A calculation method for determining the fouling thermal resistance of a direct air-cooled condenser tube bundle

Abstract

The patent application discloses a kind of calculation method for determining the fouling thermal resistance of direct air-cooled condenser tube bundle, comprising the following steps: S1, determine whether air-cooled condenser ventilation is blocked;S2, collect the design parameters of each structure of air-cooled condenser;S3, record various parameters;S4, condenser overall heat transfer coefficient, air side convective heat transfer coefficient and tube wall heat transfer coefficient;S5, obtain the fouling thermal resistance of tube bundle outer surface;S6, determine the fouling thermal resistance of tube bundle outer surface under the clean state of air-cooled condenser;S7, determine the fouling thermal resistance of tube bundle outer surface under the normal operating state of air-cooled condenser;S8, determine the surface cleaning state of air-cooled condenser heat exchange tube bundle;The beneficial effects of the present application are that, by this method, the fouling thermal resistance of air-cooled generator set direct air-cooled condenser heat exchange tube bundle can be quantitatively analyzed and evaluated, which can provide quantitative data of air-cooled condenser heat exchange tube bundle cleaning state for power plant operation and maintenance, and provide basis for air-cooled condenser condition-based maintenance and power plant energy saving.

Description

Technical Field

[0001] This invention patent relates to a calculation method for determining the fouling thermal resistance of tube bundles in a direct air condenser. Background Technology

[0002] As the most important heat exchanger equipment in air-cooled generator units of power plants, direct air-cooled condensers play a crucial role in dissipating the heat from the unit's exhaust steam to the environment. With the development of power units towards larger capacity and higher parameters, the performance of air-cooled condensers in power plants has an increasingly significant impact on the economic efficiency of the plant. Taking a 600MW air-cooled turbine unit as an example, every 1 kPa increase in condenser pressure directly increases the unit's coal consumption for power generation by approximately 1.2 g / kW·h, resulting in an additional consumption of approximately 3240 tons of standard coal annually. The fouling thermal resistance of the heat transfer tubes on the outer surface of the air-cooled condenser is a significant factor affecting condenser heat exchange. However, because the performance of the air-cooled condenser is affected by multiple factors during normal operation, such as unit load, air cooling flow rate, ambient air temperature, and the cleanliness of the external heat transfer tubes, simply monitoring changes in the air-cooled condenser pressure cannot objectively assess changes in the fouling thermal resistance of the condenser's heat transfer tubes. Therefore, there is an urgent need for a test and calculation method to determine the fouling thermal resistance of the tube bundles in direct air-cooled condensers to address the current challenges in the field.

[0003] Among existing patents, the closest is application number CN202111499181.3, entitled "Online Monitoring System and Control Method for Fouling Thermal Resistance of Water Source Heat Pump Heat Exchanger." This patent discloses a system including a data acquisition unit, an analog module, a PLC control unit, and an output unit. The analog module sends data signals acquired by the data acquisition unit to the PLC control unit. The PLC control unit calculates the real-time fouling thermal resistance of the heat exchanger based on the data signals and controls the output unit to output corresponding information. The real-time monitoring system and control method for water source heat pump heat exchangers provided by this invention acquires necessary data through a data acquisition unit to obtain the real-time fouling thermal resistance of the heat exchanger. By continuously monitoring the real-time fouling thermal resistance, it uses this resistance as an indicator for heat exchanger cleaning and compares it with a preset critical value to obtain an accurate heat exchanger cleaning time. However, the formula for calculating fouling thermal resistance in this patent does not consider the influence of the refrigerant viscosity coefficient on thermal resistance, making it incomplete and resulting in inaccurate data.

[0004] Another application, CN201210109422.3, entitled "Measuring Device and Method for Fouling Thermal Resistance and Thermal Conductivity," discloses a device consisting of a constant-temperature heating device, a container, a temperature measuring device, and a timer. The constant-temperature heating device maintains a constant temperature of T. A clean container containing a mass m of liquid to be heated is placed in the constant-temperature heating device for heating. By measuring the temperature of the liquid to be heated and the time n from the initial temperature t1 to the final temperature t2, the thermal resistance can be calculated using the thermal resistance calculation formula derived in this invention. The experiment is then repeated with a container containing fouling to obtain the thermal resistance with fouling. The difference between the thermal resistance with fouling and the thermal resistance without fouling is the fouling thermal resistance, and the ratio of the fouling thickness to the fouling thermal resistance is the thermal conductivity of the fouling. The conclusion in this patent that the fouling thermal resistance is determined by the difference between the thermal resistance with fouling and the thermal resistance without fouling is unscientific. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a calculation method for determining the fouling thermal resistance of the tube bundle of a direct air-cooled condenser that is easy to implement on-site, which facilitates the monitoring of the fouling condition of the tube bundle of the air-cooled condenser, avoids excessive loss of thermal efficiency, and is simple and practical.

[0006] The technical solution of the present invention is as follows:

[0007] A calculation method for determining the fouling thermal resistance of a direct-air condenser tube bundle includes the following steps:

[0008] S1. Determine if the air-cooled condenser ventilation is blocked; ensure that the condenser ventilation is normal when determining the tube bundle fouling thermal resistance.

