CN116384017B - Design method of dry-wet combined cooling tower - Google Patents

Design method of dry-wet combined cooling tower Download PDF

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CN116384017B
CN116384017B CN202310664511.2A CN202310664511A CN116384017B CN 116384017 B CN116384017 B CN 116384017B CN 202310664511 A CN202310664511 A CN 202310664511A CN 116384017 B CN116384017 B CN 116384017B
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air
water
tower
wet
dry
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CN116384017A (en
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徐清华
张早校
张强
刘沐林
陈刚
宋建功
徐文文
高翔
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Shandong Lanxiang Environmental Technology Co ltd
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    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F30/00—Computer-aided design [CAD]
    • G06F30/10—Geometric CAD
    • G06F30/17—Mechanical parametric or variational design
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F30/00—Computer-aided design [CAD]
    • G06F30/20—Design optimisation, verification or simulation
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F2119/00—Details relating to the type or aim of the analysis or the optimisation
    • G06F2119/08—Thermal analysis or thermal optimisation
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F2119/00—Details relating to the type or aim of the analysis or the optimisation
    • G06F2119/14—Force analysis or force optimisation, e.g. static or dynamic forces
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B30/00—Energy efficient heating, ventilation or air conditioning [HVAC]
    • Y02B30/70—Efficient control or regulation technologies, e.g. for control of refrigerant flow, motor or heating

Abstract

The application discloses a design method of a dry-wet combined cooling tower, which is characterized in that annual evaporation loss qy is compared with evaporation loss qn of the dry-wet combined cooling tower, if qy > qn is calculated, the requirement of water saving rate is not met, and the relative humidity eta is increased; if the water saving rate requirement is met, performing defogging calculation, and judging whether the defogging requirement of the cooling tower is met; and performing resistance calculation and fan model selection, and finally outputting a design result. Has the following advantages: the annual water saving rate is used as a guide to calculate the structural parameters and the operating parameters of the cooling tower more accurately, so that the parameters such as the air parameters of the cooling tower, the evaporation loss and the like are obtained, the calculation of the water loss and the calculation of the defogging effect of the cooling tower can be accurately achieved, the design of the cooling tower is optimized, the evaporation water loss is accurately controlled, and the better water saving effect of the cooling tower is achieved.

Description

Design method of dry-wet combined cooling tower
Technical Field
The application relates to a design method of a dry-wet combined cooling tower, and belongs to the technical field of cooling tower control.
Background
In recent years, the economic and productivity development is rapid, and the industrial development of coal chemical industry, petrochemical industry, power plants and the like is rapid, but the problem of water resources is also urgent. The dry-wet combined cooling tower has high heat efficiency, water saving and fog eliminating capacity and wide market prospect. However, in the existing design calculation method, in order to simplify the calculation process, most of the design calculation method assumes that the air of the wet section outlet tower is in a saturated state, the heat change caused by the water change is ignored, and most of the calculation of the evaporation loss adopts coefficient method calculation, so that the calculation of the evaporation loss and the air state parameter of the outlet tower is inaccurate.
Disclosure of Invention
Aiming at the defects, the application provides a design method of a dry-wet combined cooling tower, which takes the annual water saving rate as a guide to calculate the structural parameters and the operating parameters of the cooling tower more accurately, obtains more accurate parameters such as the air parameters of the outlet tower, the evaporation loss and the like, and can accurately calculate the water loss and the defogging effect of the cooling tower, thereby optimizing the design of the cooling tower, accurately controlling the evaporation water loss and achieving better water saving effect of the cooling tower.
