Quantitative judgment method for heat exchange effect of industrial waste heat recovery heat exchanger

Abstract

The invention relates to a quantitative judgment method for the heat exchange effect of an industrial waste heat recovery heat exchanger, and the method comprises the following steps: step 1, collecting operation parameters, step 2, determining a reference period, step 3, adjusting and testing the working condition of the reference period, step 4, calculating the heat exchange temperature difference of the reference period, and step 5, screening parameters of a comparison period and calculating the heat exchange temperature difference. Step 6, calculating a dimensionless heat exchange temperature difference ratio; according to the technical scheme, the problems that the heat exchanger is unclear in state and uneconomical in operation in application practice are solved, and guidance is provided for economical operation of the heat exchanger and efficient recovery of complementary energy.

Description

Technical Field

[0001] The invention relates to a quantitative evaluation method for heat exchange effects of a cold and hot fluid heat exchanger applicable to industrial waste heat recovery occasions, and belongs to the technical field of waste heat recovery heat exchange. Background Art

[0002] Waste heat recovery is a commonly used energy-saving technology measure in industry. By recycling the heat carried by by-products or waste gas and waste water, energy consumption and environmental pollution are reduced. In industrial waste heat recovery, the most common method is to use a heat exchanger to heat the hot and cold fluids, using the hot fluid (usually waste gas and waste water, etc.) to heat the cold fluid (usually cold water and cold air, etc.) for production, such as various preheaters in the metallurgical industry.

[0003] Since the cold and hot fluids in industrial waste heat recovery applications contain a large amount of impurities such as dust, dust and impurities are easily accumulated on the surface of the heat exchanger during operation to form a dust scale layer that hinders heat transfer, resulting in degradation of the heat exchange effect of the heat exchanger or even failure. Timely mastering the heat exchange state of the heat exchanger is of great significance for achieving economic operation of the heat exchanger, reducing energy consumption, reducing pollution emissions, and improving energy efficiency. Industrial waste heat recovery applications have unfavorable conditions such as large fluctuations in heat source load, extremely unstable parameters, and extremely high impurity content. The above problems are superimposed on the dust accumulation on the surface of the heat exchanger, making the waste heat recovery heat exchanger different from other heat exchangers, and it is difficult to quantitatively evaluate the impact of its body state on the heat exchange effect. Therefore, there is an urgent need for a new solution to solve the above technical problems. Summary of the invention

[0004] The present invention aims at the problems existing in the prior art and provides a method for quantitatively judging the heat exchange effect of an industrial waste heat recovery heat exchanger. This technical solution eliminates the influence of fluctuations in cold and hot fluid parameters and quantitatively evaluates only the degree of influence of changes in the state of the heat exchange equipment itself on the heat exchange effect.

[0005] In order to achieve the above object, the technical solution of the present invention is as follows: a quantitative evaluation method for heat exchange effect of an industrial waste heat recovery heat exchanger, performing the following steps:

[0006] Step 1: Collect and record the operating parameters of the four physical quantities of hot fluid flow, hot fluid temperature, cold fluid flow, and cold fluid temperature during the normal operation of the heat exchanger for a long period of time, verify that the data conforms to the normal distribution, and calculate the mean C and standard deviation δ of the four parameters during normal operation; select ψ between 1 / 3 and 1 / 4, and determine two operating intervals of [C-ψδ, C] and (C, C+ψδ] based on the operating data of the four parameters, which cover a load interval of 20%-26% near the mean;

[0007] Step 2: Set the reference period for evaluating the heat exchanger effect. This reference time is usually preferably when the heat exchanger surface is in a clean state. For a heat exchanger that is in continuous operation and whose surface does not have a clean state, any moment when the equipment is operating stably and normally can be selected.

[0008] Step 3: Conduct adjustment tests on the heat exchanger under different operating conditions during the reference period. For the case where 4 parameters are independent, according to the factor random test, adjust the 4 parameters respectively, so that it operates in the two intervals of [C - ψδ, C] and (C, C + ψδ] in sequence, and record the inlet and outlet temperatures of the cold (hot) fluid under the corresponding operating conditions. A total of 2 4 = 16 tests are required; for the case where 3 parameters are independent, then adjust the 3 independent parameters, and the other one is kept in follow-up. A total of 2 3 = 8 tests are required.

[0009] Step 4: Take the heat transfer temperature difference of the cold (hot) fluid as the characterization index. According to the results of the adjustment tests in Step 3, calculate the heat transfer temperature difference of the cold (hot) fluid under each operating condition respectively, determine it as the reference temperature difference Δt0 under this operating condition, and establish a reference temperature difference data table. For the case where 4 parameters are independent, the data table contains 16 values; for 3 parameters independent, the data table contains 8 values.

