DE102015102400A1 - Method for operating a refrigerant circuit, in particular in low-load operation - Google Patents

Abstract

The present invention relates to a method for operating a refrigerant circuit (1), in which a compressor (2), a high-pressure heat exchanger (3), an electrically controllable expansion element (4) and a low-pressure heat exchanger (5) are connected in sequence in a conventional manner , In the case of generic methods, at least one regular load condition and at least one low-load condition are defined. For a given regular load condition, the mass flow of the refrigerant and the opening degree of the electric expansion device (4) are regulated in accordance with a predetermined target function in a regular operation. When a low-load condition of the refrigerant circuit (1) is detected, the refrigerant circuit is operated according to another control strategy. The invention is characterized in that, upon detecting a low-load condition of the refrigerant cycle (1), it is switched to a low-load operation, and when switching to the low-load operation, the opening degree of the electric expansion element (4) remains the same or increased.

Description

The present invention relates to a method for operating a refrigerant circuit in which, in a conventional manner, a compressor, a high-pressure heat exchanger, an electrically controllable expansion element and a low-pressure heat exchanger are connected in sequence one after the other. In the case of generic methods, at least one regular load condition and at least one low-load condition are defined. For a given regular load condition, the mass flow of the refrigerant and the opening degree of the electric expansion device are regulated in accordance with a predetermined target function in a regular operation. If a low load condition of the refrigerant circuit is detected, the refrigerant circuit is operated according to another control strategy. The main field of application is directed towards motor vehicle air conditioning, whereby the invention can in principle be applied to any type of air conditioning with the aid of a refrigerant circuit.

In the following, regular operations refer to operating conditions in which the refrigerant circuit provides system-specific cooling capacity or heating capacity in such a way that continuous operation is possible in principle (in the following, only "cooling capacity" is used, and depending on the configuration of the refrigerant circuit, the heating capacity can also be used). In particular, the refrigerant circuit is regulated according to a clear target function. Regular operations known to the person skilled in the art include a maximum operation, in which the refrigerating capacity of the refrigerant circuit is to be optimized, and an efficiency mode, in which the COP (coefficient of performance) is to be maximized. In practice, the overheating at the evaporator / low-pressure heat exchanger or the supercooling at the condenser / high-pressure heat exchanger are controlled in a conventional manner. Typically, when the desired refrigeration capacity is reduced (e.g., in accordance with air conditioning requirements), the compressor typically has to be downshifted to adjust the actual refrigeration capacity down through the lower mass flow rate. Because of the lower mass flow, the opening degree of the expansion device is subsequently reduced further to maintain the pressure difference between the high-pressure section and the low-pressure section at this lower mass flow.

Low load conditions hereinafter refer to operating conditions in which the refrigerant circuit provides a comparatively low refrigeration capacity and continuous operation of the compressor can become problematic sooner or later with an unchanged control strategy. Such cases occur, for example, at a low, but present air conditioning desire. The main problem is that the mass flow of the refrigerant drops. When falling below a certain minimum mass flow of the compressor is no longer reliably controlled. To avoid this, it is known to switch the compressor in alternating operation between a switched-off state and a just acceptable minimum operation episodic back and forth. This constant switching on and off in turn means that there may be temperature fluctuations in the area of the air conditioning system. Furthermore, torque fluctuations occur, which may have effects on the load calculation of the internal combustion engine of the motor vehicle. This may then lead to slightly increased fuel consumption. Low mass flow also causes low lubricant recirculation of refrigerant carried with the refrigerant. To solve this problem, oil separators are known, among other things, which collect lubricating oil at unfavorable points in the refrigerant circuit, possibly transport it to a suitable location and feed it back into the refrigerant circuit. Alternatively, is from the DE 198 26 730 A1 known to limit the cooling capacity and the mass flow of the refrigerant to a minimum value down, so that a guaranteed minimum mass flow sufficient oil return is ensured.

The abovementioned solutions are either complicated in terms of apparatus or they disadvantageously limit the operating range of the refrigerant circuit.

It is the object of the present invention to improve a generic refrigerant circuit and a method for operating the same in view of the problems mentioned above and in particular to specify such that the refrigerant circuit has a stable operating range at low load.

