WO2011072679A1 - A vapour compression system with split evaporator - Google Patents

A vapour compression system with split evaporator Download PDF

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Publication number
WO2011072679A1
WO2011072679A1 PCT/DK2010/000100 DK2010000100W WO2011072679A1 WO 2011072679 A1 WO2011072679 A1 WO 2011072679A1 DK 2010000100 W DK2010000100 W DK 2010000100W WO 2011072679 A1 WO2011072679 A1 WO 2011072679A1
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WO
WIPO (PCT)
Prior art keywords
evaporator
compressor
compression system
vapour compression
path
Prior art date
Application number
PCT/DK2010/000100
Other languages
French (fr)
Inventor
Claus Thybo
Lars Finn Sloth Larsen
Gunda Mader
Original Assignee
Danfoss A/S
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Publication date
Priority to DKPA200901349 priority Critical
Priority to DKPA200901349 priority
Application filed by Danfoss A/S filed Critical Danfoss A/S
Publication of WO2011072679A1 publication Critical patent/WO2011072679A1/en

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Classifications

    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25BREFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B5/00Compression machines, plants or systems, with several evaporator circuits, e.g. for varying refrigerating capacity
    • F25B5/02Compression machines, plants or systems, with several evaporator circuits, e.g. for varying refrigerating capacity arranged in parallel
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25BREFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B1/00Compression machines, plants or systems with non-reversible cycle
    • F25B1/10Compression machines, plants or systems with non-reversible cycle with multi-stage compression
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28HEAT EXCHANGE IN GENERAL
    • F28DHEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D1/00Heat-exchange apparatus having stationary conduit assemblies for one heat-exchange medium only, the media being in contact with different sides of the conduit wall, in which the other heat-exchange medium is a large body of fluid, e.g. domestic or motor car radiators
    • F28D1/02Heat-exchange apparatus having stationary conduit assemblies for one heat-exchange medium only, the media being in contact with different sides of the conduit wall, in which the other heat-exchange medium is a large body of fluid, e.g. domestic or motor car radiators with heat-exchange conduits immersed in the body of fluid
    • F28D1/04Heat-exchange apparatus having stationary conduit assemblies for one heat-exchange medium only, the media being in contact with different sides of the conduit wall, in which the other heat-exchange medium is a large body of fluid, e.g. domestic or motor car radiators with heat-exchange conduits immersed in the body of fluid with tubular conduits
    • F28D1/0408Multi-circuit heat exchangers, e.g. integrating different heat exchange sections in the same unit or heat exchangers for more than two fluids
    • F28D1/0417Multi-circuit heat exchangers, e.g. integrating different heat exchange sections in the same unit or heat exchangers for more than two fluids with particular circuits for the same heat exchange medium, e.g. with the heat exchange medium flowing through sections having different heat exchange capacities or for heating/cooling the heat exchange medium at different temperatures
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28HEAT EXCHANGE IN GENERAL
    • F28DHEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D1/00Heat-exchange apparatus having stationary conduit assemblies for one heat-exchange medium only, the media being in contact with different sides of the conduit wall, in which the other heat-exchange medium is a large body of fluid, e.g. domestic or motor car radiators
    • F28D1/02Heat-exchange apparatus having stationary conduit assemblies for one heat-exchange medium only, the media being in contact with different sides of the conduit wall, in which the other heat-exchange medium is a large body of fluid, e.g. domestic or motor car radiators with heat-exchange conduits immersed in the body of fluid
    • F28D1/04Heat-exchange apparatus having stationary conduit assemblies for one heat-exchange medium only, the media being in contact with different sides of the conduit wall, in which the other heat-exchange medium is a large body of fluid, e.g. domestic or motor car radiators with heat-exchange conduits immersed in the body of fluid with tubular conduits
    • F28D1/0408Multi-circuit heat exchangers, e.g. integrating different heat exchange sections in the same unit or heat exchangers for more than two fluids
    • F28D1/0426Multi-circuit heat exchangers, e.g. integrating different heat exchange sections in the same unit or heat exchangers for more than two fluids with units having particular arrangement relative to the large body of fluid, e.g. with interleaved units or with adjacent heat exchange units in common air flow or with units extending at an angle to each other or with units arranged around a central element
    • F28D1/0435Combination of units extending one behind the other
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25BREFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B39/00Evaporators; Condensers
    • F25B39/02Evaporators

Abstract

A vapour compression system (1) comprising a compressor (2), a condenser (3), an expansion device (4) and an evaporator (5) arranged along a refrigerant path is disclosed. The evaporator (5) comprises at least two evaporator paths (5a, 5b, 5c, 5d) arranged fluidly in parallel between the expansion device (4) and the compressor (2). Each evaporator path (5a, 5b, 5c, 5d) is fluidly connected to the compressor (2) via a separate suction line (7a, 7b, 7c), and the suction pressure in each of the suction lines (7a, 7b, 7c) is distinct from the suction pressure in each of the other suction line(s) (7a, 7b, 7c). The separate suction lines (7a, 7b, 7c) allows the evaporator paths (5a, 5b, 5c, 5d) to be operated at different evaporator temperatures and pressures. Thereby a large temperature change of a secondary flow (6) across the evaporator (5) can be obtained while the suction pressure of a part of the mass flow is kept at a high level. The work required by the compressor (2) is reduced and energy is conserved.