In order to solve the technical problems, the application adopts the following technical scheme:
a design method of a dry-wet combined cooling tower comprises the steps of combining annual evaporation loss qy with evaporation loss q of the dry-wet combined cooling tower n Comparing, if qy>q n If the water saving rate is not satisfied, the relative humidity eta is increased;
if the water saving rate requirement is met, performing defogging calculation, and judging whether the defogging requirement of the cooling tower is met; resistance calculation and fan shape selection are carried out, and finally, a design result is output;
calculating evaporation loss q of dry-wet combined cooling tower according to relative water saving rate n ；
The annual evaporation loss is obtained by summing the non-summer evaporation loss and the summer evaporation loss, the dry section and the wet section of the cooling tower are operated simultaneously when the non-summer evaporation loss is calculated, the outlet air temperature of the dry section and the inlet water temperature of the wet section are obtained through the heat exchange efficiency of the dry section, and then the evaporation water loss of the wet section is calculated; when the evaporation loss in summer is calculated, the cooling tower only runs for cooling in a wet section, and the evaporation water loss in the wet section is calculated;
the annual evaporation water loss calculation under the condition of pure wet section of the cooling tower is carried out by an iteration method, and the specific method comprises the following steps: in order to solve the differential equation set of air and water state parameters, the temperature of cooling water in a tower is divided into a plurality of small differential units along the water flow direction, the water outlet mass of the tower is obtained according to mass conservation on the assumption of the water outlet air moisture content omega o of the tower, and the air moisture content and the air enthalpy value of each differential unit are obtained through differential equation differentiation of heat transfer and mass transfer until the water outlet air moisture content of the tower is obtained through recalculation, so that preparation is made for the next iteration condition.
Further, the method comprises the steps of:
step S1: determining ambient weather conditions, given calculated initial air conditions;
the environmental meteorological conditions include: the ambient atmospheric pressure Pa, the ambient air dry bulb temperature theta and the ambient air wet bulb temperature tau in the whole year, and the relative humidity phi, the saturated steam partial pressure ptheta corresponding to the dry bulb temperature and the moisture content omega of the inlet air are calculated according to a thermodynamic calculation formula i The enthalpy value hi of the inlet air and the density ρi of the inlet wet air;
step S2: determining a cooling task and a water-saving task, and giving the design requirement of a cooling tower;
the cooling tasks include: the cooling circulating water quantity q of the single tower, the water temperature t1 of the circulating water entering the tower and the water temperature t2 of the water exiting the tower; the water-saving tasks include: a relative water saving rate epsilon;
step S3: determining the structure and operating parameters of the finned tube and the structural parameters of the tower;
step S4: and determining the performance parameters of the filler.
Further, the method comprises the following steps:
step S5: calculating annual evaporation water loss under the condition of pure wet sections of the cooling tower, and giving the water loss before dry-wet combined water-saving design;
assuming the wet section air-water ratio lambda, obtaining characteristic number through thermodynamic performance parameterWherein A and n are thermal performance parameters of the filler of the cooling tower;
the formula for calculating the air moisture content is as follows:
；
the calculation formula of the air enthalpy value is as follows:
；
wherein eta is the relative humidity,the specific heat of water, omega is the air moisture content of each small-section differential unit, t is the cooling water inlet temperature of each small-section differential unit, q is the mass of cooling water, dt is the difference between the cooling water inlet and outlet temperatures of each small-section differential unit>The saturated air moisture content corresponding to the water temperature is G is air quantity, < >>For the saturated air enthalpy corresponding to the water temperature, γ is the latent heat of evaporation of water, ++>Is the saturated moisture content of air, +.>Is the saturation enthalpy of air; />The Lewis number is a proportionality constant for representing mass transfer and heat transfer, and the specific values are as follows: />。
Further, the calculated air moisture content is used as a starting condition of a new iteration to be recalculated until the difference between the calculated tower outlet air moisture content and the assumed air moisture content meets the solving accuracy, and then the cooling number is obtained by an integral method:
wherein t1 is the water temperature of the tower entering and t2 is the water temperature of the tower exiting;
the air-water ratio is regulated by changing the air quantity entering the tower, so that the difference between N1 and N2 meets the accuracy, and the evaporation loss under the pure wet operation is obtained。
Further, the method comprises the following steps:
step S6: calculating the evaporation loss of the dry-wet combined cooling tower according to the relative water saving rateAssuming the dry-wet ratio delta of the dry-wet combined cooling tower, obtaining the heat load of the dry section of the dry-wet combined cooling tower>Tower outlet water temperature of dry section +.>。
Further, the method comprises the following steps:
step S7: carrying out heat transfer calculation of a dry section of the dry-wet combined cooling tower, solving to obtain the length of the finned tube, wherein the calculation of the length of the finned tube comprises the following steps:
obtaining a heat transfer coefficient;
the heat transfer coefficient is calculated by adopting the heat transfer coefficient obtained by experiments on the premise of being capable of carrying out the experiments, and if no experimental data exists, the heat transfer coefficient can be estimated according to the following calculation process:
the convective heat transfer coefficient outside the tube is obtained by the property parameters of the air and the structural parameters of the finned tube,wherein->Is the heat conductivity coefficient of air, db is the outer diameter of the finned tube, ρ is the air density, vmax is the flow velocity of air at the narrowest position of the finned tube bundle, m is the dynamic viscosity of air, P is the spacing of the fins, T is the thickness of the fins, df is the outer diameter of the fins, pra is the prandtl number of air;
obtaining the convection heat transfer coefficient in the finned tube from the property parameters of water and the flow velocity of waterWherein->Is the heat conductivity coefficient of water, di is the inner diameter of the finned tube, re is the Reynolds number of water, and Prw is the Plantt number of water;
recalculating the total heat transfer coefficient of the finned tubeWherein Ro, ri and Rw are respectively the thermal resistances of dirt outside the tube, dirt inside the tube and the tube wall, and beta is the fin ratio of the fin tube.
Further, the fin tube length calculation further includes the steps of:
obtaining the heat exchange area of the finned tubeWherein Tln is the log mean temperature; then, the fin tube length is determined>Wherein n is 1 For the number of tube rows, n 2 Comparing the calculated fin tube length L with the assumed fin tube length L0 for each row of tubes, and re-assuming if the difference is too largeThe fin tube length is calculated until accuracy is met.
Further, the method comprises the following steps:
step S8: performing non-summer evaporation loss calculation, and preparing for calculating annual evaporation loss;
at this time, the dry section and the wet section run simultaneously, and the number of heat transfer units is calculated firstWherein Cmin is the smaller value of the heat capacity flow rates of air and water, A is the heat exchange area, and the heat exchange efficiency is calculated again>Where e is a natural constant and C is the ratio of the smaller to the larger of the heat capacity rates of air and water. Calculating the air temperature of the tower outlet at the dry section through heat exchange efficiencyWater temperature entering the tower of the wet section +.>The method comprises the steps of carrying out a first treatment on the surface of the t4 replaces t1, and then the evaporation loss and the air tower outlet state parameters of the wet section are solved according to the step S5;
step S9: performing evaporation loss calculation in summer, preparing for calculating annual evaporation loss, at the moment, only operating wet section cooling, and enabling the water temperature entering a tower to be t1, wherein the solving method is the same as that of the step S5;
step S10: checking evaporation loss, and judging whether the requirement of water saving rate is met or not: summing the evaporation losses calculated in the step S8 and the step S9 to obtain annual evaporation loss qy and the evaporation loss q of the dry-wet combined cooling tower n Comparing, if qy>q n Then, η is increased, and the process returns to step S6 to perform calculation.
Further, the method comprises the following steps:
step S11: performing defogging calculation, and judging whether defogging requirements of a cooling tower are met or not: solving the mixed air quantity, air temperature and moisture content by combining the tower outlet air state parameters of the dry section and the wet section through the law of mass conservation and energy conservation, judging whether the mixed air parameters and external air parameters have intersection points with saturated air lines on a temperature-humidity diagram through a mapping method, and outputting a solving result;
determining ventilation air quantity according to integral length parameters and ventilation speed of finned tube boxW is the number of finned tubes in the finned tube box, V is the ventilation speed, and the temperature of the air discharged from the tower is obtained through an energy conservation equation according to the thermal load。
Further, the method comprises the following steps:
step S12: resistance calculation and fan model selection: the total resistance of the dry-wet combined cooling tower is calculated firstWherein->For the resistance coefficient of the individual components, +.>For the air density of the air flowing through the individual components, +.>And (3) carrying out fan type selection by combining the air flow rates of the air flowing through all the components and the air volume obtained in the step (S11).
