How to humidify and dehumidify the constant temperature and humidity test chamber The constant temperature and humidity tester is mainly used to test the performance of materials in various environments and to test the heat, cold, dry and moisture resistance of various materials. For electronics, electrical appliances, communications, instruments, vehicles, plastic products, metals, food, chemicals, building materials, medical, aerospace and other products used for testing quality. Constant temperature and humidity testing machine in order to achieve the test conditions, inevitably need to humidify and dehumidify the operation of the test chamber, this article intends to use a large number of methods in the high temperature and humidity heat test box to analyze the various methods, pointing out their respective Advantages and disadvantages and recommended conditions. Enhanced Printing Machine,Embossed Hot Stamping,Digital Varnish Printer,Enhanced Type Heat Transfer Machine Shenzhen Textalk Graphic Technology Co.,Ltd , https://www.textalkdigital.com
Humidity is represented in many ways. For experimental equipment, the concept of relative humidity is often 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 expressed as a percentage. According to the saturated pressure properties of water vapor, the saturation pressure of water vapor is only a function of temperature. Regardless of the air pressure at which water vapor can be exposed, people have sought to express the relationship between water vapor saturation pressure and temperature through a large amount of experiments and collations. Measured in large quantities should be Goff Gretchen's formula. It is used by the current meteorological department to prepare the humidity checklist.
The process of humidification is actually increasing the partial pressure of water vapor. The initial method of humidification is spraying water on the wall of the test chamber. By controlling the water temperature, the saturation pressure of the water surface is controlled. The water on the surface of the tank wall forms a large surface, and in this surface, the relative humidity in the test chamber is increased by adding vapor pressure to the inside of the tank through diffusion. This method appeared in the 1950s. Since the control of humidity at the time was mainly a simple switch quantity adjustment using a mercury-electricity contact type conductivity meter, the adaptability to the control of the water temperature of the large delay hot water tank was poor, so the control transition process was long and could not meet the alternating damp heat The need for more humidification requirements, more importantly, when spraying on the wall, there is inevitably water droplets on the test sample to form different levels of pollution. At the same time there are certain requirements for the drainage inside the box. This method was quickly replaced by steam humidification and shallow water pan humidification. However, this method still has some advantages. Although its control transition process is long, the humidity fluctuation is small after the system is stable, and it is more suitable for a constant damp heat test. In addition, the heat of vaporization during humidification does not increase the extra heat in the system. Also, the spray water has a dehumidifying effect when the temperature of the spray water is controlled to be lower than the point temperature required by the test.
As the programmable constant temperature and humidity test chamber develops from constant damp heat to alternating damp heat, a relatively rapid humidification reaction capability is required, and when the spray humidification cannot meet the requirements, the steam humidification and the shallow water tray humidification method are started to be adopted in large quantities and developed. .
The saturation pressure of water vapor rises with the increase of water temperature. When the water temperature reaches the boiling point, the water vapor saturation pressure will exceed 100 Kpa at a standard atmospheric pressure. At this time, a special humidified steam boiler will blow out steam to constant temperature. Wet inside the test chamber. This humidification process will be completed quickly. Therefore, it is widely used in alternating heat and humidity 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 is mixed with the low-humidity air, part of the water vapor condenses into water and releases the heat of vaporization, which is generated in the box. Extra heat, sometimes to open the compressor to balance this part of the heat. 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 effect of refrigeration, so that the humidity in the tank will decrease. In order to maintain the test conditions, the amount of humidification will be increased to further increase the inside of the box. Heat. There will even be constant humidification, and at the same time the cooling will continue to dehumidify.
The use of steam humidification has a fast humidification, and can meet the need for a large amount of humidification in the temperature rising section of the constant temperature and humidity test chamber. Therefore, this method is widely used. Its main drawback is the introduction of superheated steam into the box, which increases the amount of heat inside the box. Special attention should be paid to the effect of superheated steam on the system during design.
The shallow water tray humidifier has the advantages of steam humidification and spray humidification. In the shallow water tray, a water tray with a large surface is designed in the test box, and a heater is placed in the water tray. The water vapor pressure on the water surface can continuously replenish moisture into the air through diffusion and exchange of liquids, and this form of humidified water vapor does not overheat. However, due to the fact that the area of ​​the water pan cannot be large, diffusion and exchange of liquids are not very severe. By appropriately heating the water in the water tray to a temperature higher than the test temperature in the tank, the surface layer of the water tray rises as the temperature rises, the vapor pressure rises and the difference between the partial pressure of moisture in the air in the tank and the water vapor increases. Water vapor diffusion and liquid exchange. 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 better than the direct steam humidification method. The disadvantage of this method is that it is difficult to obtain low humidity because of the diffusion of the water plate and the exchange of liquid substance during the low humidity test. Using cooling to reduce 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 water in the water tray from being unfavorably caused by the low temperature test, the water is usually discharged out of the box, and the use of the equipment is somewhat troublesome. In addition, when the test box is not used for a long period of time, microbes in the water tray are prone to affect the cleaning of the equipment.
With the continuous change of test requirements, the test product must be charged during the test process and emit a large amount of heat. At this time, the compressor is usually used to cool the sample. In the cooling process, the evaporator and the air must be exchanged for heat and mass. The greater the heat, the more intense the heat and mass exchange, and 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 kind of supercooled steam humidification method has emerged. .
The production of sub-cooled steam is usually ultrasonic, high-pressure spray, centrifugal spray and so on. They convert water into microscopic water mist, which is in a metastable state. If it gets heat, it will turn into steam. If it doesn't get heat, it will turn into water drop. In the case of the sample's heating, the fog will absorb the heat emitted by the sample and convert it into latent heat of water, making the water vapor inside the box. The pressure rises to the humidity required for the test. The latent heat of water vapor is released on the refrigerating evaporator, and humidification is accomplished through the exchange of heat and mass of this crucible. This method has proved to be very effective in practice and has been used in some test cases.
The method of dehumidification is currently used most widely in two ways. One is a method of freezing and dehumidifying, and the other is a method of dehumidification using a solid moisture absorber. The former is to condense the moisture in the air on the surface cooler to form water or frost. Because the test process of the damp-heat box is usually long, the frost on the surface cooler will affect the dehumidification effect. Generally, this phenomenon should be avoided as much as possible. To prevent the frost from freezing, the temperature of the surface cooler should be controlled at 0°C. the above. When the humidity in the box is described by the dew point, the dew point temperature is about 5-7°C. This dew point temperature has been able to meet the requirements of the current test methods, and it is very convenient to use at the same time, so it is the most widely used. When a lower dew point is required, a solid absorbent is usually used as a further moisture absorber. The surface moisture vapor pressure of this type of 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 expensive to use or to purchase specialized equipment. It is only used when there are special tests. Such as the internal combustion engine in the low temperature or operation of the test of the low temperature box, requires the addition of a large amount of air in the tank for fuel combustion. In order to prevent a large amount of frost in the fresh air from affecting the refrigeration in the evaporator of the cold box, a dehumidifier that is made of solid moisture and can be continuously operated is needed. At present, the dehumidifier is very expensive.