Method for operating a vapour compression system using a subcooling value

Abstract

A vapor compression system comprises a compressor, a condenser, an expansion device, e.g. in the form of an expansions valve, and an evaporator arranged along a refrigerant path. A method for operating the vapor compression system comprises the steps of: obtaining a superheat value being representative for the superheat of refrigerant entering the compressor; obtaining a subcooling value being representative for the subcooling of refrigerant entering the expansion device; and operating the expansion device on the basis of the obtained superheat value and on the basis of the obtained subcooling value. The subcooling value is taken into account when operating the expansion device, because variations in the subcooling value have significant influence on the refrigerating capacity of the evaporator at a given opening degree of the expansion device, thereby resulting in a more stable operation of the system. The system may further comprise an internal heat exchanger.

Claims

1. A method for operating a vapour compression system, the vapour compression system comprising a compressor, a condenser, an expansion device and an evaporator arranged along a refrigerant path, the method comprising the steps of: obtaining a superheat value being representative for the superheat of refrigerant entering the compressor, measuring a temperature of the refrigerant entering the expansion device, obtaining a subcooling value based at least in part on the measured temperature of the refrigerant entering the expansion device, the subcooling value being representative for the subcooling of refrigerant entering the expansion device, and operating the expansion device only on the basis of the obtained superheat value during start-up of the vapour compression system, and operating the expansion device on the basis of the obtained superheat value as well as on the basis of the obtained subcooling value during normal operation of the vapour compression system.

2. The method according to claim 1, wherein the vapour compression system further comprises an internal heat exchanger arranged to provide heat exchange between refrigerant flowing from the evaporator towards the compressor and refrigerant flowing from the condenser towards the expansion device, and wherein the step of obtaining a superheat value comprises measuring a temperature of refrigerant leaving the internal heat exchanger in a direction towards the compressor, and the step of obtaining a subcooling value comprises measuring a temperature of refrigerant leaving the internal heat exchanger in a direction towards the expansion device.

3. The method according to claim 2, wherein the refrigerant flowing from the evaporator towards the compressor and the refrigerant flowing from the condenser towards the expansion device flow in parallel in the internal heat exchanger.

4. The method according to claim 2, further comprising the step of measuring a temperature of refrigerant flowing from the evaporator towards the internal heat exchanger.

5. The method according to claim 1, wherein the step of operating the expansion device comprises controlling an opening degree of the expansion device.

6. A vapour compression system comprising a compressor, a condenser, an expansion device and an evaporator arranged along a refrigerant path, and an internal heat exchanger arranged to exchange heat between refrigerant flowing from the evaporator towards the compressor and refrigerant flowing from the condenser towards the expansion device, the vapour compression system further comprising a first sensor arranged to measure a value being representative for a superheat value, said first sensor being arranged in the refrigerant path between the internal heat exchanger and the compressor, a second sensor arranged in the refrigerant path between the internal heat exchanger and the expansion device to measure a temperature of refrigerant leaving the internal heat exchanger in a direction towards the expansion device, the measured temperature being representative for a subcooling value, a third sensor arranged to measure a value being representative for a superheat value, said third sensor being arranged in the refrigerant path between the evaporator and the internal heat exchanger, and a controller adapted to operate the expansion device only on the basis of the values obtained by means of the third sensor during start-up of the vapour compression system, and on the basis of values obtained by means of the first sensor as well as on the basis of values obtained by means of the second sensor during normal operation of the vapour compression system.

7. The vapour compression system according to claim 6, wherein the internal heat exchanger is a parallel flow heat exchanger.

8. The method according to claim 1, wherein the vapour compression system further comprises an internal heat exchanger arranged to provide heat exchange between refrigerant flowing from the evaporator towards the compressor and refrigerant flowing from the condenser towards the expansion device, and wherein the step of obtaining a superheat value comprises measuring a temperature of refrigerant leaving the internal heat exchanger in a direction towards the compressor, and the step of measuring the temperature of the refrigerant entering the expansion device comprises measuring a temperature of refrigerant leaving the internal heat exchanger in a direction towards the expansion device.

