EP4649271A1

EP4649271A1 - A method for avoiding flooding in a vapour compression system

Info

Publication number: EP4649271A1
Application number: EP23836476.4A
Authority: EP
Inventors: Erhan KAPICI; Lars Finn Sloth Larsen; Torben Green
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 2023-01-13
Filing date: 2023-12-20
Publication date: 2025-11-19
Also published as: WO2024149588A1; CN120225821A

Abstract

A method for controlling a vapour compression system (1) is disclosed. A superheat value (13) of refrigerant leaving the evaporator (5) is monitored, and a quantity (15) being representative for a variance of the monitored superheat value (13) is calculated. The expansion device (4) is operated in order to obtain a superheat value (13) of refrigerant leaving the evaporator (5) which is substantially equal to a reference superheat value (14). The reference superheat value (14) is decreased as long as the monitored superheat value (13) exhibits values which are above the reference superheat value (14) as well as below the reference superheat value (14). In the case that the monitored superheat value (13) exhibits only values above a lower superheat limit, where the lower superheat limit is a superheat level which is below the reference superheat value (14) by an amount corresponding to the quantity (15) being representative for a variance of the monitored superheat value (13), decreasing of the reference superheat value (14) is discontinued.

Description

A METHOD FOR AVOIDING FLOODING IN A VAPOUR COMPRESSION SYSTEM

FIELD OF THE INVENTION

The present invention relates to a method for controlling a vapour compression system, such as a refrigeration system, an air condition system or a heat pump, in a manner which results in a low superheat value of refrigerant leaving the evaporator without risking flooding of the evaporator.

BACKGROUND OF THE INVENTION

Vapour compression systems, such as refrigeration systems, air condition systems or heat pumps, normally comprise a compressor unit with one or more compressors, a heat rejecting heat exchanger, at least one expansion device and at least one evaporator arranged in a refrigerant path. Refrigerant flowing in the refrigerant path is compressed by the compressors of the compressor unit before being supplied to the heat rejecting heat exchanger. In the heat rejecting heat exchanger, heat exchange takes place between the refrigerant and the ambient or a secondary fluid flow across the heat rejecting heat exchanger, in such a manner that heat is rejected from the refrigerant.

The refrigerant leaving the heat rejecting heat exchanger is supplied to the expansion device, where it undergoes expansion before being supplied to the evaporator. The refrigerant being supplied to the evaporator is in the form of a mixture of liquid and gaseous refrigerant. When passing through the evaporator, the liquid part of the refrigerant is at least partly evaporated, while heat exchange takes place with the ambient or a secondary fluid flow across the evaporator, in such a manner that heat is absorbed by the refrigerant. Finally, the refrigerant is once again supplied to the compressor unit.

Thus, refrigerant flowing in the refrigerant path is alternatingly compressed by the compressors and expanded by the expansion device, while heat exchange takes place in the heat rejecting heat exchanger and the evaporator, respectively.

It is desirable that liquid refrigerant is present along the entire length of the evaporator, because thereby the entire length of the evaporator is used for evaporating refrigerant, and thereby the potential capacity of the evaporator for providing cooling is fully utilised. This provides an energy efficient operation of the vapour compression system.

On the other hand, it should be avoided that liquid refrigerant passes through the evaporator, because this may result in liquid refrigerant reaching the compressor unit, and this may cause damage to the compressors. A situation where liquid refrigerant passes through the evaporator is sometimes referred to as flooding of the evaporator.

Therefore, it is normally attempted to control the vapour compression system, in particular an opening degree of the expansion device, in such a manner that all of the liquid refrigerant has been evaporated shortly before reaching the outlet of the evaporator.

In order to obtain this, the superheat value of refrigerant leaving the evaporator may be monitored. The superheat is defined as the temperature difference between the evaporating temperature and the temperature of refrigerant leaving the evaporator. Thus, a high superheat value indicates that the temperature of the refrigerant leaving the evaporator is significantly higher than the evaporating temperature. This is an indication that all of the liquid refrigerant has been evaporated well before reaching the outlet of the evaporator, and that energy is therefore used for heating the gaseous part of the refrigerant passing through the evaporator, that the heat exchange taking place in the evaporator is not optimal, and that the vapour compression system is therefore not operated in an energy efficient manner.

On the other hand, zero superheat indicates that the temperature of the refrigerant leaving the evaporator is equal to the evaporating temperature. This is an indication that liquid refrigerant is present along the entire length of the evaporator, and that the potential capacity of the evaporator is thereby fully utilised and the vapour compression system is operating in an energy efficient manner. However, at zero superheat it is not possible to determine whether all of the refrigerant has been evaporated exactly when the outlet of the evaporator is reached, or if a significant amount of liquid refrigerant is in fact allowed to pass through the evaporator and potentially reach the compressors.

Accordingly, it is normally attempted to control the vapour compression system in such a manner that a superheat value is obtained, which is small, but positive, i.e. greater than zero. Thereby it is obtained that the vapour compression system is operated in an energy efficient manner without risking that liquid refrigerant reaches the compressors.

In order to control the vapour compression system in the manner described above, an appropriate reference superheat value may be selected, and the expansion device of the vapour compression system may be operated based on a comparison between the actual superheat value of refrigerant leaving the evaporator and the selected reference superheat value, and in order to cause the actual superheat value to approach the reference superheat value. The actual superheat value is normally determined from one or more measured values obtained by means of appropriate sensors. The measured values could, e.g., include temperature values and/or pressure values. For instance, the superheat value could be calculated from a measurement of the temperature of refrigerant leaving the evaporator in combination with measurements of one or more of an evaporating temperature of the evaporator, a temperature of refrigerant entering the evaporator, and a pressure of refrigerant leaving or entering the evaporator.

