temperature is included in the reduced control quantities, such as the reduced speed, i.e., an increase in the temperature at the compressor causes the reduced speed to increase. It would therefore be desirable to have a temperature control for the entry of the compressor series in particular for the cool-down or the pump-down phase, which ensures an uninterrupted pump-down with simultaneous cool-down.

• Erfassen einer Ist-Drehzahl für jeden Verdichter, wobei die Ist-Drehzahl die momentane Drehzahl des Verdichters ist,
• Erfassen eines Ist-Eintrittsdrucks, sowie einer Ist-Eintrittstemperatur am Eingang des am weitesten stromauf angeordneten, ersten Verdichters der Serie, wobei die Strömungsrichtung der Serie insbesondere von der Saugseite der Verdichter in Richtung steigenden Drucks weist, und wobei die Ist-Eintrittstemperatur und der Ist-Eintrittsdruck insbesondere die momentan herrschende Temperatur bzw. der momentan herrschende Druck am Eingang des ersten Verdichters ist,
• Vorgeben einer Maximaldrehzahl für jeden Verdichter der Serie sowie eines Soll-Eintrittsdrucks für den ersten Verdichter der Serie, wobei die Maximaldrehzahl insbesondere die höchste zulässige Drehzahl des jeweiligen Verdichters ist, bei der ein stabiler Betrieb des jeweiligen Verdichters gewährleistet ist, und wobei der Soll-Eintrittsdruck der am Eingang des ersten Verdichters zu erreichende Druck ist,
• Bestimmung eines Drehzahlindex für jeden Verdichter der Serie aus der Maximaldrehzahl und der Ist-Drehzahl jedes Verdichters,
• Bestimmung eines Proportionalwertes aus der Abweichung des Ist- vom Soll-Eintrittsdruck,
• Bestimmung eines Prioritätswertes aus dem kleineren von beiden Werten: Proportionalwert und dem kleinsten Drehzahlindex aller Verdichter der Serie (bevorzugt wird der Prioritätswert dem kleineren der beiden besagten Werte gleichgesetzt)
  • wobei wenn der Proportionalwert kleiner als der kleinste Drehzahlindex aller Verdichter der Serie ist, der Prioritätswert aus dem Proportionalwert bestimmt wird, und
  • wobei wenn der Proportionalwert größer als der kleinste Drehzahlindex aller Verdichter der Serie ist, der Prioritätswert aus dem kleinsten Drehzahlindex aller Verdichter der Serie bestimmt wird um die Priorität der Reglung auf der Regelung der Eintrittstemperatur am ersten Verdichter zu basieren,
• Ermitteln einer Soll-Eintrittstemperatur für den ersten Verdichter der Serie und einer Soll-Drehzahl für jeden Verdichter mit Hilfe des Pioritätswerts,
• Einstellen der Ist-Eintrittstemperatur des ersten Verdichters, auf die ermittelte Soll-Eintrittstemperatur,
• Einstellen der Ist-Drehzahl für jeden Verdichter auf die ermittelte Soll-Drehzahl.

This problem is solved by the method according to claim 1. For this purpose, the following steps are provided:

• Detecting an actual speed for each compressor, the actual speed being the instantaneous speed of the compressor,
• Detecting an actual inlet pressure, as well as an actual inlet temperature at the inlet of the most upstream, the first compressor of the series, wherein the flow direction of the series, in particular from the suction side of the compressor in the direction of increasing pressure, and wherein the actual inlet temperature and the actual Inlet pressure is in particular the currently prevailing temperature or the currently prevailing pressure at the inlet of the first compressor,
• Specifying a maximum speed for each compressor of the series and a target inlet pressure for the first compressor in the series, wherein the maximum speed is in particular the highest allowable speed of the respective compressor, in which a stable operation of the respective compressor is ensured, and wherein the target inlet pressure is the pressure to be reached at the inlet of the first compressor,
• Determining a speed index for each compressor of the series from the maximum speed and the actual speed of each compressor,
• Determination of a proportional value from the deviation of the actual and desired inlet pressure,
• Determining a priority value from the smaller of the two values: Proportional value and the smallest RPM index of all compressors of the series (preferably the priority value is set equal to the smaller of the two said values)
  • if the proportional value is smaller than the smallest speed index of all the compressors in the series, the priority value is determined from the proportional value, and
  • wherein if the proportional value is greater than the smallest RPM index of all the compressors of the series, the priority value is determined from the lowest RPM index of all the compressors of the series to base the priority of the regulation on the regulation of the inlet temperature at the first compressor,
• Determining a desired inlet temperature for the first compressor of the series and a desired speed for each compressor using the Pioritätswerts,
• Setting the actual inlet temperature of the first compressor to the determined target inlet temperature,
• Setting the actual speed for each compressor to the determined target speed.