Description

A VAPOUR COMPRESSION SYSTEM WITH SPLIT EVAPORATOR FIELD OF THE INVENTION
The present invention relates to a vapour compression system, such as a refrigeration system, e.g. an air condition system, the vapour compression system comprising a compressor, a condenser, an expansion device and an evaporator, the evaporator comprising at least two evaporator paths arranged fluidly in parallel.
BACKGROUND OF THE INVENTION
In vapour compression systems fluid medium, such as refrigerant, is circulated along a refrigerant path wherein the components of the vapour compression system are arranged. The fluid medium is compressed in a compressor. The compressed fluid medium is then fed to a condenser, where the compressed fluid medium is condensed, the fluid medium leaving the condenser thereby being substantially in a liquid state. The fluid medium is then fed to an expansion device, where it is expanded before entering an evaporator. In the evaporator the fluid medium is evaporated before once again entering the compressor, thereby completing the cycle.
As the fluid medium is evaporated in the evaporator, heat exchange takes place between the fluid medium and a secondary fluid flow across the evaporator, thereby cooling the fluid of the secondary fluid flow. This may be used for providing refrigeration to a closed volume, such as a room or a refrigeration entity, e.g. of the kind used in supermarkets. In the case that the difference between the temperature of the incoming secondary fluid flow and the desired outlet temperature is relatively large, it is necessary to control the operation of the vapour compression system in such a manner that the evaporator temperature, and thereby the pressure in the evaporator, is very low, in order to ensure a sufficiently high refrigeration capacity. This is undesirable, since it is very energy consuming, in particular because a relatively high amount of energy is consumed by the compressor in order to compress the low pressure fluid medium leaving the evaporator.
For instance, in the case that the vapour compression system is an air condition system, the fluid of the secondary fluid flow is air which is refrigerated, due to heat exchange with the fluid medium evaporating in the evaporator, in order to reduce the temperature inside an enclosure, such as a room. In some cases it may be required to reduce the temperature of air flowing across the evaporator from approximately 26°C to approximately 10°C in order to obtain a desired temperature of the enclosure. In this case the evaporator temperature must be maintained below 10°C.
US 2,215,327 discloses an air condition system comprising an evaporator with two evaporator coils arranged fluidly in parallel in the refrigerant path. The evaporator coils are further arranged in series with respect to the path of the air circulated across the evaporator. One of the evaporator coils is maintained at a higher refrigerant pressure and surface temperature than the other evaporator coil. The evaporator coil with the higher surface temperature is used for lowering the temperature of the air passing over the evaporator, and the evaporator coil with the lower temperature is used for lowering the temperature of the air passing over the evaporator as well as for lowering the humidity of the air passing over the evaporator. In order to maintain the evaporator coils at different pressures, each evaporator coil is provided with a suction pressure control valve which controls the flow of refrigerant through the corresponding evaporator coil. The valves are of the same construction, but are adjusted to maintain different refrigerant pressures in the evaporator coils. The suction pressure control valves are arranged fluidly between the evaporator coils and a common suction line being fluidly connected to the compressor. The suction pressure control valves reduce the pressure of the refrigerant leaving the evaporator coils, and the refrigerant pressure prevailing in the common suction line is therefore lower than the refrigerant pressure of the refrigerant leaving at least one of the evaporator coils. Accordingly, the energy consumed by the compressor in order to compress the refrigerant received via the common suction line is relatively high.
DESCRIPTION OF THE INVENTION
It is, thus, an object of embodiments of the invention to provide a vapour compression system which provides a high refrigeration capacity with reduced energy consumption as compared to prior art systems.
According to a first aspect the invention provides a vapour compression system comprising a compressor, a condenser, an expansion device and an evaporator arranged along a refrigerant path, the evaporator comprising at least two evaporator paths arranged fluidly in parallel between the expansion device and the compressor, wherein each evaporator path is fluidly connected to the compressor via a separate suction line, and wherein the suction pressure in each of the suction lines is distinct from the suction pressure in each of the other suction line(s). In the present context the term 'vapour compression system' should be interpreted to mean any system in which a flow of 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 compressor may be in the form of a single compressor, or it may be a compressor rack comprising two or more compressors.