Compared with the prior art, the application has the following technical effects:
1. the heat and mass transfer equation in the cooling tower calculation is modified, in order to calculate the evaporation loss and the air state parameter of the tower more accurately, the assumption condition of the conventional technology is not adopted, and the whole equation set is solved by adopting an iteration method, so that the parameters such as the more accurate air parameter of the tower and the evaporation loss are obtained, and the design of the cooling tower is optimized, so that the cooling tower is more water-saving and energy-saving.
2. The cooling tower is designed by taking the annual water saving rate as a design standard, so that the evaporation water loss is accurately controlled, the better water saving effect of the cooling tower is achieved, the calculation of the water loss and the calculation of the defogging effect of the cooling tower can be accurately achieved through the cooling tower designed by the method, and the calculation accuracy of the water loss can be improved by about 20%.
Drawings
In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below. Like elements or portions are generally identified by like reference numerals throughout the several figures. In the drawings, elements or portions thereof are not necessarily drawn to scale.
FIG. 1 is a general flow chart of a method of designing a wet and dry combined cooling tower in accordance with the present application;
FIG. 2 is a flow chart of the evaporation loss calculation in the present application;
FIG. 3 is a flow chart of the fin tube length calculation in the present application;
FIG. 4 is a flow chart of the calculation of the water temperature of the tower outlet at the dry section in the application.
Detailed Description
Embodiment 1, as shown in fig. 1, is a design method of a dry-wet combined cooling tower, which adopts a micro-element method to perform accurate design calculation, and comprises the following steps:
step S1: determining ambient weather conditions, given calculated initial air conditions;
the environmental meteorological conditions include: the ambient atmospheric pressure Pa (kPa), the ambient air dry bulb temperature theta (DEG C) and the ambient air wet bulb temperature tau (DEG C) all the year round, and the relative humidity phi, the saturated steam partial pressure ptheta corresponding to the dry bulb temperature and the moisture content omega of the inlet air are calculated according to a thermodynamic calculation formula i The enthalpy value hi of the inlet air and the density ρi of the inlet wet air;
step S2: determining a cooling task and a water-saving task, and giving the design requirement of a cooling tower;
the cooling tasks include: the cooling circulating water quantity q (m 3/h) of a single tower, the water temperature t1 of the circulating water entering the tower and the water temperature t2 of the circulating water exiting the tower; the water-saving tasks include: relative water saving rate epsilon.
Step S3: determining the structure and operating parameters of the finned tube and the structural parameters of the tower;
the fin tube structure parameters include: the number of the finned tube boxes, the inner diameter and the outer diameter of the finned tubes, the thickness of the fins, the distance between the finned tubes, the number of tube rows, the number of each tube row, the width of the finned tube boxes, the thermal resistance coefficient of dirt inside and outside the tubes, the material of the base tube and the material of the fins; operating parameters of the finned tube: tube pass number, fin tube box ventilation speed; the structural parameters of the tower comprise tower length, tower width, air inlet height, wall flow rate and air inlet quantity.
Step S4: determining a filler performance parameter;
the filler performance parameters include: filler capacity performance parameters, filler resistance performance parameters.
Step S5: and calculating annual evaporation water loss under the condition of pure wet sections of the cooling tower, and giving the water loss before dry-wet combined water-saving design.
The design of the dry-wet combined cooling tower is mainly to save more water than the pure wet cooling tower, and the water saving rate is also relative to the pure wet cooling tower, so that the water consumption of the cooling tower in the pure wet condition needs to be calculated at first.
As shown in fig. 2, the specific calculation process is as follows:
assuming the wet section air-water ratio lambda, obtaining characteristic number through thermodynamic performance parameterWherein A and n are thermal performance parameters of the filler of the cooling tower.
