WO2009039850A1

WO2009039850A1 - A method and a control system for controlling an opening degree of a valve

Info

Publication number: WO2009039850A1
Application number: PCT/DK2008/000333
Authority: WO
Inventors: Harald Alnor; Christian Bendtsen
Original assignee: Danfoss A/S
Priority date: 2007-09-25
Filing date: 2008-09-24
Publication date: 2009-04-02

Abstract

A method for controlling an opening degree of a valve arranged in a vapour compression system, such as a refrigeration system or a heat pump. The vapour compression system comprises a compressor, a condenser, a valve and an evaporator arranged in a refrigerant flow path. According to a first aspect, the method comprises the steps of obtaining a superheat value for the system and comparing it to a reference superheat value, determining a maximum opening degree of the valve, and controlling the opening degree of the valve in accordance with the comparing step, and in such a manner that the opening degree does not exceed the maximum opening degree. The maximum opening degree is determined dynamically as a function of a mean temperature difference across the evaporator. Thereby the it is ensured that it is possible to open the valve as much as possible, while keeping the operation of the system safe in the sense that the risk that liquid refrigerant is allowed to pass through the evaporator is minimised due to the maximum opening degree. According to a second aspect, an adjustment value is calculated on the basis of the superheat value, the reference superheat value and a generated control signal. Thereby a faster response to changes in the operating conditions for the vapour compression system is obtained.

Description

A METHOD AND A CONTROL SYSTEM FOR CONTROLLING AN OPENING DEGREE OF A VALVE

FIELD OF THE INVENTION

The present invention relates to a method for controlling an opening degree of a valve arranged in a refrigerant flow path of a vapour compression system, such as a refrigeration system or a heat pump. According to the present invention, the opening degree of the valve is controlled in such a manner that the vapour compression system is operated with an optimal superheat, thereby minimising energy consumption, and in such a manner that the system is capable of reacting fast to changes in superheat during operation. The invention further relates to a control system for performing the method.

BACKGROUND OF THE INVENTION

In vapour compression systems, such as refrigeration systems, a valve is normally arranged in the flow path at a position upstream relatively to an evaporator. The valve is used for controlling a flow of refrigerant through the evaporator. The valve may be controlled in such a manner that it is attempted to obtain an optimal superheat for the evaporator, i.e. a superheat which is as low as possible, but positive. An optimum superheat is an indication that the potential refrigeration capacity is utilised to the greatest extent possible, and energy consumption can thereby be minimised.

US 2006/0213208 discloses a control system and a method for controlling a valve determining a flow of refrigerant through an evaporator. Temperature sensors are used for measuring the temperatures, Tj_n and T_out of a refrigerant at the entrance and exit of the evaporator. The superheat is calculated from the measured temperatures as T_Sh=T_ourTj_n. The calculated superheat, T_Sh, is compared to a preferred superheat, T₅, and the instant variation, T_pvo=T_sh-T_s, is used for calculating an opening degree of the valve. Accordingly, the opening degree of the valve is calculated on the basis of the measured refrigerant temperature, and the valve is operated in order to attempt to obtain an optimum superheat.

However, the method described in US 2006/0213208 reacts relatively slowly to changes in superheat during operation of the refrigeration system.

EP 0 811 136 discloses a method for controlling the superheat temperature of the refrigerant in an evaporator arrangement of a refrigeration system or heat pump system. The superheat temperature is controlled in dependence on a comparison between desired and actual values, the desired value of the superheat temperature being varied automatically in dependence on the difference between a reference value and a periodically determined function of a number of sampled values of a temperature of the refrigerant. The function characterizes the variability of a number of sampled values of the temperature of the refrigerant at the output of the evaporator arrangement about a mean value of the sampled values. A large variability indicates instability of the temperature of the refrigerant at the outlet of the evaporator. Thus, the method of EP 0 811 136 aims at controlling the superheat temperature in such a manner that such instabilities are avoided.

