JP2010255905A - Industrial heating system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an industrial heating system with high energy efficiency. <P>SOLUTION: The industrial heating system is equipped with a heat pump device (12) passing a first fluid, a heat exchange device (14) transferring heat from the first fluid to a second fluid, and a heating chamber (16) transferring heat from the second fluid to an object. The heat pump device (12) is equipped with a multi-stage compression part (22) including a first compression part (22A) and a second compression part (22B) and compressing the first fluid, a heat radiation part (23) with at least one part disposed in the heat exchange device (14) and passing the first fluid from the multi-stage compression part (22), an expansion part (24) expanding the first fluid from the heat radiation part (23), an endoergic part (21) including a first endoergic part (21A) and a second endoergic part (21B) and wherein the first fluid from the expansion part (24) absorbs heat from the outside, and a regenerator (28) including a first regenerator (28A) and a second regenerator (28B) and transferring one part of heat of the first fluid compressed by the multi-stage compression part (22) to the first fluid heading toward the multi-stage compression part (22). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an industrial heating system.

  As an industrial heating system, a configuration in which heat of steam generated by a boiler is transmitted to an object is generally known (for example, see Patent Document 1). In addition, a configuration is known in which heat from a heat pump or refrigerator medium is transmitted to an object.

JP-A-6-249450

  An object of the present invention is to provide an industrial heating system with high energy efficiency.

  According to the aspect of the present invention, a heat pump device through which the first fluid flows, a heat exchange device in which heat from the first fluid is transmitted to the second fluid, and a heating chamber in which heat from the second fluid is transmitted to the object The heat pump device includes a first compression unit and a second compression unit, and a multi-stage compression unit that compresses the first fluid, and at least a part of the heat pump device is disposed in the heat exchange device, A heat radiating portion through which the first fluid flows, an expansion portion for expanding the first fluid from the heat radiating portion, a first heat absorption portion and a second heat absorption portion, and the first fluid from the expansion portion is external A heat absorption part that absorbs the heat of the gas, a first regenerator, and a second regenerator, and a part of the heat of the first fluid compressed by the multistage compression part is transferred to the first fluid toward the multistage compression part An industrial heating system comprising a regenerator for communicating is provided.

  According to this heating system, the object is heated by the second fluid heated using the heat from the heat pump. By dividing the heat pump elements, system design flexibility and energy efficiency can be improved.

It is the schematic which shows 1st Embodiment. It is a figure which shows the structure of a heat pump typically. It is a figure which shows typically an example of the temperature change of exhaust air and a working fluid.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing a heating system S1.

  As shown in FIG. 1, the heating system S1 includes a heat pump device 12 having a heat pump (heat pump circuit) 20 through which a working fluid (first fluid) flows, a heat exchange device 14, and a heating chamber (heating) in which an object is heated. Device) 16 and a control device 70. The control device 70 comprehensively controls the entire system. The configuration of the heating system S1 can be variously changed according to design requirements.

  In the present embodiment, heat from the working fluid of the heat pump device 12 is transmitted to the object in the heating chamber 16 through the air. That is, in the heat exchange device 14, heat from the working fluid of the heat pump device 12 is transmitted to the air. Heated air from the heat exchange device 14 is supplied to the heating chamber 16. In the heating chamber 16, heat from the air is transmitted to the object.

  In this embodiment, in the heating chamber 16, the heat from the heated air is transmitted directly or indirectly to the object. For example, in the heating chamber 16, heated air can directly contact the object. Alternatively, in the heating chamber 16, another substance can be interposed between the heated air and the object. In other embodiments, heat from the working fluid of the heat pump device 12 can be transferred to the object via a fluid other than air.

  In the present embodiment, the heating object is an industrial part or an industrial material. For example, in the heating chamber 16, at least a part of the industrial part or the industrial material is dried. Alternatively, at least a part of the industrial part or the industrial material is heat-treated in the heating chamber 16. Sludge, paper, wood, resin, chemicals, chemicals, sand, household waste, industrial waste, crafts, craft materials, electrical parts, electrical equipment, paints, industrial clothing, machine parts, mechanical products, food, foodstuffs Various objects such as food products can be heated.

