A design method for cross-flow heat exchanger with adjustable tube length

Abstract

The present invention relates to a design method for a cross-flow heat exchanger with adjustable tube length. The present invention does not specify the length of the heat exchanger. According to the changes in the temperature field inside and outside the heat exchanger, the structural layout of each heat exchange unit is accurately designed in sections to ensure that the heat exchange efficiency of each heat exchange unit is maximized as much as possible. At the same time, heat exchanger structures of various sizes under the same working conditions are designed to reduce the volume of the heat exchanger and reduce the manufacturing cost.

Description

Technical Field

[0001] The invention belongs to the technical field of heat exchangers and relates to a design method of a cross-flow heat exchanger with adjustable tube length. Background Art

[0002] Cross-flow heat exchangers are widely used in industrial fields such as converters, boilers, cracking furnaces, heating furnaces, steelmaking furnaces, exhaust gas catalytic incinerators, hot blast furnaces, etc. The current design methods of cross-flow heat exchangers all adopt the segmented design method, because it involves the problem of peripheral flow distribution, and the premise of segmentation is that the length of the single tube needs to be determined in advance. This leads to the fact that the length of the heat exchanger can only be designed according to a fixed length, but in some designs, only the upper limit of the heat exchanger size is given, and no specific size is given. Within the range of not exceeding the upper line and satisfying the conditions such as pressure loss, the smaller the volume of the heat exchanger, the better, such as air separation equipment. The current segmented design method not only needs to be designed when the length of the heat exchanger is known, but also the temperature difference field inside and outside the heat exchanger has not been taken into account during the design process. This leads to the inaccurate design of the heat exchanger. In order to achieve the rated heat exchange efficiency, it is often necessary to reserve excess heat exchange area, which increases the volume of the heat exchanger, while increasing the manufacturing cost of the heat exchanger and the pressure drop of the fluid. Moreover, it is very troublesome to design multiple sizes, and repeated calculations are required.

[0003] Invention patent application document CN109959283 discloses a method and system for thermal verification of a high-temperature cooler. The invention obtains the inlet parameters of the cooler and the number of heat exchange tube groups in the cooler, where each heat exchange tube group contains multiple rows of tube bundles; the inlet parameters of the cooler are used as the inlet conditions of the first row of tube bundles, and the outlet conditions of each row of tube bundles are used as the inlet conditions of the next row of tube bundles, thereby calculating the heat transfer and pressure drop of each row of tube bundles row by row. The invention uses too long a basic unit when calculating the temperature gradient in the tube, resulting in an inaccurate calculated temperature gradient, and there are no restrictions, which is not very helpful for the optimization of the heat exchanger's structural design.

[0004] The article "Study on Segmented Calculation of Shell and Tube Heat Exchangers Based on Matlab" introduces a segmented process design and calculation of the overall shell and tube heat exchanger from the perspective of the change of fluid physical properties along the shell direction. This article is aimed at the shell and tube heat exchanger based on the baffle structure, and does not consider the different internal and external temperature differences of each heat exchange unit of the heat exchanger. In addition, it requires a fixed length, and the designed heat exchanger structure is not accurate, which increases the design cost of the heat exchanger.

[0005] Invention patent CN113742999A discloses a design method and device for a printed circuit board heat exchanger, which determines the number of fins included in the heat exchanger plate based on the designed heat exchange capacity and operating conditions of the heat exchanger and the heat exchange area of ​​a single fin; determines the position of each fin based on the artificial fish school algorithm; and determines the direction of each fin based on the shape characteristics of the heat exchanger plate and the operating conditions. This patent only provides a design method for the heat exchanger fins, and the tube length is still manually determined and fixed. The human intervention is too large, and the designed heat exchanger structure is relatively simple.

[0006] Aiming at the shortcomings of the existing cross-flow heat exchanger design calculation method, the present invention proposes a cross-flow heat exchanger design method with adjustable tube length, which improves the accuracy of calculating the temperature gradient of the heat exchanger, optimizes the structure of the heat exchanger, and designs a heat exchanger with small size and cost saving. Summary of the invention

[0007] The purpose of the present invention is to provide a design method for a cross-flow heat exchanger with adjustable tube length. The length of the heat exchanger is not specified. According to the changes in the temperature field inside and outside the heat exchanger, the structural layout of each heat exchange unit is accurately designed in sections to ensure that the heat exchange efficiency of each heat exchange unit is maximized as much as possible. At the same time, heat exchanger structures of various sizes under the same working conditions are designed to reduce the volume of the heat exchanger and reduce the manufacturing cost.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] A design method for a cross-flow heat exchanger with adjustable tube length comprises the following steps:

[0010] 1) Tubular heat exchanger parameters include: Upper limit of heat exchange tube length L s And the lower limit of heat exchange tube length L x , cold fluid inlet temperature T in and the cold fluid outlet temperature T out , enhanced heat exchange structure, number of strokes m; the parameter selection range of the enhanced heat exchange structure is based on the upper limit of the heat exchange tube length L s And the lower limit of heat exchange tube length L x Confirm; the enhanced heat exchange structure is fins fixed inside and outside the tube wall;

[0011] 2) Determine the number of strokes m according to the heat exchange temperature difference. If the temperature difference is greater than or equal to 300°C, use a double stroke, and if the temperature difference is less than 300°C, use a single stroke. The single stroke is a stroke unit as a whole, and the double stroke divides the high temperature section and the low temperature section into two stroke units for calculation. According to the temperature difference, take 10°C-30°C as a heat exchange unit, and divide each stroke unit into n heat exchange units for calculation;

[0012] 3) Assume that the total length of a stroke unit is L. Given the inlet and outlet temperatures of the cold fluid in this stroke unit, when calculating the first heat exchange unit, select a strengthened heat exchange structure and calculate the heat transfer coefficient δ. The heat transfer coefficient δ is the sum of the convective heat transfer coefficient and the thermal conductivity coefficient. Assume the length of the heat exchange unit is L1, and the flow rate is distributed according to the ratio of the assumed heat exchange unit length L1 to the assumed total length L. Determine the inlet temperature T nin and the outlet temperature T nout of the cold fluid for each heat exchange unit; Given the inlet temperature T rin of the hot fluid, according to the first law of thermodynamics:

[0013] Q1 = mc p △t

[0014] where m is the mass velocity of the fluid, c p is the specific heat capacity, and △t is the temperature difference on the cold side of the heat exchange unit;

[0015] Heat transfer equation:

[0016] Q2 = AδTc

[0017] where A is the heat exchange area and Tc is the logarithmic mean temperature difference;

[0018] Judge whether the assumed heat exchange unit length L1 meets the requirement that the error between the heat transfer quantity Q1 and Q2 is within 3%. If it meets the requirement, then L1 is the tube length of this heat exchange unit; if it does not meet the requirement, adjust L1. If Q1 > Q2, then let L1 = L1 + 0.01. If Q1 < Q2, then let L1 = L1 - 0.01 until the heat transfer quantity is satisfied, and determine the tube length of this heat exchange unit;

[0019] 4) Repeat the above steps until the tube lengths L n of each of the n heat exchange units are obtained. Add up the tube lengths of the n heat exchange units to find the total tube length L j ; According to the difference between L j and L, give corresponding heat exchange area compensation measures to meet the requirement of heat transfer quantity;

[0020] The heat exchange area compensation measures for each heat exchange unit are shown in Table 1:

[0021] Table 1 Compensation Measures

[0022]

[0023] 5) After completing one calculation, the calculation results of the first n - 2 segments remain unchanged. Return to the (n - 1)-th segment, change the strengthened heat exchange structure, find a corresponding tube segment length, repeat step 4), complete the calculation, and then return to step 5) again until all the strengthened heat exchange structure parameters within the given range are calculated once;

[0024] 6) Keep the first n - 3 segments unchanged, change the number of heat exchange structures in the (n - 2)-th segment, repeat step 5), and then change the number of enhanced heat exchange structures in the (n - 2)-th segment again, repeat step 5), and so on, until all the parameters of the enhanced heat exchange structures in one stroke unit are calculated, complete the design of the heat exchanger, output all the designed heat exchanger structures, that is, complete the design of multi-size structures;

[0025] 7) For the case where the span between the upper and lower limits of the tube length is relatively large, it is possible to return to step 3), change the assumed total tube length to L, and repeat steps 3) to 6);

[0026] 8) With the goal of optimizing the overall performance of the heat exchanger, that is, considering the volume, mass, pressure loss, etc. of the heat exchanger, select the eligible heat exchanger design structure to complete the design of the cross-flow heat exchanger with adjustable tube length.

