WO2025002861A1

WO2025002861A1 - Determining a setpoint for operation of a cooling system

Info

Publication number: WO2025002861A1
Application number: PCT/EP2024/066677
Authority: WO
Inventors: Roozbeh Izadi-Zamanabadi; Yihan Pan
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 2023-06-27
Filing date: 2024-06-14
Publication date: 2025-01-02
Anticipated expiration: 2025-12-27

Abstract

A cooling system (10) includes a compressor (12) arranged for feeding compressed refrigerant to a condenser (18) and at least one inlet valve (36) with variable opening degree (OD) for feeding at least partly condensed refrigerant to an evaporator (34). Th evaporator (34) comprises a plurality of evaporator outlet pipes (46) connected to a common return pipe (44) arranged for feeding refrigerant back to the compressor (12). At least one temperature sensor (S1) is arranged on one of the evaporator outlet pipes (46a) for sensing an outlet pipe temperature Tout. In a calibrating phase the cooling system (10) is operated with varying opening degree (OD) of the inlet valve (36). The outlet pipe temperature Tout is monitored. A rapid change of the outlet pipe temperature Tout in response to the varying opening degree (OD) is identified and a control set point value (OHset) is determined based on the outlet pipe temperature Tout. In an operating phase the cooling system (10) is operated by controlling, based on a reading of the temperature sensor (S1), the opening degree (OD) of the inlet valve (36) to achieve the control set point (OHset).

Description

Determining a setpoint for operation of a cooling system

The invention relates to a method of operating a cooling system.

A cooling system may e.g. comprise at least a compressor arranged for feeding compressed refrigerant to a condenser and at least one inlet valve for feeding at least partly condensed refrigerant to an evaporator. The evaporator may comprise a plurality of evaporator outlet pipes connected to a common return pipe arranged for feeding refrigerant back towards the compressor. Such cooling systems may be operated in different modes, such as e.g. in DX (direct expansion) mode in which the refrigerant is fully evaporated and comprises an amount of superheat after passing through the evaporator, or in pump overfeed mode in which the refrigerant is evaporated only partly and still has a substantial portion of liquid after passing through the evaporator. A mode of operation with particularly promising properties is referred to as WDX (“wet” DX) and characterized by the refrigerant being only partly evaporated in the evaporator, but up to a high vapor quality of e.g. above 8o% .

WO 2020/ 109213 Al discloses a cooling system and operating method therefor with a direct expansion cooling circuit for an ammonia refrigerant. A compressor is provided to compress ammonia vapor. A condenser is provided to condense the ammonia vapor to obtain liquid ammonia. An evaporator is provided to evaporate the liquid ammonia. A superheat vapor quality sensor is arranged at a conduit between at least a portion of the evaporator and the compressor. The superheat vapor quality sensor comprises a heating element and a temperature sensing element. The superheat vapor quality sensor is disposed to deliver a sensor signal S indicative of a superheat vapor quality X of refrigerant flowing through the conduit from an output of the temperature sensing element. The superheat vapor quality sensor is arranged on a wall of a horizontally arranged portion of the conduit in a position forming an angle of more than 120° to a vertical upward direction. It may be considered an object to propose a method of operating a cooling system which allows efficient and well-controlled operation despite an easy, secure setup and installation procedure.

The object is achieved by a method according to claim i. Dependent claims refer to preferred embodiments of the invention.

According to the invention, the cooling system comprises at least a compressor, a condenser, and an evaporator with an inlet valve having a variable opening degree. The system may optionally comprise any number of different additional elements or components, such as e.g. an accumulator, further evaporators with inlet valves, etc. While the invention may be used with different types of refrigerant, the refrigerant is preferably ammonia.

The compressor is arranged for feeding compressed refrigerant to a condenser. The inlet valve is provided for feeding refrigerant to an evaporator where it is at least partly evaporated. Depending on the opening degree OD of the inlet valve, the mass flow of refrigerant into the evaporator maybe regulated. The inlet valve is preferably controllable, i.e. an opening degree thereof maybe automatically adjusted to a desired value, e.g. by means of a motor, servo or other type of actuator.

