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Title:
A VAPOUR COMPRESSION SYSTEM COMPRISING A SECONDARY EVAPORATOR
Document Type and Number:
WIPO Patent Application WO/2017/081157
Kind Code:
A1
Abstract:
A vapour compression system (1) comprising an ejector (9), a primary evaporator (17) and a secondary evaporator is disclosed. The vapour compression system (1) further comprises a flow control device (19) arranged with an inlet communicating with a liquid outlet (25) of the receiver (11), and an outlet of the flow control device (19) supplying refrigerant to the secondary evaporator, the secondary evaporator communicating with a secondary inlet (27) of the ejector (9). Thereby the receiver (11) pressure can be optimized irrespective of the refrigerant in the secondary evaporator and the primary (17) and secondary evaporators can provide cooling power simultaneously in an energy efficient manner.

Inventors:
MADSEN KENNETH BANK (DK)
SCHMIDT FREDE (DK)
FREDSLUND KRISTIAN (DK)
PRINS JAN (DK)
Application Number:
PCT/EP2016/077275
Publication Date:
May 18, 2017
Filing Date:
November 10, 2016
Export Citation:
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Assignee:
DANFOSS AS (DK)
International Classes:
F25B41/00; F25B1/10; F25B5/00; F25B40/02; F25B41/04
Domestic Patent References:
WO2010036480A22010-04-01
WO2009041959A12009-04-02
Foreign References:
DE112013003452T52015-04-23
CN102042721B2012-07-04
Other References:
NAKAGAWA M ET AL: "PERFORMANCE OF TWO-PHASE EJECTOR IN REFRIGERATION CYCLE", PROCEEDINGS OF THE INTERNATIONAL CONFERENCE ON MULTIPHASE FLOW, XX, XX, 8 June 1998 (1998-06-08), pages 1 - 08, XP008037195
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Claims:
CLAIMS

1. A vapour compression system (1), the vapour compression system (1) comprising a compressor unit (3) comprising one or more compressors (5), a heat rejecting heat exchanger (7), an ejector (9), a receiver (11), at least one expansion device (15) and at least one primary evaporator (17) arranged in a refrigerant path, the ejector (9) having a primary inlet (23) connected to an outlet of the heat rejecting heat exchanger (7) and an outlet (29) connected to the receiver (11), the vapour compression system (1) further comprising a flow control device (19) arranged with an inlet communicating with a liquid outlet (25) of the receiver (11), and an outlet of the flow control device supplying refrigerant to a secondary evaporator, the secondary evaporator communicating with a secondary inlet (27) of the ejector (9).

2. A vapour compression system (1) according to claim 1, wherein the secondary evaporator is or forms part of an air conditioning unit (40) .

3. A vapour compression system (1) according to claim 1, wherein the secondary evaporator is or forms part of a subcooler (21).

4. A vapour compression system (1) according to any of the preceding claims, wherein a gaseous outlet (31) of the receiver (11) communicates with an inlet of the compressor unit (3).

5. A vapour compression system (1) according to any of the preceding claims, wherein at least one of the compressors (5) of the compressor unit (3) is connectable to a gaseous outlet (31) of the receiver (11).

6. A vapour compression system (1) according to any of the preceding claims, wherein the compressor unit (3) comprises one or more valve arrangements, each valve arrangement being arranged to selectively connect an inlet of a compressor (5) of the compressor unit (3) to a gaseous outlet (31) of the receiver (11) or to an outlet of the primary evaporator(s) (17).

7. A vapour compression system (1) according to any of the preceding claims, wherein at least one of the compressors (5) of the compressor unit (3) has an inlet being connected to the gaseous outlet (31) of the receiver (11), and at least one of the compressors (5) of the compressor unit (3) has an inlet which is connected to an outlet of the primary evaporator(s) (17).

8. A vapour compression system (1) according to any of the preceding claims, wherein the flow control device (19) is or comprises a passively controlled valve.

9. A vapour compression system (1) according to claim 8, wherein the passively controlled valve defines an opening degree which is variable in response to a pressure difference across said valve.

10. A vapour compression system (1) according to any of claims 1-7, wherein the flow control device (19) is or comprises an actively controlled valve.

Description:
A VAPOUR COMPRESSION SYSTEM COMPRISING A SECONDARY EVAPORATOR FIELD OF THE INVENTION

The present invention relates to a vapour compression system such as a refrigeration system, air conditioning system, or a heat pump. The vapour compression system according to present invention comprises a secondary evaporator.

BACKGROUND OF THE INVENTION

In some vapour compression systems an ejector is arranged in a refrigerant path, at a position downstream relative to a heat rejecting heat exchanger. Thereby refrigerant leaving the heat rejecting heat exchanger is supplied to a primary inlet of the ejector. An ejector is a type of pump which uses the Venturi effect to increase the pressure energy of fluid at a suction inlet (or secondary inlet) of the ejector by means of a motive fluid supplied to a motive inlet (or primary inlet) of the ejector. Thereby, arranging an ejector in the refrigerant path as described above will cause the refrigerant to perform work, and thereby the power consumption of the vapour compression system is reduced as compared to the situation where no ejector is provided.

An outlet of the ejector is normally connected to a receiver, in which liquid refrigerant is separated from gaseous refrigerant. The liquid part of the refrigerant may be supplied to an evaporator, via an expansion device, and the gaseous part of the refrigerant may be supplied to a compressor unit comprising one or more compressors. The refrigerant from the gaseous outlet of the receiver has not been subjected to the expansion in the expansion device.

