Compressed air can be used for many different purposes including in pneumatic tools and certain industrial processes. The compression of air can also cause significant problems for these systems, however. This is because air always contains some amount of moisture, dependent on temperature and pressure. When air is compressed or heated, the volume of water it can hold increases as the dew point raises and this water will eventually condense back into a free liquid.
Condensation occurring inside pipes and sensitive equipment can be disastrous because it leads to corrosion and increases the growth of bacteria and mold. Any system that uses unfiltered compressed air is at risk of suffering damage and will require more maintenance to remain functional. To prevent these problems, an air dryer is typically used to remove much of the water from compressed air before it can damage equipment.
How Do Air Dryers Work?
Several different types of air dryers are commonly used, including:
- Refrigerated
- Desiccant
- Membrane
Refrigerated air dryers remove moisture from compressed air by cooling it. This removes moisture from the air because it lowers the dew point and decreases the amount of water vapor that the air can hold, causing it to condense. The system can then siphon this moisture away and feed the dry, compressed air onward to where it is needed.
Desiccant air dryers use desiccant materials to remove water from compressed air. These materials trap water through adsorption and can produce a much lower dew point than refrigerated systems. Desiccant air dryers are also able to function at very low temperatures while a refrigerated air dryer can not. They’re not effective as a first pass system though and moist air needs to be passed through a refrigerated air dryer before a desiccant air dryer can handle it properly.
Membrane air dryers are the last of the major systems for removing moisture from compressed air. They’re cheap to run and can often produce the lowest dew points of all three systems but they are delicate and can’t be used effectively for drying large volumes of air. For this reason, refrigerated and desiccant air dryers are the most commonly used systems for large-scale purposes.
Heated vs. in-Line Desiccant Air Dryers
Desiccant air dryers are the most effective system for the removal of moisture from compressed air at any level of scale where a very low level of remaining moisture is necessary. There are different kinds of desiccant air dryers though, and these differences affect the maintenance and costs involved in running the air dryer.
A desiccant air dryer can exist as a dual-tower (regenerative) unit containing two separate portions of desiccant material, or as an in-line system. Dual-tower desiccant dryers have a drying tower and a regenerating tower, and this separated design allows the desiccant material within to be regenerated and reused. By alternating the direction of air to only flow through either the drying or regenerating tower, the air can either be dried or used to remove moisture from the desiccant material so that it can be used in the drying process once more.
Many twin tower units use a heater to increase the efficacy of the regenerative process. While this makes the system as a whole much more efficient, it also adds significantly to running costs. Heating requires a lot of electricity and also contributes to increased maintenance costs by making the whole unit more complex.
In-line systems forego the regeneration of desiccant and this can greatly lower the cost of electricity required for this kind of unit. The desiccant in these systems needs to be replaced periodically, however, so this kind of system still has downsides. It’s not a system that can run continuously and can’t be used for any process that requires a constant, uninterrupted flow of dry air.
The choice of whether a heated or in-line desiccant air dryer is better for a given situation largely boils down to a few things but the most significant of these is how often the system will need to be run. An in-line unit can’t run continuously, making it automatically unfit for use in any system that requires a constant supply of dry air.
It is, however, the most economical and low-maintenance solution in any situation where very dry air is required but continuous supply isn’t. In-line systems can be run for shorter periods to reduce operating costs and the need for maintenance but they do still require the knowledge of how to replace spent desiccant as needed.
For some in-line systems, the whole tank of desiccant needs to be removed and replaced to prepare it to run again after the desiccant is spent. Other designs allow for fresh desiccant to be added to the unit from the top and spent desiccant to be siphoned from the bottom, making replacement much easier and quicker.
Ultimately, both kinds of desiccant air dryers are highly effective when used for the right purpose and that’s why it’s important to understand the differences and how each system works. Used correctly, either kind of unit can protect expensive or sensitive equipment from the dangers of moist air.
