Dehumidification of air is possible after the condensation priciple or the sorption priciple.

Sorption is a generic term for operations that lead to enrichment of a substance within a phase or at an interface between two phases. The enrichment within a phase is more specifically called absorption; enrichment at an interface is called adsorption. In 1909 J.W. McBain 1909 coined the term “sorption” for processes which can not be clearly distinguished in adsorption and absorption. The absorbing material called sorbent. (From Wikipedia, abridged)


Air flows over a cold surface for dehumidification. The dew point reached corresponds to the temperature to which the air can be cooled. In order to avoid icing on the surface of the cooler this principle is limited to dew points ≥ +10 ° C.

Example for condensing dehumidification: supply air dehumidification

  • ambient air with 32 ° C and RH 40%, corresponding to a water vapor content of 12.7 g / kg or dew point of 16.7 ° C
  • cooling with chilled water 6/12 °C to 10 °C and RH 98% reduces the water vapor content to 7.8 g / kg (dew point = + 10 ° C)
  • after reheating to 20 ° C this air has a relative humidity of approx. 50%


Here only a description of air dehumidification with desiccant rotors is given. Systems with liquid adsobents or fixed desiccant containers will not be discussed. The dehumidification of air is performed in the desiccant wheel. In the process air sector, water vapor molecules from the air flowing bind to the desiccant material; now usually silica gel. In the reactivation air sector the vapor pressure difference is reversed by heated air. The bound water vapor molecules are released to the air flowing through. The efficiency of this process is highly dependent on the rotor technology. High-quality rotors turn slowly with 6 … 12 revolutions per hour. The rotors are divided into a 270° process air sector and a 90° reactivation air sector. The ratio of process air flow to reactivation air flow is about 3:1. A small additional purge sector is only required for frost points below – 20 °C. The advantage of “energy recovery” through a purge sector is usually compensated by the reduction of the active dehumidification sector area; the overall energy required to remove humidity from an air stream (measured as KJ per kg moisture) is unaffected.

A rotor sectoral division of 270° / 90° is used for regeneration air temperatures of approx. 120° C. If a lower reactivation air temperature is desired for using low-cost energy (like waste or solar heat) the rotor is divided into 2 equal sections. When using the same air flow in both the process and reactivation air sector sufficient dehumidification capacity is available already at an reactivation air temperature of 70 °C

Rotor designs

Desiccant rotor dehumidification, example 1: low dew point dehumidification for tableting, target condition 20 % RH at 20 °C

Outside air with 32 °C and RH 40 % corresponding to a water vapor content of 12.7 g/kg or a dew point of 16.7 °C

  • precooling with cooling water 6/12 °C to 10 °C and RH 98 % reduces the water vapor content to 7.8 g / kg (dew point: + 10 °C)
  • desiccant dehumidification to an outlet temperature of 30 °C and RH 3.6% corresponding to a water vapor content of 1.0 g/kg or frost point of -16 °C
  • reactivation air temperature used is 120 °C
  • when cooled to 20 °C this air has a water vapor content of 1.0 g/kg or a frost point of – 16°C, resulting in a supply air with a relative humidity of approx. 7 %

Desiccant rotor dehumidification, example 2: Dehumidification at low ambient temperature; i.e. in unheated storage room for steel or in a classic car garage

  • ambient air conditions with 10 °C and RH 85 % corresponding to a water vapor content of 6.5 g/kg or a dew point of +7.6 ° C
  • desiccant dehumidification with an outlet temperature 30 ° C and RH 4.5 % corresponding to a water vapor content of 1.2 g/kg or frost point of -13 °C
  • reactivation air temperature used is 120 °C
  • after cooling the dry air supply to the room a temperature of 10 °C the relative humidity ist still only approx. 15 %
  • This means that even with some leakage of ambient air into the storage there is sufficient dehumidification available to protect steel against corrosion (requires < RH 50 %)

Sorption, example 3: Sorptive cooling – DEC system

Outside air with 30 °C and 40 % RH corresponding to a water vapour content of 12 g/kg or dew point of 16.7 °C is cooled using the proper combination of dehumidification, heat recovery and humidification.

System requirements:
air supply 6 000 kg/h at 21,2 °C and RH 65 %
heat load room 13,3 kW

Overview of system operating data for summer conditions:

exit air

  • 2. heat load room resulting in exit air with 26 °C, RH 54%:  3 kW
  • 3. exit air humidification for outlet conditions of 20 °C, RH 98 %
  • 4. exit air heating to 65 °C, RH 9 %, 78 kW supplied by thermal solar panels
  • 5. exit air energy recovery 10 kW via desiccant rotor to diagram point 5, outlet conditions 51 °C, RH 21%

inlet air

  • 6. design ambient air conditions: 30 °C, RH 40 %
  • 7. to 8.: cooling via precooler from 50 °C at fan outlet to 21 ° C
  • 9. cooling / energy recovery via desiccant rotor
  • 1. inlet air conditions 21,2 °C and RH 65 %