The evaporator comprises a plurality of evaporator outlet pipes connected to a common return pipe. Thus, between the common inlet and outlet of the evaporator the refrigerant pipes branch out into multiple parallel passes, each terminating, after traversing the evaporator, into the common return pipe of the evaporator. The common return pipe is connected to feed refrigerant back to the compressor. While the return connection between the common return pipe and the compressor could be a direct connection, it is preferably an indirect connection via at least one additional interposed component, such as an accumulator which will be further explained below.

A temperature sensor is arranged on one of the evaporator outlet pipes. As will be further explained below, there may be provided at least one further temperature sensor on another one of the evaporator outlet pipes, so for clarity the temperature sensor may be referred to as a first temperature sensor arranged on a first evaporator outlet pipe. The temperature sensor is suited and arranged for sensing a temperature of the refrigerant within the evaporator outlet pipe, here referred to as the outlet pipe temperature Tout. The temperature sensor may be any type of element delivering a temperature dependent sensor signal, preferably an electrical value (such as e.g. current, voltage, frequency, resistance). For example, the temperature sensor may e.g. be a thermocouple or resistance temperature detector (RTD), such as for example a Ptiooo element.

For example, the temperature sensor maybe provided on an outside wall of the outlet pipe (but in thermal contact therewith), but could also be arranged fully or partly embedded or protruding into the interior of the outlet pipe to achieve thermal coupling with the refrigerant. The temperature sensor maybe a passive sensor component, but could optionally also comprise an active component such as a heating element.

According to the invention, a calibrating phase is conducted to determine a control setpoint value and the system is operated in an operating phase operation of the cooling system controlled based on the determined control setpoint value. As will be further explained for preferred embodiments, control based on the determined control setpoint value is not necessarily the only type of operation of the system, so it should be understood that control during the operating phase maybe effected e.g. in different operating modes during different time intervals, only one of which may e.g. be based on the determined control setpoint value. Further, the calibrating phase may be applied once or multiple times, e.g. to determine and/ or update the control setpoint value in timely spaced manner to adjust operation during the operating phase(es) to potential changes in the system or external operating conditions.

In the calibrating phase, the method according to the invention provides operating the cooling system with varying opening degree OD of the inlet valve while monitoring the outlet pipe temperature Tout for identifying a rapid change of the outlet pipe temperature Tout sensed by the temperature sensor in response to the varying opening degree OD, and determining a control setpoint value based on the outlet pipe temperature Tout. While calibration operation with a varying opening degree may follow any regular or irregular pattern of opening degree values, it is preferred to provide a gradually (continuously or stepwise) increasing or decreasing opening degree over at least a substantial portion of the control range. Particularly preferred is a gradually increasing opening degree, particularly preferably linear.

During calibration operation with varying opening degree of the inlet valve, the outlet pipe temperature Tout is measured, e.g. intermittently or continuously. A rapid change of the outlet pipe temperature in response to the varying opening degree may be detected by suitable processing and evaluation. Detection of the rapid change maybe achieved in different ways, preferably by considering an outlet pipe temperature Tout alone or in connection with other values, signals and/or measurements. For example, the rapid change may be determined by considering the first, second, and/ or higher order derivative of the outlet pipe temperature signal Tout. Alternatively, or in addition, a variance of the outlet pipe temperature signal Tout may be considered. Preferably, the rapid chance may be detected by determining and evaluating a rate of change of the outlet pipe temperature Tout over the opening degree (or over time, if the opening degree is steadily varied over time). A rapid change may be understood to be a stronger variation than before or after, and may preferably refer to a value or region (i.e. multiple consecutive values of opening degrees) where a rate of change ATout has a higher absolute value than outside of the region, thus particularly preferably a maximum or minimum rate of change. As will become apparent in connection with preferred embodiments, in the case of steady increase of the opening degree the preferred spot is a high negative value of the rate of change of the outlet pipe temperature Tout, most preferably the minimum.

