In order to achieve test conditions in a hot and humid environment Climatic Test Chambers, it is inevitable to humidify and dehumidify the test chamber. This article intends to analyze a large number of various methods currently used in hot and humid test chambers and point out their respective advantages and disadvantages. Recommended conditions of use.
There are many ways to express humidity. As far as test equipment is concerned, the concept of relative humidity is usually used to describe humidity. Relative humidity is defined as the ratio of the partial pressure of water vapor in the air to the saturated vapor pressure of water at that temperature and is expressed as a percentage. It can be seen from the properties of water vapor saturation pressure that the saturation pressure of water vapor is only a function of temperature and has nothing to do with the air pressure that water vapor can handle. Through a large number of experiments and collation, people have sought to express the relationship between water vapor saturation pressure and temperature, which has been studied by engineering and The most widely used measurement method should be Goff's formula. It is currently used by meteorological departments to compile humidity lookup tables.
The process of humidification is actually to increase the partial pressure of water vapor. The initial humidification method is to spray water on the wall of the test chamber, and control the saturation pressure of the water surface by controlling the water temperature. The water on the surface of the box wall forms a larger surface, and water vapor pressure is added to the box through diffusion on this surface to increase the relative humidity in the test box. This method appeared in the 1950s. Since the humidity control at that time was mainly based on simple switching volume adjustment with mercury electrical contact conductivity meters, the adaptability to the control of water temperature in hot water tanks with large lag was poor, so the control transition process was long and could not meet the requirements of alternating humidity and heat. There is a need for a large amount of humidification, and more importantly, when spraying the box wall, water droplets will inevitably splash on the test sample, causing varying degrees of contamination to the test sample. At the same time, there are also certain requirements for drainage in the box. This method was soon 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 stabilized, so it is more suitable for constant humidity and heat tests. In addition, if the water vapor is not overheated during the humidification process, it will not add additional heat to the system. In addition, when the temperature of the spray water is controlled to be lower than the key temperature required by the test, the spray water has a dehumidifying effect.
As the damp heat test develops from constant damp heat to alternating damp heat, faster humidification response capabilities are required. When spray humidification can no longer meet the requirements, steam humidification and shallow water pan humidification methods have begun to be widely adopted and developed.
The saturation pressure of water vapor increases with the increase of water temperature. When the water temperature reaches the boiling point, at a standard atmospheric pressure, the saturation pressure of water vapor will exceed 100Kpa. At this time, a special humidification steam boiler will spray steam into the test chamber. Internal humidification. This humidification process will be completed quickly. Therefore, it is widely used in alternating heat and humidity chambers. In many cases, the temperature of the steam is always higher than the temperature required by the test conditions. At this time, after the high-humidity steam is mixed with the lower-humidity air, part of the water vapor will condense into water and release the heat of vaporization, which will generate heat in the box. Extra heat, sometimes in order to balance this part of the heat, it is often necessary to turn on the compressor for refrigeration. When the refrigeration temperature is improperly controlled, frost may form on the evaporator, affecting the refrigeration effect. At the same time, the refrigeration will produce a dehumidification effect, which will reduce the humidity in the box. In order to maintain the test conditions, the humidification amount will be increased, further increasing the additional moisture in the box. heat. There may even be constant humidification, cooling and dehumidification at the same time.
The use of steam humidification has fast humidification and can meet the needs of large humidification amounts in the heating section of the alternating heat and humidity test. Therefore this method is widely used. Its main disadvantage is that superheated steam is introduced into the box, which increases the heat in the box. Special attention should be paid to the impact of superheated steam on the system during design.
The shallow water tray humidifier has the advantages of both steam humidification and spray humidification methods. The shallow water tray is 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 water surface can continuously replenish water vapor into the air through diffusion and convective mass exchange, and through this form of humidification, the water vapor does not overheat. However, since the area of the water pan cannot be very large, diffusion and convection mass exchange are not very intense. By properly heating the water in the water pan to make it higher than the test temperature in the box, the water vapor pressure on the surface of the water pan increases as the temperature rises, and the difference between the water vapor partial pressure in the air in the box increases, aggravating the Water vapor diffusion and convective mass exchange. When the humidification requirements of the test chamber are met, the water temperature in the water pan is not required to be too high. At this time, the superheat of 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 and convection exchange of fluids in the water pan during low humidity tests. Using refrigeration to lower the water temperature can reduce the humidity. Since the current hot and humid box has been integrated with the low-temperature box, in order to prevent the water in the water tray from causing disadvantages during low-temperature tests, the water usually has to be drained out of the box, which adds a certain amount of trouble to the use of the equipment. In addition, when the test chamber is not used for a long time, microorganisms can easily breed in the water pan and affect the cleaning of the equipment.
As the test requirements continue to change, the test sample needs to work with electricity and emit a large amount of heat during the test. At this time, a compressor is usually used to refrigerate it. During the refrigeration process, the evaporator and the air need to exchange heat and mass, and the test sample generates heat. 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 steam humidification is used, it is difficult to achieve the high humidity conditions required by the test. Therefore, in order to meet the test requirements, another supercooled steam humidification method has emerged. .
Supercooled steam is usually produced by ultrasonic method, high-pressure spray method, centrifugal spray, etc. They convert water into micron-sized water mist, which is in a metastable state. If it gets heat, it will be converted into steam; if it doesn't get heat, it will turn into water droplets. When the test sample is heated, the mist absorbs the heat emitted by the test sample and vaporizes, converting this heat into the latent heat of water, making the water vapor in the box The pressure rise reaches the humidity required by the test. The latent heat of water vapor is released on the refrigeration evaporator, and humidification is completed through this equal-enthalpy heat and mass exchange. This method has proven to be very effective in practice and has been used in some test chambers.
There are currently two most widely used dehumidification methods, one is the freezing dehumidification method, and the other is the solid hygroscopic agent 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 hot and humid box is usually long, frosting on 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 surface cooler should be controlled at 0°C. above. At this time, when the humidity in the box is described by dew point, the dew point temperature is about 5 to 7°C. This dew point temperature can meet the requirements of the current test methods and is very convenient to use, so it is the most widely used. When lower dew points are required, solid moisture absorbers are often used to further absorb moisture. The surface water vapor pressure of this type of hygroscopic agent is on the order of hundreds to dozens of ppm, and a dew point temperature of about -70°C can be obtained. This method is very inconvenient to use or very expensive to purchase special equipment. It is only used in some tests with special requirements. For example, when the internal combustion engine is operated at low temperature or in a low-temperature box for running tests, it is required to add a large amount of air to the box for fuel combustion. In order to prevent the water vapor in the new air from forming a large amount of frost on the evaporator of the low-temperature box and affecting the refrigeration, a rotary dehumidifier that is made based on the solid moisture absorption principle and can operate continuously is needed. At present, the price of this kind of dehumidifier is very expensive.