EP1325269A1

EP1325269A1 - Vehicle air conditioning device using a supercritical cycle

Info

Publication number: EP1325269A1
Application number: EP01980592A
Authority: EP
Inventors: Mohamed Ben Yahia
Original assignee: Valeo Climatisation SA
Current assignee: Valeo Systemes Thermiques SAS
Priority date: 2000-10-12
Filing date: 2001-10-09
Publication date: 2003-07-09
Anticipated expiration: 2021-10-09
Also published as: FR2815397B1; DE60118588D1; US6786057B2; AU2002212405A1; EP1325269B1; US20030159452A1; DE60118588T2; WO2002031416A1; JP2004511747A; FR2815397A1; ES2261492T3

Abstract

The invention concerns an air conditioning unit wherein the expansion device (4) of the refrigerating fluid loop is controlled to set the internal heat exchanger efficiency θ, represented by the equation: θ = Tec Tse/ Tsr Tse, wherein: Tec, Tse and Tsr are respectively temperatures at the compressor input (1), and at the evaporator output (5) and at the fluid cooler output (2), at a reference value such that the fluid leaves the evaporator in a completely gaseous state and without overheating.

Description

Vehicle air conditioning device using a supercritical cycle

The invention relates to an air conditioning device, in particular for the passenger compartment of a vehicle, and to a method for controlling a loop of refrigerant fluid in such a device, said loop containing a compressor capable of receiving the fluid in the state gaseous and compressing it, a fluid cooler capable of cooling the fluid compressed by the compressor, at substantially constant pressure, by transferring heat to a first medium, a pressure reducing valve capable of lowering the pressure of the fluid leaving the fluid cooler bringing it at least partly to the liquid state and an evaporator capable of passing the liquid state of the fluid in the gaseous state coming from the pressure reducer, at substantially constant pressure, by taking heat from a second medium for cooling the space to be conditioned, the fluid thus vaporized then being sucked in by the compressor, the loop also containing an internal heat exchanger allowing the circulating fluid culant in a first path of the internal exchanger, between the fluid cooler and the expansion valve, c-ced-r heat to the fluid flowing in a second path of the internal exchanger, between the evaporator and the compressor.

To avoid the harmful effects on the environment of fluorinated compounds traditionally used as refrigerants in the air conditioning of motor vehicles, the use of carbon dioxide C0 ₂ is recommended.

This compound has a relatively low critical pressure, which is exceeded during compression of the fluid by the compressor, so that the fluid is then cooled without phase change by the fluid cooler which replaces the condenser of the traditional loop. In the absence of a phase change, only the lowering of the temperature of the fluid in the cooler allows dissipation of thermal energy. This dissipation generally takes place in an atmospheric air flow, it is necessary that the temperature of the fluid entering the cooler be significantly higher than atmospheric temperature. This is why we use the internal heat exchanger, which warms the fluid when it circulates between the evaporator and the cooler and cools it when it circulates between the cooler and the expansion valve.

The efficiency η of the internal heat exchanger, represented by equation [1]

= ± SΞ i_à [1] ^x sι sé

in which T _ec , T _se and T _sr are respectively the temperatures at the inlet of the compressor, at the outlet of the evaporator and at the outlet of the cooler, is a decreasing function of the flow of fluid passing through it, according to l equation [2] η = aQ ^b [2], a and b being constants characteristic of the internal exchanger.

The above is only true when the internal heat exchanger receives fluid in the gaseous state from the evaporator. If on the contrary it receives fluid in the liquid state, its effectiveness is greatly reduced.

The object of the invention is to optimize the operation of the loop so as to avoid this drawback.

On the other hand, for the air flow cooled by the evaporator to be at a homogeneous temperature, the evaporator must not have an overheating zone, in other words that the fluid vaporizes until the end of its path in one evaporator.

Another object of the invention is to satisfy this condition. The invention relates in particular to a method of the kind defined in the introduction, and provides for monitoring a first condition capable of revealing the presence of fluid in the liquid state in said first path, and for reducing the flow rate of the fluid in the loop when said first condition is satisfied.

This regulation mode, based on a thermodynamic principle, allows rapid stabilization of the loop regime, without oscillation. In particular, it prevents the appearance of a cold spike when the vehicle accelerates.

Optional, complementary or alternative features of the invention are set out below:

- Said first condition consists in that the efficiencyη of the internal heat exchanger, represented by the equation

[i] ^:

„_ T ^A θc - T ^• ' ^• se r -i T 1 - ™ Z ^ τ ^~ ~ ^[1]

J- sr ¹ se

in which T _ec , T _se and T _sr are respectively the temperatures at the inlet of the compressor, at the outlet of the evaporator and at the outlet of the cooler, is less than a reference value η ₀ .

