Falling liquid film type evaporator

Abstract

To provide a falling liquid film type evaporator which can improve heat transfer performance and suppress dry-out of a heat transfer tube surface.SOLUTION: Heat transfer tubes 42 of a falling liquid film type evaporator 40 includes a first tube group 42A and a second tube group 42B in this order from above in an area except for a liquid reserve part 44 within a shell 41. An arithmetic average roughness Ra of an outer wall of the heat transfer tube in the first tube group is larger than that of an outer wall of the heat transfer tube in the second tube group.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a falling film evaporator. [Background technology]

[0002] A refrigeration system generally has a compressor that compresses gasified refrigerant and discharges the compressed refrigerant, a condenser that condenses and liquefies the refrigerant discharged from the compressor, an expansion valve that expands the refrigerant condensed in the condenser, and an evaporator that evaporates and gasifies the refrigerant expanded by the expansion valve and draws the gasified refrigerant into the compressor.

[0003] A falling film evaporator is known as an evaporator for such a refrigeration device. In a falling film evaporator, the refrigerant liquid is sprayed by a spraying mechanism such as a sprayer installed in the upper part of the shell to supply the refrigerant liquid to the heat transfer tube group. The refrigerant liquid sprayed by the spraying mechanism first forms a liquid film on the top tube row. Then, the refrigerant gas evaporated on the surface of the top tube row flows to the evaporator outlet above, and the refrigerant liquid that does not evaporate falls to the heat transfer tubes in the lower row and forms a liquid film, and this process is repeated. As the refrigerant liquid reaches the lower rows, the flow state of the refrigerant liquid and the upward flow of the gasified refrigerant cause disturbances and deviations in the liquid film, which can cause dryout and a decrease in heat transfer performance.

[0004] In this regard, for example, Patent Document 1 below discloses a falling film evaporator in which axially grooved tubes are arranged at predetermined positions in a tube bank in order to form uniform liquid films on all heat transfer tubes, thereby improving the axially non-uniform liquid film state. Furthermore, Patent Document 1 discloses that nucleate boiling is promoted by applying thermal spraying and surface roughening treatment to the outer surface of the groove in which the axially grooved tube is provided, thereby improving the efficiency of the falling film evaporator. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 61-140788 Summary of the Invention [Problem to be solved by the invention]

[0006] In the falling film evaporator disclosed in Patent Document 1, the refrigerant liquid accumulates in the grooves of the axially grooved tubes, which leads to poor liquid drainage and a thick liquid film. If the liquid film is thick, the thermal resistance increases and the heat transfer performance decreases at low loads and low liquid film flow rates.

[0007] On the other hand, when the liquid film flow rate is high, the refrigerant liquid is scattered by the grooves of the axially grooved tube that are arranged perpendicular to the flow direction of the refrigerant liquid, making it difficult for the refrigerant liquid to be supplied to the heat transfer tube located directly below, and dryout is likely to occur.

[0008] The present invention has been made to solve the above problems, and an object of the present invention is to provide a falling film evaporator that can improve heat transfer performance and suppress dryout on the surface of the heat transfer tube. [Means for solving the problem]

[0009] A falling film evaporator that achieves the above object includes a shell, a plurality of heat transfer tubes provided in an internal space of the shell and exchanging heat with a refrigerant by water flowing inside the shell, a liquid supply mechanism provided in an upper part of the shell and distributing the liquefied refrigerant toward the heat transfer tubes to a row of the heat transfer tubes, a liquid reservoir provided in a lower part of the shell in which the liquefied refrigerant is stored, and a refrigerant pump that pumps the refrigerant stored in the liquid reservoir up to the liquid supply mechanism. The heat transfer tubes include a first tube group and a second tube group, in this order from above, in a region of the shell excluding the liquid reservoir, and the arithmetic mean roughness of the outer walls of the heat transfer tubes in the first tube group is greater than the arithmetic mean roughness of the outer walls of the heat transfer tubes in the second tube group. Effect of the Invention

