Dehumidification method of high-low temperature damp heat test box

High-low temperature hot and humid test chambers are common in instrumentation, materials, and electronic products, and are mainly used to inspect various performance indexes in high-low temperature or hot and humid environments. In actual operation, the humidifying and dehumidifying operation is performed in order to achieve the test conditions. The humidification method must have been known to you, so how do you dehumidify it?

The definition of humidity refers to 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 known from the nature 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 at which water vapor can be located. Through a large number of experiments and sorting to seek to express the relationship between water vapor saturation pressure and temperature, which has been widely used in engineering and measurement should be Goff Geliqi formula, often customers will mention the humidity range 0-100%. RH, this is actually a wrong range of values, 100% of the humidity can not be called humidity, because the humidity refers to the water vapor, and 100% of the humidity has completely formed a water droplet, and 0% of the humidity has been It is a completely dry environment (such as a high temperature of 100°C or a low temperature of 0°C), so the correct humidity range should be 30-98% Rh. If low humidity is required, it can be reduced to 10% or 20%.

Dehumidification principle of high and low temperature test chamber: when the refrigeration system is working, the temperature of the fin surface of the evaporator will be lower than the dew point temperature of the test temperature. When the air is circulated, condensation will occur when the moisture passes through the evaporator surface, thereby achieving the purpose of dehumidification.

High and low temperature test chamber humidity and heat exchange principle:

When the air of the test chamber passes through the open water surface, it undergoes heat and moisture exchange with the water surface. According to the difference in water temperature, only sensible heat exchange may occur. There may be both sensible heat exchange and wet exchange, and there is also latent heat exchange. In the direct contact between the air and the water surface, close to the water surface, due to the irregular movement of water molecules, a saturated air boundary layer with a temperature equal to the water surface temperature is formed, and the concentration of water vapor molecules or the partial pressure of water vapor depends on the The saturated air temperature of the boundary layer.

If the molecular concentration of water vapor in the test chamber is greater than the molecular concentration of water vapor in the air above it (ie, the partial pressure of water vapor in the boundary layer is greater than the partial pressure of water vapor in air), the number of molecules of water vapor in the air will increase; otherwise, it will decrease. . The former is called "evaporation" and the latter is called "condensation". During the evaporation process, the reduced water vapor molecules in the boundary layer are replenished by the water molecules that jump out of the water surface. During condensation, there are too many water vapor molecules in the boundary layer. Returning to the surface of the water, it can be seen that the sensible heat exchange between air and water depends on the temperature difference between the boundary layer and the air above it, while the wet exchange and the resulting latent heat exchange depend on the water vapor molecules between the two. The difference in concentration or pressure difference.

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. In order to prevent the surface cooler from frost formation, 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, required to add a lot of air to 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.