Water distribution method of natural ventilation countercurrent wet cooling tower under peak regulation working condition

Abstract

The invention discloses a water distribution method of a natural ventilation countercurrent wet cooling tower under a peak regulation working condition, which comprises the following steps of: arranging an electric valve at the intersection of a water distribution tank and a water distribution pipeline by utilizing the characteristic that the peripheral cooling effect of the cooling tower is good, and controlling the flow of cooling water in the water distribution pipeline by the electric valve; the water distribution pipelines are annularly arranged, so that water distribution control of inner and outer ring flow can be better carried out, and meanwhile, larger outer ring water distribution flow can be obtained; when the circulating water flow is reduced, the number of the closed electric valves is determined according to a cooling tower water distribution control system, and the electric valves on the inner side are sequentially closed from inside to outside, so that the pressure head of the nozzle is also improved, and the sprayed cooling water can better cover the whole filler area; the atomizing nozzles are used, the contact area of water and air is increased, and the heat exchange efficiency of the water distribution area is improved; the direct impact of water flow on the filler can be relieved to a certain extent, and the damage risk of the filler is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of cooling tower water distribution, and in particular to a water distribution method for a natural ventilation countercurrent wet cooling tower under peak load conditions. Background Art

[0002] Natural ventilation countercurrent wet cooling tower is a cooling equipment widely used in power and other industries. Its working principle is to exchange heat between hot water and air through natural ventilation, so that the hot water can be cooled and recycled. In this process, the performance of the water distribution system directly affects the cooling efficiency and operation stability of the cooling tower.

[0003] At present, the main water distribution methods for the operation of the unit are full-tower water distribution and peripheral water distribution. The nozzles of the full-tower water distribution are distributed throughout the cooling tower to ensure that the water can be evenly sprinkled on the filler. This water distribution method is often used in normal operating conditions and situations where high cooling effects are required. Full-tower water distribution can make full use of the entire cooling area of ​​the cooling tower, ensure high cooling efficiency, maintain a high vacuum in the condenser, and enable the unit to operate stably. Peripheral water distribution only distributes circulating water to the peripheral area of ​​the cooling tower, that is, the part close to the wall of the cooling tower. In low-temperature environments such as winter or when operating at partial load, the heat demand inside the cooling tower is relatively reduced, and the cooling capacity of the entire tower is not required. At this time, the use of peripheral water distribution can reduce unnecessary water evaporation and heat loss inside, thereby achieving the purpose of energy saving.

[0004] In actual operation, due to changes in production processes, seasonal changes and other factors, especially the impact of peak load regulation of power plants, the circulating water flow rate of the cooling tower will change. The traditional natural ventilation countercurrent wet cooling tower water distribution system has limited regulation capabilities and is difficult to adapt to different working conditions and load changes. The water distribution system needs to be able to flexibly adjust the water flow distribution to ensure the best cooling effect.

[0005] In view of the problems existing in the prior art, the present invention provides a water distribution method for a natural ventilation countercurrent wet cooling tower under peak load conditions.

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

[0007] A water distribution method for a natural ventilation countercurrent wet cooling tower under peak load conditions comprises the following steps:

[0008] Step 1) The cooling tower is divided into four symmetrical sector areas with the central shaft as the center and the mutually perpendicular water distribution tanks as the boundaries;

[0009] Step 2) arranging the water distribution pipes in the cooling tower in a ring shape, and the water distribution pipes are respectively connected to the water distribution tanks;

[0010] Step 3) An electric valve is provided at the intersection of the water distribution tank and the water distribution pipeline, and the electric valve is used to control the flow of cooling water in the water distribution pipeline;

[0011] Step 3) When the unit is in peak load operation, the circulating water flow rate is reduced, and the number of electric valves to be closed is determined according to the cooling tower water distribution control system, and the inner electric valves are closed in sequence from the inside to the outside, so as to fully utilize the cooling capacity of the outer side to cool the circulating cooling water, and at the same time increase the pressure head of the nozzle so that the sprayed cooling water can better cover the entire packing area;

[0012] Step 4) When the circulating water flow rate increases, the number of electric valves to be opened is determined according to the cooling tower water distribution control system, and the outer electric valves are opened in sequence from the outside to the inside, making full use of the cooling capacity of the outside to cool the circulating cooling water, while increasing the nozzle pressure head so that the sprayed cooling water can better cover the entire filler area.

[0013] Furthermore, an atomizing nozzle is provided on the water distribution pipeline to increase the contact area between water and air and improve the heat exchange efficiency of the water distribution area.

