Where are the faults in the high and low temperature impact chambers?
The method of high and low temperature impact test box was soon replaced by steam humidification and shallow water tray 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. 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 the relative humidity in the test chamber is increased by adding vapor pressure to the inside of the tank through diffusion in the tank. 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. As the damp heat test progresses from constant moisture heat to alternating damp heat, a relatively rapid humidification reaction capability is required, and when the spray humidification is no longer sufficient, the steam humidification and shallow water tray humidification methods have been widely adopted and developed.

The saturation pressure of water vapor in high and low temperature impact test chamber increases with the increase of water temperature. When the water temperature reaches the boiling point, the water vapor saturation pressure will exceed 100Kpa at a standard atmospheric pressure, and a special humidified steam boiler will blow out. Steam, humidify 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 increase section of the alternating wet heat test. 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. During the design, special attention should be paid to the effect of superheated steam on the system. Shallow water tray humidifiers have the advantages of steam humidification and spray humidification. The shallow water tray is a water tray with a large enough surface in the test chamber. Place heater. 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.

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.

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