One example of such a method for controlling a vapour compression system is described in US 10,612,826 B2, disclosing an HVAC system including an electronic expansion valve (EEV) and a controller in communication with the EEV. The superheat is measured and compared to a substantially fixed superheat setpoint, and the EEV is controlled so as to obtain that the measured superheat value approaches the superheat setpoint. To this end a setpoint position value for the EEV is calculated as an average of a maximum EEV setpoint position value and a minimum EEV setpoint position value, and the EEV is operated at this position. The maximum EEV setpoint position is a position of the EEV where the measured superheat value is smaller than the superheat setpoint by a specified error value, and the minimum EEV setpoint position value is a position of the EEV where the measured superheat value is larger than the superheat setpoint by the specified error value. Thus, the position of the EEV is selected such that the superheat value remains within a range defined by the superheat setpoint by +/- the specified error value. The error value may, e.g., be 2° F.

In the case that one of the sensors applied for determining the actual superheat value is malfunctioning, or provides erroneous, inaccurate or offset measured values, the actual superheat value determined based thereon will also be erroneous, inaccurate or offset. This might lead to a situation in which zero superheat has in fact been reached, but from the measured values it appears that the actual superheat value is higher than the reference superheat value. In this case, the controller will attempt to control the vapour compression system so as to lower the superheat value further, in order to approach the reference superheat value. However, since zero superheat has in fact already been reached, this will not cause a decrease of the measured superheat value. Instead, the control measures applied may cause flooding of the evaporator and introduce a risk of liquid refrigerant leaving the evaporator and entering the suction line to an extent which may cause damage to the compressors.

DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a method for controlling a vapour compression system, in which the risk of flooding of the evaporator is reduced. It is a further object of embodiments of the invention to provide a method for controlling a vapour compression system in which possibly malfunctioning or erroneous sensors are handled without degrading the ability of the system to operate in an energy efficient manner.

The invention provides a method for controlling a vapour compression system, the vapour compression system comprising a compressor unit, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant path, the method comprising the steps of: monitoring a superheat value of refrigerant leaving the evaporator,

- calculating a quantity being representative for a variance of the monitored superheat value,

- comparing the monitored superheat value to a reference superheat value, and operating the expansion device in order to obtain a superheat value of refrigerant leaving the evaporator which is substantially equal to the reference superheat value,

- decreasing the reference superheat value as long as the monitored superheat value exhibits values which are above the reference superheat value as well as below the reference superheat value, and in the case that the monitored superheat value exhibits only values above a lower superheat limit, where the lower superheat limit is a superheat level which is below the reference superheat value by an amount corresponding to the quantity being representative for a variance of the monitored superheat value, discontinuing decreasing of the reference superheat value.

Thus, the method according to the invention is a method for controlling a vapour compression system. In the present context the term 'vapour compression system' should be interpreted to mean any system in which a flow of fluid medium, such as 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 vapour compression system comprises a compressor unit comprising one or more compressors, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant path. Refrigerant circulating the refrigerant path is compressed by the compressors of the compressor unit before being supplied to the heat rejecting heat exchanger. In the heat rejecting heat exchanger, heat exchange takes place between the refrigerant and the ambient or a secondary fluid flow across the heat rejecting heat exchanger, in such a manner that heat is rejected from the refrigerant. The heat rejecting heat exchanger may be in the form of a condenser, in which case the refrigerant is at least partly condensed when passing through the heat rejecting heat exchanger. As an alternative, the heat rejecting heat exchanger may be in the form of a gas cooler, in which case the refrigerant passing through the heat rejecting heat exchanger is cooled, but remains in a gaseous or trans-critical state.

Refrigerant leaving the heat rejecting heat exchanger is supplied to the expansion device, where it undergoes expansion before being supplied to the evaporator. The refrigerant being supplied to the evaporator is in a mixed state of gaseous and liquid refrigerant. In the evaporator, the liquid part of the refrigerant is at least partly evaporated, while heat exchange takes place between the refrigerant and the ambient or a secondary fluid flow across the evaporator, in such a manner that heat is absorbed by the refrigerant. Finally, the refrigerant leaving the evaporator is supplied to the compressor unit, via a suction line.

The vapour compression system may comprise two or more expansion devices and two or more evaporators. In this case each expansion device supplies refrigerant to one of the evaporators, and the evaporators, along with their respective expansion devices, are arranged fluidly in parallel between the heat rejecting heat exchanger and the suction line. This is, e.g., relevant in refrigeration systems with several cooling entities, such as a supermarket refrigeration system with several display cases or cabinets. In this case each evaporator is arranged in thermal contact with a refrigerated volume of one of the cooling entities.

Thus, the refrigerant circulating the refrigerant path is alternatingly compressed by the compressors of the compressor unit and expanded by the expansion device, while heat exchange takes place in the heat rejecting heat exchanger and the evaporator.

In the method according to the invention, a superheat value of refrigerant leaving the evaporator is monitored. As described above, the superheat value of refrigerant leaving the evaporator is defined as the temperature difference between the evaporating temperature of the refrigerant and the actual temperature of the refrigerant leaving the evaporator. Furthermore, as described above, the superheat value is a relevant control parameter for ensuring energy efficient operation of the vapour compression system, and for preventing that liquid refrigerant reaches the compressor unit. The superheat value may be measured directly, or it may be derived from two measured parameters. This will be described in further detail below. Furthermore, a quantity being representative for a variance of the monitored superheat value is calculated. The quantity may, e.g., be an appropriate variance of the monitored superheat value. As an alternative, it may be a variance of another parameter which is related to the superheat value, e.g. the temperature of refrigerant leaving the evaporator.

