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Direct Humidification In Room Or In Duct?

Air 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 the earth’s ecosystem.

Heat is continuously exchanged between human being 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 pressure between the moisture content on a given surface, for example, human being 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 a situation of 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 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

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
  • Performance monitoring
  • Project closeout
CAREL Heatstream Titanium

When the problems due to low relative humidity are known in advance, a ducted solution is generally the simplest choice to manage, and can be correctly sized during the second stage of design, called requirements definition and planning. In order to accommodate a humidification system that meets the requirements of high efficiency, the ducting must have suitable features, determined in the design stage, when it is still possible to adapt the configuration of the AHU. Ducted humidification allows easier management and maintenance than direct humidification.

When relative humidity is underestimated or neglected in stage two of AHU design, problems will occur in the fourth stage, called performance monitoring. A retrofit solution will thus, be required. In these cases, it is usually advisable to opt for an in-room humidification system, so as to reduce the cost of investment and avoid having to carry out major work on the existing HVAC system, which moreover may often not be suitable for traditional ducted humidification.

All in-room humidification solutions feature a distribution system for delivering steam (isothermal system) or droplets of water (adiabatic system), which may include the use of blowers or be directly incorporated into the humidifier. In both cases, suitable free space must be provided near the distribution system to avoid condensation. The design stage is crucial for correct evaluation of the humidification load, the choice of the humidifier and the position of the distribution system in the points where most required. The chosen solution is always a compromise between technical performance, surrounding conditions and return on investment. Special attention must be paid to the specific application, to the free heights available, to the presence and location of people in the air-conditioned space and to the existing air-conditioning and distribution system.

CAREL Humifog Direct

CAREL offers different in the room humidification solutions, both isothermal and adiabatic. Isothermal humidifiers (heaterSteam, gaSteam, humiSteam, compactSteam) feature high energy consumption, however, can be installed in spaces with low free heights, as the steam generated is easily absorbed by the surrounding air. Furthermore, hygiene is guaranteed without the use of additional systems.

Adiabatic solutions (humiDisk, MC, humiSonic, humiFog Direct, humiFog Multizone Direct), on the other hand, require on average more free space to guarantee complete evaporation of the droplets of water, however feature lower energy consumption and can partially cool internal thermal loads (1 l of evaporated water has an equivalent cooling capacity of 690 W), thus saving on the air-conditioning system.

In the industrial applications mentioned above, where cost reduction, time to market, final product quality and energy saving requirements are increasingly demanding, humidity control is essential to ensure continuity of the business. In older facilities that undergo renovations and upgrades to meet market requirements, direct humidification plays a key role. In commercial applications, on the other hand, in-room humidification is adopted to ensure comfort for a limited investment.



Massimiliano Maistro
CAREL Application Specialist – HVAC Industrial