wobei k ein Proportionalitätsfaktor ist.
Über den Prioritätswert wird also insbesondere festgelegt, welcher der beiden Werte, der Proportionalwert oder der kleinste der Drehzahlindices, zum Regeln der Verdichterserie verwendet wird. Wenn der Prioritätswert beispielsweise dem Proportionalwert entspricht, dann liegt die Priorität der Regelung auf der Druckregelung (also insbesondere dem Pump-Down), da der Proportionalwert insbesondere die Druckdifferenz als Steuerwert widerspiegelt. Wenn der Prioritätswert dem kleinsten Drehzahlindex entspricht, dann liegt die Priorität der Reglung insbesondere auf der Regelung der Eintrittstemperatur am ersten Verdichter. Bei dieser Regelung sollen die Drehzahlen der Verdichter nicht weiter steigen.The proportional value is in particular proportional to the difference between the desired inlet pressure and the actual inlet pressure: $$\mathit{prop}=-k\left(p_{\mathit{should}}-p_{\mathit{is}}\right).$$

where k is a proportionality factor.
In particular, the priority value determines which of the two values, the proportional value or the smallest of the rotational speed indices, is used to regulate the compressor series. If the priority value corresponds, for example, to the proportional value, then the priority of the regulation lies on the pressure regulation (thus in particular the pump-down), since the proportional value particularly reflects the pressure difference as a control value. If the priority value corresponds to the lowest speed index, then the priority of the control is in particular the regulation of the inlet temperature at the first compressor. In this scheme, the speeds of the compressor should not increase.

To determine the setpoint speed for each compressor, the respective inlet temperature is detected, in particular at the input of each compressor of the series.
By means of the method according to the invention, the pump-down process can take place parallel to the cool-down. As a result of the method according to the invention, the temperature no longer drops as soon as the cooling-down process is ended. In addition, the temperature of the fluid at the outlet is already regulated in a temperature range, which are favorable for downstream components, such as heat exchangers.

Another advantage is that overspeeds are avoided in all compressors, since in particular a reduction in the inlet temperature pulls lower speeds. Moreover, it is advantageous in the method according to the invention that the pump-down process without interruption, which would be necessary for example due to excessive speeds of the compressors, can take place.

It is also advantageous that the influence of unwanted heat input through the environment, ie from the outside, can be minimized. Furthermore, it is particularly advantageous that during pump-down operation, the desired inlet temperature can be regulated transiently and automatically. The inventive method is especially suitable for temperature control in supercritical helium pumps.

, wobei i der den jeweiligen Verdichter bezeichnende Index ist.In an advantageous variant of the invention, it is provided that the speed index for each compressor is equal to the ratio (quotient) of the difference between the maximum speed n _{i, max} and the actual speed n _{i of} the respective compressor and the maximum speed: $$D_i=\frac{n_{i.\mathit{Max}}-n_i}{n_{i.\mathit{Max}}}=1-\frac{n_i}{n_{i.\mathit{Max}}}$$

where i is the index denoting the respective compressor.

Particularly preferably, the priority value influences the control such that, if the smallest speed index of all compressors is smaller than the proportional value, the actual inlet temperature is lowered, in particular by stepwise or continuous lowering of the determined target inlet temperature, until the proportional value is smaller than the smallest speed index is, and that in particular the actual speed of the respective compressor is not increased as long as the smallest speed index is smaller than the proportional value. The proportional value is used in particular for controlling the actual input pressure.

In a preferred embodiment of the invention, the actual speed of each compressor from a reduced actual speed and the target speed of each compressor is determined from a reduced target speed, wherein the reduced actual speed of the actual speed and an actual temperature is determined at the input of the respective compressor, and wherein the reduced target speed is determined from the desired speed and the actual temperature at the input of the respective compressor. The detailed conversion of reduced quantities into real / absolute quantities is shown below in an exemplary formula.

bzw. $$\mathit{in}{t}_{t=n+1}=\mathit{in}{t}_{t=n}+\mathit{PW}\cdot \frac{\mathrm{\Delta }t}{T_{\mathit{int}}}$$

In one variant of the invention, an integral value is determined from the priority value, the integral value being used in particular for determining the reduced setpoint rotational speed. The integral value is in particular composed of the proportional value prop or, in general, the priority value to the integral value int _{t = n +1} . In this case, the proportional value prop or the priority value PW with a Cycle time Δ t multiplied by an integral time T divided _int int and to the integral value of the previous cycle _{t = n is} added: $$\mathit{in}{t}_{t=n+1}=\mathit{in}{t}_{t=n}+\mathit{prop}\cdot \frac{\mathrm{\Delta }t}{T_{\mathit{int}}}$$

respectively. $$\mathit{in}{t}_{t=n+1}=\mathit{in}{t}_{t=n}+\mathit{PW}\cdot \frac{\mathrm{\Delta }t}{T_{\mathit{int}}}$$

