2.1 Several Air Humidity Concepts Related to Condensation The atmosphere around the Earth's surface is a mixture of dry air and water vapor. At a certain temperature, the air can contain a maximum amount of water vapor. When the content of water vapor in the air reaches a maximum value, it is called saturated air. In order to measure the amount of water vapor in the air, according to different uses, people introduced the concepts of absolute humidity, moisture content, relative humidity and dew point temperature.
The absolute humidity of the air. Refers to the mass of water vapor contained in a unit volume of air.
Moisture content. It refers to the amount of water vapor in 1kg dry air that directly reflects the increase or decrease in the amount of water vapor in the humid air.
The relative humidity of the air. Refers to the ratio of the absolute humidity of the air to the saturated absolute humidity at the same temperature, often expressed as a percentage, which directly reflects the degree of near-saturation of the air at a certain temperature. The air humidity in our daily life is relative humidity. When the relative humidity is 0, the air is dry air, and when the relative humidity is 100, the air is saturated air. The higher the relative humidity, the moister the air.
Dew point temperature. In the case where the air moisture content is constant, the air temperature is lowered, the absolute humidity of the saturation becomes smaller, and the relative humidity increases. When the temperature drops to a certain temperature, its relative humidity increases to 100 and the air reaches saturation. This temperature is called the dew point temperature. Dew condensation occurs when the air temperature is below the dew point temperature. The greater the moisture content of the air, the lower the temperature reduction when it reaches saturation, ie, the higher its dew point temperature. Conversely, the lower the moisture content, the lower the dew point temperature. Therefore, the dew point temperature can also indicate the degree of air humidity.
Causes the relative humidity of the air to be too high or even causes condensation. Various moisture sources continuously emit moisture and increase the absolute humidity. According to the different sources of wet can be divided into: First, the new amount of rheumatism. If the outdoor high-temperature, high-humidity air enters the lower temperature area of ​​the factory, it will increase the relative humidity, and even cause condensation. The second is the wetness of the water surface and wet surface. In Example 1, the substation is located next to the Weihe River, and the Weihe River is the main source of surface moisture. In the underground plant water pump layer in Example 3, due to leakage of water pipes and equipment, dew condensation, wall or roof cracks or capillary seepage, the water accumulated during maintenance or cleaning will form a wet surface on the equipment and the envelope structure. Like the uncovered gutters, still pools and other open waters, it continuously emits moisture to the air, which is related to the temperature, humidity, and flow rate of the indoor air. The third is wetness in the gaseous state. Water in the interior of the enclosure structure of the underground powerhouse can reach the rock wall through the capillary of the rock in the form of liquid and gas. When the partial pressure of water vapour in the outside air of the surface building envelope structure is higher than that of the inside, the moisture will also pass through the capillary of the envelope structure in a gaseous form and enter the room. Although there is no obvious visible moisture on the inner surface of the building envelope structure, However, this kind of wetness is always in progress. This direct transmission of water vapor in the containment structure is called gaseous propagation. Fourth, the body is wet. When there are many people in the factory, the amount of moisture should be considered. However, it can be neglected in a factory with less personnel.
Due to the presence of an endothermic source, the temperature drops and the relative humidity increases. As in Example 2, when the air inside the box suddenly drops due to the outside air temperature, the outside air forms the main source of heat absorption.
When the hot and cold air meet, the relative humidity at the junction increases. As in Example 3, under natural ventilation conditions, when the hot air at the motor layer and the cold air at the pump layer meet at the cable layer, the relative humidity increases and serious condensation occurs.
Excessive air humidity or condensation on the electrical equipment is not difficult to see from the three examples of this article, do not attach importance to the impact of air humidity on the operation of electrical equipment, let the free development of humidity issues, will seriously affect the normal operation of electrical equipment, and even cause ACCIDENT. Its impact on the work of electrical equipment is mainly reflected in the following aspects: When the air humidity is high or even when condensation occurs, the insulation of the electrical equipment is easy to damp and the insulation performance is reduced. For example, a relay coil that is normally not energized on a motor or equipment control panel can work in a relatively high relative humidity environment for a long period of time. Moisture will enter the coil insulation, causing the coil to become wet and the insulation to fall. For electrical equipment that directly depends on air as the insulating medium, the insulation capacity will be greatly reduced under high relative humidity or even saturated conditions. If condensation forms on the insulating surface or inside of the device, it will directly cause the insulation “flashover†of the electrical device.
When the air humidity is high or even when the condensation occurs, the electrochemical reaction occurs on the surface of the metal structure of the electrical equipment to form an “electrochemical corrosionâ€. When the electrical connection points have significant condensation, the moisture deposited on the surface of the metal structure dissolves the salt on the surface of the equipment. The formation of the electrolyte, under the action of an external electric field or due to the different metal of the electrochemical reaction occurs, so that the metal structure is subject to changes in oxidation corrosion, crisp, layering, pulverization, thus losing the metal properties. In addition, the heat released by the electrochemical reaction at the same time, long-term effect on bakelite, epoxy resin and other insulating materials will increase the aging, premature loss of insulation properties.
Scientific research shows that the critical values ​​of relative humidity of air for metal oxide corrosion are 70% for steel, 60% for copper, 76% for aluminum, 63% for iron, and 60% for zinc. The metal is stored in an environment above the critical value. The rate of oxidative corrosion continues to accelerate as the relative humidity of the air increases.
