REFRIGERATION SYSTEM AND METHOD OF OPERATING A REFRIGERATION SYSTEM

Abstract

A refrigeration system comprising an evaporator, and a method of operating a refrigeration system. The evaporator comprises: a first fluid volume upstream of a second fluid volume, and a plurality of channels fluidly connecting the first fluid volume and the second fluid volume. The system further comprises a flow restrictor arranged to prevent fluid flow through at least a first channel of the plurality of channels in response to a pressure difference between the first fluid volume and the second fluid volume being less than a predetermined threshold, and to permit fluid flow through the first channel in response to the pressure difference being greater than the predetermined threshold.

Claims

1. A refrigeration system comprising an evaporator, the evaporator comprising: a first fluid volume upstream of a second fluid volume, and a plurality of channels fluidly connecting the first fluid volume and the second fluid volume; the system further comprising a flow restrictor arranged to prevent fluid flow through at least a first channel of the plurality of channels in response to a pressure difference between the first fluid volume and the second fluid volume being less than a predetermined threshold, and to permit fluid flow through the first channel in response to the pressure difference being greater than the predetermined threshold.

2. A refrigeration system as claimed in claim 1, wherein the flow restrictor is mechanical.

3. The refrigeration system as claimed in claim 1, wherein the flow restrictor comprises a piston moveable between a first position and a second position, and a biasing mechanism arranged to urge the piston to the first position.

4. The refrigeration system as claimed in claim 1, wherein the flow restrictor comprises an actuable flap arranged to permit or prevent fluid flow through the first channel.

5. The refrigeration system as claimed in claim 1, wherein the first and second fluid volumes are adjacent each other in a header of the evaporator.

6. The refrigeration system as claimed in claim 5, wherein the flow restrictor is within the header.

7. The refrigeration system of claim 1, wherein the flow restrictor is outside the first fluid volume and the second fluid volume.

8. The refrigeration system as claimed in claim 1, wherein the flow restrictor is configured to automatically limit a pressure drop between the first fluid volume and second fluid volume.

9. A method of operating a refrigeration system to transition from a first mode of operation to a second mode of operation, the refrigeration system comprising an evaporator comprising a first fluid volume upstream of a second fluid volume, and a plurality of channels fluidly connecting the first fluid volume and the second fluid volume; the method comprising permitting or preventing fluid flow through a first channel of the plurality of channels based on a pressure difference between the first fluid volume and the second fluid volume.

10. The method as claimed in claim 9, comprising permitting fluid flow through the first channel in response to the pressure difference exceeding a predetermined threshold.

11. The method as claimed in claim 9, comprising preventing fluid flow through the first channel in response to the pressure difference falling below the predetermined threshold.

12. The method as claimed in claim 9, comprising regulating fluid velocity within the plurality of channels by increasing and/or decreasing the number of channels through which fluid flows.

13. The method as claimed in claim 9, comprising maintaining turbulence in the plurality of channels within a predetermined range.

14. The method as claimed in claim 9, comprising limiting a pressure drop across the evaporator by permitting fluid flow through the first channel.

15. The method as claimed in claim 9, comprising using the refrigeration system as claimed in any of claims 1 to 8.

Description

FIGURES

[0048] Certain preferred embodiments of the invention will be described below by way of example only and with reference to the drawings in which:

[0049] FIG. 1 shows a schematic of a portion of an evaporator with a flow restrictor in a first configuration;

[0050] FIG. 2 shows the portion of the evaporator of FIG. 1 with the flow restrictor in a second configuration;

[0051] FIG. 3 shows a schematic of a portion of an evaporator with a flow restrictor in a first configuration; and

[0052] FIG. 4 shows the portion of the evaporator of FIG. 3 with the flow restrictor in a second configuration.

DESCRIPTION

[0053] FIG. 1 shows a schematic of a portion of a multi-pass heat exchanger, specifically a two-pass flooded evaporator 100 of a refrigeration system, the flooded evaporator comprising a first fluid volume 110 and a second fluid volume 120. A plurality of channels 130 in the form of a bundle of tubes fluidly connect the first fluid volume 110 to the second fluid volume 120 so that all fluid flow from the first fluid volume 110 to the second fluid volume 120 is via the plurality of channels 130. The arrangement of the channels 130 is only shown schematically by the arrows on the left of FIG. 1.