The evaporator comprises at least two evaporator paths. The evaporator paths are arranged fluidly in parallel along the refrigerant path between the expansion device and the compressor. Thus, a flow of refrigerant leaving the expansion device is split into a number of parallel refrigerant flows corresponding to the number of evaporator paths, and is fed to each of the parallel evaporator paths. The refrigerant then passes the evaporator via each of the parallel evaporator paths. The evaporator paths may, e.g., be in the form of separate evaporators, individual evaporator tubes or coils within a single evaporator or separate compartments of a single evaporator tube, as long as the evaporator paths are arranged fluidly in parallel as described above. Each of the evaporator paths is fluidly connected to the compressor via a separate suction line. Thus, refrigerant leaving a given evaporator path is supplied directly to the compressor via an associated suction line, without being mixed with refrigerant leaving the other evaporator path(s). Thereby the suction pressure in a given suction line is determined by the operation of the evaporator path which is connected to that suction line, including the evaporator
temperature and the pressure prevailing in the evaporator path, and it is independent of the operation of the other evaporator path(s). Accordingly, different evaporator temperatures of two evaporator paths results in different suction pressures in the two suction lines connected to the two evaporator paths.
In order to ensure that distinct pressures are obtained in the evaporator paths, the expansion device should be capable of delivering refrigerant which has been expanded to distinct pressures. This could, e.g., be obtained by providing an expansion device which splits the refrigerant path into a number of parallel refrigerant paths, corresponding to the number of evaporator paths, during expansion. As an alternative, the refrigerant path may be split into a number of parallel refrigerant paths, corresponding to the number of evaporator paths, prior to the expansion. In this case a separate expansion device may be connected into each refrigerant path, thereby providing separate expansion of the refrigerant flowing in each of the parallel refrigerant paths. Thereby each expansion device delivers expanded refrigerant to one of the evaporator paths.
The separate suction lines, thus, makes it possible to operate the evaporator paths at different pressures, and thereby at different evaporator temperatures, similarly to the air condition system of US 2,215,327, but without the
requirement of suction pressure control valves arranged at the outlets of the evaporator paths, i.e. the pressure of refrigerant leaving a given evaporator path is not lowered prior to being fed to the compressor.
The suction pressure in each of the suction lines is distinct from the suction pressure in each of the other suction line(s). Thereby the evaporator
temperatures of the evaporator paths are also distinct, allowing a secondary fluid flow across the evaporator to be gradually cooled by successive
evaporator paths. Thereby a desired target temperature can be reached, without requiring that all of the evaporator paths have a very low evaporator temperature. Thus, though some of the evaporator paths may have a very low temperature, this will only apply to part of the total mass flow, the remaining part of the mass flow having a higher temperature and thereby a higher suction pressure. Furthermore, since the refrigerant leaving a given evaporator path is fed directly to the compressor without being mixed with refrigerant leaving the other evaporator path(s), the pressure of refrigerant reaching the compressor is the highest possible. For instance, refrigerant flowing via an evaporator path with a relatively high evaporator temperature, and thereby a relatively high suction pressure in the associated suction line, has a relatively high pressure when it reaches the compressor. Accordingly, the energy consumed by the compressor in order to compress this part of the mass flow is much smaller than would be the case if the refrigerant had passed through a pressure reduction valve, lowering the pressure, before being mixed with the refrigerant flow from the other evaporator path(s). Thereby the energy consumption of the
compressor is minimised and reduced considerably as compared to the energy consumption of the compressor of prior art vapour compression systems, including the air condition system of US 2,215,327.
The evaporator paths may be arranged in series along a flow direction of a secondary flow across the evaporator. According to this embodiment, the secondary flow across the evaporator passes each of the evaporator paths sequentially. The evaporator may advantageously be controlled in such a manner that evaporator temperatures of the evaporator paths changes monotonously along the flow direction of the secondary flow, the secondary flow thereby 'experiencing' a gradual temperature change.
Thus, according to one embodiment, a first evaporator path may be arranged upstream along the secondary flow direction relative to a second evaporator path, and the suction pressure in a first suction line fluidly interconnecting the first evaporator path and the compressor may be higher than the suction pressure in a second suction line fluidly interconnecting the second evaporator path and the compressor. In this case the evaporator temperature of the first, upstream, evaporator path is higher than the evaporator temperature of the second, downstream, evaporator path. Thereby the temperature of the secondary flow is first decreased from a first level to a second level by means of the first evaporator path and then further decreased from the second level to a third level by means of the second evaporator path. It should be noted that the evaporator may comprise additional evaporator paths arranged between the first evaporator path and the second evaporator path, upstream relative to the first evaporator path and/or downstream relative to the second evaporator path.