[0010] In the falling film evaporator configured in this manner, the arithmetic mean roughness of the heat transfer tubes in the first tube group, where a relatively large amount of refrigerant liquid exists, is large, promoting boiling and improving heat transfer performance. In addition, the arithmetic mean roughness of the heat transfer tubes in the second tube group, where a relatively small amount of refrigerant liquid exists, is small, so that the refrigerant liquid flows into the areas where the liquid film is locally thin due to capillary action, thereby suppressing dryout. From the above, it is possible to provide a falling film evaporator that can improve heat transfer performance and suppress dryout on the heat transfer tube surface. [Brief description of the drawings]

[0011] [Figure 1] 1 is a system diagram showing a refrigeration device system according to an embodiment of the present invention. [Diagram 2] FIG. 1 is a schematic diagram showing a falling film evaporator. [Diagram 3] The left image is an image showing how boiling occurs when the arithmetic mean roughness is small, and the right image is an image showing how boiling occurs when the arithmetic mean roughness is large. [Figure 4] 1 is a graph showing the relationship between arithmetic mean roughness and heat transfer coefficient. [Diagram 5] The left image is an image showing the liquid film thickness when the arithmetic mean roughness is small, and the right image is an image showing the liquid film thickness when the arithmetic mean roughness is large. [Figure 6] 1 is a graph showing the relationship between the arithmetic mean roughness and the film Reynolds number at which dry patches were always observed on the surface. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] An embodiment of the present invention will be described with reference to Fig. 1 to Fig. 6. In the description of the drawings, the same elements are given the same reference numerals, and duplicated description will be omitted. The dimensional ratios in the drawings are exaggerated for the convenience of explanation, and may differ from the actual ratios.

[0013] FIG. 1 is a system diagram showing a refrigeration system 1 according to this embodiment. FIG. 2 is a schematic diagram showing a falling film evaporator 40. In the following description, a refrigerant in a gasified state is referred to as a refrigerant gas, and a refrigerant in a liquefied state is referred to as a refrigerant liquid. The refrigerant is not particularly limited, but an HFO refrigerant, an HCFO refrigerant, or the like can be used. The falling film evaporator 40 may also be simply referred to as an evaporator 40.

[0014] First, with reference to FIG. 1, a schematic configuration of a refrigeration system 1 will be described.

[0015] The refrigeration system 1 according to this embodiment includes a compressor 10, a condenser 20, an expansion valve 30, an evaporator 40, a receiver 50, and a control unit 60.

[0016] The compressor 10 compresses the sucked refrigerant gas and discharges the compressed refrigerant gas at a high temperature and pressure. The compressor 10 is driven by an inverter 11, and the rotation speed of the compressor 10 is controlled by the inverter 11 and a control unit 60. The compressor 10 is, for example, a reciprocating type, but may be a screw type.

[0017] The condenser 20 exchanges heat between the high-temperature, high-pressure refrigerant gas discharged from the compressor 10 and external air or cooling water, condenses the refrigerant gas, and discharges it as a low-temperature, high-pressure refrigerant liquid.

[0018] The expansion valve 30 expands the refrigerant liquid that flows out of the condenser 20, reducing its pressure.

[0019] The evaporator 40 evaporates and gasifies the refrigerant liquid expanded by the expansion valve 30, and causes the refrigerant gas to be drawn into the compressor 10. In the evaporator 40, the refrigerant liquid is evaporated by heat exchange with water flowing inside the heat transfer tube 42, and returns to the compressor 10. A detailed configuration of the evaporator 40 will be described later.

[0020] The receiver 50 is provided between the condenser 20 and the expansion valve 30. The receiver 50 is provided to absorb fluctuations in the amount of refrigerant in the evaporator 40 due to load fluctuations.

[0021] The control unit 60 controls, for example, the start, stop, and rotation speed of the compressor 10. The control unit 60 is, for example, a CPU.

[0022] The configuration of the evaporator 40 will be described in detail below.

