CN113418405A - Method and system for preventing nuclear power condenser from freezing - Google Patents

Method and system for preventing nuclear power condenser from freezing Download PDF

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Publication number
CN113418405A
CN113418405A CN202110609441.1A CN202110609441A CN113418405A CN 113418405 A CN113418405 A CN 113418405A CN 202110609441 A CN202110609441 A CN 202110609441A CN 113418405 A CN113418405 A CN 113418405A
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cooling pipe
condenser
temperature
steam
inlet
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王学华
苏秀丽
孙世民
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China General Nuclear Power Corp
China Nuclear Power Engineering Co Ltd
CGN Power Co Ltd
Shenzhen China Guangdong Nuclear Engineering Design Co Ltd
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China General Nuclear Power Corp
China Nuclear Power Engineering Co Ltd
CGN Power Co Ltd
Shenzhen China Guangdong Nuclear Engineering Design Co Ltd
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28B—STEAM OR VAPOUR CONDENSERS
    • F28B9/00—Auxiliary systems, arrangements, or devices
    • F28B9/005—Auxiliary systems, arrangements, or devices for protection against freezing
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28B—STEAM OR VAPOUR CONDENSERS
    • F28B11/00—Controlling arrangements with features specially adapted for condensers

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Abstract

The invention discloses a method and a system for preventing a nuclear power condenser from freezing, wherein the method comprises the following steps: s1: detecting the current inlet coolant temperature of the condenser cooling pipe; s2: judging whether the outer surface of the cooling pipe is frozen or not at the current inlet coolant temperature according to the freezing prediction model; s3: if so, part of the outlet coolant heated by the cooling pipe flows back to the inlet of the cooling pipe, the temperature of the coolant entering the inlet of the cooling pipe of the condenser is increased, the speed of steam icing on the outer surface of the cooling pipe can be effectively relieved or eliminated, the reactor of the nuclear power unit in a cold plant site is in a hot stop state in winter, the outer surface of the cooling pipe of the condenser cannot be iced, the safety of condenser equipment is improved, and the hidden danger caused by icing of a titanium pipe of the condenser is eliminated.

Description

Method and system for preventing nuclear power condenser from freezing
Technical Field
The invention relates to the technical field of safety of condenser equipment in a nuclear power plant, in particular to a method and a system for preventing a nuclear power condenser from freezing.
Background
The condenser of the nuclear power plant is a surface heat exchanger, the casing of the condenser receives the exhaust steam of a low-pressure cylinder of a steam turbine or the exhaust steam of a main steam bypass exhaust system and the exhaust steam of other systems of a two-loop, circulating cooling seawater flows in the titanium tube bundle of the condenser, and the steam outside the tube bundle is condensed into water and is gathered in a hot well. The condenser maintains the back pressure required by the low pressure cylinder of the steam turbine through the combined action of condensed exhaust and continuous non-condensed gas extracted by a condenser vacuum system.
The nuclear power plant circulating water system provides necessary cooling water flow to the condenser through two independent pipelines so as to ensure the performance of a power station. The cooling water is pressurized by the circulating water pump and then enters the condenser through the circulating water inlet gallery, and then the circulating water is discharged into the sea through the drainage channel.
When a nuclear power plant reactor is started, a conventional island condenser and a circulating water system are required to be operated, a steam turbine is not operated at the moment, the working condition has no problem in a plant site in the south, but in a cold plant site, once the nuclear power plant reactor is in a hot stop stage for a long time in winter, the circulating water system is started to provide cooling water for the condenser, but only a small amount of steam in a nuclear island is discharged into the condenser, and the steam outside a titanium pipe can be frozen when circulating seawater lower than zero enters the condenser. Once the icing phenomenon occurs around the condenser tube bundle, on one hand, if the icing range surrounds the vacuumizing part of the condenser, the vacuum pump cannot pump out non-condensable gas from the condenser, so that the backpressure of the condenser is abnormal, and the problem is brought to normal debugging or starting of a unit; on the other hand, if the icing range is too large, the gravity of the ice blocks and the expansion of the ice blocks among the tube bundles can influence the stress of the tube bundles, and if the stress exceeds the limit, the tube bundles of the condenser can be damaged, so that the unit can be stopped to cause short-term economic loss, and meanwhile, a tube blocking measure needs to be taken, so that the power of the unit is influenced, further long-term economic loss is brought, and the equipment safety of the condenser is influenced.
Disclosure of Invention
The invention aims to solve the technical problem of providing a method and a system for preventing a nuclear power condenser from icing aiming at least one defect in the prior art.
The technical scheme adopted by the invention for solving the technical problems is as follows: a method for preventing a nuclear power condenser from icing is constructed, and comprises the following steps:
s1: detecting the current inlet coolant temperature of the condenser cooling pipe;
s2: judging whether the outer surface of the cooling pipe is frozen or not at the current inlet coolant temperature according to the freezing prediction model;
s3: and if so, partially refluxing the outlet coolant heated by the cooling pipe to the inlet of the cooling pipe.
Preferably, in the method for preventing the nuclear power condenser from icing, the icing prediction model is a relation function of icing thickness and icing time of different condensers at different inlet coolant temperatures.
Preferably, in the method for preventing icing of nuclear power condenser, the relation function is
Figure BDA0003095402600000021
Wherein t is the freezing time; lambda is the thermal conductivity of the ice layer; ρ is the density of the coolant; gamma is the latent heat released by the coolant when it condenses to ice at zero degrees; t iswIs the average temperature of the coolant in the cooling tube; r is the outer radius of the ice layer; r0Is the outer radius of the cooling tube.
Preferably, in the method for preventing icing of a nuclear power condenser according to the present invention, in step S3, the step of returning a part of the outlet coolant heated by the cooling pipe to the inlet of the cooling pipe includes:
s31: detecting the inlet coolant temperature of the cooling pipe before backflow, the outlet coolant temperature of the cooling pipe before backflow and the inlet coolant temperature of the cooling pipe after backflow, and calculating a backflow ratio according to a backflow ratio model;
s32: and partially refluxing the outlet coolant heated by the cooling pipe to the inlet of the cooling pipe according to the reflux ratio.
Preferably, in the method for preventing the nuclear power condenser from being frozen, the reflux ratio model is as follows:
the reflux ratio (inlet coolant temperature of the cooling pipe after the reflux-inlet coolant temperature of the cooling pipe before the reflux)/(outlet coolant temperature of the cooling pipe before the reflux-inlet coolant temperature of the cooling pipe before the reflux).
Preferably, in the method for preventing icing of a nuclear power condenser, the method further includes:
s4: judging whether the temperature of the inlet coolant of the cooling pipe after backflow is less than a threshold value;
s5: and if so, introducing auxiliary steam into the steam space outside the cooling pipe.
Preferably, in the method for preventing the nuclear power condenser from icing, the threshold value is zero degree.
Preferably, in the method for preventing icing of a nuclear power condenser, auxiliary steam is introduced into a steam space outside the cooling pipe, and the method includes the following steps:
s51: calculating a minimum auxiliary steam mass flow into the steam space to maintain a saturated steam temperature in the steam space greater than zero based on an inlet coolant temperature;
s52: and introducing auxiliary steam into the steam space according to the minimum auxiliary steam mass flow.
Preferably, in the method for preventing icing of a nuclear power condenser, step S51 includes:
s511: based on heat balance calculationFirst overall heat transfer coefficient K of condenserTAnd calculating a second overall heat transfer coefficient K of the condenser according to the inlet coolant temperature;
s512: according to the first overall heat transfer coefficient KTThe relation equal to the second overall heat transfer coefficient K is calculated to obtain the saturated steam temperature t in the steam spacesThe heat load Q of the condenser is greater than zero;
s513: and calculating to obtain the minimum auxiliary steam mass flow M introduced into the steam space according to the heat load Q of the condenser and by using an auxiliary steam mass flow calculation model.
Preferably, in the method for preventing icing of nuclear power condenser, the first overall heat transfer coefficient
Figure BDA0003095402600000041
Q is the condenser heat load; a is the cooling area of the cooling pipe; LMTD is the mean temperature difference of the logarithm of the condenser;
mean temperature difference of logarithm of the condenser
Figure BDA0003095402600000042
tsIs the saturated steam temperature of the steam space at condenser pressure; t is t1Is the inlet coolant temperature; t is t2Is the outlet coolant temperature;
the heat load Q of the condenser is W multiplied by Cp×(t2-t1) (ii) a W is the current inlet coolant flow; cpIs the specific heat capacity at the average temperature of the coolant in the cooling tube;
the second overall heat transfer coefficient K ═ K0βtβmβc，
Figure BDA0003095402600000043
K0The basic heat transfer coefficient is calculated according to the outer diameter of a cooling pipe of the condenser and the flow velocity in the pipe; beta is atAn inlet coolant temperature correction factor; beta is amThe correction coefficient is the pipe material and the wall thickness of the cooling pipe; beta is acThe condenser cleaning coefficient; c. C1Is a coefficient corresponding to the outer diameter of the cooling pipe; v is the average flow velocity in the cooling tube;
the auxiliary steam mass flow calculation model is
Figure BDA0003095402600000044
M is the minimum auxiliary steam mass flow; q is the condenser heat load; h is auxiliary latent heat of steam.