This object is achieved by a method having the features of claim 1 and by a refrigerant circuit having the features of claim 11. Advantageous training and further developments emerge from the dependent claims.

The invention is based on the idea that by reducing the enthalpy difference of the refrigerant in the evaporator, the refrigerating capacity to be set can also be achieved with a higher than the minimum mass flow.

The method according to the invention is now characterized in that, when a low-load condition of the refrigerant circuit is detected, it is switched to a low-load mode, wherein the Switching to the low-load operation, the opening degree of the electric expansion device is maintained or increased. In departure from known methods, the expansion device is not further closed at reduced load, as would automatically set for non-externally controlled expansion organs - as is the case, for example, in self-regulating thermostatic expansion valves. In addition, by using an electrical expansion device, the degree of opening can be actively controlled and thus kept constant or even increased. As will be explained below, the operating range of the refrigerant circuit can be expanded without the compressor having to be alternately turned off and on.

According to one embodiment of the invention, an accumulator is arranged in the refrigerant circuit between the low-pressure heat exchanger and the compressor, which determines the physical state of the refrigerant at the downstream exit point of the accumulator. By this measure known per se, a simpler control of the enthalpy difference at the low-pressure heat exchanger or evaporator is made possible. This is advantageous when using an electrically controllable expansion element, because then the outlet state is fixed to the evaporator and the entry state can be selectively varied by the electrically controlled expansion element.

According to one embodiment of the invention, it is provided that one or more parameters for determining the delivery rate of the compressor is / are determined and the determined delivery rate is compared with a predetermined minimum delivery rate of the compressor. The delivery rate is the mass flow at the outlet of the compressor and is thus a measure of the value of the systemic mass flow of the refrigerant circuit. The low-load condition of the refrigerant cycle is detected in this case by approaching or reaching the minimum delivery rate of the compressor, and to further reduce the refrigerant capacity of the refrigerant cycle, the delivery rate of the compressor is maintained at least at the minimum delivery rate level and the degree of opening of the electric expansion element is increased. The achievement of the minimum delivery rate of the compressor thus represents a first low-load condition. By opening the expansion device at not further lowered delivery rate of the compressor, the coefficient of performance of the refrigerant circuit is slightly reduced, but this is the cooling capacity (product of mass flow and enthalpy) further lowered, whereby the refrigerant circuit continues to operate continuously. A changeover operation between on and off compressor can thus be avoided.

The parameters for determining the delivery rate of the compressor typically include adjustable parameters such as e.g. the compressor speed or the current load or momentary drive power of the compressor. In practice, e.g. In the test bench or in the vehicle, boundary conditions of the refrigerant circuit are examined which give an indication of a too low delivery rate of the compressor. In doing so, e.g. Limit parameters such as the current load of the compressor as a function of evaporator or ambient temperature determined. The actual mass flow is not determined directly. It is typically only the underrun of one or more of the previously determined, possibly also combined limit values checked, which indicate a minimum delivery rate. The first low load condition depends on the type of compressor and must be determined separately for each compressor type.

According to a further embodiment of the invention, it is provided that one or more parameters for determining a lubricant flow of lubricant carried in the circulating refrigerant is detected and at least by means of one of the parameters for determining the lubricant flow, a limit condition is established, which indicates a marginally low lubricant flow. By determining the limit condition, the low load condition of the refrigerant circuit is detected. Approaching or achieving a marginally low lubricant flow thus represents a second low-load condition. The second low-load condition is predetermined by the design of the refrigerant circuit or the measures and requirements for lubricant recirculation.

The first and second low load condition may occur independently of each other, if necessary. Typically, the low lubricant flow limit condition occurs earlier than the minimum compressor delivery rate when there are no or only poor oil separators in the refrigerant circuit. In the case of lubricant recirculation via oil separator, however, the reverse case can also occur in that the minimum delivery rate of the compressor is reached earlier. Both low load conditions can thus also be tested and applied simultaneously in an iterative process.

According to an advantageous embodiment of the invention, it is provided that parameters for determining the delivery rate of the compressor and / or the limit condition for the lubricant flow are each stored as a parameterized characteristic map in a control unit of the refrigerant circuit and read from this. This allows one simple technical implementation with an automatic control.

