This invention relates to a refrigeration circuit comprising a first compressor device, a heat-rejecting heat exchanger, a first expansion device, a receiver having an upper portion and a lower portion, a second expansion device, and a first evaporator. The refrigeration circuit further comprises a flow path between the upper portion of the receiver and a compressor, the pressure side of which is in flow communication with the entrance of the heat-rejecting heat exchanger.

The refrigeration circuit preferably is of the type designed for CO ₂ as a refrigerant, but is not limited thereto.

The refrigeration circuit is of the two stage expansion type, wherein the refrigerant first is expanded in first stage expansion. The first stage expansion provides cooling to complete condensation of the refrigerant in the receiver. Furthermore, the section of the refrigeration circuit extending from the receiver to the compressor device is at a substantially lower pressure level than the remaining section of the refrigeration circuit extending from the compressor device to first expansion device.

It is an object of the invention to provide a refrigeration circuit with an improved receiver.

It is a further object of the invention to provide a refrigeration circuit with a receiver outputting from its upper portion flash gas having substantially no liquid droplets therein.

It is a still further object of the invention to provide a refrigeration circuit with a receiver outputting a sub-cooled liquid refrigerant.

In accordance with one embodiment of the invention there is provided a refrigeration circuit for circulating a refrigerant in a predetermined flow direction, comprising in flow direction a first compressor device, a heat-rejecting heat exchanger, a first expansion device, a receiver having in its interior an upper portion, being in flow communication with the first expansion device, and a lower portion, a second expansion device being in flow communication with the lower portion of the receiver, and a first evaporator; and comprising a further flow path between the upper portion of the receiver and the suction side of a compressor, the pressure.side of which is in flow communication with the entrance of said heat- rejecting heat exchanger; wherein at least one element of the group consisting of the following elements (a) and (b) is provided: (a) a second heat exchanger is arranged in said upper portion of said receiver, the entrance of the second heat exchanger being in flow communication with the exit of said heat-rejecting heat exchanger and the exit of the second heat exchanger being in flow communication with the entrance of said first expansion device, (b) said further flow path comprises a third expansion device and, downstream thereof, a third heat exchanger arranged in said lower portion of said receiver.

The second heat exchanger arranged in the upper portion of the receiver exchanges heat against the vapour contained in the upper portion of the receiver. Any liquid droplets that may be present in the upper portion of the receiver will be evaporated and entrained into the further flow path.

The third expansion device and the third heat exchanger arranged in lower portion of the receiver provide sub-cooling the liquid in the lower portion of the receiver. Such sub-cooled liquid refrigerant results in more efficient cooling effect by the first evaporator and reduces the formation of refrigerant vapour in the section of the circuit extending from the receiver to the second expansion device.

Ail in all the improved receiver provides for a more perfect separation into a gaseous phase of the refrigerant having substantially no content of liquid droplets, and a liquid phase that is sub-cooled and has less tendency to vapour formation.

The first compressor device may be a single compressor or a parallel group of several compressors. The compressor device may be of the type comprising a control of its performance, for example by way of controlling its rotational speed

dependent on the pressure level of the compressed gaseous refrigerant to be achieved.

The compressor associated to the further flow path starting from the upper portion of the receiver, may be a further compressor. The suction side of such further compressor may be at a higher pressure level than the suction side of the first- mentioned compressor device, or may be a substantially the same pressure level as the first-mentioned compressor device. It is possible to combine the compressor, that is associated to the further flow path, with the first-mentioned compressor device, either by using one and the same compressor for compressing the gaseous refrigerant coming from the second expansion device as well as the gaseous refrigerant coming from the upper portion of the receiver, or by combining the further compressor, that is associated to the further flow path, into a parallel group of compressors forming the first compressor device.

In accordance with an embodiment of the invention, the refrigeration circuit further comprises a branch circuit, branching off from a location located in a section of said circuit which section extends from said lower portion of said receiver to the entrance of said second expansion device; the branch circuit comprising in flow direction a fourth expansion device, a second evaporator, and a second compressor device; and the branch circuit, at its downstream end, being in flow communication with the suction side of said first compressor device.