Preferably, in the method for preventing icing of a nuclear power condenser according to the present invention, the step S511 further includes:
correcting the model for the first overall heat transfer coefficient K according to the inlet coolant flow and inlet coolant temperature of the cooling tubes and the overall heat transfer coefficientTCorrecting to obtain a corrected first overall heat transfer coefficient Kc。
Preferably, in the method for preventing the nuclear power condenser from icing, the overall heat transfer coefficient correction model is Kc＝KT×Fv×Ft，
Figure BDA0003095402600000051
Wherein, FvAn inlet coolant flow correction factor; ftAn inlet coolant temperature correction factor; vDThe flow velocity in the cooling pipe under the designed working condition is adopted; vTThe flow velocity in the cooling tube at the current inlet coolant temperature; beta is atDCorrecting coefficient for inlet coolant temperature under design condition; beta is atTThe correction factor for the inlet coolant temperature at the current inlet coolant temperature.
Preferably, in the method for preventing icing of a nuclear power condenser according to the present invention, the step S512 further includes:
according to the steam temperature correction model, the saturated steam temperature t of the steam spacesCorrecting to obtain saturated steam temperature t of the steam space under inlet coolant temperature and flow conditions when the temperature is corrected to the design working conditionsc。
Preferably, the prevention of nuclear power in the present inventionIn the method for freezing the condenser, the steam temperature correction model is
Figure BDA0003095402600000052
Wherein, t1DThe inlet coolant temperature at the design working condition; wDThe inlet coolant flow is designed under the working condition; cpcIs the specific heat capacity at the average temperature of the coolant in the cooling tube; x is the modified first overall heat transfer coefficient KTThe latter logarithmic mean temperature difference coefficient; e is a natural constant; and A is the cooling area of the cooling pipe.
The invention also constructs a system for preventing the nuclear power condenser from freezing, which comprises a regulating device and a reflux device connected with the outlet and the inlet of the cooling pipe of the condenser;
the adjusting device comprises:
the detection module is used for detecting the current inlet coolant temperature of the condenser cooling pipe;
the prediction module is used for judging whether the outer surface of the cooling pipe is frozen or not at the current inlet coolant temperature according to the freezing prediction model;
and the control module is used for controlling the reflux device to reflux the outlet coolant part heated by the cooling pipe to the inlet of the cooling pipe when the prediction module judges that the outlet coolant part is the outlet coolant part.
Preferably, in the system for preventing a nuclear power condenser from being frozen, the backflow device includes a collecting device, a backflow pipeline communicated with the collecting device and the inlet of the cooling pipe, and a backflow valve arranged on the backflow pipeline and in communication connection with the control module;
the collecting device is communicated with the outlet of the cooling pipe, is higher than the inlet of the cooling pipe, is used for collecting the outlet coolant heated by the cooling pipe, and partially reflows the outlet coolant to the inlet of the cooling pipe through the reflow pipeline by utilizing the gravity of the height difference.
Preferably, in the system for preventing the nuclear power condenser from icing, the collecting device is an overflow weir.
Preferably, in the system for preventing icing of a nuclear power condenser, the inlet of the cooling pipe is an inlet of a coolant delivery pump connected with the cooling pipe.
Preferably, in the system for preventing the nuclear power condenser from being frozen, the freezing prediction model is a relation function of the freezing thickness and the freezing time of different condensers at different inlet coolant temperatures.
Preferably, in the system for preventing the nuclear power condenser from icing, the relation function is
Figure BDA0003095402600000071
Wherein t is the freezing time; lambda is the thermal conductivity of the ice layer; ρ is the density of the coolant; gamma is the latent heat released by the coolant when it condenses to ice at zero degrees; t iswIs the average temperature of the coolant in the cooling tube; r is the outer radius of the ice layer; r0Is the outer radius of the cooling tube.
Preferably, in the system for preventing icing of a nuclear power condenser according to the present invention, the control module includes:
the backflow ratio calculating module is used for detecting the inlet coolant temperature of the cooling pipe before backflow, the outlet coolant temperature of the cooling pipe before backflow and the inlet coolant temperature of the cooling pipe after backflow, and calculating a backflow ratio according to a backflow ratio model;
and the reflux ratio control module is used for controlling the reflux device to reflux the outlet coolant part heated by the cooling pipe to the inlet of the cooling pipe according to the reflux ratio.
Preferably, in the system for preventing a nuclear power condenser from being frozen, the reflux ratio model is as follows:
the reflux ratio (inlet coolant temperature of the cooling pipe after the reflux-inlet coolant temperature of the cooling pipe before the reflux)/(outlet coolant temperature of the cooling pipe before the reflux-inlet coolant temperature of the cooling pipe before the reflux).
Preferably, in the system for preventing the nuclear power condenser from icing, the system further comprises an auxiliary steam device;
the adjusting device also comprises a judging module used for judging whether the temperature of the inlet coolant of the cooling pipe after backflow is less than a threshold value;
the control module is also used for controlling the auxiliary steam device to introduce auxiliary steam into the steam space outside the cooling pipe when the judgment module judges that the steam space is positive.
Preferably, in the system for preventing icing of a nuclear power condenser, the auxiliary steam device comprises an auxiliary steam input device and an auxiliary steam pipeline connected with the steam space outside the cooling pipe and the auxiliary steam input device.
Preferably, in the system for preventing the nuclear power condenser from being frozen, the threshold value is zero degree.
Preferably, in the system for preventing icing of a nuclear power condenser according to the present invention, the control module includes:
the auxiliary steam mass flow calculation module is used for calculating the minimum auxiliary steam mass flow which is introduced into the steam space for keeping the saturated steam temperature in the steam space to be greater than zero according to the temperature of the inlet coolant;
and the auxiliary steam mass flow control module is used for controlling the auxiliary steam input device to introduce auxiliary steam into the steam space outside the cooling pipe through the auxiliary steam pipeline according to the minimum auxiliary steam mass flow.
Preferably, in the system for preventing icing of a nuclear power condenser, the auxiliary steam mass flow calculation module includes:
a heat transfer coefficient calculation module for calculating a first overall heat transfer coefficient K of the condenser according to the heat balanceTAnd calculating a second overall heat transfer coefficient K of the condenser according to the inlet coolant temperature;
a heat load calculation module for calculating a heat transfer coefficient K according to the first overall heat transfer coefficientTOff equal to the second overall heat transfer coefficient KCalculating the saturated steam temperature t in the steam spacesThe heat load Q of the condenser is greater than zero;
and the steam flow calculation module is used for calculating the minimum auxiliary steam mass flow M introduced into the steam space according to the heat load Q of the condenser and by using an auxiliary steam mass flow calculation model.
Preferably, in the system for preventing icing of nuclear power condenser, the first overall heat transfer coefficient
Figure BDA0003095402600000081
Q is the condenser heat load; a is the cooling area of the cooling pipe; LMTD is the mean temperature difference of the logarithm of the condenser;
mean temperature difference of logarithm of the condenser
Figure BDA0003095402600000091
tsIs the saturated steam temperature of the steam space at condenser pressure; t is t1Is the inlet coolant temperature; t is t2Is the outlet coolant temperature;
the heat load Q of the condenser is W multiplied by Cp×(t2-t1) (ii) a W is the current inlet coolant flow; cpIs the specific heat capacity at the average temperature of the coolant in the cooling tube;
the second overall heat transfer coefficient K ═ K0βtβmβc，
Figure BDA0003095402600000092
K0The basic heat transfer coefficient is calculated according to the outer diameter of a cooling pipe of the condenser and the flow velocity in the pipe; beta is atAn inlet coolant temperature correction factor; beta is amThe correction coefficient is the pipe material and the wall thickness of the cooling pipe; beta is acThe condenser cleaning coefficient; c. C1Is a coefficient corresponding to the outer diameter of the cooling pipe; v is the average flow velocity in the cooling tube;
the auxiliary steam mass flow calculation model is
Figure BDA0003095402600000093
M is the minimum auxiliary steam mass flow; q is the condenser heat load; h is auxiliary latent heat of steam.
Preferably, in the system for preventing a nuclear power condenser from icing according to the present invention, the auxiliary steam mass flow calculation module further includes:
a heat transfer coefficient correction module for correcting the first overall heat transfer coefficient K according to the inlet coolant flow and the inlet coolant temperature of the cooling pipe and an overall heat transfer coefficient correction modelTCorrecting to obtain a corrected first overall heat transfer coefficient Kc。
Preferably, in the system for preventing the nuclear power condenser from icing, the overall heat transfer coefficient correction model is Kc＝KT×Fv×Ft，
Figure BDA0003095402600000094
Wherein, FvAn inlet coolant flow correction factor; ftAn inlet coolant temperature correction factor; vDThe flow velocity in the cooling pipe under the designed working condition is adopted; vTThe flow velocity in the cooling tube at the current inlet coolant temperature; beta is atDCorrecting coefficient for inlet coolant temperature under design condition; beta is atTThe correction factor for the inlet coolant temperature at the current inlet coolant temperature.