The pressure in the receiver is a parameter relevant for vapour compression system control optimization.

US 2013/0111944 Al discloses a system that comprises a compressor, a heat rejection heat exchanger, first and second ejectors, first and second heat absorption heat exchangers, and first and second separators, the first and second ejectors, heat absorption heat exchangers and separators arranged in respective refrigerant paths are arranged in series. DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a vapour compression system, which can be operated in a more energy efficient manner than prior art systems.

Accordingly, the invention provides a vapour compression system, the vapour compression system comprising a compressor unit comprising one or more compressors, a heat rejecting heat exchanger, an ejector, a receiver, at least one expansion device and at least one primary evaporator arranged in a refrigerant path, the ejector having a primary inlet connected to an outlet of the heat rejecting heat exchanger and an outlet connected to the receiver, the vapour compression system further comprising a flow control device arranged with an inlet communicating with a liquid outlet of the receiver, and an outlet of the flow control device supplying refrigerant to a secondary evaporator, the secondary evaporator communicating with a secondary inlet of the ejector.

In the present context the term 'vapour compression system' should be interpreted to mean any system in which a flow of fluid medium, such as refrigerant, circulates and is

alternatingly compressed and expanded, thereby providing either refrigeration or heating of a volume. Thus, the vapour compression system may be a refrigeration system, an air condition system, a heat pump, etc.

The vapour compression system comprises a compressor unit, comprising one or more compressors, a heat rejecting heat exchanger, an ejector, a receiver, at least one expansion device and at least one primary evaporator arranged in a first part of the refrigerant path.

This part of the refrigerant path is in the following referred to as the primary refrigerant path. The primary refrigerant path may be regarded as a primary cooling circuit, providing cooling for refrigerated volume(s) associated with the primary evaporator(s).

The vapour compression system further comprises a flow control device and a secondary evaporator. The flow control device, the secondary evaporator, the ejector and the receiver are arranged in a second part of the refrigerant path. This part of the refrigerant path is in the following referred to as the secondary refrigerant path. The secondary refrigerant path may be regarded as a secondary cooling circuit, providing cooling for a refrigerated volume, or a secondary fluid flow, associated with the secondary evaporator. Accordingly, the ejector and the receiver form part of both the primary cooling circuit and the secondary cooling circuit. The connection between the primary cooling circuit and the secondary cooling circuit, and the specifics of the secondary cooling circuits will be described in further detail below.

In the primary cooling circuit, each expansion device is arranged to control a supply of refrigerant to a specific primary evaporator or a set of primary evaporators wherein the primary evaporators run with the same refrigerant pressure at their inlets and wherein their otlets are joined to form a common refrigerant path. The heat rejecting heat exchanger could, e.g., be in the form of a condenser, in which refrigerant is at least partly condensed, or in the form of a gas cooler, in which refrigerant is cooled, but remains in a gaseous or trans-critical state. The expansion device(s) could, e.g., be in the form of expansion valve(s). Each primary evaporator may be associated with a specific refrigerated volume. In this case, each expansion device may control the cooling of a specific refrigerated volume and the refrigerated volumes can then be controlled individually. The refrigerated volumes could, e.g., be various display cases of a supermarket.

Thus, refrigerant flowing in the primary refrigerant path is compressed by the compressor(s) of the compressor unit. The compressed refrigerant is supplied to the heat rejecting heat exchanger, where heat exchange takes place with an ambient space, or with a secondary fluid flow across the heat rejecting heat exchanger, in such a manner that heat is rejected from the refrigerant flowing through the heat rejecting heat exchanger. The heat rejecting heat exchanger could, e.g., be in the form of a condenser or in the form of a gas cooler. From the heat rejecting heat exchanger, the refrigerant is supplied to the primary inlet of the ejector. As the refrigerant passes through the ejector, the pressure of the refrigerant is reduced, and the refrigerant leaving the ejector will normally be in the form of a mixture of liquid and gaseous refrigerant, due to the expansion taking place in the ejector.

The refrigerant is then supplied to the receiver, where the refrigerant is separated into a liquid part and a gaseous part. The gaseous part of the refrigerant in the receiver may be supplied to the compressor unit. Thereby the gaseous refrigerant is not subjected to the expansion done by the expansion device(s) and the compressor unit needs to increase the pressure of refrigerant from the gaseous outlet of the receiver less than refrigerant from the outlet(s) of the primary evaporator(s). This results in different energy comsumptions of compressors with the same mass flow depending on where the compressor sucks from.

From the liquid outlet of the receiver, the refrigerant path is split to separate the primary and the secondary refrigerant paths. The secondary refrigerant path will be described in further detail below. In the primary refrigerant path, refrigerant from the liquid outlet of the receiver is supplied to the expansion device(s), where the pressure of the refrigerant is reduced, before the refrigerant is supplied to the primary evaporator(s). Each expansion device supplies refrigerant to a specific primary evaporator, and therefore the refrigerant supply to each primary evaporator can be controlled individually by controlling the corresponding expansion device. The refrigerant being supplied to the primary evaporator(s) is thereby in a mixed gaseous and liquid state. In the primary evaporator(s), the liquid part of the refrigerant is at least partly evaporated, while heat exchange takes place with the ambient or with a secondary fluid flow across the primary evaporator(s), in such a manner that heat is absorbed by the refrigerant flowing through the primary evaporator(s). Finally, the refrigerant is supplied to the compressor unit.

