Refrigerated Air Dryers

Selecting the Right Refrigerated Air Dryer and Pressure Dew Point

Refrigerated air dryers are among the most commonly used compressed-air dryers in workshops and industry. They cool the compressed air in a controlled manner. In the process, the air first drops below its dew point, causing some of the water vapor it contains to condense and be separated as liquid. The compressed air is then reheated and fed into the compressed-air network with a significantly lower water vapor content.

The current product range covers flow rates from 0.4 to 36 m³/min. Depending on the size, maximum operating pressures of 14 or 16 bar are specified. However, to make the correct selection, it is not enough to simply compare the compressor’s nominal flow rate with the name of the dryer. It is also crucial to determine which pressure dew point must be achieved under actual operating conditions.

What does “pressure dew point” mean?

The pressure dew point is the temperature to which compressed air can be cooled at its current operating pressure before the water vapor it still contains begins to condense. It is therefore a direct indicator of the residual moisture content of the compressed air. The lower the pressure dew point, the drier the compressed air.

A pressure dew point of, for example, +3 °C does not mean that the compressed air leaves the dryer at +3 °C. Rather, it means that as long as the compressed air does not cool to approximately +3 °C or below under the prevailing pressure in the downstream network, no new condensate should form from the remaining water vapor. However, if a line cools below this value, water may condense again.

Why +3 °C Is Sufficient for Many Workshop and Industrial Facilities

Current refrigerated air dryers are designed to achieve a pressure dew point of approximately +3 °C under their reference conditions. This range is often sufficient for compressed-air lines within production halls and workshops at normal temperatures. The lines there generally remain well above the pressure dew point.

The situation is different when compressed-air lines are routed through unheated areas or outdoors in the winter. If the line or the consumer equipment can cool down to temperatures around 0 °C or below, a pressure dew point of +3 °C is not sufficient to reliably prevent the risk of condensation or icing. For such applications, desiccant air dryers with significantly lower pressure dew points are often used.

Pressure dew point is not the same as atmospheric dew point

The pressure dew point refers specifically to compressed air at operating pressure. If the air is expanded to atmospheric pressure, the relationship between moisture content and dew point changes. Therefore, the pressure dew point of a compressed-air dryer should not be equated with the normal weather or room dew point.

Similarly, the air temperature measured on the display or in the piping is not automatically the pressure dew point. A suitable dew point measurement is required to make a reliable statement about humidity.

Reference Conditions and Actual Performance

The rated capacities of the current refrigerated air dryers are specified according to defined reference conditions. For RDX units, the rated volume flows refer to conditions of approximately 7 bar operating pressure, +35 °C compressed air inlet temperature, and +25 °C ambient temperature. Under these conditions, a pressure dew point of approximately +3 °C is specified.

If the inlet temperature or ambient temperature rises significantly, or if the dryer is subjected to a higher flow rate, it must dissipate more heat and moisture. This reduces its usable capacity, and the actual achievable pressure dew point may be higher. In the product data, correspondingly higher pressure dew points—up to approximately +10 °C—are listed for less favorable boundary conditions.

0.4 to 36 m³/min for various system sizes

The current product range is divided into several performance categories: 0.4 to 1.8 m³/min for compact systems, 2.4 to 4.1 m³/min for medium compressed air requirements, 5.2 to 7.7 m³/min for larger workshop and industrial systems, and 10 to 18 and 20 to 36 m³/min for high continuous air flow rates. The decisive factor is always the dryer capacity required under real-world conditions.

Combine cyclone separators, filters, and refrigerated air dryers

A refrigerated air dryer is part of a complete compressed-air treatment system. Free condensate can be removed even before the dryer reaches it using a cyclone separator. Suitable filters reduce particles and oil aerosols. The refrigerated air dryer then performs the actual moisture reduction to a defined pressure dew point. If very low residual oil levels are also required, an activated carbon adsorber can be installed after the drying and fine filtration stages.

Refrigerated air dryer or desiccant air dryer?

For many general workshop and industrial applications, the refrigerated air dryer is both cost-effective and technically suitable. However, if pressure dew points significantly below 0 °C are required—for example, due to outdoor piping, the risk of freezing, or particularly sensitive processes—a desiccant air dryer is the more suitable technology. The choice should therefore be based not only on price but, above all, on the required pressure dew point.

Buy Refrigerated Air Driers

Anyone looking to purchase a refrigerated air dryer should be aware not only of the flow rate but also of the desired pressure dew point and the actual operating conditions. Of particular importance are the inlet temperature, ambient temperature, operating pressure, and the lowest temperature in the compressed air system. This helps prevent a dryer that is nominally large enough from becoming overloaded under actual operating conditions or the achieved pressure dew point from being too high for the application.