How does a
dehumidifier work?

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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

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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 controlHygrostat 01
If for some reason the dehumidifier only needs to be working within a given range of RH-values a hygrostat can easily be connected to the dehumidifier. Set the degree of relative humidity required. The hygrostat stops the dehumidifier when the required RH-value is reached, and automatically starts it up again if the relative humidity of the air increases above the desired RH-value.

Temperature control
If the room temperature is outside the operating range (3-30°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 componentsIllu _2-3b

CDT 30, 30 S, 40, 40 S with capillary tube

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

 

  

 

Illu _2-2b

 

 

CDT 20, CDT 60 and CDT 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. 

  



Defrosting

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.

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