In any event, the calculated quantity is representative for the variance of the monitored superheat value, and thereby it reflects in which manner and how much the superheat value varies or fluctuates. A low variance indicates that the superheat value is stable, whereas a high variance indicates that the superheat value is unstable.

For instance, the quantity being representative for the variance of the monitored superheat value may be of a kind which represents that the superheat value is within a range around the mean superheat value defined by the variance, for at least 95% of the time, or it may represent a standard deviation of a distribution of measured superheat values, e.g. multiplied by an appropriate factor.

The monitored superheat value is compared to a reference superheat value, and the expansion device is operated in order to obtain a superheat value of refrigerant leaving the evaporator which is substantially equal to the reference superheat value. Accordingly, the expansion device is operated in order to control the refrigerant supply to the evaporator in such a manner that the reference superheat value is obtained. This may, e.g., include adjusting an opening degree of the expansion device or modulating a duty cycle of the expansion device. The control of the expansion device may, thus, be a standard setpoint control with the reference superheat value as the setpoint. The reference superheat value represents a superheat value which it is desired to obtain, e.g. in order to ensure energy efficient operation of the vapour compression system.

As described above, energy efficient operation of the vapour compression system is obtained when the actual superheat value of refrigerant leaving the evaporator is small, but positive. Therefore, it is desirable to apply a reference superheat value which is as low as possible, in order to cause the actual superheat value to also be as low as possible. Accordingly, while the control of the vapour compression system described above is performed, the reference superheat value is gradually decreased, e.g. continuously or in a stepwise manner. This is done as long as the monitored superheat value exhibits values which are above the reference superheat value as well as below the reference superheat value.

When the superheat is monitored, discrete measured or derived values are obtained or sampled. These values are not identical, but will fluctuate to some extent and define a distribution with a mean and a variance, e.g. a normal distribution. Ideally, the measured or derived values will fluctuate about a mean value which is identical or close to the reference superheat value. When this is actually the case, some of the measured or derived values will be above the reference superheat value and some of the measured or derived values will be below the reference superheat value, i.e. the monitored superheat value exhibits values which are above as well as below the reference superheat value. When this situation occurs, it is an indication that the vapour compression system is capable of driving the actual superheat value down to the level represented by the currently applied reference superheat value. Accordingly, it may be safe to lower the actual superheat value even further, without risking flooding of the evaporator, and therefore the reference superheat value is lowered fu rther.

On the other hand, if all of the measured or derived superheat values are, e.g., above the reference superheat value, this is an indication that the vapour compression system is not capable of driving the actual superheat value further down, and that the evaporator may be flooded, or that flooding is approaching. Since the measured or derived superheat values do not in themselves indicate that zero superheat has been reached, there might be a discrepancy between the actual, real superheat value and the measured or derived superheat values, which could, e.g., be caused by one or more faulty, erroneous or offset sensors. In this case it may not be safe to lower the reference superheat value further. Furthermore, if significantly more of the measured or derived superheat values are above than below the reference superheat value, this could be an indication that the situation described above is approaching.

Accordingly, in the case that the monitored superheat value exhibits only values above a lower superheat limit, decreasing of the reference superheat value is discontinued. This situation may, e.g., indicate that the vapour compression system is not capable of driving the superheat value below the lower superheat limit, and the lower superheat limit may, accordingly, be seen as representing zero superheat, even though the measured or derived superheat value indicates a positive superheat. The lower superheat limit is a superheat level which is below the reference superheat value by an amount corresponding to the quantity being representative for a variance of the monitored superheat value.

Thus, it is assumed that the measured or derived superheat values fluctuate within a certain band or interval about a mean value which is at or near the reference superheat value, where the size of the band or interval depends on the variance of the monitored superheat value. In the case that a situation is approaching where significantly more of the measured or derived superheat values are above than below the reference superheat value, and the mean of the distribution of the monitored superheat value therefore starts 'moving away' from the reference superheat value, it may be expected that, at some point, there will no longer be measured or derived superheat values below the lower superheat limit. Therefore, when this occurs, it is a good indication that the reference superheat value should not be reduced further, and that the currently applied reference superheat value does in fact represent a minimum safe and desirable superheat value, even though the monitored superheat values indicate a higher superheat. As described above, this may be due to a malfunctioning, erroneous or offset sensor, and this is appropriately handled and taken into account by the method according to the invention. A sensor may, e.g., be offset if it is mislocated and/or poorly insulated.

Thus, the method according to the invention allows the vapour compression system to be operated at a very low actual superheat value, and thereby highly energy efficient, without risking flooding of the evaporator. In particular, it is not required to apply a safety margin on the reference superheat value in order to handle possible erroneous or offset sensor measurements.

During the step of decreasing the reference superheat value, the reference superheat value may be calculated based on the quantity being representative for a variance of the monitored superheat value and on a minimum acceptable superheat value, where the minimum acceptable superheat value represents a lower boundary for a range of superheat values which ensure safe and/or appropriate operation of the vapour compression system, e.g. with regard to preventing that liquid refrigerant reaches the compressor unit. The minimum acceptable superheat value may typically be a small, but positive value, such as 1-5 K, e.g. approximately 2 K or 3 K, thereby ensuring that the superheat remains positive. The reference superheat value may, e.g., be calculated by adding the quantity being representative for a variance of the monitored superheat value to the minimum acceptable superheat value.

