CN113883916A - Air cooling island minimum anti-freezing flow calculation method considering multiple influence factors - Google Patents
Air cooling island minimum anti-freezing flow calculation method considering multiple influence factors Download PDFInfo
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- F28—HEAT EXCHANGE IN GENERAL
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Abstract
The invention provides a method for calculating the lowest anti-freezing flow of an air cooling island by considering various influence factors, which considers the influence of exhaust steam parameter change, uneven steam flow distribution and dirt thermal resistance, combines the analysis of the heat exchange performance of an air cooling condenser with variable working condition calculation, obtains a guide value of the lowest anti-freezing flow according to data collected by a unit operation measuring point and through a series of calculation and correction, and considers the convenience of use while ensuring the accuracy.
Description
Technical Field
The invention relates to a method for calculating the lowest anti-freezing flow of an air cooling island by considering various influence factors.
Background
Although China has used some foreign design and operation experiences and some special operation experiences accumulated by the China to the present, the freezing problem of the pipe bundle of the air-cooled radiator in winter can be solved to a greater extent, in the alpine region of China, the lowest temperature in winter can reach about minus 50 ℃, and some shortages exist in the anti-freezing in the alpine region, so that great hidden dangers are brought to the safe, economical and stable operation of the direct air-cooled unit.
Steam in the single-row pipe air-cooled condenser exchanges heat with air outside the pipe through the heat exchange pipes and the fins for condensation, if the air flow outside the pipe is too large or the steam flow in the pipe is too small, most of the steam can be condensed into water in the pipe bundle in advance along the pipe pass, the steam can be continuously cooled when flowing downwards along the pipe wall, the condensate water is gradually changed from saturated water into supercooled water, and the supercooling degree is increased more and more. When the temperature drops below 0 ℃, the fluid is slowly frozen. After the freezing phenomenon occurs, the steam flow channel becomes narrower and narrower, the phenomena of steam flow velocity reduction, flow suspension and the like can occur, the freezing area is further increased, and even the tube bundle of the whole cooling unit is frozen to be damaged, so that the machine set is shut down. Therefore, in order to ensure the safe and stable operation of the direct air cooling unit, the research on the anti-freezing technology of the air cooling island is significant.
For a typical direct air-cooling unit, the air-cooling condenser is composed of a large number of single-row serpentine finned tubes. There are two cases of freezing of the fluid inside the pipe:
(1) when the external environment temperature is lower than 0 ℃, if the steam flow in the condenser tube bundle does not reach the minimum anti-freezing flow specified in the design within the limited time (generally 2 hours), the steam is condensed in the front section of the radiator tube bundle, the condensed water is supercooled in the rear section of the tube bundle, the condensed water is gradually changed from saturated water to supercooled water, and the supercooling degree is increased more and more until the condensed water is frozen;
(2) when the external environment temperature is lower than 0 ℃, the steam turbine exhaust pipeline and the steam distribution pipeline are not tightly sealed or the steam flow is too low, so that a large amount of air exists in the finned tubes. The volume of non-condensable gas in the finned tube at the top end of the countercurrent unit is large, the non-condensable gas cannot be discharged in time, and the tube bundle of the air-cooled condenser at the top end of the countercurrent unit is gradually blocked. The flow of the fluid in the finned tubes of the radiator is blocked, so that the flow speed is reduced, the front half section of the fluid in the single-row tubes is condensed, and the supercooling and even icing phenomena can occur in the rear half section.
From the above, the main reason for freezing of the air cooling island is the low steam flow. Therefore, the key to ensuring enough steam in the air cooling island is the normal starting and running of the direct air cooling unit in winter. A minimum anti-freezing flow is required to be set to prevent the steam discharge of the unit from being smaller than the minimum anti-freezing flow.
At present, the minimum anti-freezing flow of the unit is a guide value directly given by a design manufacturer, and is generally a flow value under the given steam discharge parameter and the ambient temperature.
The existing method for determining the lowest anti-freezing flow of the air cooling island mainly has the following problems:
(1) the reference meaning of the lowest anti-freezing flow value directly provided by the manufacturer is limited, and the use is inconvenient
The minimum anti-freezing flow guide value given by a design manufacturer is generally a flow value under the given steam discharge parameter and the ambient temperature, only represents a plurality of typical working conditions, and the value is also discrete. In the actual operation of the unit, the working conditions of the unit often deviate from the typical working conditions, and the minimum anti-freezing flow guide value cannot be directly used. The lowest antifreeze flow is obtained by an interpolation method, which may bring extra errors, so that the obtained value deviates from the true value. In particular, if the evaluation value is lower than the true value, the operator carries out this, which increases the risk of freezing the tube bundle. Therefore, the reference significance of the lowest anti-freezing flow value directly provided by a manufacturer is limited, and the use is inconvenient.
