Dehumidifiers and air drying

Moisture can be removed from any material which is to be dried, by passing air over it which has a lower water vapour pressure. Also, in removing this moisture, the latent heat of evaporation must be supplied, either directly by heating, or by taking sensible heat from the airstream which is carrying out the drying process. 

Air will leave the evaporator section with a reduced moisture content, but at a low temperature and high percentage saturation. In this state, it may not be effective in removing moisture from any subsequent process, as it will be too cold. 

The refrigeration method of drying air is the most energy efficient, down to a lower limit of about 0.005 kg/ kg moisture content at atmospheric pressure. Equipment to work at frosting conditions can be duplicated, one evaporator defrosting while the other is operating. Below this limit, chemical or adsorption drying must be used [61]. 

Depending on the end use of the compressed air, some or all of the condenser heat can be used to re-heat the cold air. This maybe necessary in winter, when distribution piping could be colder than the evaporator. When the air is released through a power tool, the final condition maybe less than 5% saturation. 

Unit driers for small compressed air systems need to have capacity control, so as to maintain a steady working dewpoint when there is a variation in air demand. 

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