According to one embodiment of the invention, the parameters for determining the lubricant flow include the opening degree of the electrical expansion element and at least one parameter for influencing the mass flow of the refrigerant. Since the enthalpy difference of the refrigerant in at least one of the heat exchangers, in particular in the evaporator, can be estimated via the opening degree of the expansion element and the mass flow itself can be approximated via the parameters for influencing the mass flow, the instantaneous cooling capacity as a product of mass flow and enthalpy difference can also be estimated at least relatively , The invention is now based on the consideration that the cooling capacity to be provided by the refrigerant circuit can be adjusted not only by setting a new value pair point but by setting any value pair point on a line. In this case, only those value-pair points can be taken into account that are not within the low-load range.

Mass flow can be measured or estimated in a variety of ways. The skilled person is here in particular sensory measured values on the suction pressure of the compressor, the control / energization of the compressor, the compressor speed, the system pressures or pressure differences in the refrigerant circuit, the air temperature and / or the air flow at the heat exchanger (the latter is in the aforementioned DE 198 26 730 A1 disclosed). The numerical value of the mass flow need not be known directly. In most cases, only parameters or limit values of parameters are known which make it possible to conclude or estimate a certain mass flow.

According to one embodiment of the invention, it is provided that during the low-load operation, the cooling capacity of the refrigerant circuit is changed, wherein the refrigerant circuit is controlled in accordance with the parametric map such that the associated parameters approximately meet the limit condition for the lubricant flow. In practice, it will not be an exact parameter curve, but a parameter corridor that delimits regular load conditions from low load conditions. In other words, one could also assume a clearly defined parameter curve, whereby the low-load operation is triggered upon reaching or approaching this curve.

In particular, it can be provided that the cooling capacity of the refrigerant circuit is changed during the low-load operation, wherein parameters for influencing the mass flow of the refrigerant are chosen such that the mass flow is kept substantially constant and the heat transfer performance of the low-pressure heat exchanger or the high-pressure heat exchanger by changing the opening degree of the electric Or, alternatively, the degree of opening of the electrical expansion element is kept constant and parameters for influencing the mass flow of the refrigerant are selected such that the heat transfer performance of the high pressure heat exchanger or the low pressure heat exchanger is controlled by changing the mass flow, wherein the held constant opening degree in the Compared to an opening degree in a regular operation is large. This control strategy is very simple, because thus only either the compressor or the expansion device is readjusted.

According to one embodiment of the invention, the presence of a low-load condition is checked before each iteration. In this case, for example, it is checked in each iteration how the regulation would be carried out in a regular mode. If this would satisfy the boundary condition, then the control is performed according to the low load control. It can be provided in particular that an operating parameter for operating the compressor is changed and is determined on the change of this operating parameter, the opening degree of the electric expansion element for a regular operation. The operating parameter for operating the compressor is e.g. determined in response to a temperature signal indicating that the desired temperature deviates from the actual temperature. The regulation does not then take place, but it is first checked whether there is an indication of the achievement of the minimum delivery rate of the compressor and / or an indication of the borderline low lubricant flow. If none of these instructions is present, the determined opening degree for regular operation is set, otherwise, if at least such an indication exists, the opening degree for the low-load operation is determined and set.

According to the invention, a refrigerant circuit is connected with a compressor, high-pressure heat exchanger, electrically controllable expansion element and low-pressure heat exchanger connected in series with a control unit, which is designed to carry out a previously mentioned method. Such a refrigerant circuit is particularly suitable for use in an automotive air conditioning system, because here, if necessary, fast - faster than at. Building air conditioning - must respond to dynamic changes in the environmental condition.

The invention will now be explained in more detail with reference to embodiments and with reference to the figures.

The 1 shows schematically the construction of a refrigerant circuit, which can be used according to an embodiment of the invention,

the 2A - 2 B show schematic Mollier diagrams for explaining the control strategy in a regular and a low load operation according to an embodiment of the invention,

the 3A - 3C show schematic characteristics for defining boundary conditions for recognizing between a regular operation and a low-load operation according to an embodiment of the invention,

the 4 schematically shows a flowchart for controlling a refrigerant circuit according to an embodiment of the invention and

the 5 schematically shows control points of parameters of the refrigerant circuit with respect to a characteristic after 3C when performing a control according to an embodiment of the invention.