(2) Irrespective of influence of exhaust parameters
The minimum anti-freezing flow guide value given by a design manufacturer is generally a flow value under a given exhaust steam parameter, and the influence caused by the change of the exhaust steam parameter, namely the change of the exhaust steam pressure, the temperature and the humidity is not considered. In the actual operation of the unit, the steam discharge parameters often deviate from given values, and the heat carried by the steam with unit mass is higher/lower than a designed value. Moreover, it can be seen by simple calculation that the degree of deviation is large and cannot be ignored.
(3) Not considering the influence caused by uneven steam flow distribution
The design of the air cooling island of the direct air cooling unit is considered according to the steam load distribution of each air cooling unit. However, in actual operation, the pressure distribution of the steam distribution pipeline is very likely to be uneven, which causes uneven distribution of the steam flow entering the condenser, and the steam flow deviation can reach about 5% during the low-load operation of the unit. Especially, the steam flow of the unit is low in the starting and stopping process, the heat load change is slow, and in addition, the steam flow distribution is uneven, so that the steam flow of part of air-cooled condenser rows is low, steam can be quickly condensed into water in the forward/reverse flow condenser, the condensed water can be quickly condensed into ice in winter under the low-temperature environment, the finned tube bundle of the air-cooled unit is easily frozen, the vacuum of the unit is damaged, and the normal operation of the unit is influenced. Therefore, if the problem of unreasonable steam distribution is not considered, even if the total steam amount reaches the standard, the phenomenon of too little steam may occur in some air-cooled condenser rows. At present, the minimum anti-freezing flow guide value given by a design manufacturer does not consider the factor, and is unfavorable for the anti-freezing work of a unit.
(4) Without considering the influence of variation of thermal resistance due to fouling
The direct air-cooling condenser adopts a finned tube structure, the arrangement of fins is compact, and the spacing between the fins is small. In western regions rich in coal and water, the environmental conditions are relatively severe, wind and sand are large, and dust is much. Dust is therefore very likely to collect on the finned tubes and, in severe cases, can also block the cooling air passages. The increase of the dirt thermal resistance can cause the heat transfer coefficient of the condenser to be reduced, the heat transfer performance to be deteriorated, the exhaust pressure of a unit to be increased, and the temperature of condensed water to be increased. In hot summer, the output of the unit can be influenced by the large dirt thermal resistance. However, in cold winter season, the thermal fouling resistance can have a positive effect on the freeze protection of the air condenser. This factor should also be considered.
In summary, the method for determining the minimum anti-freezing flow rate of the air cooling island by the guide value provided by the design manufacturer ignores the influence caused by the change of various influence factors, reduces the reference significance, is inconvenient to use, and is difficult to meet the requirement of anti-freezing work of the air cooling island in winter.
Disclosure of Invention
The invention provides a method for calculating the lowest anti-freezing flow of an air cooling island by considering various influence factors, which considers the influence of exhaust steam parameter change, uneven steam flow distribution and dirt thermal resistance, combines the analysis of the heat exchange performance of an air cooling condenser with variable working condition calculation, obtains a guide value of the lowest anti-freezing flow according to data collected by a unit operation measuring point and through a series of calculation and correction, and considers the convenience of use while ensuring the accuracy.
A method for calculating the lowest anti-freezing flow of an air cooling island by considering various influence factors comprises the following steps:
(1) calculating fouling thermal resistance;
(2) calculating the head-on wind speed of the air-cooled condenser in a natural ventilation state;
(3) calculating the minimum anti-freezing flow under the target working condition;
(4) and (4) repeating the step (3), and calculating the lowest antifreezing flow under different environmental temperatures and steam exhaust parameters.
Further, the step (1) specifically comprises the following steps:
(1-1) searching historical data recorded by a unit to obtain operation data of the air-cooled condenser row in a certain time period after the air-cooled condenser row is put into operation in winter;
(1-2) calculating the number of heat transfer units according to the operation data;
(1-3) calculating the test heat exchange coefficient of the cooling island in the selected time period;
(1-4) calculation of fouling resistance Using equation (5)
In the formula (5), K is the test heat exchange coefficient of the air cooling island in the selected time period; k0Designing a heat exchange coefficient for the air-cooled condenser; w/m2·℃。
Further, the step (2) specifically comprises the following steps:
(2-1) calculating the heat exchange coefficient of the air-cooled condenser in a natural ventilation state;
(2-2) searching parameters related to the calculation of the head-on wind speed by utilizing the performance curve of the air cooling island;
and (2-3) iteratively calculating to obtain the value of the head-on wind speed in the natural ventilation state.
