WO2009103472A1

WO2009103472A1 - Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit

Info

Publication number: WO2009103472A1
Application number: PCT/EP2009/001063
Authority: WO
Inventors: Bernd Heinbokel
Original assignee: Carrier Corporation
Priority date: 2008-02-20
Filing date: 2009-02-16
Publication date: 2009-08-27
Also published as: EP2242966B1; EP2242966A1

Abstract

A method of controlling a heat-rejection heat exchanging side of a refrigerant circuit comprises the steps of providing a heat-rejection heat exchanging side comprising at least one heat-rejection heat exchanger (4), wherein a refrigerant is cooled against a secondary medium; obtaining a refrigerant condensation temperature (T_c) in the heat-rejection heat exchanging side; obtaining a refrigerant outlet temperature (T_ro); obtaining a secondary medium inlet temperature (T_smi); calculating a relative subcooling value by relating the refrigerant condensation temperature (T_c), the refrigerant outlet temperature (T_ro), and the secondary medium inlet temperature (T_smi); and controlling the relative subcooling value with regard to a reference relative subcooling value.

Description

K 74 418/8

METHOD OF CONTROLLING A HEAT-REJECTION HEAT EXCHANGING SIDE OF A REFRIGERANT CIRCUIT

The invention relates to a method of controlling a heat-rejection heat exchanging side of a refrigerant circuit and a refrigerant system adapted to carry out said method.

Refrigerating systems using a condenser for transferring heat from the refrigerant to the environment are well-known in the art. It is also known to manually set a desired amount of subcooling for the refrigerant in the condenser. Sub- cooling shall herein be understood as cooling the refrigerant below its condensation temperature. Taking into account said desired amount of subcooling (also referred to as reference subcooling value), the subcooling of the refrigerant is then controlled by a control algorithm. If the reference subcooling value is set too low, gaseous state refrigerant may exit the condenser (as the refriger- ant temperature is generally not perfectly uniform upon exiting the condenser), which can lead to instabilities in the condenser control. Setting the reference subcooling value too high leads to an unnecessarily high refrigerant pressure in the condenser. Both scenarios lead to an energetically inefficient operation of the condenser and the overall refrigerating system.

Accordingly, it would be beneficial to provide an efficient control of a heat-rejection heat exchanging side of a refrigerant circuit, which is applicable to a wide range of operating conditions and a great variety of heat exchanger constructions.

Exemplary embodiments of the invention include a method of controlling a heat-rejection heat exchanging side of a refrigerant circuit. The method comprises the steps of providing a heat-rejection heat exchanging side comprising at least one heat-rejection heat exchanger, wherein a refrigerant is cooled against a secondary medium; obtaining a refrigerant condensation temperature

[l:\8\74\74418\090216_application_text.odt] 2009-02-16 09:14 in the heat-rejection heat exchanging side; obtaining a refrigerant outlet temperature; obtaining a secondary medium inlet temperature; calculating a relative subcooling value be relating the refrigerant condensation temperature, the refrigerant outlet temperature, and the secondary medium inlet temperature; and controlling the relative subcooling value with regard to a reference relative subcooling value.

Embodiments of the invention are described in greater detail below with reference to the Figures wherein:

FIG. 1 shows a schematic of a portion of a refrigerant circuit comprising a condenser;

FIG. 2 shows a schematic of a portion of another refrigerant circuit comprising a condenser and a subcooling unit;

FIG. 3 shows an exemplary function of the temperature of a refrigerant and of a secondary medium over the length of a condenser.

FIG. 1 shows a portion of a refrigerant circuit comprising a set of compressors 2, a condenser 4, and an expansion device 6. The set of compressors 2 and the condenser 4 as well as the condenser 4 and the expansion device 6 are connected by refrigerant conduits, respectively. As indicated by the arrows in FIG. 1, a refrigerant is flown through the set of compressors 2, to the condenser 4, through the condenser 4, to the expansion device 6, and through the expansion device 6. The set of compressors 2 represent the inlet to the heat-rejection heat exchanging side of the refrigerant circuit, whereas the expansion device 6 represents the outlet of the heat-rejection heat exchanging side of the refrigerant circuit.