Thus, at least part of the refrigerant flowing in the primary cooling circuit is alternatingly compressed by the compressor(s) of the compressor unit and expanded by the expansion device(s), while heat exchange takes place at the heat rejecting heat exchanger and at the primary evaporator(s). Thereby cooling or heating of one or more volumes can be obtained.

The secondary cooling circuit will now be described in detail. From the liquid outlet of the receiver, refrigerant is supplied to the flow control device, which controls the refrigerant flow to the secondary evaporator. The liquid part of the refrigerant passing through the secondary evaporator is at least partly evaporated, while heat exchange takes place with the ambient, or with another fluid flow across or in the secondary evaporator, in such a manner that heat is absorbed by the refrigerant flowing through the secondary evaporator. Finally, refrigerant from the primary evaporator outlet(s) is supplied to the secondary inlet of the ejector. The ejector increases the pressure of the refrigerant before it reaches the receiver. Thus, the ejector may ensure that any pressure drop that may have occurred in the flow control device does not affect the pressure in the receiver.

This allows the receiver pressure to be optimized irrespective of the pressure of refrigerant leaving the secondary evaporator, which may be controlled on the basis of the cooling power required from the secondary evaporator. It is advantageous to be able to optimize the pressure in the receiver irrespective of the required cooling power of the secondary evaporator, because the pressure in the receiver may be optimized with respect to several aspects. These aspects will be described below.

The pressure in the receiver is relevant for the cooling capacity of the primary evaporator(s). If the cooling power needed from the primary evaporator(s) is relatively high, the pressure in the receiver is normally also relatively high and vice versa. Moreover, relatively high ambient temperatures typically lead to a relatively high cooling power need and at realtively high ambient temperatures, the optimum receiver pressure is also typically relatively high.

Furthermore, the vapour compression system may be configured so that refrigerant from the gaseous outlet of the receiver is supplied to at least one receiver compressor instead of being mixed with refrigerant from the primary evaporator(s) prior to being supplied to the compressor unit. Refrigerant leaving the primary evaporator(s) has been expanded and thus has a lower pressure than refrigerant from the gaseous outlet of the receiver. The pressure of mixed refrigerant from the gaseous outlet of the receiver and from the primary evaporator(s) then has a lower pressure than refrigerant from the gaseous outlet of the receiver. This pressure drop may be avoided by supplying refrigerant from the gaseous outlet of the receiver to the receiver compressor(s). Moreover, if the pressure of refrigerant from the gaseous outlet of the receiver is relatively high, the receiver compressor(s) needs to lift the pressure of this refrigerant relatively little.

If refrigerant from the gaseous outlet of the receiver is supplied to the receiver

compressor(s), further optimization of the pressure in the receiver is relevant. The higher the pressure in the receiver, the less work is needed by the receiver compressor(s). An upper limit for the pressure in the receiver is set by the preference of having the receiver compressor(s) running as opposed to having the main compressor(s) of the compressor unit running. At some point, the pressure in the receiver may be so high that refrigerant in the receiver is predominantly in a liquid state. In this case, there may not be sufficient gaseous refrigerant to supply the receiver compressor(s). This point sets the upper limit for the pressure in the receiver.

The pressure in the receiver may also be optimized with respect to other parameters. In any case, the pressure in the receiver is advantageously optimized with respect to all relevant aspects. In a vapour compression system according to the present invention, the pressure in the receiver may be optimized with respect to all relevant parameters for optimum

performance of the primary cooling circuit, while at the same time, the refrigerant pressure in the secondary cooling circuit is optimized with respect to the optimum performance of the secondary evaporator. Furthermore, any pressure decrease that may occur between the liquid outlet of the receiver and the secondary evaporator may be lifted by the ejector. The ejector may lift this possible pressure drop more energy efficiently than a compressor might do.

Accordingly, a vapour compression system according to the present invention may be operated in a highly energy efficient manner. The flow control device may be an expansion device, such as an expansion valve, e.g. in the form of a mechanical valve or an electronic valve, or any other type of flow control device. The secondary evaporator may form at least part of e.g. a subcooler or an air conditioning system. The flow control device may be operated in accordance with the cooling power needed in the secondary evaporator.

The secondary evaporator may be or form part of an air conditioning unit.

In this case, the secondary refrigerant path and secondary cooling circuit is arranged to provide air conditioning, by means of the secondary evaporator being or forming part of an air conditioning unit. In the air conditioning unit, heat exchange may take place between the ambient and refrigerant evaporating in the secondary evaporator, or between a secondary fluid flow across the secondary evaporator and the ambient. Thus, according to this embodiment, the heat exchange taking place in the secondary evaporator is used for controlling an air temperature inside a room.

The refrigerant pressure in the secondary evaporator may be controlled on the basis of the cooling power required from the air conditioning unit, independently of the control of the primary cooling circuit, because the refrigerant leaving the secondary evaporator is supplied to the secondary inlet of the ejector instead of being supplied directly to the receiver. The cooling power required from the air conditioning unit may vary significantly over time and irrespective of the cooling power required from the primary evaporator(s). Moreover, the primary evaporator(s) and the air conditioning unit may provide cooling for separate areas with different cooling requirements. It is thus particularly useful to be able to control the pressure in the receiver irrespective of the cooling power required from the air conditioning unit.