According to this embodiment, the reference superheat value is calculated with due consideration to the minimum acceptable superheat value, and while taking the variance of the superheat value into account. Thereby the reference superheat value can be selected in such a manner that the variance of the superheat value will not cause the superheat value to decrease below the minimum acceptable superheat value, possibly except on rare occasions and/or briefly.

As an alternative, the reference superheat value may be calculated by adding the calculated quantity being representative for a variance of the monitored superheat value to a minimum measured superheat value. In the present context the term 'minimum measured superheat value' should be interpreted to mean the lowest superheat value which has been measured during a predefined preceding time interval. Accordingly, the minimum measured superheat value may be regarded as representing a lower boundary of a range of superheat values around a mean value of the measured superheat values, within which superheat values have actually been measured during the predefined preceding time interval. Accordingly, selecting a reference superheat value which is above the lower boundary by an amount which corresponds to the variance of the monitored superheat value provides an appropriate safety margin towards zero superheat. This embodiment is similar to the embodiment described above, except that in this case, the superheat level selected as the 'minimum acceptable superheat value' is a variable entity, which depends on the actually measured superheat values, rather than being a fixed value.

The method may further comprise the step of increasing the reference superheat value in the case that the monitored superheat value has exhibited only values above the lower superheat limit for a predefined period of time. According to this embodiment, the decrease of the reference superheat value is not merely discontinued when the monitored superheat value starts 'moving away' from the reference superheat value, the reference superheat value is also increased in order to quickly move the vapour compression system away from an operating state where there is a risk of flooding of the evaporator.

As an alternative, the method may further comprise the step of increasing the reference superheat value in the case that the monitored superheat value has exhibited only values above the reference superheat value for a predefined period of time.

In the case that the evaporator is in fact flooded, or close to flooding, this may cause a difference between the monitored superheat value and the reference superheat value which is so significant that essentially all of the measured or derived superheat values are above the reference superheat value. In this case it may be concluded that the reference superheat value has been decreased too much, e.g. to a level which in fact corresponds to a negative value of the actual superheat. Thus, it may not be sufficient to discontinue decreasing the reference superheat value in order to prevent liquid refrigerant from leaving the evaporator and entering the suction line. Therefore, when this situation is detected, the reference superheat value is increased in order to bring it to a level which represents a positive actual superheat value. However, as long as the monitored superheat value exhibits values below the reference superheat value, but above the lower superheat limit, the currently applied reference superheat value is simply maintained.

The method may further comprise the step of discontinuing decreasing of the reference superheat value in the case that the quantity being representative for the variance of the monitored superheat value decreases below a predefined threshold value. According to this embodiment, the behaviour of the quantity being representative for the variance of the monitored superheat value is further monitored. When the superheat value approaches zero superheat, a high variance of the superheat value is expected. However, if zero superheat is reached, and possibly flooding of the evaporator, the superheat value becomes stable, i.e. the variance of the superheat value decreases significantly. Thus, when this behaviour is observed, more particularly when the variance decreases below a certain threshold value, it is an indication that zero superheat has been reached, and therefore the decrease of the reference superheat value should be discontinued.

The step of calculating a quantity being representative for a variance of the monitored superheat value may comprise calculating the variance of the monitored superheat value. According to this embodiment, the actual variance of the monitored superheat value is calculated. The actual variance may be applied directly as the quantity being representative for the variance of the monitored superheat value for the purpose of calculating the lower superheat limit. As an alternative, the quantity applied for calculating the lower superheat limit may be derived from the actual variance, e.g. by multiplying the actual variance by an appropriate factor. Alternatively, another suitable quantity may be calculated, e.g. a variance of the temperature of refrigerant leaving the evaporator. As described above, the superheat value is the temperature difference between the evaporating temperature and the actual temperature of refrigerant leaving the evaporator. The temperature of the refrigerant leaving the evaporator may be expected to vary to a greater extent than the evaporating temperature. Therefore, the variance of the temperature of refrigerant leaving the evaporator may be regarded as a suitable representation of the variance of the superheat value. As another alternative, the calculated quantity may be a quantity which is proportional to the variance of the superheat value or the variance of the temperature of refrigerant leaving the evaporator.

The step of operating the expansion device in order to obtain a superheat value of refrigerant leaving the evaporator which is substantially equal to the reference superheat value may comprise adjusting an opening degree of the expansion device. The opening degree of the expansion device defines the refrigerant supply to the evaporator, in the sense that an increase in opening degree increases the refrigerant supply to the evaporator and a decrease in opening degree decreases the refrigerant supply to the evaporator. Accordingly, adjusting the opening degree of the expansion device is a suitable way for adjusting the refrigerant supply to the evaporator in order to increase or decrease the superheat value.

The method may further comprise the steps of: monitoring the opening degree of the expansion device, and increasing the reference superheat value in the case that an increase in opening degree of the expansion device exceeds a threshold increase value.

According to this embodiment, the opening degree of the expansion device is monitored, while the vapour compression system operates, in order to determine whether or not it is likely that zero superheat has been reached, without the monitored superheat value indicating that this is the case. If zero superheat has in fact been reached, but the monitored superheat value is still above the reference superheat value, then the controller of the vapour compression system will instruct the expansion device to increase the opening degree in order to increase the supply of refrigerant to the evaporator, and thereby decrease the superheat value in an attempt to reach the reference superheat value. However, since zero superheat has in fact already been reached, increasing the opening degree will not provide the desired result, and therefore the controller may instruct the expansion device to increase the opening degree even further. Thus, when this situation occurs, a significant increase in opening degree of the expansion device may be observed. Accordingly, if the increase in opening degree of the expansion device exceeds a threshold increase value, this is an indication that zero superheat has in fact been reached, and it is therefore appropriate to increase the reference superheat value in order to avoid flooding of the evaporator.