[0009] S2. Collect all structural design parameters of the air-cooled condenser;

[0010] S3. Record various parameters of the air-cooled condenser under two test conditions: 100% heat load and 90% heat load.

[0011] S4. Calculate the overall heat transfer coefficient of the condenser, the air-side convective heat transfer coefficient, and the tube wall heat transfer coefficient under the two test conditions in step S3.

[0012] S5. Calculate the thermal resistance of the fouling on the outer surface of the tube bundle based on the data obtained from steps S1 to S4.

[0013] S6. Determine the thermal resistance of the outer surface fouling of the air-cooled condenser tube bundle under clean conditions.

[0014] S7. Determine the thermal resistance of the external surface fouling of the air-cooled condenser tube bundle under normal operating conditions.

[0015] S8. Determine the surface cleanliness of the heat exchange tube bundle of the air condenser.

[0016] Furthermore, step S1 includes:

[0017] S11. Direct air condenser tube bundle ventilation test under clean baseline conditions; all fans maintain a frequency of 50Hz, and record the total power consumption P of the fans. t0 Environmental pressure p a0 Ambient temperature t a0 ;

[0018] S12. Ventilation test of the direct air condenser tube bundle under normal operating conditions; fan control frequency at f t1 Hz; record the total power consumption P of the fan. t1 Environmental pressure p a1 Ambient temperature t a1 ;

[0019] S13. Determine if the ventilation volume of the air-cooled condenser system is normal; based on the following formula...

[0020]

[0021] Where P t1 ′ represents the expected total power consumption of the wind turbine, f t0 The fan frequency is 50Hz under clean baseline conditions; f t1 The fan control frequency under normal operating conditions, in Hz; ρ t0 ρ t1 These are the air densities under clean baseline and normal operating conditions, respectively, which can be calculated based on the environmental pressure and temperature parameters under each condition.

[0022] Based on the expected value P of the total power consumption of the wind turbine in the above formula t1 If P t1 <P t1 The error message indicates a blockage in the air-cooled condenser's ventilation. The system should be inspected for blockages in the tube bundles or fan inlets and outlets. If P... t1 =P t1 The '' indicates that the air condenser is in normal ventilation condition.

[0023] Furthermore, the structural design parameters in step S2 include heat exchange area (area of ​​bare tube and fin area), elliptical tube size, rectangular fin size, fin thickness, fin spacing, number of heat exchange units, size of each heat exchange unit, and windward surface area.

[0024] Furthermore, step S3 includes conducting two test conditions of the air-cooled condenser under normal condenser ventilation conditions. The two test conditions are conducted at 100% heat load and 90% heat load of the condenser, respectively, and the following data are recorded under the two test conditions: air-cooled condenser fan frequency, air-cooled condenser fan power consumption, condenser inlet pressure, ambient pressure, ambient temperature, and condenser inlet steam flow rate.

[0025] Furthermore, step S4 includes the following steps:

[0026] S41. Calculate the condenser heat load under operating conditions; use the following formula:

[0027] Q T =F exh ×[h1(pex / 1000,x)-h2(pex / 1000)]

[0028] In the formula: Q T The condenser operating heat load is expressed in W.

[0029] h1 is the exhaust enthalpy calculated according to the 1997 version of the steam characteristic formula (the steam enthalpy is calculated from the steam pressure and steam dryness fraction) of the International Steam and Water Federation, in kJ / kg; x is the design dryness fraction of the low-pressure cylinder exhaust.

[0030] h2 is the saturated water enthalpy calculated according to the 1997 version of the steam characteristic formula (saturated water enthalpy calculated from steam pressure) of the International Steam Water Federation, in kJ / kg;

[0031] pex is the condenser inlet pressure, in kPa;

[0032] F exh The inlet steam flow rate of the air-cooled condenser is expressed in kg / s.

[0033] S42. Calculate the overall heat transfer coefficient of the condenser under operating conditions.

[0034] First, calculate the logarithmic mean temperature difference (LMTD):

[0035]

[0036] In the formula, t 1T The inlet air temperature of the air-cooled condenser is the ambient temperature, expressed in °C; t 2T The outlet air temperature of the air-cooled condenser is expressed in °C (t). sT This represents the saturation temperature corresponding to the condenser inlet pressure, in °C.

[0037] Obtain the outlet air temperature t of the air condenser 2TFirst, calculate the enthalpy h of the air outlet of the air condenser using the following formula. air Then find the outlet air temperature t corresponding to the enthalpy value. 2T ,

[0038] h air (p a ,t ao ) = h air (p a ,t ai )+Q T / F air

[0039] In the formula, t ai The inlet air temperature of the air-cooled condenser is the ambient temperature, expressed in °C.

[0040] t ao The outlet air temperature of the air-cooled condenser is expressed in °C.

[0041] p a Environmental pressure, in kPa;

[0042] h air This is a function for calculating the enthalpy of air based on ambient pressure and temperature. The unit of enthalpy is kJ / kg.