Furthermore, the pressure drop that may occur in the secondary cooling circuit for the purpose of air conditioning is lifted by the ejector and not by the compressor unit. This is particularly advantageous as the ejector may lift the possible pressure drop in a more energy efficient manner than the compressor unit.

A vapour compression system according to present embodiment is able to provide adjustable cooling power to an air conditioning unit, while at the same time controlling the vapour compression system with optimum pressure in the receiver. Thus a vapour compression system according to present embodiment may be operated in a highly energy efficient manner.

The secondary evaporator may be or form part of a subcooler. In this case, the second refrigerant path comprises a subcooler or a secondary evaporator forming part of a subcooler. The subcooler in the secondary refrigerant path according to this embodiment is arranged to exchange heat with refrigerant flowing in the primary refrigerant path between the liquid outlet of the receiver and the expansion device(s). Moreover, the subcooler may cool down refrigerant in the primary refrigerant path before it reaches the expansion device(s) and primary evaporator(s). The colder refrigerant is less likely to spontaneously form gas when e.g. the liquid refrigerant is expanded by the expansion device(s). Such formed gaseous refrigerant is known as flash gas and can at least to some extent be avoided by subcooling refrigerant, i.e. the colder the liquid refrigerant is, the less likely it is to boil when it is expanded. This is advantageous as flash gas in general usually degrades the energy efficiency of the vapour compression system. Also, the subcooled refrigerant contains more cooling power than not subcooled refrigerant. Moreover, liquid subcooled refrigerant may absorb more heat in the primary evaporator(s) before it evaporates. Accordingly, the same amount of refrigerant provides more cooling power if is subcooled, than if it is not.

The amount of subcooling is related to the refrigerant pressure in the secondary evaporator. Because the refrigerant leaving the secondary evaporator is supplied to the secondary inlet of the ejector instead of being supplied directly to the receiver, the pressure in the receiver may be optimized irrespective of the amount of subcooling desired. Accordingly, a vapour compression system according to present embodiment may be controlled to have optimum pressure in the receiver, while at the same time limiting flash gas by subcooling refrigerant.

Furthermore, the pressure drop that may occur in the secondary cooling circuit for the purpose of subcooling is at least partly lifted by the ejector. This is particularly advantageous as the ejector may lift the possible pressure drop in a more energy efficient manner than the compressor unit.

Furthermore, the compressor capacity of the compressor unit may be relatively small, as the compressor unit need not lift the possible pressure drop due to subcooling. This compression work is done by the ejector. The compressor capacity of the compressor unit is directly related to the installation cost of the vapour compression system. A vapour compression system according to the present embodiment thus may be operated in a highly energy efficient manner, while possibly also lowering the installation cost of the vapour compression system.

As an alternative, the refrigerant path from the liquid outlet of the receiver may continue through the subcooler before the refrigerant path is split to separate the primary and the secondary refrigerant paths. In this case, refrigerant supplied to the secondary evaporator via the flow control device in the secondary refrigerant path has been subcooled. Accordingly, the cooling power of the subcooler is increased without changing the refrigerant mass flow in the secondary refrigerant path as described for the primary evaporator(s) above. A gaseous outlet of the receiver may communicate with an inlet of the compressor unit.

In this case, the vapour compression system is configured so that refrigerant from the gaseous outlet of the receiver may be supplied to the receiver compressor(s) instead of being mixed with refrigerant in the refrigerant path interconnecting the primary evaporator(s) and the compressor unit. A bypass valve may be arranged in a refrigerant path interconnecting the gaseous outlet of the receiver and the refrigerant path interconnecting the primary evaporator(s) and the compressor unit. The bypass valve may control if the refrigerant from the gaseous outlet of the receiver is at least partly supplied to the refrigerant path interconnecting the primary evaporator(s) and the compressor unit. This may be

advantageous if the compressor(s) connected to the gaseous outlet of the receiver is not able to remove the necessary amount of gaseous refrigerant from the receiver. It may also be advantageous if the amount of gaseous refrigerant to be removed from the receiver is not sufficient to supply the compressor(s) connected to the gaseous outlet of the receiver.

Refrigerant leaving the primary evaporator(s) has been expanded and thus has a lower pressure than refrigerant from the gaseous outlet of the receiver. The pressure of mixed refrigerant from the gaseous outlet of the receiver and from the primary evaporator(s) then has a lower pressure than refrigerant from the gaseous outlet of the receiver. This pressure drop may be avoided by supplying refrigerant from the gaseous outlet of the receiver to the receiver compressor(s) as descrived above.

Furthermore, the refrigerant circulating in the secondary cooling circuit for the purpose of subcooling or air conditioning is at least partly lifted by the ejector as the refrigerant in the secondary refrigerant path is supplied to the secondary inlet of the ejector. The compressed gaseous refrigerant from the secondary inlet of the ejector is then supplied to the receiver. From the gaseous outlet of the receiver, the refrigerant is supplied to the receiver

compressor(s) which compresses the refrigerant further. This compression work is lowered because the refrigerant in the secondary refrigerant path has already been compressed in the ejector.

Accordingly, a vapour compression system according to present embodiment may be operated in a highly energy efficient manner. At least one of the compressors of the compressor unit may be connectable to a gaseous outlet of the receiver. In this case, the vapour compression system is configured so that at least one of the compressors of the compressor unit can selectively connect to the gaseous outlet of the receiver. This enables the compressor(s) to be switched according to the varying

requirements of the vapour compression system and thus the compressor capacity of the compressor unit may be utilized in an optimum manner. Accordingly, the compressor unit needs lees compressor capacity and the installation cost of a vapour compression system according to present embodiment is reduced.