[0014] Furthermore, the cooling tower water distribution control system control method comprises the following steps:

[0015] Step 1) Establish a cooling tower model and divide the fluid grid according to the actual cooling tower parameters, and construct the water distribution area, filler area and rain area models;

[0016] Step 2) establishing the relationship between the circulating water outlet temperature and each valve opening, circulating water flow, inlet temperature, and ambient temperature, wind direction, and wind speed; that is, setting each valve opening, circulating water flow, inlet temperature, and ambient temperature, wind direction, and wind speed control parameters, using the circulating water outlet temperature calculated by numerical simulation as the output result, thereby establishing a database through a neural network algorithm to provide a data source for the control of cooling tower water distribution;

[0017] Step 3) The data obtained by the numerical simulation are verified using the corresponding monitoring point data during the actual operation of the cooling tower to increase the reliability of the numerical simulation results;

[0018] Step 4) Based on the data in the database, the monitoring point parameters before the cooling tower water distribution control are used as input parameters, and the circulating water outlet temperature is controlled by controlling the valve opening.

[0019] Furthermore, the specific process of step 1) is as follows:

[0020] Step 1.1, carry out proportional 3D geometric modeling of cooling tower according to CAD engineering drawing. Under normal operating conditions, the power of air flow in the tower comes from the density difference between the air inside and outside the tower; set a cylindrical calculation domain to simulate the environment outside the tower, the boundaries on both sides of the calculation domain outside the tower are set as pressure inlet, the top of the calculation domain is set as pressure outlet, and the bottom is set as wall. The pressure and temperature of the inlet and outlet of the cooling tower are taken as environmental values; the boundary conditions of droplets in the tower are: the inner wall of the tower is set as the reflection boundary, and the pool surface at the bottom of the tower is set as the escape boundary, so as to capture droplets from the water distribution surface;

[0021] Step 1.2: The air entering the cooling tower is regarded as a continuous medium, and the physical process of the air in the cooling tower is described by the NS equations; in the three areas of the water distribution area, the filling area and the rain area, the circulating water in the water distribution area and the rain area transfers heat and mass with the air in the form of droplets, which is described by a discrete phase model based on the Lagrangian method; the water distribution area of ​​the cooling tower is artificially divided into a corresponding number of water distribution areas according to the number of water distribution pipes, and the water distribution surface is equally divided along the radial direction;

[0022] Step 1.3: When circulating water droplets flow through the packing area, they extend on the packing surface to form a liquid film. The circulating water droplets exchange heat with the air in the form of a liquid film inside the packing. The interaction between the air and water movement, including the heat transfer, mass transfer and momentum exchange processes, is realized as the source term of the air-water control equation.

[0023] Furthermore, the numerical simulation in step 2) adopts SIMPLE algorithm, the turbulence model is the standard k-ε model, and the component transport equation is applied to realize the change of air humidity.

[0024] The design ideas of the present invention are as follows:

[0025] The present invention utilizes the good cooling effect of the outer periphery of the cooling tower, and sets an electric valve at the intersection of the water distribution trough and the water distribution pipe, so that the electric valve controls the flow of cooling water in the water distribution pipe. The water distribution pipe is arranged in a ring, so that the water distribution control of the inner and outer circle flow can be better performed, and a larger outer circle water distribution flow can be obtained at the same time. When the circulating water flow rate is reduced, the number of closed electric valves is determined according to the cooling tower water distribution control system, and the inner electric valves are closed in sequence from the inside to the outside, which also increases the pressure head of the nozzle, so that the sprayed cooling water can better cover the entire packing area. Use an atomizing nozzle to increase the contact area between water and air and improve the heat exchange efficiency of the water distribution area. To a certain extent, the direct impact of water flow on the packing can be alleviated, reducing the risk of damage to the packing.

[0026] Beneficial effects of the present invention:

[0027] 1) The present invention can better control the water distribution of the inner and outer circle flows and obtain a larger outer circle water distribution flow through the provision of an annular water distribution pipeline.

[0028] 2) The present invention sets an electric valve at the intersection of the water distribution trough and the water distribution pipeline. By controlling the electric valve under peak load conditions, the cooling capacity of the outside is fully utilized to cool the circulating cooling water, ensuring the pressure head of the nozzle so that the sprayed cooling water can better cover the entire filler area.