Further, the step (3) specifically includes the following steps:
(3-1) calculating a target working condition heat exchange coefficient considering the influence of fouling thermal resistance;
(3-2) calculating the number of heat transfer units under the target working condition;
(3-3) calculating the heat exchange amount under the target working condition;
(3-4) calculating the lowest antifreezing flow of the target working condition;
and (3-5) correcting the uneven distribution of the steam flow.
When obtaining the historical data, the following requirements need to be met:
1) before winter, the tube bundle of the air-cooled condenser is completely cleaned;
2) the inside and the outside of the air cooling island are not cleaned again until now in a selected time period;
3) in a selected time period, the operation measuring point of the required data works normally;
4) the length of the selected time period is 1-2 hours, each parameter in the time period is stable without large fluctuation, and the data acquisition frequency is 20 seconds/time;
5) and the selected time period has high unit load capacity, and all air cooling condenser rows are put into operation.
Wherein the number of heat transfer units is calculated by equation (1):
in the formula (1), QpThe heat release of the steam of the air cooling island is kW; t is ts1The temperature of the condensate water, DEG C; t is ta1The temperature of cold air at the inlet of an air cooling fan is unit ℃; da1Is the inlet air quantity m of the air cooling fan3/s；Cpa1The specific heat capacity is constant pressure of cold air, kJ/kg DEG C; rho is the density of cold air, kg/m3；
In the formula (1), Da1The calculation formula is as follows:
in the formula (2), DaM is the air quantity of the air cooling fan at rated rotating speed3/s；naThe rated rotating speed of the air cooling fan is r/min; n isa1The rotating speed of the air cooling fan under the test working condition is obtained by data acquisition and has a unit of r/min;
in the formula (1), QpThe calculation formula of (2) is as follows:
Qp＝Dp·(hp-hs1) (3)
in the formula (3), DpThe steam flow entering the air cooling island is t/h; h ispThe enthalpy value of the steam turbine exhaust steam is kJ/kg; h iss1The temperature of the condensate obtained by data acquisition is given as the enthalpy of the condensate, kJ/kg.
Wherein the experimental heat transfer coefficient of the air cooling island in the selected time period is calculated by the formula (4):
in the formula (4), A is the heat exchange area of the air cooling island, m2。
Wherein, the heat exchange coefficient of the air condenser under the natural ventilation state is calculated by the formula (6):
in the formula (6), αitIs the steam side heat transfer coefficient, W/m2The temperature is controlled; lambda is the heat conductivity coefficient of the tube wall, W/m2·℃；αotIs the air side heat transfer coefficient, W/m2·℃；Ai、Am、AoRespectively the heat exchange area of the steam side, the pipe wall and the air side, m2；ηoThe heat exchange efficiency of the fins is improved;
in the formula (6), αotThe calculation formula of (2) is as follows:
in the formula (7), vtAnd voThe windward speed m/s of the air cooling condenser under the natural ventilation working condition and the design working condition respectively; alpha is alphaoFor designing the convective heat transfer coefficient at the air side under the working condition, W/m2·℃。
Wherein, the numerical value of the windward speed in the natural ventilation state is obtained through the iterative calculation of the formula (9):
in the formula (9), Δ is a difference; v. oftThe windward speed is m/s under the natural ventilation state; a. theyIs the frontal area, m2(ii) a Impartation of vtAn extremely low initial value and increased in steps of 0.001 m/s; when the difference value delta is lower than a preset absolute value or a preset relative value, the iteration is ended, and the v at the momenttValue is natural ventilationHead-on wind speed in the state.
Wherein, the target working condition heat exchange coefficient considering the influence of fouling thermal resistance is calculated by the formula (10):
the number of heat transfer units under the target working condition is calculated by the formula (11);
the target working condition heat exchange quantity is calculated by the formula (12):
Qpm＝(ts1m-ta1m)Ay·vt·Cpa1tm·ρtm(1-exp(1-NTUm)) (12)
in the formula (12), ts1mThe temperature of the condensate water is the lowest value defined by design;
the minimum antifreeze flow of the target working condition is calculated by the formula (13):
in the formula (13), hs1mThe enthalpy value of the lowest condensate temperature defined by corresponding design, kJ/kg; h ispmThe steam exhaust enthalpy value is the target working condition.
