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Direct air cooling system and equipment

key word:Heat exchange element


Product description

Direct air cooling system and equipment

 

1. Introduction

1.1. Purpose

The direct air cooling system and equipment are mainly used for steam turbines in power plants. Through the heat exchange effect of surface heat exchange tube bundle, the steam turbine exhaust steam flowing in the tube bundle is cooled into condensate by the ambient air flowing outside the heat exchange tube bundle. Because the direct air cooling system and equipment can save a lot of cooling water required by conventional wet cooling units, it has been widely used in power construction in water shortage areas.

1.2. Model meaning

Direct air cooling system mainly includes air-cooled condenser system, air supply system, steel structure system, exhaust pipe system, condensate system, air-cooled condenser cleaning system, drainage system, vacuum pumping system, electrical system, instrument and control system, etc. The equipment mainly includes heat exchange tube bundle, wind turbine equipment and other auxiliary equipment and components.

1.3. Structural features

The core technology of direct air cooling system: the direct air cooling system involves many specialties and subsystems, including the overall design and optimization of air cooling system, the overall control optimization of air cooling system, the design of large diameter thin-wall negative pressure exhaust pipe, the structural design of air cooler support platform, the influence of Meteorological environment on air cooler, the design and manufacture of direct air cooler, and the optimization design of large-diameter axial-flow fan and selection, and so on.

Overall design and optimization of air cooling system: according to the conditions of the area where the power plant is located, according to the different assumed values of initial temperature difference (ITD), several groups of wind speed schemes are calculated respectively. At the same time, with the active cooperation of the steam turbine plant, the unit performance with different last stage blade length is compared and analyzed, and the multiple groups of data are combined for calculation, so as to obtain the minimum total cost, meet the noise requirements of the plant boundary and site layout Set the optimization scheme required.

Overall control optimization of air cooling system: the requirement for automatic control of direct air cooling system is to timely adjust the speed of axial flow fan according to the changes of ambient temperature and steam turbine load, so as to make the cooling capacity of air cooling radiator adapt to the requirements of exhaust back pressure of air-cooled steam turbine, and ensure the safe and economic operation of air-cooled steam turbine generator set.

Design of large diameter thin-wall negative pressure exhaust pipe: the design control of exhaust pipe with diameter over 4m in large capacity direct air cooling system includes: plate and shell structure design of exhaust pipe with diameter thickness ratio greater than 50mm and steam distribution pipe series under external pressure, stability design of ring stiffened cylindrical shell, cylinder cone combined shell and cylinder ball combined shell structure under external pressure; The stress distribution of exhaust pipe under the action of continuous load, temperature load, earthquake load, wind load and foundation settlement difference, etc.; the thrust and torque of exhaust pipe to the exhaust device of low pressure cylinder of steam turbine is reduced; the pressure reduction measures of the whole exhaust pipe system are limited by production process and layout space.

The structural design of air cooler support platform is shown in Fig. 27-1, which shows the structural model of 2 × 135MW direct air cooling unit steel platform. The supporting steel structure platform of 2 × 300MW unit is 130m in length, 60m in width and 5m in height. It supports nearly 10000 tons of air cooler equipment at an altitude of 30m above the ground and bears complex dynamic loads. Therefore, the platform must be safe and reliable.

Influence of meteorological environment on air cooler: the change of air temperature and surrounding wind environment has great influence on the normal operation of air cooling system. If the system structure design is not reasonable, the formation of heat reflux will not only reduce the efficiency of the air cooling system, but also seriously threaten the safe operation of the unit. Therefore, it is necessary to conduct wind tunnel physical model test and numerical simulation test for air cooling system. According to the test results, the thermal calculation and airflow simulation test reports used to evaluate the system performance under different wind conditions are obtained. It is very important to take targeted measures to improve the face wind speed of air cooler and reduce the impact of strong wind on air cooling system.

