Refrigeration system.

0001The invention relates to a refrigeration system to be operated on board a motor vehicle with a compressor, a condenser, an evaporator, and between the condenser and evaporator, an electrically controllable thermostatic expansion valve, which furthermore has an electrically controllable actuating element.

0002It is known to provide a control circuit in refrigeration systems for motor vehicles, which act on electronically controllable actuating means for influencing the compressor power, the condenser power and / or the evaporator power (DE-OS 37 06 152). Refrigeration systems of the type mentioned at the outset have also already been proposed. in which the transmission rod determining the flow cross-section is moved by a membrane, on one side of which pressure and temperature of the refrigerant vapor behind the evaporator and on the other side of which a control medium is active, which can be influenced externally (patent application P 38 29 101.0 from August 17, 1988, according to European patent application 89 111 082.7-2206 / 0 356 642).

0003The object of the invention is to further develop a refrigeration system of the type mentioned in the introduction that, while maintaining a maximum limit for the temperature in the line between the compressor and the condenser, optimal operating conditions are always guaranteed.

0004This object is achieved by the measures specified in the characterizing part of patent claim 1. Advantageous developments of the invention are defined in the subclaims.

0005The invention for the first time creates a refrigeration system with which it is possible to work behind the evaporator with optimal superheating of the refrigerant vapor. This guarantees the highest possible performance of the evaporator with optimal efficiency of the entire refrigeration system. On the other hand, however, it is ensured that the maximum permissible temperature in the line between the compressor and the condenser is not exceeded and thus the refrigeration system as a whole is not damaged.

0006Embodiments of the invention and its advantageous developments are described in more detail below with reference to the accompanying drawings. They represent:<ul id="ul0001" list-style="none"><li>Figure 1 shows an embodiment;</li><li>FIG. 2 shows an electrically controllable thermostatic expansion valve;</li><li>Figure 3 shows the enthalpy diagram for the circuit of Figure 1;</li><li>FIG. 4 different operating states of a motor vehicle in the enthalpy diagram according to FIG. 3;</li><li>Figure 5 shows a second embodiment;</li><li>6 (a), (b), (c) different schematic representations of the implementation of the control device 100;</li><li>FIG. 7 shows a generalized representation of the exemplary embodiments according to FIGS. 1 and 5;</li><li>Figure 8 (a), (b), (c) the dependence of the electrical power (N) of the electrically actuated actuator of the expansion valve, the temperature (t₁) behind the compressor and the temperature (t₂) of the refrigerant vapor behind the evaporator depending on overheating (Δt_ü) behind the evaporator in one embodiment;</li><li>Figure 9 shows the performance of the evaporator (Q_V) and the temperature (t₁) of the refrigerant vapor behind the compressor depending on the overheating (Δt_ü);</li><li>Figure 10 shows the course of the temperature (t₁) behind the compressor, and the course of overheating (Δt_ü) behind the evaporator in various operating states of a motor vehicle refrigeration system;</li><li>Figure 11 shows the course of the temperature (t_LiK) in the headspace of a motor vehicle using the refrigeration system according to the invention in the operating states according to FIG. 10;</li><li>FIG. 12 shows a low-cost version of the control device 100.</li></ul>

0007In the refrigeration system according to FIG. 1, refrigerant vapor (suction gas) is drawn in by a compressor 1 at low pressure and low temperature and brought to a higher pressure. The refrigerant is heated. In the condenser 2, the liquefaction takes place at high pressure with heat being released to the environment. The liquefied refrigerant then reaches an expansion valve 4, in which it passes through a throttle point 3 (see FIG. 2). The throttling leads to a decrease in pressure and temperature with partial evaporation of the liquid refrigerant. The refrigerant then reaches the evaporator 5. There, the refrigerant is supplied to the environment to be cooled, that is to say, for example, the interior of a motor vehicle. The remaining part of the refrigerant is evaporated. From the evaporator 5, the refrigerant vapor (suction gas) reaches the suction gas chamber 6 of the expansion valve 4 and from there to the heat exchanger 5 ', which, for example is a fuel cooler of a motor vehicle. From the heat exchanger 5 ', the refrigerant vapor reaches the compressor 1; the cycle begins there again.

