What are the main differences between refrigerated and desiccant air dryers?
Refrigerated Air Dryers
A refrigerated air dryer chills the compressed air to 33 – 40 degrees Fahrenheit at which water vapor is condensed into a liquid and is then expelled from the system via a water-trap and automatic drain. The cool dry air then is typically re-heated to around room temperature before exiting the dryer and going to your production lines. This helps reduce condensation on the compressed air piping.
There are two types of refrigerated dryers; cycling and non-cycling.
Non-Cycling Air Dryers:
Non-Cycling air dryers allow the refrigeration circuit to run continually. They control their temperature by the use of a hot gas bypass valve and cycling of the evaporator fan to maintain a tight temperature range. These dryers are the most cost effective and are typically very reliable while maintaining a fairly consistent dew point average of 38 degrees F (as long as it’s properly sized and maintained). These dryers also are available for high inlet temperature applications typically found on reciprocating type air compressor systems.
Cycling Air Dryers:
There are three types of cycling air dryers; thermal mass, digital scroll, and variable speed drive. All of these cycling type refrigerated dryers are designed to reduce energy usage and save you money. They conserve energy by either cycling the refrigeration system, or through the use of a VFD drive. While cycling dryers are the most efficient, they have more “moving parts” which can potentially lead to more maintenance. Cycling type dryers also require a higher capital investment.
In comparison to a desiccant type air dryer, the benefit of cycling and non-cycling type refrigerated dryers is their minimal required maintenance and a lower capital investment. This is the most common type of dryer used in most service type and manufacturing industries. If your application only requires dry air without any visible moisture present (ISO Quality Classes 4, 5 & 6), this may be the best option for you.
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Desiccant Air Dryers
Desiccant type compressed air dryers remove all water vapor down to a -40 or -100 degree dew point depending on the application. Instead of refrigerating the compressed air, the dryer adsorbs the moisture with the use of Activated Alumina and Molecular Sieve type desiccants. Desiccant dryers are made up of two towers that hold the desiccant beads. The system alternates between the two, allowing one to dry the compressed air while the other is regenerating the desiccant material. These types of dryers typically consume between 5 – 18% of your compressed air supply depending on the type of controls. Dryers with optional purge controllers regenerate only when absolutely needed, thus offering energy savings that may be worth the additional capital cost.
There are several types of desiccant air dryers; heatless, heated, and blower purge. When it comes time for regeneration of the desiccant material, a heatless dryer simply uses up to 18% of the rated capacity of the dryer to purge back through the saturated tower for the regeneration process. A heated type dryer heats a lower flow of purge air (approximately 5 – 7%) to regenerate the desiccant material. Lastly, a blower purge dryer uses very little or no compressed air in the regeneration process, but instead uses heat and a blower. The advantages of heated and blower purge regenerative air dryers is that they use little or no compressed air in their regeneration process. This saves capacity and energy on your compressed air equipment.
One of the benefits of a desiccant air dryer over a refrigerated dryer is that it can operate at a consistently low dew point. If your application requires ultra-dry air (ISO Quality Classes 1, 2 & 3) or is likely to operate in below freezing conditions, this is the best option for your compressed air system. Keep in mind that they should only be used when necessary as they have a much larger initial investment, higher operation costs, and increased maintenance expenses.