Design and manufacture of direct air cooler: there are mainly three types of pipe used in direct air cooling system of thermal power plant, namely three row pipe, double row pipe and single row pipe. The three rows of tubes are made of wound elliptical steel tubes - elliptical steel fins; the double row tubes are made of sheathed elliptical steel tubes - Rectangular Steel fins; the single row tubes are made of steel aluminum composite base tubes and aluminum finned tubes, which are combined by vacuum brazing process. Single row tube is the main heat exchange element of direct air cooling system. However, due to the limitation of material source, its cost is high, which brings great initial investment burden to the owner. Therefore, it is very important to reduce the cost of single row tube and develop air-cooled heat exchanger with higher heat transfer efficiency and lower price.

Optimization design and selection of large diameter axial flow fan: the noise level of direct air cooling system increases due to the use of large axial flow fan. According to GB 12348-2008 "Emission standard of noise at boundary of industrial enterprises", class II noise standard is ≤ 60dB (A) in daytime and ≤ 50dB (A) at night. Therefore, the factory boundary (or virtual plant boundary) should be as far away as possible from the air cooling platform, or the fan with high efficiency and low noise should be selected, or the noise elimination board should be set in the wind wall to reduce the wind speed of the windward side of the air cooling radiator, and the belt drive should be used for the speed reducer to reduce the noise.

1.3.1. Structure description. The flow chart of direct air cooling system is shown in figure 27-2. The exhaust steam of the steam turbine is sent to the outdoor air-cooled condenser through the thick exhaust pipe. The axial-flow cooling fan makes the air flow through the surface of the radiator and condenses the exhaust steam into water. The condensate is pumped back to the regenerative system of the steam turbine.

The working principle of the direct air cooling system (see Fig. 27-3) is that the exhaust steam discharged from the steam turbine is introduced into the steel radiator of the air-cooled condenser through the pipeline, and the ambient air directly cools it into condensate water, thus reducing the intermediate cooling medium required for conventional secondary heat exchange, and the heat exchange temperature difference is large.

1.3.2. Description of important parts:

1.3.2.1. The air-cooled condenser is composed of two parts: forward flow tube bundle (referring to the same relative flow direction of steam and condensate) and counter current (referring to the opposite flow direction of steam and condensate).

There are generally more than 300 tunnels in 330MW units. The width of each tube bundle is 2-3m; the tube bundle height is 9-11m for downstream flow and 9-10m for counter current. The cooling area of air-cooled condenser is generally 3-10.

Generally, 6-12 tube bundles form an air-cooled condenser cooling unit. The typical cooling unit is shown in figure 27-4. The cooling unit of air-cooled steam consumer is arranged in isosceles triangle with the top angle of about 60° to form A-shaped structure. There are 3-6 tube bundles on both sides of A-shaped frame. A typical 300MW unit consists of six rows of air-cooled condensers, each of which has five cooling units, four of which are downstream cooling units and one of which is mixed cooling unit. A fan set is arranged under the cooling unit. Partition walls shall be set between different cooling units to avoid the mutual influence of adjacent cooling units and the shutdown of adjacent fans, so as to reduce the ventilation efficiency; for the entire air-cooled condenser, except for the condenser air duct, all gaps shall be sealed with anti-corrosion plate to ensure that the condenser is not bypassed. In order to facilitate maintenance and operation, the partition wall between cooling units is provided with access doors.

A steam distribution pipe is set on the air-cooled condenser, and the pipe diameter decreases gradually with the number of connected units; the condensate collection header is set under the air-cooled condenser, through which the uncondensated steam flows into the counter current unit.

The cooling tube bundle is installed on A-shaped support frame. The upper end of the tube bundle is fixed with the support frame, and the lower end can move freely up and down. Lifting facilities are arranged in the A-shaped support frame and the horizontal footpath of the platform to facilitate the maintenance of fans, motors and other equipment.

1.3.2.2. The main equipment of air-cooled condenser tube bundle system includes cooling elements, heat exchange tube bundle (including counter flow tube bundle and forward flow tube bundle), tube bundle header and steam distribution pipe, etc.

Finned tube is the core of cooling system, and its performance directly affects the cooling effect of air cooling system. The basic requirements for the performance of finned tube are: good heat transfer performance, good temperature resistance, good heat shock resistance, good atmospheric corrosion resistance, easy to clean dust, strong enough pressure resistance, low pressure drop in the tube, small air resistance, good mechanical vibration resistance and low manufacturing cost.

