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    • 专利摘要:
    18 ABSTRACT A method for controlling an opening degree of an expansion device (4) arranged in a vapourcompression system (1) is disclosed. An actual pinch being the difference betweentemperature of a secondary fluid flow (6) across the evaporator (5), at a position after thesecondary fluid flow (6) has passed the evaporator (5), and an evaporating temperature ofrefrigerant passing through the evaporator (5), is obtained. It is determined whether or notinternal instability is present in the vapour compression system (1), and a setpoint pinchvalue is decreased in the case that internal instability is not present in the vapourcompression system (1) and increased in the case that internal instability is present in thevapour compression system (1). An opening degree of an expansion device (4) is controlled in order to obtain an actual pinch which is substantially equal to the setpoint pinch value.
    公开号:SE1750501A1
    申请号:SE1750501
    申请日:2017-04-26
    公开日:2017-11-19
    发明作者:Wurtz Albrecht；NORD Björn；Gurnett Björn；Olsson Erik
    申请人:Danfoss Värmepumpar Ab；
    IPC主号:
    专利说明:

    A METHOD FOR CONTROLLING AN EXPANSION DEVICE FIELD OF THE INVENTION The present invention relates to a method for controlling an expansion device, such as anexpansion valve, arranged in a vapour compression system. According to the method of theinvention, the expansion device is controlled on the basis of the pinch.
    BACKGROUND OF THE INVENTION Vapour compression systems, such as refrigeration systems, air condition systems or heatpumps, normally comprise a compressor, a heat rejecting heat exchanger, an expansiondevice and an evaporator arranged along a refrigerant path. Refrigerant flowing in therefrigerant path is alternatingly compressed by the compressor and expanded by theexpansion device. Heat exchange takes place in the heat rejecting heat exchanger and in theevaporator, in such a manner that heat is rejected from the refrigerant when passing throughthe heat rejecting heat exchanger and heat is absorbed by the refrigerant when passingthrough the evaporator. The expansion device controls the supply of refrigerant to the eVa pOFatOF.
    In many vapour compression systems the expansion device is controlled on the basis of asuperheat of refrigerant leaving the evaporator, and in order to obtain a small, but positive,superheat. Thereby it is obtained that the vapour compression system is operated in anefficient and stable manner without risking that liquid refrigerant passes through the evaporator and reaches the compressor.
    US 6,318,101 Bl discloses a method for controlling an electronic expansion valve used in arefrigeration cycle for a heat pump. The expansion valve is controlled to maintain minimumpinch for ensuring proper flooded cooler exchange performance by monitoring the deltatemperature between the cooler fluid and the saturated suction temperature. The dischargesuperheat is monitored to protect the compressor from liquid slugging. If the dischargesuperheat is lower than the expected value, the expansion valve opening is adjusted. Acontroller monitors certain system variables which are used to determine the optimal positionof the expansion valve to optimize the system performance, the proper discharge superheatvalue, and the appropriate refrigerant charge.
    DESCRIPTION OF THE INVENTION It is an object of embodiments of the invention to provide a method for controlling anexpansion device arranged in a vapour compression system, in which stable and energy efficient control of the vapour compression system is ensured.
    It is a further object of embodiments of the invention to provide a method for controlling anexpansion device arranged in a vapour compression system, in which a minimum pinch is obtained without risking instabi|ity of the vapour compression system.
    The invention provides a method for controlling an opening degree of an expansion devicearranged in a vapour compression system, the vapour compression system furthercomprising a compressor, a condenser and an evaporator arranged in a refrigerant path, theexpansion device controlling a supply of refrigerant to the evaporator, the method comprising the steps of: - obtaining a temperature of a secondary fluid flow across the evaporator, at a position after the secondary fluid flow has passed the evaporator, - obtaining an evaporating temperature of refrigerant passing through the evaporator, - obtaining an actual pinch as the difference between the obtained temperature of the secondary fluid flow and the obtained evaporating temperature, - providing a setpoint pinch value, - determining whether or not internal instabi|ity is present in the vapour compression system, - decreasing the setpoint pinch value in the case that internal instabi|ity is not presentin the vapour compression system and increasing the setpoint pinch value in the case that internal instabi|ity is present in the vapour compression system, and - controlling an opening degree of the expansion device in order to obtain an actual pinch which is substantially equal to the setpoint pinch value.