[0054] The evaporator 100 also comprises a header 140 (or water box) within which the first fluid volume 110 and the second fluid volume 120 are defined. The header 140 may be any suitable shape. A partition or wall 142 within the header 140 separates the first fluid volume 110 from the second fluid volume 120. An evaporator inlet 112 is immediately upstream of the first fluid volume 110 and provides fluid flow thereto in use, and an evaporator outlet 122 is immediately downstream of the second fluid volume 120 and receive fluid flow therefrom in use. Therefore, in use, fluid flows from the evaporator inlet 112 to the evaporator outlet 122, via the first fluid volume 110, the plurality of channels 130, and then the second fluid volume 120.

[0055] In FIG. 1, inlets 132 of the plurality of channels 130 are shown, which inlets 132 receive fluid flow from the first fluid volume 110. Thus, fluid enters the plurality of channels 130 via the inlets 132 adjacent the first fluid volume 110. Also shown are outlets 134 of the plurality of channels 130, which provide flow to the second fluid volume 120. The inlets 132 and outlets 134 may be defined by a tubesheet or the like. The inlet 132 may therefore be inlets 132 of a first bundle of tubes of the plurality of channels 130 (e.g. flowing right to left in the figure), and the outlets 134 may be outlets 134 of a second bundle of tubes of the plurality of channels 130 (e.g. flowing left to right in the figure). Each bundle of tubes may be one pass of the evaporator.

[0056] Although the plurality of channels 130 are shown schematically, the plurality of channels 130 may have any suitable geometry between the inlets 132 and outlets 134. Further, although it is not shown, the flooded evaporator 100 may comprise a second header on the opposite side of the evaporator 100 to the header 140 (e.g. in place of the vertical arrow leftmost in the figure). The second header may receive fluid flow from the inlets 132 and the corresponding ones of the plurality of channels 130 (e.g. from the first bundle of tubes), and may provide fluid flow to the outlets 134 via the corresponding channels of the plurality of channels 130 (e.g. the second bundle of tubes). That is, fluid flow between the inlets 132 and outlets 134 may be via the second header.

[0057] The plurality of fluid channels 130 are fluidly isolated from each other except at the first fluid volume 110 and the second fluid volume 120 (and at the second header where the second header is provided). That is, fluid may only flow between each of the plurality of channels 130 in the header 140 (and in the second header where it is used). Fluid therefore cannot flow directly between the channels of the plurality of channels 130.

[0058] A flow restrictor 150 is located within the wall 142 and is thereby disposed partially in the first fluid volume 110 and partially in the second fluid volume 120. The flow restrictor 150 is therefore subject to a pressure difference 170 between the first fluid volume 110 and the second fluid volume 120 (depending on the pressure drop through the evaporator). The flow restrictor 150 comprises a piston 152 and a spring 154. The piston 152 is moveable between a first position in which the spring 154 is fully extended, and a second position in which the spring 154 is compressed. The spring 154 is therefore arranged to bias the piston 152 to the first position e.g. against a force from the pressure difference 170.

[0059] The flow restrictor 150 comprises mechanical couplings or rods 156 which are coupled to opposed ends of the piston 152. Actuable flaps 160 are coupled to respective ends of the rods 156 opposite the piston 152. The actuable flaps 160 are each pivotably coupled to the tubesheet defining the inlets 132 and outlets 134 of the plurality of channels 130. The flaps 160 are arranged to cover some of the inlets 132 and some of the outlets 134. The flaps are therefore arranged to reduce the number of inlets 132 receiving flow from the first fluid volume 110, and to reduce the number of outlets 134 permitting fluid flow into the second fluid volume 120.

[0060] FIG. 1 shows the flooded evaporator 100 and flow restrictor 150 arrangement in a first configuration, during negative brine temperature operation of the refrigeration system. In this configuration, a lower fluid flow is needed, but fluid velocity through the evaporator and channels 130 needs to be promoted in order to ensure sufficient turbulence therein for efficient heat transfer.

