A method for evaluating the performance of a heat exchanger based on flow

Abstract

The present application relates to the technical field of heat exchanger performance evaluation, and particularly relates to a method for evaluating heat exchanger performance based on flow, which comprises: obtaining the dimensionless RTD characteristic variance of a hot fluid side of a heat exchanger and the dimensionless RTD characteristic variance of a cold fluid side of the heat exchanger under standard working conditions; obtaining the flow pressure drop Δp h of the hot fluid side of the heat exchanger and the flow pressure drop Δp c of the cold fluid side of the heat exchanger; determining a performance evaluation index EEI of the heat exchanger; and determining a probability distribution function F(EEI) satisfied by performance data of the heat exchanger according to statistical hypothesis testing. The performance evaluation index EEI of the present application is only related to structural features of the heat exchanger and is not affected by parameters such as flow rate and material characteristics, so the method has good stability. Through the performance evaluation index, the probability distribution law satisfied by heat exchangers of the same type under different structural parameters can be statistically determined, so that the heat exchangers of the same type can be classified and evaluated in performance.

Description

Technical Field

[0001] This invention relates to the field of heat exchanger performance evaluation technology, and more specifically to a method for evaluating heat exchanger performance based on flow. Background Technology

[0002] Heat exchangers are widely used in industries such as chemical, oil refining, metallurgy, and pharmaceuticals, and are indispensable key equipment in the production process. The function of a heat exchanger is to transfer the energy of a hot fluid to a cold fluid during the process, generally to increase the temperature of the cold fluid or decrease the temperature of the hot fluid, depending on the process objective.

[0003] The efficiency of a heat exchanger represents the ratio between the specific technological performance exhibited during equipment operation and the energy consumption required to achieve that performance. A high efficiency level indicates the economic viability of the heat exchanger. There are many types of heat exchangers, with common types including shell-and-tube, coaxial, and plate heat exchangers. Currently, most heat exchanger performance evaluations directly assess the overall heat transfer coefficient (OTC). However, the testing and calculation methods for the OTC differ among different types of heat exchangers, and the testing methods are relatively difficult, exhibiting significant dispersion and lacking universality. This has resulted in the absence of a unified and reliable efficiency assessment method for heat exchangers both domestically and internationally. Only a few evaluation methods exist for specific heat exchangers; for example, the standard NB / T4700 includes an efficiency assessment method for plate heat exchangers, but this method cannot be applied to other types of heat exchangers.

[0004] Since it is difficult to evaluate heat exchanger performance directly through heat transfer, performance can be assessed by focusing on the fluid flow process. The flow process is fundamental to heat transfer, and the fluid flow within the heat exchanger is a major factor affecting its performance. Therefore, evaluating the flow process can also reflect the heat exchanger's performance.

[0005] Residence Time Distribution (RTD) is an effective tool for flow research, used to evaluate the performance of equipment by measuring the RTD characteristics of the fluid within it. Measuring the RTD of fluids in equipment is simple and convenient; it only requires adding a tracer at the equipment inlet during operation and monitoring the change in tracer concentration at the outlet to obtain the RTD. Therefore, this method is not limited by equipment type or structure and has universal applicability. For a long time, RTD has been mainly used for backmixing mode analysis and reaction rate calculation in chemical reactors, but its application in the study of process equipment such as heat exchangers has been limited.

[0006] Therefore, it is necessary to propose a more reasonable technical solution to solve the technical problems existing in the current technology. Summary of the Invention

[0007] To overcome at least one of the aforementioned defects, this invention proposes a method for evaluating the performance of a heat exchanger based on flow characteristics. This method quantitatively describes the deviation of the fluid flow in the heat exchanger from the ideal condition based on the RTD (Resolution Tolerance) of the fluid in the heat exchanger, thereby evaluating the performance of the heat exchanger.

[0008] To achieve the above objectives, the evaluation method disclosed in this invention can adopt the following technical solution:

[0009] A method for evaluating heat exchanger performance based on flow includes:

[0010] Obtain the dimensionless RTD characteristic variance of the heat exchanger's hot fluid side under standard operating conditions. Dimensionless RTD characteristic variance on the cold fluid side ,

[0011]

[0012]

[0013] in, , , , ; Let V be the variance of the residence time of the fluid on the hot fluid side. Let V be the variance of the residence time of the fluid on the cold fluid side. The average residence time of the fluid on the hot side. The average residence time of the fluid on the cold fluid side. The characteristic flow length on the hot fluid side. The characteristic flow length on the cold fluid side. The average velocity of the fluid on the hot side. The average flow velocity of the fluid on the cold fluid side. Let be the distribution density function of the residence time of the hot fluid side obtained through RTD experiments. This is the distribution density function of the residence time of the cold fluid side obtained through RTD experiments. t This refers to the residence time of the fluid in the heat exchanger.

