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:

heating and ventilation
condensation dehumidifying
adsorption dehumidifying

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).

Dehumidifying by ventilation and heating:
1 - air removed outside; 2 - air sucked from outside; 3 - extractor fan; 4 - heater; 5 - heated air

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 (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.

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

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