    The invention provides a method for controlling an opening degree of an expansion device arranged in a vapour compression system. The expansion device could, e.g., be in the form of an expansion valve, such as an electronic expansion valve. As an alternative, the expansion device may be an ejector or another kind of expansion device.
    In the present context the term “vapour compression system' should be interpreted to meanany system in which a flow of fluid medium, such as refrigerant, circulates and isalternatingly compressed and expanded, thereby providing either refrigeration or heating of avolume. The vapour compression system comprises a compressor, a condenser, theexpansion device and an evaporator arranged in a refrigerant path. Refrigerant flowing in therefrigerant path is compressed by the compressor before being supplied to the condenser. Inthe condenser, heat exchange takes place with the ambient or with a secondary fluid flowacross the condenser, in such a manner that heat is rejected from the refrigerant passingthrough the condenser. The refrigerant is then supplied to the expansion device, where itundergoes expansion before being supplied to the evaporator. Accordingly, the expansiondevice controls the supply of refrigerant to the evaporator. In the evaporator, heat exchangetakes place with a secondary fluid flow across the evaporator, in such a manner that heat isabsorbed by the refrigerant passing through the evaporator. Furthermore, at least part of therefrigerant passing through the evaporator is evaporated. Finally, the refrigerant is once again supplied to the compressor.
    Accordingly, refrigerant flowing in the refrigerant path is alternatingly compressed by thecompressor and expanded by the expansion device, while heat exchange takes place in thecondenser and the evaporator as described above. Thereby the vapour compression systemcan be used for providing heating or cooling to a volume. Accordingly, the vapourcompression system could be a refrigeration system, an air condition system, a heat pump, etc.
    According to the method of the invention, a temperature of a secondary fluid flow across theevaporator, at a position after the secondary fluid flow has passed the evaporator, isobtained. Accordingly, the temperature is obtained after heat exchange has taken placebetween the refrigerant flowing through the evaporator and the secondary fluid flowingacross the evaporator. Thus, the temperature is obtained at a position where the secondary fluid has been cooled, and is therefore expected to have a minimum temperature.
    The secondary fluid may be in the form of a gas, such as air, or it may be in the form of a liquid, such as water or brine.
    Furthermore, an evaporating temperature of refrigerant passing through the evaporator isobtained. The evaporating temperature being obtained may, e.g., be the dew point of the refrigerant at the pressure prevailing in the evaporator. As an alternative, the evaporating temperature being obtained may, e.g., be the bubble point of the refrigerant at the pressureprevailing in the evaporator. As another alternative, the evaporating temperature beingobtained may be an average or a weighted average of the dew point and the bubble point.Depending on the refrigerant applied, the dew point and the bubble point may besubstantially identical, or there may be a difference between the dew point and the bubblepoint. In any event, the evaporating temperature being obtained provides informationregarding at which temperature phase change between liquid and gaseous states, or viceversa, takes place for the applied refrigerant, and at the pressure prevailing in the eVa pOFatOF.
    Next, an actual pinch is obtained as the difference between the obtained temperature of thesecondary fluid flow and the obtained evaporating temperature. Since the actual pinch isderived from the obtained actual temperature values, it represents the conditions which are actually prevailing in the vapour compression system.
    When the pinch is small, the difference between the temperature of the cooled secondaryfluid and the evaporating temperature is small. Accordingly, the secondary fluid is cooled to atemperature which is close to the evaporating temperature, when passing across theevaporator. Thus, the vapour compression system operates efficiently in this case, and it isdesirable that the pinch is small. However, a very small pinch may cause undesirableinstabilities in the vapour compression system, and a too small pinch should therefore beavoided.