[0027] In step 2) above, 10 ≤ n ≤ 20. If n is less than 10, the calculation accuracy is somewhat rough. If it is greater than 20, the increase in calculation accuracy is limited, and the calculation time is increased.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] The present invention provides a design method for a cross-flow heat exchanger with adjustable tube length. It solves the problems of cumbersome design of multi-size heat exchangers and inaccurate thermal design caused by not considering the temperature difference inside and outside the heat exchanger. Through this calculation method, structural layout schemes of various sizes can be provided for the design of heat exchangers, improving the design accuracy of heat exchangers. Without changing the heat transfer efficiency, it reduces the volume of the heat exchanger, decreases the mass of the heat exchanger, saves manufacturing costs, and reduces fluid pressure loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is the calculation flow chart of the present invention.

[0031] Figure 2 Schematic diagram of the uniform structure of the heat exchange tubes (prior art).

[0032] Figure 3 is the schematic diagram of the variable volume of the heat exchange tubes (the present invention). DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] In order to make the purpose, technical solutions and advantages of the present invention clearer, the following embodiments are used to further illustrate the specific implementation manners of the present invention. The following embodiments are used to specifically illustrate the content of the present invention. These embodiments are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0034] See Figure 1 、 Figure 3 , a design method for a cross-flow heat exchanger with adjustable tube length, including the following steps:

[0035] 1) The parameters of the tubular heat exchanger include: the upper limit Ls and the lower limit Lx of the heat exchange tube length, the inlet temperature T of the cold fluid in and the outlet temperature T of the cold fluid out , the enhanced heat transfer structure, and the number of passes m; among them, the parameter selection range of the enhanced heat transfer structure is determined according to the upper limit Ls and the lower limit Lx of the heat exchange tube length; the enhanced heat transfer structure is the fins fixed inside and outside the tube wall.

[0036] 2) Determine the number of passes m according to the heat transfer temperature difference. When the temperature difference is greater than 300 °C, use a double pass; when the temperature difference is less than 300 °C, use a single pass; the single pass is an entire pass unit, and the double pass divides the high-temperature section and the low-temperature section into two pass units for calculation; according to the temperature difference, with 10 °C - 30 °C as one heat exchange unit, each pass unit is divided into n heat exchange units for calculation; 10 ≤ n ≤ 20.

[0037] 3) Assume that the total length of a pass unit is L. Given the inlet and outlet temperatures of the cold fluid in this pass unit, when calculating the first heat exchange unit, select the enhanced heat transfer structure and calculate the heat transfer coefficient δ. The heat transfer coefficient δ is the sum of the convective heat transfer coefficient and the thermal conductivity; assume that the length of the heat exchange unit is L1, and the flow rate is distributed according to the ratio of the assumed heat exchange unit length L1 to the assumed total length L; determine the inlet temperature T of the cold fluid in each heat exchange unit nin and the outlet temperature T nout ; given the inlet temperature T of the hot fluid rin , according to the first law of thermodynamics:

[0038] Q1 = mc p △t

[0039] where m is the fluid mass velocity, c p is the specific heat capacity, and △t is the temperature difference on the cold side of the heat exchange unit;

[0040] Heat transfer equation:

[0041] Q2 = AδTc

[0042] where A is the heat transfer area and Tc is the logarithmic mean temperature difference;

[0043] Judge whether the assumed heat exchange unit length L1 satisfies that the error between the heat transfer quantity Q1 and Q2 is within 3%. If it satisfies, then L1 is the tube length of this heat exchange unit; if it does not satisfy, adjust L1. If Q1 > Q2, then let L1 = L1 + 0.01. If Q1 < Q2, then let L1 = L1 - 0.01 until the heat transfer quantity is satisfied, and determine the tube length of this heat exchange unit.

[0044] 4) Repeat the above steps until the tube lengths L of each of the n heat exchange units are obtained n , and add the tube lengths of the n heat exchange units to obtain the total tube length L j; According to L j The corresponding heat transfer area compensation measures are given based on the difference from L to meet the heat transfer requirement;

[0045] The heat transfer area compensation measures for each heat exchange unit are shown in Table 1.

[0046] 5) After completing one calculation, the calculation results of the first n - 2 segments remain unchanged. Return to the (n - 1)-th segment, change the enhanced heat transfer structure, find a corresponding pipe segment length, repeat step 4), complete the calculation, and then return to step 5) again until all the enhanced heat transfer structure parameters within the given range are calculated once;

[0047] The first n - 3 segments remain unchanged. Change the number of heat exchange structures in the (n - 2)-th segment, repeat step 5), and then change the number of heat exchange structures in the (n - 2)-th segment again and repeat step 5), and so on until all the enhanced heat transfer structure parameters of one stroke unit are calculated, complete the heat exchanger design, and output all the designed heat exchanger structures, that is, complete the multi-size structure design.

