Dual Temperature Refrigeration Circuit

The present invention relates to a combined medium and low temperature refrig- eration circuit and particularly to a respective CO ₂ refrigeration circuit and a corresponding method.

Such dual temperature refrigeration circuits are known to circulate a refrigerant in a predetermined flow direction through a heat-rejecting heat exchanger, a me- dium temperature refrigeration consumer, a low temperature refrigeration consumer and a compressor unit which returns the refrigerant pressure to the high pressure present in the heat rejecting heat exchanger. Generally, the medium and low temperature refrigeration consumers are arranged in parallel in the circuit, i.e. the refrigerant leaving the heat-rejecting heat exchanger is branched so that part thereof flows through the medium temperature refrigeration consumer and the remainder through the low temperature refrigeration consumer. The compressor unit generally is a two stage compressor unit with a low pressure compressor set and a medium pressure compressor set with the low pressure compressor set being connected to the exit of the low temperature refrigeration consumer and com- presses the refrigerant leaving the same up to a pressure level which corresponds to the pressure level present at the outlet of the medium temperature refrigeration consumer. Both the refrigerant leaving the outlet of the medium temperature refrigeration consumer as well as the refrigerant leaving the outlet of the low pressure compressor are directed to the inlet of the medium pressure compressor which compresses the refrigerant up to the required high pressure as present in the heat rejecting heat exchanger. While such dual temperature refrigeration circuit is satisfying the refrigeration needs, there still is a requirement for improving the efficiency.

Accordingly, it is an object of the present invention to provide a dual temperature refrigeration circuit and a corresponding method which satisfies the cooling requirement and which is relatively simple and inexpensive, but of high efficiency.

In accordance with the present invention, this problem is solved by a combined medium and low temperature refrigeration circuit for circulating a refrigerant in a predetermined flow direction, comprising in flow direction

(a) a heat-rejecting heat exchanger ;

(b) a medium temperature refrigeration consumer ; (c) a medium pressure vapor separator having a vapor portion and a liquid portion connected to the medium temperature refrigeration consumer unit;

(d) a low temperature refrigeration consumer connected to the liquid portion of the medium pressure vapor separator; and

(e) a compressor unit having an inlet connected to the vapor portion of the me- dium pressure vapor separator and the low temperature refrigeration consumer.

As compared to the prior art dual temperature refrigeration circuits, the medium and low temperature refrigeration consumers are arranged in line so that the re- frigerant will initially be used in the medium temperature refrigeration consumer and subsequently the remaining liquid portion of the refrigerant will be used in the low temperature refrigeration consumer. The medium temperature refrigeration consumer may be arranged and/or operated so that the exiting refrigerant is a two-phase refrigerant with part thereof being liquid and the remainder gaseous. Such two-phase refrigerant is separated in the medium pressure vapor separator with the gaseous portion thereof being compressed and returned to the heat rejecting heat exchanger and the liquid portion thereof being used for the low temperature refrigeration consumer.

As compared to the conventional parallel arrangement of medium and low temperature refrigerations consumers, the serial arrangement directs substantially cooler liquid refrigerant but with a reduced pressure towards the low temperature refrigeration consumer. The refrigerant leaving the low temperature refrigeration consumer will be compressed in the compressor unit either directly to the pressure level as required for the heat rejecting heat exchanger or in a first stage up to an intermediate pressure level, for example the pressure level of the gaseous medium pressure refrigerant, and in a second step up to the pressure level as present in the heat rejecting heat exchanger. Accordingly, the compressor unit may comprise a plurality of individual compressors, it may also comprise one or two

compressor sets, for example a low pressure compressor set and a medium pressure compressor set. Each of the low and medium compressor sets may also comprise one or a plurality of individual compressors.

It is to be noted that due to the lower temperature and the lower pressure as pre- sent in the serially connected low temperature refrigeration consumer as compared to the parallel arrangement, it is possible to substantially reduce the dimensions of the low temperature refrigeration consumer, i.e. the reduced pressure for example requires far less wall thickness, etc.

The refrigeration circuit may further comprise a high pressure vapour separator having a vapor portion and a liquid portion, which is arranged between the heat rejecting heat exchanger and the medium temperature refrigeration consumer, and having its vapor portion connected to the inlet of the compressor unit and its liquid portion connected to the medium temperature refrigeration consumer.

The high pressure vapor separator also allows to reduce the temperature and pressure in advance of the medium temperature refrigeration consumer. This increases the efficiency of the medium temperature refrigeration consumer and allows smaller dimensions thereof.