0008FIG. 2 shows, as a possible implementation, an electronically controllable thermostatic expansion valve, as already described in German patent application P 38 29 101.0, which was filed on August 27, 1988, there as an exemplary embodiment according to FIG. 8. The suction gas space 6 acts on one side of the membrane 8 in the control head 7. The closed space on the other side of the membrane 8 is filled with a control medium 9. Depending on how high the pressure p₂ and temperature t₂ in the suction gas chamber 6 are, a certain position of the membrane 8 is established, which determines the flow cross-section of the throttle point 3 via the transmission rod 10, at the lower end of which the throttle body 11 is provided. A spring 12 acts on the throttle body 11 from below, the compression of which can be adjusted by a spindle 13. If the temperature t₂ in the suction gas chamber 6 rises, this leads to an expansion of the control medium 9 and thus to a setting of the membrane 8 at a lower level. The flow cross section of the throttle 3 is increased accordingly. This increases the mass flow of the refrigerant; this leads to increased cooling and thus to a temperature drop in the suction gas space 6.

0009The expansion valve 4 according to FIG. 2 also has a PTC heating plate 90 (positive temperature coefficient), which can be electrically heated via a connecting line 91, so that in addition to the control described, a further external control is possible. If, for example, an electrical signal is applied to the heating plate 90, this leads to an expansion of the control medium 9 and thus to a further opening of the throttle point 3. The entire control head 7 is enclosed by an insulating cap 92. The PTC heating element has the property of limiting the heating output by increasing the resistance above a certain temperature, as a result of which the entire valve head is protected from excessive temperatures.

0010Via line 91 the driving signal arrives at the heating plate 90 from a control device 100. This derives the driving signal from one or more measurement signals, which is recorded, for example, by a sensor symbolically represented only by a point 101, which measures the temperature t 1 in the Detects line 102 between compressor 1 and condenser 2 or in the compressor itself.

0011The purpose of this control can be seen from FIGS. 3 and 4. It is the aim of the control of the expansion valve 4<ul id="ul0002" list-style="dash"><li>on the one hand to work with a relatively large performance-optimal "overheating" of the refrigerant vapor behind the evaporator 5, however</li><li>on the other hand, for safety reasons, a maximum temperature t_Max behind the compressor 1, for example of 150 °, not to be exceeded.</li></ul>

0012The "overheating" of the refrigerant vapor behind the evaporator 5 influences the part of the liquid refrigerant which remains dissolved in the oil added to the lubrication of the compressor and thus does not contribute to cooling. However, this proportion of refrigerant dissolved in the oil decreases with increasing overheating.

0013Investigations showed that the optimal overheating for maximum cooling capacity and effectiveness of the cooling system is at relatively high values ​​(approx. 8K according to FIG. 9).

0014Overheating refers to the temperature difference between the refrigerant vapor and the refrigerant present as a wet vapor mixture at the same pressure. For clarification, reference is first made to FIG. 3. There the curve K denotes the saturation line of the vapor or the liquid. It includes the wet steam area. The lines AB and A'B ', BC and B'C', as well as the lines CD and DA and DA 'denote the changes in state in the compressor 1, in the condenser 2, in the expansion valve 4 and in the evaporator 5 (including heat exchanger 5'). The temperature values ​​are entered for clarification. When designing the entire system, which is characterized by points A and B, the overheating is Δt_ü behind the evaporator 5, for example 5 ° C. Under the assumed operating conditions, this leads to the refrigerant vapor behind the compressor 1 having the maximum permissible temperature t_Max reached, for example, 150 ° C.

0015In another design, which is characterized by the points A 'and B', the overheating is Δt_ü after the evaporator 5, for example 3 ° C with the result that only a temperature of the refrigerant vapor of about 130 ° C is reached behind the compressor 1. FIG. 3 also shows the pressure drop Δp occurring when it passes through the expansion valve 4_EX and the pressure loss Δp occurring in the evaporator 5_V registered.

0016The problem with refrigeration systems in motor vehicles is that the operating conditions change constantly; the performance of the compressor, which is driven by the motor of the motor vehicle, changes continuously with the speed. In FIG. 4, typical operating states are shown in the enthalpy diagram, namely the operating states 64/2 (64 km / h; 2nd gear), 32/2 (32 km / h; 2nd gear) and IT (<u style="single">I.</u>dle-<u style="single">T</u>est = idle) entered. These are common driving values ​​in automotive engineering, where maximum cooling capacity = minimum internal temperature is required. For example, the IT operating state is critical, in which the vehicle is at a standstill and the compressor is driven by the vehicle engine running at idling speed. The ventilation of the condenser is the most unfavorable in this case, so that particularly high pressures and hot gas temperatures can occur. The driving state 32/2 is relatively unproblematic; The driving condition 64/2, in turn, is more problematic, as critical hot gas temperatures occur due to high compressor speeds.

