Dehumidifier
A dehumidifier (colloquially dehumidifier ) is a device that lowers the absolute humidity in a room (also called a room dehumidifier ).
purpose
Dehumidifiers are used to dry new and old buildings as well as in historical buildings, after the occurrence of water damage and for rooms in which high amounts of water vapor occur, such as B. Swimming pools. The focus here is on preventing condensation (and thus the formation of mold ) on thermal bridges in the building (e.g. poorly insulated exterior walls). In addition, they are used in combination with air humidifiers to keep the humidity in a room constant.
Smaller dehumidifiers (usually with adsorbents) can also be used in vehicles to prevent the vehicle windows from misting up from the inside.
Mode of action
The common dehumidifiers work according to three fundamentally different physical methods:
- Air cooling with water excretion ( condensation );
- Absorption in hygroscopic liquids;
- Adsorption of the water vapor on an adsorbent .
When water condenses , an energy of approx. 0.62 Wh / g is released. In the case of condensation dehumidifiers, this energy must be dissipated immediately by cooling the air; in the case of adsorption dehumidifiers, it must be removed later by regenerating saturated absorbers. Typically the output is given in condensate per day . Converted, 10 kg of condensate per day corresponds to a cooling capacity of typically 300 W (theoretical lower limit: 260 W)
The drying performance depends on the temperature. Air with a temperature of 20 ° C can absorb a maximum of 17 g of water per m³. A room with a floor area of 20 m² therefore contains less than 0.8 liters of water in the air. At 10 ° C the value falls by half. In addition, the lower temperature reduces the evaporation rate. Damp basement walls are therefore very difficult to dry with air dehumidifiers, but rather with additional heating.
The energy balance is significantly more favorable if a dehumidifier also serves as a heater. Because then the condensation heat of the water and the heat loss from the device are available as heating energy.
Air cooling with water elimination
In the case of air cooling with water separation ( condensation drying), the air to be dried is passed through a heat exchanger by means of a fan . The water runs off the cooled surfaces of the heat exchanger and is collected or diverted in a condensate container. It is of crucial importance in this process that the surface temperature of the heat exchanger is lower than the dew point temperature of the air.
The heat exchanger can, for. B. be cooled with tap water, well water or brine . In dehumidifiers for household use and in building drying devices, the dew point is not reached by a closed cooling circuit: A compressor is built into the drying device, which ensures the circulation of refrigerant in a cooling circuit ( compression refrigeration machine ). The warm, moist air is sucked in by a fan and directed to the evaporator (in which the refrigerant evaporates at a low temperature), where the humidity condenses. The air then flows over the refrigerant condenser to cool it and is returned to the room. Since this process works with temperatures close to the freezing point, the evaporator can easily freeze up. This leads to a constant decrease in the dehumidification performance up to a total failure of the system. Therefore, only devices that have an "automatic defrosting process" are practical. A distinction is made between electronic defrosting and automatic hot gas defrosting.
As a rule, the dehumidifiers that work in this way are equipped with an electronic hygrometer and switch off (or to lower power) as soon as a certain value of the relative humidity is reached. In devices with a condensate container, the water level is monitored and the device switches off when the condensate container is full.
For household use there are also inexpensive devices with a Peltier element as a cooling machine. However, their performance is considerably lower and they do not work as efficiently either, since a Peltier element achieves a lower coefficient of performance than a compression refrigeration machine.
Absorption in hygroscopic liquids
When absorbing in hygroscopic liquids, the air to be dried is passed over a hygroscopic liquid by means of a fan. This usually consists of an aqueous salt solution of lithium chloride , lithium bromide or calcium chloride . The water vapor passes into the hygroscopic solution and dilutes it. The absorption capacity of the solution increases with increasing pressure, decreasing temperature and increasing water vapor concentration in the air. Due to the heat of absorption that is released, it may be necessary to cool the liquid or the dried air.
The hygroscopic liquid needs to be regenerated after a certain time. This is usually done by heating outside of the rooms to be dried or by dissipating the resulting steam.
Adsorption of water vapor
During the adsorption of the water vapor, the air to be dried is passed over an adsorbent by means of a fan. In technical applications it is mostly silica gel , often also a so-called molecular sieve or salts ( deliquescence ). The water vapor is deposited on the adsorbent and condenses there. The adsorption capacity of the adsorbent increases with decreasing temperature and increasing water vapor concentration in the air. Due to the heat of adsorption and condensation released, it may be necessary to cool the adsorbent or the dried air.
The adsorbent needs to be regenerated after a certain period of time. This is usually done by drying with hot air. In the case of molecular sieves, re-drying can take place. This happens primarily in systems in which the air to be dried is compressed. The re-drying process usually takes place in two drying containers. In a container, the compressed air is sent through the adsorber, which removes the moisture from this air (by attaching it to the granulate of the molecular sieve). Then some of the compressed and dried air is passed through the second container against the atmosphere. As a result of the expansion, the air becomes considerably more absorbent for moisture and removes the previously accumulated water molecules from the molecular sieve. This process is switched back and forth between the two containers at regular intervals so that a higher or lower percentage of the dried air is available for further use, depending on the desired degree of dryness of the compressed outlet air.
literature
- Recknagel, Sprenger, Schramek: Pocket book for heating and air conditioning . 68th edition, R. Oldenbourg Verlag Munich, Vienna, 1997, pp. 1190-1194, ISBN 3-486-26214-9