The refrigeration circuit may further comprise an intermediate expansion device between the heat rejecting heat exchanger and the high temperature vapor separator. By means of the intermediate expansion device, a two-phase refrigerant can be generated even if the refrigerant leaving the heat rejecting heat exchanger is purely gaseous. The expansion device can be a controllable expansion device in order to control the conditions in the high pressure vapor separator, like temperature, pressure, proportion of the liquid to gaseous refrigerant, etc.

The refrigeration circuit may further comprise a pressure regulated valve in a line between the vapor portion of the high pressure vapor separator and a compressor unit. For example the proportion between the gaseous and liquid refrigerant in the high pressure vapor separator can be controlled by means of such pressure regulated valve. Another type of controllable valve may also be provided for. It is possible to connect this controllable valve as well as any other controllable element in the circuit to an individual control or alternatively to the overall control of the circuit.

A refrigeration consumer may comprise at least one expansion device and at least one evaporator. The expansion device can be a controllable expansion device for controlling the condition in the evaporator and particularly the condition of the re- no frigerant at the outlet of the evaporator. By controlling the expansion device, the velocity of the refrigerant flow through the evaporator can be controlled so that the refrigerant at the outlet can have any condition between two-phase refrigerant and super heated refrigerant. Also, the refrigeration performance and thus the temperature next to the refrigeration consumer can be controlled thereby.

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The compressor unit may comprise a low pressure compressor set, a medium pressure compressor set and a high pressure vapor compressor. The low pressure compressor set can be connected to the low temperature refrigeration consumer, the medium pressure compressor set can be connected to the liquid por-

120 tion of the medium pressure vapor separator and the high pressure vapor compressor can be connected to the vapor portion of the high pressure vapor separator. The low pressure compressor set and the medium pressure compressor set can form a two stage compressor with the outlet of the low pressure compressor set being connected in the inlet of the high pressure compressor set. While the

125 high pressure compressor set compresses gaseous refrigerant from the medium pressure to the high pressure as present in the heat rejecting heat exchanger, the pressure difference over the high pressure vapor compressor will typically be substantially less. There may be one or a plurality of high pressure vapor compressors. Typically, a single high pressure vapor compressor will be sufficient. The

130 high pressure vapor compressor can be a controllable compressor.

The refrigeration circuit may further comprise a medium pressure expansion device and an intermediate pressure vapor separator with the medium pressure expansion device connected to the liquid portion of the medium pressure vapor

135 separator, the liquid portion of the intermediate pressure vapor separator connected to the low temperature refrigeration consumer, and the vapor portion of the intermediate pressure vapor separator being connected to the compressor unit. Such additional medium pressure expansion device and a joint intermediate pressure vapor separator may further reduce the temperature and the pressure of the

140 refrigerant before it will finally be directed to the low temperature refrigeration consumer and the dimensions of the low temperature refrigeration consumer can fur-

ther be reduced. It is to be noted that together with the reduction of the dimension of the respective refrigeration consumer, the temperature and pressure reduction as provided through the vapor-liquid separation also allows substantially smaller

145 dimensions for the conduits leading towards the respective refrigeration consumer.

The compressor unit may further comprise an intermediate compressor between the vapor portion of the intermediate pressure vapor separator and the medium pressure compressor set. Similar to the high pressure vapor compressor, the in- 150 termediate compressor is compressing only over a reduced pressure difference as compared to the low pressure compressor set. This applies particularly, if the intermediate compressor operates merely between the intermediate pressure and the medium pressure levels. Again, a plurality of intermediate compressors can be provided. It is possible to use a controllable intermediate compressor.

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The refrigeration circuit may further comprise a pressure regulating valve between the vapor portion of the intermediate pressure vapor separator and the compressor unit and the intermediate compressor, respectively.

160 The refrigeration circuit may further comprise a superheat sensor associated to the exit of the low temperature refrigeration consumer and connected to a control for securing superheat of the refrigerant. Control can be a local superheat control which controls the refrigeration consumer's expansion valve, etc. but can also be the general refrigeration circuit control.

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The refrigeration circuit may use a refrigerant which is working also in a super critical condition, for exam pie CO ₂ .

Another embodiment of the invention relates to a refrigeration apparatus compris- 170 ing a refrigeration circuit in accordance with the embodiment of the present invention. The refrigeration apparatus can be a refrigeration system for a supermarket, and industrial refrigeration system, etc. In case of a supermarket refrigeration system, the medium temperature refrigeration consumer(s) can be display cabinet and the likes, for example for milk products, meat, vegetables and fruits, with a 175 medium refrigeration level of less than 10 ⁰ C down to around 0°C. The low temperature refrigeration consumer(s) can be freezers with a refrigeration level of - 20 ⁰ C and below.