  In the heat pump device 12, the heat pump 20 is a circuit that transfers heat between a plurality of objects using a change in the state of the working fluid by a cycle composed of steps of evaporation, compression, condensation, and expansion. A heat pump cycle generally has the advantage of being relatively energy efficient.

  In the present embodiment, the heat pump 20 has a heat absorption part 21, a compression part 22, a heat radiation part 23, and an expansion part 24, which are connected via a conduit.

  In the heat absorption part 21, the working fluid flowing in the main path 25 absorbs the heat of the heat source outside the cycle. In the present embodiment, the heat absorption part 21 of the heat pump 20 can absorb (recover) the heat (exhaust heat) of the exhaust fluid (exhaust gas, exhaust air) from the heating chamber 16.

  In the present embodiment, the heating chamber 16 has an exhaust pipe (exhaust path) 162 through which exhaust air from the heating chamber 16 flows. In the present embodiment, a part of the exhaust pipe 162 is thermally connected to the conduit of the heat absorbing unit 21. The heat of the exhaust air is transferred to the working fluid flowing through the heat absorbing portion 21 relatively directly. According to this configuration, especially when the exhaust air from the heating chamber 16 is relatively dry, exhaust heat (sensible heat) can be efficiently recovered. The exhaust pipe 162 may include a fluid drive unit such as a pump, a flow rate control unit (not shown) such as a valve, and an exhaust gas processing device such as a filter, as necessary. Alternatively, or in addition, the heat absorption part 21 of the heat pump 20 may be configured to absorb the heat of a heat source (for example, air or a cold heat supply device) other than the fluid discharged from the heating chamber 16.

  In another embodiment, the heat of the exhaust air from the heating chamber 16 can be transmitted to the working fluid flowing through the heat absorbing unit 21 via another medium. For example, the conduit through which the heat medium that receives the heat of the exhaust air from the heating chamber 16 flows can be thermally connected to the conduit of the heat absorbing unit 21.

  In this embodiment, the heat absorption part 21 has the 1st heat absorption part 21A and the 2nd heat absorption part 21B which were mutually independent. The first heat absorption part 21A and the second heat absorption part 21B are separated. In the present embodiment, the first heat absorption part 21A and the second heat absorption part 21B are substantially in a non-series relationship with respect to the flow of the working fluid. That is, the working fluid from the first heat absorbing part 21A does not substantially go to the second heat absorbing part 21B. The working fluid from the second heat absorption part 21B also does not substantially go to the first heat absorption part 21A.

  In the present embodiment, the first heat absorbing portion 21A and the second heat absorbing portion 21B are substantially aligned along the flow of the exhaust air. The first heat absorption part 21A absorbs part of the exhaust heat from the heating chamber 16, and the second heat absorption part 21B absorbs the remaining part of the exhaust heat. That is, the exhaust pipe 162 of the heating chamber 16 includes a first portion 164 that is thermally connected to the conduit of the first heat absorbing portion 21A, and a second portion 166 that is thermally connected to the conduit of the second heat absorbing portion 21B. Have The exhaust air from the heating chamber 16 flows through the first portion 164 earlier than the second portion 166. Heat from the first portion 164 through which the relatively high-temperature exhaust air flows is transferred to the working fluid flowing through the first heat absorbing portion 21A. Heat from the second portion 166 through which the relatively low temperature exhaust air flows is transferred to the working fluid flowing through the second heat absorbing portion 21B.

  In the first heat absorption part 21A and the second heat absorption part 21B, the working fluid that has received heat evaporates. In the present embodiment, the evaporation temperature of the working fluid in the first heat absorption part 21A is higher than the evaporation temperature of the working fluid in the second heat absorption part 21B. The 1st evaporator 211 is comprised including the 1st part 164 of the exhaust pipe 162, and the conduit | pipe of the 1st heat absorption part 21A. The 2nd evaporator 212 is comprised including the 2nd part 166 of the exhaust pipe 162, and the conduit | pipe of the 2nd heat absorption part 21B. The first and second evaporators 211 and 212 have a counterflow type heat exchange structure in which a relatively high temperature fluid (working fluid in the exhaust pipe 162) and a relatively low temperature fluid (working fluid) flow opposite to each other. Can have. Alternatively, the first and second evaporators 211 and 212 may have a parallel flow type heat exchange structure in which a relatively high temperature fluid and a relatively low temperature fluid flow in parallel.