And correcting the lowest antifreezing flow of the target working condition by using a formula (14) in consideration of uneven steam flow distribution:
in the formula (I), the compound is shown in the specification,
the correction coefficient is a correction coefficient with uneven steam flow distribution, and the value is 1.05; dpmxI.e. the comprehensive examinationAnd (4) taking various influence factors into consideration to obtain the target working condition minimum anti-freezing flow value.
The invention has the beneficial effects that:
the method can quickly and accurately calculate the minimum anti-freezing flow of the direct air-cooling unit in winter after comprehensively considering various influence factors, is convenient for operators to evaluate the current freezing risk of the unit, further makes targeted adjustment, provides basis for the start/stop of the direct air-cooling unit in winter, selects proper start/stop parameters, and finally achieves the purpose of improving the winter operation safety of the direct air-cooling unit.
(1) Good accuracy and high practicability
The calculation method of the invention takes into account a plurality of influencing factors, including: the method has the advantages that the exhaust steam parameter change, the steam flow distribution unevenness, the dirt thermal resistance and the like are calculated based on the real performance of the air cooling island, the design value is more accurate than the design value which is directly used, and the applicability is effectively improved.
(2) The implementation method is simple
The invention does not need to use extra instruments, only needs to use the self-carried operation measuring points of the unit, has clear calculation process, reasonably simplifies and processes some complex factors, can be executed by a computer, and has simple implementation method.
(3) Improve the winter anti-freezing capability of the direct air cooling unit
The method can help operators to evaluate the current freezing risk of the unit, further make targeted adjustment, and effectively avoid freezing caused by insufficient steam flow of the air cooling island.
(4) Providing basis for starting/stopping of direct air cooling unit in winter
The low steam flow makes the direct air cooling unit difficult to start/stop in winter, and the invention can provide a basis for the start/stop in winter, is beneficial to operators to select proper start/stop parameters and reduces the risk of freezing during the start/stop of the unit.
In conclusion, the invention overcomes the defects of the prior method, has obvious improvement on the aspects of accuracy, practicability and implementation convenience, and can provide help for the normal running and starting/stopping of the direct air cooling unit in winter.
Drawings
FIG. 1 provides a performance graph for a designer.
Fig. 2 is a schematic diagram of an air cooling island system according to an embodiment.
In fig. 2: the device comprises an air cooling island, a steam inlet pressure measuring point 1, a steam inlet temperature measuring point 2, a steam isolating valve 3, condensed water temperature measuring points 4, a condensed water tank 5 and an air cooling fan 6.
FIG. 3 is a flow diagram of an implementation of the invention.
Detailed Description
The present invention is further described with reference to the following specific embodiments, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of changes or substitutions within the technical scope of the present invention, and the present invention shall be covered thereby.
The technical scheme provided by the invention is detailed in the following by combining specific embodiments.
Two 660MW ultra-supercritical direct air cooling units are built in the China Huajin boundary in the third period, the turbine generator units are arranged in a full high position, and the schematic layout of the air cooling island is shown in figure 2.
The exhaust steam exhausted by the steam turbine is led out of a steam engine room column A through a main steam exhaust pipeline through a flow guide tee joint, and is divided into eight steam exhaust branch pipes to the top of the air cooling condenser after passing through a section of horizontal straight pipe. Steam enters from a header at the upper part of the air-cooled condenser, and is condensed after surface heat exchange with air; condensed water is collected by a condensed water pipe and is discharged to a condensed water tank or is connected to a temporary system for discharging; the units adopt an 8 x 8 arrangement scheme (total 64 fans), namely each unit consists of 8 rows of air-cooled condensers, and each row of air-cooled condensers is provided with 8 air-cooled condenser units; the air-cooled condenser consists of a concurrent flow tube bundle and a countercurrent flow tube bundle.
The main design data are shown in table 1.
TABLE 1
The minimum winter antifreeze flow provided by the design manufacturer is shown in table 2.
TABLE 2
Note: the steam discharge parameters corresponding to the data in the table are steam discharge pressure 15kPa (a) and steam discharge enthalpy 2382.2 kJ/kg.
According to the method, the fouling resistance of the air cooling condenser is calculated firstly.
Step 1 calculates fouling resistance.
Step 1.1, searching historical data recorded by the unit, and selecting the operation data of the unit in the latest time period after winter.