The threshold increase value may depend on a difference between the monitored superheat value and the reference superheat value. According to this embodiment, the threshold increase value is not a fixed value, but varies in accordance with how close the monitored superheat value is to the reference superheat value. For instance, in the case that the difference between the monitored superheat value and the reference superheat value is large, then a relatively high threshold increase value may be applied. On the other hand, in the case that the difference between the monitored superheat value and the reference superheat value is small, then a somewhat lower threshold increase value may be applied. This is due to the fact that when the difference between the monitored superheat value and the reference superheat value is large, it may be expected that a large increase in opening degree of the expansion device is required in order to drive the monitored superheat value towards the reference superheat value. However, when the difference between the monitored superheat value and the reference superheat value is small, it may be expected that only a small increase in opening degree of the expansion valve is required.

Alternatively or additionally, the threshold increase value may depend on a ratio between the opening degree of the expansion device and the monitored superheat value. This is similar to the embodiment described above. For instance, if a large increase in opening degree is applied when the monitored superheat value is low, this is an indication that flooding may be occurring, and it is therefore appropriate to increase the reference superheat value. According to one embodiment, the threshold increase value may be calculated as a function of the ratio between the opening degree of the expansion device and the monitored superheat value, multiplied by the difference between the monitored superheat value and the reference superheat value.

The step of operating the expansion device may be performed by means of a proportional integral (PI) controller. According to this embodiment, the expansion device is operated in accordance with a standard PI control strategy with the reference superheat value as the setpoint value. Alternatively, another suitable control strategy may be applied.

The step of monitoring a superheat value of refrigerant leaving the evaporator may comprise measuring a temperature of refrigerant leaving the evaporator and an evaporating temperature of the evaporator, and calculating the superheat value from the measured temperatures.

As described above, the superheat value is the temperature difference between the evaporating temperature and the temperature of refrigerant leaving the evaporator. Therefore, if the temperature of refrigerant leaving the evaporator and the evaporating temperature are measured, the superheat value can readily be derived by subtracting the measured evaporating temperature from the measured refrigerant temperature.

The temperatures may, e.g., be measured by means of temperature sensors arranged in the refrigerant path at the outlet of the evaporator and inside the evaporator, respectively.

As an alternative, the step of monitoring a superheat value of refrigerant leaving the evaporator may comprise measuring a temperature of refrigerant leaving the evaporator and a pressure of refrigerant leaving or entering the evaporator, and calculating the superheat value from the measured temperature and pressure.

For a given refrigerant, the evaporating temperature depends on the pressure inside the evaporator. Thus, knowing the type of refrigerant applied in the vapour compression system, the evaporating temperature can be derived from the pressure prevailing in the evaporator. The pressure at the inlet or at the outlet of the evaporator provides a suitable measure for the pressure prevailing in the evaporator. Accordingly, the evaporating temperature can be at least approximately derived from a refrigerant pressure measured at the inlet or at the outlet of the evaporator. This allows the superheat value to be derived in the manner described above. As another alternative, the step of monitoring a superheat value of refrigerant leaving the evaporator may comprise measuring a temperature of refrigerant leaving the evaporator and a temperature of refrigerant entering the evaporator, and calculating the superheat value from the measured temperatures.

Similarly to the embodiment described above, the evaporating temperature can also be derived from the temperature of refrigerant entering the evaporator. As described above, the refrigerant entering the evaporator is in a mixed liquid and gaseous state, i.e. it is in a two- phase state. Therefore, for pure substances, the refrigerant temperature measured at the inlet of the evaporator is in fact the evaporating temperature. However, some refrigerants consist of more substances which means that the temperature depends on the quality (fraction of liquid and gas) known as glides. For such refrigerants, it is still possible to derive the evaporating temperature from the refrigerant temperature at the inlet of the evaporator. Thus, it is also possible to derive the superheat value, based on the derived evaporating temperature, in the manner described above.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 is a diagrammatic view of a vapour compression system being controlled in accordance with a method according to an embodiment of the invention,

Fig. 2 is a diagrammatic view of a vapour compression system being controlled in accordance with a method according to an alternative embodiment of the invention,

Fig. 3 is a block diagram illustrating a control loop of a method according to an embodiment of the invention,

Fig. 4 shows three graphs illustrating superheat, variance of the superheat and opening degree of an expansion device, respectively, of a vapour compression system being controlled in accordance with a method according to an embodiment of the invention, and

Fig. 5 is a flow chart illustrating a method according to an embodiment of the invention. DETAILED DESCRIPTION OF THE DRAWINGS

Fig . 1 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method according to an embodiment of the invention. The vapour compression system 1 comprises a compressor unit 2, a heat rejecting heat exchanger 3, an expansion device 4 and an evaporator 5 arranged in a refrigerant path. A fan 6 is arranged to drive a secondary fluid flow across the heat rejecting heat exchanger 3.

During operation of the vapour compression system 1, refrigerant flowing in the refrigerant path is compressed by means of the compressor(s) of the compressor unit 2 before being supplied to the heat rejecting heat exchanger 3. When the refrigerant passes through the heat rejecting heat exchanger 3, heat exchange takes place between the refrigerant and the secondary fluid flow driven by the fan 6, in such a manner that heat is rejected from the refrigerant.

