How does a
dehumidifier work?

Illu _2-0NY Bred

How does a mobile dehumidifier work?

The basic functional principle of a condense drying dehumidifier is really quite simple. A fan draws in humid air and carries it through a refrigerated evaporator. The air is cooled well below its dew point. The water condenses on the cold surface of the evaporator and drips into a water container or is led directly to a drain. Then the cold dry air continues through a hot condenser which heats it up and returns it to the room to pick up new humidity. This procedure is continued until the desired condition is achieved.

Temperature and airflow

Illu _2-1 Bred 

Temperature and RH-value
1. 25°C 70% RH
2. 17°C 88% RH
3. 18°C 85% RH mixed air flow
4. 33°C 35% RH


In the illustrated example 25°C hot air with 70% RH (relative humidity) (1) enters the evaporator. Inside the refrigerated evaporator (2) the air temperature drops to 17°C and the RH increases to 88%, resulting in condensation and the water is dripping off into a container.

To remove all of the water even with relatively dry air conditions, it is important that not all the air is cooled down by the evaporator as there is a risk that the dew point cannot be fully achieved. Instead only part of the air is led through the evaporator to ensure maximum condensation while the rest is by-passed as shown above. This results in a mixed 18°C and 85% RH air fl ow between the evaporator and the condenser (3). When passing the hot condenser the mixed air flow will ensure that the condenser is sufficiently cooled. The final result is an outlet air temperature from the dehumidifier of 33°C and 35% RH (4). The temperature is increased because energy is added by the compressor and by the latent heat from the condensation process.

Humidity control

The internal hygrostat on the display allows control of exactly how much you want to lower the relative humidity. Set the degree of relative humidity required, and the hygrostat will automatically stop the dehumidification process when the value is met. This way you do not risk possible damage from drying out materials too much, and you get a much more energy efficient dehumidification process.

Older CDTs do not come with a built-in hygrostat, but an external hygrostat can easily be connected to all CDTs if needed.

Temperature control

If the room temperature is outside the operating range (3-32°C) the dehumidifier stops. It starts up again automatically when the room temperature is once again within the operating range. This means that the dehumidifier will keep running as long as the room temperature remains within the operating range, continuously reducing the RH-value.  

Principal functionality of the various components:

CDT 30 and 30 S with capillary tube

1: Compressor
2: Evaporator
3: Condenser
4a: Capillary tube
5: Liquid line drier
6: Solenoid valve
7: Fan
8: Receiver

      Diagram 01 



Illu _2-2b


CDT 40, 40S, 60 and 90 with thermostatic expansion valve

1: Compressor
2: Evaporator
3: Condenser
4b: Thermostatic expansion valve
5: Liquid line drier
6: Solenoid valve
7: Fan
8: Receiver



The compressor (1) takes hot gas from the low pressure side and presses it into the condenser (3). The fan (7) draws the cold air from the evaporator (2) through the condenser (3) where it is heated up by the hot gas. In this process the gas is cooled down and ends up as liquid in the receiver (8).

The now high pressure liquid refrigerant is passed through a liquid line drier (5) that removes any unwanted moisture from the refrigerant. The refrigerant is then passed through a capillary tube or a thermostatic expansion valve (4a/4b) to reduce the pressure before it enters the evaporator (2), where it reaches its boiling point and turns back into a low pressure hot gas.

Capillary TubeBasically a capillary tube and a thermostatic valve serve the same purpose. Namely to reduce the pressure from high to low level and to control the flow of refrigerant through the evaporator. At low pressure levels the heat from the air drawn through the outside of the evaporator will turn all the refrigerant inside the evaporator into gas.

The capillary tube is a static resistance. All the refrigerant has to pass through a long thin tube, reducing the pressure.

The thermostatic expansion valve is a dynamic resistance. The sensor provides feedback to the valve, causing the valve to open a little or vice versa. If the evaporator does not get sufficient refrigerant the sensor temperature will increase, causing the valve to open a bit and vice versa. 

Thermostatic ValveCompared to a capillary tube a thermostatic expansion valve can compensate for differences in the RH-value and the temperature of the air drawn into the dehumidifier. This clearly makes it the better solution when it comes to larger dehumidifiers, but it is a more expensive solution and no significant difference in performance is achieved when using it in smaller units. 



Depending on the room temperature and the RH-value of the air, the evaporator will run very cold. In general lower air temperature means lower evaporator temperature. If the air temperature is below approximately 15-20°C (depending on the relative humidity) ice will start forming on the surface of the evaporator.

If the ice is allowed to accumulate on the evaporator, it will reduce the dehumidification capacity of the unit. To prevent this, defrosting is carried out by means of hot gas from the compressor.

Illu _2-3b

1: Compressor
2: Evaporator
3: Condenser
4a: Capillary tube
5: Liquid line drier
6: Solenoid valve
7: Fan
8: Receiver


When the set temperature of 5°C is reached on the surface of the evaporator a timer is activated and after 30 minutes the solenoid valve (6) opens, and hot gas starts to flush to the evaporator, efficiently melting the ice on the surface. When the set temperature is reached the solenoid valve closes and the system returns to normal active mode again.


Selecting the right size dehumidifier can be challenging. There are several factors to look for when selecting the right dehumidifier.

In the box to the right you can get a free copy of Dantherm's Guide how to select the right size of dehumidifier.

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