As mentioned above, the evaporator paths may be in the form of individual evaporators. As an alternative, the evaporator paths may be in the form of parallel evaporator tubes of a single evaporator. As another alternative, the evaporator paths may be in the form of individual paths formed inside a single evaporator tube, e.g. by means of separating walls formed inside the tube.
The evaporator may comprise and inlet header and/or an outlet header. The inlet header and/or the outlet header may be split into at least two header volumes, each header volume being fluidly connected to an evaporator path. In the case that the evaporator comprises an inlet header, the inlet header ensures that the refrigerant is distributed among the evaporator paths in an appropriate manner. In the case that the evaporator comprises an outlet header, the outlet header interconnects each evaporator path with the associated suction line. As an alternative to an inlet and/or outlet header, the evaporator paths may be fluidly connected directly to the expansion device and/or to the associated suction line.
The expansion device may be or comprise a thermostatic expansion valve. According to this embodiment, the expansion device further controls the supply of fluid medium to the evaporator. As an alternative, the expansion device may be or comprise an orifice, a capillary tube or any other suitable kind of expansion device.
The compressor may comprise an outlet opening being fluidly connected to the condenser, and two or more inlet openings, each inlet opening being fluidly connected to a suction line. According to this embodiment, the compressor is in the form of a single compressor, receiving refrigerant from each of the suction lines separately via the inlet openings, and delivering a single flow of refrigerant via the outlet opening.
The compressor may, in this case, comprise at least two sub-compressors, each sub-compressor having an inlet opening being fluidly connected to a suction line. According to this embodiment, refrigerant from a given suction line is supplied to a corresponding sub-compressor. The refrigerant leaves the compressor in a single flow.
At least a first sub-compressor may have an outlet opening being fluidly connected to an inlet opening of a second sub-compressor. Thereby, the refrigerant supplied to the first sub-compressor by one suction line is initially compressed to reach a certain pressure level before it is mixed with the refrigerant supplied directly from another suction line to the second sub- compressor at the inlet opening of the second sub-compressor. The second sub-compressor then compresses the refrigerant supplied directly to the second sub-compressor by the corresponding suction line, as well as the refrigerant supplied by the first sub-compressor. It should be noted that the compressor could comprise further sub-compressors, and that these may be interconnected in a similar manner, i.e. the second sub-compressor may have an outlet opening being fluidly connected to an inlet opening of a third sub-compressor. Thus, each sub-compressor increases the pressure level of a given part of the mass flow of refrigerant sufficiently to allow this part of the mass flow to mix with another part of the mass flow. The last sub-compressor in the chain of sub- compressors formed in this manner delivers the final flow of refrigerant from the compressor to the condenser.
Thus, the first sub-compressor may advantageously be adapted to increase the pressure of refrigerant received from the corresponding suction line to a pressure level being substantially equal to a pressure level of refrigerant supplied to the second compressor from the corresponding suction line.
At least one of the sub-compressors may be arranged inside another one of the sub-compressors. This is, e.g., the case when the compressor is a multistage scroll compressor. In this case the sub-compressors may have spiral shapes arranged in an interleaved manner or one inside the other. Thereby a very compact design is obtained.
As an alternative, the compressor may be in the form of a compressor rack comprising two or more compressors or subcompressors, each having a single inlet opening and a single outlet opening. In this case each suction line may be fluidly connected to the inlet opening of a given compressor or subcompressor, i.e. each compressor or subcompressor receives refrigerant from a single suction line via its inlet opening. Furthermore, each compressor or
subcompressor delivers refrigerant via its outlet opening, and the refrigerant delivered by the compressors or subcompressors may be mixed to form a single flow of refrigerant. The vapour compression system may be a refrigeration system, such as an air condition system or a heat pump. 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 schematic view of a first prior art vapour compression system, Fig. 2 is a pressure-enthalpy diagram illustrating the operation of the vapour compression system of Fig. 1,
Fig. 3 is a schematic view of a second prior art vapour compression system,
Fig. 4 is a pressure-enthalpy diagram illustrating the operation of the vapour compression system of Fig. 3, Fig. 5 is a schematic view of a vapour compression system according to a first embodiment of the invention,
Fig. 6 is a pressure-enthalpy diagram illustrating the operation of the vapour compression system of Fig. 5,
Fig. 7 is a schematic view of a vapour compression system according to a second embodiment of the invention,
Fig. 8 is a schematic view of a vapour compression system according to a third embodiment of the invention, and
Figs. 9-13 illustrate various designs of an evaporator for use in a vapour compression system according to an embodiment of the invention. DETAILED DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic view of a first prior art vapour compression system 1. The vapour compression system 1 comprises a compressor 2, a condenser 3, an expansion device 4 and an evaporator 5, arranged along a refrigerant path. A secondary fluid flow, illustrated by arrows 6, flows across the evaporator 5 in such a manner that heat transfer takes place between refrigerant flowing through the evaporator 5 and the fluid of the secondary fluid flow 6.