The refrigerant leaving the heat rejecting heat exchanger 3 is supplied to the expansion device 4, where it undergoes expansion before being supplied to the evaporator 5. When passing through the evaporator 5, heat exchange takes place between the refrigerant and air inside a refrigerated volume arranged in thermal contact with the evaporator 5, in such a manner that heat is absorbed by the refrigerant, while the liquid part of the refrigerant is at least partly evaporated . Accordingly, cooling is thereby provided to the refrigerated volume. Finally, the refrigerant is once again supplied to the compressor unit 2.

The supply of refrigerant to the evaporator 5 is controlled by means of the expansion device 4. The supply of refrigerant is controlled in order to obtain a superheat value of refrigerant leaving the evaporator 5 which is equal to a reference superheat value. To this end, an opening degree or a duty cycle of the expansion device 4 is adjusted, e.g. according to a setpoint control strategy, e.g . applying a PI controller.

The reference superheat value which is applied for the control of the expansion device 4 may be provided in the following manner. While the vapour compression system 1 operates as described above, the superheat value of refrigerant leaving the evaporator 5 is monitored . The superheat value is required for the setpoint control of the expansion device 4 described above. However, it is also applied as an input to a process for determining whether or not to adjust or change the reference superheat value, as will be described below.

The superheat value may be derived from measurements of the temperature of refrigerant leaving the evaporator 5 and one or more of the evaporating temperature in the evaporator 5, the pressure of refrigerant leaving the evaporator 5, the pressure of refrigerant entering the evaporator 5 and the temperature of refrigerant entering the evaporator 5.

Furthermore, a quantity being representative for a variance of the monitored superheat value is calculated. The calculated quantity may be a variance of the actual superheat value, e.g. in the form of a standard deviation or a mean deviation, or it could be a variance of another parameter which is related to the superheat value, e.g. the temperature of refrigerant leaving the evaporator 5.

While the vapour compression system 1 is operated as described above, the reference superheat value is decreased, e.g. continuously or in a stepwise manner, as long as the monitored superheat value exhibits values which are above the reference superheat value as well as below the reference superheat value. As long as this is the case, it can safely be assumed that it is possible to further lower the actual superheat value without flooding the evaporator 5, and that it is therefore safe to decrease the reference superheat value further in order to obtain an even lower actual superheat value.

However, in the case that the monitored superheat value exhibits only values above a lower superheat limit, where the lower superheat limit is a superheat level which is below the reference superheat value by an amount corresponding to the quantity being representative for the variance of the monitored superheat value, this may be an indication that the actual superheat value is in fact lower than the superheat value which is derived from measurements from one or more sensors. Accordingly, zero superheat may in fact have been reached, even though the monitored superheat value is positive and higher than the reference superheat value. In this case, it will not be possible to lower the superheat value further, and therefore it is no longer appropriate to continue to decrease the reference superheat value. Accordingly, when this is detected, decreasing of the reference superheat value is discontinued.

Thus, minimum superheat is obtained without risking flooding of the evaporator 5, even if one more of the sensors relied on for monitoring the superheat value is erroneous or offset.

Fig. 2 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method according to an alternative embodiment of the invention. The vapour compression system 1 of Fig. 2 is very similar to the vapour compression system 1 of Fig. 1, and it will therefore not be described in detail here.

The vapour compression system 1 of Fig. 2 comprises a number of expansion devices 4, two of which are shown, each being arranged to supply refrigerant to a separate evaporator 5. Each of the evaporators 5 is arranged in thermal contact with a separate refrigerated volume. Thus, each of the expansion devices 4 is controlled in order to allow or prevent a flow of refrigerant to the respective evaporators 5, in order to obtain a respective reference superheat value for refrigerant leaving the respective evaporators 5. The reference superheat value is adjusted essentially in the manner described above with reference to Fig. 1.

Fig. 3 is a block diagram illustrating a control loop of a method according to an embodiment of the invention. In the control loop of Fig. 3, an opening degree of an expansion device 4 supplying refrigerant to an evaporator 5 of a vapour compression system is controlled.

The temperature, Tout, of refrigerant leaving the evaporator 5 is measured and supplied to subtraction unit 7. Furthermore, the evaporating temperature, T_e, of the evaporator 5 is obtained and supplied to the subtraction unit 7. The evaporating temperature, T_e, may be measured directly, or it may be derived from measurements of one or more measured parameters, e.g. the pressure of refrigerant leaving the evaporator 5, the pressure of refrigerant entering the evaporator 5 and/or the temperature of refrigerant entering the evaporator 5.

In the subtraction unit 7, the evaporating temperature, T_e, is subtracted from the refrigerant temperature, Tout, thereby obtaining the superheat of refrigerant leaving the evaporator 5. Accordingly, the superheat value is monitored.

At calculation block 8, a variance of the superheat value, in the form of a standard deviation of the superheat value, is calculated. The variance is supplied to an algorithm 9 in which an appropriate reference superheat value is calculated with due consideration to the variance of the superheat value. The algorithm 9 seeks to lower the reference superheat value to the greatest possible extent without risking flooding of the evaporator 5. Thus, as long as it is considered safe, while taking the variance of the superheat value into account, the algorithm 9 lowers the reference superheat value, e.g. continuously or in a stepwise manner. This is, e.g., considered safe if the monitored superheat value exhibits values which are above the reference superheat value as well as below the reference superheat value. The resulting reference superheat value is supplied to a flooding protection algorithm 10.