According to the different metal materials used in the smooth tube and fin, the radiator used in direct air cooling power plant can be divided into steel tube fin radiator and aluminum tube fin radiator. In the 20th century, the development of circular finned tube and circular finned tube has gone through three stages: first, the development of circular finned tube in the 20th century and the third stage in the 20th century. The first three types in figure 27-5 are steel tube fin heat exchangers, and the fourth type is aluminum tube fin heat exchangers.

At present, multi row pipes have been basically eliminated.

The three row tube is a kind of oval finned tube with oval steel tube. The size of the base tube is 72mm × 20mm, the fin size is 94mm × 46.7mm, the fin spacing of the first row of tube is 5mm, and the spacing of the second and third rows of fins is 3mm (or 4mm). It is reported that this kind of cooling element has been used in some large-scale air-cooled power plants in China.

Hot dip galvanized large diameter elliptical steel pipe (100 mm × 20 mm) with rectangular steel fins and two rows of tubes has been used for many years,

The anti freezing performance of thermal engineering is good. According to some data, the heat transfer of two rows of large diameter oval tubes with hot-dip galvanized rectangular fins is equivalent to that of round tubes with round fins. The specific dimensions are as follows: oval tube size is 100mm × 20mm, wall thickness is 1.5mm; rectangular fin size (width × length) is 49mm × 119mm, and fin thickness is 0.5mm; the fin spacing of the first row of tubes facing the wind is 4mm; the fin spacing of the second row of pipe on the windward side is 2.5mm.

The brazed aluminum serpentine finned tube with large diameter flat steel tube and the hot rectangular finned tube on the flat steel tube are two kinds of tube bundles currently used in single row tubes, which have been widely used in power plants with capacity of 300MW and above in China.

Compared with multi row tube bundle, single row tube bundle has the following advantages:

a. Because the heat exchange area on both sides of the tube bundle is fully utilized, the total heat exchange area of the air-cooled condenser is reduced, so the cost is reduced and the manufacturing cycle is shortened.

b. The air flow resistance is small, and the power consumption of the air cooling fan is reduced, so the operation cost is reduced and the noise is also reduced.

c. The problem of non condensable gas accumulation in the tube bundle is solved, the condensate flow is more smooth, and the supercooling degree of condensate and the risk of freezing in winter are reduced.

d. Compared with the double row tube bundle, it is easier to clean. For cold regions, single row tube bundle should be used. At present, it is widely used in three north regions of Italy, Spain and China.

e. The core tube of single row tube is generally flat tube, which has a large ratio of longitudinal to width, which is conducive to the separation of vapor and liquid, antifreeze and increase the inlet velocity of steam.

There are basically two kinds of manufacturing methods for single row pipes, one is brazing aluminum fins on flat steel pipes, as shown in Fig. 27-6; the other is hot-dip galvanized steel fins on flat steel pipes. The main technical parameters of a single row pipe are shown in table 27-1. See table 27-2 for performance comparison of different finned tube bundles of condenser in direct air cooling system of 600MW unit.

Table 27-1 main technical parameters of single row pipe

Item Parameter
Single row pipe form Flat tube serpentine aluminum fin
Pipe diameter (mm) 219×19
Wall thickness (mm) 1.5
Fin geometry Serpentine fin
Fin height (mm) 19
Fin thickness (mm) 0.25
Fin spacing (mm) 2.3
Tube material Steel aluminum composite plate
Fin material Alluminum
Number of pipe rows 1

 

Table 27-2 performance comparison of different finned tube bundles of condenser in direct air cooling system of 600 MW Unit

