Dehumidifier

A dehumidifier ( colloquially dehumidifier ) is a device that is the absolute humidity in a room ( then known as Raumentfeuchter ) or containers lowers.

Purpose

Dehumidifiers for drying of new buildings, used after the occurrence of water damage and for rooms that require large amounts of water vapor, such as swimming pools. Here is the prevention of condensation (and mold ) of thermal bridges of structure ( exterior walls ) in the foreground. In addition, they are used in combination with a humidifier to keep the humidity constant in a room.

Operation

The usual dehumidifiers work by three fundamentally different physical methods:

• Air cooling with water excretion ( condensation );
• Absorption in hygroscopic liquids;
• Adsorption of water vapor on an adsorbent.

In the condensation of water an energy of about 0.62 Wh / g is free. This energy must be dissipated immediately by cooling the air, or subsequently by regeneration of saturated absorber. Typically, the performance is measured in condensate per day. Converted corresponding to 10 kg of condensate per day a power of 300 W typical (theoretical lower limit: 260 W)

The drying capacity depends on the temperature. Air with a temperature of 20 ° C can hold a maximum of 17 g / m³ of water. Hence A room with a floor area of ​​20 m² contains less than 0.8 liters of water in the air. At 10 ° C, the value drops to half. In addition, the lower temperature reduces the evaporation rate. Damp basements walls can therefore be very bad dry through dehumidifiers, but rather by additional heating.

The energy balance is significantly less expensive if a dehumidifier is used simultaneously as a heater. Because then stand the heat of condensation of the water and the heat loss of the device as heat energy available.

Air cooling with water excretion

Wherein the air cooling with water separation (condensation drying) the air to be dried is passed by means of a fan via a heat exchanger. As cooling agents are here eg tap water, well water or brine used. The water runs down on the cooled surfaces of the heat exchanger and is collected in a condensate container. Is critical in this process that the surface temperature of the heat exchanger is lower than the dew point of the air. In dehumidifiers for household use or in Bautrocknungsgeräten the dew point is effected by a closed cooling circuit: In the drying unit, a compressor is installed, which ensures a circulation of refrigerant in a refrigeration cycle. The warm humid air is sucked by a fan and cooled abruptly to the evaporator surface. The humidity condensed and the water is collected in a container and discharged via a hose or is derived.

Since this process is carried out at temperatures near the freezing point, it is thereby easy to icing of the evaporator. Therefore only devices are workable, that have an " automatic defrost ".

In general, the so -working dehumidifiers are equipped with an electronic hygrometer and turn off (or on low power) when a certain value of relative humidity is reached. For units with a condensate tank, the water level is monitored and turned off the device at condensate tank is full.

Absorption in hygroscopic liquids

In the absorption in hygroscopic liquids to be dried air is passed by a fan through a hygroscopic liquid. This is usually from an aqueous salt solution of lithium chloride, lithium or calcium chloride. The steam passes into the hygroscopic solution and diluted it. The absorbance of the solution increases with increasing pressure, decreasing temperature and increasing water vapor concentration in the air. Through the released heat of absorption cooling of the liquid or dried air may be necessary.

The hygroscopic liquid required after a certain time of regeneration. This is generally done by heating to be dried outside the premises with or derivative of the resulting vapor.

Adsorption of water vapor

The adsorption of water vapor the air to be dried is fed by a fan over an adsorbent. In technical applications, there are usually silica gel, often also called molecular sieve. The water vapor accumulates on the adsorbent and condenses there. The adsorption capacity of the adsorbent increases with decreasing temperature and increasing water vapor concentration in the air. Through the released adsorption and condensation heat cooling of the adsorbent or the dried air may be necessary.

The adsorbent needed for a certain period of regeneration. This is usually done by drying with hot air. At the molecular sieve can take place dries. This happens especially in systems in which the air to be dried is compressed. The re-drying process usually takes place in two dry containers. In a vessel, the compressed air is passed through the adsorber which this moisture from the air ( by binding to the granules of the molecular sieve ). Then, a portion of the compressed and dried air is passed through the second container to the atmosphere. Through the expansion of the air is considerably more receptive to moisture and removes the molecular sieve, the previously accumulated water molecules. Periodically, this process between the two containers is switched back and forth, so that a depending on the desired degree of dryness of the compressed air output higher or lower percentage of air dried ready for further use.