[0043] F air Cooling air flow rate for air-cooled condensers, in kg / s;

[0044] Cooling airflow is calculated using the following formula:

[0045]

[0046] In the formula, ρ ao ρ aG These are the air densities for the operating condition and the design-assured operating condition, respectively, calculated based on the ambient pressure and temperature for the corresponding conditions, in kg / m³. 3 ;P o P g The power consumption of the wind turbine under operating conditions and design-guaranteed operating conditions are respectively expressed in kW; q ao q aG The total ventilation volume of the fan under operating conditions and design guarantee conditions, in m³. 3 / s;

[0047] The overall heat transfer coefficient U of the condenser t Calculate using the following formula:

[0048]

[0049] In the formula, U t The overall heat transfer coefficient of the condenser is W / (m²).2 ·℃); A is the total heat exchange area of ​​the air-cooled condenser, in m². 2 LMTD stands for Logarithmic Temperature Difference, in °C.

[0050] S43. Calculate the heat transfer resistance R of the pipe wall. pw The following formula is used:

[0051]

[0052] In the formula, D po With D pi These are the outer and inner diameters of the heat exchanger tubes, in meters (m); R pw The heat transfer resistance of the tube wall is expressed in m²·K / W; k p is the thermal conductivity of the heat exchange tube, in W / m·K; L represents the length of the heat exchange tube, in m; N is the number of heat exchange tubes.

[0053] S44. Calculate the air-side heat transfer coefficient using the following formula:

[0054]

[0055] Among them, t fin s fin h fin These are fin spacing, fin thickness, and fin height, respectively, in mm;

[0056] k a is the thermal conductivity of air, measured in W / m·K;

[0057] μ a Aerodynamic viscosity, in Pa·s;

[0058] C pa Specific heat of air at constant pressure, expressed in kJ / kg·℃;

[0059] G maxa Mass flow rate per unit area, unit: kg / m² 2 ,

[0060] By G maxa =ρ a ×v maxa calculate,

[0061] Where v maxa The face velocity of the tube bundle is expressed in m / s, v. maxa =q ao / A ym ;

[0062] Where A ym The air-cooled condenser's frontal surface area, in meters (m²). 2 ;

[0063] C tr This is a coefficient related to the number of tube rows in the heat exchanger, with the following values: 0.78 for single row; 0.88 for double row; 0.93 for triple row; and 0.97 for quadruple row.

[0064] When calculating the above physical properties of the air medium, the characteristic temperature is taken as the inlet temperature t of the air condenser. ai and outlet temperature t a0 The average air temperature.

[0065] Furthermore, step S5 includes,

[0066] S51, finned heat exchanger tube fin efficiency φ fn Calculation

[0067]

[0068] In the formula, BesselI(X,n) is the modified Bessel function of the first kind, X is used to calculate the value of the function, and n is the order of the Bessel function; Besselk(X,n) is the modified Bessel function of the second kind, X is used to calculate the value of the function, and n is the order of the Bessel function; x e x b w, y b Here are the structural parameters of the finned heat exchanger tube, where k is the thermal conductivity of the fin material; u b The heat dissipation coefficient of the heat exchanger tube; u e The overall heat dissipation coefficient of the finned heat exchanger tube;

[0069] S52. The formula for calculating the thermal resistance of fouling on the outer surface of the tube bundle is as follows:

[0070]

[0071] In the formula, h i The condensation heat transfer coefficient inside the heat exchange tube is taken as 10000 W / (m²). 2 ·℃);

[0072] R fi The thermal resistance of the fouling inside the pipe is taken as zero.

[0073] φ fn For the finned heat exchanger tube fin efficiency;

[0074] A Pi The heat exchange area inside the heat exchange tube, in meters (m²). 2 ;

[0075] A fn The heat exchange area of ​​the finned tube is expressed in meters (m²). 2 ;

[0076] C is the correction factor for the heat transfer coefficient on the air side;

[0077] R fo Thermal resistance of fouling on the outer surface of the tube bundle, in meters. 2 ·℃) / W;

[0078] R pw The heat transfer resistance of the tube wall is determined in step S43; the unit is m. 2 K / W;

[0079] After step 5 is completed, if the fouling thermal resistance of the tube bundle outer surface is the same under both operating conditions, then the assumed correction factor C for the air-side heat transfer coefficient is correct, and the calculated fouling thermal resistance of the tube bundle outer surface under both operating conditions is the obtained fouling thermal resistance of the tube bundle outer surface; otherwise, the correction factor C for the air-side heat transfer coefficient is re-assumed, and the fouling thermal resistance of the tube bundle outer surface is recalculated.

[0080] Furthermore, step S6 includes,

[0081] Two cleaning operating conditions are selected for the condenser tube bundle under the clean baseline state. According to steps S3-S5, the thermal resistance R of the external surface fouling of the condenser tube bundle under the clean state can be obtained. foj .

[0082] Furthermore, step S7 includes,

[0083] Selecting two normal operating conditions of the condenser tube bundle under normal operating conditions, the external surface fouling thermal resistance R of the condenser under normal operating conditions can be obtained according to steps S3-S5. foz ;

[0084] Furthermore, step S8 includes the thermal resistance R of the external surface dirt under normal operating conditions. foz Thermal resistance R of external surface dirt under clean baseline conditions foj Compare and determine the surface cleanliness of the heat exchanger tube bundle of the air condenser;

[0085] If (R) foz -R foj ) / R foj If the surface cleanliness of the heat exchange tube bundle of the air condenser is greater than 20%, it is in poor condition and requires high-pressure water flushing and mechanical cleaning.