The compressor unit may comprise one or more valve arrangements, each valve

arrangement being arranged to selectively connect an inlet of a compressor of the compressor unit to a gaseous outlet of the receiver or to an outlet of the evaporator(s).

In this case, the refrigerant flow to at least one compressor of the compressor unit is selectively connected to the gaseous outlet of the receiver or to the outlet(s) of the primary evaporator(s) by a valve arrangement. Such valve arrangements may comprise manually or automatically operated mechanical valves, electronic valves, etc. By operating the valves, at least one of the compressors may be selectively switched between being supplied with refrigerant from the gaseous outlet of the receiver or the outlet(s) of the primary

evaporator(s). The selective switching of the valve arrangement may ensure that available compressor capacity is appropriately allocated, according to the current requirements, between compressing refrigerant received from the gaseous outlet of the receiver and compressing refrigerant received from the outlet(s) of the primary evaporator(s). Thus, sufficient compressor capacity can be provided for these two purposes with a minimal total compressor capacity, and the available compressor capacity can be utilised efficiently. Thus the compressor unit may comprise fewer compressors offering lower installation costs of the vapour compression system. At least one of the compressors of the compressor unit may have an inlet being connected to the gaseous outlet of the receiver, and at least one of the compressors of the compressor unit has an inlet which is connected to an outlet of the evaporator(s).In this case, a compressor in the compressor unit may be connected to the gaseous outlet of the receiver and/or to an outlet of the primary evaporator(s). If, for instance, the compressor unit comprises two compressors, the first compressor may be permanently connected to the gaseous outlet of the receiver and the second compressor may be permanently connected to an outlet of the primary evaporator(s). In this case the first compressor may be regarded as a dedicated receiver compressor and the second compressor as a dedicated main

compressor. A bypass valve may be arranged to allow refrigerant from the gaseous outlet of the receiver to reach the second compressor as well. This allows for a simple compressor unit arrangement while at the same time providing versatile receiver pressure optimization means.

Valve arrangement may allow one or more of the compressors of the compressor unit to receive refrigerant either from the gaseous outlet of the receiver or from the outlet(s) of the primary evaporator(s). Thereby the available compressor capacity can be appropriately distributed among these two purposes, as described above. For example, the compressor unit may comprise three compressors, wherein the first compressor is permanently connected to the gaseous outlet of the receiver, the second compressor is permanently connected to an outlet of the primary evaporator(s), and the third compressor is connected via a valve arrangement to both the gaseous outlet of the receiver and an outlet of the primary evaporator(s). The valve arrangement may be configured to allow the third compressor to receive refrigerant either from the gaseous outlet of the receiver or from the outlet of the primary evaporator(s). This allows for optimum distribution of the available compressor capacity of the compressor unit, as described above. Accordingly, a vapour compression system according to present invention allows the vapour compression system to be operated in a highly energy efficient manner, while also minimizing the total compressor capacity of the compressor unit and thus minimizing installation costs.

The flow control device may be or comprise a passively controlled valve.

In this case the refrigerant flow to the secondary evaporator is controlled by a passively controlled valve or a flow control device comprising a passively controlled valve. The passively controlled valve may e.g. be in the form of a flow control valve and/or an expansion valve expanding the refrigerant before it reaches the secondary evaporator. This is particularly advantageous as passively controlled valves add a minimum to vapour compression system installation costs and allows for simple control of the vapour

compression system.

The passively controlled valve may define an opening degree which is variable in response to a pressure difference across said valve.

In this case, the refrigerant flow in the secondary cooling circuit is controlled in response to a pressure difference across the passively controlled valve. Moreover, according to the present embodiment, the refrigerant flow in the secondary cooling circuit scales with the pressure in the receiver. This is particularly advantageous as it allows for simple control of the refrigerant flow in the secondary cooling circuit. Furthermore, if the secondary cooling circuit comprises a subcooler, the refrigerant flow in said circuit may advantageously be directly linked to the pressure in the receiver, as the need for subcooling may be relatively constant with respect to the optimum pressure in the receiver.

A passively controlled valve defining an opening degree in response to a pressure difference across the valve is known in the art and is possible to manufacture at relatively low cost. Accordingly, the valve adds relatively little to the overall manufacturing and installation cost of the vapour compression system.

The flow control device may be or comprise an actively controlled valve.

In this case, the actively controlled valve may be controlled on the basis of any relevant parameter. Moreover, the refrigerant flow in the secondary cooling circuit may be optimized with respect to a parameter of choice. The actively controlled valve may e.g. be controlled to keep the pressure in the secondary evaporator constant. It may also be controlled to maintain a particular superheat, i.e. a constant temperature difference between the temperature of refrigerant at the outlet of the secondary evaporator and the evaporation temperature of the refrigerant at the given pressure. It may also be controlled on the basis of the pressure in the receiver.