- We also monitor a second condition capable of revealing the existence of an overheating zone in the evaporator, and increasing the flow rate of the fluid in the loop when said second condition is satisfied.

- Said second condition consists in that the efficiencyη as defined in claim 2 is greater than or equal to a reference value η ₀ .

- The flow rate of the fluid is adjusted substantially to the maximum value compatible with an efficiency η not less than the reference value. - We adopt as reference value, whatever the value of the flow, the value η _m taken by the efficiency η when the flow is maximum and that said second path does not contain any fluid in the liquid state.

- We adopt as reference value, for a determined value Q _p of the flow, the value η _p taken by the efficiency η when said second path does not contain any fluid in the liquid state.

The flow rate is adjusted by acting on the regulator.

- To evaluate η on the basis of equation [1], a value measured by means of a "sensor" in thermal contact with the fluid is used for at least one of said temperatures.

- To evaluate η on the basis of equation [1], we use to represent T _sê the temperature of an air flow having swept the evaporator and constituting said second medium.

- We compare T _ec with a set value T _{ec cons} such that

and it is considered that η is less and greater than the reference value when T _ec is less and greater than said reference value respectively.

- The compressor is of the variable displacement type with external control.

- The compressor compresses the fluid to a supercritical pressure.

The invention also relates to an air conditioning device, in particular for the passenger compartment of a vehicle, suitable for implementing the method as defined above, comprising a coolant loop as defined, means for monitoring to monitor a first condition capable of revealing the presence of fluid in the liquid state in said second path, and optionally a second condition capable of revealing the existence of an overheating zone in the evaporator, and means for controlling the flow of the fluid in the loop according to the result of this monitoring.

The device according to the invention can include at least some of the following features:

10

- The monitoring means include means for evaluating the temperatures T _ëc , T _se and T _sr respectively at the inlet of the compressor, at the outlet of the evaporator and at the outlet of the cooler, means for calculating from

15. these the efficiency η of the internal heat exchanger, on the basis of equation [1] -

20 and means for comparing the efficiency η with a reference value.

- Loop and to define from it said reference value.

25

- The means for evaluating said temperatures comprise at least one temperature sensor in thermal contact with the fluid.

30 - The means for evaluating the temperature T _se include a temperature sensor in thermal contact with an air flow having swept the evaporator.

The features and advantages of the invention will be explained in more detail in the description below, with reference to the accompanying drawings.

FIG. 1 is a graph showing the variation in the efficiency η as a function of the flow rate Q of the fluid, for an exchanger typical internal heat usable in the process and in the device according to the invention.

Figure 2 is a circuit diagram of a coolant loop belonging to a device according to the invention.

Figure 3 is a block diagram illustrating the method and the device according to the invention.

Figure 2 shows the known structure of an air conditioning loop of the passenger compartment of a motor vehicle using carbon dioxide as a refrigerant in a supercritical thermodynamic cycle. A compressor 1 compresses the fluid to bring it to the supercritical state, after which the "luide passes through ^~ a ^~ fluid cooler 2. The fluid leaving the cooler 2 travels along a path 3-1 of a heat exchanger internal 3, then goes through a pressure reducer, 4 to reach a 5 "evaporator. Downstream of the evaporator, the fluid passes through a reservoir 6 then travels a path 3-2 of the internal exchanger 3 before returning to the compressor 1. The paths 3-1 and 3-2 are located side by side and at against the current, that is to say that the input el and the output si of the path 3-1 are adjacent respectively to the output s2 and to the input e2 of the path 3-2. Under these conditions, we define for the internal exchanger an efficiency η given by equation [1]

η = ^{± ec} _ ^{X sé} [i]

^T sr ^T se

in which T _ec , T _se and T _sr are respectively the temperatures of the fluid at the inlet of the compressor 1 (or at the outlet s2), at the outlet of the evaporator 5 (or at the inlet e2) and at the outlet from cooler 2 (or inlet el).

It is noted that, when the internal exchanger is traversed exclusively by fluid in the gaseous state, the efficiencyη is a decreasing function of the mass flow rate Q of the fluid in the loop, according to a curve of which an example is represented by the curve C ₂ in Figure 1. This curve extends from point A to point B corresponding respectively to the minimum and maximum flow rates that can be obtained in the loop. Between these, it only depends on the geometrical characteristics of the internal exchanger and the nature of the fluid.

The above condition is only satisfied if the thermal load of the loop is sufficient to allow the evaporator to completely vaporize the fluid up to its maximum flow rate. Otherwise, the efficiency follows the curve C ₁ only up to a point L corresponding to the limit flow that can be vaporized in the evaporator. Beyond this limit flow, the internal exchanger receives fluid in the liquid state from the evaporator, which suddenly decreases the efficiency depending on the curve section (- ₂ approximately-vertical, "followed" by a substantially horizontal section C ₃ for which the efficiency is practically zero.