The following requirements need to be satisfied:
1) before winter, the tube bundle of the air-cooled condenser is completely cleaned;
2) the inside and the outside of the air cooling island are not cleaned again until now in a selected time period;
3) in a selected time period, the operation measuring point of the required data works normally;
4) the length of the selected time period is 1-2 hours, each parameter in the time period is stable without large fluctuation, and the data acquisition frequency is 20 seconds/time;
5) and the selected time period has high unit load capacity, and all air cooling condenser rows are put into operation.
The length of the selected time period is 1.5 hours, and the data acquisition frequency is 20 seconds/time; the above requirements are met by other aspects of the assembly.
The main data obtained are shown in Table 3.
TABLE 3
And 1.2, calculating the number of heat transfer units according to the acquired historical data of the unit.
The following formula is used:
in the formula (1), QpThe heat release of the steam of the air cooling island is kW; t is ts1The temperature of the condensate water, DEG C; t is ta1The temperature of cold air at the inlet of the air cooling fan is DEG C; da1Is the inlet air quantity m of the air cooling fan3/s；Cpa1The specific heat capacity is constant pressure of cold air, kJ/kg DEG C; rho is the density of cold air, kg/m3；
In the formula (1), Da1The calculation formula is as follows:
in the formula (2), DaM is the air quantity of the air cooling fan at rated rotating speed3/s；naThe rated rotating speed of the air cooling fan is r/min; n isa1The rotating speed of the air cooling fan under the test working condition is obtained by data acquisition, r/min;
in the formula (1), QpThe calculation formula of (2) is as follows:
Qp＝Dp·(hp-hs1) (3)
in the formula (3), DpThe steam flow entering the air cooling island is t/h; h ispThe enthalpy value of the steam turbine exhaust steam is kJ/kg; h iss1The enthalpy of the condensed water is obtained by the temperature of the condensed water obtained by data acquisition, kJ/kg;
in the formula (1), the exhaust enthalpy h of the steam turbinetThe value of the power is calculated by an energy balance method, and the actual power of the final group is obtained by deducting the mechanical loss and the motor loss according to the actual electric power of the unit in the selected time period and the power generated by the previous groups according to the steam flow and the actual enthalpy dropSo as to calculate the actual enthalpy drop and then calculate the exhaust enthalpy; the specific procedures are not described in detail within the present invention.
The calculated result is: NTU 1.812.
Step 1.3, the experimental heat exchange coefficient of the air cooling island in the selected time period is calculated.
Using the formula (4),
in the formula (4), A is the heat exchange area of the air cooling island, m2；
The calculation result is as follows: K34.551W/(° c. m)2)；
Step 1.4 calculate fouling resistance
The heat exchange coefficient of the air-cooled condenser is generally smaller than the designed value due to the influence of dirt thermal resistance; the fouling thermal resistance is obtained by comparing the test value and the design value of the heat exchange coefficient, and the formula is as follows:
in the formula (5), K0Designing heat exchange coefficient, W/m for air cooling condenser2·℃；
Using equation (5), the result is: rf＝0.00204(℃·m2)/W。
In winter, all air-cooled condenser rows with steam isolation valves are closed, only the starting row (without the steam isolation valves) is reserved, and all fans stop running, which is the best anti-freezing running state that the unit can adopt under normal conditions, and the lowest anti-freezing flow can ensure that the temperature of condensed water in each row of the air-cooled island is not lower than a certain specified value at the moment.
Although the fans of all the air cooling units are closed, air still passes through the air cooling fin radiator, so that the fin tubes are cooled, namely, forced ventilation heat exchange is replaced by natural ventilation heat exchange; for the sake of simplifying the analysis, the natural ventilation mentioned here only means the air volume formed by the convection of air under the distribution effect of the temperature field of the radiator itself, and the influence of the natural air on the air cooling system under the effect of the atmospheric environment is not considered temporarily.
The flow resistance and the heat exchange characteristic of the air transverse-sweeping finned tube bundle are mainly influenced by the head-on wind speed, and are less influenced by the environment temperature and the wall surface temperature of the finned tube bundle; therefore, the head-on wind speed of the natural ventilation heat exchange can take a fixed value; this value is calculated in steps below.
And 2.1, calculating the heat exchange coefficient of the air-cooled condenser in a natural ventilation state.
According to the basic theory of heat transfer, the total thermal resistance of the air-cooled condenser comprises condensation heat exchange thermal resistance at a steam side, heat conduction thermal resistance at a pipe wall, convection heat exchange thermal resistance at an air side and fouling thermal resistance; therefore, the calculation formula of the heat exchange coefficient of the air-cooling condenser is as follows:
in the formula (6), αitIs the steam side heat transfer coefficient, W/m2The temperature is controlled; lambda is the heat conductivity coefficient of the tube wall, W/m2·℃；αotIs the air side heat transfer coefficient, W/m2·℃；Ai、Am、AoRespectively the heat exchange area of the steam side, the pipe wall and the air side, m2；ηoThe heat exchange efficiency of the fins is improved.
