Desiccant Air Dryers vs. Refrigerated Air Dryers – Which Do I Need for My Compressor?
Many industrial processes require clean, dry air – air free of moisture. The presence of water, dust, bacteria, microorganisms and industrial acids can ruin products and foul processes. For instance, food and beverage manufacturers often use air in their packaging processes. By retaining moisture, the shelf life of the food items will be diminished. This is only one example, but by removing moisture and contaminants, it will help protect your downstream equipment and reduce maintenance costs and downtime.
Compressed air naturally contains moisture, so to help with this, a dryer is used to keep the air supply free of moisture. Customers are aware that they require a dryer but when it comes to choosing one, they’re often left scratching their heads over which dryer to choose.
While it’s always best to consult your compressed air expert, ultimately, the decision often comes down to the acceptable moisture content or dew point required for your application, in addition to the impact of purge air for regeneration in desiccant type applications. In this article, we’ll discuss that and more, and whether you should consider refrigerated and/or desiccant air dryers for your application.
Refrigerated Air Dryers
Where they are used
Many industries use refrigerated air dryers. These dryers are applied in general purpose compressed air applications – things like assembly plants, heavy industrial and standard production lines with pneumatically controlled equipment.
Refrigerated air dryers offer a pressure dew point (PDP) of 38°F (3°C). The cost of operation is typically lower than that of other drying technologies.
How refrigerated air dryers work
Saturated compressed air enters the system and is precooled in the air/air heat exchanger. Precooled air then moves downstream through the air/refrigerant heat exchanger. The heat exchanger’s vertical profile design reduces condensed moisture by nearly 99% using gravitational force. To reliably prevent separated droplets from reentering the airstream, condensate collects in a large reservoir with subsequent recirculation where flow velocity is significantly reduced. Accumulated condensate is then discharged from the dryer via drain. The dried, cold process air passes back through the heat exchanger to be reheated — reducing relative air humidity and recovering up to 60% cooling capacity.
Advantages of refrigerated air dryers
Refrigerated air dryers offer several benefits, such as:
Lower upfront cost
Low maintenance costs
Low operating cost
Important note
It’s important to note that refrigerated air dryers work in a way that leaves a small amount of water vapor present in the air versus removing 100% of the moisture. In other words, these dryers are not recommended for applications that are moisture sensitive.
Desiccant Air Dryers
Desiccant air dryers are used when applications require very dry compressed air or when the compressed air supply runs in cold environments. The primary difference between desiccant dryers and refrigerated dryers is that a desiccant dryer can achieve a much lower pressure dewpoint. Desiccant dryers are ideal for applications requiring an extremely low dew point, as they can achieve dew points as low as -100°F (-73°C).
Desiccant air dryers are typically used in sensitive applications, such as pharmaceutical, food & beverage, high quality electronics, semiconductors or automotive body shops (paint booths).
How Desiccant Dryers Work
Desiccant dryers have a dual tower design in which both vessels are filled with desiccant material. Saturated, compressed air passes through vessel one where the desiccant adsorbs moisture, lowering the dew point to expel dry compressed air. Once vessel one reaches a set level of saturation, the air switches to pass through vessel two. While the air is passing through vessel two, vessel one dries and regenerates the desiccant material. When vessel two reaches a set level of saturation, the air switches to pass through vessel one.
Advantages of Desiccant Dryers
Advantages of desiccant dryers include:
Low PDP Pressure Dew Point
Reasonable operating costs
Ideal for sensitive environments, where moisture cannot be tolerated
Different Types of Desiccant Air Dryers
The types of desiccant dryers include heatless, heated reactivated and blower purge, each product category has its specific purge rate. The heatless desiccant air dryer has an average purge rate of 15%. Heat reactivated products have an average purge rate of 6%, and blower purge typically have a 2 – 3% purge.
As an example, a 3000 cfm (85.0 m³/min) compressor using a standard (heatless) dryer would have a 15% purge rate, or 450 cfm (12.7 m³/min) of purge. The higher your flow, the more significant energy and maintenance costs will come into play, and it presents a good time to look at alternative dryer options.
Some Facilities Use Both Refrigerated and Desiccant Dryers
In some facilities, users may find they need both a refrigerated and desiccant dryer, where a desiccant dryer is only needed for point-of-use applications or processes. An example of this may be an auto body shop. The shop would generally use a refrigerated air dryer for tools and general air uses, but then they would use a desiccant dryer for the paint booth, where clean, dry air is required.