Preferably, in the system for preventing a nuclear power condenser from icing according to the present invention, the auxiliary steam mass flow calculation module further includes:
a steam temperature correction module for correcting the saturated steam temperature t of the steam space according to the steam temperature correction modelsThe correction is carried out so that the correction is carried out,
obtaining a saturated steam temperature t of the steam space at inlet coolant temperature and flow rate corrected to design conditionssc。
Preferably, in the system for preventing the nuclear power condenser from icing, the steam temperature correction model is
Figure BDA0003095402600000101
Wherein, t1DThe inlet coolant temperature at the design working condition; wDThe inlet coolant flow is designed under the working condition; cpcIs the specific heat capacity at the average temperature of the coolant in the cooling tube; x is the modified first overall heat transfer coefficient KTThe latter logarithmic mean temperature difference coefficient; e is a natural constant; and A is the cooling area of the cooling pipe.
By implementing the invention, the following beneficial effects are achieved:
by adopting the anti-icing design scheme of the condenser cooling pipe of the nuclear power plant, the current inlet coolant temperature of the condenser cooling pipe is detected; judging whether the outer surface of the cooling pipe is frozen or not at the current inlet coolant temperature according to the freezing prediction model; if the temperature of the coolant entering the inlet of the cooling pipe of the condenser is higher than the temperature of the coolant entering the inlet of the cooling pipe of the condenser, the speed of steam icing on the outer surface of the cooling pipe can be effectively relieved or eliminated, the reactor of the nuclear power unit in a cold plant site is in a hot stop state in winter, the outer surface of the cooling pipe of the condenser cannot be iced, the safety of condenser equipment is improved, and the hidden danger caused by icing of a titanium pipe of the condenser is eliminated.
Drawings
The invention will be further described with reference to the accompanying drawings and examples, in which:
FIG. 1 is a flow chart of a method of preventing icing in a nuclear power condenser according to the present invention;
FIG. 2 is a model for calculating the growth rate of an ice layer outside a cooling tube of a condenser according to the present invention;
FIG. 3 is a graph of icing thickness (R-R) for different inlet coolant temperatures0) A graph of icing time t;
FIG. 4 is a schematic illustration of control functions for different inlet coolant temperatures and minimum auxiliary steam mass flow;
FIG. 5 is a block diagram of the regulating device, the reflux device and the auxiliary steam device of the present invention;
FIG. 6 is a schematic view of the connection of the condenser, the reflux unit and the auxiliary steam unit of the present invention;
FIG. 7 is a block diagram of a control module of the present invention;
FIG. 8 is a block diagram of an auxiliary steam mass flow calculation module of the present invention.
Detailed Description
For a more clear understanding of the technical features, objects and effects of the present invention, embodiments of the present invention will now be described in detail with reference to the accompanying drawings.
It should be noted that the flow charts shown in the drawings are only exemplary and do not necessarily include all the contents and operations/steps, nor do they necessarily have to be executed in the order described. For example, some operations/steps may be decomposed, and some operations/steps may be combined or partially combined, so that the actual execution sequence may be changed according to the actual situation.
The block diagrams shown in the figures are functional entities only and do not necessarily correspond to physically separate entities. I.e. these functional entities may be implemented in the form of software, or in one or more hardware modules or integrated circuits, or in different networks and/or processor means and/or microcontroller means.
The first embodiment is as follows:
the nuclear power condenser comprises a cooling pipe for introducing a coolant and a steam space positioned outside the cooling pipe. In some embodiments, the cooling tubes are titanium tubes and the coolant is water, such as seawater. And after circulating coolant is introduced into the cooling pipe, steam in the steam space outside the cooling pipe is condensed into water. However, in cold plants, when the circulating coolant below zero enters the condenser, the steam on the outer surface of the cooling tube will be frozen, so as to relieve and eliminate the freezing on the outer surface of the cooling tube of the condenser, as shown in fig. 1, the invention discloses a method for preventing the nuclear power condenser from freezing, which comprises the following steps:
step S1: detecting the current inlet coolant temperature of a condenser cooling pipe;
step S2: judging whether the outer surface of the cooling pipe is frozen or not at the current inlet coolant temperature according to the freezing prediction model;
step S3: and if so, partially returning the outlet coolant heated by the cooling pipe to the inlet of the cooling pipe.
Specifically, in step S2, the icing prediction model is a function of icing thickness and icing time for different condensers at different inlet coolant temperatures. For example, the relationship function is
Figure BDA0003095402600000121
Wherein t is the freezing time, hr; lambda is the thermal conductivity coefficient of the ice layer, w/m DEG C; ρ is the density of the coolant, kg/m 3; gamma is the latent heat released by the coolant when condensing into ice at zero degrees, J/kg; t iswIs the average temperature, deg.C, of the coolant in the cooling tube; r is the outer radius of the ice layer, mm; r0Is the outer radius of the cooling tube, mm.
Wherein, as shown in FIG. 2, the outer diameter of the cooling pipe is assumed to be 2R0The temperature of the cylindrical portion surrounded by the surface of the cooling pipe is constantly equal to the average temperature T of the coolant in the cooling pipewThe outer periphery of the cooling tube is surrounded by zero-degree steam (condensed water), and the steam on the outer surface of the cooling tube is first condensed into a thin ice layer due to the cooling effect of the coolant, and then the ice layer gradually grows radially toward the outer periphery. And the two-phase interface of ice and water is a coaxial cylindrical surface with the diameter of 2R at the time t, after the time d tau, the two-phase interface is expanded to R + dR, and dR is the thickness of the ice layer expanded after the time d tau.
From the heat balance relationship of the microring dR (volume 2R pi dR × 1), the heat quantity flowing out from the inner cylindrical surface of the microring by heat conduction is equal to the latent heat released by the condensation of water in the volume of the microring to ice according to the heat transfer principle, and the mathematical relationship can be expressed by the following formula (1):
Figure BDA0003095402600000131
wherein λ is a thermal conductivity of the ice layer (═ 2.22w/m · c); ρ is the density of the coolant, for example, the density of water (═ 913kg/m 3); gamma is the latent heat released by the coolant when it freezes to ice at zero degreesLatent heat released by water when it condenses to ice at zero degrees (335000J/kg), for example;
Figure BDA0003095402600000135
is the temperature gradient of the inner cylindrical surface of the micro-element ring. The reason why the heat transfer effect of the outer cylindrical surface of the micro-ring is not considered in the formula is that it is assumed that the temperature of the external water is constantly zero and the temperature gradient is inevitably zero. After mathematical processing is carried out on the expression (1), the relation between the icing time t and the outer radius R of the ice layer represented by the expression (4) can be obtained.
Figure BDA0003095402600000132
Figure BDA0003095402600000133
Figure BDA0003095402600000134
2R for condenser heat exchange tube outer diameter data0The icing thickness (R-R) at different inlet coolant temperatures can be calculated by substituting 22.22mm and various physical constants into the formula (4)0) And the relation curve with the icing time t is shown in fig. 3, so that whether the outer surface of the cooling pipe is iced or the thickness of the ice layer on the outer surface of the cooling pipe of the condenser is predicted or the time length of icing on the outer surface of the cooling pipe at the current inlet coolant temperature is judged.
As can be seen from fig. 3, 3 rules can be seen: 1) for a fixed inlet coolant temperature (e.g. T)w-2.5 ℃), the thickness of the ice increases with the passage of the icing time; 2) the growth rate of the ice layer gradually slows down with the passage of time (as shown by the slope of the curve becoming smaller), because the temperature difference of the ice layer is constant, and the temperature gradient (driving force of heat conduction) gradually decreases as the ice layer grows thicker; 3) the lower the inlet coolant temperature, the faster the rate of icing due to the temperature gradient of the ice layer andthe square root of the inlet coolant temperature is proportional.
In some embodiments, according to the photograph of the actual condenser ice returned from the site, ice blocks with the thickness of about 300mm are formed between the steam channels (the channels shaped like slits) at the upper part of the tube bundle, and the icing time according to the formula (4) is at least about 1 week, which is basically consistent with the actual investment time of the spraying water system. It can be concluded from this that the main reason for icing is due to the continuous dosing of the spray water system during a period of approximately one week with inlet coolant temperature below zero.
In addition, it should be noted that: in the actual icing process of the condenser, the temperature of steam (condensed water) sprayed from the upper part is generally higher than 0 ℃, so that sensible heat (approximately equal to 4180J/kg. ℃) needs to be released firstly during icing, then latent heat is released to solidify into ice, and the actual icing speed of the condenser is slower than the calculation result of the model.
From the above calculation, it is understood that the lower the temperature of the inlet coolant of the condenser cooling pipe is, the higher the speed of freezing of the steam on the outer surface of the cooling pipe is, because the temperature gradient of the ice layer is proportional to the square root of the temperature of the inlet coolant. Therefore, when the outer surface of the cooling pipe is judged to be frozen at the current inlet coolant temperature according to the freezing prediction model, part of the outlet coolant heated by the cooling pipe flows back to the inlet of the cooling pipe, the temperature of the coolant entering the inlet of the cooling pipe of the condenser is increased, and the speed of freezing of steam on the outer surface of the cooling pipe can be effectively relieved.
