Generally the buildings with HVAC systems may have an increased risk of sick building syndrome (SBS) and building-related illness (BRI), if IAQ were not well maintained.  Microorganisms are brought into indoor environment by people, air currents, water, construction materials and equipment. The diluted indoor environment requires special attention to ensure healthful indoor air quality (IAQ) to protect occupants against infection of harmful outdoor pollutants. Maintaining indoor humidity is an important aspect of maintenance of indoor thermal comfort environment in various residential HVAC applications. Various means for efficiently controlling the indoor humidity described
here in detail.


Humidity control is a critical aspect of treating air for air conditioning to maintain outdoor supply of fresh air. Moisture affects building materials, the comfort of building occupants, and, in many cases, the work performed in the facility — particularly, if the function of the building is the handling and processing of hygroscopic materials. In addition to making you feel warmer, humid conditions cause a lot of other problems around the home, including mold, mildew, and property damage.

-Fatigue, headache, heat exhaustion, and heat stroke; Poor sleep; Higher blood pressure; Worse allergies and asthma; Respiratory issues.
-Pest and insect risks; Mold, mildew, and moisture; Dust mites; Peeling paint and wallpaper; Viruses.
-Damaged electronics and Sticky doors and windows.

Even though an air conditioner naturally cools the air through its refrigeration cycle, you may be in need of extra dehumidification power. Given its importance, humidity is ironically one of the more misunderstood aspects influencing the internal environment of a building. There are usually two ways to control indoor humidity efficiently by use of conventional and advanced mechanisms. The first one utilises mechanical means—compression refrigeration systems—to cool the air and so to dehumidify it; the latter removes the water vapour from the air by transferring it towards a desiccant material (adsorption or absorption). In the mechanical dehumidification field, a proper control of ambient temperature and humidity can be obtained by means of an air handling unit (AHU) which treats outside air alone, while re-circulating air is treated by a simple cooling coil. The use of hybrid systems with desiccant wheel for these applications has provided the following main results: remarkable savings in operating costs and lower plant costs; a notable reduction of the power electric demand; a better control of ambient humidity. A primary concern with dehumidification-system design is energy consumption. The majority of the energy expended for dehumidification is for the removal of moisture from the air stream because condensing moisture out of air is a change in the state of the vapor. Cooling 1 lb of water 1 F requires approximately 1 Btu of energy, while condensing 1 lb of water vapor to water takes approximately 1,000 Btu.

Function and type of dehumidifiers

A dehumidifier reduces and regulates the humidity in the air. They can be used for single rooms and apartments or large commercial areas, such as swimming pools and storage warehouses.

Heat Pump Dehumidifier – These dehumidifiers need a fan, heat pump, and heat exchange coils to remove moisture from the air. Using a fan, air gets sent past the heat exchange coils, which are extremely cold. Moisture then condenses and collects. Dehumidifying Ventilator – Similar to how it sounds, this humidifier works by using an exhaust fan to expel air outside. It uses a sensor and exhaust fan and is most often used in basements, attics, and crawlspaces.

Chemical Absorbent Dehumidifier – Also known as desiccant dehumidifiers, they consist of hydrophilic materials, such as silica gel. Many residential units contain single-use desiccant-type cartridges, gel, and powder.

Typical systems

Dehumidification is accomplished in a variety of ways. The method used often is dependent on the specific temperature and humidity conditions required in the space and the climate conditions at the site. The systems used in a specific application are chosen based on their effectiveness and efficiency in achieving the desired conditions. In addition, a life-cycle-cost analysis that compares first cost, operating costs, and maintenance costs is applied. Typical methods of dehumidification include the application of cooling, liquid and solid desiccants, and compression.

Conventional cooling and dehumidification (condensation dehumidifiers)

The most common method of dehumidification is the use of cooling coils. This method typically involves a coil in an air stream with a fluid circulated through the coil at a temperature below the dew point of the air stream. The amount of moisture removed is relative to the temperature of the fluid (and coil). Typical fluids used are chilled water, refrigerants, glycol solutions, or engineered fluids. Because the coil temperature is below the dew point of the air stream, the moisture that condenses on the cooling coils is collected in a pan and removed.

