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Methods for humidification and dehumidification in environmental test chambers


In order to achieve test conditions in a humid and hot environment test chamber, it is inevitable to perform humidification and dehumidification operations on the test chamber. This article intends to analyze the various methods that are widely used in the humid and hot environment test chamber, point out their respective advantages and disadvantages, and recommend the conditions for use.

There are many methods for expressing humidity, and in terms of testing equipment, the concept of relative humidity is usually used to describe humidity. The definition of relative humidity refers to the ratio of water vapor pressure in the air to the saturated vapor pressure of water at that temperature, expressed as a percentage. From the properties of water vapor saturation pressure, it can be seen that the saturation pressure of water vapor is only a function of temperature, and is not related to the air pressure at which water vapor can reach. People have sought to express the relationship between water vapor saturation pressure and temperature through a large number of experiments and organization, among which the Gofglitz formula has been widely used in engineering and metrology. It is currently used by meteorological departments to compile humidity check tables.

The process of humidification is actually to increase the water vapor pressure. The initial humidification method was to spray water onto the wall of the test chamber, and control the surface saturation pressure of the water by controlling the water temperature. The water on the surface of the box wall forms a larger surface, on which water vapor pressure is added to the box through diffusion, causing an increase in relative humidity in the test box. This method emerged in the 1950s. Due to the fact that humidity control at that time was mainly achieved through simple on-off adjustment using a mercury electric contact type conductivity meter, the adaptability to controlling the water temperature of a large lag hot water tank was poor. Therefore, the transition process of control was long and could not meet the high requirements of alternating humidity and heat for humidification. More importantly, during the spraying of the box wall, water droplets inevitably sprayed on the test sample, forming varying degrees of pollution. There are also certain requirements for drainage inside the box. This method was quickly replaced by steam humidification and shallow water plate humidification. But this method still has some advantages. Although its control transition process is long, the humidity fluctuation is relatively small after the system is stable, making it more suitable for conducting constant humidity and heat tests. In addition, not overheating the water vapor during the humidification process will not increase the additional heat in the system. Furthermore, when the spray water temperature is controlled below the required key temperature for the test, the spray water has a dehumidifying effect.

With the development of humid heat tests from constant to alternating humid heat, it is required to have a fast humidifying reaction ability. When spray humidifying cannot meet the requirements, steam humidifying and shallow water pan humidifying methods have been 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 atmosphere pressure, the saturation pressure of water vapor will exceed 100Kpa. At this time, a special humidification steam boiler will spray steam to humidify the test chamber. This humidification process will be completed quickly. Therefore, it is widely used in alternating humidity and heat boxes. In many cases, the temperature of the steam is always higher than the required temperature for the test conditions. When high humidity steam is mixed with low humidity air, a portion of the water vapor will condense into water and release vaporization heat, generating additional heat inside the box. Sometimes, in order to balance this portion of heat, it is necessary to turn on the compressor for refrigeration. When the cooling temperature is not properly controlled, it may cause frost on the evaporator and affect the cooling effect,