Refrigerated air dryers cool the compressed air to 3° Celsius. This causes condensation of the water vapour into liquid water. Using an inbuilt water separator, the liquid gets removed. The dew-point is dictated by its ability to cool the air to the desired dew-point temperature, and then the efficiency of the liquid removal separator. This is a suitable option for applications which don’t need a critical dew-point.
There are 3 common types of refrigeration dryers:
- Direct Expansion Refrigeration Dryers. This is the most common and stable type of refrigeration dryer. But as the dryer is constantly running, it can be the costliest to run.
- Thermal Mass Refrigeration Dryers. Thermal mass dryers have a large mass, usually a water tank or sand, which gets cooled down by the refrigeration circuit. The heat exchange with the compressed air then takes place between the air circuit and the thermal mass. Once the thermal mass cools down, the refrigeration compressor can be turned off to save energy.
- Variable Speed Refrigeration Dryer. This type of dryer uses invertor technology to vary the speed of the refrigeration compressor. It aims to match the cooling requirements of the dryer to the water vapour loading of the incoming compressed air. This can be more energy efficient than a direct expansion dryer. Yet the cost and complexity means they’re often only suitable for larger compressed air flows.
Refrigerated air dryers tend to be used in industrial and manufacturing facilities.
Membrane dryers use hollow-fibre membranes which diffuse the moisture from the compressed air to atmosphere. To remove the permeated moisture from the membrane, this requires a constant flow of compressed air called ‘sweep air’. The sweep air can be as much as 50% of the dryer’s capacity, which reduces the efficiency.
If a process requires higher quality compressed air, we recommend a desiccant dryer. This works by passing compressed air saturated with water vapour over a bed of adsorbent desiccant material. The longer the air is in contact with the desiccant, the drier it becomes.
The desiccant material has a limit to how much moisture it can absorb. To achieve a constant dew-point, the desiccant must get regenerated.
To supply critical dry air, a desiccant dryer can be used in spray paint operations, laboratories and printers.
Some manufacturers have an integrated dryer option on their air compressors. This is a good option to consider with limited space. It can also provide savings on the initial installation costs.
When selecting a dryer for your compressed air system, the first step should be to define the air quality requirements. Dryers play an important role in providing clean, dry air. The initial investment can result in savings by avoiding costly downtime due to contamination or moisture damage. To protect against other contaminants, such as oil or microorganisms, filtration is key.