In this embodiment, in order to optimize the hot water reflux ratio, the step S3 of partially refluxing the outlet coolant heated by the cooling pipe to the inlet of the cooling pipe includes:
step S31: and detecting the inlet coolant temperature of the cooling pipe before backflow, the outlet coolant temperature of the cooling pipe before backflow and the inlet coolant temperature of the cooling pipe after backflow, and calculating the backflow ratio according to the backflow ratio model. Wherein the reflux ratio model is: the reflux ratio (inlet coolant temperature of the cooling pipe after the reflux-inlet coolant temperature of the cooling pipe before the reflux)/(outlet coolant temperature of the cooling pipe before the reflux-inlet coolant temperature of the cooling pipe before the reflux).
Step S32: and partially refluxing the outlet coolant heated by the cooling pipe to the inlet of the cooling pipe according to the reflux ratio.
The calculation of the inlet coolant temperature at different reflux ratios for an auxiliary steam mass flow of 210kg/s at an inlet coolant temperature of minus 2.5 degrees is shown in table 1 below, which shows a comparison of the inlet coolant temperature for different hot water reflux ratios of the condenser cooling tubes:
TABLE 1
Figure BDA0003095402600000151
It can be seen from the above table that the temperature of the coolant at the inlet of the condenser cooling pipe can be effectively increased by adopting the hot water reflux measure, the reflux ratio is increased from 10% to 40%, the temperature of the coolant at the inlet can be increased from minus 2.5 ℃ to minus 0.4 ℃ on the premise that the mass flow of the auxiliary steam is not changed, the heat transferred to the coolant in the cooling pipe by the auxiliary steam can be fully utilized by adopting the hot water reflux measure, the steam temperature outside the cooling pipe can be increased by the auxiliary steam, meanwhile, the temperature of the coolant at the inlet can be increased by the hot water reflux measure, and the icing phenomenon on the outer surface of the condenser cooling pipe can be effectively eliminated.
As can be seen from the data in table 1, when a unit is started under low temperature conditions, the temperature of the water intake of the started unit may be lower than zero even if hot water is refluxed because the flow rate of the circulating water is large, and the condenser may be frozen. Therefore, from the viewpoint of the process system, it is still necessary to take appropriate measures for suppressing the ice formation on the outer surface of the condenser cooling pipe.
Therefore, the method for preventing the nuclear power condenser from icing further comprises the following steps:
step S4: and judging whether the inlet coolant temperature of the returned cooling pipe is less than a threshold value. Wherein the threshold is zero degrees.
Step S5: if so, introducing auxiliary steam into the steam space outside the cooling pipe.
Although the temperature of the coolant at the inlet of the circulating water pump can be increased by the measure of hot water backflow, the temperature of the coolant at the inlet of the circulating water pump is still lower than zero, in order to prevent the outer surface of the cooling pipe of the condenser from freezing, auxiliary steam needs to be introduced when the circulating coolant enters the cooling pipe of the condenser to enable the temperature of the outer surface of the cooling pipe to be higher than zero, the freezing phenomenon at the outer surface of the cooling pipe of the condenser can be effectively avoided, the quantity of introduced steam quality needs to be calculated and determined, the waste of steam working media can be caused by the fact that the mass flow of the introduced steam is too large, and the anti-freezing effect cannot be achieved due to the fact that the mass flow of the introduced steam is too small. Meanwhile, the mass flow of the introduced steam is related to the temperature of the inlet coolant in real time, and the mass flow of the introduced auxiliary steam is dynamically adjusted according to the temperature of the inlet coolant, so that energy waste can be avoided, and the economical efficiency is improved.
The following table 2 shows basic parameters of a condenser of a nuclear power plant in a certain cold plant site, and the minimum auxiliary steam mass flow required for preventing the condenser of the nuclear power plant from freezing can be obtained according to the basic parameters of the condenser and a related calculation formula.
TABLE 2
Name of item Unit of Parameter(s)
Types of / Single back pressure, surface type
Cooling area m2 36913
Cooling tube material / Ti
Cooling tube specification / Φ22×0.5
Number of cooling tubes / 32104
Effective length of cooling pipe mm 16470
Flow rate of circulating water m3 27
Therefore, step S5 specifically includes:
step S51: calculating a minimum auxiliary steam mass flow into the steam space to maintain the saturated steam temperature in the steam space greater than zero based on the inlet coolant temperature;
step S52: and introducing auxiliary steam into the steam space according to the minimum auxiliary steam mass flow.
Step S51 specifically includes:
step S511: calculating a first overall heat transfer coefficient K of the condenser according to the heat balanceTAnd calculating a second overall heat transfer coefficient K of the condenser according to the inlet coolant temperature.
Wherein the first overall heat transfer coefficient
Figure BDA0003095402600000171
Q is condenser heat load, W; a is the cooling area of the cooling pipe, m2Looking up a table 2; LMTD is condenser logarithm average temperature difference, degree centigrade; kTIs the first overall heat transfer coefficient, W/(m)2·K)。
Logarithmic mean temperature difference of condenser
Figure BDA0003095402600000172
tsIs the saturated steam temperature, deg.C, of the steam space under condenser pressure; t is t1Inlet coolant temperature, deg.C; t is t2Outlet coolant temperature, deg.C; delta t is the temperature rise, DEG C, of the cooling pipe coolant; delta t is the heat transfer end difference of the condenser, DEG C; LMTD is the mean temperature difference of the logarithm of the condenser, DEG C.
Condenser heat load Q ═ W × Cp×(t2-t1) (ii) a W is the current inlet coolant flow, kg/s; cpJ/(kg. K) is the specific heat capacity at the average temperature of the coolant in the cooling pipe; q is condenser heat load, W; t is t1Inlet coolant temperature, deg.C; t is t2Is the outlet coolant temperature, deg.C.
Second overall heat transfer coefficient K ═ K0βtβmβc，
Figure BDA0003095402600000173
K0Is a basic heat transfer coefficient calculated according to the outer diameter of a cooling pipe and the flow velocity in the pipe of the condenser, W/(m)2·K)；βtAn inlet coolant temperature correction factor; beta is amThe correction coefficient is the pipe material and the wall thickness of the cooling pipe; beta is acThe condenser cleaning coefficient; c. C1The following table 3 is a table for coefficients corresponding to the outer diameters of the cooling pipes; v is the average flow velocity in the cooling tube, m/s.
Step S512: according to a first overall heat transfer coefficient KTThe saturated steam temperature t in the steam space is calculated according to the relation equal to the second overall heat transfer coefficient KsThe heat load Q of the condenser is greater than zero;
step S513: calculating to obtain the minimum auxiliary steam mass flow M introduced into the steam space according to the heat load Q of the condenser and by using an auxiliary steam mass flow calculation model;
wherein, the auxiliary steam mass flow calculation model is
Figure BDA0003095402600000174
M is the minimum auxiliary steam mass flow rate, kg/s; q is condenser heat load, W; h is auxiliary latent heat of steam, J/kg.
TABLE 3
d(mm) 16～19 22～25 28～32 35～38 41～45 48～51
c1 2747 2705 2664 2623 2582 2541
In some embodiments, to obtain a more accurate first overall heat transfer coefficient KcStep S511 further includes:
inlet coolant according to cooling pipeFlow and inlet coolant temperature, and overall heat transfer coefficient modification model versus first overall heat transfer coefficient KTCorrecting to obtain a corrected first overall heat transfer coefficient Kc。
The overall heat transfer coefficient correction model is Kc＝KT×Fv×Ft，
Figure BDA0003095402600000181
Wherein, KcFor the modified first overall heat transfer coefficient, FvAn inlet coolant flow correction factor; ftAn inlet coolant temperature correction factor; vDThe flow velocity in the cooling pipe under the designed working condition is m/s; vTThe flow velocity in the cooling pipe at the current inlet coolant temperature, m/s; beta is atDCorrecting coefficient for inlet coolant temperature under design condition; beta is atTThe correction factor for the inlet coolant temperature at the current inlet coolant temperature.
In some embodiments, to obtain a more accurate saturated steam temperature t of the steam spacesStep S512 further includes:
correcting the saturated steam temperature t of the steam space according to the steam temperaturesCorrecting to obtain saturated steam temperature t of steam space under inlet coolant temperature and flow rate conditions when the temperature is corrected to design working conditionsc。
The steam temperature correction model is
Figure BDA0003095402600000182
Figure BDA0003095402600000183
Wherein, tscFor correcting the saturated steam temperature t of the steam space at inlet coolant temperature and flow rate to the design conditionsc，℃，t1DThe inlet coolant temperature at design conditions, DEG C; Δ tcTemperature rise at the inlet coolant temperature and flow rate conditions corrected to the design conditions is measured at DEG C; δ tcFor correction to designThe difference of heat transfer ends at the inlet coolant temperature and flow rate under working conditions, DEG C; wDThe inlet coolant flow is kg/s under the design working condition; cpcJ/(kg. K) is the specific heat capacity at the average temperature of the coolant in the cooling pipe; x is the modified first overall heat transfer coefficient KTThe latter logarithmic mean temperature difference coefficient; e is a natural constant, about 2.72; a is the cooling area of the cooling pipe, m2And (6) looking up a table 2.