[0086] Furthermore, the cleanliness baseline state refers to the state of the air-cooled condenser after high-pressure water flushing and mechanical cleaning of the finned heat exchange surfaces following a major overhaul of the unit.

[0087] The beneficial effects of adopting the above technical solution are as follows:

[0088] This invention, in principle, employs external measurement and calculation methods. It primarily measures parameters such as the inlet steam flow rate, condenser inlet pressure, fan power consumption, ambient pressure, and ambient temperature of the air-cooled condenser during normal operation, and then uses appropriate methods to calculate the fouling thermal resistance of the air-cooled condenser heat exchanger tube bundle. This invention solves the problem of accurately and quantitatively assessing the fouling thermal resistance of the heat exchanger tube bundle in power plant air-cooled condensers during normal operation. It provides quantitative data on the cleanliness status of the air-cooled condenser heat exchanger tube bundle for power plant operation and maintenance, providing a basis for condition-based maintenance of air-cooled condensers and energy conservation and consumption reduction in power plants. This invention solves the technical challenge of periodically and accurately assessing the cleanliness status of the heat exchanger tube bundle in large-scale unit air-cooled condensers, enabling monitoring and judgment of condenser cleanliness and abnormal conditions, thus meeting the needs of power plant economic analysis.

[0089] By adopting the evaluation method of the present invention, it is possible to periodically or cyclically evaluate the changes in the thermal resistance of the fouling on the outer surface of the heat transfer tubes of the direct air condenser through on-site testing, and provide guidance for taking appropriate operation and maintenance strategies. Attached Figure Description

[0090] Appendix Figure 1 This is a flowchart of the steps of the present invention;

[0091] Appendix Figure 2 This is a flowchart of step S1;

[0092] Appendix Figure 3 Flowchart for step S4

[0093] Appendix Figure 4 This is a schematic diagram of the structure of a finned heat exchanger tube. Detailed Implementation

[0094] The following detailed embodiments further illustrate the present invention.

[0095] A calculation method for determining the fouling thermal resistance of a direct-air condenser tube bundle includes the following steps:

[0096] S1. Determine if the air-cooled condenser ventilation is blocked, and ensure that the condenser ventilation is normal when determining the thermal resistance of the tube bundle fouling.

[0097] A more detailed technical solution is that step S1 involves the following steps to determine whether the air-cooled condenser ventilation is blocked, ensuring normal condenser ventilation when determining the tube bundle fouling thermal resistance:

[0098] S11. Ventilation test of the direct air-cooled condenser tube bundle under clean baseline conditions. All fans maintain a frequency of 50Hz, and the total power consumption P of the fans is recorded. t0 Environmental pressure p a0 Ambient temperature t a0 ;

[0099] The cleanliness baseline test for air-cooled condensers refers to a test conducted after the finned heat exchange surfaces of the air-cooled condenser have undergone thorough high-pressure water flushing and mechanical cleaning. This is typically performed after a major unit overhaul.

[0100] S12. Ventilation test of the direct air condenser tube bundle under normal operating conditions; fan control frequency at f t1 Hz; record the total power consumption P of the fan. t1 Environmental pressure p a1 Ambient temperature t a1 ;

[0101] S13. Determine whether the ventilation volume of the air-cooled condenser system is normal.

[0102]

[0103] Where P t1 ′ represents the expected total power consumption of the wind turbine, f t0 The fan frequency is 50Hz under clean baseline conditions; f t1 The fan control frequency under normal operating conditions, in Hz; ρ t0 ρ t1 These are the air densities under clean baseline and normal operating conditions, respectively, which can be calculated based on the environmental pressure and temperature parameters under each condition.

[0104] Based on the above formula, and based on the expected value P of the total power consumption of the wind turbine... t1 If P t1 <P t1 The error message "' indicates a blockage in the air-cooled condenser ventilation. The system should be inspected for blockages in the tube bundles or fan inlets and outlets to resolve the issue. If P..." t1 =P t1 The '' indicates that the air condenser is in normal ventilation condition.

[0105] S2. Under normal condenser ventilation conditions, two operating conditions of the air-cooled condenser were tested on-site. The two tests were conducted at 100% and 90% condenser heat loads respectively (hereinafter referred to as the two operating conditions). The following data were recorded for each test condition: air-cooled condenser fan frequency, air-cooled condenser fan power consumption, condenser inlet pressure, ambient pressure, ambient temperature, and condenser inlet steam flow rate.

[0106] S3. Collect all structural design parameters of the air-cooled condenser. This includes heat exchange area (area of ​​bare tubes and fins), elliptical tube dimensions, rectangular fin dimensions, fin thickness, fin spacing, number of heat exchange units, dimensions of each heat exchange unit, and windward surface area.

[0107] S4. Calculate the overall heat transfer coefficient of the condenser, the air-side convective heat transfer coefficient, and the tube wall heat transfer coefficient under the two operating conditions respectively.

[0108] A more detailed technical solution is as follows: Step S4, calculating the overall heat transfer coefficient of the condenser, the air-side convective heat transfer coefficient, and the tube wall heat transfer coefficient under the two operating conditions, uses the following method:

[0109] S41. Calculate the condenser heat load under operating conditions. Use the following formula:

[0110] Q T =F exh ×[h1(pex / 1000,x)-h2(pex / 1000)]

[0111] In the formula: Q T The condenser operating heat load is expressed in W.