Name Pipe bundle
Three row pipes [1] Two rows of pipes [2] Single rwo of pipes [3]
Manufacture Zhangjiakou bakduel heat exchanger Co., Ltd Harbin Air Conditioning Co., Ltd. Imported from abroad
(BDT) technology (German GEA Technology) (Belgium H-L Technology)
Name of finned tube All steel, hot dip galvanized oval tube with oval fin All steel, hot-dip galvanized elliptical tube sleeve rectangular fins, fins with spoiler holes Silicon aluminum alloy serpentine fins are brazed on steel flat tubes, and there are concave convex pits on the fins
Fin tube parameters      
Geometric dimension (mm)
Air flow length (mm) 2×90.5+95=276 120+125=245 1×200=200
Fin smoothness Smooth   With spoiler    
Air side resistance Small Middle
Winging ratio (average) 9.94 13.85 12.75
Tube bundle size (including frame) 9.8×2.7×0.55 9×3×0.52 9×3×0.25
m×m×m
Effective length (m) and number of tube bundles Downstream; counter-current all 9.6m161 pieces Downstream 8.9m115 pieces Downstream 8.9m49 pieces
Counter-current 8.33mm115 pieces Counter-current 8.33mm49 pieces
Windward area of tube bundle  Aw(m2/piece) 24.36 Downstream 24.57counter-current 23 Downstream 24.57counter-current 23
Fin area Aal(m2/piece) 2344.42 Downstream 3085counter-current 2889 Downstream 2570counter-current 2400
Finned tube
appearance Oval steel tube wrapped with elliptical steel sheet Oval steel tube sleeve rectangular steel sheet Serpentine aluminum sheet of flat steel tube
Maximum working temperature 300 350 120
Connection between fin and base tube The fins are connected and wound on the base tube by winding machine, and then the outer surface is hot-dip galvanized The fins are sheathed on the base tube by a casing machine, and then the outer surface is hot-dip galvanized to fill the gap The fins are brazed on both sides of the flat tube with silicon aluminum and sealed with silicon material
Corrosion protection Surface hot dip galvanizing Surface hot dip galvanizing Silicon aluminum coating on the outer surface
Fin spacing Smm 5,3,3 4,2.5 2.8
Mass Heavier Heavy Light
Fin processing Smooth Small holes with turbulence There are turbulent bump waves
Heat and air side wind resistance Heat transfer performance 97.4%, wind resistance 63% Heat transfer performance 100%, wind resistance 100% Heat transfer performance 105%, wind resistance 110%
μ=2.8m/s
Pipe bundle
Connection between finned tube and tube sheet Welding
Advantage (1) The wind resistance of the air side of the tube bundle is the smallest, and the service power and noise are also small. ACC system is widely used in the world (1) It has a large ratio of length to width, and the maximum filling degree can be obtained in the auxiliary condensing pipe;
(2) The finned tube has the most economical material and high mechanical strength. (2) The thermal characteristics are extraordinary, which can be 3 kPa lower than the annual back pressure of multi row tubes at 0 ;
(3) The fins are smooth, not easy to gather sand and dust, and the effect of high pressure water washing is complete. (3) There is no non condensable gas gathering in the tube bundle, which solves the problem of antifreeze theoretically.
  (4) The steam pressure drop inside the tube bundle is 0.5kpa and the end difference is 1.2
Dis-advantage There are many tube rows and each bundle is slightly heavier It is a high wind tube bundle, with a lot of auxiliary power and high noise Complex structure and high unit price
Main equipment cost of an air cooling unit (10000 yuan) (including 8 pieces of tube bundle, A-frame air duct, header, partition wall and fan) appa. 222112% appa.  199100% appa.  553278%

 

1.3.2.3. The main equipment of air supply system includes fan, reducer, motor, frequency converter, fan inlet duct and fan sealing plate, protective net, vibration switch, anti vibration and vibration reduction facilities, etc.

At present, forced ventilation is adopted in direct air-cooled condenser. The direct air-cooled condenser with natural ventilation has not been put into practice yet, which is technically feasible, but whether it can compete with forced ventilation in terms of technical and economic indicators still needs to pass the practical test. In addition, the regulation of natural ventilation only depends on blinds when changing working conditions, which is less than that of forced ventilation, which is also a major disadvantage of natural ventilation. At present, the main mode of forced draft condenser is duty cycle.

The fan system is composed of axial fan, drive motor, reducer and motor frequency converter. These equipments are supplied by mature manufacturers at home and abroad. The following focuses on the axial flow fan and motor frequency converter, which are crucial to the cooling performance.