The vapour compression system 1 of Fig. 1 operates in the following manner. Refrigerant is compressed in the compressor 2. The compressed refrigerant then passes through the condenser 3, and the refrigerant leaving the condenser 3 is at least partly in a liquid form. The refrigerant is then expanded in the expansion device 4 before entering the evaporator 5 where it is at least partly evaporated, the refrigerant leaving the evaporator 5 thereby being in a substantially gaseous form. The gaseous refrigerant then enters the
compressor 2, via suction line 7, thereby completing the cycle.
When the refrigerant is evaporated in the evaporator 5, heat is extracted from the secondary fluid flow 6, thereby lowering the temperature of the secondary fluid flow 6. The evaporator 5 of the vapour compression system 1 of Fig. 1 comprises only a single evaporator path, i.e. the entire temperature decrease of the secondary fluid flow 6 must be provided by this evaporator path. In the case that the desired decrease in temperature is very high, the evaporator
temperature of the evaporator 5 must be very low. This leads to a very low suction pressure in the suction line 7. As a consequence, the work required by the compressor 2 in order to compress the refrigerant is relatively high.
Fig. 2 is a pressure-enthalpy (log(p)-h) diagram illustrating the variations in pressure and enthalpy of the refrigerant during operation of the vapour compression system 1 of Fig. 1. From point 8 to point 9 the refrigerant is condensed in the condenser 3. The pressure remains constant while the enthalpy decreases. The refrigerant leaving the condenser 3 defines a positive subcooling.
From point 9 to point 10 the refrigerant is expanded in the expansion device 4. The pressure is decreased while the enthalpy remains constant. From point 10 to point 11 the refrigerant passes through the evaporator 5. The pressure remains constant while the enthalpy increases. The refrigerant leaves the evaporator 5 with a positive superheat.
From point 11 to point 8 the refrigerant is compressed in the compressor 2. The pressure as well as the enthalpy increases during this step. The increase in enthalpy is illustrated by arrow 12. In the vapour compression system 1 of Fig. 1 the enthalpy of the entire mass flow of refrigerant needs to be increased by the amount indicated by arrow 12. The work performed by the compressor 2 during the compressing step is determined by the product of the mass flow and the increase in enthalpy. Accordingly, a relatively large amount of work needs to be performed by the compressor 2 in the situation illustrated in Figs. 1 and 2.
Fig. 3 is a schematic view of a second prior art vapour compression system 1. The vapour compression system 1 is very similar to the one illustrated in Fig. 1 , and it will therefore not be described in detail here. In the vapour compression system 1 of Fig. 3 the evaporator 5 comprises a first evaporator path 5a and a second evaporator path 5b arranged fluidly in parallel along the refrigerant path and in series along the flow direction defined by the secondary fluid flow 6. Thus, refrigerant leaving the expansion device 5 is divided into two parallel flows, one passing through the first evaporator path 5a and the other passing through the second evaporator path 5b.
A pressure control valve 13 arranged in the refrigerant path between the outlet of the first evaporator path 5a and the suction line 7 allows the pressure in the first evaporator path 5a to be distinct from the pressure in the second
evaporator path 5b. Thereby the evaporator temperature of the first evaporator path 5a can also be distinct from the evaporator temperature of the second evaporator path 5b. Thereby the secondary fluid flow 6 can be cooled in two steps, first passing across the first evaporator path 5a and then passing across the second evaporator path 5b. Typically, the evaporator temperature of the first evaporator path 5a will be higher than the evaporator temperature of the second evaporator path 5b, and thereby the pressure prevailing in the first evaporator path 5a will be higher than the pressure prevailing in the second evaporator path 5b. Fig. 4 is a pressure-enthalpy (log(p)-h) diagram illustrating the variations in pressure and enthalpy of the refrigerant during operation of the vapour compression system 1 of Fig. 3. This is similar to the diagram of Fig. 2, and it will therefore not be described in detail here.
In the diagram of Fig. 4, point 10a illustrates the entrance of the first evaporator path 5a, and point 10b illustrates the entrance of the second evaporator path 5b. Thus, from point 10a to point 11a refrigerant passes through the first evaporator path 5a, and from point 10b to point 11 b refrigerant passes through the second evaporator path 5b. It is clear that the pressure in the first evaporator path 5a is higher than the pressure in the second evaporator path 5b. Furthermore, the enthalpy of the refrigerant leaving the first evaporator path 5a is higher than the enthalpy of the refrigerant leaving the second evaporator path 5b.