In a preferred embodiment of the invention, an actual total pressure ratio is determined, the actual total pressure ratio being equal to the quotient of an actual discharge pressure, which corresponds to the pressure at an outlet of the compressor located farthest downstream, and the actual inlet pressure of the first compressor ,

In one variant of the invention, a capacitance factor is determined from the actual total pressure ratio and a proportional integral value determined from the priority value and the integral value, wherein the reduced target rotational speed for each compressor is determined as a function value of a control function assigned to the respective compressor in particular assigns a reduced desired speed to each value pair of capacity factor and model total pressure ratio, which is determined in particular from the actual total pressure ratio.

Fig. 1

eine schematischen Darstellung des erfindungsgemäßen Verfahrens.

The following description of the figures describes advantageous embodiments and examples as well as further features of the method according to the invention. It shows:

Fig. 1

a schematic representation of the method according to the invention.

Aus dem Ist- und Soll-Eintrittsdruck p_ist, p_soll, sowie dem Ist-Gesamtdruckverhältnis π_ist lässt sich ein Kapazitätsfaktor X bestimmen, der für alle Verdichter V₁, V₂, V₃, V₄ gleich ist. Mit diesem Kapazitätsfaktor X wird für jeden Verdichter V₁, V₂, V₃, V₄ über eine dem jeweiligen Verdichter V₁, V₂, V₃, V₄ zugeordnete Regelungsfunktion F, die beispielsweise in Form einer Tabelle oder eines Polynoms für jeden Verdichter vorberechnet ist, die jeweilige reduzierte Soll-Drehzahl n_{1 soll, red} ,n_{2 soll, red} ,n_{3 soll, red} ,n_{4 soll, red}, ermittelt, so dass die Verdichter V₁, V₂, V₃, V₄ der Serie möglichst wirtschaftlich arbeiten.In FIG. 1 is a schematic diagram shown schematically, with which the inventive method can be performed. Four compressors V _1, V _2, V _3, V ₄ are arranged in series and have, on their suction side of a respective inlet pressure p, p _{is the first} p _2, p _3, and a temperature at its input _T, T _1, T _2, T _3. Upstream of the first compressor V _{1 of} the series, is an inlet for cool fluid having a temperature T _coldbox (for example 200K, 100K, 50K, 20K and / or 4K) that can be supplied in particular via a valve to the fluid to be cooled. For each compressor V ₁ , V ₂ , V ₃ , V ₄ the temperature T _ist , T ₁ , T ₂ , T ₃ detected at its entrance. In the case of the first compressor V ₁ , this is the actual inlet temperature T _ist . Furthermore, the actual pressure p _ist , p ₁ , p ₂ , p ₃ at the input of the respective compressor V ₁ , V ₂ , V ₃ , V _{4 is} detected. _Is from the actual inlet pressure p and the actual discharge pressure p ₄ is calculated an actual overall pressure ratio _is π which n for the determination of reduced speeds _{1 to, red,} n _{2 is intended, red,} n _{is to 3, red,} n _{4 should , red} the compressor V ₁ , V ₂ , V ₃ , V ₄ is used: $${\pi }_{\mathit{is}}=\frac{p_4}{p_{\mathit{is}}}$$

_Is from the actual and desired inlet pressure p, p _should, as well as π _is the actual overall pressure ratio can be a capacity factor X determine which is the same for all compressors V _1, V _2, V _3, V. ₄ With this capacity factor X is for each compressor V ₁ , V ₂ , V ₃ , V ₄ via the respective compressor V ₁ , V ₂ , V ₃ , V ₄ associated control function F, for example in the form of a table or a polynomial for each Compressor is precalculated, the respective reduced target speed n _{1 should, red} , n _{2 should, red} , n _{3 should, red} , n _{4 should, red} , determined so that the compressor V ₁ , V ₂ , V ₃ , V _{4 of} the series work as economically as possible.