[0023] As shown in FIGS. 1 and 2, the evaporator 40 includes a shell 41, a heat transfer tube 42, a spray 43, a liquid reservoir 44, a storage portion 45, and a refrigerant pump 46.

[0024] Shell 41 extends along the longitudinal direction (the direction perpendicular to the paper surface of FIG. 2) and has a cylindrical shape. Note that shell 41 is not limited to a cylindrical shape. Inside shell 41, refrigerant gas and refrigerant liquid are mixed.

[0025] The heat transfer tube 42 is provided in the internal space of the shell 41 so as to extend along the longitudinal direction of the shell 41. Water with which the refrigerant exchanges heat flows inside the heat transfer tube 42. The water has a high temperature relative to the refrigerant, and the refrigerant liquid that has exchanged heat with the water becomes a refrigerant gas and moves to the compressor 10.

[0026] As shown in Fig. 2, the heat transfer tubes 42 have, in order from the top, a first tube group 42A, a second tube group 42B, and a third tube group 42C. As shown in Fig. 2, the first tube group 42A and the second tube group 42B are provided in a region above the liquid reservoir 44 in the shell 41. As shown in Fig. 2, the third tube group 42C is provided in the liquid reservoir 44 in the shell 41.

[0027] The heat transfer tubes 42 in the first tube group 42A are arranged in four vertical rows and eleven tubes in the horizontal direction, although there is no particular limitation thereto. The heat transfer tubes 42 in the second tube group 42B are arranged in four vertical rows and eleven tubes in the horizontal direction, although there is no particular limitation thereto. The heat transfer tubes 42 in the third tube group 42C are arranged in two vertical rows, with nine tubes in the horizontal direction in the upper row and five tubes in the horizontal direction in the lower row, although there is no particular limitation thereto.

[0028] The arithmetic mean roughness Ra of the heat transfer tubes 42 in the first tube group 42A is larger than the arithmetic mean roughness Ra of the heat transfer tubes 42 in the second tube group 42B. The arithmetic mean roughness Ra does not have to be gradually decreased from the top to the bottom in the first tube group 42A and the second tube group 42B. However, preferably, the heat transfer tubes 42 are configured so that the arithmetic mean roughness Ra is gradually decreased from the top to the bottom. As a method for forming the arithmetic mean roughness Ra of the first tube group 42A and the second tube group 42B, for example, sandblasting can be used. By using sandblasting in this way, a predetermined arithmetic mean roughness Ra can be formed easily and at low cost. In addition, since sandblasting can be used for materials such as steel and stainless steel, which are difficult to process fins, the choice of materials is expanded.

[0029] A preferred numerical range for the arithmetic mean roughness Ra of the heat transfer tubes 42 in the first tube group 42A will be described below with reference to Fig. 3 and Fig. 4. The left diagram in Fig. 3 is an image diagram showing the state of boiling when the arithmetic mean roughness Ra is small, and the right diagram in Fig. 3 is an image diagram showing the state of boiling when the arithmetic mean roughness Ra is large. Fig. 4 is a graph showing the relationship between the arithmetic mean roughness Ra and the heat transfer coefficient.

[0030] In the first tube group 42A provided in the upper part, the amount of refrigerant liquid is large, so that dry-out is unlikely to occur, and therefore it is possible to improve the heat transfer performance by promoting boiling.

[0031] The inventors compared the left and right diagrams in Fig. 3 and found that by increasing the arithmetic mean roughness Ra, the density of the foaming points is reduced, and the groove height of the recesses 90 increases with the increase in the arithmetic mean roughness Ra, which promotes the growth of boiling bubbles, thereby promoting nucleate boiling and improving heat transfer performance. However, as the arithmetic mean roughness Ra increases, the number of recesses 90 that contribute to boiling decreases, and it was found that there is an optimal arithmetic mean roughness Ra at which heat transfer performance is improved. Hereinafter, the optimal arithmetic mean roughness Ra of the heat transfer tubes 42 in the first tube group 42A will be described with reference to Fig. 4.