    On the other hand, when the pinch is large, the temperature of the cooled secondary fluid issignificantly higher than the evaporating temperature. This is an indication that the vapour compression system is operating in an inefficient manner, and this is undesirable.
    Furthermore, a setpoint pinch value is provided. The setpoint pinch value represents adesired level for the pinch, and the vapour compression system should be operated in such amanner that this desired pinch value is reached. The setpoint pinch value could, e.g., initiallybe a value which is empirically known to provide relatively efficient operation of the vapourcompression system, without risking instabilities. Such an empirical value will provide asuitable starting point for the setpoint pinch value. For instance, the initial setpoint pinchvalue could be selected on the basis of an estimated cooling capacity. The estimated coolingcapacity could, e.g., be derived from the speed of the compressor. However, the setpointpinch value may subsequently be adjusted in accordance with the prevailing operatingconditions. This will be described in further detail below.
    Next it is determined whether or not internal instability is present in the vapour compressionsystem. Internal instability could, e.g., be in the form of liquid refrigerant passing through the eva porator.
    In the case that it is determined that internal instability is not present in the vapourcompression system, the setpoint pinch value is decreased. In this case, it can be assumedthat the vapour compression system operates in a stable manner, and it may therefore beconsidered safe to lower the pinch to a level below the current setpoint pinch value. Asdescribed above, it is desirable to operate the vapour compression system at as low a pinchas possible, without risking instability. Thus, in order to obtain this, the setpoint pinch value is decreased.
    On the other hand, in the case that it is determined that internal instability is present in thevapour compression system, the setpoint pinch value is increased. In this case, the instabilitymay be due to the pinch having reached a too low level. Accordingly, in order to restoreinternal stability, the pinch should be increased to a level above the current setpoint pinch value. In order to obtain this, the setpoint pinch value is increased.
    Finally, the opening degree of the expansion device is controlled in order to obtain an actualpinch which is substantially equal to the setpoint pinch value. Since refrigerant is supplied tothe evaporator via the expansion device, controlling the opening degree of the expansiondevice determines how much refrigerant is supplied to the evaporator. Increasing theopening degree of the expansion device increases the refrigerant flow towards theevaporator. This results in more cooling being provided by the evaporator, and thereby adecrease in the temperature of the secondary fluid flow across the evaporator, at a positionafter the secondary fluid flow has passed the evaporator. Accordingly, the pinch is alsodecreased. Similarly, decreasing the opening degree of the expansion device decreases therefrigerant flow towards the evaporator, resulting in an increase in the temperature of the secondary fluid, and thereby increasing the pinch.
    Thus, according to the method of the invention, the pinch is used as a control parameter forthe opening degree of the expansion device. Furthermore, the setpoint pinch value is decreased as long as an internal instability is not present in the vapour compression system.However, as soon as an internal instability is detected, the setpoint pinch value is increased, in order to increase the actual pinch to a level which restores internal stability.
    Accordingly, the method of the invention ensures that the vapour compression system isoperated with a pinch, which is as small as possible, without risking internal instability of the vapour compression system.
    The step of determining whether or not internal instability is present in the vapour compression system may comprise the steps of: - monitoring a temperature difference between a temperature of secondary fluid before passing the evaporator and a temperature of refrigerant leaving the evaporator, - comparing the monitored temperature difference to a threshold value, and - in the case that the monitored temperature difference is larger than the thresholdvalue, determining that an internal instability is present in the vapour compression system.
    According to this embodiment, the temperature of secondary fluid before passing theevaporator is obtained. Accordingly, the temperature is obtained before heat exchange hastaken place between the refrigerant flowing through the evaporator and the secondary fluid, i.e. before the secondary fluid has been cooled.