[0061] In FIG. 1, the pressure difference 170 between the first fluid volume 110 and the second fluid volume 120 is less than a predetermined threshold required to move the piston 152 against the biasing action of the spring 154. As such, the actuable flap 160 in the first fluid volume is in a closed position, covering some (but not all) of the inlets 132 of the plurality of channels 130 and thereby preventing fluid flow through the corresponding ones of the channels 130. The actuable flap 160 in the second fluid volume 120 is also in a closed position, covering some (but not all) of the outlets 134 of the plurality of channels 130, preventing fluid flow through corresponding ones of the channels 130.

[0062] Since the flow restrictor 150 prevents fluid flow through multiple of the plurality of channels 130, and therefore permits fluid flow though only some of the plurality of channels 130, the total flow cross-section through the evaporator 100 is reduced and the fluid velocity of the flow is increased. As such, turbulence is maintained at a sufficient level to ensure efficient heat transfer.

[0063] FIG. 2 shows the flooded evaporator 100 of FIG. 1 in a second configuration, during positive brine temperature operation of the refrigeration system. During such conditions, a higher flow rate is needed but an increase in the pressure drop across the evaporator 100 is not desirable.

[0064] In FIG. 2, the pressure difference 170 between the first fluid volume 110 and the second fluid volume 120 has increased beyond the predetermined threshold, and the piston 152 has therefore been moved by the pressure difference 170 against the biasing action of the spring 154 into its second position. The rods 156 have therefore also moved the actuable flaps 160 to their open positions, so that the inlets 132 and outlets 134 of the plurality of channels 130 that were covered, are no longer covered. As such, fluid flow is permitted through the corresponding channels 130 by movement of the flaps 160.

[0065] Since the flow restrictor 150 permits fluid flow through all of the plurality of channels 130, the total flow cross-section through the evaporator is increased. Therefore, a higher flow rate through the evaporator is achieved, and the pressure-drop may be controlled.

[0066] The flooded evaporator 100 and flow restrictor 150 therefore enable the refrigeration system to operate in two modes, each mode requiring substantially different flow characteristics through the evaporator 100. In a first mode which needs less fluid flow through the evaporator 100 (e.g. a negative temperature brine mode), the flow restrictor 150 is arranged to prevent flow through some of the plurality of channels 130 and thereby reduce the total flow cross-section through the evaporator 100. A reduced heat transfer surface (i.e. fewer channels 130 carrying fluid for heat transfer) is acceptable during this mode because less cooling capacity is delivered from the refrigeration cycle at the lower temperature. The reduced cross-section maintains fluid flow velocity at a level sufficiently high to maintain desirable turbulence in the channels 130, and hence maintain efficient heat transfer. In a second mode which needs more fluid flow through the evaporator 100 (e.g. a positive temperature brine mode), the flow restrictor 150 is arranged to permit flow through the plurality of channels (e.g. through all of the plurality of channels 130) and thereby increase the total flow cross-section through the evaporator 150. In such a mode, an increased heat transfer surface (i.e. more channels 130 carrying fluid for heat transfer) is needed due to the higher cooling capacity delivered by the refrigeration cycle. The increased cross-section allows increased total fluid flow without making a pressure drop across the evaporator too large.

[0067] Moreover, the flow restrictor 150 automatically transitions between configurations in response to the pressure difference 170 between the first fluid volume 110 and second fluid volume 120. It is therefore operable to automatically limit the pressure drop between the first fluid volume 110 and the second fluid volume 120, thereby ensuring efficient operation of the refrigeration system. The spring 154 may be configured (e.g. during assembly) to provide the mode transition at the desired time, e.g. depending upon parameters of the system such as fluid physical properties, heating/cooling requirements, and so on.

[0068] FIG. 3 shows an alternative flooded evaporator 100 and flow restrictor 150. Therein, the first fluid volume 110 is divided into a primary fluid volume 114 and an auxiliary fluid volume 116. The primary fluid volume 114 receives fluid flow from the evaporator inlet 112 regardless of the configuration of the flow restrictor 150 (e.g. always receives flow from the evaporator inlet 112 during use. The auxiliary fluid volume 116 receives fluid flow from the evaporator inlet 112 only if the flow restrictor 150 permits such flow. The inlets 132 of the plurality of channels 130 are divided between the primary fluid volume 114 and the auxiliary fluid volume 116, and therefore only receive fluid from either one or the other.