[0029] 3) The present invention uses an atomizing nozzle to increase the contact area between water and air and improve the heat exchange efficiency of the water distribution area; to a certain extent, it can alleviate the direct impact of water flow on the filler and reduce the risk of damage to the filler.

[0030] 4) The present invention does not have a strict division between inner and outer circles. The inner and outer circles can be divided according to the range of the water distribution area, that is, the area where the electric valve is opened, which is more flexible. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the arrangement of the water distribution pipeline inside the cooling tower of the present invention;

[0032] Figure 2 It is a control flow chart of the present invention. DETAILED DESCRIPTION

[0033] The present invention is further described below in conjunction with the accompanying drawings.

[0034] like Figure 1 As shown, the central shaft is used as the center and the mutually perpendicular water distribution troughs are used as boundaries to divide the area into four symmetrical sectors. Figure 1 The water distribution pipe is arranged in a circular shape to better control the water distribution flow of the inner and outer circles, and at the same time obtain a larger water distribution flow of the outer circle. An electric valve is set at the intersection of the water distribution trough and the water distribution pipe to control the water distribution flow of the water distribution area.

[0035] When the unit is in peak load operation, the circulating cooling water flow is low. Since the cooling coefficient on the outside of the cooling tower is higher than that on the inside, the cooling capacity on the outside is fully utilized to cool the circulating cooling water. Due to the reduction in the circulating water flow, the water flow in the water distribution pipe will decrease, the nozzle pressure head will decrease, and the distribution of the water distribution will lead to poor cooling effect. Therefore, the number of electric valves to be closed is determined according to the cooling tower water distribution control system, and the inner electric valves are closed from the inside to the outside. When the circulating water flow increases, in order to avoid excessive pressure head in the nozzle, the outer electric valves are opened from the outside to the inside in the same way. This method can not only fully utilize the cooling capacity on the outside to cool the circulating cooling water, but also make the pressure in the water distribution pipe higher, thereby increasing the nozzle pressure head, so that the sprayed cooling water can better cover the entire packing area.

[0036] In addition, the circulating cooling water passes through the water distribution area, filler area and rain area in the cooling tower in turn, and the use of atomizing nozzles increases the contact area between water and air and improves the heat exchange efficiency of the water distribution area. To a certain extent, it can alleviate the direct impact of water flow on the filler and reduce the risk of damage to the filler.

[0037] The main heat and mass transfer areas of the natural ventilation countercurrent wet cooling tower include the water distribution area, the filler area and the rain area. Under windless conditions, the cooling loads of the water distribution area, the filler area and the rain area account for 10%, 70% and 20% of the total load of the cooling tower, respectively. Among them, the filler area is the most important cooling area, and its thermal performance has an important influence on the thermal performance of the entire tower. The water distribution area is the starting area for heat and mass transfer between air and water in the cooling tower, which determines the spatial distribution of circulating water in the tower. The rain area is the heat and mass exchange area that the outside air first contacts and enters, and it is the key area for uniform ventilation and wind distribution throughout the tower.

[0038] When the circulating cooling water flow is low, the water density of the whole tower water distribution does not reach the rated value due to the large spraying area, which will cause the circulating water droplets in the unit volume of filler to be unable to cover the filler surface, and the cooling capacity of the unit volume of filler cannot be fully utilized. The peripheral water distribution spray area has a larger water density, which is closer to the rated water density of the filler, and its overall cooling effect will be better than the whole tower water distribution.

[0039] Therefore, in the case of deep peak-shaving conditions with low loads and only a single low-speed circulating water pump is enabled, the cooling tower adopts an outer ring water distribution operation mode, which is more economical. However, the inner and outer ring divisions in the current water distribution method are fixed. The existing large-scale natural ventilation cooling tower water distribution layout is: inner square and outer circle, with a copper coin pattern water distribution layout. The inner zone is from the central shaft to 1 / 2 of the radius, which is a square of 1 / 2 radius by 1 / 2 radius. The outer zone is from 1 / 2 of the radius to the inner wall of the outer edge of the cooling tower, and its shape is the remaining part of the circle minus the square. The water distribution area of ​​the inner zone is about 32%, and the water distribution area of ​​the outer zone is about 68%. That is, the water distribution volume of the outer zone is much larger than that of the inner zone.