Accordingly, a vapour compression system according to present embodiment may be controlled to optimize several parameters and thus may be operated in a highly efficient manner.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in further detail with reference to the accompanying drawings in which

Fig. 1 is a diagrammatic view of a vapour compression system comprising a subcooler according to a first embodiment of the invention,

Fig. 2 is a diagrammatic view of a vapour compression system comprising a subcooler according to a second embodiment of the invention,

Fig. 3 is a diagrammatic view of a vapour compression system comprising a subcooler according to a third embodiment of the invention, Fig. 4 is a diagrammatic view of a vapour compression system comprising an air conditioning unit according to a fourth embodiment of the invention, and

Fig. 5 is a logP-h diagram for a vapour compression system according to an embodiment of the invention. DETAILED DESCRIPTION OF THE DRAWINGS

It should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. Fig. 1 is a diagrammatic view of a vapour compression system 1 according to a first embodiment of the invention. The vapour compression system 1 comprises a compressor unit 3 comprising two compressors 5, a heat rejecting heat exchanger 7, an ejector 9, a high pressure valve 24, a receiver 11, a bypass valve 13, an expansion device 15, and a primary evaporator 17 arranged in a primary refrigerant path. The vapour compression system 1 further comprises a flow control device 19 and a subcooler 21. The flow control device 19, the subcooler 21, the ejector 9 and the receiver 11 form a secondary refrigerant path. As such, the ejector 9 and the receiver 11 form part of both the primary refrigerant path and the secondary refrigerant path.

Refrigerant flowing in the primary refrigerant path is compressed by the two compressors 5 of the compressor unit 3. The compressed refrigerant is supplied to the heat rejecting heat exchanger 7, where heat exchange takes place in such a manner that heat is rejected from the refrigerant.

The refrigerant leaving the heat rejecting heat exchanger 7 may be supplied to a primary inlet 23 of the ejector 9, before being supplied to the receiver 11. The refrigerant leaving the heat rejecting heat exchanger 7 may also be supplied to a refrigerant path interconnecting the heat rejecting heat exchanger 7 and the receiver 11 via a high pressure valve 24. If the high pressure valve 24 is closed, all refrigerant leaving the heat rejecting heat exchanger 7 is supplied to the receiver 11 via the ejector 9. Refrigerant leaving the heat rejecting heat exchanger 7 may also be supplied to the receiver via the high pressure valve 24, if the valve 24 is open. This is particularly advantageous when the ejector 9 is not running or when the refrigerant flow from the heat rejecting heat exchanger 7 to the receiver 11 is larger than the flow capacity of the ejector 9. When refrigerant leaving the heat rejecting heat exchanger 7 passes through the ejector 9 or the high pressure valve 24, the refrigerant undergoes expansion. Thereby the pressure of the refrigerant is reduced, and the refrigerant being supplied to the receiver 11 is in a mixed liquid and gaseous state. In the receiver 11 the refrigerant is separated into a liquid part and a gaseous part. The liquid part of the refrigerant in the primary refrigerant path is supplied to the primary evaporator 17, via a liquid outlet 25 of the receiver 11 and the expansion device 15. In the primary evaporator 17, the liquid part of the refrigerant is at least partly evaporated, while heat exchange takes place in such a manner that heat is absorbed by the refrigerant. The refrigerant leaving the primary evaporator 17 is supplied to the compressors 5 of the compressor unit 3.

Thus, at least part of the refrigerant flowing in the primary refrigerant path is alternatingly compressed by the compressors 5 of the compressor unit 3 and expanded by the expansion device 15, while heat exchange takes place at the heat rejecting heat exchanger 7 and at the primary evaporator 17. Thereby cooling or heating of a volume can be obtained and the primary refrigerant path may be regarded as a primary cooling circuit.

Refrigerant flowing in the secondary refrigerant path is compressed by the ejector 9. This compression is facilitated by the expansion of refrigerant in the primary refrigerant path supplied to primary inlet 23 of the ejector 9, while refrigerant in the secondary refrigerant path supplied to the secondary inlet 27 of the ejector 9 is compressed. Moreover, the refrigerant from the primary 23 and secondary inlet 27 of the ejector is mixed to obtain a common pressure of refrigerant at the outlet 29 of the ejector 9. From the outlet 29 of the ejector 9 refrigerant is supplied to the receiver 11. As mentioned above, the ejector 9 and the receiver 11 form part of both the primary and the secondary refrigerant path. From the liquid outlet 25 of the receiver 11, refrigerant in the secondary refrigerant path is supplied to the flow control device 19. This flow control device 19 expands the refrigerant and/or controls the refrigerant flow to the subcooler 21 arranged to cool down refrigerant flowing in the part of the primary refrigerant interconnecting the receiver 11 and the expansion device 15. In the subcooler 21, the liquid part of the refrigerant is at least partly evaporated to exchange heat with refrigerant flowing in the primary refrigerant path. In this way, the subcooler 21 cools down refrigerant in the primary refrigerant path before it reaches the expansion device 15 and the primary evaporator 17. This has at least two effects. Firstly, the colder refrigerant is less likely to spontaneously form gas when e.g. the liquid refrigerant is expanded by the expansion device 15. Such formed gaseous refrigerant is known as flash gas and can at least to some extent be avoided by subcooling refrigerant, i.e. the colder the liquid refrigerant is, the less likely it is to boil when it is expanded. This is advantageous as flash gas in general usually degrades the energy efficiency of the vapour compression system 1. Secondly, the subcooled refrigerant contains more cooling power than not subcooled refrigerant. Moreover, liquid subcooled refrigerant may absorb heat in the primary evaporator 17 before it evaporates. Accordingly, the same amount of refrigerant provides more cooling power if is subcooled, than if it is not.

From the subcooler 21, refrigerant in the secondary refrigerant path is supplied to the secondary inlet 27 of the ejector 9. From the gaseous outlet 31 of the receiver 11, refrigerant may be supplied via the bypass valve 13 to the refrigerant path interconnecting the primary evaporator 17 and the compressor unit 3.