In such embodiment, the branch circuit provides low temperature cooling, for example for deep-freezing purposes. As compared to such low temperature cooling, the second expansion device and the first evaporator provide for medium temperature cooling, for example for keeping food and beverages at a temperature level of O to 10 ⁰ C.

The refrigeration circuit may comprise one or several second expansion devices/first evaporators, arranged in parallel, and one or several fourth expansion devices/second evaporators, arranged in parallel, if any.

The refrigerant in the refrigeration circuit may be a one-component refrigerant or a multiple-components refrigerant.

In the preceding description, reference has been made to various expansion devices. It should be stressed that expansion devices of various constructions and designs may be provided. A quite common form of expansion device is an expansion valve. The expansion device may be a throttling device or a throttle valve. The expansion device, depending on its location, the temperature level, and the pressure level, may serve to expand liquid refrigerant to gaseous refrigerant or may expand gaseous refrigerant from a higher pressure level to a lower pressure level.

This invention further relates to a refrigeration apparatus comprising a refrigeration circuit as disclosed in the present application.

The refrigeration apparatus of this invention may be provided as a heat pump. The technical elements of cooling apparatus and heat pumps are the same. With the cooling apparatus, the purpose of cooling is the primary purpose, and the related generation of heat is normally a side effect. With heat pumps, the generation of heat is the desired purpose, whereas the related cooling effect of the evaporator(s) is normally considered a less useful side effect. This invention also discloses a heat pump having a circuit as disclosed in the present application. Such circuit may be designated a refrigeration circuit because it contains a refrigerant undergoing condensation and evaporation. Some times people prefer to use the term working fluid rather than to use the term refrigerant when describing a heat pump.

A refrigeration circuit containing CO ₂ as a refrigerant may be a circuit operated in transcritical cycle, or may be a circuit operated in subcritical cycle, or may be a circuit operable in transcritical cycle or in subcritical cycle depending on parameters such as environmental temperature and pressure level after the compressor device. In typical applications such as cooling temperature sensitive products, deep-freezing, cooling buildings, the refrigeration circuit does not reach a subcritical temperature level at the heat-rejecting heat exchanger, at least in summer time season; the circuit is operated in transcritical cycle. In such a situation the heat-rejecting heat exchanger operates as a gas cooler. In case of a subcritical cycle, the heat-rejecting heat exchanger operates as a combined gas cooler and condenser.

The main functions of the receiver are to permanently keep available a sufficient quantity of liquid refrigerant and to provide a separation between liquid refrigerant and gaseous refrigerant (vapour). In case of transcritical cycle, the condensation of refrigerant by means of flash cooling provided by the first expansion device is a further function.

The refrigeration apparatus/heat pump of this invention has a number of preferred fields of application. The most important are cooling food and beverages in shops, restaurants or other locations of storage; cooling other temperature-sensitive products such as pharmaceuticals; deep-freezing; cooling buildings of any sort; cooling cars and any other type of vehicles in the broad sense, such as aircrafts, ships, railway cars etc.

This invention further relates to a refrigeration method. In an embodiment of the invention the refrigeration method comprises at least one step of the group of steps consisting of (i) operating a heat source in said upper portion of said receiver, (ii) operating a heat sink in said lower portion of said receiver.

An exemplary embodiment of the invention will be described in the following. The features of such embodiment are preferred features of the refrigeration circuit of this invention:

Fig. 1 shows a diagram of a refrigeration circuit for elucidating the basic configuration of such a circuit;

Fig. 2 shows a receiver/separator on a larger scale, which may be incorporated in the refrigeration circuit of Fig. 1.

The total refrigeration circuit shown in Fig. 1 comprises a first-described (basic) circuit, a second-described further flow path, and a third-described branch circuit, and some additional elements.

