The principle of humidification and dehumidification in constant temperature and humidity chambers has evolved from steady-state damp heat to alternating damp heat, which demands a faster response in the humidification process. When traditional spray humidification fails to meet these requirements, steam humidification and shallow water pan methods have become widely adopted and have seen significant development over time.
Humidity can be expressed in various ways, but in the context of testing equipment, relative humidity is the most commonly used metric. Relative humidity refers to the ratio of the actual water vapor pressure in the air to the saturated vapor pressure at the same temperature, typically expressed as a percentage. It's well known that the saturation vapor pressure of water depends solely on temperature and not on atmospheric pressure. Through extensive experimentation and data collection, a clear relationship between temperature and water vapor saturation pressure has been established. This relationship is often calculated using the Goff-Gratch formula, which is still widely used by meteorological departments for compiling humidity charts.
To meet the required test conditions, constant temperature and humidity test chambers must effectively control both temperature and humidity. This paper reviews the different humidification techniques used in such chambers, discussing their respective advantages and disadvantages, and provides recommendations based on application needs.
The humidification process essentially involves increasing the partial pressure of water vapor within the chamber. The earliest method involved spraying water onto the chamber walls, where the water surface’s saturation pressure was controlled by adjusting the water temperature. As water evaporated from the tank walls, it increased the relative humidity inside the chamber through diffusion. This technique was introduced in the 1950s, but due to the limitations of early humidity control systems—mainly mercury-electric contact type conductivity meters—the temperature control of hot water tanks had significant lag, leading to slow transitions and insufficient performance for alternating heat and humidity cycles. Moreover, water droplets from wall sprays often fell onto test samples, causing contamination.
Additionally, this method required careful drainage management inside the chamber. As a result, it was soon replaced by more efficient methods like steam and shallow water pan humidification. However, the older method still offers some benefits. Although its control transition is slower, once stable, the humidity fluctuation is minimal, making it more suitable for constant damp heat tests. Also, the moisture added during the process doesn't introduce additional heat into the system. Furthermore, if the sprayed water is kept below the dew point temperature of the test environment, it can even help in dehumidifying the chamber.
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