In the case that the superheat temperature differs significantly from the desired superheat temperature, the method disclosed in EP 0 811 136 will result in the valve being opened very much relatively abruptly. This introduces the risk that liquid refrigerant is accidentally allowed to pass through the evaporator. This is very undesirable since it has the consequence that the potential refrigeration capacity of the refrigerant is not utilised efficiently.

SUMMARY OF THE INVENTION

It is, thus, an object of the invention to provide a method of controlling a valve arranged in a vapour compression system in such a manner that energy consumption is minimised during operation.

It is a further object of the invention to provide a method of controlling a valve arranged in a vapour compression system, the method being capable of reacting faster to changes in superheat during operation than similar prior art methods.

It is an even further object of the invention to provide a method of controlling a valve arranged in a vapour compression system, the method reducing the risk that liquid refrigerant is passed through the evaporator

It is an even further object of the invention to provide a control unit for controlling a valve arranged in a vapour compression system, the control unit being adapted to cause minimisation of energy consumption during operation.

It is an even further object of the invention to provide a control unit for controlling a valve arranged in a vapour compression system, the control unit being able to react faster to changes in superheat during operation than similar prior art control units.

It is an even further object of the invention to provide a control unit for controlling a valve arranged in a vapour compression system, the control unit being able to reduce the risk that liquid refrigerant is allowed to pass through the evaporator. According to a first aspect of the invention the above and other objects are fulfilled by providing a method for controlling an opening degree of a valve arranged in a vapour compression system, the vapour compression system further comprising a compressor, a condenser and an evaporator, the compressor, the condenser, the valve and the evaporator being arranged in a refrigerant flow path, the method comprising the steps of:

- obtaining a superheat value for the vapour compression system,

- comparing the obtained superheat value to a reference superheat value,

- determining a maximum opening degree for the valve, and

- controlling the opening degree of the valve in accordance with the comparing step in order to control the superheat value to be equal to or close to the reference superheat value, and in such a manner that the opening degree of the valve does not exceed the maximum opening degree,

wherein the maximum opening degree is determined dynamically as a function of a mean temperature difference across the evaporator.

In the present context the term 'vapour compression system' should be interpreted to mean any system in which a flow of refrigerant circulates and is alternatingly compressed and expanded, thereby providing either refrigeration or heating of a volume. Thus, the vapour compression system may be a refrigeration system, an air condition system, a heat pump, etc.

The compressor may be a single compressor, but it could also be two or more compressors, e.g. forming a compressor rack. Similarly, the vapour compression system may comprise only a single evaporator, but it may, alternatively, comprise two or more evaporators, preferably arranged in parallel in the refrigerant flow path. In this case the evaporators may advantageously be arranged in such a manner that each evaporator provides refrigeration or heating to a separate volume. This is, e.g., the case in a refrigeration system of the kind having a number of individually refrigerated display cases. Such refrigeration systems are often used in food stores, such as supermarkets. When two or more evaporators are used for providing refrigeration/heating to separate volumes, the supply of refrigerant to an evaporator is normally controlled independently of the supply of refrigerant to the other evaporator(s). Thus, in this case each evaporator is preferably provided with a valve for controlling the supply of refrigerant to that particular evaporator. As an alternative, two or more evaporators may provide refrigeration/heating to a common volume. This is sometimes the case in larger storage room which need to be refrigerated to cool or freeze the goods which are stored in the storage room.

The superheat value for the vapour compression system is typically the difference between the temperature of gaseous refrigerant leaving the evaporator and the evaporation temperature, i.e. the temperature at which the refrigerant evaporates in the evaporator. Accordingly, the step of obtaining a superheat value preferably comprises measuring, or otherwise obtaining, these two temperatures, and calculating the difference. It may be difficult to measure the evaporation temperature directly, and this temperature is therefore often estimated on the basis of other measured quantities, e.g. the temperature of the refrigerant at the inlet of the evaporator or the pressure of the refrigerant at the outlet of the evaporator. Thus, the step may be performed by measuring the evaporation pressure and calculating the evaporation temperature, measuring the temperature of gaseous refrigerant leaving the evaporator, and calculating the difference between these two temperatures.