[0014] Obtain the flow pressure drop on the hot fluid side of the heat exchanger. and flow pressure drop on the cold fluid side :

[0015]

[0016]

[0017] in, The characteristic drag coefficient on the hot fluid side, This represents the characteristic drag coefficient on the cold fluid side; and , and These are all determined by the structural characteristics of the hot and cold fluid sides, and are unrelated to flow velocity, etc.

[0018] Determine the performance evaluation index (EEI) for heat exchangers:

[0019]

[0020] in, The density of the fluid on the hot side. The density of the fluid on the cold fluid side;

[0021] Determine the efficiency probability distribution function of the heat exchanger By summarizing the performance data of heat exchangers of the same type under different structural parameters, and based on statistical hypothesis testing, determining the normal distribution law that the probability distribution of the performance index of a certain type of heat exchanger satisfies, the probability distribution function of the performance of that type of heat exchanger can be obtained. .

[0022] In the evaluation method disclosed in this invention, the performance evaluation index EEI is only related to the structural characteristics of the heat exchanger and is not affected by parameters such as flow rate and material properties, thus the method has excellent stability. This performance evaluation index can be used to statistically analyze the probability distribution of a certain type of heat exchanger under different structural parameters, thereby classifying and evaluating the performance of that type of heat exchanger.

[0023] Furthermore, in this invention, the efficiency evaluation index (EEI) of the heat exchanger can be calculated using the above method. Based on this value, the efficiency level of the corresponding heat exchanger can be evaluated. Specifically, this can be achieved in the following way: Two efficiency evaluation indexes (EEI) are set, and corresponding medium-high efficiency and medium-low efficiency thresholds are obtained in the efficiency probability distribution function. The interval from the efficiency value corresponding to the medium-high efficiency threshold to an efficiency value of 1 is the high-efficiency interval; the interval from the efficiency value corresponding to the medium-high efficiency threshold to the efficiency value corresponding to the medium-low efficiency threshold is the medium-efficiency interval; and the interval from the efficiency value corresponding to the medium-low efficiency threshold to an efficiency value of 0 is the low-efficiency interval. When using this scheme, if the efficiency level of the heat exchanger is in the high-efficiency range, it means that the heat exchanger is economical and energy-saving, and can be recommended for production and use; if the efficiency level of the heat exchanger is in the medium-efficiency range, the design unit can consider it appropriately based on its own costs; if the efficiency level of the heat exchanger is in the low-efficiency range, it indicates that the heat exchanger with these structural parameters is not suitable for production and use.

[0024] Furthermore, due to significant structural differences between different types of heat exchangers, their EEI performance calculation results vary considerably. Here, we optimize the calculation and explain one possible choice: When using a shell-and-tube heat exchanger, its performance evaluation index EEI is equal to the product of the tube-side and shell-side performance ratios. The performance of the shell-side fluid in the shell-and-tube heat exchanger is used as an equivalent to its performance evaluation index, i.e.:

[0025] .

[0026] For the shell side of a shell-and-tube heat exchanger, due to the relatively large inner diameter of the equipment and the presence of internal components such as baffles, the variance of the fluid residence time ranges from 0 to... If the dead zone volume of the shell side is large, the residence time of the fluid will be highly dispersed, and its variance will be very large, even approaching that of the shell side. For the tube side of a shell-and-tube heat exchanger, since the tube side typically consists of hundreds or thousands of heat exchange tubes, the fluid flow model in the tube side can be considered as an axial diffusion model, i.e. 0, therefore the efficiency ratio of the pipeline is 0. It is almost a constant number.

[0027] Since the performance evaluation index EEI is the product of the tube-side and shell-side performance ratios, and for shell-and-tube heat exchangers, the tube-side performance ratio is... Since it is a constant, the performance evaluation index EEI of shell-and-tube heat exchangers can be simplified in the manner described above.

[0028] Compared with the prior art, some of the beneficial effects of the technical solution disclosed in this invention include:

[0029] The performance evaluation index EEI of this invention is only related to the structural characteristics of the heat exchanger and is not affected by parameters such as flow rate and material properties, thus the method has excellent stability. This performance evaluation index can be used to statistically analyze the probability distribution of a certain type of heat exchanger under different structural parameters, thereby classifying and evaluating the performance of that type of heat exchanger. Attached Figure Description

[0030] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 For the fluid in the heat exchanger E ( t A schematic diagram showing the changes in the curve and its variance.

[0032] Figure 2 This is an example of the classification and evaluation of heat exchanger performance levels. Detailed Implementation

[0033] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments.