In the 1 is schematically the structure of a refrigerant circuit 1 which can be used according to an embodiment of the invention. Generic refrigerant circuits comprise for this purpose at least one compressor 2 , a capacitor 3 , an expansion valve 4 and an evaporator 5 , In the refrigerant circuit 1 is from the compressor 2 compressed refrigerant to the condenser 3 where the refrigerant heated by the compression is in heat exchange with an ambient fluid, eg air, for heat dissipation. From the condenser 3 the refrigerant becomes an expansion valve 4 guided, the flow is relaxed and cooled. The cooled refrigerant is passed through the evaporator 5 passed, in which it is again in heat-conducting contact with an ambient fluid while it is heated to evaporate again with cooling of the ambient fluid itself, in turn, from the compressor 2 to be sucked.

The section downstream from the compressor 2 to the expansion valve 4 is also called the high pressure section, the section downstream of the expansion valve 4 to the compressor 2 is also referred to as a low pressure section. Depending on the operating mode, the heat output of the high-pressure heat exchanger and / or the cooling capacity of the low-pressure heat exchanger can be used for the purposes of air conditioning. The following is only about the cooling capacity of the refrigerant circuit 1 spoken, it is clear to those skilled in the art that this is associated with a corresponding heat output in the high pressure section. Depending on the refrigerant circuit, the high-pressure heat exchanger also functions as a condenser 3 or gas cooler called. The low pressure heat exchanger is typically an evaporator 5 , In the refrigerant circuit 1 For example, additional components may be present, such as internal heat exchangers, bypasses and switching means, which will not be discussed here.

The following embodiments relate to a refrigerant circuit 1 for an automotive air conditioning system in the cooling mode, in which an evaporator 5 As a low-pressure heat exchanger to produce refrigeration according to an air conditioning request. However, the invention can also for other modes of operation of the refrigerant circuit 1 be used, in particular for a heat pump operation in which the of the high-pressure heat exchanger or condenser 3 generated heat to be used for the air conditioning target.

In the refrigerant circuit 1 According to the embodiment, an electrically controllable expansion valve 4 used to the refrigerant circuit 1 to be able to control or regulate exactly as required. This is the electric expansion valve with an electronic control unit 7 connected to receive from those signals to change its opening degree. The compressor 2 is also with the control unit 7 connected to set the operating parameters for its control, for example, a current value or a corresponding speed or flow rate. The refrigerant circuit 1 further optionally comprises an accumulator 6 at the exit of the evaporator 5 on. The accumulator 6 has the function, not yet evaporated refrigerant at the outlet of the evaporator 5 to separate already evaporated refrigerant and mix again in a defined ratio. The accumulator thus provides a defined refrigerant state at the outlet of the evaporator 5 one.

An actual temperature is detected by a temperature sensor (not shown). In the event of a deviation between the setpoint and the actual temperature, the refrigerant circuit then becomes 1 automatically readjusted. If an increased cooling capacity is required, the compressor becomes 2 highly regulated, ie the performance of the compressor 2 increased, for example, by this supplied with more power or otherwise the speed or delivery rate is increased. In the reverse case, the compressor is downshifted, ie the performance of the compressor 2 decreased. On a change in the delivery rate of the compressor 2 becomes the expansion valve 4 readjusted to the associated pressure difference between the High pressure section and the low pressure section to compensate and again set the optimal COP operating point (eg by adjusting a defined hypothermia).

In the 2A is schematically a phase diagram 10A shown, which describes the state of the refrigerant, if this the refrigerant circuit 1 according to 1 flows through in a regular mode. The phase diagram is a so-called Mollier diagram showing the enthalpy on the x-axis and the representation of the pressure on the y-axis. The refrigerant is a prior art refrigerant known per se for use in automotive air conditioning systems, such as, but not limited to, R134a, R1234yf, or R744. The refrigerant has three phases below a critical pressure, namely a liquid phase I, a wet steam phase II and a superheated steam phase III. In the wet steam phase II, vaporous and boiling refrigerant coexist, while in the superheated steam phase III only gaseous, possibly superheated refrigerant occurs.