While in principle the calibration and choice of setpoint may be conducted fully or in part manually, it is preferably effected automatically in a programmed manner, i.e. a programmed control device provides the varying opening degree values, processes the measured temperature values and determines the setpoint based on the rapid change criteria. In at least a portion of the operating phase, the cooling system is operated by controlling the opening degree OD of the inlet valve based on a reading of the temperature sensor. Thus, the inlet valve is operated with a controlled opening degree depending on an output of the temperature sensor, i.e. dependent on the outlet pipe temperature. Control may preferably be effected as feedback loop control, which may be of any type, such as e.g. proportional, integral, derivative or any combination thereof, preferably using the previously determined control setpoint value (or a value dependent therefrom, such as calculated based on the control setpoint value) as setpoint and the outlet pipe temperature (or a value dependent therefrom) as process variable.

The operating method according to the invention allows to easily and reliably provide control of the cooling system to achieve efficient operation. It has surprisingly been found that the criteria of a rapid temperature change is a clear indication for identifying a suitable setpoint, in particular for operation of the cooling system in WDX mode, i.e. with the refrigerant not fully evaporated but to a high vapor quality. This observation maybe explained if it is considered that a favorable operating point for the WDX regime is if there is a small but noticeable amount of unevaporated refrigerant, such as e.g. roughly io%. The unevaporated refrigerant will tend to form small droplets which may provide exceptionally efficient cooling in contact the inner surface of a pipe of the evaporator .Since an advantageous operating point may thus be determined by the proposed calibration, this may be used in an automated fashion, such that determination of a setpoint may be effected partly or fully automatically, such as by execution of an operating program, e.g. executed on a programmable control device.

In a preferred embodiment, the step of identifying the rapid change of the outlet pipe temperature Tout in the calibrating phase may include identifying a region (i.e. consecutive values of the opening degree) where a rate of change ATout has a higher absolute value than outside of the region. While it is preferred to determine the setpoint quite precisely corresponding to the absolute highest rate of change, i.e. depending on the direction of change either the maximum or minimum, it may also be acceptable to choose a setpoint close to the maximum/ minimum, i.e. within the identified region. Different types of values may be used for the setpoint value, such as e.g. a temperature value as sensed by the temperature sensor. However, it is preferred not to use an absolute temperature value as setpoint directly, but an overheat setpoint value. The overheat setpoint value may preferably be calculated from one or more measurements taken from the system, preferably as a temperature difference, most preferably using the reading of the temperature sensor and an evaporation temperature TE_Vap.. The evaporation temperature may e.g. be determined based on one or more measurements taken from the system, preferably a measurement of the pressure at one or more of the evaporator outlet pipes or the common return pipe. While the evaporation temperature maybe measured or calculated specifically at the time/setting where the highest rate of change is found, it is preferred to use a time averaged evaporation temperature value for the determination of the overheat setpoint value. This avoids errors in determining the setpoint due to sudden fluctuations. The evaporation temperature may e.g. be averaged over a time period of several minutes or even several hours.

In a most preferred embodiment, the overheat setpoint value may thus be calculated as the difference between the measured temperature (e.g. reading of the temperature sensor) at the time / setting where the highest rate of change is found and the time averaged evaporation temperature.

Thus, control may preferably be effected based on overheat values, i.e. both the process variable and the setpoint may be overheat values.

The step of controlling the opening degree OD of the inlet valve in the operating phase preferably includes determining a difference between a value dependent on the reading of the temperature sensor and the control setpoint value OHset. Further preferred, the value may be an overheat value dependent on a reading of the temperature sensor and an evaporation temperature.

