Compressed air systems are essential in industrial operations, where they are considered the fourth utility . These systems are the energy source that regularly supplies dry, compressed air. Meanwhile, clean, dry, compressed air lowers the operating costs of any facility. However, during air compression, the water vapour from the atmospheric air condenses into liquid water when the air or gas cools past the saturation point . As a result, moisture will form in the compressed air, damaging the system and causing problems in the operation of solenoid valves, pneumatic systems and air motors.
This is the reason why facilities with compressed air systems should also invest in compressed air dryers. Compressed air should be free from moisture at all times. This article will give you an overview of how air dryers work and what are their different types.
How do air dryers work?
A compressed air dryer separates water vapour or moisture from industrial process air. It largely reduces the amount of moisture in compressed air, thereby suppressing dew point. Dew point is the temperature at which air or the vapour condenses into liquid water. What makes this excess moisture undesirable is that it can contaminate the equipment or products and cause frozen pipes, which can lead to corrosion and other problems .
Typically, the compressor first draws in and then compresses humid air. The process increases the air temperature and subsequently cools the air, condensing water vapour out of the unit and drained into the tanks, hoses or pipes flowing away from the compressor . The air can also be over-compressed, which reduces the temperature, drains of condensate and returns to the regular pressure. Another way of drying compressed air is by using a special material that absorbs water from the air and then discharging it from the compressor . The way air dryers work to protect the compressed air from moisture varies according to the type of air dryers used.
1. Refrigerant type
Refrigerant type is the most commonly used type because of its relative cost-effectiveness and ease of maintenance. This type is appropriate for applications without special requirements needed.
Refrigerated air dryers work like those of home refrigerators or air-conditioning systems. The difference is the objects being cooled. They remove moisture from the air by quickly cooling it, condensing it and then draining it off.
The two variants of refrigerated air dryers are cycling and non-cycling.
Cycling dryers use a refrigerant to cool down a glycol or aluminium mass surrounding the air passage. A thermostat-controlled heat sink then cools down the compressed air. This type saves in operating costs because it only powers up to satisfy demand. It can control the refrigerant compressor when loads are reduced and the temperature is lower. Cycling dryers also maintain the dew point, which is variable, at a constant temperature.
Non-cycling dryers shut down from time to time and restart to maintain the required temperature. In this type, the refrigerant circulates continuously through the system, which results in its quick responses for load changes and its capability for continuous operations. However, it cannot save energy even during periods of low airflow. In contrast to the cycling type, non-cycling dryers can maintain a consistent dew point.
2. Desiccant type
Desiccant dryers, which are largely similar to the refrigerant type, are appropriate for applications that require extremely dry air or involve subfreezing temperatures. This type removes water from compressed air through adsorption; the water adsorbs on porous desiccant, which is commonly activated alumina or silica gel.
Given that the desiccants show no chemical reaction with water, there is no need for replenishment. Instead, the air dryer regenerates or dries them out. With this regeneration process, desiccant dryers usually have two towers: one for air drying and the other for regenerating desiccants. A less common type is single-tower desiccant dryers, which features a single tower with desiccants that dry the air coming in.
Heated, heatless and heat of compression desiccant air dryers are the three types of desiccant dryers.
- Heatless (no internal or external heaters)
This type contains no heating source and diverts dried air to the regenerative tower, using purge air to remove moisture.
- Heated (internal or external heaters)
This type uses a source of heat within the drying tower to remove moisture from the desiccant air dryer, which reduces the need for purge air.
- Heat of compression
This type uses both towers simultaneously to maintain a uniform temperature.
3. Deliquescent/chemical type
Chemical air dryers involve deliquescent chemical or chemical desiccants that absorb moisture in the air, whether it is condensed or a vapour. During the water removal process, the desiccant is consumed, and it must be regularly replenished. To achieve a successful chemical dryer operation and prevent these chemicals from building up over time, a high-quality filtration system must be in place.
4. Membrane type
Membrane air dryers are gas separation devices that use permeable membrane microtubes in a bundle to extract water vapour from compressed air. The material used in the microtubes only allows water vapour to pass but traps nitrogen and other molecules. This results in only dry air remaining in the tubes. Membrane air dryers are becoming popular because they require no electricity and little maintenance. Their operation is also quieter than those of others.
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 “Refrigeration: Why Do We Need to Dry Air Before It Enters Compressors?” Quincy Compressor, 6 Mar. 2020.
 “Understanding Different Types of Air Compressor Dryers.” Quincy Compressor, 21 Aug. 2020.
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