[0034] The existing methods for evaluating the performance of heat exchangers have significant limitations. This embodiment optimizes these methods to overcome the shortcomings of the prior art.

[0035] Example

[0036] This embodiment provides a method for evaluating heat exchanger performance based on flow, including:

[0037] S01. Obtain the dimensionless RTD characteristic variance of the heat exchanger's hot fluid side under standard operating conditions. Dimensionless RTD characteristic variance on the cold fluid side :

[0038]

[0039]

[0040] in, , , , ; Let V be the variance of the residence time of the fluid on the hot fluid side. Let V be the variance of the residence time of the fluid on the cold fluid side. The average residence time of the fluid on the hot side. The average residence time of the fluid on the cold fluid side. The characteristic flow length on the hot fluid side. The characteristic flow length on the cold fluid side. The average velocity of the fluid on the hot side. The average flow velocity of the fluid on the cold fluid side. Let be the distribution density function of the residence time of the fluid on the hot fluid side. Let be the distribution density function of the residence time of the fluid on the cold fluid side. t This refers to the length of stay.

[0041] S02. Obtain the flow pressure drop of the heat exchanger on the hot fluid side. and flow pressure drop on the cold fluid side :

[0042]

[0043]

[0044] in, The characteristic drag coefficient on the hot fluid side, This represents the characteristic drag coefficient on the cold fluid side;

[0045] S03. Determine the performance evaluation index EEI for the heat exchanger:

[0046]

[0047] in, The density of the fluid on the hot side. The density of the fluid on the cold fluid side;

[0048] S04. Determine the efficiency probability distribution function of the heat exchanger. By summarizing the performance data of heat exchangers of the same type under different structural parameters, and based on statistical hypothesis testing, determining the normal distribution law that the probability distribution of the performance index of a certain type of heat exchanger satisfies, the probability distribution function of the performance of that type of heat exchanger can be obtained. .

[0049] In the evaluation method disclosed in this embodiment, the performance evaluation index EEI is only related to the structural characteristics of the heat exchanger and is not affected by parameters such as flow rate and material properties, so the method has good stability. This performance evaluation index can be used to statistically analyze the probability distribution of a certain type of heat exchanger under different structural parameters, thereby classifying and evaluating the performance of that type of heat exchanger.

[0050] Appendix Figure 1 As shown, for heat exchangers, it is generally desirable that the RTD model of the fluid in the heat exchanger is a plug flow model or an axial diffusion model, that is, fluids that enter the equipment simultaneously at a certain moment experience as similar a residence time as possible before flowing out of the equipment simultaneously. The variance of their residence time is then considered. 0. However, due to limitations imposed by the equipment walls and internal components, the residence time of the fluid within the equipment is dispersed; this ideal situation is impossible, and the variance of the residence time is 0. The only way to minimize the variance of fluid residence time in the equipment is to modify its structure. Therefore, according to the definition of the performance evaluation index EEI, a higher EEI value indicates a higher efficiency ratio of the heat exchanger, meaning better economic performance.

[0051] In this embodiment, the efficiency evaluation index (EEI) of the heat exchanger can be calculated using the above method. Based on this value, the efficiency level of the corresponding heat exchanger can be evaluated. Specifically, this can be achieved in one of the following ways: Two EEI values ​​are set, and corresponding medium-high efficiency and medium-low efficiency thresholds are obtained in the efficiency probability distribution function. The interval from the efficiency value corresponding to the medium-high efficiency threshold to an efficiency value of 1 is the high-efficiency interval; the interval from the efficiency value corresponding to the medium-high efficiency threshold to the efficiency value corresponding to the medium-low efficiency threshold is the medium-efficiency interval; and the interval from the efficiency value corresponding to the medium-low efficiency threshold to an efficiency value of 0 is the low-efficiency interval. When using this scheme, if the heat exchanger's efficiency level is in the high-efficiency range, it means that the heat exchanger is economical and energy-saving, and can be recommended for production and use; if the heat exchanger's efficiency level is in the medium-efficiency range, the design unit can consider it appropriately based on its own costs; if the heat exchanger's efficiency level is in the low-efficiency range, it indicates that the heat exchanger with these structural parameters is not suitable for production and use.

[0052] Under standard operating conditions, the efficiency ratios of shell-and-tube heat exchangers with different structural parameters are statistically analyzed. Based on statistical hypothesis testing, the probability distribution function F(EEI) satisfied by the efficiency index EEI of shell-and-tube heat exchangers with different structural parameters can be obtained. This function can then be used to classify the efficiency levels of shell-and-tube heat exchangers with different structural parameters. Figure 2 As shown, in one example, the performance threshold for medium-high performance is set to 0.75, and the performance threshold for medium-low performance is set to 0.25. Based on this:

[0053] (1) Shell-and-tube heat exchangers whose corresponding efficiency values ​​are in the range of 0.75 to 1 can be rated as high-efficiency heat exchangers.