    Furthermore, a temperature of refrigerant leaving the evaporator is obtained. In the casethat the liquid part of the refrigerant passing through the evaporator is evaporated wellbefore the outlet of the evaporator, the gaseous refrigerant in the evaporator will be heatedto a temperature above the evaporating temperature. In this case the cooling capacity of theevaporator is not utilised in an efficient manner. On the other hand, in the case that liquidrefrigerant is passing through the evaporator, the temperature of refrigerant leaving theevaporator is substantially at the evaporating temperature. Liquid refrigerant passing throughthe evaporator is undesirable, since the compressor may suffer damage in the case thatliquid refrigerant reaches the compressor. It is therefore desirable to operate the vapourcompression system in such a manner that liquid refrigerant is present throughout the entireevaporator, but liquid refrigerant is not allowed to leave the evaporator and enter the suctionline. When this is obtained, the temperature of refrigerant leaving the evaporator is slightlyabove the dew point of the refrigerant. The temperature difference between the dew pointand the temperature of the refrigerant leaving the evaporator is normally referred to as thesuperheat, and the vapour compression system is often operated in order to obtain a superheat which is small, but positive.
    When the temperature difference between the temperature of secondary fluid before passingthe evaporator and the temperature of refrigerant leaving the evaporator is small, this is anindication that the temperature of refrigerant leaving the evaporator is relatively high,approaching the temperature of the secondary fluid to be cooled. This is also an indication that the superheat of refrigerant leaving the evaporator is relatively high. Accordingly, in this case there is no risk that liquid refrigerant is passing through the evaporator and entering thesuction line, and thereby the risk of internal instability being introduced in the vapour compression system is minimal.
    On the other hand, when the temperature difference between the temperature of secondaryf|uid before passing the evaporator and the temperature of refrigerant leaving the evaporatoris small, this is an indication that the temperature of refrigerant leaving the evaporator isrelatively low, and possibly approaching the dew point. Accordingly, this is also an indicationthat the superheat of refrigerant leaving the evaporator is relatively low, and therefore, inthis case, there is a risk that liquid refrigerant may pass through the evaporator and enterthe suction line. Accordingly, in this case there is a risk of internal instability being introduced in the vapour compression system.
    Therefore, according to this embodiment, the monitored temperature difference is comparedto a threshold value, and in the case that the monitored temperature difference is larger thanthe threshold value, it is determined that an internal instability is present in the vapourcompression system. The threshold value could, e.g., be selected in such a manner that itreflects a temperature difference, above which there is a considerable risk that liquidrefrigerant may pass through the evaporator, thereby causing internal instability of thevapour compression system. It is an advantage of this embodiment that this can be obtained without directly obtaining or deriving the superheat.
    As an alternative, the step of determining whether or not internal instability is present in the vapour compression system may comprise the steps of: - monitoring a superheat of refrigerant leaving the evaporator, - comparing the monitored superheat to a threshold value, and - in the case that the monitored superheat value is smaller than the threshold value, determining that an internal instability is present in the vapour compression system.
    This is similar to the embodiment described above. However, according to this embodiment, the superheat is directly monitored.
    The step of comparing the monitored temperature difference or the monitored superheat to athreshold value may comprise comparing the monitored temperature difference or the monitored superheat to a time filtered temperature difference or superheat.
    According to this embodiment, a controller keeps track of a filtered value of the temperaturedifference or the superheat, and this time filtered value is used as the threshold value.Accordingly, the threshold value is not a fixed value, but rather reflects historical values ofthe temperature difference or superheat. Thereby it is possible to reveal, due to comparison,if the temperature difference or superheat suddenly changes significantly. This is anadvantage, since such sudden changes may indicate that internal instability is being introduced in the vapour compression system.
    The step of obtaining an evaporating temperature may comprise measuring a pressure ofrefrigerant passing through the evaporator and deriving the evaporating temperature based on the measured pressure.
    For a given refrigerant, the dew point as well as the bubble point is uniquely determined bythe pressure of the refrigerant. Accordingly, once the pressure has been measured, theevaporating temperature can be determined, e.g. by means of a look-up table. The pressuremay, e.g., be measured by means of a pressure sensor arranged at the inlet or at the outletof the evaporator.