  In the present embodiment, the two-stage recovery of exhaust heat by the first endothermic part 21A and the second endothermic part 21B is advantageous for improving the heat recovery rate. In addition, another exhaust heat recovery unit may be provided.

  The compression unit 22 compresses the working fluid by a compressor or the like. At this time, the temperature of the working fluid usually increases. In the present embodiment, the compression unit 22 has a multistage compression structure. The number of compression stages is set according to the specification of the system S1, and is 2, 3, 4, 5, 6, 7, 8, 9, or 10 or more. Among the various compressors such as an axial flow compressor, a centrifugal compressor, a reciprocating compressor, and a rotary compressor, a compressor suitable for compressing the working fluid is applied. Power is supplied to the compressor. In the compression unit 22 having a multistage compression structure, a multiaxial compression structure or a coaxial compression structure can be applied. The compression ratio (pressure ratio) of the compression unit 22 is set according to the specifications of the system S1.

  In the present embodiment, the compression unit 22 has a two-stage compression structure including a first compression unit 22A that is a front stage and a second compression unit 22B that is a rear stage. As the working fluid used in the heat pump 20, various known heat media such as a fluorocarbon medium (HFC 245fa, R134a, etc.), ammonia, water, carbon dioxide, air, etc. are used according to the specifications and heat balance of the system S1. It is done. In the present embodiment, the first compressor 22A boosts the low-pressure working fluid to an intermediate pressure. The second compressor 22B boosts the working fluid from the first compressor 22A to a relatively high pressure (including a supercritical pressure region).

  In the present embodiment, the working fluid from the first heat absorption part 21 </ b> A is supplied (injected) between the stages of the compression part 22. In multistage compressors, effective intercooling is advantageous for reducing compression power. Further, such interstage injection can be preferably applied to an integral casing type multistage compressor.

  The heat radiating unit 23 has a conduit through which the working fluid compressed by the compressing unit 22 flows, and gives heat of the working fluid flowing in the main path 25 to an object outside the cycle. In the present embodiment, the heat radiating portion 23 includes a first heat radiating portion 23A and a second heat radiating portion 23B. A first heat radiating portion 23A and a second heat radiating portion 23B are arranged in that order along the flow direction of the working fluid. The number of heat radiation units is set according to the specifications of the system S1, and is 2, 3, 4, 5, 6, 7, 8, 9, 10, or 11 or more. The first heat radiating part 23A is arranged at a position downstream of the compression part 22 in the flow direction of the working fluid, and the second heat radiating part 23B is arranged at a position downstream of the first heat radiating part 23A. In the present embodiment, the heat radiating portions 23 </ b> A and 23 </ b> B are disposed in the heat exchange device 14.

  The expansion unit 24 expands the working fluid by a pressure reducing valve, a turbine, or the like. When a turbine is used, power can be taken out from the expansion unit 24, and the power may be supplied to the compression unit 22, for example.

  In this embodiment, the expansion part 24 has a first expansion part 24A and a second expansion part 24B. The first expansion portion 24A expands the working fluid toward the first heat absorption portion 21A. The second expansion unit 24B expands the working fluid toward the second heat absorption unit 21B.

  In the present embodiment, the heat pump 20 further includes a regenerator 28. The regenerator 28 transfers part of the heat of the working fluid compressed by the compression unit 22 to the working fluid toward the compression unit 22. Due to the increase in the initial input temperature of the working fluid to the compression unit 22, the power of the compression unit 22 is reduced.

  In the present embodiment, the regenerator 28 heats the working fluid from the second heat absorption part 21B (second evaporator 212) toward the compression part 22. The regenerator 28 includes a first regenerator 28A used for heat exchange in a relatively low temperature region and a second regenerator 28B used for heat exchange in a relatively high temperature region. The first regenerator 28A has a configuration in which a high temperature side conduit 310 and a low temperature side conduit 315 are thermally connected. For example, both conduits are placed in contact with or adjacent to each other. Similarly, the 2nd regenerator 28B has the structure by which the high temperature side conduit | pipe 320 and the low temperature side conduit | pipe 325 were comprised thermally. The first and second regenerators 28A and 28B can have a countercurrent heat exchange structure in which a low-temperature fluid and a high-temperature fluid flow oppositely. Alternatively, the first and second regenerators 28A and 28B may have a parallel flow type heat exchange structure in which a high-temperature fluid and a low-temperature fluid flow in parallel.