[0040] The proportional three-dimensional geometric modeling of the cooling tower is carried out according to the CAD engineering drawing. Under normal operating conditions, the power of the air flow in the tower comes from the density difference of the air inside and outside the tower. At the same time, a cylindrical calculation domain is set to simulate the environment outside the tower. The boundaries on both sides of the calculation domain outside the tower are set as pressure inlets, the top of the calculation domain is set as a pressure outlet, and the bottom is set as a wall. The pressure and temperature of the cooling tower inlet and outlet are both taken as environmental values. The boundary conditions of the droplets (discrete phase) in the tower are: the inner wall of the tower is set as a reflection boundary, and the pool surface at the bottom of the tower is set as an escape boundary to capture droplets from the water distribution surface.

[0041] The air entering the cooling tower is regarded as a continuous medium, and the physical process of the air in the cooling tower can be described by the NS equations;

[0042] The movement of air in the cooling tower is considered to be continuous, and its heat flow characteristics in the tower can be described by the continuity equation, momentum equation and energy equation, which have the general form:

[0043]

[0044] Where ρ is the density of the continuous phase, kg / m 3 ;u i is the velocity value in each direction, m / s; ψ is the general form of the variable; Γ is the generalized diffusion coefficient; S i is the source item.

[0045] In the three areas of water distribution area, filling area and rain area, the circulating water in the water distribution area and rain area transfers heat and mass with the air in the form of droplets, which can be described by a discrete phase model based on the Lagrangian method. The water distribution area of ​​the cooling tower is artificially divided into a corresponding number of water distribution areas according to the number of water distribution pipes, and the water distribution surface is equally divided along the radial direction.

[0046] When circulating water droplets flow through the packing area, they extend on the packing surface to form a liquid film. The circulating water droplets exchange heat with the air in the form of a liquid film inside the packing. The interaction between air and water movement, including processes such as heat transfer, mass transfer and momentum exchange, is realized as the source term of the air-water control equation.

[0047] The heat and mass transfer in the packing area is relatively complex. According to the thermal parameters of the packing and experimental data, the mass transfer coefficient in the packing area can be expressed as:

[0048] β xv =Bg m q n

[0049] In the formula, β xv is the internal mass transfer coefficient, kg / (m 3 ·s); g is the mass flow rate of wet air, kg / (m 2 ·s); q is the water density, kg / (m 2 ·s); B, m, n are experimental constants.

[0050] The evaporation rate per unit volume of filler is calculated as follows:

[0051] m evp =β xv (ω sat,Tw -ω a )

[0052] In the formula, m evp is the evaporation per unit volume of filler, kg / (m 3 ·s);ω sat，Tw is the saturated humidity content of wet air at the circulating water drop temperature; ω a The humidity content of humid air.

[0053] The resistance of the packing area can be expressed as:

[0054]

[0055] Where: ΔP is the resistance of the packing area, N / m 2 ; γ a is the specific gravity of moist air, N / m 3 ; v is the wet air velocity, m / s; M is the empirical coefficient determined by the test.

[0056] Figure 2 The flowchart of the cooling tower water distribution control system of the present invention generally includes two parts: a database establishment process and a cooling tower water distribution control process. The database establishment is to establish the relationship between the circulating water outlet temperature and each valve opening, circulating water flow, inlet temperature and the temperature, wind direction and wind speed of the environment in the cooling tower water distribution system through numerical simulation, and the latter result is obtained by changing the former. Using the database to obtain a large amount of data in advance can greatly improve the control efficiency in the actual cooling tower water distribution process.

[0057] The SIMPLE algorithm is used in the numerical simulation, the standard k-ε model is used as the turbulence model, and the component transport equation is applied to realize the change of air humidity.

[0058] The specific process is as follows: First, according to the actual cooling tower parameters, a cooling tower model is established and the fluid grid is divided. For a specific cooling tower, the model is also fixed; secondly, the water distribution area, filler area and rain area models are constructed; then, the valve opening, circulating water flow, inlet temperature, and ambient temperature, wind direction, wind speed, etc. are set as control parameters, and the circulating water outlet temperature obtained by numerical simulation is used as the output result, so as to establish a database through the neural network algorithm to provide a data source for the control of cooling tower water distribution. For the data obtained by numerical simulation, it is necessary to use the corresponding monitoring point data during the actual operation of the cooling tower for verification to increase the reliability of the numerical simulation results. The second part is the cooling tower water distribution control process, which takes the monitoring point parameters before the cooling tower water distribution control as input parameters, and controls the circulating water outlet temperature by controlling the valve opening based on the data in the database.