The obtained superheat value is compared to a reference superheat value. The reference superheat value is a superheat value at which it is desired to operate the vapour compression system, preferably a superheat value which ensures that the potential refrigeration capacity of the system is utilised in an optimum manner. Accordingly, the vapour compression system is operated in such a manner that it is attempted to achieve a superheat value for the vapour compression system which is as close as possible to the reference superheat value. The opening degree of the valve determines the supply of refrigerant to the evaporator, and it therefore has a significant impact on the superheat value of the vapour compression system. Thus, in order to control the superheat value of the vapour compression system as described above, the opening degree of the valve is operated in accordance with the comparing step.

The reference superheat value may be a substantially fixed value, e.g. a value which is initially set or a value which is only adjusted, e.g. manually, at large time intervals. Alternatively, the reference superheat value may be a dynamical value which is automatically and continuously adjusted in response to changing operating conditions of the vapour compression system. This will be described further below.

The method comprises the step of determining a maximum opening degree for the valve. The opening degree of the valve is controlled in such a manner that it does not exceed the maximum opening degree. Thereby it is ensured that substantial changes in the opening degree of the valve can not be performed in such a manner that the maximum opening degree is exceeded, and the risk of liquid refrigerant passing through the evaporator is accordingly considerably reduced. The maximum opening degree is determined dynamically, i.e. it is determined automatically and continuously, and thereby dynamical changes in the operating conditions of the vapour compression system, in particular substantial changes in superheat, can be taken into account when the maximum opening degree for the valve is determined. In particular, the maximum opening degree is determined as a function of a mean temperature difference across the evaporator. The temperature difference may be a difference in temperature of ambient air entering and leaving the evaporator, a difference in temperature of refrigerant entering and leaving the evaporator, or any other suitable temperature difference which provides information about the performance of the vapour compression system. This will be explained in further detail below.

Thus, according to the present invention the opening degree of the valve is controlled in order to obtain an optimum superheat value, and in such a manner that a maximum opening degree is not exceeded. Furthermore, the maximum opening degree is determined dynamically and in accordance with a mean temperature difference across the evaporator, i.e. the applied maximum opening degree is always determined in such a manner that the relevant and current mean temperature difference across the evaporator is taken into account. Accordingly, the risk of liquid refrigerant passing through the evaporator is reduced due to the maximum opening degree. At the same time, the dynamical determination of the maximum opening degree ensures that the maximum opening degree is adjusted to the actual load on the system, while allowing the opening degree to be changed as much as possible under the given circumstances, thereby allowing the vapour compression system to operate as efficiently as possible.

The step of determining a maximum opening degree may comprise the steps of: - obtaining the current opening degree for the valve,

- dividing the current opening degree by a mean temperature difference across the evaporator, and

- obtaining an average of the result of the division step.

According to this embodiment, an average, preferably a temporal average, of the ratio between the current opening degree of the valve and the mean temperature difference across the evaporator is also taken into account when the maximum opening degree is determined. Thus, according to this embodiment, the maximum opening degree reflects the load on the evaporator and the necessary opening degree to balance this load. Thereby the opening degree of the valve is limited to the amount of refrigerant which the evaporator is actually capable of evaporating under the given circumstances, in particular in a given load situation. This helps in preventing that liquid refrigerant is allowed to pass through the evaporator.

Preferably, the step of determining a maximum opening degree may further comprise the step of multiplying the obtained average by the mean temperature difference across the evaporator. According to this embodiment, the mean temperature difference across the evaporator is directly reflected in the maximum opening degree, i.e. the maximum opening degree is promptly altered in response to a change in mean temperature difference.

The step of obtaining an average may comprise feeding the result of the division step to a low pass filter. As an alternative, the average may be calculated in an ordinary manner. The method may further comprise the step of multiplying the obtained average by a factor, said factor being larger than 1. Thereby it is ensured that the maximum opening degree is always allowed to increase when the current opening degree is limited by the maximum opening degree. Thereby the maximum opening degree will always allow the valve to be opened a little bit more, and a situation where the opening degree becomes relatively small, but the maximum opening degree prevents it from increasing, is prevented. Furthermore, the result of this step may further be multiplied by the current value of the mean temperature difference across the evaporator.