[0112] h1 is the exhaust steam enthalpy calculated according to the 1997 version of the steam characteristic formula (the steam enthalpy is calculated from the steam pressure and steam dryness) of the International Steam Water Federation, in kJ / kg;

[0113] x represents the design dryness of the low-pressure cylinder exhaust;

[0114] h2 is the saturated water enthalpy calculated according to the 1997 version of the steam characteristic formula (saturated water enthalpy calculated from steam pressure) of the International Steam Water Federation, in kJ / kg;

[0115] pex is the condenser inlet pressure, in kPa;

[0116] F exh The inlet steam flow rate of the air-cooled condenser is expressed in kg / s.

[0117] S42. Calculate the overall heat transfer coefficient of the condenser under operating conditions. Use the following formula:

[0118] The logarithmic mean temperature difference (LMTD) is calculated using the following formula:

[0119]

[0120] In the formula, t 1T The inlet air temperature of the air-cooled condenser is the ambient temperature, expressed in °C; t 2T The outlet air temperature of the air-cooled condenser is expressed in °C (t). sT is the saturation temperature corresponding to the condenser inlet pressure, in °C.

[0121] Obtain the outlet air temperature t of the air condenser 2T First, calculate the enthalpy of the air at the outlet of the air-cooled condenser using the following formula, then find the corresponding air temperature at the outlet of the air-cooled condenser:

[0122] h air (p a ,t ao ) = h air (p a ,t ai )+Q T / F air

[0123] In the formula, t ai The inlet air temperature of the air-cooled condenser is the ambient temperature, in °C.

[0124] t ao p represents the outlet air temperature of the air-cooled condenser, in °C. a Environmental pressure, in kPa;

[0125] h air This is a function for calculating the enthalpy of air based on ambient pressure and temperature. The unit of enthalpy is kJ / kg.

[0126] F air The airflow rate for cooling the air-cooled condenser is expressed in kg / s; the airflow rate is calculated using the following formula:

[0127]

[0128] In the formula, ρ ao ρ aG These are the air densities for the operating condition and the design-assured operating condition, respectively; calculated based on the ambient pressure and temperature for the corresponding operating conditions, in kg / m³. 3 ;P o P g The power consumption of the wind turbine under operating conditions and design-guaranteed operating conditions are respectively expressed in kW; q ao q aG The total ventilation volume of the fan under operating conditions and design guarantee conditions, in m³. 3 / s;

[0129] The overall heat transfer coefficient of the condenser is calculated using the following formula:

[0130]

[0131] In the formula, U t The overall heat transfer coefficient of the condenser is W / (m²). 2 ·℃); A is the total heat exchange area of ​​the air-cooled condenser, m 2 LMTD stands for Logarithmic Temperature Difference, in °C.

[0132] S43. Calculate the thermal resistance of the pipe wall. Use the following formula:

[0133]

[0134] In the formula, D po With D pi These are the outer and inner diameters of the heat exchanger tubes, in meters (m); R pw The heat transfer resistance of the tube wall, in meters (m). 2 ·K / W; k p is the thermal conductivity of the heat exchange tube, in W / m·K; L represents the length of the heat exchange tube, in m; N is the number of heat exchange tubes.

[0135] S44. Calculate the air-side heat transfer coefficient. Use the following formula:

[0136]

[0137] Among them, t fin s fin h fin These are the fin spacing, fin thickness, and fin height, respectively, in mm; k a ρ is the thermal conductivity of air, measured in W / m·K; μ a C is the aerodynamic viscosity, in Pa·s. pa Specific heat of air at constant pressure, expressed in kJ / kg·℃;

[0138] G maxa Mass flow rate per unit area, kg / m² 2 , by G maxa =ρ a ×v maxa calculate;

[0139] v maxa The oncoming wind speed of the tube bundle is given by v. maxa =q ao / A ym Calculation; A ym The area of ​​the air-cooled condenser's frontal surface, in m² 2 C tr This is a coefficient, related to the number of tube rows in the heat exchanger, and its values ​​are shown in the table below:

[0140] Table 2-1 C tr Coefficient values

[0141] Number of pipes <![CDATA[Coefficient C tr value]]> 1 0.78 2 0.88 3 0.93 4 0.97

[0142] When calculating the above physical properties of the air medium, the characteristic temperature is taken as the inlet temperature t of the air condenser. ai and outlet temperature t a0 The average air temperature.

[0143] S5. Calculate the thermal resistance of the fouling on the outer surface of the tube bundle based on the data obtained from steps S1 to S4.

[0144] For two operating conditions, assuming a correction factor C for the air-side heat transfer coefficient, calculate the fouling thermal resistance of the tube bundle outer surface for each condition. If the fouling thermal resistances of the two tube bundles are the same, then the assumed correction factor C for the air-side heat transfer coefficient is correct, and the calculated fouling thermal resistance of the tube bundle outer surface for both operating conditions is the obtained fouling thermal resistance. Otherwise, re-assume the correction factor C for the air-side heat transfer coefficient and calculate the fouling thermal resistance of the tube bundle outer surface.

