HUMIDITY CONTROL
Direct or duct?
Humidification control in HVAC systems generally comes as direct or ducted and the difference between the two is discussed in this report from the Carel Industrial HVAC marketing unit.
A
ir is a mixture of gases, comprising nitrogen (78%), oxygen (21%) and others (1%), including water vapour
which, despite only being present in small quantities, plays a fundamental role in our planet’s ecosystem. Heat is continuously exchanged between
us and the environment, and this can cause situations of comfort or discomfort, depending on how hard the body has to work to maintain heat balance. As regards convection, heat exchange is mainly affected by the air temperature and speed, while evaporation is also affected by the relative humidity. The difference in partial pressures between the moisture content on a given surface, for example, our skin, and the humidity of the air, tends to balance itself naturally, in accordance with the principles of physics. As a result, we perceive dryness of the skin when the air humidity is too low, or a damp feeling when it is high. Feeling comfortable does not correspond to a precise humidity and temperature value, rather it is subjective and depends on other aspects, including physical activity and the amount of
clothing worn, which substantially affect heat exchange.
The following representation on the psychrometric chart, taken from ASHRAE Fundamentals (2001, 8.12), shows the comfort zones in winter and summer, for sedentary activity.
To create and maintain comfort, mechanical systems are used to manage indoor temperature and humidity values. In commercial environments, such as offices, hotels, libraries and museums, special attention is placed on humidity control not only for comfort, but also to avoid the spread of bacteria, especially in winter.
While on one hand humans are more
sensitive to changes in temperature than in relative humidity, on the other, the hygroscopic materials used in industrial processes are more sensitive to changes in humidity than in temperature. In such cases, humidity must be kept within a tight range, so as to preserve the products’ properties and workability. In many industrial environments, including printing, tobacco, processing of plastics and textiles, the production capacity and final product quality
To better understand the difference between these, it is worth looking at the main stages in the design of an HVAC system: Project initiation Requirements definition and planning Project execution Performace monitoring Project closeout
When the problems due to low relative humidity
are strongly affected by relative humidity and the possible formation of electrostatic discharges. Hygroscopic materials naturally tend to absorb or give off moisture, so as to reach equilibrium with the surrounding air; consequently even small variations in relative humidity can lead to considerable production waste and jamming of production lines. To solve this problem, mechanical systems
are needed to control humidity. An HVAC system consists of several subsystems, including humidification control, which can be managed in two ways: Ducted humidification Direct humidification
38 September 2020
www.acr-news.com
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