The method may further comprise the step of calculating the mean temperature difference across the evaporator on the basis of two or more measured temperature values. As mentioned above, these temperature values may advantageously include one or more of the following, non- exhaustive, list of temperatures. Temperature of air entering the evaporator, temperature of air leaving the evaporator, temperature of refrigerant entering the evaporator, temperature of refrigerant leaving the evaporator and evaporation temperature.

Thus, the step of calculating the mean temperature difference across the evaporator may be performed using the formula:

where ΔT_m is the mean temperature difference across the evaporator, S₃ is an inlet temperature of air supplied across the evaporator, S₄ is an outlet temperature of air supplied across the evaporator, and T_ev is an evaporation temperature of the evaporator. This particular formula reflects the load on the evaporator, i.e. the performance which the evaporator needs to deliver, in a very realistic manner, and calculating the mean temperature difference as described above accordingly results in a maximum opening degree which reflects the current operating conditions for the evaporator in a very realistic manner.

The method may further comprise the step of supplying the determined maximum opening degree to a proportional integral (Pl) controller. Preferably, the Pl controller is further provided with information relating to the result of the comparison step. In this case the Pl controller is preferably used for calculating an opening degree of the valve on the basis of the comparison step as well as in accordance with the maximum opening degree.

The method may further comprise the step of dynamically determining the reference superheat value. According to this embodiment, the reference superheat value is automatically and substantially continuously determined in such a manner that changes in operating conditions for the vapour compression system can be taken into account.

The step of dynamically determining the reference superheat value may comprise the steps of:

- monitoring an outlet refrigerant temperature at an outlet of the evaporator,

- measuring an instability value of the monitored outlet refrigerant temperature,

- comparing the instability value to a reference instability value, thereby obtaining an error value, and - integrating the error value to obtain the reference superheat value.

According to this embodiment the reference superheat value is determined in a manner which is very similar to the manner described in EP 0 811 136. This preferably functions in the following manner. When the superheat becomes very small, i.e. when the refrigerant temperature at the outlet of the evaporator becomes close to the evaporation temperature, then the refrigerant temperature at the outlet becomes unstable in the sense that it starts oscillating at a relatively high frequency. Accordingly, the high frequency part of this temperature signal is an indication of the degree of instability of the temperature. Thus, an instability value can be derived from the temperature signal, based on the high frequency part of the signal. The derived instability value may then be compared to a reference instability value, and an error value, e.g. the difference between the derived instability value and the reference instability value, is obtained. The reference instability value is preferably a value indicating an acceptable level of instability of the refrigerant temperature at the outlet of the evaporator. Finally, the error value is integrated to obtain the reference superheat value.

Thus, if the superheat value of the vapour compression system becomes so low that the refrigerant temperature at the outlet opening of the evaporator becomes unstable and starts oscillating at a high frequency, then the instability value increases. This will cause the error signal to increase, and since the reference superheat value is the integrated error signal, the reference superheat value will also increase. Since the vapour compression system is operated to obtain a superheat value which is equal to or close to the reference superheat value, this has the effect that the superheat value is increased, i.e. the refrigerant temperature at the outlet opening of the evaporator is allowed to increase as compared to the evaporating temperature. Thereby the refrigerant temperature at the outlet opening of the evaporator is pulled away from the unstable region. On the other hand, when the outlet temperature has been pulled away from the unstable region, the instability value decreases, and this eventually has the consequence that the reference superheat value is decreased. Accordingly, the reference superheat value is continuously adjusted in such a manner that it is attempted to obtain a value which is as low as possible, while ensuring that the refrigerant temperature at the outlet opening of the evaporator remains stable.

The method may further comprise the steps of:

- generating a control signal on the basis of the comparing step, and

- calculating an adjustment value on the basis of the obtained superheat value, the reference superheat value and the generated control signal,

wherein the step of controlling the opening degree of the valve is further performed in accordance with the adjustment value.