In a preferred embodiment, the system may be controlled, at least during a part of the operating phase, as a WDX system (“wet” direct expansion), i.e. without overheat such that the refrigerant in the common return pipe is partly in liquid and partly in vapor form. This type of operation has been found to use the evaporator to a high degree of its capacity. A preferred range of the average vapor quality in the return pipe during WDX operation is below the overheat range which starts above ioo%, The preferred values for the average vapor quality in the return pipe during WDX operation are higher than 8o%, particularly preferably 85% or above.

In a preferred embodiment, the temperature sensor is arranged on a first evaporator outlet pipe. The first evaporator outlet pipe on which the temperature sensor is placed may in principle be chosen arbitrarily, but is preferably determined by a comparison of the heat load of the parallel evaporator pipes. Placement of the temperature sensor on an evaporator outlet pipe with high heat load allows increased sensitivity and thus advantageous control properties. Preferably, the heat load of the first evaporator outlet pipe is higher than an average heat load of the evaporator outlet pipes of the evaporator. Particularly preferably, the evaporator outlet pipe with the highest heat load is chosen as first evaporator outlet pipe.

It should be noted that “heat load” generally refers to the amount of heat absorbed by one evaporator pipe, thus determining the vapor quality at the corresponding outlet pipe. Under given operating conditions, such as nominal operation of the evaporator, the heat load may be determined by a comparison of the vapor quality of the refrigerant in the evaporator outlet pipes, where the highest vapor quality indicates the highest heat load. As the skilled person will recognize, there may be cases where the vapor quality does not accurately reflect the effectively absorbed amount of heat, e.g. in cases of significant differences between the evaporator pipes. Thus, it should be recognized that the term “heat load” as used here for ease of reference is an expression referring to the comparison of vapor quality among the evaporator outlet pipes. The evaporator outlet pipe which under given (e.g. nominal) operating conditions first reaches 100% vapor quality (i.e. “runs dry”) if the flow of refrigerant through the evaporator is reduced is considered to have the highest heat load.

While it may be possible to operate the cooling system in WDX mode during the entire operating phase, a preferred embodiment of the operating method provides a choice of operation in different operating modes, particularly WDX and DX (direct expansion). Control of the system may e.g. be switched one or more times between WDX and DX mode. In DX mode the refrigerant is fully evaporated in the evaporator, i.e. the vapor quality in the common return pipe is 100%, preferably with an amount of overheat.

While in principle operation in both control modes may be possible using the same temperature sensor, it has proven advantageous to use more than one temperature sensor, i.e. besides the first temperature sensor additionally at least a second temperature sensor. The second temperature sensor may be arranged e.g. on the common return pipe or on a second evaporator outlet pipe different from the first evaporator pipe. Preferably, the second temperature sensor may be arranged on an evaporator outlet pipe with a lower-than-average heat load, particularly on the evaporator outlet pipe with the lowest heat load.

According to a preferred embodiment, the cooling system is controlled during the operating phase in a first operating mode as a WDX system based on the reading of the first temperature sensor and in a second operating mode as a DX system based on the reading of the second temperature sensor.

In a preferred embodiment of the invention, the cooling system may include an accumulator connected between the common return pipe and the compressor, which may have a vessel for liquid and vapor refrigerant, which may be used to separate vapor and liquid. The compressor is preferably connected to a top portion of the accumulator to feed evaporated refrigerant to the compressor. In a particularly preferred embodiment, there may be heating means provided to heat and partly evaporate liquid contained in the accumulator. Heating may particularly preferably be achieved by a conduit of refrigerant from the condenser to the evaporator.

In the accumulator, a liquid level sensor may be provided. If the cooling system including the accumulator is operated alternatively in WDX (first operating mode) and DX mode (second operating mode), the amount of liquid in the accumulator may rise during WDX mode, but will decrease during DX operation where the refrigerant is fully evaporated. The operating mode maybe chosen dependent on the liquid level within the accumulator. For example, if the liquid level in the accumulator reaches or surpasses a specified upper threshold, DX operation may be activated to decrease the amount of liquid. If the liquid level falls below a specified lower threshold, WDX operation may be chosen for further operation.