Thus, at least part of the refrigerant flowing in the secondary refrigerant path is alternatingly compressed by the ejector 9 and expanded by the flow control device 19. The expansion in the flow control device also cools down the refrigerant. Heat exchange takes place at the subcooler 21. Thereby cooling of refrigerant flowing in the part of the primary refrigerant interconnecting the receiver 11 and the expansion device 15 is provided, and the secondary refrigerant path may be regarded as a secondary cooling circuit.

The amount of subcooling is related to the refrigerant pressure in the subcooler 21. Because the refrigerant leaving the subcooler 21 is supplied to the secondary inlet 27 of the ejector 9 instead of being supplied directly to the receiver 11, the pressure in the receiver 11 may be optimized irrespective of the amount of subcooling desired. Accordingly, a vapour

compression system 1 according to the first embodiment may be controlled to have optimum pressure in the receiver 11, while at the same time limiting flash gas by subcooling refrigerant.

Furthermore, the expansion work leading to a drop in refrigerant pressure that occurs in the flow control device 19 in the secondary cooling circuit for the purpose of subcooling is lifted by the ejector 9 and not by the compressor unit 3. This is particularly advantageous as the ejector 9 may lift the pressure drop in a more energy efficient manner than the compressor unit 3. Furthermore, the compressor capacity of the compressor unit 3 may be relatively small, as the compressor unit 3 need not lift the pressure drop due to subcooling. This compression work is done by the ejector 9. The compressor capacity of the compressor unit 3 is directly related to the installation cost of the vapour compression system 1. A vapour compression system 1 according to the first embodiment thus may be operated in a highly energy efficient manner, while possibly also lowering the installation cost of the vapour compression system 1.

Fig. 2 is a diagrammatic view of a vapour compression system 1 according to a second embodiment of the invention. The vapour compression 1 of Fig. 2 is similar to that of Fig. 1 except for the following differences. The vapour compression 1 of Fig. 2 does not comprise a high pressure valve 24. Accordingly, in the vapour compression 1 of Fig. 2, refrigerant leaving the heat rejecting heat exchanger 7 is always supplied to the primary inlet 23 of the ejector 9. Also, the compressor unit 3 is arranged with a number of main compressors 35, of which two are shown, connected to the outlet of the primary evaporator 17. The compressor unit 3 is further arranged to have dedicated receiver compressor 33 connected to the gaseous outlet 31 of the receiver 11.

The vapour compression system 1 according to the second embodiment is thus configured so that refrigerant from the gaseous outlet 31 of the receiver 11 can be supplied to the receiver compressor 33 instead of being mixed with refrigerant in the refrigerant path interconnecting the primary evaporator 17 and the main compressors 35. The bypass valve 13 controls if the refrigerant from the gaseous outlet 31 of the receiver 11 is at least partly supplied to the refrigerant path interconnecting the primary evaporator 17 and the main compressors 35. This may be advantageous if the receiver compressor 33 is not able to remove the necessary amount of gaseous refrigerant from the receiver 11. It may also be advantageous if the amount of gaseous refrigerant to be removed from the receiver 11 is not sufficient to supply the receiver compressor 33 with enough refrigerant to keep it running in an energy efficient manner.

Refrigerant leaving the primary evaporator 17 has been expanded and thus has a lower pressure than refrigerant from the gaseous outlet 31 of the receiver 11. The pressure of mixed refrigerant from the gaseous outlet 31 of the receiver 11 and from the primary evaporator 17 then has a lower pressure than refrigerant from the gaseous outlet 31 of the receiver 11. This pressure drop is avoided by supplying refrigerant from the gaseous outlet 31 of the receiver 11 to the receiver compressor 33 as is the case in the second embodiment. Thus, a vapour compression system 1 according to the second embodiment may be operated in a highly efficient manner.

Fig. 3 is a diagrammatic view of a vapour compression system 1 according to a third embodiment of the invention. The vapour compression 1 of Fig. 3 is similar to that of Fig. 2 except for the following difference. A valve arrangement in the form of a three way valve 37 is arranged to distribute compressor capacity of the compressor unit 3 to where it is needed. As described with reference to the second embodiment, the main compressor 35 with a main compressor capacity is always connected to the outlet of the primary evaporator 17 and the receiver compressor 33 with a receiver compressor capacity is always connected to the gaseous outlet 31 of the receiver 11. However, according to the third embodiment, the three way valve 37 is arranged to distribute the compressor capacity of a distributable compressor 39 with a distributable compressor capacity to add to the main compressor capacity or to the receiver compressor capacity.

According to the third embodiment, refrigerant from the gaseous outlet 31 of the receiver 11 is always supplied to the receiver compressor 33. It may also be supplied via the three way valve 37 to the switchable compressor 39 and/or via the bypass valve 13 to the refrigerant path interconnecting the primary evaporator 17 and the main compressor 35. Furthermore, refrigerant from the primary evaporator 17 is always supplied to the main compressor 35. It may also be supplied via the three way valve 37 to the switchable compressor 39.

In a vapour compression system 1 according to the third embodiment, the selective distribution of the distributable compressor capacity by means of the three way valve 37 and the bypass valve 13 may ensure that the compressor capacity of the compressor unit is distributed to where compressor capacity is needed. Thus the compressor 3 unit may comprise fewer compressors offering lower installation costs of the vapour compression system. Also, the selective distribution of compressor capacity allows a vapour compression system 1 according to the third embodiment to be operated in a highly energy efficient manner.

