Why to dehumidify air
Dehumidify air is to remove moisture from the air. This process is often called Dehumidification, which the moisture or water vapor or the humidity is removed from the air to achieve low relative humidity
Why do we need dehumidification?
Malaysia weather is a tropical rainforest climate, which is typically hot and wet throughout the year and rainfall is both heavy and frequent. Which show that our weather is humid throughout the years and with relative humidity level is ranging from 60 – 80%RH, and sometimes may goes as high as 95%. Dehumidification is needed to remove the excess moisture in the air which would cause problems such as:
Maintaining an ideal relative humidity is important for health and comfort. ASHRAE Standards 62.1-2016 (The American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommends indoor relative humidity below 65% to reduce microbial growth. EPA 2012 (United States Environmental Protection Agency) also recommends indoor relative humidity between 30 and 60% to reduce mold growth.
Method of dehumidification
There are two common ways to remove the moisture from the air: by cooling it to condense the water vapor, and by passing the air over a desiccant, which pulls moisture from the air through differences in vapor pressures.
Most people are familiar with the principle of condensation. When air is cooled down below its dewpoint temperature, moisture will condensate on the surface and air is dehumidified by the process of cooling and condensation. After the air is cooled and dehumidified, the air will be pre-heated before it is returned to the treated room. Most of the refrigerant type dehumidifiers such as residential dehumidifier, portable dehumidifier, commercial dehumidifier, industrial dehumidifier, ducted / mounted dehumidifier and wall mounted dehumidifier use this configuration.
This system is efficient if the treated room air is both warm and very moist. Typically, refrigerant dehumidifier is recommended for treated room temperature at 22 – 30 deg. C and to achieve humidity level at 40% – 60%RH.
The table show below is the recommendation of refrigerant dehumidifier based on area size.
(with height at 10ft)
|RH Required||Room Temperature||Propose Capacity||Application|
|100 – 200 sq ft||45 – 60%||23 – 30°C||20 -35L/day||Light Usage|
|200 – 400 sq ft||45 – 60%||23 – 30°C||35 -55L/day||Light to Medium|
|400 – 600 sq ft||45 – 60%||23 – 30°C||55 – 75L/day||Light to Medium|
|600 – 800 sq ft||45 – 60%||23 – 30°C||110 – 160L/day||Medium to Heavy|
|800 – 1000 sq ft||45 – 60%||23 – 30°C||160 -210L/day||Medium to Heavy|
Note: This table is only a general guideline. It would require more detail information to select the suitable dehumidifier.
Desiccant dehumidifiers is different from cooling-based dehumidifiers. Instead of cooling the air to condense its moisture, desiccants dehumidifiers attract moisture from the air by creating an area of low vapor pressure at the surface of the desiccant wheel. The pressure exerted by the water in the air is higher, so the water molecules move from the air to the desiccant and the air is dehumidified.
Desiccant dehumidifiers make use of changing vapor pressure to dry the air continuously in a repeating cycle, which is also based on the adsorption principal. Treated air is simply passed through a porous wheel of solid desiccant and its moisture in the air is absorbed and the humidity level is lowered. After the desiccant wheel absorb the moisture, it will become saturates and to be used again it must be ‘recharged’ or also called as ‘regenerated’ by heating it up until the absorbed moisture is evaporated. The evaporated moisture will be driven off the desiccant wheel and goes right back to outdoor air or to surrounding ambient, which is not the treated room. The desiccant dehumidifier is designed so that the treated air is separated from the incoming air stream to dry the wheel, which will be exhausted to the outside.
The desiccant wheel is slowly rotating in the system to continuously absorb moisture to keep the treated air dry and regenerated using heater to help remove the absorb moisture from the desiccant. Roughly 3⁄4 of the time the desiccant will be absorbing moisture out of the treated room air, and the remaining 1⁄4 of the time it will recharge.
This system is efficient if the treated room air is both low in temperature and relative humidity. Typically, desiccant dehumidifier is recommended for treated room temperature at 22 deg. C and below, and to achieve humidity level at 5% – 40%RH.
Cooling-based dehumidifiers VS Desiccant dehumidifiers
Both cooling-based dehumidifiers / refrigerant dehumidifier and desiccant dehumidifier can remove moisture from air. So the question arises which type of dehumidifier to be used to achieve good result? There are no simple answers to this question, but there are some general guidelines that have emerged in the industry.
This type of dehumidifiers is more economical and more energy efficient than desiccants dehumidifier when the treated air temperature and moisture levels is high. It is typically applied in conditions of 22 – 30 deg. C, and to achieve relative humidity at 40% – 60%RH. They are very seldom use in room conditions that are 20 deg. C and below, because condensate might freeze on the coil, thus reducing moisture removal capacity.
Desiccant have useful advantages when treating ventilation air for building HVAC systems, and dehumidifying the fresh air with the desiccant system decrease the installed cost of the centralized cooling system, and eliminates thick coils with high air and liquid-side pressure drops. This help saves considerable fan and pump energy.
Desiccants are especially efficient when drying air to create low relative humidity at low temperature conditions. Typically, it is recommended for applications with room temperature below 20 deg. C, and to achieve relative humidity at 5% – 40%RH.
Applications for dehumidifiers