The principle of heat and moisture exchange of constant temperature and humidity incubator air and water surface direct contact
The compressor of the constant temperature and humidity incubator is the heart of the refrigeration system. It sucks low-temperature and low-pressure gas into high-temperature and high-pressure gas, releases heat by condensing into liquid, and takes away the heat through the fan, so the hot air is the reason below the constant temperature and humidity incubator, and then passes Throttling to low-pressure liquid, followed by low-temperature low-pressure gas through the evaporator and finally back to the compressor; the refrigerant absorbs heat in the evaporator to complete the gasification process and absorbs heat to achieve the purpose of cooling, and completes the cooling process of the constant temperature and humidity incubator .
In the case of low temperature and high humidity in a constant temperature and humidity incubator, the added steam and air are not fully mixed, or local condensation occurs in contact with the wall of the box, which not only reduces the amount of added steam, but also releases heat to make the humid air in the box The temperature rises; plus the aforementioned ε ′> ε, so it is not an isothermal humidification process, and the temperature inside the box will increase somewhat. Steam humidification, such as humidification with electric heating, is divided into open type and closed type. Open responsiveness is slow 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 lag, but it needs to be equipped with a pressure reducing valve, solenoid valve, drain pipe, etc. The structure is complex, and it is mostly used in large artificial climate chambers. The open type is mostly used in small and medium-sized constant temperature and humidity incubators. In the past, the constant temperature and humidity incubator produced by Sida Test Instrument Factory used open electric heating steam humidification.
When the air of the constant temperature and humidity incubator is in direct contact with the water surface, as a result of the irregular movement of water molecules close to the water surface, a saturated air boundary layer with a temperature equal to the temperature of the water surface is formed, and the concentration of water vapor molecules or water vapor fraction The pressure depends on the saturated air temperature of the boundary layer. When the air passes through the open water surface, heat and moisture exchange occurs with the water surface. Depending on its water temperature, only sensible heat exchange may occur; it may also have both sensible heat exchange, wet exchange, and latent heat exchange. Sensible heat exchange is the temperature difference between air and water, heat transfer due to heat conduction, convection and radiation, and latent heat exchange is the result of the vaporization of water vapor in the air to absorb the 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.
If the temperature of the boundary layer is higher than the temperature of the air above it, heat is transferred from the boundary layer to the air; otherwise, heat is transferred from the air to the boundary layer. If the concentration of water vapor molecules in the boundary layer is greater than the concentration of water vapor molecules in the air above it, 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 replaced by the water molecules jumping out of the water surface; during the condensation process, too many 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 moisture exchange and the resulting latent heat exchange depend on the concentration difference or the difference of the water vapor molecules between the two Pressure difference. The principle of humidity is to heat the water by electricity to generate steam in the water tank. The steam enters the constant temperature and humidity incubator through the spray tube to humidify the air in the tank. The super-atmospheric steam coming out of the steam boiler is decompressed and sprayed into a constant temperature and humidity incubator for humidification.
The air flow in the constant temperature and humidity incubator passes through the surface of the shallow water tray in the box, and the saturated air boundary area whose temperature is equal to the temperature of the water surface exchanges moisture and heat. When the concentration of vapor molecules in the boundary area is greater than the concentration of water vapor molecules in the airflow passing through, it is humidification, otherwise it is dehumidification.
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