Embodiments of the invention will be described with reference to the drawings, in which

Fig. 1 shows a schematic representation of an embodiment of a cooling system;

Fig. 2 shows a schematic representation of an evaporator with multiple pipes;

Fig. 3a shows a diagram of a first example of a temperature curve of an evaporator outlet pipe temperature Tout and of a rate of change ATout of the temperature curve;

Fig. 3b shows a diagram of a second example of a temperature curve of the evaporator outlet pipe temperature Tout and rate of change ATout.

Figure 1 shows an embodiment of a cooling system io operated with ammonia as refrigerant.

A cooling circuit of the cooling system io comprises a compressor 12 to compress ammonia vapor n contained in the upper portion of a suction accumulator 14 filled with gaseous ammonia 11 and a rest of liquid ammonia 20 accumulated at the bottom. Compressed ammonia vapor obtained from the compressor 12 is supplied through a conduit 16 to a condenser 18, where it condenses at least partly to collect as liquid ammonia 20 in a collector 22.

The hot liquid ammonia 20 is supplied through a first conduit 24 and a second conduit 28 to evaporators 32. The first conduit 24 comprises a heating spiral 26 in the suction accumulator 14 for the heated liquid refrigerant to aid in evaporating liquid ammonia 20 there.

In the example shown, the cooling system 10 comprises two identical evaporators 32 connected in parallel. The skilled person will recognize that different embodiments of the cooling system io may comprise a different number of evaporators 32, such as only one or more than two evaporators. In the following, only one of the evaporators 32 connected in parallel will be described.

The liquid ammonia 20 is supplied through a controllable evaporator inlet valve 36 to the evaporator 32. The evaporator 32, as shown in Fig. 2, comprises a plurality of parallel evaporator tubes 34 in thermal contact with an air flow of a ventilator schematically indicated by an arrow. The evaporator 32 has a common liquid inlet 40 and a common outlet 44. The evaporator 32 has multiple passes, i.e. evaporator tubes 34 passing from inlet 40 to outlet 44 in parallel. Thus, the common inlet 40 branches into multiple evaporator inlet pipes 42 and there are multiple evaporator outlet pipes 46 leading into the common outlet 44.

Back in Fig. 1, the common outlet 44 of the evaporator 32 is joined to the common outlet of the other evaporator(s) leading into a return conduit 38 leading to the accumulator 14.

The cooling system 10 comprises a number of sensors connected to a control unit 50, namely a first temperature sensor Si and a second temperature sensor S2 arranged on different ones of the evaporator outlet pipes 46a, 46b as shown in Fig. 2 to measure the temperature of the refrigerant within the evaporator outlet pipes 46a, 46b, a pressure and temperature sensor 54 arranged on the common return pipe 44 to measure temperature and pressure of the refrigerant in the common return pipe 44, and a liquid level sensor 52 arranged within the accumulator 14 to sense the level of the liquid refrigerant 20 therein. The control unit 50 is connected to the sensors Si, S2, 52, 54 to receive sensor signals. The control unit 50 is further connected to the controllable inlet valve 36 to set a desired opening degree OD of the inlet valve 36.

The first and second temperature sensors Si, S2 serve different purposes for different modes of operation as will be explained below. The first temperature sensor Si is used in a WDX operating mode whereas the second temperature sensor S2 is used for control in a DX operating mode. At the evaporator 32, the first temperature sensor Si is arranged on the first evaporator outlet tube 46a which is characterized by the highest heat load among the evaporator outlet tubes 46 and the second temperature sensor S2 is arranged on the second evaporator outlet tube 46b which has the lowest heat load. As explained above, the “heat load” here refers to the portion of evaporated refrigerant, i.e. the vapor quality as compared between the evaporator outlet tubes 46. If under given operating conditions of the cooling system 10 the opening degree OD of the inlet valve 36 is gradually reduced, the refrigerant in the first evaporator outlet tube 46a will first reach a “dry” state with 100% vapor quality, and the refrigerant in the second evaporator outlet tube 46b will be the last to reach 100% vapor quality.