    As an alternative, the step of obtaining an evaporating temperature may comprise measuringthe evaporating temperature. According to this embodiment, the evaporating temperature ismeasured directly, e.g. by means of a temperature sensor arranged, e.g., at the inlet of theevaporator. As an alternative, the temperature sensor may be arranged on a pipe interconnecting the expansion device and the evaporator.
    Decrease of the setpoint pinch value may prevented if the setpoint pinch value has beenincreased, until a change in external operating conditions of the vapour compression systemhas been detected. According to this embodiment, once the setpoint pinch value has beenincreased, due to an internal instability of the vapour compression system having beendetected, the setpoint pinch value is not allowed to be decreased again, unless the externaloperating conditions of the vapour compression system change. Thereby it is prevented that internal instability is once again introduced in the vapour compression system.
    The step of controlling an opening degree of the expansion device may comprise deriving anevaporating pressure setpoint value and controlling the opening degree by means of aproportional integral derivative (PID) control loop, based on the derived evaporating pressure setpoint value.
    According to this embodiment, the opening degree of the expansion device is controlled in response to the evaporating pressure, and by means of an ordinary PID control loop.
    However, the evaporating setpoint value is derived in such a manner that it corresponds tothe setpoint pinch value. Thereby, controlling the opening degree of the expansion deviceaccording to the evaporating pressure setpoint results in the opening degree being controlledin such a manner that the actual pinch becomes substantially equal to the setpoint pinch value.
    The method may further comprise the step of increasing a gain of the PID control loop in thecase that a change in external operating conditions for the vapour compression system isdetected. In some cases, increasing the setpoint pinch value may be an insufficient measurefor quickly restoring internal stability of the vapour compression system. In this case a gainof the PID control loop may be increased in order to restore internal stability faster. Thismay, e.g., be required in the case that the external operating conditions for the vapour compressions system have changed.
    The gain being increased could be any gain associated with the PID control loop. It is notruled out that two or more different gains associated with the PID control loop are increased.For instance, the PID control loop may define three different gains, i.e. Kp and Kd being innerloop gains, fulfilling the condition Kp/Kd being constant, and Kl. In this case, any combinationof these gains may be increased.
    The method may further comprise the step of increasing responsivity of the PID control loopin the case that an increase in cooling capacity of the vapour compression system isdetected. The responsivity of the PID control loop could, e.g., be increased by increasing again of the PID control loop. This could, e.g., be performed by monitoring the speed of thecompressor. For each speed or speed interval, an appropriate gain, e.g. an appropriate valueof Kp, may be specified, e.g. in a table. The appropriate value of the relevant gain is thenselected, based on the actual speed of the compressor.
    The method may further comprise the steps of: - monitoring a cooling capacity of the vapour compression system, and - increasing the setpoint pinch value in the case that an increase in the cooling capacityof the vapour compression system is detected.
    According to this embodiment, the cooling capacity of the vapour compression system ismonitored. This could, e.g., include monitoring the speed of the compressor. In the case that an increase in the cooling capacity of the vapour compression system is detected, it may be concluded that a change in the external operating conditions has occurred, causing the increase in cooling capacity. As a precaution, the setpoint pinch value is then increased.
    The secondary fluid flowing across the evaporator may be a brine. In the present context, theterm “brine' should be interpreted to be water with an additive providing freeze protection. Inthis case the secondary fluid is a liquid. As an alternative, the secondary fluid could beanother kind of liquid, such as water. As another alternative, the secondary fluid could be gaseous, e.g. in the form of an air flow.
    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, and Fig. 2 illustrates temperature variations of refrigerant passing through an evaporator of a vapour compression system and secondary fluid passing across the evaporator.
    DETAILED DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagrammatic view of a vapour compression system 1 being controlled inaccordance with a method according to an embodiment of the invention. The vapourcompression system 1 comprises a compressor 2, a condenser 3, an expansion device 4, in the form of an expansion valve, and an evaporator 5 arranged in a refrigerant path.