According to this embodiment a faster response to changes in the operating conditions for the vapour compression system can be obtained, due to the calculated adjustment value. For instance, in the case that the comparing step reveals that the superheat value for the vapour compression system is very far from the reference superheat value, then it may be desirable to increase the opening degree of the valve as fast as possible in order to reach the reference superheat value quickly. However, it should still be ensured that the maximum opening degree is not exceeded. The faster response by the system which is obtained due to the adjustment value ensures that the energy consumption of the vapour compression system can be reduced, because the vapour compression system is operated in an optimum manner for a larger part of the time. The step of calculating an adjustment value may be performed using a piecewise linear function of the obtained superheat value, the reference superheat value and the generated control signal.

According to a second aspect of the invention the above and other objects are fulfilled by providing a method for controlling an opening degree of a valve arranged in a vapour compression system, the vapour compression system further comprising a compressor, a condenser and an evaporator, the compressor, the condenser, the valve and the evaporator being arranged in a refrigerant flow path, the method comprising the steps of:

- obtaining a superheat value for the vapour compression system,

- comparing the obtained superheat value to a reference superheat value,

- generating a control signal on the basis of the comparing step,

- calculating an adjustment value on the basis of the obtained superheat value, the reference superheat value and the generated control signal, and

- controlling the opening degree of the valve on the basis of the generated control signal and on the basis of the adjustment value, and in order to control the superheat value to be equal to or close to the reference superheat value.

As described above, the method according to the second aspect of the invention provides a faster response to changes in the operating conditions, in particular changes in superheat, for the vapour compression system. The step of calculating an adjustment value may be performed using a piecewise linear function of the obtained superheat value, the reference superheat value and the generated control signal.

Alternatively or additionally, the method may further comprise the step of dynamically obtaining a reference superheat value, and the step of dynamically determining the reference superheat value may comprise the steps of:

- monitoring an outlet refrigerant temperature at an outlet of the evaporator,

- comparing the instability value to a reference instability value, thereby obtaining an error value, and

- integrating the error value to obtain the reference superheat value.

According to a third aspect of the invention the above and other objects are fulfilled by providing a control unit for controlling a vapour compression system, the vapour compression system comprising a compressor, a condenser, a valve and an evaporator arranged in a refrigerant flow path, the control unit comprising:

- means for obtaining a superheat value for the vapour compression system,

- means for comparing the obtained superheat value to a reference superheat value, - means for dynamically determining a maximum opening degree for the valve as a function of a mean temperature difference across the evaporator, and

- means for controlling the opening degree of the valve in response to an output generated by the comparing means and in accordance with the dynamically determined maximum opening degree.

It should be noted that a person skilled in the art would readily recognise that any feature described in combination with the first aspect of the invention could also be combined with the second and third aspects of the invention, any feature described in combination with the second aspect of the invention could also be combined with the first and third aspects of the invention, and any feature described in combination with the third aspect of the invention could also be combined with the first and second aspects of the invention.

The control unit according to the third aspect of the invention may advantageously be used for performing the method of the first aspect of the invention and/or the method of the second aspect of the invention.

The vapour compression system is a refrigeration system, a heat pump, or any other suitable kind of vapour compression system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanying drawings in which

Fig. 1 is a schematic view of a refrigeration system in which a method according to an embodiment of the invention can be employed, Fig. 2 is a diagrammatic view of a control system for controlling a valve in accordance with an embodiment of the invention,

Fig. 3 shows a detail of Fig. 2,

Fig. 4 shows another detail of Fig. 2, and

Fig. 5 is a graph illustrating a piecewise linear function which can be used in a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic view of a refrigeration system 1 comprising a compressor 2, a condenser 3, an expansion valve 4 and an evaporator 5, the compressor 2, the condenser 3, the expansion valve 4 and the evaporator 5 being interconnected in a refrigerant flow path. The expansion valve 5 is controlled by means of a control unit 6 in a manner which will be described further below.