The basic circuit, when beginning with a compressor device 6 and progressing in flow direction of the CC^-refrigerant, comprises the following elements:

compressor device 6 or 6 and 6'; conduit 7; heat-rejecting heat exchanger 1 (gas cooler and/or condenser); conduit 2; first expansion valve a; receiver 3; conduit 4; two parallel second expansion valves b and c; two parallel evaporators E2 and E3; conduit 5 back to compressor device 6.

The compressor device 6 comprises three parallel compressors and a further compressor 6' to be described in more detail further below. The suction sides of the three compressors are supplied by a common supply space 20. Typically, the compressor device 6 compresses the supplied gaseous CO ₂ to a pressure in the range of 50 to 120 bar, whereby the temperature of the gaseous compressed CO ₂ is increased to about 50 to 15O ⁰ C. In subcritical operation the pressure of the compressed gaseous CO ₂ would typically be in the range of 40 to 70 bar.

The heat-rejecting heat exchanger removes heat from the CO ₂ . In subcritical operation, the CO ₂ is typically cooled to 10 to 30 ⁰ C and condensed in the heat- rejecting heat exchanger 1; in this case heat exchanger 1 works as a combined gas cooler and condenser. In transcritical operation, the CO ₂ is typically cooled to a temperature of 25 to 45°C, without condensation of a substantial part of the CO ₂ , in the heat-rejecting heat exchanger; in this case it works as a gas cooler. In order to remove heat from the CO ₂ , the heat exchanger 1 is gas cooled or liquid (water) cooled.

The vapour or liquid/vapour mixture or liquid CO ₂ in subcritical operation, is expanded by the expansion valve a provided next to the receiver 3, thereby providing flash gas in an upper portion of the receiver 3. Typically, the pressure level in the interior of the receiver 3 is 30 to 40 bar. A lower portion of the receiver

3 contains liquid CO ₂ - The receiver 3 also acts as a separator of liquid CO ₂ and CO ₂ vapour.

By the expansion valves b and c the liquid CO ₂ is expanded to typically a temperature of minus 15 to O ⁰ C, resulting in a pressure level of typically 20 to 35 bar. The evaporators E2 and E3 next to the expansion valves b and c serve to allow for a complete evaporation of the CO ₂ and provide large cool surfaces, from where the cooling proper originates, typically by air moving by the "cool air is heavier than warm air" principle or moving by forced ventilation.

The compressor device 6 and the receiver 3 are typically mounted in a common metal frame, also supporting the control equipment of the refrigeration apparatus. The (first) heat exchanger 1, that is a heat-rejecting heat exchanger, normally stands some distance away from the compressor device 6 and the receiver 3 and the expansion valve 8, for example outside a building, where it can be cooled best. It is important to note that only the section of the basic circuit extending from the pressure side of the compressor device 6 to the exit side of the expansion valve 8 is at the high pressure level of typically 50 to 120 bar. The remaining section of the basic circuit extending from the exit side of the expansion valve a to the suction side of the compressor device 6 is at two substantially lower pressure levels, namely typically 30 to 40 bar in front of the expansion valves b and c and typically 25 to 30 bar in front of the compressor device 6. As a consequence, the second- mentioned section of the basic circuit may be designed for such lower pressure levels, i.e. by using tubes having thinner walls, by using less sophisticated connections where CO ₂ is flowing, and by using evaporators adapted to the relatively low pressure level.

There is a further flow path, starting at an exit side of the upper portion (vapour portion) of the receiver 3 with a conduit 12 and containing an expansion valve e or throttle valve, and finally leading to the entrance side of the compressor device 6 via a conduit 11. The expansion valve e serve to reduce the pressure of the gaseous CO ₂ to the level existing at the suction side of the compressor device 6.

As an alternative, the expansion valve e may be dispensed with, and there is just a conduit 12, 15 from the upper portion of the receiver 3 to the further compressor 6'. The suction side of such further compressor 6' is at a higher pressure level that the suction side 20 of the compressor device 6. The pressure sides of all the compressors 6 and 6' have the same pressure level. Rather than providing the further compressor 6', it is possible to feed from line 15 into one or several of the compressors of the compressor device 6, but at a stage after a first compression stage, so that the flash gas is fed into the compressor device 6 at the right pressure level of the compressors.

