Methods of dehumidifying :

Air dehumidifying is a process that consists in decreasing the contents of steam

contained in the air. Air can be dehumidified in several ways:

1-Heating and ventilation 2-Condensation dehumidifying

3-Adsorption dehumidifying 4- Liquid-desiccant systems
5- Solid-desiccant systems 6- Compression
7- Spray dehumidifiers

1-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).

1- air removed outside 2- air sucked from outside 3- extractor fan 4- heater
5- heated air
2- Condensation dehumidifying :

This method consists in removing humidity from the air by cooling it below the dew
point which causes outdropping 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 outdropping 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.

Condensation dehumidifying

1- humid air 2- filter 3- evaporator 4- dripping tray 5- condensate tank

6- dehumidified and cooled air
7- condenser 8- extractor fan 9- dehumidified and heated air
3- Adsorption dehumidifying :

This method consists in removing humidity from the air by its absorbing 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, washability, 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.

Adsorption dehumidifying

1- filter 2- humid air 3- rotor 4- dehumidified air 5- extractor fan

6- regeneration air 7- heater
8- hot regeneration air 9- humid regeneration air
4- Liquid-desiccant systems :

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.

5- Solid-desiccant systems :

Another type of dehumidfication 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.
6- Compression :

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.

7- Spray dehumidifiers :

When water is chilled below the atmospheric dewpoint, atmospheric water will
condense onto it faster than water evaporates from it. Spray dehumidifiers mix
sprays of chilled water and air to capture atmospheric moisture. They also capture
pollutants and contaminants like pollen, for which purpose they are sometimes
called "air washers".

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.

REFERENCES :

1- Gatley, D.P. (2002). Understanding Psychrometrics, American Society of Heating,

Refrigerating and Air-Conditioning Engineers Inc., Atlanta.

2- Gatley, D.P. (2000, September, October, November). “Dehumidification

Enhancements for 100-Percent-Outside-Air AHUs,” HPAC Engineering.

3- Harriman, L. (June, 2003). “20 Years of Commercial Desiccant Systems,” HPAC

Engineering.