The remainder of the refrigerant circuit is not shown in FIG. 1, as its concrete structure is irrelevant to the invention. In the simplest case the refrigerant is flown through the expansion device 6, through an evaporator, and back to the set of compressors 2. The remainder of the refrigerant circuit may also com- prise two separate portions, for example a refrigerator portion and a freezer

H:\8\74\74418\090216_application_text.odt] 2009-02-16 09:14 portion. In this case the refrigerant flow is split up after expansion device 6, advancing to two different evaporators, wherein each of the evaporators may be associated with an additional expansion device. There may also be an additional compressor on the way back from the evaporator of the freezer portion to the set of compressors 2. The invention may also be applied to a refrigerant circuit using CO₂ as a refrigerant, which may be operated transcritically at times. In this case the refrigerant may be flown to a refrigerant collector after the expansion device 6. From there it may reach an evaporator through a second expansion device or it may be split up to reach a plurality of evaporators through a plurality of expansion devices, wherein the evaporators can in any combination belong to a refrigerator portion or to a freezer portion of the refrigerant system. In this case, the invention is generally applied to the refrigerating system, when it is operated subcritically, i.e. when condensation takes place in the heat-rejection heat exchanging side of the refrigerant circuit. The invention may also be applied to any other design of the remainder of the refrigerant circuit.

The condenser 4 is shown as an air-cooled condenser with a fan (illustrated schematically in FIG. 1) blowing air over a structure, through which the refrigerant is flown. That means that air is acting as a secondary medium in the exem- plary embodiment of FIG. 1. A controller (not shown) is associated with the heat-rejection heat exchanging side of the refrigerant circuit. The controller is connected to a plurality of sensors for receiving a number of momentary values of system parameters. Based on these values, the controller carries out a control algorithm and controls one or more actuators in order to take appropriate measures.

In the exemplary embodiment, three sensors are of particular relevance. A temperature sensor measures the ambient air temperature, particularly the air temperature in the shade in close proximity to the condenser. This sensed temper- ature represents the temperature the air initially has when interacting with the refrigerant in the condenser 4, i.e. when blown at the condenser 4 by the fan. This value is hereinafter referred to as secondary medium inlet temperature

I smi-

I \8\74\74418\090216_apphcatιon_text odt] 2009-02-16 09 14 A second sensor measures the temperature of the refrigerant upon leaving the condenser 4, herein referred to as refrigerant outlet temperature T_ro. This sensor may be located at the end - in refrigerant flow direction - of the condenser 4, somewhere in the refrigerant conduit between condenser 4 and ex- pansion device 6, and particularly right before the expansion device 6.

A third sensor measures the refrigerant pressure in the heat-rejection heat exchanging side of the refrigerant circuit. As the refrigerant pressure is nearly constant between the compressor 2 and the expansion device 6, this refriger- ant pressure sensor can be located anywhere between these two elements. It can be located in the conduit between the set of compressors 2 and the condenser 4, in the condenser 4, in the conduit between the condenser 4 and the expansion device 6, and particularly right before the expansion device 6. It is apparent that a combined temperature and pressure sensor can be used between the condenser 4 and the expansion device 6 in order to sense the refrigerant outlet temperature T_ro and the refrigerant pressure. Using given knowledge about the properties of the refrigerant, the controller can calculate the refrigerant condensation temperature T_c from the momentary refrigerant pressure condition.

The controller receives the obtained values of the refrigerant condensation temperature T_c, the refrigerant outlet temperature T_ro, and the secondary medium inlet temperature T_smi. It relates these values in order to yield a relative sub- cooling value. In the exemplary embodiment of FIG. 1, the controller computes the difference between the refrigerant condensation temperature T₀ and the refrigerant outlet temperature T_ro. According to the definition above, this difference represents an actual subcooling value for the refrigerant. Furthermore, the controller computes the difference between the refrigerant condensation temperature T_c and the secondary medium inlet temperature T_smi. As the secondary medium inlet temperature is the minimum temperature that the refrigerant could be cooled down to using the condenser 4, the computed difference is referred to as maximum potential subcooling value of the refrigerant. A relative subcooling value RSC is then calculated by dividing the actual subcooling value by the maximum potential subcooling value. These operations can be ex- pressed by the formula

(l:\8\74\74418\090216_application_text.odt) 2009-02-16 09:14

In other words, the relative subcooling value is a measure of which portion of the possible subcooling is achieved by the present refrigerating system under the momentary system conditions.