Another embodiment of the present invention relates to a method for operating a 18Q combined medium and low temperature refrigeration circuit for circulating a refrigerant in a predetermined flow direction, the refrigeration circuit comprising in flow direction a heat-rejecting heat exchanger, a medium temperature refrigeration consumer , a low temperature refrigeration consumer and a compressor unit each thereof having an inlet and an outlet, respectively, wherein the method com- 185 prises the following steps:

(a) separating the liquid medium pressure refrigerant leaving the outlet of the low temperature refrigeration consumer from the gaseous refrigerant leaving the same outlet;

190 (b) directing the gaseous medium pressure refrigerant towards the inlet of the compressor unit; and

(c) directing the liquid medium pressure refrigerant towards the inlet of the low temperature refrigeration consumer.

195 The method may further comprise the following steps:

(d) separating the liquid high pressure refrigerant leaving the outlet of the heat- rejecting heat exchanger from the gaseous refrigerant leaving the same outlet

200 (e) directing the gaseous high pressure refrigerant towards the inlet of the compressor unit; and

(f) directing the liquid high pressure refrigerant towards the inlet of the low temperature refrigeration consumer.

205 The method may further comprise in advance of step (d) the step of expanding the high pressure refrigerant leaving the heat rejecting heat exchanger and preferably regulating the pressure of the gaseous high pressure refrigerant.

The method may further comprise the following steps:

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(g) expanding the liquid medium pressure refrigerant to an intermediate pressure; (h) separating the liquid intermediate pressure refrigerant from the gaseous intermediate pressure refrigerant;

(i) directing the gaseous intermediate pressure refrigerant towards the inlet of the 215 compressor unit; and

(j) directing the liquid gaseous intermediate pressure refrigerant towards the low temperature refrigeration consumer, and preferable regulating the pressure of the gaseous intermediate pressure refrigerant; and preferably (k) controlling the superheat of the gaseous low pressure refrigerant entering the 220 inlet of the compressor unit.

Embodiments of the present invention are described in greater detail below with reference to the figures wherein

225 Fig. 1 is a combined medium and low temperature refrigeration circuit in accordance with a first embodiment of the present invention; and

Fig. 2 is a combined medium and low temperature refrigeration circuit in accordance with a second embodiment.

230

Fig. 1 shows a combined medium and low temperature refrigeration circuit 2 for circulating a refrigerant in a predetermined flow direction as indicated by the arrows, comprising in flow direction a heat rejecting heat exchanger 4, a plurality of medium temperature refrigeration consumers 6, a medium pressure vapor separa-

235 tor 8 having a vapor portion 10 and a liquid portion 12, a plurality of low temperature refrigeration consumers 14 as well as a compressor unit 16. The compressor unit 16 comprises a plurality of individual compressors 18 some of which are grouped together to compressor sets, like a low pressure 2-stage compressor set comprising a first stage low pressure compressor set 20 and a second stage low

2 ₄ o pressure compressor set 22. There also is a medium pressure compressor set 24, the input 26 thereof being connected to the vapor portion 10 of the medium pressure vapor separator 8. The compressors and compressor sets may all be located at the same location, but may also be located at different places within the circuit 2. The outlets 28 and 30 of the second stage low pressure compressor set 22 and

245 the medium pressure compressor set 24 are connected with the inlet 32 of the heat rejecting heat exchanger 4. The heat rejecting heat exchanger 4 can be a conventional condenser in case of a conventional refrigerant and can be a gas- cooler in case of a refrigerant which is at least partially operated in a supercritical condition.

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A high pressure vapor separator 34 having a vapor portion 36 and a liquid portion 38 is provided between the heat rejecting heat exchanger 4 and the medium temperature refrigeration consumers 6. Particularly, the vapor portion 36 of the high pressure vapor separator 34 is connected with the outlet 40 of the heat rejecting 255 heat exchanger 4 via heat exchanger outlet line 42. An expansion device 44 is located in the heat exchanger outlet line 42. The liquid portion 38 of the high pressure vapor separator 34 is connected to the medium temperature refrigeration

consumers 6 by means of a high pressure liquid line 46. The vapor portion 36 of the high pressure vapor separator 34 is connected by means of the high pressure vapor line 48 to a high pressure vapor compressor 50. A pressure regulated valve 52 is arranged in the high pressure vapor line 58. Each of the medium and low temperature refrigeration consumers 6 and 14, respectively, may comprise at least one expansion device 54 and 56, respectively, and at least one evaporator 58 and 60, respectively. Of course, there may be a single as well as a plurality of low and medium temperature refrigeration consumers, respectively.