Furthermore, the variance of the superheat value and the monitored superheat value are supplied to the flooding protection algorithm 10. In the flooding protection algorithm 10, a lower superheat limit is derived in the form of a superheat level which is below the superheat reference value, received from algorithm 9, by an amount corresponding to the variance of the monitored superheat value, or to an appropriate quantity being representative for the variance of the monitored superheat value. The received signal representing the monitored superheat value is then compared to the derived lower superheat limit. In the case that the monitored superheat value exhibits values which are above the lower superheat limit as well as below the lower superheat limit, this is an indication that the actual superheat value has not yet reached zero superheat, and that it is therefore possible and safe to lower the superheat value further. Thus, when this is the case, the reference superheat value received from algorithm 9 is applied.

On the other hand, in the case that the monitored superheat value exhibits only values above the lower superheat limit, this is an indication that zero superheat has actually been reached, even though the monitored superheat indicates positive superheat values above the reference superheat value. This could, e.g., be due to one or more of the sensors applied for obtaining the temperature, Tout, of refrigerant leaving the evaporator 5 and/or the evaporating temperature, T_e, of the evaporator 5 being erroneous and/or offset. Accordingly, when this is the case, it is no longer considered safe to lower the reference superheat value further, and therefore further decrease of the reference superheat value is discontinued, and a previous value for the reference superheat value is maintained, even if a decreased reference superheat value is received from the algorithm 9.

The reference superheat value output from the flooding protection algorithm 10 is supplied to a subtraction unit 11, which also receives the monitored superheat value. In the subtraction unit 11, the superheat value is subtracted from the reference superheat value, thereby obtaining an error signal, which is supplied to a PI controller 12. The PI controller 12 then controls the opening degree of the expansion device 4, based on the error signal, and in accordance with a standard PI control strategy.

Fig. 4 shows three graphs illustrating various parameters being relevant for a method for controlling a vapour compression system according to an embodiment of the invention, as a function of time. The top graph illustrates monitored superheat value 13 and applied reference superheat value, the middle graph illustrates a quantity 15 being representative for a variance of the monitored superheat value, in the form of the standard deviation of the monitored superheat value, and the lower graph illustrates opening degree 16 of the expansion device.

Initially, and up to a point in time corresponding to approximately 4-10⁴ s, the vapour compression system operates in a relatively stable manner. The monitored superheat value 13 exhibits values which are above the applied reference superheat value 14 as well as below the applied reference superheat value 14, i.e. the monitored superheat value 13 fluctuates in an appropriate manner about the applied reference superheat value 14. Furthermore, the quantity 15 being representative for the variance of the superheat value remains at a relatively stable and constant level, and the opening degree 16 of the expansion device fluctuates appropriately about a mean value being approximately 40% of the fully open position. As long as this is the case, it can be concluded that zero superheat has not been reached, and that it is therefore safe to decrease the reference superheat value 14 in order to attempt to lower the actual superheat value further.

Accordingly, from the point in time corresponding to approximately 4-10⁴ s, the reference superheat value 14 is gradually decreased. It can be seen that this causes the variance of the superheat value to decrease. This is due to the fact that the actual superheat value of the refrigerant leaving the evaporator approaches zero superheat, and therefore the monitored superheat value 13 no longer fluctuates as previously, but approaches a substantially constant level which represents actual zero superheat. Accordingly, the monitored superheat value 13 eventually stabilises at approximately 4K.

A superheat value of 4K is normally considered to be far from zero superheat, in the sense that there is no risk of flooding of the evaporator, and it should be possible to lower the superheat value further. Therefore, the monitored superheat value 13 in itself does not indicate that zero superheat has been reached, and therefore the decrease of the reference superheat value continues. However, in reality, zero superheat has been reached, and the fact that the monitored superheat value 13 indicates 4K is most likely the result of one or more sensors being erroneous and/or offset.

It can be seen that, in an attempt to drive the monitored superheat value 13 down towards the reference superheat value 14, the opening degree 16 of the expansion device is increased drastically and eventually reaches a fully open position. However, as described above, zero superheat has in fact already been reached, and it is therefore not possibly to lower the monitored superheat value 13 below the 4K limit. Thus, the increased opening degree 16 does not have the desired impact on the monitored superheat value 13.

As described above, the monitored superheat value 13 alone does not indicate that zero superheat has been reached. However, the behaviour of the monitored superheat value 13, the quantity 15 being representative for the variation of the monitored superheat value and the opening degree 16 of the expansion device illustrated in Fig. 4 indicate that this is the case. Therefore, when such behaviour is detected, it can be duly taken into account that one or more sensors are erroneous and/or offset, and the reference superheat value 14 can be adjusted accordingly, by applying the method according to the invention, so that flooding of the evaporator is avoided. Fig. 5 is a flow chart illustrating a method for controlling a vapour compression system according to an embodiment of the invention. The process is started at step 17. At step 18 a superheat value of refrigerant leaving the evaporator is monitored, and a quantity being representative for a variance of the monitored superheat value is calculated.

At step 19, a reference superheat value is selected as a starting point for the control of the vapour compression system. At step 20, a lower superheat limit is calculated as a superheat level which is below the selected reference superheat value by an amount corresponding to the quantity being representative for the variance of the monitored superheat value.

At step 21 the vapour compression system is operated according to the selected superheat reference value. In particular, an opening degree of an expansion device controlling a refrigerant supply is adjusted in order to obtain a superheat value of refrigerant leaving the evaporator which is equal to the superheat reference value. During this, monitoring of the superheat value and calculation of the quantity being representative for the variance of the monitored superheat value are continued.

