What you need to know about compressed air desiccant dryers
In this article, we outline the need for dry compressed air, look at the operation of traditional heatless desiccant dryers, and reveal the benefits of the latest zero-purge desiccant dryer developments.
Why industry needs really dry air
As any physics student can tell you, the air around us always contains some moisture in the form of water vapour, and that's not always a good thing. When it comes to compressed air, this moisture can cause a lot of headaches. It can lead to high maintenance costs, shorten the lifespan of your equipment, and decrease performance of your air-using devices.
In the worst scenario, untreated air can cause pneumatic equipment to fail prematurely. It can also result in a high rate of product rejection, increased leaks, disruptions in your control systems, and corrosion that shortens the life of your pipework.
Choosing a reliable desiccant dryer is therefore essential to protect production systems and processes, maintain the quality of end products and minimize energy costs.
Dryer technologies
Regenerative-desiccant-type dryers are preferred in the pharmaceutical, chemical, and food and beverage industries. These dryers use a porous desiccant to adsorb moisture, retaining large amounts of water in a small quantity of desiccant. The desiccant, housed in two towers, requires regeneration after reaching its capacity. Compressed air flows through one tower while the other regenerates by reducing pressure and passing dried purge air through the desiccant bed.
Regeneration air can be heated internally or externally to reduce purge air needs and can also be supplied by a blower. These dryers typically have regeneration cycles based on time, dew point, or both, depending on the type: heatless desiccant dryers or heated blower purge dryers. Understanding heatless air dryer working principles is essential for optimizing performance and efficiency.
Heatless desiccant dryers
Heatless dryers use only compressed air as a purge. Because of their simple design, heatless desiccant dryers are often preferred for extreme environments. This sector includes operations in remote areas, hazardous sites with explosive gases and powders, and applications with a high inlet air temperature.
- The drying process: Wet compressed air flows upward through the desiccant which adsorbs the moisture, from bottom to top.
- The regeneration process: Dry air from the outlet of the drying tower is expanded to atmospheric pressure and sent through the saturated desiccant, forcing the adsorbed moisture out. Following desorption, the blow-off valve is closed and the vessel is re-pressurized.
- Switching: After regeneration, the functions of both towers are switched.
Heated blower purge desiccant dryers
These desiccant dryers utilize ambient air from an external blower, heat, and minimal compressed air.
- The drying process: Wet compressed air flows upward through the desiccant which adsorbs the moisture, from bottom to top.
- The regeneration process: The blower takes ambient air. Then blows it over the external heater. Subsequently, heated air is sent through the saturated desiccant, forcing the adsorbed moisture out, from top to bottom.
- Switching: After regeneration, the functions of both towers are switched.
Energy efficiency in desiccant dryers
Pressure drop is a crucial factor in a dryer's performance. A dryer's energy consumption is influenced by its internal pressure drop and the efficiency of its regeneration process. If a desiccant dryer experiences a high pressure drop, the compressor discharge pressure must be set higher, leading to increased energy and operating costs.
The need for extra pressure requires more energy to produce and, in extreme cases, may necessitate a larger output compressor to compensate for output loss through the dryer, which can be as much as 20%. Therefore, it is important to select desiccant dryers that offer a very low pressure drop—below 0.2 bar for most models—and the most efficient regeneration process.
In recent years, desiccant dryers incorporate state-of-the-art energy management control with built-in Dewpoint Dependent Switching. The principle is simple: a dewpoint sensor delays switching towers until the dewpoint falls. This cycle time extension can generate energy savings of up to 90%.
Comparing life cycle costs of Heatless vs. Heated blower dryers
When calculating lifecycle costs, a heatless desiccant dryer may seem more expensive due to its substantial use of compressed air for purging during regeneration. Typically, the average purge air consumption is about 16-20% of the rated flow capacity of a heatless dryer. Despite these losses, heatless dryers remain popular for their simplicity and reliability.
However, when comparing the total cost of ownership (TCO) over a 10-year period, the heated blower purge type of dryer has a significant advantage, reducing costs by approximately 40% compared to the heatless desiccant method. Although the power costs for the heating process are about 30%, this is considerably outweighed by the compressed air output losses from the purging operation, which account for 80% of the TCO.
Newer and more advanced dryer technologies are now available. For example, the latest models in Atlas Copco’s BD+ range guarantee a dew point down to -40°C and can achieve -70°C. Additionally, the zero purge losses achieved by the industry-leading BDZP dryers, operating with an inlet flow rate of 300 l/s at a system pressure of 7 Bar G, could result in annual savings of £10-14k, based on average kW/hr rates and continuous 24/7 operation throughout the year.
Chart that shows the relative life cycle cost of desiccant dryers over a 10 year period
Conclusion
The overall conclusion is that for more demanding installations where a negative dew point is required adsorption dryers are regarded as the only valuable solution. But it is a solution that comes with a caveat.
Although the initial purchase price of a heatless desiccant dryer may appear to be attractive, the running costs are extremely high in comparison to other dryer systems. This type of dryer can best be used in applications with a low amount of running hours, or as a back-up unit in a compressed air network to take over in case of issues with the main dryer. On balance, it would appear that dryers featuring heated blower purge with zero purge cooling are the most economical alternative.