Starting from the entrance of the evaporator 5 the refrigerant is in a cool and at least partially condensed state (point 11A in the Mollier diagram). By continuous heat transfer between the evaporator 5 and the ambient fluid evaporates the refrigerant (point 12A ) from the condenser 2 sucked and compressed (point 13A ). With steady heat transfer between the ambient fluid and the condenser 3 the refrigerant is cooled until complete condensation and the fully condensed refrigerant is in the downstream section of the condenser 3 further subcooled (point 14A ). When flowing through the expansion valve 4 the refrigerant is depressurized and optionally passes into the wet steam phase II (back to point 11A where the circuit closes).

In a first regular operation, the aim is to obtain the coefficient of performance, defined as the quotient of the heat transfer rate, here by the evaporator 5 provided cooling capacity 16A , and that of the compressor 2 used mechanical power to optimize under given conditions. Since the exact COP value can not be measured, typically a defined subcooling of the refrigerant after the condenser 3 regulated. To achieve this, when the required cooling capacity is reduced, the compressor becomes pressurized 2 down-regulated to reduce the mass flow, and the opening degree of the expansion valve 4 reduced. Due to the lower mass flow, the refrigerant is then cooled or heated more. In the Mollier diagram 10A This is characterized in that the horizontal lines between the state points 11A - 12A and 13A - 14A get longer. If, as provided in the embodiment, the point 12A through the use of a rechargeable battery 6 is fixed, the point wanders 14A , which is the exit condition at the condenser 3 defined, further into the liquid phase I, causing hypothermia 15A increased and thus up to a certain point the coefficient of performance (COP) is increased.

For the sake of completeness, it is also mentioned that there may be further regular operations, for example maximum operation, in which, regardless of the coefficient of performance, maximum heat transfer is attempted. In this case, the compressor becomes 2 typically operated at maximum delivery rate.

Regular operation is only above a certain flow rate of the compressor 2 possible. Depending on the design of the refrigerant circuit 1 and the type of compressor 2 can not be fallen below a certain minimum delivery rate. Therefore, it is known to switch off the refrigerant circuit temporarily in such a low-load operation.

In the 2 B is schematically a phase diagram 10B shown, which describes the state of the refrigerant, if this the refrigerant circuit 1 according to 1 flows through in a low load operation according to the invention. The difference to the phase diagram 10A essentially consists in that, despite low power output of the compressor 2 the expansion valve 4 is relatively wide open. In the example shown occurs at the outlet of the capacitor 3 at the point 14B no more hypothermia. In other words, with successive power reduction of the refrigerant circuit 1 when detecting a low load condition in a low load operation with a different from the regular operation control logic switched by at least the first time switching to this low load operation, so if at least the previous iteration step was calculated and controlled according to regular operation, the opening degree of the electric expansion element 4 is not further reduced, but remains the same or increased. The consequences of this regulatory strategy are explained in more detail in the following examples.

In the 3A - 3C schematically illustrated are characteristic curves for defining boundary conditions for detecting between a regular operation and a low-load operation according to various embodiments of the invention. In this case, only the control of the compressor as parameters 2 (here represented by an applied electric current) as an indication of the flow rate / mass flow of the refrigerant (shown here in the x-axis) and the degree of opening of the expansion device 4 (shown here in the y-axis) used. The numerical examples are in arbitrary units.

The criteria for defining such a characteristic are expediently determined separately for a specific configuration of a refrigerant circuit. The criteria can be combined with each other arbitrarily, especially multi-dimensional maps with other parameters are conceivable.

In the 3A - 3C First, diagrams for determining low load conditions according to an embodiment of the invention are shown. The measuring points 21a - 21f show experimentally determined points, in which in the specific configuration, a sufficient lubricant recirculation of refrigerant carried in the refrigerant was determined. The measuring points 22a - 22d show corresponding points at which the lubricant recirculation is marginally low. The measuring points 23a - 23b are points where lubricant recirculation must definitely be considered inadequate.