As mentioned above, the compressor unit 16 comprises a plurality of compressor sets 20, 22 and 24 as well as individual compressors 18, 50. The compressor unit 16 comprises a plurality of inlets at different pressure levels with the inlets 62 to the first stage compressor set 20 of the low pressure compressor set 20, 22 being at the lower most pressure level, the inlet 26 to the medium pressure compressor set 24 being at a higher medium pressure level and the inlet 64 to the high pressure vapor com pressor 50 being as compared therewith at the highest level.

Subsequent, the operation of a refrigeration circuit in accordance with Fig. 1 will be described with reference to a CO ₂ refrigeration circuit 2. In use, the pressure of the refrigerant in the heat rejecting heat exchanger 4 can be up to 120 bar and is typically approximately 85 bar in "summer mode" and approximately 45 bar in "winter mode". The refrigerant flowing through the heat exchanger outlet line 42 is expanded in expansion valve 44 which reduces the pressure to between approximately 30 and 40 bar and preferably 36 bar with such pressure being typically independent from a "winter mode" and "summer mode".

The high pressure receiver or high pressure vapor separator 34 collects and sepa- rates liquid and gaseous refrigerant in the liquid and vapor portions 38 and 36, respectively. The high pressure liquid line 46 directs the liquid refrigerant from the liquid portion 38 to the expansion devices 54 of the medium temperature refrigeration consumers 6.

The medium temperature refrigeration consumers 6 cool down to approximately 1 to 10 ⁰ C. They can be arranged or controlled so that there is a two-phase refrigerant present at the outlet thereof. Such two-phase refrigerant is fed to the medium pressure vapor separator 8 where it is collected and separated in a vapor portion

10 and liquid portion 12, respectively. Gaseous refrigerant from the vapor portion 10 is directed to the inlet 26 of the medium pressure compressor 24. The input pressure is typically between 20 and 30 bar and approximately 26 bar, which results in a temperature of the refrigerant of approximately -10 ⁰ C in the medium temperature refrigeration consumers 6. A high pressure line 66 returns the compressed, hot, gaseous refrigerant to the heat rejecting heat exchange 4. Similar, the gaseous refrigerant from the vapor portion 36 of the hight pressure vapor separator 34 is directed through high pressure vapor line 48 and pressure controlled valve 52 to the inlet 64 of the high pressure vapor compressor 50 and returned to high pressure line 66 to the heat rejecting heat exchanger 4.

The liquid refrigerant from the liquid portion 12 of the medium temperature vapor separator 8 is directed to and through the low temperature refrigeration consumers 14. The low temperature refrigeration consumers 14 are arranged or controlled so as to provide super heated gaseous refrigerant only to the inlet 68 of the first stage low temperature compressor set 20. A superheat sensor (not shown) can be associated to the exit of the low temperature refrigeration consumers 14 or the inlet 68 to the first stage low temperature compressor set 20 in order to ensure that no liquid refrigerant may enter the first stage low temperature compressor set 20. Additionally and/or alternatively, an internal heat exchanger (not shown) can be provided for between the low temperature refrigeration consumers 14 and the inlet 68.

The pressure at the inlet 68 is typically between 8 and 20 bar and preferably approximately 12 bar which results in a temperature of the refrigerant of approximately -37°C in the low temperature refrigeration consumers 14.

Again, the gaseous refrigerant is returned through the low pressure compressor 26 and 22, respectively and the high pressure line 66 to the heat rejecting heat exchanger 4.

The embodiment of Fig. 2 corresponds substantially to the embodiment of fig. 1. Accordingly, corresponding elements are indicated with corresponding reference numbers. The main difference between the two embodiments results in a different routing of the liquid refrigerant leaving the liquid portion 12 of the medium temperature vapor separator 8 as compared to Fig. 1. Particularly, a medium pressure

330 expansion device 70 and an intermediate pressure vapor separator 72 having a vapor portion 74 and a liquid portion 76 are additionally arranged subsequent to the medium temperature vapor separator 8. Moreover, a pressure regulated intermediate valve 78 as well as an intermediate compressor 80 connect the vapor portion 74 of the intermediate pressure vapor separator 72 to the intermediate

335 pressure level between first stage low pressure compressor set 20 and second stage low pressure compressor set 22. Again, the intermediate compressor 80 can be a single compressor or a plurality of compressors and may further be a controllable compressor. The pressure difference over intermediate compressor 80 is substantially less than the pressure difference over the first stage low pressure

340 compressor set 20. The pressures and temperatures are by and large the same as with the embodiment of Fig. 1 and for the intermediate vapor separator 72 the saturation temperature is approximately half way between the low temperature and medium temperature evaporator temperatures.

345 The pressure regulated valves 52 and 78 allow to hold or control the back pressure feeding into the expansion valves for the medium and low temperature cases.

350