[0010] Step 5: When the heat exchanger has been operating for a period of time or when it is necessary to understand the change in the heat transfer state, record the operating data of 4 physical quantities, namely the hot fluid flow rate, hot fluid temperature, cold fluid flow rate, and cold fluid temperature of the heat exchanger during this period; take either of the two intervals of [C - ψδ, C] and (C, C + ψδ] as the target interval, screen the records in the operating data that meet the target interval, calculate the (cold) hot fluid temperature difference, and calculate the average value when there are multiple conforming ones to obtain the actual heat transfer temperature difference Δt i . If there is no conformity, record again until the requirements are met.

[0011] Step 6: Define a dimensionless index - heat transfer temperature difference ratio K that characterizes the change in the heat exchanger heat transfer effect, K = Δt i / Δt0; According to the comparison period, screen the operating intervals of the independent parameters corresponding to Δt i , look up the matching Δt0 in the reference temperature difference data table, calculate the heat transfer temperature difference ratio K, and obtain the value of the change in the heat transfer effect during the comparison period relative to the reference period. Generally, 0 < K ≤ 1. The smaller the K value, the worse the heat transfer effect and the more serious the deterioration; in special cases, K > 1 indicates that the heat transfer effect has improved compared to the reference period.

[0012] The beneficial effects of the present invention are as follows: 1) the technical solution selects ψ between 1 / 3 and 1 / 4 according to the normal theory to determine the 20%-26% operating interval near the heat exchanger operating center as a representative comparison interval; 2) the solution establishes a comparative benchmark period for the heat exchange effect of the heat exchanger; 3) adjustment tests under different working conditions are carried out according to parameter independence; 4) the heat exchange temperature difference of the cold (hot) fluid is used as a characterization index of the heat exchange capacity, and the benchmark period Δt0 and the comparison period Δt0 are determined respectively. i ; 5) Define the dimensionless index - heat exchange temperature difference ratio K as a measure of the degree of change in heat exchange effect. K<1 means that the heat exchange has deteriorated, and the smaller K is, the more serious the deterioration is. This achieves an intuitive and quantitative description of the change in the heat exchange effect of the heat exchanger, solves the problem of unclear heat exchanger status and uneconomical operation in application practice, and provides guidance for the economic operation of the heat exchanger and efficient recovery of surplus energy. DETAILED DESCRIPTION

[0013] In order to deepen the understanding of the present invention, a detailed description is given below in conjunction with embodiments.

[0014] Embodiment 1:

[0015] The present embodiment is a method for quantitatively evaluating the heat exchange effect of an industrial waste heat recovery heat exchanger, which involves a gas preheater that heats coal gas with industrial waste gas, wherein the hot fluid is flue gas and the cold fluid is coal gas. Flue gas is produced after the combustion of coal gas, and there are no independent variables between coal gas and flue gas, so there are three independent variables: hot flue gas temperature, cold coal gas temperature, and hot flue gas flow / cold coal gas flow 2-choose-1. The cold coal gas flow is used as the adjustment scalar, and the cold coal gas temperature rise is used as the characterization index.

[0016] Here are the steps:

[0017] Step 1: Collect and record the operating parameters of the four physical quantities, including hot flue gas temperature, cold gas temperature, hot flue gas flow rate, and cold gas flow rate, which are counted hourly within 3-5 days during normal production of the gas preheater. After verifying that the data conforms to the normal distribution, calculate the mean C and standard deviation δ of these four parameters during normal operation, which are 280℃, 30℃, and 320,000 m 3 / h, 200,000 m 3 / h, the standard deviations δ are 20℃, 6℃ and 40,000 m 3 / h, 20,000m 3 / h; Select ψ = 1 / 3, the four parameter operating ranges are: hot flue gas temperature [273.3, 280], (280, 286.7]; gas temperature [28, 30], (30, 32]; flue gas volume [30.7, 32], (32, 33.3]; gas volume [19.3, 20], (20, 20.7]. The operating range covers 26% of the load range.

[0018] Step 2: The preheater is a newly modified and put into operation equipment. One month after the equipment is put into operation is selected as the evaluation base period.

[0019] Step 3. During the baseline period, adjust the operating parameters for testing. Since the heat exchanger has only three independent variables, select cold gas temperature, cold gas flow rate and hot flue gas temperature as adjustment variables, and the hot flue gas flow rate as a driven variable and does not participate in the adjustment. Perform a factorial randomized experimental design according to 3 factors and 2 levels, as shown in the following table. In the table, 1 and 2 in the hot flue gas temperature, cold gas temperature, and cold gas flow rate represent the low and high parameter value intervals of the parameters, respectively. The test was carried out according to the operating sequence for a total of 8 times, and the cold gas temperature (inlet temperature) and gas outlet temperature were recorded.