[0032] 4, the horizontal axis indicates the arithmetic mean roughness Ra, and the vertical axis indicates the heat transfer coefficient. 2 4, that is, under conditions where boiling is predominant. As can be seen from Fig. 4, it was found that the heat transfer coefficient is improved by approximately two times by setting the arithmetic mean roughness Ra to 3 µm, for example, compared to the arithmetic mean roughness Ra (0.4 µm) of a smooth tube that has not been sandblasted. From these experimental results, it is preferable that the arithmetic mean roughness Ra of the heat transfer tubes 42 in the first tube group 42A is 2 to 5 µm.

[0033] Next, a preferred numerical range of the arithmetic mean roughness Ra of the heat transfer tubes 42 in the second tube group 42B will be described with reference to Fig. 5 and Fig. 6. The left diagram in Fig. 5 is an image diagram showing the liquid film thickness when the arithmetic mean roughness Ra is small, and the right diagram in Fig. 5 is an image diagram showing the liquid film thickness when the arithmetic mean roughness Ra is large. Fig. 6 is a graph showing the relationship between the arithmetic mean roughness and the film Reynolds number at which dry patches were always observed on the surface.

[0034] In the second tube group 42B provided below, the amount of refrigerant liquid is small, so evaporation and boiling in a thin liquid film state coexist, and evaporation is the dominant factor in heat transfer. Therefore, in the second tube group 42B, it is necessary to reduce the arithmetic mean roughness Ra to suppress dryout and stabilize the flow state.

[0035] The inventors compared the left and right diagrams in FIG. 5 and found that when the arithmetic mean roughness Ra increases, the refrigerant liquid arranged around the heat transfer tube 42 bridges as shown in the right diagram in FIG. 5, increasing the liquid film thickness and decreasing the heat transfer performance.

[0036] Therefore, by forming an arithmetic mean roughness Ra that is lower than the liquid film thickness of the refrigerant liquid, it is possible to suppress a decrease in heat transfer performance. Furthermore, it is possible to suppress dry-out by supplying refrigerant liquid to the area where the liquid film is locally thin by capillary action. Hereinafter, the optimal arithmetic mean roughness Ra of the heat transfer tubes 42 in the second tube group 42B will be described with reference to FIG. 6.

[0037] In FIG. 6, the horizontal axis indicates the arithmetic mean roughness Ra, and the vertical axis indicates the film Reynolds number at which dry patches were always observed on the surface during the evaporation heat transfer experiment. The film Reynolds number is a value that renders the liquid film flow rate dimensionless using the refrigerant physical property values. The results shown by circles in FIG. 6 are the results when the heat flux on the heat transfer surface of the heat transfer tube 42 is 5 kW / m 2 The results shown by the squares in FIG. 6 are for a region where evaporation is dominant, and the results are for a region where the heat flux on the heat transfer surface of the heat transfer tube 42 is 20 kW / m 2 The results are shown for the region where boiling is dominant.

[0038] As can be seen from FIG. 6, the heat flux of the water flowing inside the heat transfer tube 42 is 5 kW / m 2 When the arithmetic mean roughness Ra gradually increases, the height of the recesses 90 increases with the increase in the arithmetic mean roughness Ra, so that the capillary force acts on the recesses 90 and the expansion of the dry patch is suppressed. As a result, the surface of the heat transfer tube continues to be wet even if the amount of refrigerant supplied is small. Also, as can be seen from FIG. 6, when the heat flux of the water flowing inside the heat transfer tube 42 is 20 kW / m 2 In this case, if the arithmetic mean roughness Ra is not about 0.5 μm or more, dry patches will appear, and if dry patches are to be prevented from appearing, the flow rate of the refrigerant must be increased. From the results of this experiment, it is preferable that the arithmetic mean roughness Ra of the heat transfer tubes 42 in the second tube group 42B is 0.5 to 2 μm.

