How does an adsorption dryer work?

How an adsorption dryer works

The hygroscopic material absorbs moisture, thereby drying the air stream. This part of the process is known as the adsorption process. To expel the moisture stored in the sorbent, hot air is then passed through the adsorption bed in the opposite direction, whereby the hot air stream absorbs the water vapour bound in the adsorption bed. Desorption therefore takes place; this process is referred to as regeneration. In an adsorption dryer, the sorption material usually consists of a highly hygroscopic silica gel, which is firmly embedded in a slowly rotating rotor that continuously rotates between two separate, counter-flowing air streams. The humid air stream is dried in the adsorption zone of the rotor. In the regeneration zone, the moisture stored in the sorption material is driven out by the hot air stream and reabsorbed by it. As the operating principle is based on sorption – a process largely independent of temperature – rather than on condensation, adsorption dryers are particularly recommended for use at low temperatures. Furthermore, adsorption dryers can achieve particularly low moisture levels, such as those required in the pharmaceutical industry, for example.

Applications Unlike condensation dehumidifiers, whose scope of application is limited by the system constraints of the refrigeration circuit used, adsorption dryers are not subject to any restrictions regarding temperature and humidity. However, due to the nature of the system, their specific power consumption is always higher than that of condensation dehumidifiers. They should therefore be used where particularly low supply air humidity (< 6g/kg) or low ambient temperatures justify the increased energy consumption, or where condensation dehumidifiers can no longer fulfil the required dehumidification task. The following diagram by Thiekötter shows a rough comparison of the specific power consumption of refrigerant-driven condensation dehumidifiers versus adsorption dryers with purely electrically driven regeneration. Connection The various air flows of the adsorption dryers must be routed through ventilation ducts. This is usually achieved using spiral ducts. The duct carrying humid air to the outside should be insulated. If outside air is used as process air, care must be taken to ensure that the humid air outlet is sufficiently far away from the outside air intake. The humid air must always be discharged to the outside.

Regeneration and temperature control

In order to expel and remove the water vapour adsorbed within the rotor, the adhesive forces acting on the surface of the sorbent must be overcome. To achieve this, the regeneration air flow must be heated accordingly. This is accomplished by means of an upstream regeneration heater. In small adsorption dryers, regeneration heating is always electric. For larger units, the regeneration heater can be designed as follows:

• Electric (standard)
• Steam
• Hot water
• Combination of electric and PWW heating coils
• Combination of electric with steam or hot water heating coils

Larger adsorption dryers offer the option of using various media for regenerating the rotor. Where possible, on-site media such as steam, hot water or PWW hot water should be used for regeneration or to support it, in order to maximise energy efficiency.

The rotor’s storage material is heated by exposing the rotor to high temperatures of around 120 °C to expel the adsorbed water vapour. The change of state in the drying sector therefore does not occur ideally adiabatically at constant enthalpy. The heat remaining in the rotor is referred to as entrained heat and leads to an overheating of the dry air stream of approximately 1.5 K per g/kg of drying capacity. With a specified entrained heat of 1.3 K/g/kg, this superheating amounts to, for example, in a drying process from 12 to 4.5 g/kg of dry air: 1.3 K/g/kg × (12 – 4.5) g/kg = 9.75 K.

Awareness of this fact is important for assessing the integration of an adsorption dryer into the overall air-conditioning concept for the room to be dried. The manufacturers’ technical calculations already take the entrained heat into account, and the actual temperature of the dry air flow is specified.

To achieve particularly low supply air humidity levels, surface coolers may need to be installed upstream. In temperature-sensitive areas, the temperature of the supply air should be regulated directly via the dryer using an aftercooler, possibly in combination with a reheating coil. Ideally, the manufacturer of the adsorption dryer will supply the required modules pre-installed and ready for connection within the dryer housing. When drying untreated outdoor air, a preheater should be provided for frost protection.

Heat recovery

When using larger adsorption dryers, factory installation of a heat recovery unit is recommended in view of the system-related high energy consumption of the regeneration heaters. In this process, the heated, humid air is passed through a cross-flow heat exchanger before being discharged to the outside, where it transfers a large proportion of its thermal energy to the air stream required for regeneration. This can significantly reduce the energy consumption of the regeneration heater.