[0069] The second fluid volume 120 also comprises a primary fluid volume 124 and an auxiliary fluid volume 126. The outlets 134 of the plurality of channels 130 are divided between the primary fluid volume 124 and the secondary fluid volume 126. The primary fluid volumes 114, 124 are therefore fluidly isolated from their respective auxiliary fluid volumes 116, 126 within the header 140.

[0070] The flow restrictor 150 also comprises a first pressure conduit 180 fluidly connecting one side of the piston 152 to the pressure at the evaporator inlet 112, and a second pressure conduit 182 fluidly connecting the other side of the piston 152 to the pressure at the evaporator outlet 122. The first pressure conduit 180 and second pressure conduit 182 therefore ensure the piston 152 is responsive to the pressure difference 170 between the primary fluid volume 114 of the first fluid volume 110 and the primary fluid volume 124 of the second fluid volume 120.

[0071] The flow restrictor 150 comprises actuable flaps 160 operable to open and close an auxiliary inlet 118 and an auxiliary outlet 128, and thereby permit or prevent fluid flow through some of the plurality of channels 130 in communication with the auxiliary fluid volumes 116, 126. The auxiliary inlet 118 provides fluid flow the auxiliary fluid volume 116, and the auxiliary outlet 128 receives fluid flow from the auxiliary fluid volume 126.

[0072] Thus, similarly to the evaporator 100 and flow restrictor 150 of FIGS. 1 and 2, the evaporator 100 and flow restrictor 150 of FIG. 3 enables operation of the refrigeration system is two distinct modes. In the first mode, shown in FIG. 3, the pressure 170 between the evaporator inlet 112 and evaporator outlet 122 is below the predetermined threshold and the refrigeration system is in a first mode (e.g. a negative brine temperature mode). Fluid flow through some (but not all) of the plurality of channels 130 is prevented by the actuable flaps 160 being in their closed positions, preventing fluid flow through the auxiliary inlet 118 and auxiliary outlet 128, and hence preventing flow through the associated channels of the plurality of channels 130.

[0073] FIG. 4 shows the evaporator 100 and flow restrictor 150 of FIG. 3 when the refrigeration system is operating in its second mode (e.g. a positive brine temperature mode). In the depicted case, the pressure difference 170 exceeds the predetermined threshold and the actuable flaps 160 are moved to their open positions, permitting fluid flow through the auxiliary inlet 118 and the auxiliary outlet 128, thereby permitting fluid flow through corresponding channels of the plurality of channels 130.

[0074] The proportion of channels of the plurality of channels 130 that are controlled by the flow restrictor 150 can be selected as required. Moreover, the strength of the spring 154 can be selected as required, so that the evaporator changes modes under predetermined conditions. Further, since the flow restrictor 150 is outside the header 140, a manual override (not shown) may be provided for a user to manually control the actuable flaps 160 e.g. manually open them after increasing a pump speed of the system, or manually close them after decreasing the pump speed of the system.

[0075] Although the evaporator in FIGS. 3 and 4 is a two-pass evaporator, since the flow restrictor 150 is outside the header 140, it could also be used with a single-pass heat exchanger, or any multi-pass heat exchanger.

[0076] The flow restrictor 150 may be arranged so that it controls the same number of inlets 132 as outlet 134, thereby helping uniform fluid velocity in the plurality of channels 130 with fluid flow therein. Alternatively, the number of channels 130 in each bundle of tubes controlled by the flow restrictor 150 may be selected based on a temperature difference between each pass of the evaporator 100.

[0077] Although FIGS. 3 and 4 show a mechanical flow restrictor 150, an electronic flow restrictor may be used instead. For example, the pressure conduits 180 and 182 could be replaced by pressure transducers, and the actuable flaps 160 could be replaced by solenoid valves, or motorised valves. An electronic controller could then be provided to open and close the valves in response to the pressures measured by the pressure transducers. The pressure transducers may also be overridden by the electronic controller if required.