In the 3A becomes a map from the measuring points 20A derived. This includes a limit 24 for a minimum opening degree of the expansion valve 4 and a limit 25 for a minimum energization of the compressor 2 , The map 20A or the limits 24 . 25 be in the control unit 7 stored and influence the control strategy in low load operation. According to this embodiment, the low load range is detected when at least one of the two limit values 24 . 25 would reach or fall below, if further regulated according to the control function in regular mode. The next iteration step of the control is then performed according to a different control strategy to avoid inadequate lubricant recirculation. It may be provided in this case, for example, that first the limit 24 - minimum opening degree of the expansion valve 4 - is achieved. In this case, the compressor 2 down-regulated as long as the cooling capacity continues to be reduced until the second limit value is reached 25 is reached. The degree of opening of the electrically controlled expansion valve remains 4 then constant. In the case of further cooling capacity to be reduced, the mass flow is finally kept substantially constant and the degree of opening of the expansion valve 4 gradually opened further. As a result, although then reduces the coefficient of performance, the heat transfer performance of the evaporator 5 but is reduced according to the lower air conditioning request, so that the compressor 2 This can continue to be operated continuously.

In the 3B becomes analogous to the 3A from the measuring points another map 20B derived. This includes a limit 26 for a minimum opening degree of the expansion valve 4 , In this case, when the low load range is detected, the expansion valve becomes 4 remain constant at a comparatively large opening degree, which this limit 26 equivalent. As long as the low load condition still applies, the cooling capacity is then only on the control of the compressor 2 regulated.

In the 3C becomes analogous to the 3A . 3B from the measuring points yet another map 20C derived. To the measured values 21a - 21f . 22a - 22d and 23a - 23b becomes a limit line 27 adapted, which marks the transition between the regular load condition and the low load condition. For larger delivery rates of the compressor 2 , So large supply current is sufficient, a relatively small opening degree of the expansion valve 4 , At further reduced delivery rate of the compressor 2 Lubricant recirculation can only be carried out as the opening degree of the expansion valve increases 4 ensure that the limit line 27 here has a rising edge. The limit line 27 may also have a minimum flow rate of the compressor 2 take into account the limit line 27 from this minimum delivery rate parallel to the y-axis (not shown). Further reduction of the cooling capacity of the refrigerant circuit 1 is then kept at constant delivery rate of the compressor 2 and only by a further opening of the electric expansion valve 4 set.

The limit line 27 runs in the map shown in sections in a straight line, but can alternatively be adapted to any of the measuring points. A possible control strategy with this characteristic 20C will be discussed below with reference to the 5 explained in more detail.

It can also be used instead of a sharply drawn limit line 27 a threshold corridor can be defined. When entering the operating point in this limit corridor can then be checked, for example, how critically further adjustment in the regular operation for the lubricant return and / or the minimum delivery rate of the compressor 2 would.

As long as a low load condition is present, in this way, even during the low load operation, the cooling capacity of the refrigerant circuit 1 be changed flexibly. In other words, the actual temperature can continue to be adapted dynamically to the desired temperature. In particular, while the refrigerant circuit 1 be controlled according to the parameterized map such that the associated parameters approximately one or more of the limit conditions 24 - 27 for the lubricant flow.

4 schematically shows a flowchart 30 for controlling a refrigerant circuit 1 according to an embodiment of the invention. In the exemplary embodiment, it is provided that, for a detected temperature difference between a desired and an actual temperature, first the compressor 2 in normal operation, the delivery rate or the mass flow of the refrigerant adapts, for example, by a modified current is fed. This change then makes the electrically controllable expansion valve 4 readjusted. If it must be temporarily switched from the regular operation in the low-load operation, it may happen that on the change in the opening degree of the expansion valve 4 according to the control logic of the low load operation of the compressor 2 must be readjusted to the deviating from the regular operation control logic of the low load operation also for the compressor 2 to take into account.

After changing the delivery rate of the compressor 2 is in an iteration step 31 the opening degree of the electric expansion valve 4 determined for regular operation. This calculated opening degree is not yet set. First, still in step 32 checks whether there is a condition for switching to a low-load operation, for example, based on at least one of the maps 20A - 20C , If there is no indication that it must be switched to the low load mode, then in one step 36 the determined opening degree for regular operation is set.