[0020] Standard sequence Run sequence Hot flue gas temperature Cold gas temperature Cold gas flow 8 1 2 2 2 1 2 1 1 1 4 3 1 2 2 2 4 1 1 2 3 5 1 2 1 6 6 2 1 2 5 7 2 1 1 7 8 2 2 1

[0021] Step 4, using the heat exchange temperature difference of cold coal gas as the characterization index, cold coal gas heat exchange temperature difference = coal gas outlet temperature - cold coal gas temperature, calculate the coal gas heat exchange reference temperature difference Δt0 under 8 working conditions according to the test results of step 3, and obtain the heat exchange reference temperature difference data table.

[0022] Step 5: After 3 months of production operation, the heat exchange status needs to be evaluated. At this time, the operation data of the four indicators of the heat exchanger, namely, the hot flue gas temperature, the cold gas flow, the cold gas temperature, and the gas outlet temperature, are collected and recorded again. The operation data are screened at the same time with the hot flue gas temperature [273.3, 280], the cold gas temperature [28, 30], and the cold gas flow [19.3, 20] to obtain the operation data under this condition. The gas temperature difference of the selected items is calculated according to the gas outlet temperature-cold gas temperature, and the average gas temperature difference Δt is calculated. i The actual heat exchange temperature difference of the gas at this time is obtained in this case: i =148°C.

[0023] Step 6: Define the dimensionless index that characterizes the change in heat exchange effect of the heat exchanger - the heat exchange temperature difference ratio K. Under the above working conditions, Δt i =148℃, according to the hot flue gas temperature [273.3, 280], cold gas temperature [28, 30], cold gas flow [19.3, 20], the corresponding Δt0 = 152℃ is obtained in the heat exchange benchmark temperature difference data table, and K = Δt i / Δt0=148 / 152=0.973.

[0024] After adopting the method of this embodiment, the quantitative evaluation result K=0.973 of the current heat exchanger is obtained, indicating that, excluding the influence of the heat source parameter change, the heat exchange effect is reduced by 0.027, or 2.7%, relative to the reference state due to the influence of the heat exchanger itself. In this way, the heat exchange state of the heat exchanger is quantified, which plays an important role in guiding the heat exchanger to increase the amount of waste energy recovery and achieve economic operation.

[0025] Embodiment 2:

[0026] The present embodiment provides a quantitative evaluation method for the heat exchange effect of an industrial waste heat recovery heat exchanger, which involves an air preheater in which industrial waste gas heats air, wherein the hot fluid is flue gas and the cold fluid is air. The hot flue gas temperature, the cold air temperature, the hot flue gas flow rate, and the cold air flow rate are all independent variables. The hot flue gas temperature drop is used as a characterization index.

[0027] The steps are as follows, which are different from Example 1:

[0028] Step 1: Collect and calculate the mean values ​​of the operating parameters of the four physical quantities of hot flue gas temperature, cold air temperature, hot flue gas flow rate, and cold air flow rate during normal production of the air preheater. The mean values ​​C are 300℃, 35℃, 200,000 m3 / h, and 120,000 m3 / h, respectively, and the standard deviations δ are 20℃, 6℃, and 20,000 m 3 / h, 20,000m 3 / h. Select ψ = 1 / 4, and the four parameter operating ranges are: hot flue gas temperature [295, 300], (300, 305]; air temperature [33.5, 35], (35, 36.5]; flue gas volume [19.5, 20], (20, 20.5]; air flow [11.5, 12], (12, 12.5], and the range covers 20% of the load range.

[0029] Step 2: The preheater has been in operation for 2 years, and the 25th month of operation is selected as the evaluation base period.

[0030] Step 3: During the reference period, a factorial randomized experimental design with 4 factors and 2 levels was conducted for 4 independent variables, with a total of 16 experiments, as shown in the following table. The experiments were carried out in the order of operation, and the hot flue gas temperature (inlet temperature) and flue gas outlet temperature were recorded in turn.

[0031]

[0032]

[0033] Step 4: Take the flue gas heat exchange temperature difference as the characterization index, flue gas heat exchange temperature difference = hot flue gas temperature - flue gas outlet temperature, and calculate the flue gas heat exchange reference temperature difference Δt0 under 16 working conditions according to the test results of step 3, and obtain the heat exchange reference temperature difference data table. The table contains the reference flue gas temperature difference data of 16 working conditions.