A control algorithm relates the relative subcooling value, and potentially other relative subcooling values at previous points in time, to a reference relative subcooling value. Many control algorithms exist that calculate appropriate measures from the momentary control error (defined as the difference between the momentary relative subcooling value and the reference relative subcooling value) - possibly in combination with control errors at previous instants in time - , with the options for appropriate measures being described below. As these control methods are well-known in the art, a detailed description thereof shall be omitted for brevity. As an example, any combination of P, I, and D elements can be used to generate an appropriate control algorithm. The parametrization of the selected type may be adapted to the particular system under consideration. Using delay elements (T elements) may also be considered when design- ing an appropriate control algorithm. A Pl control algorithm has been found to yield good results.

In response to the control algorithm results the controller may control the performance of the set of compressors 2 and/or the flow rate through the expan- sion device 6, using those as actuators. Each of these measures or their combination will have an influence on the refrigerant pressure in the heat-rejection heat exchanging side of the refrigerant circuit, thereby affecting the refrigerant condensation temperature T₀. This closes the control loop, as the refrigerant condensation temperature T_cis obtained and fed back to the controller. In other words, the controller has appropriate means at its disposal to have an influence on the refrigerant condensation temperature T₀, which affects the momentary relative subcooling value, which in turn makes it possible for the controller to bring the relative subcooling value to a desired reference relative subcooling value.

|l \8\74\74418\090216_apphcatιon_text odt] 2009-02-16 09 14 The refrigerant condensation temperature T_c may also be influenced by controlling the refrigerant amount in the heat-rejection heat exchanging side of the refrigerant circuit. This may be effected by providing a bypass-structure to the heat-rejection heat exchanging side of the refrigerant circuit, wherein the bypass-structure comprises a refrigerant collector. The refrigerant collector may be used to temporarily store refrigerant deducted from the refrigerant circuit. This results in an efficient control of the refrigerant amount and, therefore, the refrigerant condensation temperature T_c in the heat-rejection heat exchanging side of the refrigerant circuit. Valves or any other suitable devices may be provided to control the amount of refrigerant in the refrigerant collector.

The reference relative subcooling value in the exemplary embodiment of FIG 1 lies between 0.5 and 0.7.

A number of modifications may be made to the refrigerating system depicted in FIG. 1, some of which will be discussed hereinafter. Instead of a set of compressors 2, the refrigerant circuit may comprise only one compressor, whose performance may be adjustable. Instead of air, the refrigerant in the condenser may be cooled against another secondary medium. Just to name some examples, the secondary medium could be air enriched with water particles, water or a brine. As is apparent to a person skilled in the art, appropriate means would be necessary to sense the temperature of the secondary medium shortly before starting the heat exchange with the refrigerant. In case the secondary medium is part of a whole secondary medium circuit, adequate circulating means (e.g. a pump), heat-rejection heat exchanging means, and guiding means (e.g. secondary medium conduits) may be necessary for operating the secondary medium circuit.

Even though the easily interpretable nature of the formula given above may be lost, there are different options for relating the refrigerant condensation temperature Tc, the refrigerant outlet temperature T_ro, and the secondary medium inlet temperature T_smι and to generate an aggregate metric, which can be controlled with regard to a reference value. For example, the numerator and the denomin- ator may be interchanged or an aggregate metric may be computed by dividing

[l:\8\74\74418\090216_application_text.odt] 2009-02-16 09:14 (Tro-Tsmi) by (Tc-Tsmi). It is also possible to rewrite the above formula in order to yield a reference refrigerant condensation temperature T_c,_ref, which may be used by the control algorithm as a direct reference point for the refrigerant condensation temperature T_c. This deduced formula is

c-^ref \ -RSC_rqf

FIG. 2 shows a portion of another refrigerant circuit which differs from the portion of FIG. 1 in that the heat-rejection heat exchanging side of the refrigerant circuit does not only comprise a condenser 4, but also a subcooling unit 8. The refrigerant flows through the condenser 4 before flowing through the subcooling unit 8. As the name subcooling unit suggests, the refrigerant is cooled down further therein after being condensed in the condenser 4. In the subcooling unit 8, the refrigerant is also cooled against a secondary medium. The sec- ondary medium of the subcooling unit 8 may be the same as or different from the secondary medium of the condenser 4. As the temperature of the secondary medium of the subcooling unit 8 before interacting with the refrigerant determines the maximum potential subcooling value, said temperature is obtained by a temperature sensor or other appropriate means and used as T_smι by the controller. Accordingly, the refrigerant outlet temperature T_ro is obtained by a sensor between the subcooling unit 8 and the expansion device 6, particularly right before the expansion device 6.