Calculating to obtain tscAnd then the corrected pressure of the condenser can be directly searched from the steam thermodynamic property table.
According to the formula, the temperature of the coolant at the inlet of the circulating water pump of a certain cold plant is at the lowest of-2.5 ℃, in order to prevent the outer surface of the cooling pipe of the condenser from being frozen, auxiliary steam is introduced into the condenser to ensure that the average temperature of the coolant in the cooling pipe is higher than 0 ℃, so that the outer surface of the cooling pipe of the condenser can be prevented from being frozen, and the minimum auxiliary steam mass flow can be obtained by calculating the temperatures of the coolants at different inlets. Table 4 below shows the estimation results of the surface temperature of the heat exchange tube at each load stage after the rush down.
TABLE 4
Figure BDA0003095402600000191
As can be seen from the above table, the auxiliary steam mass flow rate has a direct relationship with the inlet coolant temperature, i.e. the higher the inlet coolant temperature, the lower the required auxiliary steam mass flow rate. The minimum steam mass flow of the condenser icing phenomenon is calculated according to the formula for a certain type of condenser, the minimum steam mass flow is matched with different inlet coolant temperatures of a plant site, the minimum steam mass flow is used for eliminating the condenser icing phenomenon, and a control function is formed, as shown in fig. 4, the auxiliary steam mass flow is controlled in a mode of interlocking with the inlet coolant temperature according to the control function, the purpose of preventing the condenser icing can be achieved, and energy can be saved.
Example two:
as shown in fig. 5, the invention also discloses a system for preventing the nuclear power condenser from freezing, which comprises: the device comprises a regulating device and a reflux device connected with an outlet and an inlet of a condenser cooling pipe;
the adjusting device comprises:
the detection module is used for detecting the current inlet coolant temperature of the condenser cooling pipe;
the prediction module is used for judging whether the outer surface of the cooling pipe is frozen or not at the current inlet coolant temperature according to the freezing prediction model;
and the control module is used for controlling the reflux device to reflux the outlet coolant part heated by the cooling pipe to the inlet of the cooling pipe when the prediction module judges that the outlet coolant part is the outlet coolant part.
In the present embodiment, as shown in fig. 6, the backflow device comprises a collecting device 2, a backflow pipeline 3 communicated with the collecting device 2 and the inlet of the cooling pipe 1, and a backflow valve 4 arranged on the backflow pipeline 3 and communicated with the control module. The collecting device 2 is communicated with the outlet of the cooling pipe 1 and is positioned higher than the inlet of the cooling pipe 1, and is used for collecting the outlet coolant heated by the cooling pipe 1 and partially reflowing the outlet coolant to the inlet of the cooling pipe 1 through the reflow pipeline 3 by using the gravity of the height difference.
In some embodiments, the collecting device 2 is an overflow weir, and in order to smoothly discharge the coolant to the open sea, the level of the overflow weir is 2-3 m higher than the coolant level at the inlet of the cooling pipe 1, and the invention provides that the outlet coolant is returned to the inlet of the cooling pipe 1 by gravity by arranging a coolant return pipe 3 on the overflow weir, preferably to the inlet of a coolant delivery pump 3 connected with the cooling pipe 1, such as the inlet of a circulating water pump of a circulating water system, according to the inherent characteristics of the circulating water system arrangement, so as to increase the coolant temperature at the inlet of the condenser cooling pipe 1 by increasing the water intake temperature.
In some embodiments, a temperature measuring point T may also be provided at the inlet of the cooling pipe 1, for example at the outlet of a circulating water pump of a circulating water system.
In the prediction module, the icing prediction model is a relation function of icing thickness and icing time of different condensers at different inlet coolant temperatures. For example, the relationship function is
Figure BDA0003095402600000201
Figure BDA0003095402600000211
Wherein t is the freezing time, hr; lambda is the thermal conductivity coefficient of the ice layer, w/m DEG C; ρ is the density of the coolant, kg/m 3; gamma is the latent heat released by the coolant when condensing into ice at zero degrees, J/kg; t iswIs the average temperature, deg.C, of the coolant in the cooling tube; r is the outer radius of the ice layer, mm; r0Is the outer radius of the cooling tube, mm.
Wherein, as shown in FIG. 2, the outer diameter of the cooling pipe is assumed to be 2R0The temperature of the cylindrical portion surrounded by the surface of the cooling pipe is constantly equal to the average temperature T of the coolant in the cooling pipewThe outer periphery of the cooling tube is surrounded by zero-degree steam (condensed water), and the steam on the outer surface of the cooling tube is first condensed into a thin ice layer due to the cooling effect of the coolant, and then the ice layer gradually grows radially toward the outer periphery. And the two-phase interface of ice and water is a coaxial cylindrical surface with the diameter of 2R at the time t, after the time d tau, the two-phase interface is expanded to R + dR, and dR is the thickness of the ice layer expanded after the time d tau.
From the heat balance relationship of the microring dR (volume 2R pi dR × 1), the heat quantity flowing out from the inner cylindrical surface of the microring by heat conduction is equal to the latent heat released by the condensation of water in the volume of the microring to ice according to the heat transfer principle, and the mathematical relationship can be expressed by the following formula (1):
Figure BDA0003095402600000212
wherein λ is a thermal conductivity of the ice layer (═ 2.22w/m · c); ρ is the density of the coolant, for example, the density of water (═ 913kg/m 3); γ is the latent heat released by the coolant when it freezes to ice at zero degrees, for example the latent heat released by water when it freezes to ice at zero degrees (═ 335000J/kg);
Figure BDA0003095402600000216
is the temperature gradient of the inner cylindrical surface of the micro-element ring. The heat transfer effect of the outer cylindrical surface of the micro-ring is not considered in the formula because it is assumed that the temperature of the external water is constant at zero degree, and the temperature gradient is inevitably zeroThe reason is that. After mathematical processing is carried out on the expression (1), the relation between the icing time t and the outer radius R of the ice layer represented by the expression (4) can be obtained.
Figure BDA0003095402600000213
Figure BDA0003095402600000214
Figure BDA0003095402600000215
2R for condenser heat exchange tube outer diameter data0The icing thickness (R-R) at different inlet coolant temperatures can be calculated by substituting 22.22mm and various physical constants into the formula (4)0) And the relation curve with the icing time t is shown in fig. 3, so that whether the outer surface of the cooling pipe is iced or the thickness of the ice layer on the outer surface of the cooling pipe of the condenser is predicted or the time length of icing on the outer surface of the cooling pipe at the current inlet coolant temperature is judged.
As can be seen from fig. 3, 3 rules can be seen: 1) for a fixed inlet coolant temperature (e.g. T)w-2.5 ℃), the thickness of the ice increases with the passage of the icing time; 2) the growth rate of the ice layer gradually slows down with the passage of time (as shown by the slope of the curve becoming smaller), because the temperature difference of the ice layer is constant, and the temperature gradient (driving force of heat conduction) gradually decreases as the ice layer grows thicker; 3) the lower the inlet coolant temperature, the faster the rate of icing, since the temperature gradient of the ice layer is proportional to the square root of the inlet coolant temperature.
In some embodiments, according to the photograph of the actual condenser ice returned from the site, ice blocks with the thickness of about 300mm are formed between the steam channels (the channels shaped like slits) at the upper part of the tube bundle, and the icing time according to the formula (4) is at least about 1 week, which is basically consistent with the actual investment time of the spraying water system. It can be concluded from this that the main reason for icing is due to the continuous dosing of the spray water system during a period of approximately one week with inlet coolant temperature below zero.
In addition, it should be noted that: in the actual icing process of the condenser, the temperature of steam (condensed water) sprayed from the upper part is generally higher than 0 ℃, so that sensible heat (approximately equal to 4180J/kg. ℃) needs to be released firstly during icing, then latent heat is released to solidify into ice, and the actual icing speed of the condenser is slower than the calculation result of the model.
From the above calculation, it is understood that the lower the temperature of the inlet coolant of the condenser cooling pipe is, the higher the speed of freezing of the steam on the outer surface of the cooling pipe is, because the temperature gradient of the ice layer is proportional to the square root of the temperature of the inlet coolant. Therefore, when the outer surface of the cooling pipe is judged to be frozen at the current inlet coolant temperature according to the freezing prediction model, part of the outlet coolant heated by the cooling pipe flows back to the inlet of the cooling pipe, the temperature of the coolant entering the inlet of the cooling pipe of the condenser is increased, and the speed of freezing of steam on the outer surface of the cooling pipe can be effectively relieved.
In the present embodiment, in order to achieve optimization of the hot water reflux ratio, as shown in fig. 7, the control module includes a reflux ratio calculation module and a reflux ratio control module.