[0039] Because the heat transfer tubes 42 in the third tube group 42C are immersed in the refrigerant liquid, the arithmetic mean roughness Ra of the heat transfer tubes 42 in the first tube group 42A is preferably set to 2 to 5 μm to promote boiling. Note that the arithmetic mean roughness Ra of the heat transfer tubes 42 in the third tube group 42C is not limited to 2 to 5 μm, and may be 0.5 to 2 μm, and may be smooth tubes that have not been subjected to sandblasting, or may be finned tubes.

[0040] The heat transfer tube 42 of the falling film evaporator 40 according to this embodiment is not formed with fins. Here, for example, if a heat transfer tube with fins is used, the flow state on the heat transfer tube surface is restricted by the fins, and the flow state of the falling liquid at a high liquid film flow rate becomes a liquid column. Therefore, there is a possibility that dryout occurs because there are areas between the liquid columns to which the refrigerant liquid is not supplied. In contrast, the heat transfer tube 42 of the falling film evaporator 40 according to this embodiment is not formed with fins and has a predetermined arithmetic mean roughness Ra by sandblasting, so that the refrigerant liquid falls in droplets over a wide flow rate range. Therefore, when the droplets collide with the heat transfer tube 42, the refrigerant liquid spreads easily, and the occurrence of dryout can be suppressed. Therefore, the refrigerant flow rate can be lowered than in the conventional falling film evaporator, and the power consumption of the refrigerant pump 46 can be reduced.

[0041] The spray 43 is used as an example of a liquid supply mechanism. The spray 43 is provided in an upper portion inside the shell 41, and radially sprays the refrigerant liquid toward the heat transfer tubes 42. The angle at which the spray 43 radially sprays toward the inside of the shell 41 is not particularly limited. The number of sprays 43 provided is not particularly limited.

[0042] The liquid reservoir 44 is provided in the lower part of the shell 41, and stores the refrigerant liquid. The liquid reservoir 44 is provided to prevent cavitation from occurring in the refrigerant pump 46.

[0043] In the liquid reservoir 44, the third tube group 42C of the heat transfer tubes 42 is disposed.

[0044] Storage portion 45 is fixed below shell 41 and stores refrigerant liquid. Storage portion 45 is provided to prevent cavitation from occurring in refrigerant pump 46. The method of fixing storage portion 45 to shell 41 is not particularly limited.

[0045] The refrigerant pump 46 pumps the refrigerant stored in the liquid reservoir 44 and the storage unit 45 up to the spray 43. The refrigerant pump 46 is a pump for liquid that can pump refrigerant liquid but cannot pump refrigerant gas. The refrigerant pump 46 is disposed on a connecting pipe 47 that connects the storage unit 45 and the spray 43.

[0046] In the evaporator 40 configured as above, the refrigerant liquid pumped up by the refrigerant pump 46 is sprayed by the spray 43 toward the heat transfer tube 42 to cool the cold water flowing inside the heat transfer tube 42. The refrigerant liquid sprayed toward the heat transfer tube 42 is stored in the liquid reservoir 44 and the storage section 45, and is pumped up by the refrigerant pump 46 to be circulated.

[0047] By using a falling film evaporator as the evaporator 40 in this manner, the amount of refrigerant used can be reduced and cooling performance can be improved compared to a flooded evaporator in which the shell is filled with refrigerant liquid.