Controlled by the control unit 50, a setpoint for WDX operating mode is determined by conducting a calibration procedure by providing varying values of opening degree OD for the inlet valve 36 while monitoring the first temperature sensor Si.

Fig. 3a shows graphs of a first example of a calibration. In the top portion of Fig. 3, the curves shows are the temperature Tout as sensed by the first temperature sensor Si, the evaporation temperature TEvap of the refrigerant at the common outlet 44 of the evaporator 32, and the opening degree OD of the inlet valve 36 over time t. The evaporation temperature TEvap may be calculated from the pressure measurement of the temperature and pressure sensor 54 arranged at the common outlet 44.

In the bottom portion of Fig. 3, the rate of change ATout is shown over time t.

Within a calibration time interval 56, the opening degree OD is gradually increased in a ramp over almost the entire control range. The temperature Tout decreases as more and more refrigerant is fed to the evaporator 32. However, the temperature curve is not linear but first shows a slowly decreasing progression and then decreases rapidly.

As visible from the bottom portion of Fig. 3, the rate of change ATout of the temperature Tout sensed by the first temperature sensor Si shows a clearly identifiable minimum value ATmin within the region of rapid decrease of the temperature Tout. The minimum ATmin is used to identify a control set point OHset as an overheat value, i.e. a temperature difference between the temperature Tout as sensed by the first temperature sensor Si and the evaporation temperature TEvap.

Fig. 3b shows the curves for OD, Tout, TEvap, and ATout for a second calibration example taken at different pressure and temperature levels. Again, in the calibration time interval 56 the opening degree OD is ramped up linearly causing the temperature Tout of the refrigerant in the first evaporator outlet pipe 46a to eventually decrease, making the desired set point identifiable by the minimum ATmin and allowing to determine the overheat control set point value OHset as the difference between the temperatures Tout and TEvap.

Using the thus determined overheat control set point value OHset, the system 10 is first controlled in a first WDX operating mode, i.e. the control unit 50 controls the opening degree OD of the inlet valve 36 in a closed-loop control using as input a measured overheat value OH (calculated as the difference between the temperature Tout as measured by the first temperature sensor Si and the evaporation temperature TEvap as determined based on the temperature and pressure sensor 54), determining a difference between the thus measured overheat value OH and the overheat control set point value OHset, and adjusting the opening degree OD in accordance with the difference (i.e., increasing the opening degree if OH is lower than OHset and increasing OD if OH is higher than OHset).

The resulting operation is WDX operation with e.g. 85-95% vapor quality in the common return pipe 44.

Thus, a certain portion of the refrigerant is fed to the accumulator 14 in liquid form. If consequently the liquid level with in the accumulator 14 rises above a lower threshold as detected by the control unit 50 based on the signal of the liquid level sensor 52, control of the system 10 is switched to DX mode based on the signal obtained from the second temperature sensor S2. Since in DX mode the refrigerant is fully evaporated, the level of liquid refrigerant 20 within the accumulator 14 will tend to decrease. In case that the level of refrigerant 20 however rises further up to the higher threshold, the system is shut down for safety reasons. If the level falls again below the lower threshold, the control unit 50 is switched back to WDX control as described above.