As described above, refrigerant leaving the first evaporator path 5a passes through pressure control valve 13 before being mixed with refrigerant leaving the second evaporator path 5b in the suction line 7. This causes a decrease in the pressure of the refrigerant leaving the first evaporator path 5a, a decrease in the enthalpy of the refrigerant leaving the first evaporator path 5a and an increase in enthalpy of the refrigerant leaving the second evaporator path 5b. Point 11c illustrates the suction pressure and enthalpy of the refrigerant flowing in the suction line 7.
From point 1 1c to point 8 the refrigerant is compressed by the compressor 2. The entire mass flow of refrigerant requires an increase in enthalpy
corresponding to the difference illustrated by arrow 12. Fig. 5 is a schematic view of a vapour compression system 1 according to a first embodiment of the invention. Similarly to the vapour compression systems 1 of Figs. 1 and 3, the vapour compression system 1 of Fig. 5 comprises a compressor 2, a condenser 3, an expansion device 4 and an evaporator 5. The evaporator 5 comprises a first evaporator path 5a and a second evaporator path 5b arranged fluidly in parallel in the refrigerant path, and in series along the direction of a secondary fluid flow 6 across the evaporator 5. The first
evaporator path 5a is fluidly connected to the compressor 2 via first suction line 7a, and the second evaporator path 5b is fluidly connected to the compressor 2 via second suction line 7b. Thus, refrigerant leaving the first evaporator path 5a is not mixed with refrigerant leaving the second evaporator path 5b before entering the compressor 2.
The separate suction lines 7a, 7b makes it possible to maintain distinct suction pressures in the two suction lines 7a, 7b, thereby providing an evaporator temperature of the first evaporator path 5a which is distinct from the evaporator temperature of the second evaporator path 5b. For instance, the evaporator temperature of the first evaporator path 5a may be higher than the evaporator temperature of the second evaporator path 5b. Thereby the temperature of the secondary fluid flow 6 can be gradually cooled, first by the first evaporator path 5a and then by the second evaporator path 5b. Thereby a low target
temperature of the secondary fluid flow 6 can be reached without requiring a very low evaporator temperature, and thereby a very low pressure, of both evaporator paths 5a, 5b.
Furthermore, since refrigerant leaving the first evaporator path 5a is not mixed with refrigerant leaving the second evaporator path 5b, the pressure and enthalpy of each refrigerant flow is maintained in the respective suction lines 7a, 7b.
Fig. 6 is a pressure-enthalpy (log(p)-h) diagram illustrating the variations in pressure and enthalpy of the refrigerant during operation of the vapour compression system 1 of Fig. 5. Similarly to the situation illustrated in Fig. 4, from point 10a to point 11a refrigerant passes through the first evaporator path 5a, and from point 10b to point 11b refrigerant passes through the second evaporator path 5b. It is clear that the pressure in the first evaporator path 5a is higher than the pressure in the second evaporator path 5b. From point 11a to point 8 refrigerant which is supplied to the compressor 2 from the first evaporator path 5a via the first suction line 7a is compressed in the compressor 2. Similarly, from point 11b to point 8 refrigerant which is supplied to the compressor 2 from the second evaporator path 5b via the second suction line 7b is compressed in the compressor 2. Due to the separate suction lines 7a, 7b, the refrigerant leaving the first evaporator path 5a is not mixed with the refrigerant leaving the second evaporator path 5b. Accordingly, the suction pressure in the first suction line 7a is determined by the pressure in the first evaporator path 5a, and the suction pressure in the second suction line 7b is determined by the pressure in the second evaporator path 5b. The enthalpy increase during compression of the refrigerant flowing in the first suction line 7a is indicated by arrow 12a, and the enthalpy increase during compression of the refrigerant flowing in the second suction line 7b is indicated by arrow 12b. It is clear from Fig. 6 that the enthalpy increase 12a of the refrigerant flowing in the first suction line 7a is significantly smaller than the enthalpy increase 12b of the refrigerant flowing in the second suction line 7b. Accordingly, for this part of the mass flow a relatively small enthalpy increase is required, and only the part of the mass flow which flows via the second suction line 7b requires the larger enthalpy increase 12b. Since the work performed by the compressor 2 is the product of enthalpy increase and mass flow, the total work to be performed by the compressor 2 is therefore reduced as compared to the situations illustrated in Figs. 1-4. Thereby energy consumption is reduced.