However, if there is an indication of the achievement of a low load condition, in one step 33 checks if the current cooling capacity is too high or too low. The degree of opening is then determined for the low-load operation, wherein at too high a cooling capacity for the opening degree of the expansion valve 4 a different from it larger (step 34 ) or, if the cooling capacity is too low, a smaller value is calculated (step 35 ). In step 36 then the calculated opening degree according to step 34 or 35 set.

In this embodiment, the program returns to regular operation after each iteration loop, and the existence of a low load condition is checked again prior to each further iteration. Alternatively, it can be provided that only switched back to the regular mode when the last determined operating point is no longer within the limit corridor.

In the 5 are schematic operating points of the refrigerant circuit with respect to a characteristic after 3C illustrated in which a control is performed according to another embodiment of the invention. Starting from the operating point 28a is the refrigerant circuit 1 in regular mode. It is neither a minimum delivery rate of the compressor 2 reached, even marginal or even low lubricant recirculation is to be feared.

At this time it is now determined that the current cooling capacity is too high. It has been detected, for example via temperature sensors, that the actual temperature in the cooling operation is below the target temperature, or that after a phase of cooling down of the passenger compartment from a too high actual temperature, the target temperature is reached. Then the delivery rate of the compressor 2 reduced and it turns a new operating point 28b one.

On a change of the current supply of the compressor 2 is a readjustment of the electric expansion valve 4 triggered. It is determined that the new operating point is assigned to the low load operation, so that now the new opening degree not according to operating point 28b' (which would be set after regular operation) reduced, but according to operating point 28c must be opened further.

After the expansion valve 4 has been opened further, the operating point is at or very close to the limit line 27 , A further reduction of the cooling capacity can now be achieved by further reducing the delivery rate of the compressor 2 (Point 28d ) and then further opening of the expansion valve 4 respectively.

The compressor 2 can be adjusted to the setting of the new opening degree of the compressor 2 according to the operating point 28c also be readjusted yourself. In this variant, when the low load range has been detected for the first time, the delivery rate of the compressor is determined 2 increased (point 28e ) and subsequently the opening degree of the expansion valve 2 slightly humiliated (point 28f ). The point 28f has one compared to the initial points 28a . 28b further open expansion valve 4 on. In particular, the mass flow of the refrigerant and the opening degree of the expansion valve 4 greater than at the comparison point 28b' which would have been adjusted in the control according to the regular operation. In this way, with a lower COP although the target cooling capacity can be adjusted in accordance with normal operation, without risking too low lubricant recirculation.

LIST OF REFERENCE NUMBERS

1

Refrigerant circulation

2

compressor

3

capacitor

4

electric expansion valve

5

Evaporator

6

accumulator

7

electronic control unit

10A, 10B

Mollier diagrams

11A-14A, 11B-14B

State points in the Mollier diagram

15A

Cooling degree

16A, 16B

Cooling capacity of the evaporator

20A-20C

Charts for determining low load conditions

21a-21f

Operating points with sufficient lubricant return

22a-22d

Operating points with critical lubricant return

23a-23b

Operating points with insufficient lubricant return

24-27

Limit condition / characteristics for compressor and expansion valve

28a-28f

successive operating points during a regular operation

30

flow chart

31-36

steps

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 19826730 A1 [0003, 0016]

Claims (11)

