Difference Between Indirect Air Cooling and Direct Air Cooling for Generators

I have done both of these kinds, direct air cooling and indirect air cooling are for the cooling method of circulating water, direct air cooling is the use of air cooling island with a large fan and the heat dissipation surface of the air cooling island to cool, you can look at the Suzhou electric field, indirect air cooling is through the intercooling tower (and the appearance of the cooling tower is similar) There is a cooling triangle around the lower part of the intercooling tower, which is cooled by air convection inside and outside the intercooling tower, you can take a look at the 660,000 machine tablets of the Qinling electric field in Shaanxi.

Power plants do have a lot of "air-cooled" units, even if some generators use "stator rotor coil double water internal cooling", but the core of such generators still needs to be forced to cool by air.

The air cooling system of the generator is a confined space, the air relies on the blades at both ends of the generator rotor as the power, the cold air enters the generator to bring out the heat, and the hot air passes through the "air cooler" and is cooled and circulated by the cooling water.

Thermal power plants (thermal power plants) or fossil fuel power plants are power plants that generate electricity by burning coal, natural gas and oil, and are designed to generate large amounts of electricity sustainably, and in many countries, most of the electricity is provided by thermal power plants.

Thermal power plants (except ferrofluid generators) convert the thermal energy generated by combustion into mechanical energy through various rotating machinery, which then drives the generator. The prime mover is usually a steam or gas turbine, but in some smaller power stations, an internal combustion engine may also be used. They all generate electricity by using the pressure drop in the process of changing high-temperature, high-pressure steam or gas through a turbine to low-pressure air or condensate.

My job is steam turbine system commissioning.

The exhaust steam of the direct air-cooled unit is cooled by surface heat exchange by an axial fan in the air condenser (air-cooled island), condensed into water, returned to the hot well (or condensate tank), and then entered the thermal system. There are many 600MW units in China, and the 660MW ultra-supercritical power plant is under construction at the Guodian Inner Mongolia Bulian Power Plant, which is estimated to be put into operation at the end of this year or early next year.

Indirect air cooling is divided into two types: Heller type and Harmon type, and its circulating water system is closed.

The Heller intercooling system adopts a hybrid condenser, the circulating water enters the condenser through the turbine to cool the exhaust steam of the steam unit (the two are mixed), and about 2-3% of the cooled circulating water enters the thermal system as condensate (condensate is a part of the circulating water), and the remaining water enters the cooling tower through the circulating pump for heat exchange. The shape of the cooling tower is the same as that of the tower of the water-cooled unit, but the lower part of the tower is vertically arranged with a circulating water cooling equipment (closed), called the cooling fan section, which carries out surface heat exchange through natural ventilation, and enters the steam turbine plant after cooling. The desulfurization tower is installed at the bottom of the cooling tower, and the middle and upper sections are used as the chimney of the boiler, that is, the cooling tower, the smoke tower, and the desulfurization tower, and the three towers are in one.

In January 11, the 660MW Heller intercooling unit of Guodian Baoji No. 2 Power Plant has been put into operation (the world's first).

The Harmon intercooling system uses a surface condenser, and its condenser works in the same way as a water-cooled unit, where the circulating water and exhaust steam are exchanged through the tube bundle. The internal arrangement of the cooling tower is different from that of the Heller type, with the cooling elements arranged almost horizontally inside the tower, and there are fans in the lower part like a direct air cooling system.

The water quality of the Heller intercooling system is condensate, and the circulating water quality of the Hamon type is the desalinated water quality.

In terms of automatic control, the Heller style is the most complex and troublesome; Direct air cooling is secondary; Harmon is the easiest.

The generators and transformers of power plants usually have air-cooled, hydrogen-cooled, water-cooled, water-cooled and other cooling methods. Direct air cooling is the use of air movement to take away heat.

The term "indirect air cooling" is rarely heard, and it literally refers to the use of some kind of intermediate ** to cool the equipment, and then use air to cool this intermediate **, of course, it also includes cooling methods that do not come into direct contact with the air (such as a transformer immersed in oil, which brings heat to the radiator, and then carries out forced air cooling).

Direct air cooling means that the exhaust steam of the steam turbine is directly cooled by air, while indirect air cooling is used to cool the exhaust steam of the steam turbine with intermediate medium, and air is used to cool the intermediate medium.