With the increase in the degree of automation of electrical equipment, the proportion of electronic products in the equipment components continues to increase, and the vast majority of electronic products are required to be stored under dry conditions. Otherwise, the capacity of the capacitor will be reduced when it is damped, and internal circuits will be prone to internal faults when the integrated circuit is damped. The moisture will also oxidize the pins and connectors of the computer's CPU, board gold fingers and electronic equipment, resulting in poor contact or poor solderability. The crystal produces oxidation and the like. Looking at the relevant data, it can be seen that storing electronic components in an environment with relative humidity of 40% can ensure their safety.
3 Prevention and control measures to solve the impact of excessive humidity or condensation on electrical equipment should be based on the principle of moisture prevention, dehumidification supplemented by the principle of the implementation, each profession must closely cooperate with comprehensive consideration, take "guide, plug, septum, seal, in addition â€
And other measures.
Dredging In order to prevent the accumulation of water around the electrical equipment and eliminate the wet source, the drainage system must be well prepared. The size and slope of the drainage ditch must not only meet the requirements of the regulations in terms of design, but also must pay great attention to the quality of the construction and must not produce backward slopes and blockages. Drains and catchment wells should be covered and closed to eliminate unnecessary open water. Also make a waterproof surface layer so that it does not leak or leak. After the construction is completed, if there is a leakage point, use a water pipe to guide the water into the drainage ditch, or fill it with grouting.
Closure For electrical equipment cabinets, waterproof and fireproof plugging shall be performed at the cable entry and exit points to shut off the gas-borne moisture source. It is advisable to use a bushing structure for underground powerhouses and a moisture-proof partition wall on the inside of the retaining wall below the generator level of a ground-level hydropower plant. The cracks and capillary permeated water of such a structure are cut by the interlayer space, and only the remaining gas is wet and the amount of moisture transmitted is small.
The technical supply and drainage pipes in the isolation workshop shall be laid as dark pipes as possible. If clear pipes are used, maintenance management should be strengthened to reduce water leakage. In order to prevent the dew condensation caused by the outer surface temperature of the tube below the dew point temperature of the air, the outer surface of the tube needs to be covered with heat insulation material.
Important electromechanical equipment for sealing shall be arranged in a dry room; automation components, terminal boxes, etc. shall be arranged in the sealed control box, and the control box shall not be placed close to a damp wall.
Dehumidification and good moisture treatment can greatly reduce the amount of moisture produced in the factory, but the dehumidification work is still indispensable. In general, the following methods should be used: heat dehydration method. In humid parts with particularly low temperatures, such as pump rooms, main valve rooms, etc., there are few equipments and large amounts of moisture are produced. It is not economical to use a comprehensive dehumidification method. The electrical equipment can be placed in a box containing an incandescent lamp, an electric heater or an infrared lamp to increase the temperature of the air in the box and reduce the relative humidity, that is, the so-called local heating and dehumidification. This measure is simple and can still solve some problems. In this method, there is no change in the moisture content of the air. Although it cannot take away the wet load, it can reduce the relative humidity of the air.
Ventilation dehumidification method. This method is to send outdoor air to the wet place and absorb the wet load and discharge it. For example, underground hydropower stations or pumping stations in the initial phase of the building envelope contain more moisture and produce a large amount of moisture, often using the method of dehumidification by ventilation, the effect is good. As long as the room air dew point temperature is higher than the blowing point temperature, this method can take away the room wet load. However, as the ratio of the indoor heat load to the wet load is different, the relative humidity in the room may be reduced or may not change or increase. The use of a ventilation dehumidification method at a relatively low temperature and humidity in a hot and humid environment cannot achieve the purpose of reducing the relative humidity.
Heating ventilation dehumidification method. In order to solve the problem of the relative humidity increasing when the ventilation and dehumidification method is applied to a low-temperature and humid area, an electric heater may be installed on the air supply system to send hot air to these parts. This method is essentially a combination of Method 1 and Method 2, so it is called hot air dehumidification, or warming and ventilation dehumidification. As in Example 2, we installed additional heaters and fans and put them into action during thunderstorms.
Because it is uneconomical to put the heater on for a long time, in the pump station or the hydropower station, the heat generation in the generator layer is large to exclude the heat load; the heat in the following parts of the generator layer is smaller to exclude the wet load. The more commonly used method for a pumping station or a hydroelectric plant is to draw the air, which has absorbed the thermal load of the heating parts such as the generator layer, to the following parts of the generator layer, so that the temperature in these areas can be increased, the relative humidity can be lowered, and also taken away. The wet load of these rooms.
Air conditioning dehumidification method. For high-temperature and high-humidity hydroelectric power plants in southern China, the outdoor ventilation temperature during summer is high. If the temperature and humidity are used for dehumidification, the temperature at the dehumidification site may be too high. The outdoor air must be cooled and dehumidified before it is sent to the factory to achieve the purpose of dehumidification. This is the air-conditioning dehumidification method.
Dehumidifier dehumidification method. This method is suitable for dehumidification in areas that are more important and are not suitable for the aforementioned methods. The dehumidifier can be used directly in the room, or can be placed on the outside of the room to send, return tube connected with it. The dehumidification principle includes freezing condensation, liquid absorption, and solid adsorption.
(Editor: Qiu Yanan) First: Guo Binbin, male, born in October 1973, from Lingchuan County, Shanxi Province, graduated from the Department of Electrical Engineering of Taiyuan University of Technology in 1996, engineer, Wanjiazhai YRDP, Shanxi Province Running Sub-branch, No. 217 Nan Nei Huan Street, Taiyuan City, Shanxi Province, 030012. AEBCRDS: lctriccg book her 3nden sweet visits
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