The capacity factor X is in particular such that it can assume values between 0 ( X _pump = 0 pumping regime) and 1 ( X _blocking = 1, blocking regime). Both the pump and the lock-up regime are operating conditions of the compressor, which must be avoided. The pumping regime corresponds to the operating conditions in which the compressor fulfills the so-called surge condition whereas the blocking regime corresponds to operating conditions which fulfill the so-called choke condition. So that the compressors are not driven into these regimes, the capacity factor X is limited to values between a minimum value X _min = X _pump + 0.05 and a maximum value X _max = X _disable - 0.1;

$$\mathit{in}{t}_{\mathit{min}}=X_{\mathit{min}}+\ln \left({\pi }_{\mathit{ist}}\right)$$

Da das gemessene Ist-Gesamtdruckverhältnis π_ist im transienten Betrieb (Pump-down) immer größer wird (der Ist-Eintrittsdruck p_ist wird immer kleiner), werden dadurch die Grenzwerte des Integralwertes auch immer größer. Im umgekehrten Fall (Pump-up), also wenn der Soll-Eintrittsdruck p_soll kleiner als der Ist-Eintrittsdruck p_ist ist, werden diese Grenzwerte immer kleiner.Also, for the integral value int _{t = n +1,} an upper and lower limit _max int and _min int int the integral value by X _max or X _min and ln from the natural logarithm of the actual overall pressure ratio (π _is) is derived: $$\mathit{in}{t}_{\mathit{Max}}=X_{\mathit{Max}}+\ln \left({\pi }_{\mathit{is}}\right)$$

$$\mathit{in}{t}_{\mathit{min}}=X_{\mathit{min}}+\ln \left({\pi }_{\mathit{is}}\right)$$

Since the measured actual total pressure ratio π _ist in transient operation (pump-down) is always larger (the actual inlet pressure p _is is getting smaller), thereby the limit values of the integral value are always larger. In the opposite case (pump-up), So when the target inlet pressure p _{should be} smaller than the actual inlet pressure _p is, these limits are getting smaller.

Wenn alle Verdichter V₁, V₂, V₃, V₄ in Serie an ihrem Designpunkt laufen, erreicht die Verdichterserie ihren Design- oder Arbeitspunkt bei einem Design-Gesamtdruckverhältnis π_Design .When the integral value int _{t = n + 1} becomes larger or smaller than the upper and lower limit values int _max , int _min , respectively, it is limited to the respective limit value. Priority value PW and integral value int _{t = n +1} are added to generate a proportional integral value PI. $$\mathit{PI}=\mathit{PW}+\mathit{in}{t}_{n+1}$$

When all compressors V ₁ , V ₂ , V ₃ , V _{4 run} in series at their design point, the compressor series reaches its design or operating point at a design total pressure ratio π _design .

$$X=\ln \left({\pi }_{\mathit{Design}}\right)+X_{\mathit{sperr}}-\ln \left({\pi }_{\mathit{ist}}\right),$$

sonstIf the proportional integral value PI is smaller than the sum of the maximum value of the capacitance factor X _max and the natural logarithm of the design total pressure ratio value π _design , then the capacitance factor X becomes the difference between the proportional integral value PI and the natural one Logarithm of the actual total pressure ratio π _is calculated. Otherwise, the proportional-integral value PI is limited to the sum of the natural logarithm of the design total pressure ratio π _design and the maximum value of the capacitance factor X _max, in particular for calculating the capacitance factor X, that is to say: $$\begin{array}{cc}X=\mathit{PI}-\ln \left({\pi }_{\mathit{is}}\right).& \mathrm{if\; PI}<\end{array}\ln \left({\pi }_{\mathit{design}}\right)+X_{\mathit{locking}}$$

$$X=\ln \left({\pi }_{\mathit{design}}\right)+X_{\mathit{locking}}-\ln \left({\pi }_{\mathit{is}}\right).$$

otherwise

bzw. $$\mathit{SF}=\exp \left(\mathrm{0,5}\ast \left(X-X_{\mathit{min}}\right)\right)\mathrm{für}X<X_{\mathit{min}}$$

gegeben sein. Damit ergibt sich: $${\pi }_{\mathit{Model}}={\pi }_{\mathit{ist}}\cdot \mathit{SF}\iff \ln \left({\pi }_{\mathit{Model}}\right)=\ln \left({\pi }_{\mathit{ist}}\right)+\mathrm{0,5}\cdot \left(X-X_{\mathit{min}/\mathit{max}}\right)$$

Based on the thus calculated capacity factor X, it is now decided in the method according to the invention how a model total pressure ratio π _{model is} determined, which is then to the control function F to determine the reduced setpoint speeds n _{1, red} , n _{2 soll, red} , n _{Let 3, red} , n _{4 should, be given red} . The model total pressure ratio π _Model is equal to the actual total pressure ratio n _ist when the determined capacity factor X is between the minimum and maximum values X _min , X _max . If the capacity factor X is outside this value range, then the model total pressure ratio π _{model is} modified via a saturation function SF. Subsequently, the capacity factor X is limited to its minimum or maximum value X _min , X _max and then, in particular together with the model total pressure ratio π _Model , forwarded to the control function F, the off these arguments the reduced target speed n _{1 should red} , n _{2 should, red} , n _{3 should, red} , n _{4 is determined, red} for the respective compressor V ₁ , V ₂ , V ₃ , V ₄ .
The saturation function SF can for values of the capacity factor X, which are not between the minimum and the maximum value X _min , X _max , for example, by $$\mathit{SF}=\exp \left(0.5*\left(X-X_{\mathit{Max}}\right)\right)\mathrm{for\; X}>X_{\mathit{Max}}$$