In Figure 3, which shows an air conditioning device according to the invention, there is the loop of Figure 1, composed of elements 1 to 6, to which are added a flow sensor 7 placed upstream of the evaporator 5 so as to measure the mass flow rate of the fluid passing through it in the liquid state, as well as two temperature sensors 10 and 11 associated with respective reading blocks 12 and 13, intended to measure the temperature of the fluid respectively between the outlet of the fluid cooler 2 and the inlet el of the path 3-1 of the internal exchanger 3, and between the outlet s2 of the path 3-2 of the latter and the inlet of the compressor 1. Another sensor 14, associated with a reading block 15, measures the temperature of an air flow F after it has passed through the evaporator 5 under the action of a blower 16, this air flow being intended to be sent into the cabin of the vehicle to adjust the temperature prevailing therein.

According to the invention, the temperature T _sr at the outlet of the cooler 2 (or at the inlet el) and the temperature of the cooled air are sent by the blocks 12 and 15 respectively to a processing block 17 also connected to the sensor of flow 7, which calculates from these measured values - with if necessary a correction to take account of the difference between the temperature of the cooled air and the temperature T _se at the outlet of the evaporator 2 (or at 1 ' input e2) - a setpoint T _{ec cons} that the temperature T _ec of the fluid should have at the input of compressor 1 (or at output s2) so that the efficiency η of the internal exchanger 3, calculated according to l 'equation [1], takes a reference value η _p equal to the ordinate of point P of the curve C ₁ which has the abscissa the flow rate Q _p measured by the sensor 7. The real value of T _ec , supplied by the block 13, is compared to this set value by a comparator 18. If T _ec <T _{ec cons} , this means that the actual efficiency is lower than the reference value, and therefore that the representative point of --l e efficiency on the graph of ia ^~ Figure 11 is below the curve C _x , so on one of s sections C ₂ and C ₃ , indicating the presence of liquid in the internal exchanger. The comparator 18 ′ then generates an error signal 19 which is transmitted to a regulator 20, which acts on a control block 21 which controls the regulator 4, so as to reduce the flow rate.

If on the contrary T _ec = T _{ec cons} , this means that the internal exchanger contains fluid entirely in the gaseous state, and that the point representing the efficiency on the graph of FIG. 1 is on the curve C ₁ . However, this equality does not make it possible to distinguish between the following three cases: either the representative point is the point L defined previously, or the representative point is located to the left of the point L, or the point L does not exist, the thermal load of the loop being sufficient so that the internal exchanger does not receive liquid whatever the flow rate of the fluid. If it is desired that the evaporator does not have a superheating zone, or that its superheating zone is minimal, it is then possible to control the regulator 4 so as to increase the flow rate by a small increment. Regulation will thus be carried out around point L if it exists, and if not, the flow will be stabilized at its maximum value corresponding to point B, ensuring a minimum overheating zone. As a variant, the mass flow rate of the fluid can be determined by other means than the sensor 7. For example, the volume flow rate of the fluid in the compressor can be determined from the displacement and the speed of the latter, and the mass flow is deduced therefrom taking into account the density of the fluid, which is a function of the nature of the latter, of the temperature and of the pressure.

In another variant, the fluid flow rate is not taken into account, and the efficiency η is compared to a reference value η _m equal to the ordinate of point B. The inequality η <η _m then means that the point representative of the efficiency is found on one of the sections C and C ₃ , below the point K of the section C ₂ having the abscissa η ^, requiring a reduction — of the ^~ "flow rate. If, here again, it is desired avoid or minimize - the evaporator overheating zone, the regulator will be controlled so as to maintain the efficiency at the value η _m , thus achieving regulation around point K, or bringing the operating point to point B The flow corresponding to point K is very close to that corresponding to point L.

Of course, instead of calculating a setpoint value T _{ec cons} using the reference value of the efficiency of the internal exchanger, one could directly compare the real efficiency, η with the reference value, and produce the signal based on this comparison. These two procedures are strictly equivalent.

Furthermore, the invention is not limited to monitoring the efficiency of the internal exchanger as an indicator of the presence of fluid in the liquid state in the first path or of the existence of a overheating zone in the evaporator. These phenomena can be detected by other means, for example using specific sensors assigned to the internal exchanger and / or the evaporator.

Although the invention has been described in detail in connection with the use of carbon dioxide, it finds a advantageous application with any refrigerant, in particular operating according to a supercritical cycle and requiring an internal heat exchanger.

