Application of Rotary Dehumidifier in Sanbanxi Hydropower Station

Application of Rotary Dehumidifier in Sanbanxi Hydropower Station

Power station overview

Sanbanxi Hydropower Station is located in the middle and lower reaches of Qingshui River, the upper reaches of the main stream of Yuanshui River, in Jinping County, Guizhou Province. The power station is equipped with 4 mixed flow turbine generator sets with a total installed capacity of 1,000 MW. The power plant building is an underground structure, and the underground cavern group consists of the main engine cave, busbar cave, and main transformer cave. The main engine cave is divided into the generator layer, busbar layer, turbine layer, volute layer, and cone layer from top to bottom; the main transformer cave is divided into 3 layers, from top to bottom, they are GIS switch station, pipeline layer, and main transformer layer. The entrance tunnel is about 500 m long and is the main traffic channel between the main engine cave and the outside world.

Problems with ventilation and air conditioning systems in factories

According to the design return visit, in-depth communication with the factory staff and on-site investigation, the factory ventilation and air conditioning system mainly has the following problems:

(1) High humidity in the factory air

The original design did not take into account the moisture dissipation in the factory and did not consider sufficient dehumidification measures (although air treatment was set up, the dehumidification capacity was limited). Especially in the humid season (April to August every year), the air in the factory is very humid (it is more obvious below the turbine layer. According to field measurements, the relative humidity is between (80% ~ 98%) RH, and even reaches 100%RH in some places). Water accumulates in many parts, condensation forms on the surface of water pipes, and the surface of equipment and pipelines rusts, which reduces the insulation performance of electrical equipment and may cause automation components to fail or malfunction, posing a safety hazard and seriously affecting the long-term stable operation of the electromechanical equipment in the factory and the health of the operators in the factory.

(2) Reservoir water temperature is too high

The original design used low-temperature water in front of the dam as a natural cooling source. However, due to insufficient water supply, the power plant operated at a low water level for a long time. The water temperature in front of the dam was high (around 17°C on average), deviating from the original design of the water intake setting value in front of the dam (14°C), and failed to meet the design requirements, resulting in the air handling unit failing to achieve the predetermined cooling and dehumidification effect.

(3) The air handling unit has been out of service for many years. Due to the rust on the windward side of the air handling unit filter section, the air handling unit has not been put into operation for many years. The air handling unit uses a natural cold source (upstream reservoir water). Although the cooling and dehumidification capabilities are not as good as those of a mechanical cold source, it does have a certain dehumidification capacity. The unit has been out of service for many years, which not only makes the air supply volume in the factory unguaranteed, but also makes the natural convection and the natural air intake generated when the exhaust fan is turned on almost without any cooling and dehumidification treatment (the inlet tunnel is equipped with a wall lining, and the dehumidification effect is very weak). In this way, the hot and humid air coming in from outside the tunnel increases the humidity in the factory, causing condensation on the local low-temperature surface.

(4) Low air volume in the factory

As the air handling units have been out of service for many years, some exhaust fans are also intermittently operated or even stopped (such as busbar exhaust fans). Only some exhaust fans in the ventilation system are running, and there is no mechanical air supply, causing the air volume in the factory to deviate significantly from the design value, and ventilation dead spots are formed in some areas, and the airflow organization is significantly different from the original design.

(5) Air duct system disorder

Some air ducts in the factory were not formed according to the design requirements (such as the vault air supply was not completed), and the entrance door opening of the installation site was not blocked according to the design requirements (an openable door was set up), resulting in turbulent air flow in the factory. The entrance door and the main factory building frequently exchanged heat and moisture due to the wind pressure outside the door. In the humid season, a large amount of humid air entered the factory, increasing the humidity in the factory.

