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2. Steam humidification, such as electric heating and humidification, divided into open and closed. The open type is slow in response and often has hysteresis, so the humidity fluctuates greatly, but the structure is simple and reliable. The closed steam pressure is greater than atmospheric pressure, between 0.1 and 0.3 MPa, without hysteresis, but it needs to be equipped with a pressure reducing valve, a solenoid valve, a drain pipe, etc. The structure is complicated and is mostly used in a large artificial climate chamber. The open type is mostly used in small and medium-sized wet heat boxes.
The principle of heat and moisture exchange in direct contact between air and water:
When the air passes through the open water surface, it undergoes heat and moisture exchange with the water surface. Depending on the water temperature, only sensible heat exchange may occur; there may be both sensible heat exchange and wet exchange, as well as latent heat exchange. The sensible heat exchange is a temperature difference between air and water, which is caused by heat conduction, convection and radiation, and latent heat exchange is the result of evaporation (or condensation) of water vapor in the air to absorb (or release) latent heat of vaporization. The total heat exchange amount is the algebraic sum of the sensible heat exchange amount and the latent heat exchange amount.
When the air is in direct contact with 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 water vapor pressure depends on the boundary layer. Saturated air temperature.
If the temperature of the boundary layer is higher than the temperature of the air above it, the boundary layer will transfer heat to the air; otherwise, the air will transfer heat to the boundary layer. If the concentration of water vapor molecules in the boundary layer is greater than the concentration of water vapor molecules on the air (ie, the partial pressure of water vapor in the boundary layer is greater than the partial pressure of water vapor in the air), the number of water vapor molecules 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 water molecules that jump out of the water surface; during the condensation process, too much water vapor molecules in the boundary layer will return to the water surface.
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, and the wet exchange and the resulting latent heat exchange depend on the concentration difference or fraction of water vapor molecules between the two. Pressure difference.
The wet principle heats the water to generate steam in the water tank. The steam enters the wet heat box through the spray tube to humidify the air inside the box.
The steam above atmospheric pressure from the steam boiler is depressurized and then sprayed into a humid heat tank for humidification.
The airflow in the hot and humid tank passes through the surface of the shallow water pan in the tank, and the temperature is equal to the saturated air boundary zone of the water surface temperature for wet heat exchange. When the concentration of steam molecules in the boundary zone is greater than the concentration of water vapor molecules in the flowing gas stream, it is humidified, and vice versa.
Analyze the principle of heat and humidity exchange in constant humidity test chamber
1. In the case of low-humidity and high-humidity of the constant-humidity constant temperature test chamber, since the added steam is not sufficiently mixed with the air or local condensation occurs in contact with the tank wall, not only the amount of added steam is reduced, but also the heat is released to the inside of the tank. The temperature of the humid air rises; plus the above ε'>ε, so it is not an isothermal humidification process, and the temperature inside the tank will increase.