0017FIG. 4 shows the typical curve of Δt for a conventional thermostatic expansion valve_ü, i.e. overheating, is drawn with a thin dash-dotted line. In the operating states IT and 64/2, the maximum permissible temperature t_Max of the refrigerant vapor reached. In the operating state 32/2, however, this is not the case. Here in point B '(see also Fig. 3) only a lower temperature is reached behind the compressor. For reasons of increasing the effectiveness of the refrigeration system in this operating state, it is therefore entirely desirable and also possible with a higher superheat Δt_ü to work, to the extent that B 'on the curve t_Max would lie.

0018The somewhat stronger and dashed line in FIG. 4 results from these requirements as the desired characteristic curve Δt_ü/Should. To put it simply, the requirement can be stated that, for reasons of effectiveness, the overheating should be, for example, 8K, depending on the refrigerant used, the type and amount of the cooling oil. However, it should be controllable down to one or two K in certain operating conditions in order to avoid inadmissibly high refrigerant vapor temperatures after the compressor and to avoid early evaporator icing.

00191 is used in the refrigerant circuit of Figure 1, an electronically controllable thermostatic expansion valve 4 and controls it in the manner shown on the basis of a signal that from the temperature t 1 in the line 102 with a sensor 101 behind the compressor 1 and before is derived from the capacitor 2, the desired characteristic can be realized with a control device 100 in which the maximum temperature t to be maintained there_Max is used for the control variable for controlling the throttle point 3 in the expansion valve 4 (cf. FIG. 2). FIG. 5 contains a simplified and systematic clarification of FIG. 1 from this point of view. It differs from FIG. 1 characterized in that instead of passing the line behind the evaporator 5 through the suction gas chamber 6 of the thermostatic expansion valve 4, the latter - as an expansion valve 104 - shown schematically at the between the evaporator 5 and the heat exchanger 5 'sensors 105 and 105', symbolized by a point, are provided, which determine the temperature t₂ and the pressure p₂ and output as a manipulated variable to the valve control systems of the expansion valve 104. This is nothing else than what is realized in the constructive design of the expansion valve 4 according to FIG. 2 in the form of the line 6, which in the construction shown has the effect that pressure p₂ and temperature t₂ on the membrane 8 are effective as manipulated variables. The action of the temperature t₂ or the pressure p₂ takes place on the control head 107 of the expansion valve 104. The control signal for the expansion valve 104 derived in the control device 100 acts via an actuating member (actuator) 190. In FIG. 2, this actuator is formed by the heating plate 91.

0020In addition, it is also provided that the control device 100 leads a further line to a switching coupling 108.

0021You can - so to speak as an "emergency brake" - in the event of too high a hot gas temperature t 1, which can no longer be regulated via the expansion valve 104, separate the driven motor 109 from the compressor 1. Furthermore, a further sensor 110, which determines the rotational speed (s) of the drive shaft, is provided on the drive shaft 109 'of the compressor 1 and is likewise supplied as an input variable to the control device 100 and can also be taken into account there to improve the control circuit side behavior.

0022A third sensor t₃ measures the evaporation surface temperature or the air temperature after evaporation to determine any evaporator icing. If the temperature falls below a minimum, the control unit 100 can also influence the actuator 190.

0023FIG. 6 (a) shows a generalization in which the control device 100 is part of a more extensive control device 200, for example a microprocessor, which also performs other tasks (compressor control, fan control). The control device 100 is used to control the overheating, ie to control the overheating Δt_ü behind the evaporator 5 such that the maximum permissible temperature t 1 behind the compressor 1 is its permissible maximum value t_Max does not exceed. The output of the control device 100 goes to the expansion valve 4 or 104, as well as the clutch 108. As a further input variable, the speed of the sensor 110 is also supplied to the control unit 100, which, however, can also control further units of the control device 200 with this information.

0024FIG. 6 (b) schematically shows a simple form of implementation, as a low-cost solution, in which the control device 100 is implemented only by a double T switch, one of which is a contact pair in the event of a change in the setting of the setpoint on the expansion valve 4 or 104 is closed, otherwise open, and the other pair of contacts 113 can be opened if the compressor coupling 108 has to be released.