The annual evaporation water loss calculation under the condition of pure wet section of the cooling tower is carried out by an iteration method, and the specific method comprises the following steps: in order to solve the differential equation set of air and water state parameters, the temperature of cooling water in a tower is divided into a plurality of small differential units along the water flow direction, the water outlet mass of the tower is obtained according to mass conservation on the assumption of the water outlet air moisture content omega o of the tower, and the air moisture content and the air enthalpy value of each differential unit are obtained through differential equation differentiation of heat transfer and mass transfer until the water outlet air moisture content of the tower is obtained through recalculation, so that preparation is made for the next iteration condition.
The starting conditions of the microcomponents can only be obtained by assuming that the column air moisture content is the boundary condition of a given differential equation set, and the calculation is continued.
Each section refers to a plurality of infinitesimal sections, a calculation formula is a derivative differential equation set, and the differential equation set is used for calculating heat and mass transfer conditions inside the cooling tower, so that evaporation loss and cooling number are calculated.
The formula for calculating the air moisture content is as follows:
；
the calculation formula of the air enthalpy value is as follows:
；
wherein eta is the relative humidity,the specific heat of water, omega is the air moisture content of each small-section differential unit, t is the cooling water inlet temperature of each small-section differential unit, q is the mass of cooling water, dt is the difference between the cooling water inlet and outlet temperatures of each small-section differential unit>The saturated air moisture content corresponding to the water temperature is G is air quantity, < >>For the saturated air enthalpy corresponding to the water temperature, γ is the latent heat of evaporation of water, ++>Is the saturated moisture content of air, +.>Is the saturation enthalpy of air; />The Lewis number is a proportionality constant for representing mass transfer and heat transfer, and the specific values are as follows: />。
And re-calculating the calculated out-tower air moisture content as a starting condition of a new iteration until the calculated out-tower air moisture content and the assumed air moisture content are different to meet the solving accuracy, and obtaining the cooling number by an integral method:
wherein t1 is the water temperature of the tower entering and t2 is the water temperature of the tower exiting.
The air-water ratio is regulated by changing the air quantity entering the tower, so that the difference between N1 and N2 meets the accuracy, and the evaporation loss under the pure wet operation is obtained。
Step S6: calculating the evaporation loss of the dry-wet combined cooling tower according to the relative water saving rateAssuming the dry-wet ratio delta of the dry-wet combined cooling tower, obtaining the heat load of the dry section of the dry-wet combined cooling tower>Tower outlet water temperature of dry section +.>。
Step S7: and performing heat transfer calculation on the dry section of the dry-wet combined cooling tower, and solving to obtain the length of the finned tube.
The specific calculation process is shown in fig. 3:
assuming the length L0 of the finned tube, determining the ventilation air quantity according to the integral length parameter and the ventilation speed of the finned tube boxW is the number of finned tubes in the finned tube box, V is the ventilation speed, and the temperature of the air discharged from the tower is obtained through an energy conservation equation according to the heat load>Wherein c a Specific heat for air; taking the average value of the temperature of air entering and exiting the tower and the temperature of water as reference values, and looking up the property table of the air and the water to obtain the property parameters of the air and the water, wherein the property parameters comprise the density, specific heat, dynamic viscosity, prandtl number, heat conductivity coefficient and density of the air.
The heat transfer coefficient is calculated by adopting the heat transfer coefficient obtained by experiments on the premise that the experiments can be carried out, if no experimental data exists, the heat transfer coefficient can be estimated according to the following calculation process:
the convective heat transfer coefficient outside the tube is obtained by the property parameters of the air and the structural parameters of the finned tube,wherein->Is the heat conductivity of air, db is the outer diameter of the finned tube, ρ is the air density, vmax is the flow velocity of air at the narrowest point of the finned tube bundle, m is the dynamic viscosity of air, P is the spacing of the fins, T is the thickness of the fins, df is the outer diameter of the fins, pra is the prandtl number of air.
Obtaining the convection heat transfer coefficient in the finned tube from the property parameters of water and the flow velocity of waterWherein->Is the thermal conductivity of water, di is the inner diameter of the finned tube, re is the Reynolds number of water, and Prw is the Plantl number of water.