[0048] 7) For the case where the span between the upper and lower limits of the pipe length is relatively large, it is possible to return to step 3), change the assumed total pipe length to L, and repeat steps 3) to 6).

[0049] 8) With the goal of the optimal overall performance of the heat exchanger, that is, comprehensively considering the volume, mass, pressure loss, etc. of the heat exchanger, select the suitable heat exchanger design structure to complete the design of the cross-flow heat exchanger with adjustable pipe length.

[0050] Example:

[0051] Taking the heat exchanger of a reheating furnace already applied in a large steel plant as an example, the air flow rate is 21000 m 3 / h, the flue gas flow rate is 25000 m 3 / h, the flue gas inlet temperature is 700 °C, the total pipe length is 2.5 m, the expected air outlet temperature is 458 °C, the air inlet temperature is 20 °C, the air side pressure loss does not exceed 2500 Pa, and the flue gas side does not exceed 180 Pa. Due to the too high heat exchange temperature and site limitations, the heat exchanger is divided into high and low temperature two strokes. The high temperature section uses smooth tubes with internal inserts to increase the air heat exchange temperature by 120 °C, and the low temperature section uses the form of internal fins with external fins to increase the air heat exchange temperature by 318 °C. According to the design method of the uniform structure, the number of internal and external fins of a single pipe in the low temperature section is calculated to be 240 and 170 respectively (as Figure 2 shown).

[0052] The high-temperature flue gas inlet temperature is 700°C, and considering the strength of the material structure, the bare tube is used, and no further calculation is required. The variable volume calculation is mainly performed on the structure of the inner fins and outer fins in the low-temperature section. According to the actual requirements, the upper and lower limits of the tube length are given as 1.5m-2.7m, the number of outer fins in each section is given as 11-30, and the number of inner fins in each section is given as 28-36. The temperature difference in the low-temperature section is 318°C, and the tube is divided into 12 heat exchange units for calculation. The calculation is performed according to the method of the present invention, such as Figure 1 The calculation process of .

[0053] It is known that the inlet temperature of the hot fluid in the low temperature section is 20℃ and the inlet and outlet temperatures of the cold fluid in this trip are 318℃. According to the actual production site conditions, it is assumed that the pipe length is 2m. According to the inlet and outlet temperatures of the cold fluid, it is divided into 12 sections, and the air temperature is increased by 26.5℃ in each section. Calculation of the first section, it is known that the inlet temperature of the hot fluid is 639℃, and the inlet and outlet temperatures of the cold fluid are 20℃ and 46.5℃. The number of fins inside the enhanced heat exchange structure is selected to be 28, and the number of fins outside is 30. Assuming that a pipe is 0.5m long, according to the first law of thermodynamics and the heat transfer equation, the heat exchange amount Q1 is much greater than Q2. The assumed pipe length is reduced by 0.5m. When the pipe length is 0.17m, the heat exchange needs are met, that is, the length of the first section is 0.17m.

[0054] Repeat the above steps until the length of 12 sections of tubes is calculated, and the total length of 12 sections is 1.74m, 1.74m<2m, and the error is less than 20%. Then a compensation mechanism is adopted to compensate for the number of heat exchange structures. According to the calculation formula in Table 1, the total number of enhanced heat exchange structures is 280, including 160 inner fins and 120 outer fins. The increased heat exchange area is (1.74 / 2)

[0055] *0.12*280=29 pieces, 16 pieces of inner fins and 13 pieces of outer fins are added, and the fins are distributed according to the proportion of the number of fins in each section to the total number of fins. According to the temperature of the cold and hot fluids and the length of the section, the corresponding inner fins are determined to be 14 pieces and the outer fins are 30 pieces. The structural layout of each section is combined together to complete a design calculation.

[0056] After completing a calculation, the calculation results of the first 10 sections remain unchanged, return to the 11th section, change the inner fins to 12 pieces and the outer fins to 30 pieces within the given range, and find a corresponding pipe section length. Repeat the above steps to complete the calculation until the number of heat exchange structures within the given range is calculated once. Keep the first 9 sections unchanged, change the number of heat exchange structures in 10 sections, repeat the above steps, and complete all designs. The designed heat exchanger size includes all lengths within 1.5m-2.7m.