[0054] (2) Shell-and-tube heat exchangers whose efficiency value F(EEI) is in the range of 0.25 to 0.75 are medium-efficiency heat exchangers.

[0055] (3) Shell-and-tube heat exchangers whose efficiency value F(EEI) is in the range of 0 to 0.25 are inefficient heat exchangers.

[0056] Due to significant structural differences among different types of heat exchangers, their EEI performance calculation results vary considerably. This section optimizes and explains one possible choice: When using a shell-and-tube heat exchanger, its performance evaluation index EEI is equal to the product of the tube-side and shell-side performance ratios. The performance of the shell-side fluid in the shell-and-tube heat exchanger is used as an equivalent to its performance evaluation index, i.e.:

[0057] .

[0058] For the shell side of a shell-and-tube heat exchanger, due to the relatively large inner diameter of the equipment and the presence of internal components such as baffles, the variance of the fluid residence time ranges from 0 to... If the dead zone volume of the shell side is large, the residence time of the fluid will be highly dispersed, and its variance will be very large, even approaching that of the shell side. For the tube side of a shell-and-tube heat exchanger, since the tube side typically consists of hundreds or thousands of heat exchange tubes, the fluid flow model in the tube side can be considered as an axial diffusion model, i.e. 0, therefore the efficiency ratio of the pipeline is 0. It is almost a constant number.

[0059] Since the performance evaluation index EEI is the product of the tube-side and shell-side performance ratios, and for shell-and-tube heat exchangers, the tube-side performance ratio is... Since it is a constant, the performance evaluation index EEI of shell-and-tube heat exchangers can be simplified in the manner described above.

[0060] The above are the embodiments listed in this example. However, this example is not limited to the optional embodiments described above. Those skilled in the art can arbitrarily combine the above methods to obtain other various embodiments. Anyone can derive other various forms of embodiments under the guidance of this example. The above specific embodiments should not be construed as limiting the scope of protection of this example. The scope of protection of this example should be defined in the claims.

Claims

1. A method for evaluating the performance of a heat exchanger based on flow, characterized by, Comprising: Obtaining the dimensionless RTD characteristic variance of the hot fluid side of a heat exchanger under standard conditions and the dimensionless RTD characteristic variance of the cold fluid side , wherein, , , , ; is the variance of the fluid residence time of the hot fluid side, is the variance of the fluid residence time of the cold fluid side, is the average residence time of the fluid of the hot fluid side, is the average residence time of the fluid of the cold fluid side, is the characteristic flow length of the hot fluid side, is the characteristic flow length of the cold fluid side, is the average flow velocity of the fluid of the hot fluid side, is the average flow velocity of the fluid of the cold fluid side, is the distribution density function of the fluid residence time of the hot fluid side obtained by the RTD experiment, is the distribution density function of the fluid residence time of the cold fluid side obtained by the RTD experiment, t is the residence time of the fluid in the heat exchanger; Obtaining flow pressure drops on the hot fluid side of a heat exchanger and the cold fluid side : wherein, is the characteristic resistance coefficient of the hot fluid side, is the characteristic resistance coefficient of the cold fluid side; determining an energy efficiency index EEI of the heat exchanger: wherein is the density of the hot fluid side fluid, is the density of the cold fluid side fluid; Determining a probability distribution function of performance of a heat exchanger .

2. The method for assessing heat exchanger performance based on flow according to claim 1, characterized in that: setting two energy efficiency index EEI values, corresponding to obtain a high-middle energy efficiency boundary point and a low-middle energy efficiency boundary point in an energy efficiency probability distribution function, an energy efficiency value corresponding to the high-middle energy efficiency boundary point to an energy efficiency value of 1 interval is a high energy efficiency interval, an energy efficiency value corresponding to the high-middle energy efficiency boundary point to an energy efficiency value corresponding to the low-middle energy efficiency boundary point interval is a medium energy efficiency interval, and an energy efficiency value corresponding to the low-middle energy efficiency boundary point to an energy efficiency value of 0 interval is a low energy efficiency interval.

3. The method for rating heat exchanger performance based on flow according to claim 1, wherein: When a shell-and-tube heat exchanger is used, its energy efficiency index EEI is equal to the product of the tube side and shell side energy efficiency ratio, and the energy efficiency of the shell side fluid of the shell-and-tube heat exchanger is equivalent to its energy efficiency index, that is: 。

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