About humidification test chamber humidification method and dehumidification method
In this paper, the methods of humidification and dehumidification of the damp heat test chamber are deeply studied and comprehensively discussed, which is helpful for the engineering application of the humidification and dehumidification methods in the development of the new damp heat test chamber.
In order to realize the test conditions, the damp heat test box inevitably needs to humidify and dehumidify the test box. This paper intends to analyze a large number of various methods currently used in the damp heat test chamber, and point out their respective advantages and disadvantages and suggestions. The conditions used.
Humidity is expressed in many ways. In the case of test equipment, the concept of relative humidity is often used to describe humidity. Relative humidity is defined as the ratio of the water vapor pressure in the air to the saturated vapor pressure of water at that temperature and is expressed as a percentage. It is known from the nature of water vapor saturation pressure that the saturation pressure of water vapor is only a function of temperature. It is independent of the air pressure at which water vapor can be used. Through a large number of experiments and collation, people have found a relationship between water vapor saturation pressure and temperature, which has been A large amount of measurement should be adopted by the Goffeglie formula. It is used by the current meteorological department to compile a humidity checklist.
The process of humidification is actually to increase the partial pressure of water vapor. The initial humidification method is to spray water to the test chamber wall, and the water surface saturation pressure is controlled by controlling the water temperature. The water on the surface of the tank wall forms a large surface on which the water vapor pressure is added to the tank by means of diffusion to increase the relative humidity in the test chamber. This method appeared in the 1950s. Because the control of humidity at that time is mainly a simple switchover adjustment of the mercury-electric contact type conductivity meter, the control of the water temperature of the hot water tank with large hysteresis is poor, so the transition process of the control is long and cannot meet the alternating heat and humidity. The need for more humidification, and more importantly, when spraying the wall of the tank, it is inevitable that water droplets will drip on the sample to form different degrees of contamination on the sample. At the same time, there are certain requirements for the drainage inside the tank. This method was quickly replaced by steam humidification and shallow water pan humidification. But this method still has some advantages. Although its control transition process is long, the humidity fluctuation is small after the system is stable, which is more suitable for constant damp heat test. In addition, the moisture in the humidification process does not increase the extra heat in the system. Also, when the temperature of the spray water is controlled to be lower than the point temperature required for the test, the spray water has a dehumidification effect.
As the damp heat test develops from constant damp heat to alternating damp heat, a relatively fast humidification reaction capability is required. When spray humidification is no longer satisfactory, steam humidification and shallow pan humidification methods are widely adopted and developed.
The saturation pressure of water vapor increases with the increase of water temperature. When the water temperature is high to the boiling point, the water vapor saturation pressure will exceed 100Kpa at a standard atmospheric pressure. At this time, a special humidification steam boiler will spray steam to the test chamber. Wet inside. This humidification process will be completed very quickly. Therefore, it is widely used in alternating wet heat boxes. In many cases, the temperature of the steam is always higher than the temperature required by the test conditions. When the high-humidity steam and the lower-humidity air are mixed, a part of the water vapor will condense into water and release the heat of vaporization, which is generated in the box. Extra heat, sometimes in order to balance this part of the heat often turn on the compressor refrigeration. When the cooling temperature is not properly controlled, the frost on the evaporator may affect the cooling effect. At the same time, the dehumidification effect will be generated due to the cooling effect, so that the humidity in the tank will decrease, and the humidification amount will be increased to maintain the test condition, further increasing the extra humidity in the tank. Heat. There will even be a phenomenon of constant humidification, cooling and continuous dehumidification.
The use of steam humidification has a fast humidification, and can adapt to the need of the alternating humidification test in the heating section to require a large amount of humidification. Therefore, this method is widely used. The main disadvantage is the introduction of superheated steam into the tank, which increases the heat inside the tank. Pay special attention to the impact of superheated steam on the system during design.
The shallow water tray humidifier has the advantages of two methods of steam humidification and spray humidification. The shallow water tray is designed with a water tray with a large enough surface in the test chamber, and a heater is placed in the water tray. The water vapor pressure on the surface of the water can be continuously replenished with water by diffusion and exchange of fluids, and the humidified water vapor in this form is not hot. However, since the area of ​​the water tray cannot be made very large, the diffusion and the exchange of the liquid are not very intense. By appropriately heating the water of the water tray to be higher than the test temperature in the tank, the surface layer of the water tray increases with the temperature, the water vapor pressure rises high, and the difference between the water vapor pressure in the air in the tank increases, which is exacerbated. Water vapor diffusion and exchange of fluids. In the case of meeting the humidification requirements of the test chamber, the water temperature in the water tray is not required to be too high, and the superheat of the water vapor is significantly reduced. This is superior to the direct steam humidification method. The disadvantage of this method is that it is difficult to obtain low humidity due to the diffusion of the water tray and the exchange of fluid during the low humidity test. Using refrigeration to lower the water temperature can reduce the humidity. Since the current wet heat box has been integrated with the low temperature box, in order to prevent the water in the water tray from being adversely affected by the low temperature test, it is usually necessary to drain the water out of the box, which increases the trouble for the use of the equipment. In addition, when the test chamber is not used for a long time, the microorganisms in the water tray are easy to breed and affect the cleaning of the equipment.
As the test requirements continue to change, the test sample is charged and a large amount of heat is emitted during the test. At this time, the compressor is usually used for cooling. During the refrigeration process, the evaporator and the air are subjected to heat exchange, and the test product is sent. The greater the heat, the more intense the heat and mass exchange, the water vapor in the test chamber will be removed by the evaporator. If the steam humidification is difficult to achieve the high humidity conditions required by the test, another subcooled steam humidification method appears to meet the test requirements. .
Supercooled steam is usually produced by ultrasonic method, high pressure spray method, centrifugal spray, and the like. They convert water into micron-sized mists that are in a metastable state. If it gets heat, it will be converted into steam. If it is not heated, it will become water droplets. When the sample is heated, the heat generated by the mist absorption test sample will be vaporized, and this heat will be converted into latent heat of water to make the water vapor inside the box. The pressure rises to the humidity required by the test. The latent heat of water vapor is released on the refrigerating evaporator, and the humidification is completed by the heat exchange of the helium. This method has proved to be very effective in practice and has been used in some test chambers.
Dehumidification methods are currently the most widely used, one is the freeze dehumidification method, and the other is the solid moisture absorption dehumidification method. The former is to condense water vapor in the air on the surface cooler to form water or frost. Since the test process of the wet heat box is usually long, the frost of the surface cooler will affect the dehumidification effect. Generally, this phenomenon should be avoided as much as possible. In order to prevent the surface cooler from frosting, the temperature of the air cooler should be controlled at 0 °C. the above. When the humidity in the tank is described by the dew point, the dew point temperature is about 5 to 7 °C. This dew point temperature has been able to meet the requirements of the current test methods, and it is very convenient to use, so it is the most widely used. When a lower dew point is required, a solid moisture absorbent is typically employed to further the moisture absorbent. The surface water vapor pressure of such a moisture absorbent is on the order of several hundred to several tens of ppm, and a dew point temperature of about -70 ° C can be obtained. This method is very inconvenient to use or expensive to purchase specialized equipment. It is only used when there are tests with special requirements. For example, when the internal combustion engine is at a low temperature or a low temperature tank for running tests, it is required to use a large amount of air in the tank for fuel combustion. In order to prevent the water vapor in the new air from affecting the refrigeration in the evaporator of the low temperature box, a rotary dehumidifier which is made by the solid moisture absorption principle and can be continuously operated is needed. At present, the price of such a dehumidifier is very expensive.
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