Reference Signs

10 cooling system

11 refrigerant vapor

12 compressor

14 accumulator

16 conduit from compressor to condenser

18 condenser

20 refrigerant liquid

22 collector

24 conduit from collector to heating spiral

26 heating spiral

28 pipe connection from heating turns to inlet valve

32 evaporator

34 evaporator tubes

36 inlet valve

38 return conduit

40 common evaporator inlet

42 evaporator inlet pipes

44 common evaporator outlet

46 evaporator outlet pipes

46a first evaporator outlet pipe

46b second evaporator outlet pipe

50 control unit

52 liquid level sensor

54 pressure and temperature sensor

56 calibration interval

51 first temperature sensor

52 second temperature sensor

Claims

1. Method of operating a cooling system (io),

- the cooling system (io) including at least a compressor (12) arranged for feeding compressed refrigerant to a condenser (18), and at least one inlet valve (36) with variable opening degree (OD) for feeding at least partly condensed refrigerant to an evaporator (34),

- the evaporator (34) comprising a plurality of evaporator outlet pipes (46) connected to a common return pipe (44) arranged for feeding refrigerant back towards the compressor (12), at least one temperature sensor (Si) being arranged on one of the evaporator outlet pipes (46a) for sensing an outlet pipe temperature (Tout), the method comprising in a calibrating phase, operating the cooling system (10) with varying opening degree (OD) of the inlet valve (36), monitoring the outlet pipe temperature (Tout), identifying a rapid change of the outlet pipe temperature (Tout) in response to the varying opening degree (OD), and determining a control setpoint value (OHset) based on said outlet pipe temperature (Tout), in an operating phase, operating the cooling system (10) by controlling, based on a reading of the temperature sensor (Si), the opening degree (OD) of the inlet valve (36) to achieve the control set point (OHset).

2. Method according to claim 1, wherein in the calibrating phase, the step of operating the cooling system (10) with varying opening degree (OD) of the inlet valve (36) includes gradually increasing or decreasing the opening degree (OD).

3. Method according to one of the preceding claims, wherein in the calibrating phase, the step of identifying the rapid change of the outlet pipe temperature (Tout) includes identifying a region of a rate of change (ATout) with a higher absolute value than outside of the region.

4. Method according to one of the preceding claims, wherein the step of identifying the rapid change of the outlet pipe temperature (Tout) includes identifying a maximum or minimum rate of change.

5. Method according to one of the preceding claims, wherein in the calibrating phase, the step of determining the control set point value (OHset) includes determining an overheat setpoint value dependent on a reading of the temperature sensor (Si) and an evaporation temperature (TEvap).

6. Method according to claim 5, wherein

- the evaporation temperature (TE_Vap) is determined based on a pressure measurement taken at at least one of the plurality of evaporator outlet pipes (46) or the common return pipe (44).

7. Method according to one of the preceding claims, wherein in the operating phase, the step of controlling the opening degree (OD) of the inlet valve (36) includes determining a difference between a value dependent on the reading of the temperature sensor (Si) and the control set point value (OHset).

8. Method according to claim 7, wherein

- the value is an overheat value dependent on a reading of the temperature sensor and an evaporation temperature.

9. Method according to one of the preceding claims, wherein in at least a part of the operating phase, the system is controlled as a WDX system with the refrigerant in the common return pipe (44) being part liquid and part vapor.

10. Method according to one of the preceding claims, wherein

- the temperature sensor (Si) is arranged on a first evaporator outlet pipe (46a) which has a heat load which is higher than an average heat load of the evaporator outlet pipes (46) of the evaporator.

11. Method according to claims 9 or 10, wherein

- the temperature sensor is a first temperature sensor (Si), and at least a second temperature sensor (S2) is provided, arranged on the common pipe or on a second evaporator outlet pipe (46b)

- wherein in the operating phase, the cooling system is controlled in a first operating mode as a WDX system based on the reading of the first temperature sensor (Si) and in a second operating mode as a DX system, with the refrigerant in the common return pipe (44) being only vapor, based on the reading of the second temperature sensor (S2).

12. Method according to one of the above claims, wherein

- the cooling system (10) includes an accumulator (14) connected between the return pipe (44) and the compressor (12), the compressor (12) being connected to a top portion of the accumulator to feed evaporated refrigerant to the compressor (12).

13. Method according to claims 11 and 12, wherein

- the first or second operating mode is chosen dependent on a liquid level within the accumulator.