Large diameter axial flow fan is adopted for large air cooling unit, and the fan blade is shown in Fig. 27-7. There are three debugging methods of fan, namely single speed, double speed and variable frequency speed regulation. In order to reduce the investment, the forward flow tube bundle is driven by a single speed motor and the counter current tube bundle is driven by a two speed motor. In recent years, most of the direct air cooling units adopt variable frequency speed regulation fan.

The function of cooling fan is to provide air for cooling steam turbine. Almost all air-cooled condensers adopt vertical axial-flow fans, as shown in Fig. 27-8. Each group of air-cooled condensers is equipped with one axial-flow fan.

The general requirements of cooling fan are as follows:

a. Large diameter requirements. Generally, the diameter of fans for units above 300 MW are 9.144 and 9.754m.

b. High static pressure efficiency requirements. η s ≥ 66% (energy consumption level of fan group operation).

c. Low noise requirement. According to the requirements of GB 12348-2008 "emission standard of environmental noise at boundary of industrial enterprises", the sound pressure level of air-cooled fans at plant boundary is less than or equal to 55dB (A), and the noise sound power level of single fan is less than or equal to 88 dB (A).

d. High hydrostatic to total pressure ratio. The required ratio is 0.74-0.81.

c. The requirements for the safety and reliability, blade interchangeability, vibration and balance characteristics of the fan.

In order to make the noise of the whole air cooling device meet the requirements of GB12348-2008, the air cooling fan adopts large diameter, full speed and low noise fan, and is equipped with low voltage frequency converter for frequency conversion and speed regulation, so as to achieve the purpose of energy saving and noise reduction. In order to further reduce the noise, some units also choose the low noise fan with streamlined fan blades.

Each unit needs to be equipped with dozens of fans. The number of fans and their performance parameters are selected according to the unit capacity, site environmental conditions and other factors (see table 27-3 and table 27-4)

Table 27-3 performance parameters of 300MW direct air-cooled condenser cooling fan

Item Downflow condenser Counter flow condenser
Fan diameterm 7.925
Fan speedr/min Variable frequency 
Fan flowm3/h 401 377
Fan pressurePa 116.2 120.6
Fan shaft powerKW 85
Wind speed at windwardm/s 2.57
Matching power of motorKW 90
VoltageV 380
Number of sets 20 6

 

Table 27-4 performance parameters of 600MW direct air-cooled condenser cooling fan

Item Flowdown (count-flow) condenser
Fan diameterm 9.14
Fan speedr/min Variable frequency
Fan flowm3/h 196.5×103
Fan pressurePa 140
Fan shaft powerKW 74
Wind speed at windwardm/s 2.8
Matching power of motorKW 110
VoltageV 380
Number of sets 56

Direct air cooling unit adopts frequency conversion technology to control the speed of drive motor of axial fan. Because the power required for speed control of the motor is reduced proportionally to the second power of torque, the energy saving is greatly realized. Compared with the traditional control method, frequency conversion speed regulation shows great advantages.

The device that converts the fixed voltage and frequency alternating current into alternating current with variable voltage and frequency is called frequency converter. Frequency converter is divided into two forms: AC-AC and AC-DC-AC. AC-AC frequency converter can directly convert power frequency AC into AC voltage and frequency can be controlled, also known as direct frequency converter; AC-DC-AC converter first changes power frequency AC into DC through rectifier, and then converts DC into AC which can be controlled by voltage and frequency, also known as indirect inverter. At present, indirect frequency converter is widely used in air cooling units. Figure 27-9 shows the structure and principle of frequency converter.

The frequency converter is composed of the main circuit and the control circuit. Because the main circuit is nonlinear (switch action), the inverter itself is the harmonic interference source, so it will affect the equipment on the power side and output side. Compared with the main circuit, the control circuit of the inverter is a small energy and weak signal circuit, which is easily interfered by other devices and causes the inverter to be unable to work. Therefore, when the frequency converter is installed and used, the following anti-interference measures must be taken for the circuit:

a. The separation distance between the control cable and the main circuit cable or other power cables is usually more than 30cm (the minimum is 10cm). When the separation is difficult, the control cable shall be laid through the iron pipe.

b. The control signal must be grounded at a single point. The grounding wire is not used as the signal path, and one strand is grounded at the DCS side.

c. The control cabinet equipped with frequency converter should be far away from large capacity frequency converter and motor as far as possible, and its control cable should also avoid these equipment with large flux leakage.

d. To prevent poor contact, the cable connection points should be regularly tightened and reinforced.