Recalculating the total heat transfer coefficient of the finned tubeWherein Ro, ri and Rw are respectively outside the tubeDirt, dirt in the tube, thermal resistance of the tube wall, and beta is the fin ratio of the fin tube.
Obtaining the heat exchange area of the finned tubeWherein Tln is the log mean temperature; then, the fin tube length is determined>Wherein n is 1 For the number of tube rows, n 2 For each row of tubes, the calculated fin tube length L is compared with the assumed fin tube length L0, and if the difference is too large, the fin tube length is again assumed to be calculated until accuracy is satisfied.
Step S8: the calculation of the evaporation loss in non-summer is performed in preparation for the calculation of the annual evaporation loss.
At this time, the dry section and the wet section of the cooling tower run simultaneously, the calculation process of the dry section is shown in figure 4, and the number of heat transfer units is calculated firstWherein Cmin is the smaller value of the heat capacity flow rates of air and water, A is the heat exchange area, and the heat exchange efficiency is calculatedWherein e is a natural constant, C is the ratio of a smaller value to a larger value in the heat capacity flow rates of air and water, and the temperature of the air discharged from the tower at the dry section is calculated by the heat exchange efficiency +.>Water temperature entering tower with wet sectionThe method comprises the steps of carrying out a first treatment on the surface of the And t4 replaces t1, and then the evaporation loss and the air tower outlet state parameters of the wet section are solved according to the step S5.
Step S9: and (5) carrying out the calculation of the evaporation loss in summer, preparing for calculating the annual evaporation loss, wherein the cooling tower only runs for cooling in a wet section, the water temperature entering the tower is t1, and the solving method is the same as that in the step S5.
Step S10: checking and judging evaporation lossWhether the water saving rate is satisfied or not: summing the evaporation losses calculated in the step S8 and the step S9 to obtain annual evaporation loss qy and the evaporation loss q of the dry-wet combined cooling tower n Comparing, if qy>q n If the water saving rate is not satisfied, increasing eta, and returning to the step S6 for calculation.
Step S11: performing defogging calculation, and judging whether defogging requirements of a cooling tower are met or not: and solving the mixed air quantity, air temperature and moisture content by combining the tower outlet air state parameters of the dry section and the wet section through the law of mass conservation and energy conservation, judging whether the mixed air parameters and the external air parameters have intersection points with saturated air lines on a temperature-humidity diagram through a mapping method, and outputting a solving result.
Step S12: resistance calculation and fan model selection: the total resistance of the dry-wet combined cooling tower is calculated firstWherein->For the resistance coefficient of the individual components, +.>For the air density of the air flowing through the individual components, +.>And (3) carrying out fan type selection by combining the air flow rates of the air flowing through all the components and the air volume obtained in the step (S11).
Step S13: and outputting design results, including dry-wet ratio, fin tube length, annual evaporation loss, fan air quantity, tower outlet air temperature, moisture content and fog-dissipating effect diagram.
The description of the present application has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the application in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiments were chosen and described in order to best explain the principles of the application and the practical application, and to enable others of ordinary skill in the art to understand the application for various embodiments with various modifications as are suited to the particular use contemplated.