[0145] S6. Select two operating conditions for the condenser tube bundle under clean baseline conditions. The two tests are carried out under 100% heat load and 90% heat load of the condenser, respectively. According to the process of steps S3-S5 above, the external surface fouling thermal resistance of the tube bundle under clean conditions can be obtained.

[0146] S7. Select two operating conditions for the condenser tube bundle under normal operating conditions. The two tests are carried out under 100% heat load and 90% heat load of the condenser, respectively. According to the process of steps S3-S5 above, the external surface fouling thermal resistance of the condenser under normal operating conditions can be obtained.

[0147] S8. By comparing the thermal resistance of the external surface dirt under normal operating conditions with that under clean baseline conditions, the surface cleanliness of the heat exchange tube bundle of the air condenser can be determined.

[0148] The more detailed technical solution for step 5, assuming a correction factor C for the air-side heat transfer coefficient, calculates the fouling thermal resistance on the outer surface of the tube bundle for the two operating conditions, using the following steps:

[0149] S51, finned heat exchanger tube fin efficiency φ fn Calculation

[0150] A schematic diagram of the finned heat exchanger tube structure is shown below. Figure 4 As shown:

[0151]

[0152] In the formula, BesselI(C,n) is the modified Bessel function of the first kind, X is used to calculate the value of the function, and n is the order of the Bessel function; Besselk(X,n) is the modified Bessel function of the second kind, X is used to calculate the value of the function, and n is the order of the Bessel function; x e x b w, y b The structural parameters of the finned heat exchanger tube are shown in the attached figure; k is the thermal conductivity of the heat exchanger tube fin material; u b The heat dissipation coefficient of the heat exchanger tube; u e The overall heat dissipation coefficient of the finned heat exchanger tube;

[0153] S52. The formula for calculating the thermal resistance of fouling on the outer surface of the tube bundle is as follows:

[0154]

[0155] In the formula, h i The condensation heat transfer coefficient inside the heat exchange tube is taken as 10000 W / (m²). 2 ·℃); R fi The thermal resistance of the fouling inside the pipe is taken as zero; φ fn For finned heat exchanger tube fin efficiency; A Pi The heat exchange area inside the heat exchange tube, in meters (m²). 2 A fn The heat exchange area of ​​the finned tube is expressed in meters (m²). 2 C is the correction factor for the air-side heat transfer coefficient. R fo For the thermal resistance of external fouling, the unit is (m). 2 ·℃) / W;R pw The heat transfer resistance of the tube wall, in meters (m). 2 K / W;

[0156] After step 5 is completed, if the fouling thermal resistance of the tube bundle outer surface is the same under both operating conditions, then the assumed air-side heat transfer coefficient correction factor C is correct, and the calculated fouling thermal resistance of the tube bundle outer surface under both operating conditions is the obtained fouling thermal resistance of the tube bundle outer surface; otherwise, the air-side heat transfer coefficient correction factor C is re-assumed, and the fouling thermal resistance of the tube bundle outer surface is recalculated.

[0157] After calculating the fouling thermal resistance on the outer surface of the tube bundle using the above formula, the fouling condition of the tube bundle can be evaluated, making it easier to grasp the fouling condition of the heat exchange tube bundle of the air-cooled condenser in a timely manner and providing data support for the safe operation of the power plant.

[0158] The principle of this invention patent adopts an external measurement and calculation method. It mainly measures parameters such as the steam flow rate at the inlet of the air condenser, the inlet pressure of the condenser, the power consumption of the fan, the ambient pressure and the ambient temperature during normal operation, and uses an appropriate method to calculate the fouling thermal resistance of the heat exchange tube bundle of the air condenser.

[0159] This invention solves the problem of accurately and quantitatively assessing the fouling thermal resistance of the heat exchanger tube bundles in power plant air-cooled condensers during normal operation. It can provide quantitative data on the cleanliness status of the heat exchanger tube bundles in air-cooled condensers for power plant operation and maintenance, and provide a basis for condition-based maintenance of air-cooled condensers and energy conservation and consumption reduction in power plants.

[0160] This invention solves the technical problem of regularly and accurately assessing the cleanliness of the heat exchange tube bundles of air-cooled condensers in large power units. It can monitor and determine the cleanliness and abnormal conditions of the condenser, thus meeting the needs of power plant economic analysis.

[0161] By adopting the evaluation method of the present invention, it is possible to periodically or cyclically evaluate the changes in the thermal resistance of the fouling on the outer surface of the heat transfer tubes of the direct air condenser through on-site testing, and provide guidance for taking appropriate operation and maintenance strategies.