Method for operating a refrigerant circuit ( 1 ) with one circuit connected sequentially to a compressor ( 2 ), a high pressure heat exchanger ( 3 ), an electrically controllable expansion organ ( 4 ) and a low pressure heat exchanger ( 5 ), in which - at least one regular load condition and at least one low load condition are defined, - for a given regular load condition the mass flow of the refrigerant and the opening degree of the electric expansion member ( 4 ) is regulated according to a predetermined objective function in a regular mode and - a low-load condition of the refrigerant circuit ( 1 ), characterized in that - upon detection of a low-load condition of the refrigerant circuit ( 1 ) is switched to a low-load operation, - wherein when switching to the low-load operation, the opening degree of the electric expansion element ( 4 ) remains the same or is increased. Method for operating a refrigerant circuit according to claim 1, characterized in that in the refrigerant circuit ( 1 ) between low-pressure heat exchanger ( 5 ) and compressor ( 2 ) an accumulator ( 6 ), which determines the physical state of the refrigerant at the downstream exit point from the accumulator ( 6 ). Method for operating a refrigerant circuit according to one of claims 1 or 2, characterized in that - one or more parameters for determining the delivery rate of the compressor ( 2 ) is determined, - the determined delivery rate with a predetermined minimum delivery rate of the compressor ( 2 ), - the low-load condition of the refrigerant circuit ( 1 ) by approaching or reaching the minimum delivery rate of the compressor ( 2 ) is detected, - wherein for further reduction of the cooling capacity of the refrigerant circuit ( 1 ) the delivery rate of the compressor ( 2 ) at least at the level of the minimum delivery rate and the degree of opening of the electrical expansion element ( 4 ) is increased. Method for operating a refrigerant circuit according to one of claims 1 to 3, characterized in that - one or more parameters for determining a lubricant flow of lubricant carried in the circulating refrigerant is detected, - at least by means of one of the parameters for determining the lubricant flow a limit condition ( 24 - 27 ), which indicates a marginally low lubricant flow, and - by determining the limit condition, the low-load condition of the refrigerant circuit ( 1 ) is recognized. Method for operating a refrigerant circuit according to claim 3 or 4, characterized in that parameters for determining the delivery rate of the compressor ( 2 ) and / or the limit condition for the lubricant flow in each case as a parameterized characteristic field in a control unit ( 7 ) of the refrigerant circuit ( 1 ) are stored and read from this. Method for operating a refrigerant circuit according to one of claims 4 or 5, characterized in that the parameters for determining the lubricant flow, the opening degree of the electric expansion element ( 4 ) and at least one parameter for influencing the mass flow of the refrigerant. Method for operating a refrigerant circuit according to one of claims 5 or 6, characterized in that - during the low-load operation, the cooling capacity of the refrigerant circuit ( 1 ), wherein - the refrigerant circuit ( 1 ) is controlled according to the parameterized characteristic map such that the associated parameters approximately correspond to the limit condition ( 24 - 27 ) for the lubricant flow. Method for operating a refrigerant circuit according to one of the preceding claims, characterized in that during the low-load operation, the cooling capacity of the refrigerant circuit ( 1 ), wherein - parameters for influencing the mass flow of the refrigerant are selected such that the mass flow is kept substantially constant and the heat transfer capacity of the high-pressure heat exchanger ( 3 ) or the low-pressure heat exchanger ( 5 ) by changing the opening degree of the electric expansion element ( 4 ) or - the degree of opening of the electrical expansion element ( 4 ) is kept constant and parameters for influencing the mass flow of the refrigerant are selected such that the heat transfer capacity ( 16B ) of the low-pressure heat exchanger ( 5 ) or the high-pressure heat exchanger ( 3 ) is controlled by changing the mass flow, wherein the constant opening degree is large compared to an opening degree in a regular operation. Method for operating a refrigerant circuit according to one of the preceding claims, characterized in that before each Iteration the existence of a low load condition is checked. Method for operating a refrigerant circuit according to one of claims 3 to 9, characterized in that - an operating parameter for operating the compressor ( 2 ), - the change of the operating parameter for operating the compressor ( 2 ) the degree of opening of the electrical expansion element ( 4 ) is checked for a regular operation, - it is checked whether an indication of the achievement of the minimum delivery rate of the compressor ( 2 ) and / or an indication of the borderline low lubricant flow is present, - if none of these indications exists, the determined opening level for the regular operation is set, - if such an indication, the opening degree for the low load operation is determined and set. Refrigerant circuit, in particular for an automotive air conditioning system comprising - a refrigerant circuit ( 1 ) with one circuit connected sequentially to a compressor ( 2 ), a high pressure heat exchanger ( 3 ), an electrically controllable expansion organ ( 4 ) and a low pressure heat exchanger ( 5 ) and - one with the refrigerant circuit ( 1 ) connected control unit ( 7 ) configured to carry out a method according to any one of claims 1-10.

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