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Dehumidification methods
Two techniques – one objective: the controlled reduction of excessive humidity
Before going into the details of the two dehumidification methods, we first have to dispel a myth:
Heating makes the air warmer, not drier
Heating is certainly not among the dehumidification methods! It is true that warm air can absorb more water than cold air. Thus, by heating the room air with a constant water content, the relative humidity level of this warm air would indeed be reduced at first.
But the hotter the air, the more it is surrounded by cold surfaces on which the moisture condenses. After all, heating alone cannot make moisture disappear – and therefore the water content of the air remains the same.
To withdraw moisture from the room air effectively and permanently, the only technical solution available is dehumidification, either by condensation or desiccation.
Condensation versus desiccation
All devices offered on the market with designations such as refrigerant dryer, condenser dryer, condensation dehumidifier, electric or Peltier dehumidifier are based on the principle of condensation.
On the other hand, there are technologies for desiccant dehumidification. These include the still widely advertised granules. However, truly effective, permanent dehumidification can only be achieved with this method when using electrical devices with hot air regeneration, more commonly known as desiccant dehumidifiers.
It is just a matter of technology
Even if many device designations on the market may vary, the devices usually belong to either of the two groups, and their names provide information on the dehumidification technology used.
Leaving aside the granules, the procedure is the same for all electrically operated devices: ambient air is sucked in by a fan, moisture is withdrawn from the air inside the device, and drier air is emitted into the room, which continuously mixes with the more humid room air until the desired humidity level is reached.
However, the dehumidification methods as well as the fields and limits of application of both device groups differ considerably:
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Condensation
As already shown in the saturation curve in chapter 1, the water absorption capacity of the air solely depends on its temperature. The lower the temperature, the less water can be bound by the air.
But what happens if the air enriched with water cools down abruptly, for example due to contact with a colder surface?
In this case, the saturation limit of 100 % RH is exceeded and the air can no longer bind the excessive moisture, which consequently condenses on the cold surface and turns into water.
Even air has to blow off steam sometimes
Since the water vapour condenses to water at this temperature limit, it is referred to as the dew point. You certainly know this phenomenon from cold glass bottles in the summer on which condensate forms, or from fogged window panes in the winter as well as bathroom mirrors while you are taking a shower. Even the foggy morning dew is a visible sign of moisture-saturated cold air.
So when air cools down, it can absorb less water vapour and the excessive moisture condenses on the colder surfaces.
This is the physical principle condenser dryers – also known as refrigerant dryers – are based on. They cool down the air flowing through the device to below its dew point and withdraw the moisture contained therein by condensation on a cold surface.
The market offer for refrigerant dryers ranges from powerful condenser dryers with compressor technology – so-called refrigerant compression dryers – to extremely compact electric or Peltier dehumidifiers with a low absolute energy demand, however, also a considerably lower active power and a substantially poorer energy balance.
Simply put, electric dehumidifiers require four times more energy than compressor devices to draw one litre of water from the air.
Desiccation
While the dehumidification process is based on the dew point in case of condensation dryers, desiccant dehumidifiers use the principle of sorption. Here, the vapour pressure gradient between the humid air and a hygroscopic sorption agent is used for water withdrawal from the air.
This category also includes dehumidification granules, although they are at best suited for keeping the inside of small, closed containers dry.
Granules – a rather bland solution
The original and main purpose of these pouches is to temporarily protect moisture-sensitive goods during transport and storage. Everyone knows the little bags that come in new handbags, electronic devices, pharmaceuticals or clothing.
Therefore, granules do not serve as a true alternative to dehumidifiers. Moreover, they are an uneconomical single-use solution requiring the user to regularly buy fresh granule bags for the receptacle since the granules are not regenerated. Much like a sponge, the desiccant permanently absorbs water from the air and must be replaced as soon as it is saturated – a highly cost-intensive and environmentally harmful procedure in the long run.
The case is different with electrical devices with hot air regeneration. They have an integrated, rotating desiccant wheel coated with highly hygroscopic materials such as silica gel or lithium chloride that withdraw the water molecules from the sucked in air flowing through the desiccant wheel.
To ensure that the desiccant wheel can take up moisture continuously, the latter must be given off again in some way. This is done by hot air generation: hot air is guided through a regeneration zone of the desiccant wheel and uses its thermal energy to remove the water vapour previously bound in the rotor from the silica gel.
For more information regarding condensation please refer to the following chapters:
Chapter 2.1: Condenser dryers with compressor technology
Chapter 2.2: Condenser dryers with Peltier technology
For more information regarding desiccation please refer to the following chapters:
Chapter 2.3: Desiccant dehumidifiers
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Practical knowledge concerning dehumidifiers – summary of all chapters
Chapter 1: Basic knowledge regarding humidity – relative is everything
Chapter 2: Overview of dehumidification methods – condensation and desiccation
Chapter 2.1: Condenser dryers with compressor technology
Chapter 2.2: Condenser dryers with Peltier technology
Chapter 2.3: Desiccant dehumidifiers
Chapter 3: Which dehumidification method for which purpose?