[0048] As described above, the falling film evaporator 40 according to this embodiment includes the shell 41, a plurality of heat transfer tubes 42 provided in the internal space of the shell 41 and exchanging heat with the refrigerant by water flowing inside, a spray 43 provided in an upper portion of the shell 41 and spraying a liquefied refrigerant toward the heat transfer tubes 42, a liquid reservoir 44 provided in a lower portion of the shell 41 in which the liquefied refrigerant is stored, and a refrigerant pump 46 that pumps the refrigerant stored in the liquid reservoir 44 up to the spray 43. In a region of the shell 41 excluding the liquid reservoir 44, the heat transfer tubes 42 include a first tube group 42A and a second tube group 42B in this order from above, and the arithmetic mean roughness Ra of the outer walls of the heat transfer tubes 42 in the first tube group 42A is greater than the arithmetic mean roughness Ra of the outer walls of the heat transfer tubes 42 in the second tube group 42B. In the falling film evaporator 40 configured in this manner, the arithmetic mean roughness Ra of the heat transfer tubes 42 in the first tube group 42A, where a relatively large amount of refrigerant liquid exists, is large, so boiling is promoted and heat transfer performance is improved. In addition, the arithmetic mean roughness Ra of the heat transfer tubes 42 in the second tube group 42B, where a relatively small amount of refrigerant liquid exists, is small, so that the refrigerant liquid flows into the area where the liquid film is locally thin by capillary action, and dryout can be suppressed. From the above, it is possible to provide a falling film evaporator 40 that can improve heat transfer performance and suppress dryout on the surface of the heat transfer tube 42.

[0049] The outer walls of the heat transfer tubes 42 are configured so that the arithmetic mean roughness Ra gradually decreases from top to bottom. The falling film evaporator 40 configured in this way can provide an arithmetic mean roughness that more appropriately corresponds to the change in the refrigerant flow rate from top to bottom, enabling more effective heat exchange. Furthermore, the heat transfer tubes 42 can be manufactured by gradually adjusting the particle size of the sandblasting particles or the sand projection time, making it easy to manufacture and change the roughness.

[0050] Furthermore, no fins are formed on the heat transfer tubes 42. With the falling film evaporator 40 configured in this manner, the refrigerant liquid spreads easily when droplets collide with the heat transfer tubes 42, making it possible to suppress the occurrence of dryout. As a result, the refrigerant flow rate can be made lower than in conventional falling film evaporators, and the power consumption of the refrigerant pump 46 can be reduced.

[0051] The present invention is not limited to the above-described embodiment, and various modifications can be made within the scope of the claims.

[0052] For example, in the above-described embodiment, no fins were formed on the heat transfer tube 42, but the surface of the heat transfer tube on which fins that do not impede the flow state are formed may be provided with projections and recesses. [Explanation of symbols]

[0053] 40 Falling film evaporator, 41 Shell, 42 Heat transfer tube, 42A 1st tube group, 42B 2nd tube group, 43 Spray, 44 Liquid reservoir, 46 Refrigerant pump.

Claims

1. A shell, a plurality of heat transfer tubes provided in an internal space of the shell and configured to exchange heat with the refrigerant through water flowing therethrough; a liquid supply mechanism provided in an upper portion of the shell and configured to distribute the liquefied refrigerant toward the heat transfer tubes to the rows of the heat transfer tubes; a liquid reservoir portion provided in a lower portion of the shell and configured to store the liquefied refrigerant; a refrigerant pump that pumps the refrigerant stored in the liquid reservoir to the liquid supply mechanism, The heat transfer tubes include a first tube group and a second tube group, arranged in this order from above, in a region of the shell excluding the liquid reservoir portion, a falling film evaporator, wherein an arithmetic mean roughness of an outer wall of the heat transfer tubes in the first tube group is greater than the arithmetic mean roughness of the outer wall of the heat transfer tubes in the second tube group.

2. 2. The falling film evaporator according to claim 1, wherein the outer walls of the plurality of heat transfer tubes are configured such that the arithmetic mean roughness gradually decreases from the upper heat transfer tube to the lower heat transfer tube.

3. 3. The falling film evaporator according to claim 1, wherein the arithmetic mean roughness of the heat transfer tubes in the first tube group is 2 to 5 μm, and the arithmetic mean roughness of the heat transfer tubes in the second tube group is 0.5 to 2 μm.

4. 3. The falling film evaporator according to claim 1, wherein the heat transfer tube is not formed with fins.

5. 3. The falling film evaporator of claim 1, wherein the refrigerant is an HFO refrigerant or an HCFO refrigerant.

Images