The reflux ratio calculation module is used for detecting the inlet coolant temperature of the cooling pipe before reflux, the outlet coolant temperature of the cooling pipe before reflux and the inlet coolant temperature of the cooling pipe after reflux, and calculating the reflux ratio according to the reflux ratio model. Wherein the reflux ratio model is: the reflux ratio (inlet coolant temperature of the cooling pipe after the reflux-inlet coolant temperature of the cooling pipe before the reflux)/(outlet coolant temperature of the cooling pipe before the reflux-inlet coolant temperature of the cooling pipe before the reflux).
And the reflux ratio control module is used for controlling the reflux device to reflux the outlet coolant part heated by the cooling pipe to the inlet of the cooling pipe according to the reflux ratio.
The calculation of the inlet coolant temperature at different reflux ratios for an auxiliary steam mass flow of 210kg/s at an inlet coolant temperature of minus 2.5 degrees is shown in table 1 below, which shows a comparison of the inlet coolant temperature for different hot water reflux ratios of the condenser cooling tubes:
TABLE 1
Figure BDA0003095402600000231
It can be seen from the above table that the temperature of the coolant at the inlet of the condenser cooling pipe can be effectively increased by adopting the hot water reflux measure, the reflux ratio is increased from 10% to 40%, the temperature of the coolant at the inlet can be increased from minus 2.5 ℃ to minus 0.4 ℃ on the premise that the mass flow of the auxiliary steam is not changed, the heat transferred to the coolant in the cooling pipe by the auxiliary steam can be fully utilized by adopting the hot water reflux measure, the steam temperature outside the cooling pipe can be increased by the auxiliary steam, meanwhile, the temperature of the coolant at the inlet can be increased by the hot water reflux measure, and the icing phenomenon on the outer surface of the condenser cooling pipe can be effectively eliminated.
As can be seen from the data in table 1, when a unit is started under low temperature conditions, the temperature of the water intake of the started unit may be lower than zero even if hot water is refluxed because the flow rate of the circulating water is large, and the condenser may be frozen. Therefore, from the viewpoint of the process system, it is still necessary to take appropriate measures for suppressing the ice formation on the outer surface of the condenser cooling pipe.
Thus, as shown in fig. 5, the system further comprises an auxiliary steam device; the adjusting device also comprises a judging module for judging whether the temperature of the inlet coolant of the cooling pipe after backflow is less than a threshold value, wherein the threshold value is zero degree; the control module is also used for controlling the auxiliary steam device to introduce auxiliary steam into the steam space outside the cooling pipe when the judgment module judges that the auxiliary steam device is yes.
In the present embodiment, as shown in fig. 6, the auxiliary steam device comprises an auxiliary steam inlet 5 and an auxiliary steam pipe 6 connected to the steam space outside the cooling pipe and to the auxiliary steam inlet 5. In some embodiments, the auxiliary steam device further comprises a steam valve 7 provided on the auxiliary steam pipe 6.
Although the temperature of the coolant at the inlet of the circulating water pump can be increased by the measure of hot water backflow, the temperature of the coolant at the inlet of the circulating water pump is still lower than zero, in order to prevent the outer surface of the cooling pipe of the condenser from freezing, auxiliary steam needs to be introduced when the circulating coolant enters the cooling pipe of the condenser to enable the temperature of the outer surface of the cooling pipe to be higher than zero, the freezing phenomenon at the outer surface of the cooling pipe of the condenser can be effectively avoided, the quantity of introduced steam quality needs to be calculated and determined, the waste of steam working media can be caused by the fact that the mass flow of the introduced steam is too large, and the anti-freezing effect cannot be achieved due to the fact that the mass flow of the introduced steam is too small. Meanwhile, the mass flow of the introduced steam is related to the temperature of the inlet coolant in real time, and the mass flow of the introduced auxiliary steam is dynamically adjusted according to the temperature of the inlet coolant, so that energy waste can be avoided, and the economical efficiency is improved.
The following table 2 shows basic parameters of a condenser of a nuclear power plant in a certain cold plant site, and the minimum auxiliary steam mass flow required for preventing the condenser of the nuclear power plant from freezing can be obtained according to the basic parameters of the condenser and a related calculation formula.
TABLE 2
Name of item Unit of Parameter(s)
Types of / Single back pressure, surface type
Cooling area m2 36913
Cooling tube material / Ti
Cooling tube specification / Φ22×0.5
Number of cooling tubes / 32104
Effective length of cooling pipe mm 16470
Flow rate of circulating water m3 27
Thus, as shown in FIG. 7, the control module includes an auxiliary steam mass flow calculation module and an auxiliary steam mass flow control module.
The auxiliary steam mass flow calculation module is used for calculating the minimum auxiliary steam mass flow which is introduced into the steam space for keeping the saturated steam temperature in the steam space to be greater than zero according to the temperature of the inlet coolant;
the auxiliary steam mass flow control module is used for controlling the auxiliary steam input device to introduce auxiliary steam into the steam space outside the cooling pipe through the auxiliary steam pipeline according to the minimum auxiliary steam mass flow.
As shown in fig. 8, the auxiliary steam mass flow calculation module includes an auxiliary steam mass flow control module, a heat load calculation module, and a steam flow calculation module.
The heat transfer coefficient calculation module is used for calculating first total heat transfer of the condenser according to heat balanceThermal coefficient KTAnd calculating a second overall heat transfer coefficient K of the condenser according to the inlet coolant temperature;
wherein the first overall heat transfer coefficient
Figure BDA0003095402600000251
Q is condenser heat load, W; a is the cooling area of the cooling pipe, m2Looking up a table 2; LMTD is condenser logarithm average temperature difference, degree centigrade; kTIs the first overall heat transfer coefficient, W/(m)2·K)。
Logarithmic mean temperature difference of condenser
Figure BDA0003095402600000261
tsIs the saturated steam temperature, deg.C, of the steam space under condenser pressure; t is t1Inlet coolant temperature, deg.C; t is t2Outlet coolant temperature, deg.C; delta t is the temperature rise, DEG C, of the cooling pipe coolant; delta t is the heat transfer end difference of the condenser, DEG C; LMTD is the mean temperature difference of the logarithm of the condenser, DEG C.
Condenser heat load Q ═ W × Cp×(t2-t1) (ii) a W is the current inlet coolant flow, kg/s; cpJ/(kg. K) is the specific heat capacity at the average temperature of the coolant in the cooling pipe; q is condenser heat load, W; t is t1Inlet coolant temperature, deg.C; t is t2Is the outlet coolant temperature, deg.C.
Second overall heat transfer coefficient K ═ K0βtβmβc，
Figure BDA0003095402600000262
K0Is a basic heat transfer coefficient calculated according to the outer diameter of a cooling pipe and the flow velocity in the pipe of the condenser, W/(m)2·K)；βtAn inlet coolant temperature correction factor; beta is amThe correction coefficient is the pipe material and the wall thickness of the cooling pipe; beta is acThe condenser cleaning coefficient; c. C1The following table 3 is a table for coefficients corresponding to the outer diameters of the cooling pipes; v is the average flow velocity in the cooling tube, m/s.
The heat load calculation module is used for calculating the heat load according to the first busCoefficient of heat transfer KTThe saturated steam temperature t in the steam space is calculated according to the relation equal to the second overall heat transfer coefficient KsThe heat load Q of the condenser is greater than zero;
and the steam flow calculation module is used for calculating the minimum auxiliary steam mass flow M introduced into the steam space according to the heat load Q of the condenser and by using the auxiliary steam mass flow calculation model.
Wherein, the auxiliary steam mass flow calculation model is
Figure BDA0003095402600000263
M is the minimum auxiliary steam mass flow rate, kg/s; q is condenser heat load, W; h is auxiliary latent heat of steam, J/kg.
TABLE 3
d(mm) 16～19 22～25 28～32 35～38 41～45 48～51
c1 2747 2705 2664 2623 2582 2541
In some embodiments, to obtain a more accurate first overall heat transfer coefficient KcAs shown in fig. 8, the auxiliary steam mass flow calculation module further includes a heat transfer coefficient correction module.
The heat transfer coefficient correction module is used for correcting the first overall heat transfer coefficient K by the overall heat transfer coefficient correction model according to the inlet coolant flow and the inlet coolant temperature of the cooling pipeTCorrecting to obtain a corrected first overall heat transfer coefficient Kc。
The overall heat transfer coefficient correction model is Kc＝KT×Fv×Ft，
Figure BDA0003095402600000271
Wherein, KcFor the modified first overall heat transfer coefficient, FvAn inlet coolant flow correction factor; ftAn inlet coolant temperature correction factor; vDThe flow velocity in the cooling pipe under the designed working condition is m/s; vTThe flow velocity in the cooling pipe at the current inlet coolant temperature, m/s; beta is atDCorrecting coefficient for inlet coolant temperature under design condition; beta is atTThe correction factor for the inlet coolant temperature at the current inlet coolant temperature.
In some embodiments, to obtain a more accurate saturated steam temperature t of the steam spacesAs shown in fig. 8, the auxiliary steam mass flow calculation module further includes a steam temperature correction module.