9. The method according to claim 3, further comprising the step of measuring a temperature of refrigerant flowing from the evaporator towards the internal heat exchanger.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 is a diagrammatic view of a vapour compression system according to an embodiment of the invention,

(3) FIG. 2 is a pressure-enthalpy diagram illustrating operation of the vapour compression system of FIG. 1, and in accordance with a method according to an embodiment of the invention, and

(4) FIGS. 3a and 3b are heat exchanger diagrams for an evaporator operated with superheat and an evaporator operated without superheat, respectively.

DETAILED DESCRIPTION

(5) FIG. 1 is a diagrammatic view of a vapour compression system 1 according to an embodiment of the invention. The vapour compression system 1 comprises a compressor 2, a condenser 3, an expansion device 4 in the form of an expansion valve, and an evaporator 5. The vapour compression system 1 further comprises an internal heat exchanger 6 arranged in the suction line of the vapour compression system 1. In the internal heat exchanger 6 heat exchange takes place between refrigerant flowing in the suction line from the evaporator 5 towards the compressor 2, and refrigerant flowing from the condenser 3 towards the expansion device 4. Thereby cool refrigerant flowing from the evaporator 5 towards the compressor 2 is heated, while hot refrigerant flowing from the condenser 3 towards the expansion device 4 is cooled. The internal heat exchanger 6 is a parallel flow heat exchanger, i.e. the two flows of refrigerant flow in parallel in the internal heat exchanger 6.

(6) As described above, the heat exchange taking place in the internal heat exchanger 6 ensures that any liquid refrigerant which may leave the evaporator 5 is evaporated in the internal heat exchanger 6, and that a superheat is created in the refrigerant, thereby ensuring that the refrigerant reaching the compressor 2 is in a substantially gaseous state. This allows the vapour compression system 1 to be operated in such a manner that liquid refrigerant is present throughout the evaporator 5 without risking that liquid refrigerant reaches the compressor 2. Thereby the potential refrigeration capacity of the evaporator 5 can be utilised to the greatest possible extent, without risking damage to the compressor 2.

(7) At the same time the internal heat exchanger 6 causes the subcooling of the refrigerant flowing in the liquid line, i.e. from the condenser 3 towards the expansion device 4, to be increased.

(8) A first sensor 7 is arranged in the refrigerant path between the internal heat exchanger 6 and the compressor 2. The first sensor 7 is arranged for measuring the temperature of the refrigerant leaving the internal heat exchanger 6. Thereby the values measured by means of the first sensor 7 are representative for the superheat of the refrigerant leaving the internal heat exchanger 6 and flowing towards the compressor 2.

(9) A second sensor 8 is arranged in the refrigerant path between the internal heat exchanger 6 and the expansion device 4. The second sensor 8 is arranged for measuring the temperature of the refrigerant leaving the internal heat exchanger 6, and possibly also the pressure of the refrigerant. Thereby the values measured by the second sensor 8 are representative for the subcooling of the refrigerant being supplied to the expansion device 4. Preferably, the second sensor 8 is arranged for measuring only a temperature value. Thereby changes in the subcooling of the refrigerant can be monitored, since changes in the temperature reflect changes in the subcooling, but absolute values of the subcooling are not obtained. However, this is often sufficient for the purpose of controlling the expansion device 4, and it is much simpler to measure the temperature than to obtain absolute values for the subcooling.

(10) A third sensor 9 is arranged in the refrigerant path between the evaporator 5 and the internal heat exchanger 6. The third sensor 9 is arranged for measuring the temperature of the refrigerant leaving the evaporator 5, and possibly also the pressure of the refrigerant. Thereby the values measured by the third sensor 9 are representative for the superheat of the refrigerant leaving the evaporator 5 and before heat exchange takes place in the internal heat exchanger 6. Values measured by the third sensor 9 may advantageously be used only during a start-up sequence of the vapour compression system 1. This will be described in further detail below.