[0057] According to the mass and pressure loss, the heat exchanger design with the best comprehensive performance is 1.85m in length, 180 external fins in the low-temperature section, and 36 internal fins to meet the heat exchange requirements. The overall volume is reduced by 7.5% and the mass is reduced by 3.2%.

Claims

1. A design method for a cross-flow heat exchanger with adjustable tube length, characterized in that: It includes the following steps: 1) Determine the number of passes m according to the heat exchange temperature difference. For a temperature difference greater than 300 °C, a double-pass is adopted; for a temperature difference less than or equal to 300 °C, a single-pass is adopted. The single-pass is an entire pass unit, and the double-pass divides the high-temperature section and the low-temperature section into two pass units for calculation. According to the temperature difference, with 10 °C - 30 °C as one heat exchange unit, each pass unit is divided into n heat exchange units for calculation; 2) Assuming that the total pipe length of a stroke unit is L, and the inlet and outlet temperatures of the cold fluid of the stroke unit are known, when calculating the first heat exchange unit, select the enhanced heat exchange structure and calculate the heat transfer coefficient δ, which is the sum of the convective heat transfer coefficient and the thermal conductivity coefficient; assuming that the length of the heat exchange unit is L1, the flow rate is distributed according to the ratio of the assumed heat exchange unit length L1 to the assumed total pipe length L; determine the inlet temperature T of the cold fluid of each heat exchange unit nin and outlet temperature T nout ; Known hot fluid inlet temperature T rin , according to the first law of thermodynamics: Q1=mc p △t m is the fluid mass velocity, c is p is the specific heat capacity, △t is the temperature difference on the cold side of the heat exchange unit; Heat transfer equation: Q2 = AδTc where A is the heat exchange area and Tc is the logarithmic mean temperature difference; Judge whether the assumed heat exchange unit length L1 satisfies that the error between the heat transfer amount Q1 and Q2 is within 3%. If it is satisfied, then L1 is the tube length of this heat exchange unit. If it is not satisfied, adjust L1. If Q1 > Q2, then let L1 = L1 + 0.01; if Q1 < Q2, then let L1 = L1 - 0.01 until the error between the heat transfer amount Q1 and Q2 is within 3%, and determine the tube length of this heat exchange unit; 3) Repeat the above step 2) until the tube length L of each of the n heat exchange units is obtained. n , add the lengths of n heat exchange unit tubes to get the total tube length L j According to L j The difference with L gives the corresponding heat exchange area compensation measures to meet the demand for heat exchange; The heat exchange area compensation measure for each heat exchange unit is: |LL j | / L<5%, the calculation result is accurate and no compensation is required; 5%≤(LL j ) / L≤20%, increase heat exchange area: L j / L*0.12*Total number of enhanced heat exchange structures; -20%≤(LL j ) / L≤-5%, reduce heat exchange area: L / L j *0.12*Total number of enhanced heat exchange structures; (LL j ) / L>20%,L-1.4*(LL j ) is the new L; (LL j ) / L<-20%,1.4*(L j -L)+L is the new L; 4) Calculate the total length of the tube L j And according to L j The difference between L and L gives the corresponding heat exchange area compensation measures, that is, a calculation is completed. After a calculation is completed, the calculation results of the first n-2 sections remain unchanged, and the n-1 section is returned to change the number of enhanced heat exchange structures, and a corresponding pipe section length is calculated. Repeat step 3) to complete the calculation, and then return to step 4) again until all the enhanced heat exchange structures within the given range are calculated once; 5) Keep the first n - 3 sections unchanged, change the number of enhanced heat transfer structures in the (n - 2)-th section, repeat step 4), and then change the number of enhanced heat transfer structures in the (n - 2)-th section again, repeat step 4), and so on until the number of enhanced heat transfer structures in one pass unit is all calculated, complete the heat exchanger design, and output all designed heat exchanger structures; 6) With the goal of the optimal overall performance of the heat exchanger, select the conforming heat exchanger design structure to complete the design of the cross-flow heat exchanger with adjustable tube length; The enhanced heat transfer structure described is a fin.

2. A design method for a cross-flow heat exchanger with adjustable tube length according to claim 1, characterized in that: In the above step 2), 10 ≤ n ≤ 20.

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