The single-row elliptical finned tube used by the air-cooled condenser mainly influences the total heat exchange thermal resistance on the convective heat exchange thermal resistance at the air side; the heat exchange thermal resistance of the pipe wall and the steam side is small, and the change value under different working conditions is small, so that the heat exchange thermal resistance can be treated as a fixed value.
In the formula (6), αotThe calculation formula of (2) is as follows:
in the formula (7), vtAnd voThe windward speed m/s of the air cooling condenser under the natural ventilation working condition and the design working condition respectively; alpha is alphaoFor designing the convective heat transfer coefficient at the air side under the working condition, W/m2·℃。
And 2.2, a relational expression of the head-on wind speed, the heat exchange quantity, the condensate temperature and other parameters is obtained.
The head-on wind speed in the natural ventilation state is obtained, and a performance curve provided by an air cooling island design party is required to be used, and is shown in figure 1; the relationship between different environmental temperatures and the heat exchange quantity and back pressure after the fan stops running can be seen in the graph; a group of data is selected from the graph, and the following relations exist among the data such as the head-on wind speed, the heat exchange quantity, the condensate temperature and the like according to the equations (1) and (4):
in the formula (8), vtThe windward speed is m/s under the natural ventilation state; a. theyIs the frontal area, m2(ii) a Other parameter definitions are the same as before.
And 2.3, calculating the value of the windward speed by iterative calculation.
From the formulae (6) and (7), KtCan be expressed as v onlytA function of a variable, i.e. Kt＝f(vt) Therefore, there is only one unknown quantity v in the formula (8)t(ii) a Formula (8) cannot be reduced to vtThe method of the present invention is obtained by performing iterative calculation, and the calculation formula is derived from the following equations (6), (7), and (8):
in the formula (9), Δ is a difference; impartation of vtAn extremely low initial value, e.g., 0.01m/s, and increased in steps of 0.001 m/s; when the difference Δ is small enough (below a preset absolute or relative value), the iteration ends, at which point vtThe value is the head-on wind speed in the natural ventilation state.
In FIG. 1Selecting data with the ambient temperature of 10 ℃ and the heat load rate of 50% as a basis, and deriving a calculation formula in the step 2.3 through calculation formulas in the step 2.1 and the step 2.2; using the formula (9), let vtThe initial value is 0.01m/s, the step length is 0.001, the iteration stop condition is set to be | delta | less than or equal to 0.005, and the result is obtained through iterative calculation: head-on wind speed v under natural ventilation statet0.394m/s, heat transfer coefficient Kt＝8.945W/(℃·m2)。
Step 3, calculating the minimum anti-freezing flow under the target working condition
The calculation of the minimum anti-freezing flow rate under the target working condition refers to the calculation of the minimum steam flow rate under the specified boundary condition.
The specified boundary conditions, namely the specified environmental temperature and the specified steam exhaust parameters, simultaneously consider the influence caused by fouling thermal resistance and flow deviation, and stop the fan to meet the condition that the temperature of the condensed water is not lower than a limit value;
the boundary conditions are specified as: ambient temperature ta1mAt-20 deg.C and exhaust pressure Ppm15kPa, the lowest value t of the condensation water temperature is limited in the operating regulationss1mTaking the influence caused by fouling resistance and flow deviation into consideration at 35 ℃, all fans are stopped, steam isolation valves in columns #1, #2, #7 and #8 are closed, and only columns #3, #4, #5 and #6 are put into operation.
And 3.1, calculating a target working condition heat exchange coefficient considering the influence of fouling thermal resistance.
Step 2.1 shows that the ambient temperature does not affect the heat transfer coefficient, so the calculation formula of the target working condition heat transfer coefficient is:
the results were: target working condition heat exchange coefficient Ktm＝8.785W/(℃·m2)。
And 3.2, calculating the number of heat transfer units under the target working condition.
The definitions of the parameters in formula (11) are the same as above.
Using equation (11), the result is; NTUm＝2.524。
Step 3.3 calculating the target working condition heat exchange quantity
Qpm＝(ts1m-ta1m)Ay·vt·Cpa1tm·ρtm(1-exp(1-NTUm)) (12)
In the formula (12), ts1mThe temperature of the condensate water is the lowest value defined by design; the other parameters are defined as above.