Fig. 4 is a diagrammatic view of a vapour compression system 1 according to a fourth embodiment of the invention. The vapour compression 1 of Fig. 4 is similar to that of Fig. 2 except for the following difference. The secondary cooling circuit comprises an air conditioning unit 40 and not a subcooler 21.

So according to the fourth embodiment, the secondary refrigerant path and secondary cooling circuit is arranged to provide air conditioning for a volume by means of the air conditioning unit 40. In the air conditioning unit 40, heat exchange takes place between the ambient and refrigerant evaporating in the air conditioning unit 40. Thus, according to this embodiment, the heat exchange taking place in the air conditioning unit 40 is used for controlling an air temperature inside a room.

The refrigerant pressure in the air conditioning unit 40 may be controlled on the basis of the cooling power required from the air conditioning unit 40, independently of the control of the primary cooling circuit, because the refrigerant leaving the air conditioning unit 40 is supplied to the secondary inlet 27 of the ejector 9 instead of being supplied directly to the receiver 11. The cooling power required from the air conditioning 41 unit may vary significantly over time and irrespective of the cooling power required from the primary evaporator 17. Moreover, the primary evaporator 17 and the air conditioning 41 unit may provide cooling power for separate areas with different cooling requirements. It is thus particularly useful to be able to control the pressure in the receiver 11 irrespective of the cooling power required from the air conditioning unit 40.

Furthermore, the expansion work leading to a drop in refrigerant pressure that occurs in the flow control device 19 in the secondary cooling circuit for the purpose of air conditioning 41 is lifted by the ejector 9 and not by the compressor unit 3. This is particularly advantageous as the ejector 11 may lift the pressure drop in a more energy efficient manner than the compressor unit 3.

Accordingly, a vapour compression system 1 according to the fourth embodiment is able to provide adjustable cooling power to an air conditioning unit 40, while at the same time controlling the vapour compression system 1 with optimum pressure in the receiver 11. Thus a vapour compression system 1 according to present embodiment may be operated in a highly energy efficient manner.

Fig. 5 is a logP-h diagram for a vapour compression system according to an embodiment of the invention. The vapour compression system could, e.g., be the vapour compression system illustrated in Fig. 1, Fig. 2 or Fig. 3.

During normal operation of the vapour compression system, in the primary refrigerant path, at point 41 refrigerant enters one or more compressors of the compressor unit being connected to the outlet of the primary evaporator. From point 41 to point 43 the refrigerant is compressed by this compressor or these compressors. Similarly, at point 45 refrigerant enters one or more compressors of the compressor unit being connected to the gaseous outlet of the receiver. From point 45 to point 47 the refrigerant is compressed by this compressor or these compressors. It can be seen that the compression results in an increase in pressure as well as in enthalpy for the refrigerant.

From points 43 and 47, respectively, to point 49 the refrigerant passes through the heat rejecting heat exchanger, where heat exchange takes place in such a manner that heat is rejected by the refrigerant. This results in a decrease in enthalpy, while the pressure remains constant. From point 49 to point 51 the refrigerant passes through the high pressure valve and undergoes expansion before it is supplied to the receiver.

Alternatively, from point 49 to point 51a the refrigerant passes through the ejector, and is supplied to the receiver. Thereby the refrigerant undergoes expansion, resulting in a decrease in the pressure of the refrigerant and a slight decrease in enthalpy as it performs compression work on refrigerant from the secondary inlet of the ejector.

Point 53 represents the liquid part of the refrigerant in the receiver, and from point 53 to point 55 the refrigerant passes through the expansion device without being subcooled, thereby decreasing the pressure of the refrigerant. Similarly, point 45 represents the gaseous part of the refrigerant in the receiver, being supplied directly to the compressors that are connected to the gaseous outlet of the receiver.

From point 55 to point 41 the refrigerant passes through the primary evaporator, where heat exchange takes place in such a manner that heat is absorbed by the refrigerant. Thereby the enthalpy of the refrigerant is increased, while the pressure remains constant. From point 53 to point 53a the refrigerant passes through the subcooler. The enthalpy difference between points 55a and 41 and the enthalpy difference between points 55 and 41 represents the absorbed heat of subcooled refrigerant and not subccoled refrigerant passing through the primary evaporator. Thus, the difference in enthalpy between point 55a and 55 corresponds to the added cooling power of the subcooled refrigerant as compared to the not subcooled refrigerant.

From point 53 to point 57 refrigerant passes through the flow control device in the secondary refrigerant path and is expanded. Then from point 57 to point 59 refrigerant passed through the subcooler, where heat exchange takes place in such a manner that heat is absorbed by the refrigerant. Thereby the enthalpy of the refrigerant is increased, while the pressure remains constant.

From point 59 to point 61 refrigerant from the subcooler is compressed by the ejector. This refrigerant is herein mixed with refrigerant from the heat rejecting heat exchanger. The mixed refrigerant can be represented by a point lying on the line between points 51a and 61 as it is composed of refrigerant represented by point 51a and by point 61. The position of the point relative to that of points 51a and point 61 depends on how much of the mixed refrigerant stems from point 51a and from point 61, i.e. it depends how much of the refrigerant at the outlet of the ejector stems from the primary inlet and the secondary inlet of the ejector, respectively.