As is apparent from FIG. 2, having a condenser and a subcooling unit in the heat-rejection heat exchanging side of the refrigerant circuit provides for an additional means of influencing the relative subcooling value. The controller may in an exemplary embodiment be adapted to switch the subcooling unit on or off. With the subcooling unit turned on, the secondary medium inlet temperature T_sm, of the subcooling unit will go into the calculation of the relative sub- cooling value, whereas the secondary medium inlet temperature T_sm, of the condenser will be looked at, when the subcooling unit is switched off. These temperatures may be different, particularly in the case of different secondary media being used. Moreover, switching the subcooling unit on/off may lead to different refrigerant outlet temperatures T_r0. Hence, the subcooling unit will

U:\8\74\74418\090216_application_textodtl 2009-02-16 09:14 provide for an increased number of degrees of freedom for the controller to reach the reference relative subcooling value.

If on the other hand the secondary medium inlet temperature T_smι and the refri- gerant outlet temperature T_ro are set by the particular condenser being used, which may have been selected based on other system aspects, the controller is still able reach a desired relative subcooling value by influencing the refrigerant condensation temperature T_c.

FIG. 3 shows an exemplary function of the temperature of the refrigerant and of the secondary medium over the length of a condenser, wherein the refrigerant and the secondary medium exhibit a counter-flow relationship. The x-axis represents the distance the refrigerant and the secondary medium travel in the condenser, which is a particularly insightful way of looking at the temperature development when refrigerant and secondary medium flow side by side. The secondary medium enters the condenser at point Xi having the secondary medium inlet temperature T_smι. It leaves the condenser at point X₂, at which point it has been heated up to the secondary medium outlet temperature T_smo. The refrigerant enters the condenser at point x₂ flowing towards point X₁. While flow- ing through the condenser, the refrigerant is first cooled down from the refrigerant inlet temperature T_n to the refrigerant condensation temperature T₀. For the most part of the length of the condenser, the refrigerant temperature remains constant at the refrigerant condensation temperature T_c. During this time energy is continuously transferred from the refrigerant to the secondary medi- um, which results in the refrigerant being condensed, but not in a further decrease in temperature. Shortly before the refrigerant reaches point Xi,it reaches a condensed state and its temperature is decreased again from there on, which represents the process of subcooling. At X₁ the refrigerant leaves the condenser having the refrigerant outlet temperature T_ro.

It is apparent from FIG. 3 which effects a change of the refrigerant condensation temperature T₀, as discussed above, has on the relative subcooling value, which is defined as (T_c-T_ro)/(T_C-T_sm,) in the exemplary embodiment of FIG. 3. However, it is also apparent to a person skilled in the art how different con- denser designs affect the relative subcooling value. The length, along which

|l:\8\74\74418\090216_applιcation_text.odt) 2009-02-16 09:14 heat exchange between the refrigerant and the secondary medium occurs, may have an influence on the difference between the refrigerant outlet temperature T_ro and the secondary medium outlet temperature T_smo- This will have an effect on the absolute value of the refrigerant outlet temperature T_ro as well, thus af- fecting the relative subcooling value. The nature of the interaction between the refrigerant and the secondary medium will have an effect on the relative sub- cooling value as well. A co-flow relationship or a counter-flow relationship or perpendicular flow directions of the refrigerant and the secondary medium or any combination thereof may be considered, when designing a condenser, and may all lead to different results in terms of relative subcooling values. However, the controller may then take appropriate measures to cause the refrigerant condensation temperature T_c to take on a value that leads to the relative subcooling value equal to the reference subcooling value.

Exemplary embodiments of the invention as described above allow for an energetically efficient control of a heat-rejection heat exchanging side of a refrigerant circuit, resulting in an optimum stable refrigerant subcooling and refrigerant pressure. They further allow for generating an aggregate metric, i.e. the relative subcooling value, which reflects a plurality of system aspects. Different ambient temperatures, which provide for varying load conditions throughout the course of a day and throughout the different seasons, have an influence on the relative subcooling value via the secondary medium inlet temperature, at least in a case when the heat-rejection heat exchanging side is cooled by air. If it is not, then the type of heat-rejection heat exchanger used, for example a condenser or a gas cooler, does not only have an influence on the refrigerant outlet temperature (as described above), but also on the secondary medium inlet temperature, both of which are reflected in the relative subcooling value. Setting a reference relative subcooling value therefore allows for an efficient control of a heat-rejection heat exchanging side of a refrigerant system, irrespective of the type of heat-rejection heat exchanger used, irrespective of the size of the whole refrigerating system, irrespective of the load conditions, irrespective of the refrigerant used, and irrespective of the season and time of day, which are associated with varying ambient temperatures. For any combination of these parameters, the control will achieve an optimum amount of subcooling. Thus it prevents re- frigerant to exit the heat-rejection heat exchanger in a gaseous state, which is