0025Figure 6 (c) shows a generalization of the concept of Figure 6 (a), which assumes that p₂ and t₂ do not act directly on the expansion valve 4 or 104 in the sense of a thermostatic control as shown in FIG. 2, but via external Sensors reach the control device 100 and are evaluated there, and that the control device 100 then controls a simple injection valve 115 that can be actuated by an electric motor.

0026To deepen the understanding of the invention, FIG. 7 shows the control loop including the measurement points in front of and behind the evaporator 5 and the compressor 1. The expansion valve (or injection valve) 4, 104 is arranged in front of the evaporator 5, its throttle point (corresponding to throttle point 3 in Figure 2) is adjusted by an actuator 120 (corresponding to the transmission rod 10 in Figure 2). This actuating member 120 is activated in its adjusting movement by the valve control system 130 (corresponding to the control media 9 in FIG. 2 in the components 8-13). A control variable x (Δt_ü) supplied, which is derived in a logic circuit 133 from the signals which are determined for temperature t₂ and pressure p₂ by sensors 105, 105 '. The graphic design of the logic circuit 133 with the information "+" and "-" takes into account the fact that an increased pressure p₂ to reduce the flow cross-section of the throttle point (eg throttle point 3 in Fig. 2), however, an increased temperature t₂ - as a result of the reduction in overheating - should lead to an increase in the flow cross section of the expansion valve 104. So you can, after appropriate conversion of the signals for t₂ and p₂ in converters 134 and 135, form the logic circuit 133 as a subtractor.

0027A manipulated variable arrives at the second input 132 of the valve control system 130 and is derived from the two variables in the link 136 - again in the form of a subtractor, for example. One size corresponds to a fixed, albeit adjustable, default setting for a setpoint of the overheating Δt_ü. In the expansion valve according to Figure 2, the presetting is carried out by the position of the spindle 13 and thus the bias of the spring 12. At this fixed preset value, the second variable w₂, which arrives at the logic circuit 136 and which is controlled by a further actuator 140, the 2 corresponds to the heating plate 90, goes out. In Figure 7, however, this is shown in a very general way, without being tied to the implementation in the form of a PTC heating plate; It goes without saying that suitable converters must also be used to ensure that the values ​​that arrive at the logic circuit 136 are mutually compatible quantities.

0028The actuator 140 in turn receives its control from the control circuit 100. It is supplied with the size w₃, which is derived from the sensor 101 in the line 102 to the compressor 1 and corresponds to the prevailing temperature t₁, which is a certain maximum temperature t_Max, as discussed above, must not exceed. At the same time, further control variables can be given to the control device 100, for example w_e.g., for example as protection against icing, or z_e.g.; the latter can be used to find further disturbance variables and / or parameters, such as compressor speed, driving speed, ambient temperature, outside temperature, etc., as parameters in the control circuit.

0029FIGS. 8 (a), (b), (c) show the characteristic curves which have arisen in a practical implementation of the invention. Figure 8 (a) shows the heating power (N, measured in watts) of a heating plate 90 (according to Figure 2), which was used in a circuit according to Figure 1, Figure 5 or Figure 7. Figure 8 (b) shows the hot gas temperature behind the compressor 1 and Figure 8 (c) shows the temperature of the refrigerant vapor after the evaporator 5. The superheat Δt is in each case on the abscissa_ü plotted in ° K. With decreasing power N of the heating plate, the overheating Δt increases_ü. At the same time, the temperature r increases behind the compressor. The temperature after the evaporator drops. So there must always be a control or regulation such that, on the one hand, t 1 does not exceed its maximum value, but on the other hand, while maintaining this maximum value, maximum cooling, ie the lowest possible temperature t 2, is obtained.

0030Figure 9 (a) shows the evaporator power Q_V (kW) and temperature t₁ behind the compressor depending on the overheating Δt_ü in ° K under various operating conditions (t_Le= Air temperature upstream of the evaporator; P_RCa= Pressure of the heated coolant vapor behind the compressor 1). The curves according to FIG. 9a, in particular the middle one, show that at an overheating temperature of Δt_ü about 8K a maximum of the evaporator power Q_V given is. From a comparison of the curves according to FIG. 9 (a) and (b), for example for t_Le= 25 ° C, P_RCa= 20bar, it follows that the maximum of the evaporator power Q_V with overheating of approx. 8K corresponds to a hot gas temperature behind the compressor of approx. 130 ° C. These are the optimal conditions of the refrigerant circuit, which should be maintained by the control and regulation according to the invention as possible in all changing operating conditions.