  In the present embodiment, the high temperature side inlet 321 of the second regenerator 28B is fluidly connected to a branch portion 232 provided between the first heat radiating portion 23A and the second heat radiating portion 23B. The branching unit 232 determines a substantial distribution of the working fluid from the first heat radiating unit 23A to the second heat radiating unit 23B and the second regenerator 28B.

  The high temperature side outlet 322 of the second regenerator 28 </ b> B is fluidly connected to the branch 234. The branch part 234 determines a substantial distribution of the working fluid with respect to the first heat absorption part 21A and the second heat absorption part 21B. The high temperature side inlet 311 of the first regenerator 28 </ b> A is fluidly connected to the branch portion 234. The high temperature side outlet 312 of the first regenerator 28A is fluidly connected to the inlet of the second heat absorber 21B. The low temperature side inlet 316 of the first regenerator 28A is fluidly connected to the outlet of the second heat absorber 21B. The low temperature side outlet 317 of the first regenerator 28A is fluidly connected to the low temperature side inlet 326 of the second regenerator 28B. The low temperature side outlet 327 of the second regenerator 28B is fluidly connected to the inlet of the first compression unit 22A.

  As shown in FIG. 1, the heat exchange device 14 has a two-stage heating structure having a first heat exchange unit 141 for a relatively high temperature region and a second heat exchange unit 142 for a relatively high temperature region. The heat exchange device 14 includes an air supply path 140 through which air taken from the outside flows toward the heating chamber 16, a fluid drive unit (not shown) such as a pump and a blower, and a flow rate control unit (such as a valve, if necessary) ( (Not shown) and, if necessary, a gas processing device such as a filter.

  In the first heat exchanging unit 141, a part of the conduit of the air supply path 140 (first heating unit 145) and a part of the conduit of the main path 25 of the heat pump 20 (second heat radiating unit 23B) are thermally connected. Has a structured. Similarly, the second heat exchange unit 142 includes another part of the conduit of the air supply path 140 (second heating unit 146) and another part of the conduit of the main path 25 of the heat pump 20 (first heat radiating unit 23A). ) And are thermally connected. The first and second heat exchange units 141 and 142 are counter-current heat exchange systems in which a low-temperature fluid (air in the air supply path 140) and a high-temperature fluid (working fluid in the heat pump 20) flow opposite to each other. Can have. Alternatively, the first and second heat exchange units 141 and 142 may have a parallel flow type heat exchange method in which a low-temperature fluid and a high-temperature fluid flow in parallel. Various known heat exchange structures for the first and second heat exchange units 141 and 142 can be employed. For example, both conduits are placed in contact with or adjacent to each other. For example, one conduit can be disposed around or within the other conduit.

  In the present embodiment, the air flowing through the air supply path 140 receives heat from the second heat radiating portion 23B before receiving heat from the first heat radiating portion 23A. In the air supply path 140, the temperature of the air rises due to heating by the second heat radiating part 23B, and the temperature further rises due to heating by the first heat radiating part 23A.

  In the present embodiment, the heating chamber 16 includes a fluid inlet portion 61, a fluid outlet portion 63, and a transfer device (not shown) as necessary. In one example, the transfer device can have various forms such as a conveyor, a transport vehicle, and a transport robot. The object to be heated is put into the heating chamber 16 and taken out from the heating chamber 16 by the transfer device. Alternatively or additionally, the heating chamber 16 may include an output unit having a gate type, a swivel type, or the like for outputting an object after heating. The output part of the heated object can have a mechanism for performing a predetermined process such as a chemical process on the heated object as necessary.

  In the present embodiment, the transfer device can move the object to be heated in the heating chamber 16 as necessary. The heating chamber 16 further includes a dehydrating device (not shown) as needed, and thereby the object can be dehydrated. During the dehydration, a flocculant can be added to the object as necessary. Various forms such as a centrifugal type, a pressure type, a pressure type, and a vibration type can be applied to the dehydration depending on the object. Dehydration reduces the volume of the object. Moreover, the heating chamber 16 can further have a preheating chamber for applying heat to the object before entering the heating chamber 16 as necessary.