Claims (4)

1. A design method of a dry-wet combined cooling tower is characterized by comprising the following steps of: the method combines annual evaporation loss qy with evaporation loss q of a dry-wet combined cooling tower n Comparing, if qy>q n If the water saving rate is not satisfied, the relative humidity eta is increased;
if the water saving rate requirement is met, performing defogging calculation, and judging whether the defogging requirement of the cooling tower is met; resistance calculation and fan shape selection are carried out, and finally, a design result is output;
the annual evaporation loss qy is obtained by summing the non-summer evaporation loss and the summer evaporation loss, the dry section and the wet section of the cooling tower are operated simultaneously when the non-summer evaporation loss is calculated, the outlet air temperature of the dry section and the inlet water temperature of the wet section are obtained through the heat exchange efficiency of the dry section, and then the evaporation water loss of the wet section is calculated; when the evaporation loss in summer is calculated, the cooling tower only runs for cooling in a wet section, and the evaporation water loss in the wet section is calculated;
the annual evaporation water loss calculation under the condition of pure wet section of the cooling tower is carried out by an iteration method, and the specific method comprises the following steps: in order to solve a differential equation set of air and water state parameters, cooling water temperature in a tower is divided into a plurality of small differential units along the water flow direction, the water outlet mass of the tower is obtained according to mass conservation on the assumption of the water outlet air moisture content omega o of the tower, and the air moisture content and the air enthalpy value of each differential unit are obtained through differential equation differentiation of heat transfer and mass transfer until the water outlet air moisture content of the tower is obtained through recalculation, so that preparation is made for the next iteration condition;
the method comprises the following steps:
step S1: determining ambient weather conditions, given calculated initial air conditions;
the environmental meteorological conditions include: the ambient atmospheric pressure Pa, the ambient air dry bulb temperature theta and the ambient air wet bulb temperature tau in the whole year, and the relative humidity phi, the saturated steam partial pressure ptheta corresponding to the dry bulb temperature and the moisture content omega of the inlet air are calculated according to a thermodynamic calculation formula i The enthalpy value hi of the inlet air and the density ρi of the inlet wet air;
step S2: determining a cooling task and a water-saving task, and giving the design requirement of a cooling tower;
the cooling tasks include: the cooling circulating water quantity q of the single tower, the water temperature t1 of the circulating water entering the tower and the water temperature t2 of the water exiting the tower; the water-saving tasks include: a relative water saving rate epsilon;
step S3: determining the structure and operating parameters of the finned tube and the structural parameters of the tower;
step S4: determining a filler performance parameter;
the method further comprises the steps of:
step S5: calculating annual evaporation water loss under the condition of pure wet sections of the cooling tower, and giving the water loss before dry-wet combined water-saving design;
assuming the wet section air-water ratio lambda, obtaining characteristic number through thermodynamic performance parameterWherein A and n are thermal performance parameters of the filler of the cooling tower;
the formula for calculating the air moisture content is as follows:
；
the calculation formula of the air enthalpy value is as follows:
；
wherein eta is the relative humidity,the specific heat of water, omega is the air moisture content of each small-section differential unit, t is the cooling water inlet temperature of each small-section differential unit, q is the mass of cooling water, dt is the difference between the cooling water inlet and outlet temperatures of each small-section differential unit>The saturated air moisture content corresponding to the water temperature is G is air quantity, < >>For the saturated air enthalpy corresponding to the water temperature, γ is the latent heat of evaporation of water, ++>Is the saturated moisture content of air, +.>Is the saturation enthalpy of air; />The Lewis number is a proportionality constant for representing mass transfer and heat transfer, and the specific values are as follows: />；
Re-calculating the calculated air moisture content as a starting condition of a new iteration until the difference between the calculated out-tower air moisture content and the assumed air moisture content meets the solving accuracy, and obtaining the cooling number by an integral method:
wherein t1 is the water temperature of the tower entering and t2 is the water temperature of the tower exiting;
the air-water ratio is regulated by changing the air quantity entering the tower, so that the difference between N1 and N2 meets the accuracy, and the evaporation loss under the pure wet operation is obtained；
Step S6: calculating the evaporation loss of the dry-wet combined cooling tower according to the relative water saving rateAssuming the dry-wet ratio delta of the dry-wet combined cooling tower, obtaining the heat load of the dry section of the dry-wet combined cooling tower>Tower outlet water temperature of dry section +.>；