respectively. $$\mathit{SF}=\exp \left(0.5*\left(X-X_{\mathit{min}}\right)\right)\mathrm{For}X<X_{\mathit{min}}$$

be given. This results in: $${\pi }_{\mathit{Model}}={\pi }_{\mathit{is}}\cdot \mathit{SF}\iff \ln \left({\pi }_{\mathit{Model}}\right)=\ln \left({\pi }_{\mathit{is}}\right)+0.5\cdot \left(X-X_{\mathit{min}/\mathit{Max}}\right)$$

This modification of the model total pressure ratio π _model ensures that in operating conditions in which the capacity factor X is saturated, the control still has an influence on the compressors V ₁ , V ₂ , V ₃ , V ₄ , because then instead of the capacity factor X, the model total pressure ratio π _{model is} changed, whereby the control function F reduced target speeds n _{1 should red} , n _{2 should, red} n _{3 should, red} n _{4 should,} call _red , which lead out of these operating conditions.

The reduced setpoint speeds n _{1 should, red} , n _{2 should, red} , n _{3 should, red} , n _{4 should, red} for each compressor V ₁ , V ₂ , V ₃ , V _4, in particular in a table (look up table). In particular, this table can be constructed by model calculations using Eulerian Turbomachine equations. According to the capacity factor X and the model total pressure ratio π _Model , in particular, a software for reading the reduced target speeds n _{1 soll, red} , n _{2 soll, red} , n _{3 soll, red} , n _{4 soll, red} n _red from the table be used. This table then corresponds in particular to the control function F and comprises at least for a plurality of capacitance factors X (for example X = 0, 0.25, 0.5, 0.75 and 1), and model total pressure ratios π _Model the respective reduced rotational speeds n _{1 soll, red} , n _{2 soll, red} , n _{3 soll,} red, n _{4 soll, red} n _red for the respective compressor V ₁ , V ₂ , V ₃ , V ₄ . Values of the capacity factor X that are not listed in the table are determined by interpolation. Furthermore, the capacity factor X is dependent on the model total pressure ratio π _Model and the reduced speeds n _{1 soll, red} , n _{2 soll, red} , n _{3 soll, red} , n _{4 soll, red} n _red chosen so that by the scheme the control function F, the actual inlet pressure p _is the desired inlet pressure p _{is intended to} equalize.

gegeben ist und der Drehzahlindex D_i durch $$D_i=1-Q_i=1-\frac{n_i}{n_{i,\mathit{max}}}$$

gegeben ist. Dabei ist n_i,max die Maximaldrehzahl des i-ten Verdichters V_i. i ist ein Index (i = 1 - 4).In order to ensure a pump-down in parallel during the cool-down of the system, that is to say simultaneously during the cooling-down phase the pressure on the suction side of the compressor V ₁ , Decrease V ₂ , V ₃ , V ₄ , it must be decided whether the actual inlet temperature _is T, at the input of the first compressor V ₁ must be reduced to high
To avoid speeds at the compressors V ₁ , V ₂ , V ₃ , V ₄ , or whether the operation without additional cooling at the input of the first compressor V ₁ can be guaranteed. For this purpose, two values are compared with each other. On the one hand, a proportional value prop _is calculated from the actual and desired inlet pressure p _ist , p _soll . And second, from a speed quota a speed index is calculated for each compressor, the speed quota by $$Q_i=\frac{n_i}{n_{i.\mathit{Max}}}$$

is given and the speed index D _i by $$D_i=1-Q_i=1-\frac{n_i}{n_{i.\mathit{Max}}}$$

given is. In this case, n _{i, max is} the maximum speed of the i-th compressor V _i . i is an index ( i = 1-4).

If the speed index D _{i of} a compressor V _i thus tends to zero, this means that the compressor V _{i is} operating near its maximum speed n _{i, max} , and no higher speeds n _{i should} be increased by an increase in the reduced set speeds n _{1, red} , n _{2 should, red} , n _{3 should, red} , n _{4 should, red} should be set.
From the set of speed indices D _i for each compressor V _i , the smallest speed index D _i is now compared with the proportional value prop. The smaller of the two values is assigned to the priority value PW, which is then used to determine further control values (such as the reduced setpoint speeds n _{1, red} , n _{2 soll} , _red , n _{3 soll} , _red , n _{4 soll} , _red , in particular using the capacity factor or the target inlet temperature T _soll ) is used. That is, if a compressor V _i already has very high speeds n _i , its speed index D _{i will be} almost or equal to zero. As a result, the control of the plant is prioritized so that cold fluid is added via a cold reservoir upstream of the input of the first compressor V ₁ , so that the actual inlet temperature T _is reduced. In particular until the proportional value is prop lower - As a result, the rotational speeds n _i V _i of the compressor, so that the speed index D _i of this compressor V _i rises again reduced. In this way Ensures economical operation of the compressor series, especially in cool-down and pump-down.