The steam temperature correction module is used for correcting the saturated steam temperature t of the steam space according to the steam temperature correction modelsCorrecting to obtain saturated steam temperature t of steam space under inlet coolant temperature and flow rate conditions when the temperature is corrected to design working conditionsc。
The steam temperature correction model is
Figure BDA0003095402600000272
Figure BDA0003095402600000273
Wherein, tscFor correcting the saturated steam temperature t of the steam space at inlet coolant temperature and flow rate to the design conditionsc，℃，t1DThe inlet coolant temperature at design conditions, DEG C; Δ tcTemperature rise at the inlet coolant temperature and flow rate conditions corrected to the design conditions is measured at DEG C; δ tcCorrecting the temperature of the inlet coolant to the heat transfer end difference at the designed working condition and under the flow condition; wDThe inlet coolant flow is kg/s under the design working condition; cpcJ/(kg. K) is the specific heat capacity at the average temperature of the coolant in the cooling pipe; x is the modified first overall heat transfer coefficient KTThe latter logarithmic mean temperature difference coefficient; e is a natural constant, about 2.72; a is the cooling area of the cooling pipe, m2And (6) looking up a table 2.
Calculating to obtain tscAnd then the corrected pressure of the condenser can be directly searched from the steam thermodynamic property table.
According to the formula, the temperature of the coolant at the inlet of the circulating water pump of a certain cold plant is at the lowest of-2.5 ℃, in order to prevent the outer surface of the cooling pipe of the condenser from being frozen, auxiliary steam is introduced into the condenser to ensure that the average temperature of the coolant in the cooling pipe is higher than 0 ℃, so that the outer surface of the cooling pipe of the condenser can be prevented from being frozen, and the minimum auxiliary steam mass flow can be obtained by calculating the temperatures of the coolants at different inlets. Table 4 below shows the estimation results of the surface temperature of the heat exchange tube at each load stage after the rush down.
TABLE 4
Figure BDA0003095402600000281
As can be seen from the above table, the auxiliary steam mass flow rate has a direct relationship with the inlet coolant temperature, i.e. the higher the inlet coolant temperature, the lower the required auxiliary steam mass flow rate. The minimum steam mass flow of the condenser icing phenomenon is calculated according to the formula for a certain type of condenser, the minimum steam mass flow is matched with different inlet coolant temperatures of a plant site, the minimum steam mass flow is used for eliminating the condenser icing phenomenon, and a control function is formed, as shown in fig. 4, the auxiliary steam mass flow is controlled in a mode of interlocking with the inlet coolant temperature according to the control function, the purpose of preventing the condenser icing can be achieved, and energy can be saved.
By implementing the invention, the following beneficial effects are achieved:
by adopting the anti-icing design scheme of the condenser cooling pipe of the nuclear power plant, the current inlet coolant temperature of the condenser cooling pipe is detected; judging whether the outer surface of the cooling pipe is frozen or not at the current inlet coolant temperature according to the freezing prediction model; if the temperature of the coolant entering the inlet of the cooling pipe of the condenser is higher than the temperature of the coolant entering the inlet of the cooling pipe of the condenser, the speed of steam icing on the outer surface of the cooling pipe can be effectively relieved or eliminated, the reactor of the nuclear power unit in a cold plant site is in a hot stop state in winter, the outer surface of the cooling pipe of the condenser cannot be iced, the safety of condenser equipment is improved, and the hidden danger caused by icing of a titanium pipe of the condenser is eliminated.
It is to be understood that the foregoing examples, while indicating the preferred embodiments of the invention, are given by way of illustration and description, and are not to be construed as limiting the scope of the invention; it should be noted that, for those skilled in the art, the above technical features can be freely combined, and several changes and modifications can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention; therefore, all equivalent changes and modifications made within the scope of the claims of the present invention should be covered by the claims of the present invention.

Claims (32)

1. A method for preventing a nuclear power condenser from icing is characterized by comprising the following steps:
s1: detecting the current inlet coolant temperature of the condenser cooling pipe;
s2: judging whether the outer surface of the cooling pipe is frozen or not at the current inlet coolant temperature according to the freezing prediction model;
s3: and if so, partially refluxing the outlet coolant heated by the cooling pipe to the inlet of the cooling pipe.
2. The method of claim 1, wherein the icing prediction model is a function of icing thickness versus icing time for different condensers at different inlet coolant temperatures.
3. The method of preventing icing in a nuclear power condenser as recited in claim 2 wherein said relationship function is
Figure FDA0003095402590000011
Wherein t is the freezing time; lambda is the thermal conductivity of the ice layer; ρ is the density of the coolant; gamma is the latent heat released by the coolant when it condenses to ice at zero degrees; t iswIs the average temperature of the coolant in the cooling tube; r is the outer radius of the ice layer; r0Is the outer radius of the cooling tube.
4. The method for preventing icing on a nuclear power condenser according to claim 1, wherein the step S3 of partially returning the outlet coolant heated by the cooling pipe to the cooling pipe inlet includes:
s31: detecting the inlet coolant temperature of the cooling pipe before backflow, the outlet coolant temperature of the cooling pipe before backflow and the inlet coolant temperature of the cooling pipe after backflow, and calculating a backflow ratio according to a backflow ratio model;
s32: and partially refluxing the outlet coolant heated by the cooling pipe to the inlet of the cooling pipe according to the reflux ratio.
5. The method for preventing the nuclear power condenser from being frozen as claimed in claim 4, wherein the reflux ratio model is as follows:
the reflux ratio (inlet coolant temperature of the cooling pipe after the reflux-inlet coolant temperature of the cooling pipe before the reflux)/(outlet coolant temperature of the cooling pipe before the reflux-inlet coolant temperature of the cooling pipe before the reflux).
6. The method of preventing icing in a nuclear power condenser according to claim 1, further comprising:
s4: judging whether the temperature of the inlet coolant of the cooling pipe after backflow is less than a threshold value;
s5: and if so, introducing auxiliary steam into the steam space outside the cooling pipe.
7. The method of preventing icing on a nuclear power condenser according to claim 6 wherein the threshold value is zero degrees.
8. The method of claim 6, wherein the step of introducing auxiliary steam into the steam space outside the cooling tube includes:
s51: calculating a minimum auxiliary steam mass flow into the steam space to maintain a saturated steam temperature in the steam space greater than zero based on an inlet coolant temperature;
s52: and introducing auxiliary steam into the steam space according to the minimum auxiliary steam mass flow.
9. The method for preventing icing on nuclear power condensers according to claim 8, wherein the step S51 includes:
s511: calculating a first overall heat transfer coefficient K of the condenser according to the heat balanceTAnd calculating a second overall heat transfer coefficient K of the condenser according to the inlet coolant temperature;
s512: according to the first overall heat transfer coefficient KTThe relation equal to the second overall heat transfer coefficient K is calculated to obtain the saturated steam temperature in the steam spacetsThe heat load Q of the condenser is greater than zero;
s513: and calculating to obtain the minimum auxiliary steam mass flow M introduced into the steam space according to the heat load Q of the condenser and by using an auxiliary steam mass flow calculation model.
10. The method for preventing icing on a nuclear power condenser according to claim 9,
the first overall heat transfer coefficient
Figure FDA0003095402590000031
Q is the condenser heat load; a is the cooling area of the cooling pipe; LMTD is the mean temperature difference of the logarithm of the condenser;
mean temperature difference of logarithm of the condenser
Figure FDA0003095402590000032
tsIs the saturated steam temperature of the steam space at condenser pressure; t is t1Is the inlet coolant temperature; t is t2Is the outlet coolant temperature;
the heat load Q of the condenser is W multiplied by Cp×(t2-t1) (ii) a W is the current inlet coolant flow; cpIs the specific heat capacity at the average temperature of the coolant in the cooling tube;
the second overall heat transfer coefficient K ═ K0βtβmβc，
Figure FDA0003095402590000033
K0The basic heat transfer coefficient is calculated according to the outer diameter of a cooling pipe of the condenser and the flow velocity in the pipe; beta is atAn inlet coolant temperature correction factor; beta is amThe correction coefficient is the pipe material and the wall thickness of the cooling pipe; beta is acThe condenser cleaning coefficient; c. C1Is a coefficient corresponding to the outer diameter of the cooling pipe; v is the average flow velocity in the cooling tube;
the auxiliary steam mass flow calculation model is
Figure FDA0003095402590000034
M is the minimum auxiliary steam mass flow; q is the condenser heat load; h is auxiliary latent heat of steam.
11. The method for preventing icing on nuclear power condensers according to claim 10, wherein the step S511 further comprises:
correcting the model for the first overall heat transfer coefficient K according to the inlet coolant flow and inlet coolant temperature of the cooling tubes and the overall heat transfer coefficientTCorrecting to obtain a corrected first overall heat transfer coefficient Kc。
12. The method for preventing icing on a nuclear power condenser according to claim 11,
the total heat transfer coefficient correction model is Kc＝KT×Fv×Ft，
Figure FDA0003095402590000041
Wherein, FvAn inlet coolant flow correction factor; ftAn inlet coolant temperature correction factor; vDThe flow velocity in the cooling pipe under the designed working condition is adopted; vTThe flow velocity in the cooling tube at the current inlet coolant temperature; beta is atDCorrecting coefficient for inlet coolant temperature under design condition; beta is atTThe correction factor for the inlet coolant temperature at the current inlet coolant temperature.
