There are two kind of humidification methods, one is "shallow water pan humidifier", another is steam humidification method.
The shallow water pan humidifier has the advantages of steam humidification and spray humidification. The shallow water pan is a water pan with a sufficiently large surface designed in the test chamber, and a heater is placed in the water pan. The water vapor pressure on the water surface can continuously replenish water vapor into the air through diffusion and convective mass exchange, while humidifying water vapor in this form does not overheat. However, due to the limited area of the water tray, diffusion and convective mass exchange are not very intense. By appropriately heating the water in the water pan to raise it above the test temperature in the chamber, the surface layer of the water pan increases with the temperature, resulting in a higher water vapor pressure and an increase in the difference between the water vapor pressure in the air and the water vapor pressure in the chamber, exacerbating 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, and at this time, the superheat of water vapor decreases significantly. This is superior to the direct steam humidification method, but the disadvantage of this method is that it is difficult to obtain low humidity during low humidity tests due to the diffusion and convective mass exchange in the water tray. Reducing water temperature through refrigeration can result in a decrease in humidity. Due to the integration of the current wet and hot chamber with the low-temperature chamber, in order to prevent the water in the water tray from causing adverse effects during low-temperature testing, it is usually necessary to discharge the water outside the chamber, which adds some trouble to the use of the equipment. In addition, when the test chamber is not used for a long time, microorganisms can easily grow in the water pan and affect the cleanliness of the equipment.
With the continuous changes in test requirements, the test object needs to work with electricity and emit a large amount of heat during the test process. At this time, a compressor is usually used to cool it. During the refrigeration process, the evaporator and air need to undergo heat and mass exchange. The greater the heat generated by the test object, the more intense the heat and mass exchange becomes. The water vapor in the test box 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 method of subcooled steam humidification has emerged.
The generation of supercooled steam usually includes ultrasonic method, high-pressure spray method, centrifugal spray method, etc. They convert water into micron sized water mist, which is in a metastable state. If it receives heat, it will be converted into steam. If it does not receive heat, it will become water droplets. In the case of heating of the test object, the mist absorbs the heat emitted by the test object and vaporizes, converting this heat into latent heat of water, causing the water vapor pressure inside the box to rise to the required humidity for the test. The latent heat of water vapor is released on the refrigeration evaporator, and humidification is achieved through this equal enthalpy heat and mass exchange. This method has been proven to be very effective in practice and has been applied in some test chambers.
There are currently two most widely used methods for dehumidification, one is freezing dehumidification, and the other is solid desiccant dehumidification. The former condenses water vapor from the air onto the surface cooler to form water or frost. Due to the usually long testing process of a humid hot chamber, frost formation in the surface cooler can affect the dehumidification effect. Generally, this phenomenon should be avoided as much as possible. To prevent frost formation in the surface cooler, the temperature of the surface cooler should be controlled above 0 ℃. When the humidity inside the chamber is described as dew point, the dew point temperature is approximately 5-7 ℃. This dew point temperature can meet the requirements of current testing methods and is very convenient to use, making it the most widely used. When a lower dew point is required, solid hygroscopics are usually used to further absorb moisture. The surface water vapor pressure of this type of hygroscopic agent ranges from hundreds to tens of PPm, and can achieve a dew point temperature of around -70 ℃. This method is inconvenient to use or expensive to purchase specialized equipment. It is only used in tests with special requirements. When using a low-temperature chamber for internal combustion engines or conducting operational tests, it is required to supplement a large amount of air in the chamber for fuel combustion. To prevent a large amount of frost formation in the evaporator of the low-temperature chamber caused by water vapor in the new air from affecting refrigeration, a rotary dehumidifier that can operate continuously and is made using the principle of solid moisture absorption is needed. At present, the commodity price of this dehumidifier is very expensive.