Rotary dehumidifier

from Wikipedia, the free encyclopedia
How it works process air dryer ULT Dry-Tec®
Process diagram for the dehumidifier

A rotary dehumidifier (also sorption rotor , sorption wheel or sorption regenerator ) ( desiccant wheel ; desiccant rotor ) is a device for dehumidifying an air stream using hygroscopic materials ( sorbents ).

description

The rotary dehumidifier consists of a rotor with many thin air ducts running through it in the axial direction. The rotor often has a honeycomb structure, comparable to a rotary heat exchanger , but with a significantly smaller cross-section of the channels. Their walls are coated with hygroscopic material. It can be silica gel , zeolites / molecular sieves or lithium chloride , with different processes acting on the sorbents. When silica gel is an adsorption of water vapor instead of (binding of water on the surface of the sorbent), while lithium chloride an absorption takes place (penetration of water into the structure of the sorbent solution process). Since these processes are usually not differentiated in everyday language usage and a partial overlap can also take place, only sorption is generally spoken of.

The sorption rotor is built into a housing and rotates continuously. It goes through 3 sectors. In the first sector, the air flow to be dehumidified flows through the rotor. There the water vapor contained is bound by the hygroscopic material, and the air flow exits with less moisture . This is possible until the absorption capacity of the sorbent is exhausted. Then the so-called breakthrough occurs and the moisture hits through the rotor, i.e. H. the air passes through undried. To prevent this, the sorbent needs to be regenerated. For this purpose, the rotor in the 2nd sector is turned into a significantly lower regeneration air flow which, due to its high temperature (40–70 ° C for lithium chloride rotors and 70–90 ° C for silica gel rotors), drives out the water vapor. The adjoining third sector is used to flush the rotor. A part of the air flow to be dehumidified is branched off and used to expel the warm and humid regeneration air and carried along with the regeneration air flow. This rinsing sector is used to somewhat cool the carrier mass of the sorbent that is heated up during regeneration. However, it is not installed in every system.

The air flow to be dehumidified is heated by three factors:

  • Enthalpy of condensation : The condensation of the water vapor molecules releases the heat of condensation .
  • Binding energy : The binding energy is released by the adhesion of the water molecules to the sorbent molecules.
  • Entrained heat : The energy that is brought into the air flow to be dehumidified by the heated carrier mass and that increases the temperature is called entrained heat.

Due to these 3 factors, the change in the state of drying does not proceed isenthalpically , as is almost the case with sorption, but with an increase in enthalpy. This leads to a heating of the air in the drying sector by 7 to 11  K depending on the regeneration temperature. Since the adsorption process cannot be cooled directly, the drying process takes place at very unfavorable temperatures . The dehumidification performance that can be achieved with a rotary dehumidifier is therefore limited and is around 70–90%.

When cooling through drying and evaporation , the rotary dehumidifier is also used the other way around in winter operation, i.e. for humidification. Since exhaust air is used for regeneration in most systems, and this has a higher humidity than the outside air in winter, the rotor is used to humidify and preheat the outside air.

Rotary dehumidifiers with silica gel coating are considered non-toxic, are mostly non-flammable (depending on the carrier material) and maintenance-free. According to the manufacturer, the dehumidification performance is almost constant over many years. Rotors with a lithium chloride coating, on the other hand, have to be removed regularly for re-soaking, as their dehumidification performance decreases over time.

The speed of rotation of the rotary dehumidifier in dehumidification mode depends on the design, the air conditions, the regeneration temperature, the volume flows and other parameters. It is approximately in the range of 8 to 20 revolutions per hour.

Rotary dehumidifiers are manufactured in sizes from approx. 0.8 to 4.5 m in diameter. From a size of around 2 m, the rotors are made from individual sectors and only assembled on site in order to improve handling and transport.

Since only temperatures between 40 and 90 ° C are required for regeneration, waste heat or solar heat can also be used for a rotary dehumidifier ( solar air conditioning ).

literature

  • VDI-Wärmeatlas Springer-Verlag, Berlin 9th edition 2002
  • Recknagel, Sprenger, Schramek pocket book for heating + air conditioning 02/03, Oldenbourg Industrieverlag

Web links