Ambient air contains water vapour. When the air is compressed, numerous key parameters increase, such as the air temperature, proportion of water vapour per volumetric unit of air and consequently, the air’s dew point temperature – or pressure dew point. Measured in degrees Celsius, it indicates the lowest temperature at which water will not condense out of compressed air – in other words, condensation will result if the air is chilled below the pressure dew point. The lower the pressure dew point, the drier the compressed air. If the compressed air is not dried following the compression process, water can condense when the compressed air cools. It can then accumulate in the downstream compressed distribution air network or even within the realms of the production process itself – which can have even more serious consequences, as water can then damage not only the compressed air system, but also downstream equipment that uses compressed air and the products being produced. So it’s extremely important to exercise due care in selecting the appropriate degree of compressed air drying for the specific process in mind.
Ultimately, it’s the degree of compressed air drying that determines the drying method, and the cost of drying the compressed air. Refrigeration drying generally provides the most efficient and cost-effective method for most applications, usually ensuring a pressure dew point of +3°C. If a lower pressure dew point is required due to the nature of the processes in question, more complex desiccant dryers or combination dryers can be used. However, these types of dryers involve higher costs as a result of the additionally required materials and increased energy consumption. In these types of dryers, the compressed air is treated using desiccants, such as silica gel or activated aluminium. During the drying phase, water vapour contained in the compressed air binds to the desiccants. Once the adsorption capacity of the desiccant is exhausted, it must itself be dried out – either continuously or at intervals, depending on its saturation. This process is called regeneration and is responsible for the greater part of desiccant dryer operating costs.
In technical terms, regeneration processes are differentiated into chamber and drum processes. In chamber regeneration, the desiccant is usually in granulate form and is contained in two separate pressure receivers. Desiccant regeneration takes place non-continuously and, depending on the type of unit, may employ/waste cold compressed air that has already been dried. In the case of dryers that have been specially adapted to the compressor, hot compressed air supplied directly from the second compressor stage is used for regeneration.
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