Claims

claims

1. Method for controlling a loop of refrigerant fluid in an air conditioning device, in particular of the passenger compartment of a vehicle, said loop containing a compressor (1) capable of receiving the fluid in the gaseous state and of compressing it , a fluid cooler (2) capable of cooling the fluid compressed by the compressor, at substantially constant pressure, by transferring heat to a first medium, a pressure reducer (4) capable of lowering the pressure of the fluid leaving the fluid cooler by bringing it at least in part to the liquid state and an evaporator (5) suitable for passing the gaseous fluid to the liquid state coming from the pressure reducer, at substantially constant pressure, by withdrawing from the heat-of a second environment to cool the space to be cooled, - the fluid _r .so vaporised then being sucked by the compressor, the loop further containing an internal .chaleur exchanger (3) to the fluid circulating in .permettant a first path (3-1) of the internal exchanger, between the fluid cooler and the expansion valve, to yield heat to the fluid flowing in a second path (3-2) of the internal exchanger, between the evaporator and the compressor, characterized in that a first condition capable of revealing the presence of fluid in the liquid state in said second path is monitored, and that the flow rate of the fluid in the loop is reduced when said first condition is satisfied.

1

2. Method according to claim 1, in which said first condition consists in that the efficiency η of the internal heat exchanger, represented by the equation

[1]

T - T η ≈ - = 2≤ SÉ [i] sr - "- se

in which T _ec , T _se and T _sr are respectively the temperatures at the inlet of the compressor, at the outlet of the evaporator and at the outlet of the cooler, is less than a reference value η _Q.

3. Method according to one of claims 1 and 2, wherein there is further monitored a second condition capable of revealing the existence of an overheating zone in the evaporator, and increasing the flow rate of the fluid in the loop when said second condition is satisfied.

4. Method according to claim 3, in which said second condition consists in that the efficiency η as defined in claim 2 is greater than or equal to a reference value η ₀ .

5. Method according to one of claims 2 and 4, wherein the fluid flow rate is adjusted substantially to the maximum value compatible with an efficiency η not less than the reference value.

6. Method according to one of claims 2, 4 and 5, wherein one adopts as reference value, whatever the value of the flow, the value η _m taken by the efficiency η when the flow is maximum and that said second path does not contain any fluid in the liquid state.

7. Method according to one of claims 2, 4 and 5, wherein one adopts as reference value, for a determined value Q _p of the flow, the value η _p taken by the efficiency η when said second path does not contain of fluid in the liquid state.

8. Method according to one of claims 2 and 4 to 7, wherein the flow rate is adjusted by acting on the regulator.

9. Method according to one of claims 2 and 4 to 8, wherein, to evaluate η on the basis of equation [1], one uses for at least one of said temperatures a value measured by means of a sensor (10, 11) in thermal contact with the fluid.

10. Method according to one of claims 2 and 4 to 9, wherein, to evaluate η on the basis of equation [1], we used to represent T _se the temperature of an air flow (F) having swept the evaporator and constituting said second medium.

11. Method according to one of claims 2 and 4 to 10, in which T _ec is compared to a set value T _{ec cons} such that

T ec cons - T se

Tic

^T sr ~ ^T se and it is considered that η is less and greater than the reference value when T _ec is less and greater than said reference value respectively.

-12. Method according to one of the preceding claims, in which the compressor is of the type with variable displacement with external control.

13. Method according to one of the preceding claims, wherein the compressor compresses the fluid to a supercritical pressure.

14. Air conditioning device, in particular of the passenger compartment of a vehicle, suitable for implementing the method according to one of the preceding claims, comprising a loop of refrigerant fluid as defined in claim 1, monitoring means (10-15, 17) for monitoring a first condition capable of revealing the presence of fluid in the liquid state in said second path, and optionally a second condition capable of revealing the existence of an overheating zone in the 'evaporator, and means (19, 20, 21) for controlling the flow rate of the fluid in the loop according to the result of this monitoring.

15. Device according to claim 14, in which the monitoring means comprise means (10-15) for evaluating the temperatures T _ec , T _se and T _sr respectively at the inlet of the compressor, at the outlet of the evaporator and at the outlet of the cooler, ^• means (17) for calculating at from these the efficiency η of the internal heat exchanger, based on equation [1]

η = l ^{βc "} l ^Ξé c ^i] ,

¹ sr - ¹ se

and means for comparing the efficiency η with a reference value.

16. Device according to claim 15, further comprising means (7) for determining the flow rate of the fluid in the loop and for defining therefrom said reference value.

17. Device according to one of claims 15 and 16, in which the means for evaluating said temperatures comprise at least one temperature sensor (10, 11) in thermal contact with the fluid.

18. Device according to one of claims 15 to 17, wherein the means for evaluating the temperature T _se comprises a temperature sensor (14) in thermal contact with an air flow (F) having swept the evaporator.