In the operation of the generator, due to the existence of current and magnetic field, there will be iron loss and copper loss, this loss is transmitted to the winding and iron core in the form of heat, it will cause heat, we will consider the temperature it bears when designing the equipment, in order to make it not overheated and overcooled, we often use air cooler to match, the air cooler plays a good cooling effect in the whole process, so that the heat of the generator will not be so high, so that it can operate normally and prolong its life.

There are often copper and iron losses when the generator is working, and the temperature of the core and coil will gradually increase as the generator works. In order to prolong the service life of the generator, the metal hose compensator, the aging rate of coil insulation is slowed down, so that the temperature of the coil cannot exceed a certain standard. Therefore, it is necessary to take measures to remove the heat generated by the coil and core in time to maintain the temperature of the coil insulation not exceeding the specified value.

The more commonly used cooling methods are air cooling, hydrogen cooling, and water cooling. Although the effect of air cooling is poor, due to the simple and reliable operation of air cooling equipment, medium and small generator sets are used more.

The air cooler uses cooling water to cool the hot air coming out of the generator before feeding it to the generator. In fact, when the generator is working, the heat generated is taken away by the cooling water, and the air only acts as a medium. Install an air cooler to extend the life of the generator and improve the efficiency of the generator.

1) Characteristics of air-cooled units.

The all-air-cooled generator has the advantages of reliable operation, simple operation and convenient maintenance. From the manufacturing point of view, the air-cooled unit has better parameters and slightly higher efficiency at the rated point. There is no water-cooled unit, such as stator water joint, water treatment and other auxiliary equipment, the structure is simple, and the manufacturing is relatively easy to achieve.

At present, large-scale generators no longer use the method of assembling the lobes in the factory and then transporting them to the construction site for joint installation, but the installation method of on-site lamination assembly. The air-cooled unit has the advantages of relatively simple installation process requirements and short installation cycle. The main problems of air cooling are the uneven axial temperature distribution of stator wire rods, the mechanical stress caused by heat, and the warping of stator lamination caused by the thermal expansion of the stator core.

For the latter problem, there are some relatively good successful experiences in technology. Due to the relatively simple structure of the air-cooled generator and the absence of special joints in the stator windings of the generator, the air-cooled generator has advantages in terms of operational reliability.

2) The characteristics of water internal cooling.

The stator winding adopts the water internal cooling method, and the heat of the winding is directly taken away by the water. The volumetric heat capacity of water is 3,500 times that of air, and the thermal conductivity is 23 times that of air, so water cooling is very effective. The ultimate capacity of water-cooled units can be times that of air-cooled units.

The size of an all-air-cooled generator is about 25% larger than that of a water-cooled generator of the same capacity. The main advantage of stator winding water internal cooling is that the internal temperature of the stator wire rod can be controlled at about 65, which reduces the stator temperature rise and improves the thermal stress; The temperature distribution of the wire rod along the axis of the stator core is uniform, which can reduce the deformation caused by uneven thermal expansion, so that the life of the main insulation can be increased, and the life of the whole machine should be longer than that of the air-cooled unit in theory. The length of the stator core of the water-cooled unit is shorter than that of the air-cooled unit, which can effectively save materials.

Less ventilation is required than in air-cooled units, and the corresponding wind loss is also reduced. Although the efficiency at rated load is lower than that of air-cooled units, the no-load loss is smaller, and the efficiency is higher when operating below the rated capacity. The stator groove of the water-cooled unit is wide and shallow, which increases the stiffness of the stator core and reduces the stator vibration.

Due to the complex structure of the stator wire rod, some manufacturing processes and installation processes are also complicated.

The main problem of the water-cooled unit is the reliability of the stator water joint structure and the water treatment device. When a sudden short circuit occurs near the generator or the generator is asynchronous, the end of the stator rod is subjected to a large electromagnetic force when closing, and here is the waterway connection of the winding. The welding quality of the water joint is also one of the keys to the reliability of the generator's operation, which has a lot to do with the manufacturer's manufacturing level.

In addition, the cooling water used for the stator winding of the generator must be clean water to remove ions, and the reliability of the cooling water system also constitutes a certain constraint on the reliability of the generator operation.

Other advantages brought about by water cooling:

1) Due to the reduction of the weight of the rotor, the lifting capacity of the plant bridge machine is reduced.

2) Due to the reduction of the height of the stator core, the overall height of the unit decreases, so the height of the plant can be reduced accordingly.

3) The weight of the rotor is reduced, so that the thrust load of the generator is reduced, which is convenient for the operation of the thrust bearing.