    Refrigerant flowing in the refrigerant path is compressed by the compressor 2 before beingsupplied to the condenser 3. In the condenser 3, heat exchange takes place between therefrigerant flowing through the condenser 3 and the ambient in such a manner that heat isrejected from the refrigerant. Furthermore, the refrigerant passing through the condenser 3is at least partly condensed, and at least part of the refrigerant leaving the condenser 3 is therefore in a liquid state.
    Next, the refrigerant is supplied to the expansion device 4, where it undergoes expansionbefore being supplied to the evaporator 5. Thereby the expansion device 4 controls therefrigerant supply to the evaporator 5, and the refrigerant entering the evaporator 5 is in the form of a mixture of liquid and gaseous refrigerant. 11 In the evaporator 5, heat exchange takes place between the refrigerant passing through theevaporator 5 and a secondary fluid flow 6 across the evaporator 5, in such a manner thatheat is absorbed by the refrigerant passing through the evaporator 5. Accordingly, thesecondary fluid 6 is cooled when passing across the evaporator 5. The secondary fluid 6could, e.g., be in the form of a brine, in the form of another kind of liquid, or in the form of a gas, such as an air flow.
    Finally, the refrigerant is once again supplied to the compressor 2. Accordingly, therefrigerant flowing in the refrigerant path is alternatingly compressed by the compressor 2and expanded by the expansion device 4, while heat exchange takes place at the condenser 3 and the evaporator 5, as described above.
    The vapour compression system 1 is further provided with a first temperature sensor 7arranged to measure the temperature of refrigerant entering the evaporator 5, and a secondtemperature sensor 8 arranged to measure the temperature of refrigerant leaving theevaporator 5. It should be noted that a pressure sensor could also be arranged at the inletand/or at the outlet of the evaporator 5 in addition to the temperature sensors 7, 8 or as a replacement for one or both of the temperature sensors 7, 8.
    The vapour compression system 1 of Fig. 1 may be operated in the following manner.Initially, a temperature of the secondary fluid flow 6 is obtained, at a position after thesecondary fluid flow 6 has passed the evaporator 5, i.e. after the secondary fluid 6 has beencooled due to heat exchange with the refrigerant flowing through the evaporator 5. This may,e.g., be obtained by means of a temperature sensor (not shown) arranged in the secondary fluid flow 6 downstream relative to the evaporator 5.
    Furthermore, an evaporating temperature of refrigerant passing through the evaporator 5 isobtained. As described above, the evaporating temperature could be the dew point of therefrigerant at the prevailing pressure level, the bubble point of the refrigerant at theprevailing pressure level, or a suitable weighted average of the dew point and the bubblepoint. The evaporating temperature may be obtained by means of the first temperaturesensor 7 and/or the second temperature sensor 8. Alternatively or additionally, theevaporating temperature may be derived from a measured value of the pressure prevailing in in the evaporator 5.
    Based on the obtained temperature of the secondary fluid flow 6 and the obtainedevaporating temperature, an actual pinch is then obtained as the difference between thetemperature of the secondary fluid flow 6 and the evaporating temperature. When the pinch is large, the temperature of the cooled secondary fluid 6 is significantly higher than the 12 evaporating temperature. This is an indication that the vapour compression system 1 isoperated in an inefficient manner, and a small pinch is therefore desirable.
    When the pinch is small, the difference between the temperature of the cooled secondaryfluid 6 and the evaporating temperature is small. Accordingly, the secondary fluid 6 has beencooled to a temperature which is close to the evaporating temperature, indicating that thevapour compression system 1 is operating in an efficient manner. However, if the pinchbecomes too small there is a risk that internal instability is introduced in the vapourcompression system 1, e.g. due to liquid refrigerant passing through the evaporator 5. Thus,even though a small pinch is desirable in order to obtain efficient operation of the vapourcompression system 1, it should be ensured that the pinch does not reach a level where internal instability is introduced in the vapour compression system 1.
    Furthermore, a setpoint pinch value is provided. The setpoint pinch value is a pinch valuewhich is desirable, e.g. in terms of efficient and/or stable operation of the vapour compression system 1.