With the current trend toward greater sustainably; this water can be reclaimed for a variety of uses, such as for makeup for cooling towers, lawn irrigation, or toilet flushing. One manufacturer has a dehumidification unit that yields drinking water. The corresponding reduction in water use can contribute to certification of green buildings.

Because, this method also reduces the temperature of the HVAC-system supply air, there is the possibility that the spaces served could be sub-cooled if the rooms do not have sufficient internal heat gain to offset the low-temperature air. This traditionally has been accomplished with reheat coils that maintain space temperature, which can represent a significant energy-cost penalty, depending on the conditions to be attained. Applications that require air temperature to be driven down for dehumidification purposes, but do not require the amount of cooling, can be provided with a heat-pipe run-around system. These systems consist of coils on either side of the cooling coil and use a refrigerant to economically pre-cool and reheat the air stream.

This method consists in removing humidity from the air by cooling it below the dew point which causes out-dropping of humidity (condensation). Condensation air dehumidifiers are used in this method. Their main elements include extractor fan, compressor, heat exchangers (condenser and evaporator), and expanding element. The extractor fan forces flow of humid air through heat exchangers. The evaporator temperature is lower than dew point temperature which causes out-dropping (condensation) of steam contained in the air on its walls. The condensate is gathered in dehumidifier tank or removed to sewage system or outside. After passing the evaporator, the cooled and dried air flows through condenser where it is heated. Thanks to this, the value of relative humidity decreases even more. Dry air from the condenser is sent back to the room it comes from.

The temperature of air coming out of dehumidifier is 3-8°C higher than the temperature of sucked air. The above- mentioned temperature increase may cause faster evaporation of water e.g. from wet walls which facilitates dehumidifying and does not pose a risk of damaging as in the case of dehumidifying by heating and ventilating. The amount of water contained in the air is effectively decreased along with expansion of operation time of machines in a closed room.

Effectiveness of condensation dehumidifiers depends on operating conditions (temperature and humidity) and on type of machines (size, manufacturer). It reaches its maximum value with higher values of temperature and relative humidity. Thus, decreasing of water contents in the air causes a decrease of efficiency of the machines. The specific nature of these machines makes it impossible to use them in temperatures below 0-5°C (depending on a model). Condensation dehumidifying is definitely more effective than and economical than drying by means of heating and ventilating primarily, thanks to eliminating exchange of air contained in the room.

The main operation principles of compressor-based condensation dryers:

-Such devices are meant to be used indoors (with closed doors and windows).
-Minimum distance from walls should be observed (usually 0.2 – 0.5 m).
-Placing the device in the centre of a room improves its efficiency.
-Do not place the dehumidifier near any heat sources.
-Do not place any object on the device.
-Do not cover the air inlet or outlet.
-Use the dryer within the temperatures specified in the manual.
-Most devices should be stored and transported in the same position in which they are operated.
-Achieving the air parameters with a dew point below 2-5°C is not possible.

Heating and ventilation

This method consists in raising air temperature in a room and applying intense ventilation. Its efficiency depends on external conditions that can even make it impossible to use the method. The lower external temperature and the higher temperature of the dehumidified room, the more effective the method. Thus, the best effects can be achieved in winter, worse in the spring and autumn, and the worst ones in summer.

In the case of dehumidifying humid walls, temperature in the room should not exceed 35°C. Higher temperatures can cause excessive increase in pressure of steam contained in walls which leads to cracking and damaging of the walls. Lack of proper ventilation (air exchange) while dehumidifying humid walls with this method causes steam to be absorbed by drier parts of walls and the ceiling. One should also mention that air heating with gas heaters gives the opposite effect as compared to the planned one. This is because burning propane-butane produces carbon dioxide and steam. In result, not only air temperature but also air humidity increases.

Using this method is connected to high costs. This is mainly a result of its low effectiveness (long time of dehumidifying) and the fact that the best effects are achieved at the largest temperature differences (large demand for heating power).

