The principle of humidification and dehumidification in constant temperature and humidity chambers has evolved from steady-state damp heat testing to alternating damp heat conditions, which demand a faster response in the humidification process. When traditional spray humidification methods fail to meet these requirements, steam humidification and shallow water pan techniques have become widely adopted and further developed.
Humidity can be expressed in various ways, but in the context of test equipment, relative humidity is most commonly used. Relative humidity is defined as the ratio of the actual water vapor pressure in the air to the saturated vapor pressure at the same temperature, usually expressed as a percentage. It's well known that the saturation vapor pressure of water depends solely on temperature, not on atmospheric pressure. Through extensive experimentation and data collection, scientists have established a clear relationship between temperature and water vapor saturation pressure, which is now widely calculated using the Goff-Gratch formula. This formula is still used today by meteorological departments to create humidity tables and reference charts.
To achieve precise test conditions, constant temperature and humidity test chambers must effectively control both temperature and humidity. This paper explores the different humidification and dehumidification methods used in such chambers, highlighting their respective advantages and disadvantages, as well as recommending suitable applications for each.
The humidification process essentially involves increasing the partial pressure of water vapor in the air. One of the earliest methods involved spraying water onto the chamber walls, where the water surface’s saturation pressure was controlled by adjusting the water temperature. The large surface area of the water on the chamber walls allowed water vapor to diffuse into the chamber, gradually increasing the relative humidity. This method was introduced in the 1950s, but due to limitations in humidity control at the time—mainly using basic mercury-electric contact type conductivity meters—the system had poor control over the hot water tank temperature, leading to long transition times. As a result, it could not meet the higher demands of alternating heat and humidity cycles. Additionally, water droplets from the wall spray often fell onto the test samples, causing contamination.
Drainage inside the chamber also posed challenges. Because of these issues, this method was soon replaced by more efficient techniques like steam and shallow water pan humidification. However, it still holds some benefits. Although the control transition is slower, once stable, the humidity fluctuations are minimal, making it ideal for constant damp heat testing. Moreover, this method doesn't add extra heat to the system. In fact, if the sprayed water is kept below the required dew point temperature, it can also help in dehumidifying the chamber.
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