Fig. 7 is a schematic view of a vapour compression system 1 according to a second embodiment of the invention. The vapour compression system 1 of Fig. 7 is essentially identical to the vapour compression system 1 shown in Fig. 5, and it is operated essentially as described above. However, in the vapour compression system 1 of Fig. 7 the compressor 2 is a multistage compressor comprising two sub-compressors 2a, 2b. Sub-compressor 2b is fluidly connected to and receives refrigerant from suction line 7b, and sub-compressor 2a receives refrigerant from suction line 7a, as well as from an outlet opening of sub-compressor 2b. As described above, the refrigerant supplied by suction line 7b is at a lower pressure level than the refrigerant supplied by suction line 7a. Accordingly, sub-compressor 2b increases the pressure level of the refrigerant supplied by suction line 7b before it is mixed with refrigerant supplied by suction line 7a and supplied to sub-compressor 2a. Sub-compressor 2a then increases the pressure of the entire mass flow of refrigerant to the outlet pressure level and delivers the compressed refrigerant to the condenser 3.
Thus, each part of the mass flow of refrigerant only receives a required enthalpy increase, and the work to be performed by the compressor 2 is minimised as described above. The compressor 2 shown in Fig. 7 may, e.g., be in the form of a multistage scroll compressor.
Fig. 8 is a schematic view of a vapour compression system 1 according to a third embodiment of the invention. The vapour compression system 1 of Fig. 8 is very similar to the vapour compression systems 1 of Figs. 5 and 7, and it will therefore not be described in detail here. However, the evaporator 5 of the vapour compression system 1 of Fig. 8 comprises a plurality of evaporator paths 5a, 5b, 5c, three of which are shown. Each of the evaporator paths 5a, 5b, 5c is fluidly connected directly to the compressor 2 via a separate suction line 7a, 7b, 7c. The vapour compression system 1 of Fig. 8 is operated essentially as described above with reference to Figs. 5 and 6. However, the plurality of evaporator paths 5a, 5b, 5c makes it possible to gradually decrease the evaporator temperature from the first evaporator path 5a towards the last evaporator path 5c. Thereby an even further portion of the mass flow of refrigerant can be maintained at a suction pressure which is as high as possible, and an even further reduction in energy consumption of the compressor 2 can be obtained.
Figs. 9-13 illustrate various designs of evaporators 5 for use in vapour compression systems 1 according to embodiments of the invention. It should be noted that the evaporator designs shown in Figs. 9-13 are merely examples, and that alternative evaporator designs could be envisaged which would fall within the scope of protection defined by the appended claims.
Fig. 9 is a schematic view of an evaporator 5 comprising three evaporator paths 5a, 5b, 5c arranged fluidly in parallel along a refrigerant path. Each evaporator path 5a, 5b, 5c receives refrigerant via a separate inlet tube 14a, 14b, 14c, and each evaporator path 5a, 5b, 5c delivers refrigerant via a separate outlet tube 15a, 15b, 15c. Each of the separate outlet tubes 15a, 15b, 15c is fluidly connected to a separate suction line (not shown) which is connected directly to a compressor (not shown). Thereby the advantages described above are obtained.
A secondary fluid flow passes across the evaporator 5 as indicated by arrows 6, thereby allowing heat transfer to take place between refrigerant evaporating in the evaporator paths 5a, 5b, 5c and the fluid of the secondary fluid flow 6.
Fig. 10 is a schematic view of an evaporator 5 of the A-coil type. The evaporator 5 comprises two evaporator paths 5a, 5b, in the form of two slaps, arranged fluidly in parallel along a refrigerant path. Each evaporator path 5a, 5b receives refrigerant via a separate inlet tube 14a, 14b, and each evaporator path 5a, 5b delivers refrigerant via a separate outlet tube 15a, 15b. Each of the separate outlet tubes 15a, 15b is fluidly connected to a separate suction line (not shown) which is connected directly to a compressor (not shown). Thereby the
advantages described above are obtained.
A secondary fluid flow passes across the evaporator 5 as indicated by arrows 6. The secondary fluid flow 6 enters between two substantially oppositely arranged parts of the evaporator 5, and a portion of the secondary fluid flow 6 passes across one part of the evaporator, while the remaining portion of the secondary fluid flow 6 passes across the substantially oppositely arranged part of the evaporator 5. The secondary fluid flow 6 passes each of the parts of the evaporator 5 in such a manner that the first evaporator path 5a is encountered first, and the second evaporator path 5b is encountered subsequently.
Fig. 11 is a schematic view of an evaporator 5 of the A-coil type, similarly to the evaporator 5 shown in Fig. 10. The evaporator 5 of Fig. 11 comprises an inlet header 16 and an outlet header 17. The inlet header 16 receives refrigerant from inlet tubes 14a, 14b and distributes the refrigerant to the evaporator paths 5a, 5b. The inlet header 16 is split into two inlet header volume 16a, 16b. A first inlet header volume 16a is arranged in fluid communication with the first evaporator path 5a, and a second inlet header volume 16b is arranged in fluid communication with the second evaporator path 5b. Similarly, the outlet header 17 receives refrigerant from the evaporator paths 5a, 5b. The outlet header 17 is split into two outlet header volumes 17a, 17b. A first outlet header volume 17a is fluidly connected to the first evaporator path 5a and to a first outlet tube 15a, and a second outlet header volume 17b is fluidly connected to the second evaporator path 5b and to a second outlet tube 15b. Each of the outlet tubes 15a, 15b is fluidly connected to a separate suction line (not shown), each suction line establishing a fluid connection to a compressor (not shown).