(11) A pressure sensor 10 is arranged in the refrigerant path near the first sensor 7 for measuring the pressure of the refrigerant leaving the internal heat exchanger 6. The measured pressure may, together with the temperature signal obtained by the first sensor 7, be used for calculating the superheat of the refrigerant entering the compressor 2.

(12) Each of the sensors 7, 8, 9, 10 communicates with a controller 11. Thus, the controller 11 receives measured values from each of the sensors 7, 8, 9, 10, and based on these measured values an output signal for an actuator 12 is generated. In response to the generated output signal, the actuator 12 operates the expansion device 4, e.g. by adjusting an opening degree, in such a manner the potential refrigeration capacity of the evaporator 5 is utilised to the greatest possible extent, without risking damage to the compressor 2 due to liquid refrigerant reaching the compressor 2.

(13) In the following the operation of the vapour compression system 1 of FIG. 1 will be described with reference to FIG. 1 and FIG. 2. FIG. 2 is a pressure-enthalpy diagram illustrating operation of the vapour compression system 1 of FIG. 1. The solid line 13 represents the operation of the vapour compression system 1 using a prior art control method where the expansion device 4 is controlled purely on the basis of a measured superheat value of refrigerant leaving the evaporator 5, and where the vapour compression system 1 is not provided with an internal heat exchanger 6. The dashed line 14 represents the operation of the vapour compression system 1 in accordance with a method according to an embodiment of to the invention.

(14) Initially, i.e. during start-up of the vapour compression system 1, the vapour compression system 1 is preferably operated in accordance with a prior art control strategy, where the expansion device 4 is operated purely on the basis of a measured value of the superheat of refrigerant leaving the evaporator 5. In this case the controller 11 operates the expansion device 4 purely on the basis of measurements performed by the third sensor 9.

(15) When the vapour compression system 1 operates properly, e.g. after approximately 10-15 minutes, the control strategy is changed to a method according to an embodiment of the invention. In this case the controller 11 operates the expansion device 4 on the basis of measurements performed by the first sensor 7 as well as on the basis of measurements performed by the second sensor 8, and the dashed line 14 of the pressure-enthalpy diagram is followed.

(16) The reason for this two-step operation of the vapour compression system 1 is that the control of the vapour compression system 1 according to an embodiment of the invention is very sensitive to disturbances in the suction pressure. During start-up of the vapour compression system 1 such variations are expected, in particular in the case that the compressor 2 is in the form of a compressor rack comprising two or more compressors which are switched on or off in order to allow the vapour compression system 1 to match the refrigeration load. During start-up of the vapour compression system 1 a lot of switching on and off of the compressors is expected, thereby resulting in significant disturbances in the suction pressure, and it is therefore advantageous to operate the vapour compression system 1 in accordance with a prior art method during start-up.

(17) Once the start-up sequence has been completed, the vapour compression system 1 is operated in accordance with a method according to an embodiment of the invention, and following the dashed line 14 of the pressure-enthalpy diagram. From point 15 to point 16 the refrigerant is compressed in the compressor 2, resulting in an increase in enthalpy as well as an increase in pressure. It is clear from FIG. 2 that during this process the enthalpy is slightly lower than it is the case when the vapour compression system 1 is operated in accordance with a prior art method, illustrated by the solid line 13.

(18) From point 16 to point 17 the refrigerant is condensed in the condenser 3. During this the pressure is maintained at a substantially constant level, while the enthalpy is reduced. From point 17 to point 18, the refrigerant passes through the internal heat exchanger 6. It is clear from FIG. 2 that this results in the enthalpy being further reduced, while the pressure stays at the substantially constant level. Accordingly, the subcooling of the refrigerant leaving the condenser 3 is increased due to heat exchange taking place in the internal heat exchanger 6. This is illustrated by the dashed portion 14a. It is clear from FIG. 2 that this additional increase in subcooling is not obtained in the prior art method, i.e. when an internal heat exchanger 6 is not provided.