    Next, it is determined whether or not internal instability is present in the vapour compressionsystem 1. This may, e.g., be performed by monitoring a temperature difference between atemperature of the secondary fluid flow 6 before passing the evaporator 5 and a temperatureof refrigerant leaving the evaporator 5. The temperature of the secondary fluid flow 6 may,e.g., be obtained by means of a temperature sensor (not shown) arranged in the secondaryfluid flow 6 at a position upstream relative to the evaporator 5. The temperature ofrefrigerant leaving the evaporator 5 may be obtained by means of the second temperaturesensor 8. The temperature difference may be compared to a threshold value, and in the casethat the monitored temperature difference is larger than the threshold value, it may be determined that an internal instability is present in the vapour compression system 1.
    In the case that it is determined that internal instability is not present in the vapourcompression sytem 1, it may be considered safe to lower the pinch further, therebyincreasing the efficiency of the operation of the vapour compression system 1. Therefore, inthis case the setpoint pinch value is decreased.
    On the other hand, in the case that it is determined that internal instability is present in thevapour compression system 1, it can be assumed that the pinch has become too small, and itis therefore desirable to increase the pinch. Therefore, in this case the setpoint pinch value is increased. 13 Finally, the opening degree of the expansion device 4, and thereby the supply of refrigerantto the evaporator 5, is controlled in order to obtain an actual pinch which is substantiallyequal to the setpoint pinch value.
    In the case that the actual pinch is smaller than the setpoint pinch value, the actual pinchmust be increased. This is obtained by decreasing the opening degree of the expansiondevice 4, thereby decreasing the supply of refrigerant to the evaporator 5. This decreases thecooling provided to the secondary fluid flow 6 due to the heat exchange taking place in theevaporator 5, thereby increasing the temperature of the secondary fluid flow 6 downstream relative to the evaporator 5. As a consequence, the actual pinch is increased.
    Similarly, in the case that the actual pinch is larger than the setpoint pinch value, the actualpinch must be decreased. This is obtained by increasing the opening degree of the expansiondevice 4, thereby increasing the supply of refrigerant to the evaporator 5. This increases thecooling provided to the secondary fluid flow 6 due to the heat exchange taking place in theevaporator 5, thereby decreasing the temperature of the secondary fluid flow 6 downstream relative to the evaporator 5. As a consequence, the actual pinch is decreased.
    Fig. 2 illustrates temperature variations of refrigerant passing through an evaporator of avapour compression system and secondary fluid passing across the evaporator. The vapourcompression system could, e.g., be the vapour compression system of Fig. 1. In the situationillustrated in Fig. 2, the secondary fluid flow is in the form of a brine.
    It can be seen that the temperature of the brine is decreased gradually as it passes acrossthe evaporator. The temperature of refrigerant passing through the evaporator issubstantially constant for a part of the evaporator. In this part of the evaporator, liquidrefrigerant is evaporated, and therefore the temperature of the refrigerant is substantiallyequal to the evaporating temperature. In the last part of the evaporator, all of the liquidrefrigerant has been evaporated, and the gaseous refrigerant is instead heated, resulting in an increase in the temperature of the refrigerant.
    The difference marked “pinch 1' represents the actual pinch, i.e. the temperature differencebetween the temperature of the secondary fluid at a position after the secondary fluid has passed the evaporator, and the evaporating temperature.
    The difference marked “superheat' represents the superheat of refrigerant leaving theevaporator, i.e. the temperature difference between the temperature of the refrigerant leaving the evaporator and the evaporating temperature. 14 The difference marked “pinch 2' represents the temperature difference between thetemperature of the secondary fluid flow at a position before the secondary fluid flow passesthe evaporator, and the temperature of refrigerant leaving the evaporator. If this differenceis large, it is an indication that there is a risk of internal instability being introduced in thevapour compression system, e.g. due to liquid refrigerant passing through the evaporator.Thus, monitoring this temperature difference can provide information regarding whether ornot internal instability is present in the vapour compression system, and therefore regardingwhether or not it can be considered safe to lower the actual pinch further.