In Fig. 1 only one compressor 2 and one evaporator 5 are shown. However, it should be understood that the compressor 2 could in fact be two or more compressors, e.g. arranged in a rack. Similarly, the refrigeration system 1 may, alternatively or additionally, comprise two or more evaporators 5, preferably arranged in parallel in the refrigerant flow path. In this case the evaporators 5 may be connected to separate refrigerated volumes, such as separate display cases in a food store, e.g. a supermarket.

Fig. 2 is a diagrammatic view of a control system for controlling a valve in accordance with an embodiment of the invention. A valve (not shown) is arranged in such a manner that an opening degree, OD, of the valve determines the supply of refrigerant to an evaporator 5. The opening degree of the valve is controlled by means of the control system.

A number of temperature sensors are arranged in the vicinity of the evaporator 5. Each temperature sensor is adapted to measure a relevant temperature and to generate a corresponding signal. The generated signals are fed to the control system and used as control parameters during control of the opening degree of the valve. In Fig. 2 TO is the evaporation temperature of the evaporator 5, i.e. the temperature at which the refrigerant evaporates in the evaporator 5. S2 is the refrigerant temperature at the outlet of the evaporator 5, i.e. the temperature of the gaseous refrigerant leaving the evaporator 5. S3 and S4 are temperatures of ambient air passing the evaporator. S3 is the temperature of such air when it enters the evaporator 5, and S4 is the temperature of the air leaving the evaporator 5.

The difference between S2 and TO is the superheat of the refrigeration system. If the superheat becomes relatively large, it is an indication that a relatively large part of the evaporator 5 contains gaseous refrigerant, and that the potential refrigeration capacity of the evaporator 5 is therefore not utilised to the greatest extent possible. It is therefore desirable to control the supply of refrigerant to the evaporator 5 in such a manner that the superheat is minimal, but positive.

Furthermore, the supply of refrigerant to the evaporator 5 should be controlled in such a manner that liquid refrigerant is not allowed to pass through the evaporator 5. When liquid refrigerant passes through the evaporator 5, the potential refrigeration capacity of the refrigerant is not utilised to the maximum extent, since some of the refrigerant has obviously not been evaporated in the evaporator 5. Furthermore, liquid refrigerant passing through the evaporator 5 may cause damage to the compressor. This is very undesirable.

TO and S2 are fed into a first summation unit 7 in order to calculate the superheat, SH. The result is fed into a second summation unit 8 along with a reference superheat value, SHref. Accordingly, the measured superheat is compared to the reference superheat value. The reference superheat value represents an optimum superheat value which it is desired to obtain. Thus, it is attempted to control the supply of refrigerant to the evaporator 5 in such a manner that the measured superheat value becomes as close as possible to the reference superheat value.

The result of the comparison performed by the second summation unit 8 is fed to a proportional integrator (Pl) 9. The Pl 9 generates a control signal, SHctrl, based on the result received from the second summation unit 8, and based on information relating to a maximum opening degree, MaxOD, of the valve. The calculation of MaxOD will be explained further below with reference to Fig. 3. Under normal circumstances the opening degree of the valve is controlled in accordance with the control signal, SHctrl, i.e. in order to obtain a superheat value which is as close as possible to the reference superheat value, while ensuring that the opening degree of the valve does not exceed MaxOD. However, in some situations it is desired that the system reacts faster than it is possible when the valve is operated purely on the basis of the measured superheat as described above. In such situations SHctrl is adjusted by a value which is calculated using a piecewise linear function (PWL) 12. This will be described further below with reference to Fig. 4.

The reference superheat value is dynamically calculated by means of a search function 10. When the superheat approaches zero, i.e. when S2 approaches TO, the S2 signal becomes unstable in the sense that the signal fluctuates with a high frequency. This is undesirable, and the aim is therefore to control the superheat to be as low as possible, but sufficiently high to ensure a stable S2 signal. Accordingly, the reference superheat is dynamically calculated to fulfil this. This is done in the following manner. The temperature S2 is fed into a stability measurement device 11. Here the S2 signal is passed through a high pass filter in order to determine how large the high frequent part of the signal is, since this is a measure of the instability of the signal as described above. The signal is then normalized and passed through a low pass filter. The result is a DC signal, σ, which is representative of the instability of the S2 signal.