The use environment of frequency converter has a direct impact on its normal function and service life. Therefore, in order to prolong the service life and improve the energy-saving effect, it is necessary to carry out regular maintenance and replace some parts of the inverter. The maintenance of frequency converter mainly includes the following three aspects:

a. dust removal shall be carried out regularly. Due to the compactness of the internal components of the frequency converter, it is difficult to clean the internal dust with the fan or vacuum cleaner. Therefore, it is necessary to disassemble the frequency converter to carry out soot blowing and dust removal. The ash deposition on the control board should be treated with brush to facilitate the heat dissipation of some components. In addition, due to the small insulation distance between the integrated diodes in the rectifier part of the frequency converter, too much dust will easily damage the rectifier block, so it is necessary to increase the frequency of soot blowing and dust removal.

b. Replace parts regularly. The service life of different types of components in frequency converter is different, and changes with the change of installation environment and use conditions. Pay attention to patrol inspection, analysis and timely replacement during use.

c. There should be good heat dissipation. The failure rate of frequency converter increases exponentially with the increase of temperature, and the service life decreases exponentially with the increase of temperature. When the ambient temperature is increased by 10℃, the service life of frequency converter is reduced by half. When the frequency converter works, the current flowing through the frequency converter is very large, and the heat generated by the frequency converter is also very large, so the influence of its heating cannot be ignored. Generally, there are several ways to solve the problem of heat dissipation: ① according to the heat generated by the cabinet, appropriately increase the size of the cabinet; ② when the inverter is initially selected, it can be considered to set the frequency conversion radiator part as water cooling; ③ install cooling fan at the air outlet of the frequency converter.

1.3.2.4. Civil engineering and steel structure system. There are two kinds of structural forms of direct air cooling system support, one is steel structure, the other is reinforced concrete structure. According to the current data, the steel structure support is more widely used. The steel structure is divided into the steel structure above the platform and the steel structure below the platform. The steel structure above the platform includes tube bundle supporting steel structure, fan lifting beam, fan bridge, unit partition wall, door, footpath, railing, maintenance vertical ladder and maintenance platform, windbreak wall, etc.; the steel structure below the platform includes rack, stairs, etc.

1.3.2.5. Exhaust pipe free system. It generally includes exhaust pipe (main steam pipe and steam distribution pipe), support and hanger, valve, safety valve, release valve, manhole, expansion joint, hot well, etc.

1.3.2.6. Condensate collection system. The condensate collection system is to collect the water condensed by the air-cooled condenser to the condensate tank or exhaust device through the condensate pipe, and then send it to the steam turbine thermal system through the condensate pump.

1.3.2.7. The cleaning system of air-cooled condenser generally includes all equipment after the water supply source interface, including flushing device, flushing device control panel, booster pump, connecting pipeline and accessories between flushing device and flushing water source

1.3.2.8. Drainage system. The exhaust pipe between the air-cooled condenser and the exhaust device can be provided with necessary drainage to ensure that there is no water in the pipe during the start-up, shutdown and normal operation of the unit.

1.3.2.9. Vacuum system. Vacuum pumping system is an important part of direct air cooling system. For condensing steam turbine unit, it is necessary to establish a certain vacuum in the cylinder and condenser of the steam turbine. During normal operation, in order to maintain the condenser vacuum, the air leaked from the vacuum system and the non condensable gas brought by steam must be continuously pumped out of the condenser. The function of vacuum system is to establish and maintain low back pressure of steam turbine and vacuum of condenser. When the unit is started, the air accumulated in some steam water pipeline systems and equipment shall be pumped out to speed up the start-up speed; during normal operation, the air and other non condensable gases in the steam and drain water and leaking into the vacuum system shall be removed in time to maintain the vacuum of the air condenser and reduce the corrosion to the equipment, etc. The shaft seal and low-pressure heater in the low-pressure part of the steam turbine also rely on the normal operation of the vacuum extraction system, so as to establish the corresponding vacuum. During the operation of condensing equipment, if the vacuum system is not tight, air will leak into it, which will reduce the vacuum and increase the partial pressure of air. Because the solubility of air is directly proportional to its partial pressure, more air will dissolve into water and the oxygen content of condensate will increase.

