Refrigerated Air Dryers
As a refrigerated air dryer chills compressed air to 33–40°F, water vapor condenses into a liquid. Then it is expelled from the system via a water trap and automatic drain. The cool, dry air is reheated to room temperature, exits the dryer, and into the production lines. This helps reduce condensation in the compressed air piping system.
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 continuously. The temperature is controlled by the use of a hot gas bypass valve and the cycling action of an evaporator fan. This maintains a consistent temperature range and dew point average of 38°F as long as it is properly sized and maintained. This type of dryer is the most cost effective and are very reliable. Non-cycling air dryers are also used in applications where the inlet temperatures are high. A reciprocating compressor system is a good example of an application with high inlet temperatures.
Cycling Air Dryers
There are three types of cycling air dryers: thermal mass; digital scroll; and variable speed drive. All these cycling-type refrigerated dryers save money by reducing energy usage. They conserve energy by either cycling the refrigeration system or through the use of a VFD (variable frequency drive). While cycling dryers are the most efficient, they have more moving parts. This can lead to more maintenance and repair issues. Cycling air dryers also come with a higher capital investment on the front end.
Compared to desiccant dryers, cycling and non-cycling refrigerated dryers require a lower capital investment and minimal maintenance. They are the most common type of dryer used in most service and manufacturing applications. If the application requires dry air without any visible moisture present (ISO Quality Classes 4, 5 & 6), this may be the best option.
Desiccant Air Dryers
Desiccant compressed air dryers remove all water vapor down to a -40 to -100° 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 desiccants. Desiccants are solids that absorb water. Desiccant dryers have two towers that contain desiccant beads. The system alternates between the two towers. This allows one tower to dry the compressed air while the other is regenerating the desiccant material. These types of dryers consume 5%–18% of the compressed air supply, depending on the type of controls. Dryers with optional purge controllers only regenerate when needed. The energy savings that a purge controller offers may be worth the extra capital cost.
There are several types of desiccant air dryers: heatless; heated; and blower purge. When regenerating the desiccant material, a heatless dryer uses up to 18% of the rated capacity of the dryer. It purges compressed air back through the saturated tower to complete the regeneration process. A heated dryer heats a lower flow of purge air (approximately 5%–7%) to regenerate the desiccant material. A blower purge dryer uses very little or no compressed air in the regeneration process. Instead, it uses heat and a blower. There is an advantage to heated and blower purge regenerative air dryers. They use little or no compressed air in their regeneration process. This saves capacity and energy used in the compressed air system.
One of the benefits of desiccant air dryers over refrigerated dryers is that they can operate at a consistently low dew point. When the application requires ultra-dry air (ISO Quality Classes 1, 2 & 3) or is likely to operate in below freezing conditions, a desiccant air dryer is the best option. But keep in mind, they have a much larger initial investment, higher operating costs, and increased maintenance expenses. Desiccant air dryers should only be used when absolutely necessary.