[0162] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from its spirit or basic characteristics. The above description is merely a preferred embodiment of the present invention; however, the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the technical scope disclosed in the present invention, based on the technical solutions and improved concepts of the present invention, should be covered within the scope of protection of the present invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects. The scope of the present invention is defined by the appended claims rather than the foregoing description, and therefore all variations falling within the meaning and scope of the equivalent elements of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A calculation method for determining the fouling thermal resistance of a direct-air condenser tube bundle, characterized in that: It includes the following steps: S1. Determine if the air-cooled condenser ventilation is blocked; ensure that the condenser ventilation is normal when determining the tube bundle fouling thermal resistance. S2. Collect all structural design parameters of the air-cooled condenser; S3. Record various parameters of the air-cooled condenser under two test conditions: 100% heat load and 90% heat load. S4. Calculate the overall heat transfer coefficient of the condenser, the air-side convective heat transfer coefficient, and the tube wall heat transfer coefficient under the two test conditions in step S3. S5. Calculate the thermal resistance of the fouling on the outer surface of the tube bundle based on the data obtained from steps S1 to S4. S6. Determine the thermal resistance of the outer surface fouling of the air-cooled condenser tube bundle under clean conditions. S7. Determine the thermal resistance of the external surface fouling of the air-cooled condenser tube bundle under normal operating conditions. S8. Determine the surface cleanliness of the heat exchanger tube bundle of the air condenser. Step S5 include, S51, finned heat exchanger tube fin efficiency φ fn Calculation; In the formula, BesselI(X,n) is the modified Bessel function of the first kind, X is used to calculate the value of the function, and n is the order of the Bessel function; Besselk(X,n) is the modified Bessel function of the second kind, X is used to calculate the value of the function, and n is the order of the Bessel function; x e x b w, y b Here are the structural parameters of the finned heat exchanger tube, where k is the thermal conductivity of the fin material; u b The heat dissipation coefficient of the heat exchanger tube; u e The overall heat dissipation coefficient of the finned heat exchanger tube; S52. The formula for calculating the thermal resistance of fouling on the outer surface of the tube bundle is as follows: In the formula, h i The condensation heat transfer coefficient inside the heat exchange tube is taken as 10000 W / (m²). 2 ·℃); R fi The thermal resistance of the fouling inside the pipe is taken as zero. φ fn For the finned heat exchanger tube fin efficiency; A Pi The heat exchange area inside the heat exchange tube, in meters (m²). 2 ; A fn The heat exchange area of ​​the finned tube is expressed in meters (m²). 2 ; C is the correction factor for the heat transfer coefficient on the air side; R fo Thermal resistance of fouling on the outer surface of the tube bundle, in meters. 2 ·℃) / W; R pw The heat transfer resistance of the tube wall is determined in step S43; the unit is m. 2 K / W; After step S5 is completed, if the fouling thermal resistance of the tube bundle outer surface is the same under both operating conditions, then the assumed correction factor C for the air-side heat transfer coefficient is correct, and the calculated fouling thermal resistance of the tube bundle outer surface under both operating conditions is the obtained fouling thermal resistance of the tube bundle outer surface; otherwise, the correction factor C for the air-side heat transfer coefficient is re-assumed, and the fouling thermal resistance of the tube bundle outer surface is recalculated.

2. The calculation method for determining the fouling thermal resistance of a direct-air condenser tube bundle according to claim 1, characterized in that: Step S1 includes: S11. Direct air condenser tube bundle ventilation test under clean baseline conditions; all fans maintain a frequency of 50Hz, and record the total power consumption P of the fans. t0 Environmental pressure p a0 Ambient temperature t a0 ; S12. Ventilation test of the direct air condenser tube bundle under normal operating conditions; fan control frequency at f t1 At Hz; record the total power consumption P of the fan. t1 Environmental pressure p a1 Ambient temperature t a1 ; S13. Determine if the ventilation volume of the air-cooled condenser system is normal; based on the following formula... Where P t1 ′ represents the expected total power consumption of the wind turbine, f t0 The fan frequency is 50Hz under clean baseline conditions; f t1 The fan control frequency under normal operating conditions, in Hz; ρ t0 ρ t1 These are the air densities under clean baseline and normal operating conditions, respectively, which can be calculated based on the environmental pressure and temperature parameters under each condition. Based on the expected value P of the total power consumption of the wind turbine in the above formula t1 If P t1 <P t1 The error message indicates a blockage in the air-cooled condenser's ventilation. The system should be inspected for blockages in the tube bundles or fan inlets and outlets. If P... t1 =P t1 The '' indicates that the air condenser is in normal ventilation condition.

3. The calculation method for determining the fouling thermal resistance of a direct-air condenser tube bundle according to claim 2, characterized in that: The structural design parameters in step S2 include heat exchange area, elliptical tube size, rectangular fin size, fin thickness, fin spacing, number of heat exchange units, size of each heat exchange unit, and windward surface area; the heat exchange area includes the area of ​​the bare tube and the fin area.

4. The calculation method for determining the fouling thermal resistance of a direct-air condenser tube bundle according to claim 3, characterized in that: Step S3 includes running two test conditions of the air-cooled condenser under normal condenser ventilation conditions. The two test conditions are conducted at 100% heat load and 90% heat load of the condenser, respectively. The following data are recorded under the two test conditions: air-cooled condenser fan frequency, air-cooled condenser fan power consumption, condenser inlet pressure, ambient pressure, ambient temperature, and condenser inlet steam flow rate.