From the priority value PW, a temperature control unit TE determines the target inlet temperature T _soll . The calculation is qualitatively such that at a low priority value PW, the target inlet temperature T _{soll is} gradually reduced. For example, the target inlet temperature T setpoint _{is set} to 90% of the measured actual inlet temperature T _ist . The downgrading to this value is realized, for example, by a ramp function. If, during the downgrading of the target inlet temperature T _soll, the speed indices still have control priority, a new downgrade of the set inlet temperature T _soll to 90% of the last measured actual inlet temperature T _{ist is} performed. At each downgrading of the target inlet temperature T _soll to 90% of the measured actual inlet temperature T _ist , it is checked whether the determined target inlet temperature T _{soll is} greater than a design temperature at the inlet of the compressor series. If the design temperature is 4K and the temperature setpoint is 3.8K, the value is limited to 4K.
Via a cooling reservoir control box C, the corresponding amount of cold fluid upstream of the inlet of the first compressor V _{1 is} acted upon by the warm fluid, so that by mixing the two differently warm fluids, the fluid has a mixing temperature which is smaller than previously measured Actual inlet temperature T _is is. At a higher priority value PW, the fluid at the inlet of the first compressor V _{1 is} not acted upon or with only a small amount of cold fluid, since the compressors V ₁ , V ₂ , V ₃ , V _{4 of} the series already not with too high speeds n _i run.

In a variant, also by an integrator, which is in particular part of a PI (proportional-integral) controller, and carries out a time integration of the priority value PW, the calculation of the target inlet temperature T _should be affected - for example, so that a Smoothing or a certain slope of a temperature ramp for T _{soll is} achieved.

wobei n_i die Drehzahl des Verdichters ist (Soll, oder Ist-Drehzahl), n_{i, red} die reduzierte Drehzahl (Soll- oder Ist Drehzahl) des Verdichters V_i, n_{i, Design} die Auslegungs- oder Design-Drehzahl des Verdichters V_i, T_i-1 ist die Temperatur am Eingang des Verdichters V_i und T_{i, Design} die Design- oder Auslegungstemperatur des Verdichters V_i. Wobei T₀ (i = 1) gleich der Ist-Eintrittstemperatur T_ist des ersten Verdichters V₁ ist. Analog dazu gilt für den reduzierten Massenstrom ṁ_red : $${\dot{m}}_{\mathit{red}}=\frac{{\dot{m}}_{\mathit{ist}}}{{\dot{m}}_{\mathit{Design}}}\cdot \frac{p_{\mathit{Design}}}{p_{\mathit{ist}}}\cdot \sqrt{\frac{T_{\mathit{ist}}}{T_{\mathit{Design}}}}$$

wobei ṁ_red der reduzierte Massenstrom durch den Verdichter ist, ṁ_ist der momentane Massenstrom, ṁ_Design den Massenstrom bezeichnet, für den der jeweilige Verdichter ausgelegt ist, p_Design den Designdruck am jeweiligen Verdichter darstellt, T_Design die Design-Temperatur ist und p_ist der Ist-Eintrittsdruck am jeweiligen Verdichter ist. Bezugszeichenliste: PW Prioritätswert prop Proportionalwert int Integralwert p_ist Ist-Eintrittsdruck am ersten Verdichter p_soll Soll-Eintrittsdruck am ersten Verdichter TE Temperaturregelungseinheit C Kühlreservoir-Kontrollbox F Regelungsfunktion X Kapazitätsfaktor D_i Drehzahlindex des i-ten Verdichters (i = 1-4) n_i Ist-Drehzahl des i-ten Verdichters (i = 1-4) n_{i, max} Maximaldrehzahl i-ten Verdichters (i = 1-4) V_i i-ter Verdichter der Serie (i = 1-4) p_i Ist-Druck am Ausgang des i-ten Verdichters, bzw. am Eingang des (i+1)-ten Verdichters (i = 1-4) n_{i, soll} Soll-Drehzahl des i-ten Verdichters (i = 1-4) n_{i soll, red} reduzierte Soll-Drehzahl des i-ten Verdichters (i = 1-4) n_{i, Design} Auslegungs- bzw. Designdrehzahl des i-ten Verdichters (i = 1-4) T_ist Ist-Eintrittstemperatur (am ersten Verdichter) T_soll Soll-Eintrittstemperatur (am ersten Verdichter) T_i Ist-Temperatur am Eingang des (i+1)-ten Verdichters, bzw. am Ausgang des i-ten Verdichters (i = 1-4) T_{i, Design} Auslegungs- bzw. Designtemperatur des i-ten Verdichters (i = 1-4) T_coldbox Temperatur des kalten Fluids SF Sättigungsfunktion π_Model Modell-Gesamtdruckverhältnis π_ist Ist-Gesamtdruckverhältnis π_Design Design-Gesamtdruckverhältnis X Kapazitätsfaktor X_min Minimalwert des Kapazitätsfaktors X_max Maximalwert des Kapazitätsfaktors PI Proportional-Integral-Wert It is important in the overall control that reduced values are used to control the system and in particular the compressors V ₁ , V ₂ , V ₃ , V ₄ . So calculated For example, the reduced speed n _i , _red a compressor V _i from the following formula: $$n_i=n_{i.\mathit{red}}\cdot n_{i.\mathit{design}}\cdot \sqrt{\frac{T_{i-1}}{T_{i.\mathit{design}}}}$$