The method further comprises the steps of:
step S7: carrying out heat transfer calculation of a dry section of the dry-wet combined cooling tower, solving to obtain the length of the finned tube, wherein the calculation of the length of the finned tube comprises the following steps:
obtaining a heat transfer coefficient;
the heat transfer coefficient is calculated by adopting the heat transfer coefficient obtained by experiments on the premise of performing the experiments, and if no experimental data exists, the heat transfer coefficient is estimated according to the following calculation process:
the convective heat transfer coefficient outside the tube is obtained by the property parameters of the air and the structural parameters of the finned tube,wherein->Is the heat conductivity coefficient of air, db is the outer diameter of the finned tube, ρ is the air density, vmax is the flow velocity of air at the narrowest position of the finned tube bundle, m is the dynamic viscosity of air, P is the spacing of the fins, T is the thickness of the fins, df is the outer diameter of the fins, pra is the prandtl number of air;
obtaining the convection heat transfer coefficient in the finned tube from the property parameters of water and the flow velocity of waterWhereinIs the heat conductivity coefficient of water, di is the inner diameter of the finned tube, re is the Reynolds number of water, and Prw is the Plantt number of water;
recalculating the total transmission of the finned tubesCoefficient of thermalWherein Ro, ri and Rw are respectively the thermal resistances of dirt outside the tube, dirt inside the tube and the tube wall, and beta is the finned ratio of the finned tube;
step S8: performing non-summer evaporation loss calculation, and preparing for calculating annual evaporation loss;
at this time, the dry section and the wet section run simultaneously, and the number of heat transfer units is calculated firstWherein Cmin is the smaller value of the heat capacity flow rates of air and water, A is the heat exchange area, and the heat exchange efficiency is calculated again>Where e is a natural constant and C is the ratio of the smaller value to the larger value of the heat capacity rates of air and water; calculating the air temperature of the tower outlet at the dry section through heat exchange efficiencyWater temperature entering the tower of the wet section +.>The method comprises the steps of carrying out a first treatment on the surface of the t4 replaces t1, and then the evaporation loss and the air tower outlet state parameters of the wet section are solved according to the step S5;
step S9: performing evaporation loss calculation in summer, preparing for calculating annual evaporation loss, at the moment, only operating wet section cooling, and enabling the water temperature entering a tower to be t1, wherein the solving method is the same as that of the step S5;
step S10: checking evaporation loss, and judging whether the requirement of water saving rate is met or not: and (3) summing the evaporation losses calculated in the step S8 and the step S9 to obtain annual evaporation loss qy, comparing the annual evaporation loss qy with the evaporation loss qn of the combined dry and wet cooling tower, if qy > qn is calculated, increasing eta, and returning to the step S6 for calculation.
2. A method of designing a wet and dry combined cooling tower as claimed in claim 1, wherein: the fin tube length calculation further includes the steps of:
obtaining the heat exchange area of the finned tubeWherein Tln is the log mean temperature; then, the length of the fin tube is obtainedWherein n is 1 For the number of tube rows, n 2 For each row of tubes, the calculated fin tube length L is compared with the assumed fin tube length L0, and if the difference is too large, the fin tube length is again assumed to be calculated until accuracy is satisfied.
3. A method of designing a wet and dry combined cooling tower as claimed in claim 2, wherein: the method further comprises the steps of:
step S11: performing defogging calculation, and judging whether defogging requirements of a cooling tower are met or not: solving the mixed air quantity, air temperature and moisture content by combining the tower outlet air state parameters of the dry section and the wet section through the law of mass conservation and energy conservation, judging whether the mixed air parameters and external air parameters have intersection points with saturated air lines on a temperature-humidity diagram through a mapping method, and outputting a solving result;
determining ventilation air quantity according to integral length parameters and ventilation speed of finned tube boxW is the number of finned tubes in the finned tube box, V is the ventilation speed, and the temperature of the air discharged from the tower is obtained through an energy conservation equation according to the thermal load。
4. A method of designing a wet and dry combined cooling tower as claimed in claim 3, wherein: the method further comprises the steps of:
step S12: resistance resistorForce calculation and fan model selection: the total resistance of the dry-wet combined cooling tower is calculated firstWherein the method comprises the steps ofFor the resistance coefficient of the individual components, +.>For the air density of the air flowing through the individual components, +.>And (3) carrying out fan type selection by combining the air flow rates of the air flowing through all the components and the air volume obtained in the step (S11).
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