Furthermore, Fig. 1 shows a branch circuit comprising the following: A conduit 8 branches off from the conduit 4 upstream of the expansion valves b and c; a (fourth) expansion valve d; a second evaporator E4; a conduit 9; a second compressor device 10, and a conduit 11 providing fluid flow connection with the suction side of the first compressor device 6. The expansion valve d and the second evaporator E4 are designed to provide an expansion of the liquid CO2 to a lower pressure level than existing at the suction side 20 of the compressor device 6. The temperature level reached at the evaporator E4 is lower than the temperature level reached at the evaporators E2 and E3, thereby providing means for deep-freezing or storing at deep-freezing temperature. Typical values are 7 to 15 bar and minus 50 to minus 25 ⁰ C in the evaporator E4.

Finally, Fig. 1 shows a conduit 13 branching off the conduit 2 (that leads from the first heat exchanger 1 to the first expansion valve a) to a heat exchanger E1, an expansion valve f being provided in such conduit 13. A conduit 14 leads from the heat exchanger E1 to the suction side of the further compressor 6'. The heat exchanger E1 exchanges heat against the CO2 flowing through the conduit 2.

Since the expansion valve f provides cool gaseous CO ₂ , the CO ₂ flowing through the conduit 2 is cooled, thereby either assisting in condensation of CO ₂ or in sub- cooling of liquid CO ₂ .

Fig. 2 shows a schematically sectional view of the receiver 3 at a larger scale than in Fig. 1. The receiver 3 has in its interior an upper portion 3a and a lower portion 3b. A quantity of liquid CO ₂ is contained in the receiver 3, filling the interior of the

receiver 3 up to a level ) 22. Depending on the operational conditions of the refrigeration circuit, the level 22 may be higher or lower than shown in Fig. 2.

The line 2 (providing a fluid flow connection between the exit of the heat exchanger 1 and the expansion valve a, cf. Fig. 1) extends into the receiver 3 and is connected to a second heat exchanger 24 arranged in the upper portion 3a of the receiver 3. There is a further conduit 26, extending outside the receiver 3 and connecting the downstream end of the second heat exchanger 24 to the interior of the upper portion 3a of the receiver 3, an expansion valve 28 being provided in such conduit 26. The expansion valve 28 produces flash gas in the upper portion 3a, which as a consequence is at a lower temperature level than the CO2 flowing through the second heat exchanger 24. Any droplets of liquid CO ₂ that may be present in the upper portion 3a, are evaporated. This minimizes the potential for erosion of the expansion valve 34 described in the following paragraph.

The expansion valve 28 has the same function as the expansion valve a shown in Fig. 1. The difference is that the conduit 2 does not lead directly to the expansion valve 28, but there is the second heat exchanger 24 upstream of the expansion valve 28. By means of the second heat exchanger 24, the gaseous CO ₂ exiting the upper portion 3a contains less condensed CO ₂ than without the provision of the second heat exchanger 24.

There is a further conduit 30 leading, outside the receiver 3, from the upper portion 3a to a third heat exchanger 32 arranged in the lower portion 3b of the receiver 3, an expansion valve 34 being provided in such conduit 30. The downstream end of the third heat exchanger 32 is connected by a conduit 36 to the suction side 20 of the compressor device 6. In other words, the expansion valve 34 replaces the expansion valve e shown in Fig. 1, and the third heat exchanger 32 is provided in addition.

By passing through the expansion valve 34 the CO ₂ becomes cooler, and the third heat exchanger 32 provides sub-cooling of the liquid CO ₂ accumulated in the lower portion 3b of the receiver 3. The liquid, sub-cooled CO ₂ exits the lower portion 3b via conduit 4, as shown in Fig. 1.

The gaseous CO ₂ flowing through the third heat exchanger 32 gets a certain overheating which reduces the risk of entrainment of liquid CO ₂ into the compressor device 6.