The vacuum pumping system is composed of suction pipe, stop valve and condenser vacuum equipment. In foreign countries, steam ejector is often used in this system. During the start-up of steam turbine, auxiliary air extractor is used to meet the requirements of steam turbine start-up within the specified period; when the steam turbine is in normal operation, the main air extractor with smaller output is used to maintain the vacuum of exhaust system. Water ring vacuum pump is widely used in direct air cooling unit, which has large output and good economy.

Water ring mechanical vacuum pump system is a new type of condenser vacuum system with superior performance. It is composed of water ring vacuum, low speed motor, steam water separator, working water cooler, steam butterfly valve, high and low water level regulators, relevant connecting pipes, valves and electrical control equipment inside the pump group.

The working flow of the water ring vacuum pump group is shown in Fig. 27-10. The gas drawn from the condenser enters the gas suction port, passes through the normally open steam operated butterfly valve, and enters the water circulation vacuum pump along the suction pipe of the pump. The pump is driven by the low speed motor through the coupling, and the mixed gas discharged from the vacuum pump enters the steam water separator through the pump outlet pipe, and the separated gas is discharged to the air through the gas discharge port. The separated water and the make-up water from the water level regulator enter the cooler together. After cooling, one of the cooling working water enters the suction pipe of the vacuum pump through the orifice plate to condense the condensable part of the gas that will enter the vacuum pump, so as to improve the pumping capacity of the vacuum pump; the other way of water directly enters the pump body as the make-up water for the working water, so as to keep the water ring stable without overheating. Generally, the cooling water of cooler can be directly taken from the inlet of condenser cooling water and the outlet of condenser cooling water.

The steam operated butterfly valve is used to isolate the air from the standby pump to the condenser before opening the standby system. A differential pressure switch (PDS) is installed at the front and rear of the butterfly valve. Only when the pressure difference between the front and rear of the steam operated butterfly valve is less than 3kPa (3.4kPa in some power plants, this value can be adjusted), the pneumatic butterfly valve will be opened, and the gas at the gas side of the condenser will be connected and pumped into the vacuum pump through the steam butterfly valve. In this way, a large amount of air will not flow back into the working condenser vacuum system due to starting the vacuum pump Ensure the normal operation of condenser equipment and system. When the vacuum of the system drops to 10kPa higher than the set value, the standby pump can be put into operation automatically by pressure switch (PS); when the suction pressure reaches the set value of 10 ~ 30KPa, the standby pump will stop again by the action of PS, so as to ensure the operation of the pumping pressure within the specified range

Water ring mechanical vacuum pump is the key equipment of water ring mechanical vacuum system. The vacuum system is usually equipped with three water ring mechanical vacuum pumps, which are used to pump air and non condensable gas in the condenser. The motor and vacuum pump are directly connected. During normal operation, one is in operation and the other is standby. When the unit is started, three pumps can be put into operation at the same time to quickly establish the condenser vacuum and speed up the unit startup.

1.3.2.10. Electrical system. Electrical system generally includes auxiliary power system, security power supply, DC system and AC uninterrupted power supply system, lighting system and maintenance system.

1.3.2.11. Instrument and control system includes distributed control system, thermal instrument equipment, etc.

1.3.3. Description of important characteristics:

1.3.3.1. The back pressure of steam turbine varies greatly. The exhaust steam of steam turbine is directly condensed by air, and its back pressure changes with the change of air temperature. In North China, the temperature difference between four seasons and even day and night is large, so it is required that the steam turbine should have a wide range of back pressure operation.