This result is fed to the search function 10, where it is compared to a reference instability value. The reference instability value can, e.g., be set manually. The comparison results in an error signal which is integrated by means of a proportional integrator (Pl). The result of this integration is supplied to the second summation unit 8 and used as the reference superheat value, SHref. Thus, if the instability of the S2 signal increases, then σ is far from the reference instability value, and the error signal therefore increases. As a consequence, SHref also increases, since it is an integrated value of the error signal. Accordingly, if the instability of the S2 signal becomes unacceptably high, then SHref is automatically increased, and the S2 temperature is consequently forced towards a region with a better stability. When the SHref has been increased sufficiently to ensure a stable S2 signal, then σ decreases, causing the error signal to decrease, and thereby SHref is also decreased. Thus, SHref is dynamically calculated to be as low as possible, while ensuring stability of the S2 signal.

Fig. 3 is a detailed view of MaxOD calculator 13 of the control system of Fig. 2. TO, S3 and S4 are supplied to a first calculating unit 14 which is adapted to calculate a representative mean temperature difference across the evaporator on the basis of the supplied measured temperatures. The mean temperature difference, ΔT_m, is calculated using the formula

_AT ^{S3 ~ S4}

In

S4-T0.

The result of this calculation is fed to a second calculating unit 15. The current opening degree, OD, of the valve is also fed to the second calculating unit 15, and the second calculating unit 15 calculates the

quantity and feeds this to an average filter 16, typically being or

Δ71 m comprising a low pass filter. The result is multiplied by a factor which is larger than one, typically approximately 1.1 , in order to prevent that the maximum opening degree becomes so small that it will not be possible to increase the actual opening degree of the valve. Finally, the average value, multiplied by the factor, is multiplied by ΔT_m at third calculation unit 17. The result is MaxOD, which is fed to the Pl 9 as described above with reference to Fig. 2. The Pl 9 ensures that the opening degree of the valve does not exceed MaxOD. Thereby it is prevented that the opening degree of the valve is increased significantly in an abrupt manner, and thereby the risk that liquid refrigerant is accidentally allowed to pass through the evaporator is reduced.

Fig. 4 is a detailed view of adjustment value calculator 18 of the control system of Fig. 2. The calculated superheat value, SH, the reference superheat value, SHref, the superheat control signal, SHctrl, as well as the maximum opening degree, MaxOD, are all supplied to a piecewise linear function (PWL) 12. The PWL 12 determines, based on the supplied values, whether or not it is necessary to adjust the SHctrl value. If it is determined that adjustment is necessary, then the PWL 12 calculates the necessary adjustment value. The calculated adjustment value is supplied to a third summation unit 19 where it is added to the SHctrl value. The resulting value is used for controlling the opening degree of the valve. A more detailed description of the function of the PWL 12 can be found below with reference to Fig. 5.

Fig. 5 is a graph illustrating the output of the PWL 12 as a function of the measured superheat value of the refrigeration system.

When the measured superheat value is equal to the reference superheat value it can be assumed that the system is operating in an optimum manner, and that an adjustment of the SHctrl signal is not required. Accordingly, the PWL output is zero in this situation.

When the measured superheat value is close to the reference superheat value, it can be assumed that only small adjustments to the SHctrl value are necessary. Accordingly, the piecewise linear function has a relatively low slope in a region around SH=SHref.

When the measured superheat is relatively far from the reference superheat, it can be expected that major adjustments in the opening degree of the valve are required in order to obtain a superheat value which is equal to or close to the reference superheat value. Normally, the Pl is not capable of altering the SHctrl signal sufficiently fast in this situation, and the response time of the system is therefore not sufficient unless the PWL provides a significant adjustment signal. Accordingly, linear functions with a somewhat larger slope are selected when the difference between the measured superheat value and the reference superheat value exceeds a predetermined threshold value.