where n _{i is} the speed of the compressor (target, or actual speed), n _{i, red is} the reduced speed (target or actual speed) of the compressor V _i , n _{i, design} the design or design speed of the compressor V _i , T _i-1 is the temperature at the inlet of the compressor V _i and T _{i, design} the design or design temperature of the compressor V _i . Where T ₀ (i = 1) is equal to the actual inlet temperature T _{ist of} the first compressor V ₁ . Analogously applies to the reduced mass flow ṁ _red : $${\dot{m}}_{\mathit{red}}=\frac{{\dot{m}}_{\mathit{is}}}{{\dot{m}}_{\mathit{design}}}\cdot \frac{p_{\mathit{design}}}{p_{\mathit{is}}}\cdot \sqrt{\frac{T_{\mathit{is}}}{T_{\mathit{design}}}}$$

where ṁ _{red is} the reduced mass flow through the compressor, ṁ _is the instantaneous mass flow, ṁ _{Design designates} the mass flow for which the particular compressor is designed, p _{Design represents} the _design pressure at the respective compressor, T _{Design is} the design temperature and p _is the actual inlet pressure at the respective compressor is. LIST OF REFERENCE NUMBERS PW priority value prop proportional value int integral value p _is Actual inlet pressure at the first compressor p _should Target inlet pressure at the first compressor TE Temperature control unit C Cooling reservoir control box F control function X capacity factor D _i Speed index of the i-th compressor (i = 1-4) _i Actual speed of the i-th compressor (i = 1-4) n _{i, max} Maximum speed of the i-th compressor (i = 1-4) V _i i-th compressor of the series (i = 1-4) p _i Actual pressure at the output of the i-th compressor, or at the input of the (i + 1) -th compressor (i = 1-4) n _{i, shall} Target speed of the i-th compressor (i = 1-4) _{i should, red} Reduced setpoint speed of the i-th compressor (i = 1-4) n _{i, design} Design or design speed of the i-th compressor (i = 1-4) T _is Actual inlet temperature (at the first compressor) T _shall Set inlet temperature (on the first compressor) T _i Actual temperature at the input of the (i + 1) -th compressor, or at the output of the i-th compressor (i = 1-4) T _{i, design} Design temperature of i-th compressor (i = 1-4) T _{cold box} Temperature of the cold fluid SF saturation function π _model Model overall pressure ratio π _is Actual total pressure ratio π _design Design overall pressure ratio X capacity factor X _min Minimum value of the capacity factor X _max Maximum value of the capacity factor PI Proportional-integral value

Claims (7)

1. Method for pressure and temperature control of a fluid, in particular helium, in a series of cryogenic compressors, comprising the steps:

   - measuring an actual rotational speed for each compressor (V₁, V₂, V₃, V₄),

   - measuring an actual inlet pressure (p_ist) and an actual inlet temperature (T_ist) at the inlet of the first compressor (V₁) of the series, arranged furthest upstream,

   - stipulating an inlet pressure setpoint (p_soll) for the first compressor (V₁) of the series,

   - determining a rotational speed index (D_i) for each compressor (V₁, V₂, V₃, v₄) from a maximum rotational speed (n_i,max) of the respective compressor and the actual rotational speed (n_i) of the respective compressor (V₁, V₂, V₃, V₄),

   - determining a proportional value (prop) from the deviation of the actual inlet pressure (p_ist) from the inlet pressure setpoint (p_soll),