  Next, an example of the heat treatment process of the heating system S1 will be described.

  As shown in FIG. 1, in the heat exchanging device 14, air is heated by heat transfer from the heat pump 20. More specifically, in the first heating unit 145, the air is heated by the heat transmitted from the second heat radiating unit 23B of the heat pump 20. In the second heating unit 146, the air is further heated by the heat transmitted from the first heat radiating unit 23A of the heat pump 20. That is, air is sequentially heated in the heat exchange device 14. In the heating chamber 16, the temperature of the object increases due to heat from the air. Air (exhaust gas etc.) from the heating chamber 16 is discharged to the outside through the fluid outlet 63 and the exhaust pipe 162. In one example, the outlet temperature of the air in the heat exchange device 14 is about 130 ° C. In one example, the temperature of the exhaust gas from the heating chamber 16 is about 100 ° C. The above numerical value is an example, and the present invention is not limited to this. The supply temperature of the heated air can be about 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, or 140 ° C. or higher.

  The energy efficiency of the boiler is generally about 0.7 to 0.8 (70 to 80%), whereas the coefficient of performance (COP) as the energy efficiency of the heat pump is generally 2.5 to 5. 0. The coefficient of performance of the heat pump changes according to the input / output temperature difference of the medium to be heated, and the coefficient of performance tends to decrease at a relatively high input / output temperature difference.

  In this embodiment, since the compression part 22 has a two-stage compression structure and the heat absorption part 21 and the regenerator 28 are each divided into two parts, it is possible to appropriately adjust the heat balance in the system S1, and the energy efficiency Can be further improved.

FIG. 2 is a diagram schematically showing the configuration of the heat pump 20.
As shown in FIG. 2, in the present embodiment, the heat pump 20 includes at least a first compression route CL1 that includes at least a second compression unit 22B and a first heat absorption unit 21A, and does not include the first compression unit 22A, and at least a first compression. Two circulation routes including the second circulation route CL2 including the portion 22A, the second compression portion 22B, and the second heat absorption portion 21B are provided.

  Specifically, in the first circulation route CL1, the working fluid circulates in the order of the first heat absorption part 21A, the second compression part 22B, the first heat radiation part 23A, the second heat radiation part 23B, and the first expansion part 24A. . In the second circulation route CL2, the working fluid is divided into the second heat absorbing part 21B, the first compressing part 22A, the second compressing part 22B, the first heat radiating part 23A, the second heat radiating part 23B, the second regenerator 28B, and the first regeneration. It circulates in order of the container 28A and the second expansion part 24B.

  In this embodiment, the heat absorption part 21, the compression part 22, the heat radiation part 23, the expansion part 24, and the regenerator 28 are each composed of two parts, and the heat balance is optimized. By providing the two circulation routes CL1 and CL2, the design flexibility of the system S1 is improved.

  In the present embodiment, the working fluid at the outlet of the second heat radiating portion 23B and the working fluid at the outlet of the second regenerator 28B substantially merge, and a part thereof serves as a heating source for the first regenerator 28A. Part of the gas flows into the first heat absorption part 21A. In the working fluid route including the first circulation route CL1 and the second circulation route CL2, the flow point balance design is facilitated by providing the junction point of both circulation routes CL1 and CL2.

  Here, it is preferable that the temperature of the working fluid after the heat is radiated by the second regenerator 28B is approximately the same as the temperature of the working fluid after the heat is radiated by the second heat radiating portion 23B. This condition setting facilitates the heat balance design and improves the flexibility of the piping design.

FIG. 3 is a diagram schematically illustrating an example of a temperature change between the exhaust air (exhaust gas) from the heating chamber 16 and the working fluid in the evaporators 212 and 213.
As shown in FIG. 3, in the first evaporator 211, heat exchange is performed between the working medium and the exhaust gas in a relatively high temperature range. Further, in the second evaporator 212, heat exchange is performed between the working medium and the exhaust gas in a relatively low temperature range. Thus, in the two-stage exhaust heat recovery, heat exchange is preferably performed even when the working fluid undergoes a phase change.