[l:\8\74\74418\090216_application_text.odt] 2009-02-16 09:14 caused by too little subcooling, and it prevents unnecessarily high refrigerant pressure in the heat-rejection heat exchanger, which is caused by too much subcooling. Thus, the exemplary embodiments lead to an optimum temperature profile over the heat-rejection heat exchanger and, therefore, to an optim- um use of the heat transfer surface area of the heat-rejection heat exchanger as well as to maximum energetic efficiency.

In a further embodiment of the invention, the step of calculating a relative subcooling value comprises calculating an actual subcooling value by subtracting the refrigerant outlet temperature from the refrigerant condensation temperature. This allows for the amount of subcooling of the refrigerant to have an influence on the aggregate metric, which is controlled by the control method. This helps in ensuring an optimum subcooling of the refrigerant.

It is possible that the calculating of a relative subcooling value comprises calculating a maximum potential subcooling value by subtracting the secondary medium inlet temperature from the refrigerant condensation temperature. Calculating this difference allows for relating the refrigerant condensation temperature to the environment surrounding the heat-rejection heat exchanger, thus generating an appropriate basis that the refrigerant subcooling value can be compared with.

The calculating of a relative subcooling value may also comprise setting the actual subcooling value in relation to the maximum potential subcooling value. This allows for yielding an aggregate metric based on the refrigerant outlet temperature, the refrigerant condensation temperature, and the secondary medium inlet temperature, which can serve as a basis for a control method that is easy to be implemented and efficient to be carried out.

The relative subcooling value RSC may particularly be calculated according to the formula

T - T RSC= ^{c r}°

T c - T smi

[l:\8\74\74418\090216_apphcation_text.odtl 2009-02-16 09:14 This formula allows for the relative subcooling value to be a metric that meaningfully reflects the subcooling state of the refrigerant under the momentary load conditions, the momentary surrounding conditions of the heat-rejection heat exchanger, and the type of heat-rejection heat exchanger being used.

It is furthermore possible that the reference relative subcooling value is above 0.5. This allows for a sufficient amount of subcooling in order to prevent refrigerant to leave the heat-rejection heat exchanger in a gaseous state. It therefore provides a stable control of the amount of subcooling of the refrigerant. The reference relative subcooling value may be between 0.5 and 0.7. This also prevents the amount of subcooling to become excessive and therefore prevents unnecessarily high refrigerant pressure in the heat-rejection heat exchanger. Depending on system considerations, the reference relative subcooling value may also be in a range from 0.3 to 0.5 or in a range from 0.7 to 0.8.

In a further embodiment, the controlling of the relative subcooling value is effected by controlling the refrigerant condensation temperature. Controlling the refrigerant condensation temperature may be effected by controlling the refrigerant pressure in the heat-rejection heat exchanging side of the refrigerant cir- cuit. As the performance of the set of compressors and/or the flow rate through the expansion device can be controlled easily, this allows for the control method to be implemented efficiently and in a cost saving manner. As there are two means of controlling the refrigerant pressure in the heat-rejection heat exchanging side of the refrigerant circuit, the flow rate through said side may be con- trolled at the same time, allowing for an efficient control adaptable to all load conditions.

In yet another embodiment, obtaining the refrigerant condensation temperature comprises sensing the refrigerant pressure in the heat-rejection heat exchan- ging side of the refrigerant circuit and calculating the refrigerant condensation temperature from the refrigerant pressure and refrigerant properties. As refrigerant properties generally relate pressure and condensation temperature, the condensation temperature can be deduced from the readings of a pressure sensor. This is advantageous as pressure sensors with short response times to pressure changes exist.

[l:\8\74\74418\090216_application_text.odt] 2009-02-16 09:14 It is furthermore possible that the refrigerant outlet temperature and the secondary medium inlet temperature are obtained by respective temperature sensors.

The refrigerant may be CO₂. It may also be R404a or any type of hydrocarbons or of halogenized fluorhydrocarbons or ammonia or any other refrigerant suitable for the particular refrigerant circuit under consideration.