Ventilation and dehumidification system renovation

Overall plan

(1) Refrigeration and rotary dehumidification system

In view of the above problems with the ventilation and air-conditioning system in the plant, it is planned to renovate the ventilation and air-conditioning system in the plant. Since the biggest problem of the air environment in the plant is the problem of moisture condensation, the primary purpose of the renovation is dehumidification, followed by cooling. Since the temperature of the underground powerhouse cave wall is relatively low (the annual average surface temperature of the dam site area is 17.8 ℃), the average temperature of the water pipes such as technical water supply and drainage is 17 ℃, and there is a certain volatility. Therefore, in order to completely solve the problem of moisture condensation, the air dew point in the plant must be processed to below 14 ℃ (taking into account the volatility of the wall and pipe water temperature), and the system equipment is required to have a certain degree of adjustability, and in poor or extreme conditions, it still has sufficient dehumidification capacity to process the air dew point to a lower state. To achieve the above purpose, it is obvious that the system design using mechanical refrigeration and rotary dehumidification combined operation is a more suitable configuration method.

Therefore, according to the actual situation of this power station, a dehumidification cooling treatment method combining mechanical refrigeration and rotary dehumidification is adopted. And in order to save energy to the maximum extent, the dehumidification air conditioning system adopts a single return air system to ensure the necessary fresh air volume in the factory environment and maximize the use of return air.

(2) New exhaust system

The exhaust system in the original design is still retained, but some fans need to be modified or replaced to meet different exhaust volume requirements. According to the temperature and humidity conditions in different seasons, the dehumidification system and the exhaust system adopt different joint operation modes.

(3) Air duct system modification

The original design was a direct-current ventilation system without return air circulation. According to the design requirements of the new system, the return air duct system is being modified in the factory to achieve a single return air system cycle and ensure the normal operation of the dehumidification and air conditioning system.

(4) The local corridor dehumidification elevation below the turbine layer is 314,

The 310, 306 m corridor is located below the turbine layer, with low temperature and high humidity. Since it is impossible to realize return air circulation and the space is small, a mobile dehumidifier (electric refrigeration and dehumidification method) is used to achieve local dehumidification and heating. According to the layout of the mobile dehumidifier and the layout of the surrounding equipment, some mobile dehumidifiers can use nozzle air supply, and it can be considered to set up a small jet fan above the corridor to ensure uniform temperature and humidity of the air in the corridor.

Through the application example of the ventilation and dehumidification transformation design of Sanbanxi Hydropower Station, it can be seen that the application of rotary dehumidification is suitable for large storage capacity and low water temperature power stations. According to the technical characteristics of the rotary dehumidification unit, in order to improve the dehumidification effect of the rotary wheel, the joint operation mode of rotary dehumidification and air conditioning cooling is usually adopted. The treated air is first pre-cooled by the mechanical cold source for preliminary dehumidification, and then the advantage of the rotary wheel's strong dehumidification ability for low-temperature humid air is brought into play, and the rotary wheel performs the second round of enhanced dehumidification to achieve the dehumidification effect of low dew point and dry air.

Since underground powerhouses are buried deep, and are usually hundreds or even thousands of meters away from the surface opening, the discharge of the regenerated hot and humid air from the rotary dehumidifier is a problem. Whether to use air ducts to discharge it outside the factory or to cool and dehumidify it on site requires certain technical and economic comparisons. For renovation projects, since there is often no location for air ducts, they can only be treated on site. On-site treatment is best cooled by reservoir water, which is relatively energy-efficient (underground powerhouses generally have reservoir water with lower water temperature); for the design of a new power station, if the distance to the opening is within five or six hundred meters, it should be more economical to discharge it outside the factory; if the distance is...

The distance to the cave entrance is 700-800 meters or even more than a kilometer. It should be more economical to treat it with reservoir water on site. Generally speaking, mechanical refrigeration is not recommended, which is contrary to the current development purpose of building a resource-saving and harmonious society.

The regeneration energy consumption of the rotary dehumidifier is a shortcoming in the application of rotary dehumidifier technology. Taking the selection calculation of the rotary dehumidifier of Sanbanxi Hydropower Station as an example, the unit with an air volume of 60,000 m3/h has a dehumidification capacity of 468 kg/h, and the regeneration power reaches 420 kW, which puts forward high requirements on the power consumption of the plant. How to study the new technology of rotary regeneration in future practice and break through the bottleneck of its own development will be a topic that the development of rotary dehumidifier technology needs to face and overcome.

It can be foreseen that with the advancement of rotary dehumidification technology, rotary dehumidification will be more widely used and developed in more and more industrial fields including hydropower stations.