Using equation (12), the result is: qpm＝149.814MW。
Step 3.4 calculation of minimum antifreeze flow of target working condition
In the formula (13), hs1mThe enthalpy value of the lowest condensate temperature defined by corresponding design, kJ/kg; h ispmThe steam exhaust enthalpy value is the target working condition.
Using equation (13), the result is: dpm＝238.445t/h。
The energy balance method mentioned in the step 1.2 is relatively complex to calculate, and most of steam discharged by a steam turbine has certain humidity and is not saturated steam; the data show that the humidity of the final-stage exhaust steam of the steam turbine does not exceed 10-12% generally, and the humidity of the intermediate reheating unit is 5-8% generally; for convenient calculation, the exhaust steam humidity in the invention is all limited; higher humidity means that the heat carried by steam of unit mass is less, so that the treatment can avoid that the calculated value is lower than the true value due to over-small humidity value, and the calculation is convenient and quick.
And 3.5, correcting the uneven distribution of the steam flow.
In actual operation, the pressure of the steam distribution pipeline is very likely to have the condition of uneven distribution, thus the steam flow entering the condenser is unevenly distributed, and the negative load of the unit is lowDuring the load operation period, the steam flow deviation can reach about 5 percent; if not for D in step 3.4pmThe calculation result is corrected, and the freezing phenomenon of individual air cooling condenser rows and units can occur; the correction calculation formula is:
in the formula (I), the compound is shown in the specification,
the correction coefficient is a correction coefficient with uneven steam flow distribution, and the value is 1.05; dpmxNamely the target working condition minimum anti-freezing flow value after comprehensively considering various influence factors.
Using equation (14), the result is: dpmx＝250.367t/h；
DpmxNamely, the difference between the target working condition minimum anti-freezing flow value after comprehensively considering various influence factors and the reference value 249.0t/h provided by a design manufacturer under the working condition is very small, and the accuracy of the method is verified.
Step 4, repeating the step 3, and calculating the lowest antifreezing flow under different environmental temperatures and exhaust steam parameters; and summarizing, listing or drawing a curve of the lowest antifreezing flow values of the target working conditions so as to facilitate use.
In conclusion, the invention relates to a method for calculating the lowest anti-freezing flow of the air cooling island by considering various influence factors, which can be applied to direct air cooling units with different capacities, has obvious improvement on the aspects of accuracy, practicability and implementation convenience, can provide help for the normal operation and start/stop of the direct air cooling units in winter, and has a certain application prospect.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents or improvements made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (10)
1. A method for calculating the lowest anti-freezing flow of an air cooling island by considering various influence factors is characterized by comprising the following steps of:
(1) calculating fouling thermal resistance;
(2) calculating the head-on wind speed of the air-cooled condenser in a natural ventilation state;
(3) calculating the minimum anti-freezing flow under the target working condition;
(4) and (4) repeating the step (3), and calculating the lowest antifreezing flow under different environmental temperatures and steam exhaust parameters.
2. The method for calculating the lowest antifreezing flow rate of the air cooling island by considering various influence factors according to claim 1, wherein the step (1) specifically comprises the following steps:
(1-1) searching historical data recorded by a unit to obtain operation data of the air-cooled condenser row in a certain time period after the air-cooled condenser row is put into operation in winter;
(1-2) calculating the number of heat transfer units according to the operation data;
(1-3) calculating the test heat exchange coefficient of the cooling island in the selected time period;
(1-4) calculation of fouling resistance Using equation (5)
In the formula (5), K is the test heat exchange coefficient of the air cooling island in the selected time period; k0Designing a heat exchange coefficient for the air-cooled condenser; unit W/m2·℃。
3. The method for calculating the lowest antifreezing flow rate of the air cooling island considering various influence factors according to claim 2, wherein the number of the heat transfer units is calculated by the following formula (1):
in the formula (1), QpFor heat release of air cooling island steam，kW；ts1The temperature of the condensate water, DEG C; t is ta1The temperature of cold air at the inlet of the air cooling fan is DEG C; da1Is the inlet air quantity m of the air cooling fan3/s；Cpa1The specific heat capacity is constant pressure of cold air, kJ/kg DEG C; rho is the density of cold air, kg/m3；
In the formula (1), Da1The calculation formula is as follows:
in the formula (2), DaM is the air quantity of the air cooling fan at rated rotating speed3/s；naThe rated rotating speed of the air cooling fan is r/min; n isa1The rotating speed of the air cooling fan under the test working condition is obtained by data acquisition, r/min;
in the formula (1), QpThe calculation formula of (2) is as follows:
Qp＝Dp·(hp-hs1) (3)
in the formula (3), DpThe steam flow entering the air cooling island is t/h; h ispThe enthalpy value of the steam turbine exhaust steam is kJ/kg; h iss1The temperature of the condensate obtained by data acquisition is given as the enthalpy of the condensate, kJ/kg.