Claims

1. A water distribution method for a natural ventilation countercurrent wet cooling tower under peak load conditions, characterized in that: The steps include: Step 1) The cooling tower is divided into four symmetrical sector areas with the central shaft as the center and the mutually perpendicular water distribution tanks as the boundaries; Step 2) arranging the water distribution pipes in the cooling tower in a ring shape, and the water distribution pipes are respectively connected to the water distribution tanks; Step 3) An electric valve is provided at the intersection of the water distribution tank and the water distribution pipeline, and the electric valve is used to control the flow of cooling water in the water distribution pipeline; Step 3) When the unit is in peak load operation, the circulating water flow rate is reduced, and the number of electric valves to be closed is determined according to the cooling tower water distribution control system, and the inner electric valves are closed in sequence from the inside to the outside, so as to fully utilize the cooling capacity of the outer side to cool the circulating cooling water, and at the same time increase the pressure head of the nozzle so that the sprayed cooling water can better cover the entire packing area; Step 4) When the circulating water flow rate increases, the number of electric valves to be opened is determined according to the cooling tower water distribution control system, and the outer electric valves are opened in sequence from the outside to the inside, making full use of the cooling capacity of the outside to cool the circulating cooling water, while increasing the nozzle pressure head so that the sprayed cooling water can better cover the entire filler area.

2. The water distribution method for a natural ventilation countercurrent wet cooling tower under peak load conditions according to claim 1, characterized in that: The water distribution pipeline is provided with an atomizing nozzle to increase the contact area between water and air and improve the heat exchange efficiency of the water distribution area.

3. The water distribution method for a natural ventilation countercurrent wet cooling tower under peak load conditions according to claim 1, characterized in that: The cooling tower water distribution control system control method comprises the following steps: Step 1) Establish a cooling tower model and divide the fluid grid according to the actual cooling tower parameters, and construct the water distribution area, filler area and rain area models; Step 2) establishing the relationship between the circulating water outlet temperature and each valve opening, circulating water flow, inlet temperature, and ambient temperature, wind direction, and wind speed; that is, setting each valve opening, circulating water flow, inlet temperature, and ambient temperature, wind direction, and wind speed control parameters, using the circulating water outlet temperature calculated by numerical simulation as the output result, thereby establishing a database through a neural network algorithm to provide a data source for the control of cooling tower water distribution; Step 3) The data obtained by the numerical simulation are verified using the corresponding monitoring point data during the actual operation of the cooling tower to increase the reliability of the numerical simulation results; Step 4) Based on the data in the database, the monitoring point parameters before the cooling tower water distribution control are used as input parameters, and the circulating water outlet temperature is controlled by controlling the valve opening.

4. The water distribution method for a natural ventilation countercurrent wet cooling tower under peak load conditions according to claim 3, characterized in that: The specific process of step 1) is as follows: Step 1.1, carry out proportional 3D geometric modeling of cooling tower according to CAD engineering drawing. Under normal operating conditions, the power of air flow in the tower comes from the density difference between the air inside and outside the tower; set a cylindrical calculation domain to simulate the environment outside the tower, the boundaries on both sides of the calculation domain outside the tower are set as pressure inlet, the top of the calculation domain is set as pressure outlet, and the bottom is set as wall. The pressure and temperature of the inlet and outlet of the cooling tower are taken as environmental values; the boundary conditions of droplets in the tower are: the inner wall of the tower is set as the reflection boundary, and the pool surface at the bottom of the tower is set as the escape boundary, so as to capture droplets from the water distribution surface; Step 1.2: The air entering the cooling tower is regarded as a continuous medium, and the physical process of the air in the cooling tower is described by the NS equations; in the three areas of the water distribution area, the filling area and the rain area, the circulating water in the water distribution area and the rain area transfers heat and mass with the air in the form of droplets, which is described by a discrete phase model based on the Lagrangian method; the water distribution area of ​​the cooling tower is artificially divided into a corresponding number of water distribution areas according to the number of water distribution pipes, and the water distribution surface is equally divided along the radial direction; Step 1.3: When circulating water droplets flow through the packing area, they extend on the packing surface to form a liquid film. The circulating water droplets exchange heat with the air in the form of a liquid film inside the packing. The interaction between the air and water movement, including the heat transfer, mass transfer and momentum exchange processes, is realized as the source term of the air-water control equation.

5. The water distribution method for a natural ventilation countercurrent wet cooling tower under peak load conditions according to claim 3, characterized in that: In the step 2), the numerical simulation adopts SIMPLE algorithm, the turbulence model is the standard k-ε model, and the component transport equation is applied to realize the change of air humidity.

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