    权利要求:
    Claims (12)
    [1] 1. A method for controlling an opening degree of an expansion device (4) arranged in a vapour compression system (1), the vapour compression system (1) further comprising a compressor (2), a condenser (3) and an evaporator (5) arranged in a refrigerant path, the expansion device (4) controlling a supply of refrigerant to the evaporator (5), the method comprising the steps of: obtaining a temperature of a secondary fluid flow (6) across the evaporator (5), at a position after the secondary fluid flow (6) has passed the evaporator (5), obtaining an evaporating temperature of refrigerant passing through the evaporator (5), obtaining an actual pinch as the difference between the obtained temperature of the secondary fluid flow (6) and the obtained evaporating temperature, providing a setpoint pinch value, determining whether or not internal instability is present in the vapour compression system (1), decreasing the setpoint pinch value in the case that internal instability is not presentin the vapour compression system (1) and increasing the setpoint pinch value in the case that internal instability is present in the vapour compression system (1), and controlling an opening degree of the expansion device (4) in order to obtain an actual pinch which is substantially equal to the setpoint pinch value.
    [2] 2. A method according to claim 1, wherein the step of determining whether or not internal instability is present in the vapour compression system (1) comprises the steps of: monitoring a temperature difference between a temperature of secondary fluid (6)before passing the evaporator (5) and a temperature of refrigerant leaving the evaporator (5), comparing the monitored temperature difference to a threshold value, and 16 - in the case that the monitored temperature difference is larger than the thresholdvalue, determining that an internal instability is present in the vapour compressionsystem (1).
    [3] 3. A method according to c|aim 1, wherein the step of determining whether or not internal instability is present in the vapour compression system (1) comprises the steps of: - monitoring a superheat of refrigerant leaving the evaporator (5), - comparing the monitored superheat to a threshold value, and - in the case that the monitored superheat value is smaller than the threshold value,determining that an internal instability is present in the vapour compression system (1)-
    [4] 4. A method according to c|aim 2 or 3, wherein the step of comparing the monitoredtemperature difference or the monitored superheat to a threshold value comprises comparingthe monitored temperature difference or the monitored superheat to a time filtered temperature difference or superheat.
    [5] 5. A method according to any of the preceding claims, wherein the step of obtaining anevaporating temperature comprises measuring a pressure of refrigerant passing through the evaporator (5) and deriving the evaporating temperature based on the measured pressure.
    [6] 6. A method according to any of claims 1-4, wherein the step of obtaining an evaporating temperature comprises measuring the evaporating temperature.
    [7] 7. A method according to any of the preceding claims, wherein decrease of the setpoint pinchvalue is prevented if the setpoint pinch value has been increased, until a change in external operating conditions of the vapour compression system (1) has been detected.
    [8] 8. A method according to any of the preceding claims, wherein the step of controlling anopening degree of the expansion device (4) comprises deriving an evaporating pressuresetpoint value and controlling the opening degree by means of a proportional integral derivative (PID) control loop, based on the derived evaporating pressure setpoint value. 17
    [9] 9. A method according to claim 8, further comprising the step of increasing a gain of the PIDcontrol loop in the case that a change in external operating conditions for the vapourcompression system (1) is detected.
    [10] 10. A method according to claim 8 or 9, further comprising the step of increasing responsivity 5 of the PID control loop in the case that an increase in cooling capacity of the vapour compression system (1) is detected.
    [11] 11. A method according to any of the preceding claims, further comprising the steps of: - monitoring a cooling capacity of the vapour compression system (1), and - increasing the setpoint pinch value in the case that an increase in the cooling capacity10 of the vapour compression system (1) is detected.
    [12] 12. A method according to any of the preceding claims, wherein the secondary fluid flowingacross the evaporator (5) is a brine.
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    同族专利:
    公开号 | 公开日
    SE543171C2|2020-10-20|
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    法律状态:
    优先权:
    申请号 | 申请日 | 专利标题
    DKPA201600304|2016-05-18|
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