4. The method for calculating the lowest antifreezing flow of the air cooling island considering the multiple influence factors as claimed in claim 3, wherein the experimental heat transfer coefficient of the air cooling island in the selected time period is calculated by the following formula (4):
in the formula (4), A is the heat exchange area of the air cooling island, m2。
5. The method for calculating the lowest antifreezing flow rate of the air cooling island by considering various influence factors according to claim 4, wherein the step (2) specifically comprises the following steps:
(2-1) calculating the heat exchange coefficient of the air-cooled condenser in a natural ventilation state;
(2-2) searching parameters related to the calculation of the head-on wind speed by utilizing the performance curve of the air cooling island;
and (2-3) iteratively calculating to obtain the value of the head-on wind speed in the natural ventilation state.
6. The method for calculating the lowest antifreeze flow rate of the air cooling island considering various influence factors according to claim 5, wherein the heat exchange coefficient of the air cooling condenser in the natural ventilation state is calculated by the following formula (6):
in the formula (6), αitIs the steam side heat transfer coefficient, W/m2The temperature is controlled; lambda is the heat conductivity coefficient of the tube wall, W/m2·℃；αotIs the air side heat transfer coefficient in W/m2·℃；Ai、Am、AoRespectively the heat exchange area of the steam side, the pipe wall and the air side, m2；ηoThe heat exchange efficiency of the fins is improved;
in the formula (6), αotThe calculation formula of (2) is as follows:
in the formula (7), vtAnd voThe windward speed m/s of the air cooling condenser under the natural ventilation working condition and the design working condition respectively; alpha is alphaoFor designing the convective heat transfer coefficient at the air side under the working condition, W/m2·℃。
7. The method for calculating the lowest antifreezing flow rate of the air cooling island considering the multiple influence factors as recited in claim 6, wherein the numerical value of the windward speed in the natural ventilation state is obtained by iterative calculation according to equation (9):
in the formula (9), Δ is a difference; v. oftThe windward speed is m/s under the natural ventilation state; a. theyIs the frontal area, m2(ii) a Impartation of vtAn extremely low initial value and increased in steps of 0.001 m/s; when the difference value delta is lower than a preset absolute value or a preset relative value, the iteration is ended, and the v at the momenttThe value is the head-on wind speed in the natural ventilation state.
8. The method for calculating the lowest antifreezing flow rate of the air cooling island by considering various influence factors according to claim 7, wherein the step (3) specifically comprises the following steps:
(3-1) calculating a target working condition heat exchange coefficient considering the influence of fouling thermal resistance;
(3-2) calculating the number of heat transfer units under the target working condition;
(3-3) calculating the heat exchange amount under the target working condition;
(3-4) calculating the lowest antifreezing flow of the target working condition;
and (3-5) correcting the uneven distribution of the steam flow.
9. The method for calculating the lowest antifreezing flow of the air cooling island considering the multiple influence factors according to claim 8, wherein the target working condition heat exchange coefficient considering the influence of the fouling thermal resistance is calculated by the following formula (10):
the number of heat transfer units under the target working condition is calculated by the formula (11);
the target working condition heat exchange quantity is calculated by the formula (12):
Qpm＝(ts1m-ta1m)Ay·vt·Cpa1tm·ρtm(1-exp(1-NTUm)) (12)
in the formula (12), ts1mThe temperature of the condensate water is the lowest value defined by design;
the minimum antifreeze flow of the target working condition is calculated by the formula (13):
in the formula (13), hs1mThe enthalpy value of the lowest condensate temperature defined by corresponding design, kJ/kg; h ispmThe steam exhaust enthalpy value is the target working condition.
10. The method for calculating the lowest antifreezing flow rate of the air cooling island considering various influence factors as recited in claim 9, wherein the target working condition lowest antifreezing flow rate is corrected by using a formula (14) in consideration of the uneven distribution of the steam flow rate:
in the formula (I), the compound is shown in the specification,
the correction coefficient is a correction coefficient with uneven steam flow distribution, and the value is 1.05; dpmxNamely the target working condition minimum anti-freezing flow value after comprehensively considering various influence factors.
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