Liquid desiccant dehumidification

Desiccant dehumidification systems often are used where the dehumidification requirement is greater than the cooling requirement. One type, the liquid desiccant system, involves spraying a liquid desiccant solution (typically, lithium chloride) through an air stream, where it absorbs moisture from the air stream and provides some cooling. The exact conditions can be varied by changing the concentration of the lithium-chloride solution. One benefit of using systems such as this is that the desiccant solutions act as a biocide for the conditioned air, which is beneficial in applications for which bacteria or viruses are least desirable. The solution then is regenerated by heating it, where it loses its moisture before spraying it across the air stream again. A hot, moist waste-air stream also is exhausted from the regenerator.

Basic principles concerning operation of adsorptive dehumidifiers:

-These machines are meant to be dehumidify air in closed rooms.
-It is possible to use the dehumidifier in the dehumidified room or outside the room.
-Remove humid air through ducts outside the dehumidified room.
-Use the shortest ducts possible.
-Use ducts made of stainless steel.
-Do not cover air inlet or outlet.

Solid desiccant dehumidification

Another type of dehumidification approach employs a solid desiccant, such as silica gel. Moist air is drawn through the desiccant, which absorbs the moisture. As the desiccant reaches its capacity, it is moved into a warmer air stream (called the reactivation air stream), where it rejects the moisture before it is exposed to the moist air stream again. This often is accomplished by running supply air through a desiccant wheel. Solid desiccants need to be regenerated, which can be accomplished using waste heat or an exhaust air stream.

To explain further, desiccant dehumidification is a simple way to obtain dry air by using desiccants (adsorbents or materials that have a natural affinity for water). A desiccant is able to take up the additional moisture given up by the air without changing its size or shape. So, an airstream can pass through a desiccant and become significantly drier without elaborate cooling, compression, or other complex systems or controls. After the drying task is complete, the desiccant is dried using hot air in a process called regeneration and is ready to dry more air. Salient features of desiccant dehumidification are as follows:

-Desiccant dehumidification is the most accepted method of dehumidification across industries.
-A desiccant dehumidifier is the most energy-efficient.
-It can maintain RH level as low 1%.

This method consists in removing humidity from the air, absorbing it by hygroscopic materials. The main devices used in this method are adsorptive dehumidifiers whose major elements include rotor along with a driving unit, extractor fans, air heater, filter, casing, and fittings. The rotor is most frequently made of properly profiled aluminium sheets (creating axial capillaries), whose surface is covered with hygroscopic substance. Such a construction results in a large increase of humidity adsorbing area. The rotor is driven by an electric motor by a transmission (belt type most often). The device is divided into a dehumidifying sector and regeneration sector in result of which dehumidified air is achieved behind the rotor. While turning, the rotor causes the humid hygroscopic material gets to the regeneration sector where hot air flows through the rotor to remove humidity that is then sent outside. Additional feature of the rotor is its high durability, wash-ability, ability of self-cleaning and preventing development of bacteria. A great advantage of this type of dehumidifying is the possibility to dehumidify air without cooling as well as dehumidifying air of sub-zero temperature. Due to a multitude of advantages of this method, it is widely used in industry, e.g. pharmaceutical, food, and in AC systems.


Some process systems use compression as a means of reducing the absolute moisture of air. Compressing air reduces its ability to hold moisture, so water condenses once the air is compressed to its saturation point. This is the first stage of dehumidification for a process such as instrument air and usually is followed by additional dehumidification (coil or desiccant) so air is not saturated.

On the horizon

Still in the laboratory, but possibly coming soon, is a new technology called electrostatic dehumidification, which uses electrostatic fields to separate moisture vapor from air streams, so that only the portion of air with less water vapor is used in the process. This is particularly exciting because of the possibility to significantly decrease the energy used for dehumidification by eliminating the need to cause water to change state from a vapor to a liquid. Depending on the specific application, this approach may become an attractive alternative to the other, more traditional methods outlined above.

Dr. (Prof.) D.B. Jani received Ph.D. in Thermal Science (Mechanical Engineering) from Indian Institute of Technology (IIT) Roorkee. Currently he is recognized Ph.D. Supervisor at Gujarat Technological University (GTU). Published more than 150 Research Articles in reputed International Conferences and Journals. Presently, he is an Associate Professor at GEC, Dahod, Gujarat Technological University, GTU, Ahmedabad (Education Department, State of Gujarat, India). His area of research is Desiccant cooling, ANN, TRNSYS, and Exergy

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