At step 22, it is investigated whether or not the monitored superheat value exhibits values which are above the reference superheat value as well as below the reference superheat value. If this is the case, the process is forwarded to step 23, where it is investigated whether or not the monitored superheat value exhibits values which are above the lower superheat limit as well as below the lower superheat value. If this is the case, it can be assumed that the superheat value can safely be lowered further without risking flooding of the evaporator. Therefore, in this case the process is forwarded to step 24, where the reference superheat value is decreased, and the process is subsequently returned to step 20 for calculation of a new lower superheat limit, based on the new, decreased reference superheat value.

In the case that step 23 reveals that the monitored superheat value exhibits only values above the lower superheat limit, this is an indication that zero superheat may be approaching, and that it is therefore not considered safe to lower the superheat value further. Therefore, in this case the process is forwarded to step 25, where it is investigated whether or not this situation has lasted for a predefined period of time. If this is not the case, the process is forwarded to step 26, where the reference superheat value is maintained, i.e. decreasing of the reference superheat value is discontinued. Subsequently, the process is returned to step 20.

In the case that step 22 reveals that the monitored superheat value exhibits only values above the reference superheat value, there is a risk that zero superheat has already been reached, and that flooding of the evaporator may be occurring or eminent. Therefore, in this case the process is forwarded to step 25, where it is investigated whether or not this situation has lasted for a predefined period of time. If this is not the case, actual flooding of the evaporator may not be occurring, and it may therefore be sufficient to maintain the present superheat level in order to prevent flooding. Therefore, in this case the process is forwarded to step 26 where the reference superheat value is maintained.

In the case that step 25 reveals that the monitored superheat value has exhibited only values above the reference superheat value, or above the lower superheat limit, for the predefined period of time, simply maintaining the current superheat level is not considered sufficient to prevent flooding of the evaporator, and therefore the process is forwarded to step 27, where the reference superheat value is increased before the process is returned to step 20 for calculation of a new lower superheat limit, based on the new, increased reference superheat value.

Claims

1. A method for controlling a vapour compression system (1), the vapour compression system (1) comprising a compressor unit (2), a heat rejecting heat exchanger (3), an expansion device (4) and an evaporator (5) arranged in a refrigerant path, the method comprising the steps of: monitoring a superheat value (13) of refrigerant leaving the evaporator (5),

- calculating a quantity (15) being representative for a variance of the monitored superheat value (13),

- comparing the monitored superheat value (13) to a reference superheat value (14), and operating the expansion device (4) in order to obtain a superheat value (13) of refrigerant leaving the evaporator (5) which is substantially equal to the reference superheat value (14),

- decreasing the reference superheat value (14) as long as the monitored superheat value (13) exhibits values which are above the reference superheat value (14) as well as below the reference superheat value (14), and in the case that the monitored superheat value (13) exhibits only values above a lower superheat limit, where the lower superheat limit is a superheat level which is below the reference superheat value (14) by an amount corresponding to the quantity (15) being representative for a variance of the monitored superheat value (13), discontinuing decreasing of the reference superheat value (14).

2. A method according to claim 1, further comprising the step of increasing the reference superheat value (14) in the case that the monitored superheat value (13) has exhibited only values above the lower superheat limit for a predefined period of time.

3. A method according to claim 1, further comprising the step of increasing the reference superheat value (14) in the case that the monitored superheat value (13) has exhibited only values above the reference superheat value (14) for a predefined period of time.

4. A method according to any of the preceding claims, further comprising the step of discontinuing decreasing of the reference superheat value (14) in the case that the quantity (15) being representative for the variance of the monitored superheat value (13) decreases below a predefined threshold value.

5. A method according to any of the preceding claims, wherein the step of calculating a quantity (15) being representative for a variance of the monitored superheat value (13) comprises calculating the variance of the monitored superheat value (13).

6. A method according to any of the preceding claims, wherein the step of operating the expansion device (4) in order to obtain a superheat value (13) of refrigerant leaving the evaporator (5) which is substantially equal to the reference superheat value (14) comprises adjusting an opening degree (16) of the expansion device (4).

7. A method according to claim 6, further comprising the steps of: monitoring the opening degree (16) of the expansion device (4), and increasing the reference superheat value (14) in the case that an increase in opening degree (16) of the expansion device (4) exceeds a threshold increase value.

8. A method according to claim 7, wherein the threshold increase value depends on a difference between the monitored superheat value (13) and the reference superheat value (14).

9. A method according to claim 7 or 8, wherein the threshold increase value depends on a ratio between the opening degree (16) of the expansion device (4) and the monitored superheat value (13).

10. A method according to any of the preceding claims, wherein the step of operating the expansion device (4) is performed by means of a proportional integral (PI) controller (12).

11. A method according to any of the preceding claims, wherein the step of monitoring a superheat value (13) of refrigerant leaving the evaporator (5) comprises measuring a temperature of refrigerant leaving the evaporator (5) and an evaporating temperature of the evaporator (5), and calculating the superheat value (13) from the measured temperatures.

12. A method according to any of claims 1-10, wherein the step of monitoring a superheat value (13) of refrigerant leaving the evaporator (5) comprises measuring a temperature of refrigerant leaving the evaporator (5) and a pressure of refrigerant leaving or entering the evaporator (5), and calculating the superheat value (13) from the measured temperature and pressure.

13. A method according to any of claims 1-10, wherein the step of monitoring a superheat value (13) of refrigerant leaving the evaporator (5) comprises measuring a temperature of refrigerant leaving the evaporator (5) and a temperature of refrigerant entering the evaporator (5), and calculating the superheat value (13) from the measured temperatures.