0031Figure 10 now shows the behavior of an air conditioning system according to Figure 1, 5 or 7 in the vehicle under various operating conditions, with t 1 in ° C in ° C, that is, the temperature behind the compressor, and in the lower range Δt_ü in ° K, i.e. the overheating. The diagrams show a typical behavior of a vehicle air conditioning system at an ambient temperature of 40 ° C, at 40% relative humidity and a solar load on the vehicle of 1000 W / m². The individual driving conditions are: IT = idle; 2/32 = 2. Speed ​​step at 32 km / h; D / 64 speed level D at 64 km / h; etc. The indication "speed step" means the shift position of the automatic transmission. The dash-dotted curve A represents the course of the temperature t₁ behind the compressor when using a conventional thermostatic expansion valve <u style="single">without</u> Control depending on the overheating. It shows that both relatively quickly in idle, after about 15 minutes and then again towards the end of the test, in 2nd gear at 64 km / h, i.e. relatively high engine speed, the temperature t₁ of the hot gas after the compressor becomes impermissibly high.

0032Curve B, drawn in a thick line, shows the course when actuation according to the invention. It can be seen that the temperature t 1 due to the regulation of the overheating below the maximum permissible value t_Max remains. One can use a higher Δt as a setpoint_ü going out, because if it gets too high you can curtail it. The hatched area is the area in which regulation or control can take place when a controllable thermostatic expansion valve is used. The overheating is entered in the lower area of ​​FIG.

0033FIG. 11 shows the course of the temperature t under the same operating conditions_Lik in the vehicle in the headspace. The dash-dotted line drawn in strong lines shows the course of the curve in a conventional one<u style="single">Not</u> controllable thermostatic expansion valve; curve B, which is drawn in a strong line, gives the profile of the headspace temperature t_Lik when controlling the thermostatic expansion valve depending on the overheating. It shows that the lowering of the temperature in the invention (curve B) takes place more strongly than in the prior art, but that at low temperatures, which would result in an excessively high temperature downstream of the compressor, as a result of the curtailment of the Overheating there is no further reduction in the headspace temperature, but a stabilization with a slight increase occurs, which is still in the range that is acceptable at all times, especially since this not only avoids excessive stress on the air conditioning system, but possibly also excessive cooling, which may be perceived as unpleasant.

0034FIG. 12 shows a simple circuit in a low-cost version, as indicated in FIG. 6 a or b, somewhat more in detail. The first safety chain consists of switching elements according to the state of the art. In the duo switch B1, the normally closed contact b₁₁ opens when the hot gas temperature is too high, for example at t₁> 150 ° C. In the duo switch B2, the normally closed contact b₂₁ opens when the evaporator freezes (anti-icing protection), for example<img file="EP0443099A2_D0001.tif" /> <0 ° C. The switch B_e.g. with further break contact b_e.g. can take over further safety functions. M1 denotes the coupling of the compressor to its drive. Are the normally closed contacts b₁₁, b₂₁, b_e.g. closed, so there is a coupling of the compressor 1 to the drive motor. In addition, the coil K₁ receives a relay current and thus closes the contact K₁ in a second chain, which is used to regulate the overheating by the expansion valve. The contacts b₁₂ and b₂₂ of the duo switches B1 and B2 and the switches B3 and Bx are open during normal operation of the refrigeration system, so that the PTC heating plate 90 of the thermostatic expansion valve 4 or 104 is not energized according to Figure 2. In this case the valve works with a high, performance-optimized overheating. If, on the other hand, one of the contacts b₁₂, b₂₂, b₃ or b_x closes, b₁₂ of the duo switch B1 e.g. at a hot gas temperature t₁> 130 ° C or b₂₂ of the duo switch B2 at an evaporator temperature t₃ <3 ° C or the switches B3 and Bx due to other critical operating conditions of the refrigeration system, then the PTC heating plate 90 of the expansion valve 4 or 104 supplied with current. In this second case there is an increase in the flow cross section of the throttle 3 and thus a reduction in the overheating. The lower overheating Δt_ü of the refrigerant after the evaporator causes the temperature t 1 to decrease again and the risk of evaporator icing is reduced by decreasing evaporator output.

0035The switches B3 and BX can be used to pick up additional influencing variables. At the same time, additional switches can be provided. The circuit according to FIG. 12 can be expanded as desired, as indicated by the arrow at the top right in FIG. 12.