[0034] Step 5: In the third year of production operation, the heat exchange status assessment needs to be carried out. At this time, the operation data of the four indicators of the heat exchanger, namely, the hot flue gas temperature, the cold air flow rate, the cold air temperature, and the flue gas outlet temperature, are collected and recorded again. The operation data are screened by hot flue gas temperature (300, 305], air temperature [33.5, 35], flue gas volume [19.5, 20], and air flow rate (12, 12.5], and the average flue gas temperature difference Δt is calculated. i . This case obtains the actual heat exchange temperature difference of the flue gas at this time Δt i =140℃.

[0035] Step 6: In the heat exchange reference temperature difference data table, according to the above working conditions, the corresponding flue gas heat exchange temperature difference Δt0 = 145°C is obtained, and the heat exchange temperature difference ratio K = Δt i / Δt0=140 / 145=0.966.

[0036] The method of this embodiment is used to obtain a quantitative evaluation result of the air heat exchanger operation after one year of operation relative to the benchmark period, K=0.966, indicating that the heat exchange effect of the heat exchanger itself is reduced by 3.4% compared with the benchmark.

[0037] It should be noted that the above embodiments are not intended to limit the protection scope of the present invention, and equivalent changes or substitutions made on the basis of the above technical solutions all fall within the protection scope of the claims of the present invention.

Claims

1. A quantitative determination method for the heat exchange effect of an industrial waste heat recovery heat exchanger, characterized in that: The method comprises the following steps: Step 1: Run parameter collection, Step 2: Determination of the base period, Step 3: Baseline period operating condition adjustment test, Step 4: Calculation of heat exchange temperature difference during the base period, Step 5: Compare the period parameter screening and heat exchange temperature difference calculation, Step 6: Calculation of dimensionless heat transfer temperature difference ratio.

2. The method for quantitatively judging the heat exchange effect of an industrial waste heat recovery heat exchanger according to claim 1, characterized in that: Step 1: Operation parameter collection, as follows: collect the operation parameters of four physical quantities, namely, hot fluid flow, hot fluid temperature, cold fluid flow, and cold fluid temperature, during a long period of normal operation, calculate the parameter mean C and standard deviation δ according to the normal distribution, select ψ between 1 / 3 and 1 / 4, and determine the appropriate parameter interval as [C-ψδ, C], (C, C+ψδ], covering the load interval of 20%-26% near the mean C.

3. The method for quantitatively judging the heat exchange effect of an industrial waste heat recovery heat exchanger according to claim 2, characterized in that: Step 2: Determine the reference period. First, select when the heat exchanger surface is kept clean. Second, select any period when the heat exchange equipment operates stably and normally.

4. The method for quantitatively judging the heat exchange effect of an industrial waste heat recovery heat exchanger according to claim 3 is characterized in that: Step 3: Baseline period operating condition adjustment test: In the base period, the values ​​of each independent parameter are adjusted according to the factorial random experiment design to run different operating condition tests in the two intervals of [C-ψδ, C] and (C, C+ψδ); according to the number of independent parameters n, 2 levels are used to perform 2 n Test.

5. The method for quantitatively judging the heat exchange effect of an industrial waste heat recovery heat exchanger according to claim 3 or 4, characterized in that: Step 4: Calculate the heat exchange temperature difference during the reference period, as follows: Based on the results of the adjustment test in step 3, select the heat exchange temperature difference of the cold or hot fluid as the characterization index, calculate the heat exchange temperature difference of the cold fluid or hot fluid under each working condition during the reference period, that is, the reference temperature difference Δt0, and establish a reference temperature difference data table; the data table contains 2 n data.

6. The method for quantitatively judging the heat exchange effect of an industrial waste heat recovery heat exchanger according to claim 5, characterized in that: Step 5: Parameter screening and heat exchange temperature difference calculation during the comparison period, as follows: When understanding the heat exchange state changes during the comparison period, record the operating data of four physical quantities, namely, hot fluid flow, hot fluid temperature, cold fluid flow, and cold fluid temperature, when the heat exchanger is running stably during this period; For the independent variable data, [C-ψδ, C] or (C, C+ψδ] is used as the target interval until the records that meet the requirements are screened out. The temperature difference of the cold fluid or hot fluid in the screened records is calculated. When multiple records meet the requirements, the average value is calculated to obtain the actual heat exchange temperature difference Δt during the comparison period. i .

7. The method for quantitatively judging the heat exchange effect of an industrial waste heat recovery heat exchanger according to claim 6, characterized in that: Step 6: Calculation of dimensionless heat transfer temperature difference ratio, specifically as follows: Define a dimensionless index - heat transfer temperature difference ratio K, K = Δt i / Δt0, to characterize the change degree of the heat exchanger heat transfer effect. Generally, 0 < K ≤ 1 indicates that the heat transfer effect deteriorates; in special cases, K > 1 indicates that the heat transfer effect improves; the smaller K is, the worse the heat transfer effect is, and the more serious the deterioration is.

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