1.3.3.2. The vacuum system is huge. The exhaust steam of steam turbine is led out by large diameter pipes, and air is used as the direct cooling medium, and the surface heat exchange is carried out through the steel radiator. The condensing exhaust steam needs a large cooling area, so the vacuum system is huge.

It consumes a lot of energy. The air required by the direct air cooling system is provided by large diameter fans, which need to consume energy. According to foreign data, the self consumption of direct air cooling system accounts for about 1.5% of the generating capacity of the unit

1.3.3.4. The overall floor area of the power plant is small. Because the air-cooled condenser is generally arranged on the top of the steam turbine room or on the elevated platform in front of the steam turbine room, the electrical equipment can still be arranged under the platform, so the land occupation of the air-cooled condenser is comprehensively utilized, and the overall floor area of the power plant is reduced.

1.3.3.5. The anti freezing measures in winter are flexible and reliable. The main antifreeze measure of indirect air cooling system is to set louvers to regulate and isolate the air entering the radiator. If the louvers are not closed tightly or the drive mechanism has mechanical or electrical faults, the radiator will freeze. The direct air cooling system can adjust the inlet air volume of air-cooled condenser by changing the fan speed, stopping the fan or reversing the fan, and using the heat absorbing air to imitate the freezing of the air-cooled condenser. The regulation is relatively flexible, and the effect is good, which has been proved by operation experience.

1.3.3.6. The dissolved oxygen content of condensate is high. Because of the huge vacuum system of direct air cooling unit, it is easy to cause oxygen inhalation in negative pressure system. Moreover, due to the high back pressure of the unit, the supercooling degree of condensate is easy to be too large, which further increases the content of dissolved oxygen in condensate.

 

2. Product appearance

Direct air cooling system usually adopts whole island bidding. The main system and equipment range of direct air cooling island system with air-cooled condenser as the core will be different according to the actual project. The outline of the system and equipment is shown in figure 27-11, and the structural dimensions vary according to the matching turbine capacity.

In general, the direct air cooling system is arranged outside and parallel to row a of the steam turbine room. The tube bundles of the air condensers of the two units are placed on an air cooling platform. The size of air cooling platform of 300MW unit: width (parallel to turbine room) is about 150m; length (perpendicular to turbine room) is about 60m; height of air-cooled condenser platform is about 35m. Fig. 27-12 shows that the structural dimensions vary according to the matching turbine capacity.

 

3. Selection method

The selective force method of direct air cooling system and equipment is very complicated. The selection of equipment and components in the direct air cooling system can only be determined after heat exchange calculation and flow field analysis are carried out according to the steam turbine exhaust parameters adopted in the project, the environmental meteorological conditions of the project and the characteristic parameters of the heat exchange tube bundle to be used. The usual options are as follows:

3.1. Determine the tube type of the tube bundle, according to the exhaust back pressure requirements under full steam temperature in summer, combined with the exhaust steam parameters of the steam turbine under TMCR condition, combined with the heat transfer characteristic parameters of the tube bundle under different face wind speeds, the optimal face wind speed and total heat transfer area are determined through optimization calculation.

3.2. According to the optimized face wind speed and total heat exchange area, the amount of heat exchange tube bundle is determined, and the fan, motor and reducer are selected.

3.3. According to the structure size of tube bundle and other equipment, the general layout is carried out, and the layout is optimized.

 

4. Scope of supply

At present, the direct air cooling system and equipment are mainly designed and supplied in complete sets. The scope of supply varies greatly according to the bidding situation of the project, and the relatively complete scope of supply is as follows:

4.1. Heat exchange tube bundle, condensate header and steam distribution pipe.

4.2. Wind turbine equipment, including fan, motor, reducer and frequency converter.

4.3. Air cooled steel structure platform and supporting components on the platform.

4.4. Wind wall of air cooling platform.

4.5. Large diameter and explosion-proof membrane exhaust compensator.

4.6. Small diameter pipe.

4.7. Butterfly valve, lifting facilities, tube bundle cleaning equipment.

4.8. Cable and bridge, maintenance and lighting facilities.

4.9. Low voltage dry type transformer, low voltage distribution cabinet.

4.10. The technical requirements of water ring vacuum pump are generally proposed by the system supplier and purchased by the owner.