(19) From point 18 to point 19 the refrigerant is expanded in the expansion device 4, resulting in a decrease in pressure, while the enthalpy is maintained at a substantially constant level. For this purpose the expansion device 4 is operated on the basis of measurements performed by the first sensor 7 as well as on the basis of measurements performed by the second sensor 8. Thus, the expansion device 4 is operated on the basis of a measured superheat value of refrigerant leaving the internal heat exchanger 6, and with due consideration to the subcooling, or at least to changes in the subcooling, of refrigerant being supplied to the expansion device 4.

(20) It is clear from FIG. 2 that the pressure level reached during the expansion step when operating the vapour compression system 1 in accordance with a method according to an embodiment of the invention is slightly higher than the pressure level reached when operating the vapour compression system 1 according to a prior art method. It is also clear that a larger part of the expansion takes place in the liquid zone when the vapour compression system 1 is operated in accordance with a method according to an embodiment of the invention.

(21) From point 19 to point 20 the refrigerant passes through the evaporator 5, and at least a part of the refrigerant undergoes evaporation. Thus, during this step the enthalpy increases while the pressure is maintained at a substantially constant level. It is clear from FIG. 2 that the refrigerant is still in the mixed phase when leaving the evaporator 5, i.e. no superheat is created in the evaporator 5, and the potential refrigeration capacity of the evaporator 5 is utilised to the maximum extent.

(22) From point 20 to point 15, the refrigerant passes through the internal heat exchanger 6. This results in the enthalpy being further increased while the pressure remains at the substantially constant level. Thereby a positive superheat is introduced in the refrigerant, represented by the line 14b, and it is thereby prevented that liquid refrigerant reaches the compressor 2. It is clear from FIG. 2 that the enthalpy decrease represented by the line 14a corresponds to the enthalpy increase represented by line 14b.

(23) FIGS. 3a and 3b are heat exchanger diagrams for an evaporator operated with superheat and an evaporator operated without superheat, respectively. The diagram of FIG. 3a corresponds to the situation illustrated by the solid line 13 in FIG. 2, and the diagram of FIG. 3b corresponds to the situation illustrated by the dashed line 14 in FIG. 2.

(24) In FIGS. 3a and 3b the solid line 21 illustrates the temperature of the flow of fluid flowing across the evaporator as a function of position along the evaporator, and the dashed line 22 illustrates the evaporator temperature as a function of position along the evaporator.

(25) In FIG. 3a the temperature of the refrigerant towards the end of the evaporator increases significantly due to the superheat introduced in the refrigerant in the evaporator. Thereby the evaporator temperature is also increased towards the end of the evaporator. In this case a temperature difference between the evaporator temperature and the temperature of the cooled fluid, ΔT, which exceeds the superheat, SH, is required. Accordingly, the evaporator temperature must be very low.

(26) In FIG. 3b, on the other hand, no superheat is introduced in the refrigerant in the evaporator. Accordingly, the requirements to the temperature difference between the evaporator and the cooled fluid, ΔT, are reduced. Accordingly, the evaporator temperature can be increased as compared to the situation illustrated in FIG. 3a, as illustrated by arrow 23.

(27) Thus, it is clear from FIGS. 3a and 3b that when the vapour compression system is operated in accordance with a method according to an embodiment of the invention, the temperature difference at the evaporator can be decreased without increasing the charge of the vapour compression system.

(28) The arrows above the lines in the graphs of FIGS. 3a and 3b illustrate the flow direction of the refrigerant flowing in the evaporator and the fluid flowing across the evaporator, respectively.

(29) Although various embodiments of the present invention have been described and shown, the invention is not restricted thereto, but may also be embodied in other ways within the scope of the subject-matter defined in the following claims.