Air Drying Units
 
Deliquescent air dryers utilize an absorptive type chemical, called a desiccant, to provide a -5°C to 7°C dew point suppression below the temperature of the compressed air entering the dryer. The moisture in the compressed air reacts with the absorptive material to produce a liquid effluent which is then drained from the dryer. Keep in mind that this effluent is typically corrosive and must be disposed of in accordance with local regulations.

While deliquescent dryers are typically used in applications such as sandblasting and logging operations, they are not recommended for industrial applications since the dried compressed air exiting the dryer may contain small amounts of the effluent which may be corrosive to downstream equipment.
    Refrigerated air dryer

Refrigerant circuit
From the compressor (F) the refrigerant gas comes in the condenser (G) where it is cooled by a high volume cooling fan / or cooling water and becomes liquid. A capillary tube or a pressure regulated expansion valve (H) regulates the flow into the air-to-refrigerant heat exchanger (C) where the refrigerant evaporates by extracting heat from the compressed air.

Refrigerated air dryers remove moisture from the compressed air through a mechanical refrigeration system to cool the compressed air and condense water and lubricant vapor. Most refrigerated dryers cool the compressed air to a temperature of approximately 2°C, resulting in a pressure dew point range of 2°C - 5°C. Keep in mind that this range is also the lowest achievable with a refrigerated design since the condensate begins to freeze at 0°C.

The refrigerant pressure is then increased in the compressor and the cycle starts again. Refrigerated dryers are available in two basic configurations: Direct Expansion (non-cycling) and cycling dryers.

Direct expansion dryers cool the compressed air in an air-to-refrigerant heat exchanger, called an evaporator. The warm compressed air flows into one side of the evaporator while low pressure, liquid refrigerant is metered into another side. The heat from the compressed air "boils" the refrigerant, reducing the temperature of the compressed air. Operation of the refrigeration compressor is continuous and therefore requires a combination of control valves to regulate refrigerant flow as the heat load from the compressed air changes.

Thermal mass dryers cool the compressed air through an intermediate fluid. Two heat exchangers, a compressed air chiller and refrigerant evaporator are fitted inside a tank which is filled with a thermal conducting fluid, which is usually a water and propylene glycol mixture. The refrigeration system removes heat from the fluid, which in turn removes heat from the compressed air.

Since the refrigeration system is used to only cool the fluid, the refrigeration compressor is "cycled off" once the fluid temperature is chilled to the required point. This cycling of the refrigeration compressor results in significant energy savings on most compressed air systems. On average, cycling dryers provide energy savings of 50 percent when compared to equally sized non-cycling designs.

Cycling dryers offer a simplified refrigeration circuit, a reduction of 60 percent or more in the required refrigerant, an elimination of dryer freeze-up potential and an increased energy savings since the dryer dew point can be raised to as high as 15°C. While the initial purchase price of a cycling dryer can be 25 percent or more above an equally sized non-cycling unit, the energy savings potential of cycling designs usually provide a payback period of less than one year.