The heat-rejection heat exchanging side of the refrigerant circuit may comprise a condenser. It is also possible that the heat-rejection heat exchanging side of the refrigerant circuit comprises a condenser and a subcooling unit. It may also comprise any combination of any number of any types of heat exchangers suitable for the refrigerating system under consideration. This allows for designing a refrigerating system that is adapted to the particular size, performance, cost, and noise requirements, with the individual heat exchangers potentially being spaced apart.

The method of the invention may be carried out by a refrigerating system com- prising a refrigerating circuit with a heat-rejection heat exchanging side and having a controller associated therewith. The advantages associated with the various embodiments of the invention may equally be attained by adapting said refrigerating system in such a way that it is capable of carrying out these features.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.

[l:\8\74\74418\090216_application_text.odtl 2009-02-16 09:14 Reference Numerals:

2 Set of compressors

4 Condenser 6 Expansion device δ Subcooling unit

T_n Refrigerant inlet temperature

Tc Refrigerant condensation temperature

T_ro Refrigerant outlet temperature Tsmo Secondary medium outlet temperature

Tsmi Secondary medium inlet temperature

[I \8\74\74418\090216_apphcatιon_text odt] 2009-02-16 09 14

Claims

1. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit, the method comprising the steps of: providing a heat-rejection heat exchanging side comprising at least one heat-rejection heat exchanger (4), wherein a refrigerant is cooled against a secondary medium; obtaining a refrigerant condensation temperature (T_c) in the heat-rejection heat exchanging side; obtaining a refrigerant outlet temperature (T_r0); obtaining a secondary medium inlet temperature (T_smι); calculating a relative subcooling value by relating the refrigerant condensation temperature (T_c), the refrigerant outlet temperature (T_r0), and the secondary medium inlet temperature (T_smi); and controlling the relative subcooling value with regard to a reference relative subcooling value.

2. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to claim 1, wherein the step of calculating a relative sub- cooling value comprises calculating an actual subcooling value by subtracting the refrigerant outlet temperature (T_ro) from the refrigerant condensation temperature (T₀).

3. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to claim 1 or 2, wherein the step of calculating a relative subcooling value comprises calculating a maximum potential subcooling value by subtracting the secondary medium inlet temperature (T_smi) from the refrigerant condensation temperature (T_c).

4. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to claim 3, wherein the step of calculating a relative subcooling value comprises setting the actual subcooling value in relation to the maximum potential subcooling value.

[I \8\74\74418\090216_applιcation_text odt| 2009-02-16 09 14

5. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to any of the previous claims, wherein the relative sub- cooling value RSC is calculated according to the formula:

RSC =

T - T.

6. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to claim 5, wherein the reference relative subcooling value is above 0.5.

7. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to claim 6, wherein the reference relative subcooling value is between 0.5 and 0.7.

8. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to any of the previous claims, wherein controlling the relative subcooling value is effected by controlling the refrigerant condensation temperature (T_c).

9. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to claim 8, wherein controlling the refrigerant condensation temperature (T₀) is effected by controlling the refrigerant pressure in the heat-rejection heat exchanging side of the refrigerant circuit.

10. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to any of the previous claims, wherein the step of obtaining the refrigerant condensation temperature (T_c) comprises sensing the refrigerant pressure in the heat-rejection heat exchanging side of the refrigerant circuit and calculating the refrigerant condensation temperature (T_c) from the refrigerant pressure and refrigerant properties.

11. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to any of the previous claims, wherein the refrigerant outlet temperature (T_ro)and the secondary medium inlet temperature (T_smi)are obtained by respective temperature sensors.

N:\8\74\74418\090216_application_text.odt) 2009-02-16 09:14

12. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to any of the previous claims, wherein the refrigerant is CO₂.

13. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to any of the previous claims, wherein the heat-rejection heat exchanging side of the refrigerant circuit comprises a condenser (4).

14. Method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to any of the previous claims, wherein the heat-rejection heat exchanging side of the refrigerant circuit comprises a condenser (4) and a subcooling unit (8).

15. Refrigerating system comprising a refrigerating circuit with a heat-rejection heat exchanging side and having a controller associated therewith, wherein the controller is adapted to carry out the method of controlling a heat-rejection heat exchanging side of a refrigerant circuit according to any of the claims 1 to 14.

[l:\8\74\74418\090216_application_text.odt] 2009-02-16 09:14