In order to ensure that the opening degree of the valve does not exceed MaxOD, the PWL output value is prevented from exceeding MaxOD- SHctrl. When this situation occurs, the opening degree of the valve is simply maintained at MaxOD.

Similarly, when the PWL output reaches -SHctrl the resulting control signal is zero, corresponding to the valve being completely closed. Since it does not make sense to have a negative opening degree, the PWL output is prevented from falling below the value -SHctrl, and the valve is simple maintained in a closed state when this situation occurs.

Claims

1. A method for controlling an opening degree of a valve arranged in a vapour compression system, the vapour compression system further comprising a compressor, a condenser and an evaporator, the compressor, the condenser, the valve and the evaporator being arranged in a refrigerant flow path, the method comprising the steps of:

- obtaining a superheat value for the vapour compression system,

- comparing the obtained superheat value to a reference superheat value,

- determining a maximum opening degree for the valve, and

2. A method according to claim 1 , wherein the step of determining a maximum opening degree comprises the steps of:

- obtaining the current opening degree for the valve,

- dividing the current opening degree by a mean temperature difference across the evaporator, and - obtaining an average of the result of the division step.

3. A method according to claim 2, wherein the step of obtaining an average comprises feeding the result of the division step to a low pass filter.

4. A method according to claim 2 or 3, further comprising the step of multiplying the obtained average by a factor, said factor being larger than 1.

5. A method according to any of the preceding claims, further comprising the step of calculating the mean temperature difference across the evaporator on the basis of two or more measured temperature values.

6. A method according to claim 5, wherein the step of calculating the mean temperature difference across the evaporator is performed using the formula:

where ΔT_m is the mean temperature difference across the evaporator, S₃ is an inlet temperature of air supplied across the evaporator, S₄ is an outlet temperature of air supplied across the evaporator, and T_ev is an evaporation temperature of the evaporator.

7. A method according to any of the preceding claims, further comprising the step of supplying the determined maximum opening degree to a proportional integral (Pl) controller.

8. A method according to any of the preceding claims, further comprising the step of dynamically determining the reference superheat value.

9. A method according to claim 8, wherein the step of dynamically determining the reference superheat value comprises the steps of:

- monitoring an outlet refrigerant temperature at an outlet of the evaporator,

- integrating the error value to obtain the reference superheat value.

10. A method according to any of the preceding claims, further comprising the steps of:

- generating a control signal on the basis of the comparing step, and

11. A method according to claim 10, wherein the step of calculating an adjustment value is performed using a piecewise linear function of the obtained superheat value, the reference superheat value and the generated control signal.

12. A method for controlling an opening degree of a valve arranged in a vapour compression system, the vapour compression system further comprising a compressor, a condenser and an evaporator, the compressor, the condenser, the valve and the evaporator being arranged in a refrigerant flow path, the method comprising the steps of:

- obtaining a superheat value for the vapour compression system,

- comparing the obtained superheat value to a reference superheat value,

- generating a control signal on the basis of the comparing step,

13. A method according to claim 12, wherein the step of calculating an adjustment value is performed using a piecewise linear function of the obtained superheat value, the reference superheat value and the generated control signal.

14. A method according to claim 12 or 13, further comprising the step of dynamically obtaining a reference superheat value.

15. A method according to claim 14, wherein the step of dynamically determining the reference superheat value comprises the steps of:

- monitoring an outlet refrigerant temperature at an outlet of the evaporator,

- integrating the error value to obtain the reference superheat value.

16. A control unit for controlling a vapour compression system, the vapour compression system comprising a compressor, a condenser, a valve and an evaporator arranged in a refrigerant flow path, the control unit comprising:

- means for obtaining a superheat value for the vapour compression system,

- means for comparing the obtained superheat value to a reference superheat value,

- means for dynamically determining a maximum opening degree for the valve as a function of a mean temperature difference across the evaporator, and

17. A control unit according to claim 16, wherein the vapour compression system is a refrigeration system.

18. A control unit according to claim 16, wherein the vapour compression system is a heat pump.