  Returning to FIG. 2, as described above, in the present embodiment, the heating system S1 employs a two-stage recovery system for exhaust heat and a two-stage heating system for supply air. These methods are combined well with the two-part heat pump 20 and contribute to the improvement of COP.

  The COP calculated for the system S1 of the present embodiment was 3.28. The temperature of the working fluid in each part of the heat pump 20 in this trial calculation is shown in FIG. In addition, these numerical values are examples, and this invention is not limited to this. As a comparative example, the COP was 2.97 when the heat absorption part 21, the expansion part 24, and the regenerator 28 were calculated as a single part.

  The preferred embodiments of the present invention have been described above, but the present invention is not limited to these embodiments. The numerical value used in the above description is an example, and the present invention is not limited to this. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit of the present invention. The present invention is not limited by the above description, but only by the appended claims.

  S1: heating system, CL1: first circulation route, CL2: second circulation route, 12: heat pump device, 14: heat exchange device, 16: heating chamber, 20: heat pump, 21: heat absorption part, 21A: first heat absorption part , 21B: second heat absorption part, 22: compression part, 22A: first compression part, 22B: second compression part, 23: heat radiation part, 23A: first heat radiation part, 23B: second heat radiation part, 24: expansion part 24A: first expansion section, 24B: second expansion section, 28: regenerator, 28A: first regenerator, 28B: second regenerator, 70: control device, 140: air supply path.

Claims (9)

A heat pump device through which the first fluid flows;
A heat exchange device for transferring heat from the first fluid to the second fluid;
A heating chamber in which heat from the second fluid is transferred to the object,
The heat pump device is
A multi-stage compression section that includes a first compression section and a second compression section and compresses the first fluid;
At least a part of the heat exchange device is disposed, and a heat radiating unit through which the first fluid from the multistage compression unit flows;
An inflating portion for inflating the first fluid from the heat radiating portion;
A heat absorption part including a first heat absorption part and a second heat absorption part, wherein the first fluid from the expansion part absorbs external heat;
A regenerator including a first regenerator and a second regenerator, and transferring a part of heat of the first fluid compressed by the multistage compression unit to the first fluid toward the multistage compression unit;
An industrial heating system comprising: The heat pump device further includes a path that fluidly connects the first heat absorption unit and the stage of the multistage compression unit,
The industrial heating system according to claim 1, wherein the first fluid from the first heat absorption unit enters between stages of the multistage compression unit.   The industrial heating system according to claim 1 or 2, wherein the first heat absorption part and the second heat absorption part are substantially in a non-series relationship with respect to the flow of the first fluid.   The industrial heating system according to any one of claims 1 to 3, wherein an evaporation temperature in the first endothermic part is higher than an evaporation temperature in the second endothermic part. The first compression unit and the second compression unit are a front stage and a rear stage, respectively, in the multistage compression unit,
The heat pump device is
A first circulation route including at least the second compression section and the first heat absorption section, and not including the first compression section;
The industrial use according to any one of claims 1 to 4, further comprising a second circulation route including at least the first compression unit, the second compression unit, and the second heat absorption unit. Heating system. The heat dissipating part is disposed between the first heat dissipating part that is the front stage, the second heat dissipating part that is the rear stage, and the first heat dissipating part and the second heat dissipating part in the flow direction of the first fluid. Including a bifurcation,
The part of the first fluid from the first heat radiating part enters the second heat radiating part via the branch part, and the rest enters the second regenerator. The industrial heating system according to claim 5.   The temperature of the first fluid after radiating heat from the second regenerator is approximately the same as the temperature of the first fluid after radiating heat from the second heat radiating unit. Industrial heating system.   The first fluid at the outlet of the second heat radiating portion and the first fluid at the outlet of the second regenerator substantially merge, and a part thereof serves as a heat source for the first regenerator, and another part. The industrial heating system according to claim 6 or 7, wherein the gas flows into the first heat absorption part. Further comprising a path through which the exhaust fluid having exhaust heat from the heating chamber flows,
A part of the exhaust heat from the heating chamber is supplied to the first heat absorption part,
The industrial heating system according to any one of claims 1 to 8, wherein the remaining part of the exhaust heat from the heating chamber is supplied to the second heat absorption unit.

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