5. The calculation method for determining the fouling thermal resistance of a direct-air condenser tube bundle according to claim 4, characterized in that: Step S4 includes the following steps: S41. Calculate the condenser heat load under operating conditions; use the following formula: Q T =F exh ×[h1(pex / 1000,x)-h2(pex / 1000)] In the formula: Q T The condenser operating heat load is expressed in W. h1 is the exhaust enthalpy calculated according to the 1997 version of the steam characteristic formula of the International Steam and Water Federation, in kJ / kg; x is the design dryness fraction of the low-pressure cylinder exhaust. h2 is the saturated water enthalpy calculated according to the 1997 version of the steam characteristic formula of the International Steam Water Federation, with the unit being kJ / kg; pex is the condenser inlet pressure, in kPa; F exh The inlet steam flow rate of the air-cooled condenser is expressed in kg / s. S42. Calculate the overall heat transfer coefficient of the condenser under operating conditions. First, calculate the logarithmic mean temperature difference (LMTD): In the formula, t 1T The inlet air temperature of the air-cooled condenser is the ambient temperature, expressed in °C; t 2T The outlet air temperature of the air-cooled condenser is expressed in °C (t). sT This represents the saturation temperature corresponding to the condenser inlet pressure, in °C. Obtain the outlet air temperature t of the air condenser 2T First, calculate the enthalpy h of the air outlet of the air condenser using the following formula. air Then find the outlet air temperature corresponding to the enthalpy value. h air (p a ,t ao )=h air (p a ,t ai )+Q T / F air In the formula, t ai The inlet air temperature of the air-cooled condenser, i.e., the ambient temperature, is expressed in °C; t ao p represents the outlet air temperature of the air-cooled condenser, in °C. a Environmental pressure, unit: kPa; h air This is a function for calculating the enthalpy of air based on ambient pressure and temperature. The unit of enthalpy is kJ / kg. F air The cooling air flow rate for the air-cooled condenser is expressed in kg / s; the cooling air flow rate is calculated using the following formula: In the formula, ρ ao ρ aG These are the air densities for the operating condition and the design-assured operating condition, respectively, calculated based on the ambient pressure and temperature for the corresponding conditions, in kg / m³. 3 ;P o P g The power consumption of the wind turbine under operating conditions and design-guaranteed operating conditions are respectively expressed in kW; q ao q aG The total ventilation volume of the fan under operating conditions and design-assured operating conditions is expressed in m³. 3 / s; The overall heat transfer coefficient U of the condenser t Calculate using the following formula: In the formula, U t The overall heat transfer coefficient of the condenser is W / (m²). 2 ·℃); A is the total heat exchange area of ​​the air-cooled condenser, in m². 2 LMTD stands for Logarithmic Temperature Difference, in °C. S43. Calculate the heat transfer resistance R of the pipe wall. pw The following formula is used: In the formula, D po With D pi These are the outer and inner diameters of the heat exchanger tubes, in meters (m); R pw The heat transfer resistance of the tube wall, in meters (m). 2 ·K / W; k p is the thermal conductivity of the heat exchange tube, in W / m·K; L represents the length of the heat exchange tube, in m; N is the number of heat exchange tubes. S44. Calculate the air-side heat transfer coefficient using the following formula: Among them, t fin s fin h fin These are fin spacing, fin thickness, and fin height, respectively, in mm; k a is the thermal conductivity of air, expressed in W / m·K; μ a Aerodynamic viscosity, in Pa·s; C pa Specific heat of air at constant pressure, expressed in kJ / kg·℃; G maxa Mass flow rate per unit area, unit: kg / m² 2 , By G maxa =ρ a ×v maxa Calculate, where v maxa The face velocity of the tube bundle is expressed in m / s, v. maxa =q ao / A ym ;where A ym The air-cooled condenser's frontal surface area, in meters (m²). 2 ; C tr This is a coefficient related to the number of tube rows in the heat exchanger, with the following values: 0.78 for single row; 0.88 for double row; 0.93 for triple row; and 0.97 for quadruple row. When calculating the above physical properties of the air medium, the characteristic temperature is taken as the inlet temperature t of the air condenser. ai and outlet temperature t a0 The average air temperature.

6. The calculation method for determining the fouling thermal resistance of a direct-air condenser tube bundle according to claim 5, characterized in that: Step S6 includes, Two cleaning operating conditions are selected for the condenser tube bundle under the clean baseline state. According to steps S3-S5, the thermal resistance R of the external surface fouling of the condenser tube bundle under the clean state can be obtained. foj .

7. The calculation method for determining the fouling thermal resistance of a direct-air condenser tube bundle according to claim 6, characterized in that: Step S7 includes... Selecting two normal operating conditions of the condenser tube bundle under normal operating conditions, the external surface fouling thermal resistance R of the condenser under normal operating conditions can be obtained according to steps S3-S5. foz .

8. The calculation method for determining the fouling thermal resistance of a direct-air condenser tube bundle according to claim 7, characterized in that: Step S8 includes the thermal resistance R of external surface dirt under normal operating conditions. foz Thermal resistance R of external surface dirt under clean baseline conditions foj Compare and determine the surface cleanliness of the heat exchanger tube bundle of the air condenser; If (R) foz -R foj ) / R foj If the surface cleanliness of the heat exchange tube bundle of the air condenser is >20%, it is in poor condition and requires high-pressure water flushing and mechanical cleaning.

9. The calculation method for determining the fouling thermal resistance of a direct-air condenser tube bundle according to claim 8, characterized in that: The cleanliness baseline state refers to the state of the air-cooled condenser after high-pressure water flushing and mechanical cleaning of the finned heat exchange surfaces following a major overhaul of the unit.

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