13. The method for preventing icing on a nuclear power condenser according to claim 12, wherein the step S512 further comprises:
according to the steam temperature correction model, the saturated steam temperature t of the steam spacesCorrecting to obtain saturated steam temperature t of the steam space under inlet coolant temperature and flow conditions when the temperature is corrected to the design working conditionsc。
14. The method for preventing icing on a nuclear power condenser according to claim 13,
the steam temperature correction model is
Figure FDA0003095402590000042
Wherein, t1DThe inlet coolant temperature at the design working condition; wDThe inlet coolant flow is designed under the working condition; cpcIs the specific heat capacity at the average temperature of the coolant in the cooling tube; x is the modified first overall heat transfer coefficient KTThe latter logarithmic mean temperature difference coefficient; e is a natural constant; and A is the cooling area of the cooling pipe.
15. A system for preventing a nuclear power condenser from icing is characterized by comprising a regulating device and a reflux device connected to an outlet and an inlet of a cooling pipe of the condenser;
the adjusting device comprises:
the detection module is used for detecting the current inlet coolant temperature of the condenser cooling pipe;
the prediction module is used for judging whether the outer surface of the cooling pipe is frozen or not at the current inlet coolant temperature according to the freezing prediction model;
and the control module is used for controlling the reflux device to reflux the outlet coolant part heated by the cooling pipe to the inlet of the cooling pipe when the prediction module judges that the outlet coolant part is the outlet coolant part.
16. The system for preventing icing on a nuclear power condenser according to claim 15, wherein the return device comprises a collection device, a return pipe in communication with the collection device and the cooling pipe inlet, and a return valve disposed on the return pipe and in communication with the control module;
the collecting device is communicated with the outlet of the cooling pipe, is higher than the inlet of the cooling pipe, is used for collecting the outlet coolant heated by the cooling pipe, and partially reflows the outlet coolant to the inlet of the cooling pipe through the reflow pipeline by utilizing the gravity of the height difference.
17. The system for preventing icing in a nuclear power condenser according to claim 16 wherein the collection device is a weir.
18. The system for preventing icing on nuclear power condensers according to claim 16, wherein the cooling tube inlet is a coolant transfer pump inlet connected to the cooling tube.
19. The system for preventing icing on nuclear power condensers according to claim 15, wherein the icing prediction model is a function of icing thickness versus icing time for different condensers at different inlet coolant temperatures.
20. The system for preventing icing on nuclear power condensers according to claim 19, wherein the relationship function is
Figure FDA0003095402590000051
Wherein t is the freezing time; lambda is the thermal conductivity of the ice layer; ρ is the density of the coolant; gamma is the latent heat released by the coolant when it condenses to ice at zero degrees; t iswIs the average temperature of the coolant in the cooling tube; r is the outer radius of the ice layer; r0Is the outer radius of the cooling tube.
21. The system for preventing icing on a nuclear power condenser according to claim 15, wherein the control module comprises:
the backflow ratio calculating module is used for detecting the inlet coolant temperature of the cooling pipe before backflow, the outlet coolant temperature of the cooling pipe before backflow and the inlet coolant temperature of the cooling pipe after backflow, and calculating a backflow ratio according to a backflow ratio model;
and the reflux ratio control module is used for controlling the reflux device to reflux the outlet coolant part heated by the cooling pipe to the inlet of the cooling pipe according to the reflux ratio.
22. The system for preventing the nuclear power condenser from being frozen as claimed in claim 21, wherein the reflux ratio model is as follows:
the reflux ratio (inlet coolant temperature of the cooling pipe after the reflux-inlet coolant temperature of the cooling pipe before the reflux)/(outlet coolant temperature of the cooling pipe before the reflux-inlet coolant temperature of the cooling pipe before the reflux).
23. The system for preventing icing on nuclear power condensers according to claim 15, further comprising an auxiliary steam device;
the adjusting device also comprises a judging module used for judging whether the temperature of the inlet coolant of the cooling pipe after backflow is less than a threshold value;
the control module is also used for controlling the auxiliary steam device to introduce auxiliary steam into the steam space outside the cooling pipe when the judgment module judges that the steam space is positive.
24. The system for preventing icing on nuclear power condensers according to claim 23, wherein the auxiliary steam device includes an auxiliary steam input device and an auxiliary steam line connected to the steam space outside the cooling tubes and the auxiliary steam input device.
25. The system for preventing icing on a nuclear power condenser according to claim 23 wherein the threshold value is zero degrees.
26. The system for preventing icing on a nuclear power condenser according to claim 23, wherein the control module comprises:
the auxiliary steam mass flow calculation module is used for calculating the minimum auxiliary steam mass flow which is introduced into the steam space for keeping the saturated steam temperature in the steam space to be greater than zero according to the temperature of the inlet coolant;
and the auxiliary steam mass flow control module is used for controlling the auxiliary steam input device to introduce auxiliary steam into the steam space outside the cooling pipe through the auxiliary steam pipeline according to the minimum auxiliary steam mass flow.
27. The system for preventing icing on nuclear power condensers according to claim 26, wherein the auxiliary steam mass flow calculation module includes:
a heat transfer coefficient calculation module for calculating a first overall heat transfer coefficient K of the condenser according to the heat balanceTAnd calculating a second overall heat transfer coefficient K of the condenser according to the inlet coolant temperature;
a heat load calculation module for calculating a heat transfer coefficient K according to the first overall heat transfer coefficientTThe relation equal to the second overall heat transfer coefficient K is calculated to obtain the saturated steam temperature t in the steam spacesThe heat load Q of the condenser is greater than zero;
and the steam flow calculation module is used for calculating the minimum auxiliary steam mass flow M introduced into the steam space according to the heat load Q of the condenser and by using an auxiliary steam mass flow calculation model.
28. The system for preventing icing on a nuclear power condenser of claim 27 wherein the first overall heat transfer coefficient
Figure FDA0003095402590000071
Q is the condenser heat load; a is the cooling area of the cooling pipe; LMTD is the mean temperature difference of the logarithm of the condenser;
mean temperature difference of logarithm of the condenser
Figure FDA0003095402590000072
tsIs the saturated steam temperature of the steam space at condenser pressure; t is t1Is the inlet coolant temperature;t2is the outlet coolant temperature;
the heat load Q of the condenser is W multiplied by Cp×(t2-t1) (ii) a W is the current inlet coolant flow; cpIs the specific heat capacity at the average temperature of the coolant in the cooling tube;
the second overall heat transfer coefficient K ═ K0βtβmβc，
Figure FDA0003095402590000073
K0The basic heat transfer coefficient is calculated according to the outer diameter of a cooling pipe of the condenser and the flow velocity in the pipe; beta is atAn inlet coolant temperature correction factor; beta is amThe correction coefficient is the pipe material and the wall thickness of the cooling pipe; beta is acThe condenser cleaning coefficient; c. C1Is a coefficient corresponding to the outer diameter of the cooling pipe; v is the average flow velocity in the cooling tube;
the auxiliary steam mass flow calculation model is
Figure FDA0003095402590000081
M is the minimum auxiliary steam mass flow; q is the condenser heat load; h is auxiliary latent heat of steam.
29. The system for preventing icing on nuclear power condensers according to claim 27, wherein the auxiliary steam mass flow calculation module further comprises:
a heat transfer coefficient correction module for correcting the first overall heat transfer coefficient K according to the inlet coolant flow and the inlet coolant temperature of the cooling pipe and an overall heat transfer coefficient correction modelTCorrecting to obtain a corrected first overall heat transfer coefficient Kc。
30. The system for preventing icing on nuclear power condensers according to claim 29, wherein the overall heat transfer coefficient correction model is Kc＝KT×Fv×Ft，
Figure FDA0003095402590000082
Wherein, FvAn inlet coolant flow correction factor; ftAn inlet coolant temperature correction factor; vDThe flow velocity in the cooling pipe under the designed working condition is adopted; vTThe flow velocity in the cooling tube at the current inlet coolant temperature; beta is atDCorrecting coefficient for inlet coolant temperature under design condition; beta is atTThe correction factor for the inlet coolant temperature at the current inlet coolant temperature.
31. The system for preventing icing on nuclear power condensers according to claim 30, wherein the auxiliary steam mass flow calculation module further comprises:
a steam temperature correction module for correcting the saturated steam temperature t of the steam space according to the steam temperature correction modelsThe correction is carried out so that the correction is carried out,
obtaining a saturated steam temperature t of the steam space at inlet coolant temperature and flow rate corrected to design conditionssc。
32. The system for preventing icing on nuclear power condensers according to claim 31, wherein the steam temperature correction model is
Figure FDA0003095402590000091
Wherein, t1DThe inlet coolant temperature at the design working condition; wDThe inlet coolant flow is designed under the working condition; cpcIs the specific heat capacity at the average temperature of the coolant in the cooling tube; x is the modified first overall heat transfer coefficient KTThe latter logarithmic mean temperature difference coefficient; e is a natural constant; and A is the cooling area of the cooling pipe.
CN202110609441.1A 2021-06-01 2021-06-01 Method and system for preventing nuclear power condenser from freezing Pending CN113418405A (en)

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