Fig. 12 is a schematic view of an evaporator 5 having a fin-and-tube design with four evaporator paths 5a, 5b, 5c, 5d, in the form of slaps arranged in an A-coil like configuration. Each slap is a two-row fin-and-tube evaporator path 5a, 5b, 5c, 5d. The inlet and outlet connections of the evaporator paths 5a, 5b, 5c, 5d are not shown in Fig. 12, but they could, e.g., be direct connections to inlet and outlet tubes as shown in Fig. 10 or inlet and outlet headers as shown in Fig. 11. In any event, the outlet connections must ensure that each evaporator path 5a, 5b, 5c, 5d is connected to a separate suction line. Fig. 13 is a schematic view of an evaporator 5 of an N-coil type. The evaporator 5 operates essentially as the evaporator 5 shown in Fig. 10.

Claims

1. A vapour compression system (1) comprising a compressor (2), a condenser (3), an expansion device (4) and an evaporator (5) arranged along a refrigerant path, the evaporator (5) comprising at least two evaporator paths (5a, 5b, 5c, 5d) arranged fluidly in parallel between the expansion device (4) and the compressor (2), wherein each evaporator path (5a, 5b, 5c, 5d) is fluidly connected to the compressor (2) via a separate suction line (7a, 7b, 7c), and wherein the suction pressure in each of the suction lines (7a, 7b, 7c) is distinct from the suction pressure in each of the other suction line(s) (7a, 7b, 7c).
2. A vapour compression system (1) according to claim 1 , wherein the evaporator paths (5a, 5b, 5c, 5d) are arranged in series along a flow direction of a secondary flow (6) across the evaporator (5).
3. A vapour compression system (1) according to claim 2, wherein a first evaporator path (5a) is arranged upstream along the secondary flow direction (6) relative to a second evaporator path (5b), and wherein the suction pressure in a first suction line (7a) fluidly interconnecting the first evaporator path (5a) and the compressor (2) is higher than the suction pressure in a second suction line (7b) fluidly interconnecting the second evaporator path (5b) and the compressor (2).
4. A vapour compression system (1) according to any of the preceding claims, wherein the evaporator paths (5a, 5b, 5c, 5d) are in the form of individual evaporators.
5. A vapour compression system (1) according to any of claims 1-3, wherein the evaporator paths (5a, 5b, 5c, 5d) are in the form of parallel evaporator tubes of a single evaporator.
6. A vapour compression system (1) according to any of the preceding claims, wherein the evaporator (5) comprises and inlet header (16) and/or an outlet header (17).
7. A vapour compression system (1) according to claim 6, wherein the inlet header (16) and/or the outlet header (17) is/are split into at least two header volumes, each header volume being fluidly connected to an evaporator path (5a, 5b, 5c, 5d).
8. A vapour compression system (1) according to any of the preceding claims, wherein the expansion device (4) is or comprises a thermostatic expansion valve.
9. A vapour compression system (1) according to any of the preceding claims, wherein the compressor (2) comprises an outlet opening being fluidly connected to the condenser (3), and two or more inlet openings, each inlet opening being fluidly connected to a suction line (7a, 7b, 7c).
10. A vapour compression system (1) according to claim 9, wherein the compressor (2) comprises at least two sub-compressors (2a, 2b), each sub- compressor (2a, 2b) having an inlet opening being fluidly connected to a suction line (7a, 7b, 7c).
11. A vapour compression system (1) according to claim 10, wherein at least a first sub-compressor (2b) has an outlet opening being fluidly connected to an inlet opening of a second sub-compressor (2a).
12. A vapour compression system (1) according to claim 11 , wherein the first sub-compressor (2b) is adapted to increase the pressure of refrigerant received from the corresponding suction line (7b) to a pressure level being substantially equal to a pressure level of refrigerant supplied to the second compressor (2a) from the corresponding suction line (7a).
13. A vapour compression system (1) according to any of claims 10-12, wherein at least one of the sub-compressors (2a, 2b) is arranged inside another one of the sub-compressors (2a, 2b).
14. A vapour compression system (1) according to any of the preceding claims, wherein the vapour compression system (1) is a refrigeration system.
PCT/DK2010/000100 2009-12-18 2010-06-29 A vapour compression system with split evaporator WO2011072679A1 (en)

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