   - determining a priority value (PW),

   wherein, if the proportional value (prop) is smaller than the smallest rotational speed index (D_i) of all the compressors (V₁, V₂, V₃, V₄) of the series, the priority value (PW) is determined from the proportional value (prop), and

   wherein, if the proportional value is greater than the smallest rotational speed index (D_i) of all the compressors (V₁, V₂, V₃, V₄) of the series, the priority value (PW) is determined from the smallest rotational speed index (D_i) of all the compressors (V₁, V₂, V₃, V₄) of the series, in order to base the priority of the control on the control of the inlet temperature at the first compressor,

   - determining an inlet temperature setpoint (T_soll) for the first compressor (V₁) of the series and a rotational speed setpoint (n_1,soll, n_2,soll, n_3,soll, n_4,soll) for each compressor (V₁, V₂, V₃, V₄) with the aid of the priority value(PW),

   - adjusting the actual inlet temperature (T_ist) of the first compressor (V₁) to the determined inlet pressure setpoint (T_soll), and

   - adjusting the actual rotational speed (n_i) for each compressor (V₁, V₂, V₃, V₄) to the determined rotational speed setpoint (n_1,soll, n_2,soll, n_3,soll, n_4,soll).
2. Method according to Claim 1, characterized in that the rotational speed index (D_i) for each compressor (V₁, V₂, V₃, V₄) corresponds to the ratio of the difference of the maximum rotational speed (n_i,max) and the actual rotational speed (n_i) of the respective compressor (V₁, V₂, V₃, V₄) and the maximum rotational speed (n_i,max).
3. Method according to either of Claims 1 and 2, characterized in that the priority value (PW) influences the control in such a way that if the smallest rotational speed index (D_i) of all the compressors (V₁, V₂, V₃, V₄) is smaller than the proportional value (prop), the actual inlet temperature (T_ist) is reduced, in particular by a stepwise reduction in the determined inlet temperature setpoint (T_soll), until the proportional value (prop) is smaller than the smallest rotational speed index (D_i), and that in particular the actual rotational speeds (n_i) of the compressors (V₁, V₂, V₃, V₄) are not increased as long as the smallest rotational speed index (D_i) is smaller than the proportional value (prop).
4. Method according to one of the preceding claims, characterized in that the actual rotational speed (n_i) of each compressor (V₁, V₂, V₃, V₄) is determined from a reduced actual rotational speed, and in that the rotational speed setpoint (n_1,soll, n_2,soll, n_3,soll, n_4,soll) of each compressor is determined from a reduced rotational speed setpoint (n_1,soll,red, n_2,soll,red, n_3,soll,red, n_4,soll,red), wherein the reduced actual rotational speed is determined from the actual rotational speed (n_i) and an actual temperature (T_ist, T₁, T₂, T₃) at the inlet of the respective compressor (V₁, V₂, V₃, V₄), and wherein the reduced rotational speed setpoint (n_1,soll,red, n_2,soll,red, n_3,soll,red, n_4,soll,red) is determined from the rotational speed setpoint (n_1,soll, n_2,soll, n_3,soll, n_4,soll) and the actual temperature (T_ist, T₁, T₂, T₃) at the inlet of the respective compressor (V₁, V₂, V₃, V₄).
5. Method according to one of the preceding claims, characterized in that an integral value (int) is determined from the priority value (PW), wherein the integral value (int) is used in particular to determine the reduced rotational speed setpoint (n_1,soll,red, n_2,soll,red, n_3,soll,red, n_4,soll,red) Of the respective compressor (V₁, V₂, V₃, V₄).
6. Method according to one of the preceding claims, characterized in that an actual overall pressure ratio (π_ist ) is determined, wherein the actual overall pressure ratio (π_ist ) correspond to the quotient from an actual outlet pressure (p₄), which corresponds to the pressure at an outlet of the compressor (V₄) that is arranged furthest downstream and the actual inlet pressure (p_ist) of the first compressor (V₁).
7. Method according to Claim 6, characterized in that a capacity factor (X) is determined from the actual overall pressure ratio (π_ist ) and a proportional-integral value which is determined from the priority value (PW) and the integral value (int), wherein the reduced rotational speed setpoint (n_1,soll,red, n_2,soll,red, n_3,soll,red, n_4,soll,red) for each compressor (V₁, V₂, V₃, V₄) is determined as a function value of a control function (F) assigned to the respective compressor (V₁, V₂, V₃, V₄), which in particular assigns a reduced rotational speed setpoint (n_1,soll,red, n_2,soll,red, n_3,soll,red, n_4,soll,red) to each value pair comprising capacity factor (X) and model overall pressure ratio (π_model ), which in particular is determined from the actual overall pressure ratio (π_ist ).

Images