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Air dryer is essential to ensure a supply of dry, ready-to-use air. But should you buy refrigerated air dryers or buy desiccant air dryers? Both have their place in the compressed air industry. However, there are some big differences in how they work, how much they cost, and how dry they will get your air. In this article, we’ll look at the differences between desiccant and refrigerated dryers and help you choose which one is right for you.
Choosing an air dryer is an important element of compressed air system design. The type of air dryer you select will have a big impact on air quality, system efficiency, and operating costs. Selecting the right type of dryer and sizing it appropriately for your system demands will ensure that you have a consistent supply of clean, dry air while controlling operating costs. There are several considerations in dryer selection:
A compressed air dryer removes moisture from the air that comes out of the compressor. Atmospheric air contains water vapor—what your local weather person will refer to as humidity. The amount of water air can hold depends on both temperature and pressure. As air is compressed, the higher pressures will cause excess moisture to fall out of the air as liquid condensation at the discharge of the air compressor. Additional moisture remains in the air stream as water vapor.
Moisture in compressed air can cause problems in the compressed air system, pneumatic equipment and manufacturing processes:
The air dryer removes excess moisture from the air and reduces the dew point, or the temperature at which condensation will appear. The Dew point is often used as a measurement of the moisture content in compressed air; the lower the dew point, the drier the air is.
Both refrigerated and desiccant air dryers remove moisture from compressed air. But there are significant differences in how they do it, how much moisture they remove, and how much they cost to operate.
Here’s a summary of the main differences:
Refrigerated vs. Desiccant Air Dryer Quick Reference Chart
Desiccant Air Dryers Refrigerated Air Dryers Application Metal finishing, paint lines, pharmaceutical, medical, food processing and other applications requiring ultra-dry air (ISO Quality Classes 1, 2 & 3); low-temperatures below freezing applications Standard manufacturing and service application removes liquid water only(ISO Quality Classes 4, 5 & 6) Typical Dew Point -40°F to -100°F +38°F Initial Investment Cost High Low Operating Cost High to Moderate Relatively Low Maintenance Cost Moderate LowRefrigerated compressed air dryers work by cooling the air. They work much like your refrigerator or freezer, using compressor coils filled with a refrigerant to chill the air to 33° to 40°F. As the air cools, water vapor condenses into liquid water, which is then drained off and disposed of. Liquid is collected in a water trap and expelled via an automatic drain. The dry compressed air is usually reheated to room temperature within the dryer before it is used.
Refrigerated dryers are the most commonly used air dryers in the manufacturing and service industries. They lower the dew point to ~38°F, which is more than adequate for powering pneumatic tools and other applications that simply require air with no visible moisture. If your application only requires dry air without any visible moisture present (ISO Quality Classes 4, 5 & 6), this type of air dryer will likely work for you. Compared to desiccant air dryers, refrigerated air dryers tend to have:
There are two types of refrigerated compressed air dryers: non-cycling and cycling.
Non-cycling air dryers allow the refrigeration circuit to run continually. They control their temperature by the use of a hot gas bypass valve and cycling of the evaporator fan to maintain a tight temperature range. These dryers are the most cost-effective and are typically very reliable while maintaining a fairly consistent dew point average of 38°F (as long as it’s properly sized and maintained). These dryers are also available for high inlet temperature applications typically found on reciprocating air compressor systems. All standard dryers no matter what type have a design criterion of 100 deg. inlet compressed air temperature,100 deg. ambient temperature and 100 PSIG pressure. Correction factors can be found in the dryer literature allowing you to adjust the dryer size for your specific conditions. Refrigerated dryers are typically oversized for summer conditions.
A cycling refrigerated air dryer reduces energy use by cycling on and off in response to demand. These dryers cost a bit more upfront but are much more energy-efficient over the long run. Maintenance costs tend to be a bit higher than for non-cycling refrigerated dryers due to their increased complexity. There are three types of cycling refrigerated compressed air dryers:
Before investing in a cycling refrigerated air dryer, you may want to calculate your energy savings to determine whether they will offset the higher capital investment. Some power companies offer rebates for these dryers. Contact us for additional information.
Desiccant compressed air dryers work by removing water vapor from the air using adsorption. An adsorptive material attracts water molecules and binds them to the surface of the material. They are full of tiny micropores, which act to increase the available surface area for adsorption. Desiccant dryers typically use activated alumina or molecular sieve desiccants. Most desiccant air dryers have two towers filled with desiccant beads to allow for continual operation. The system alternates between the two, allowing one to dry the compressed air while the other is regenerating the desiccant material.
Desiccant air dryers are more expensive than refrigerated air dryers, both in their initial capital investment and in operating and maintenance costs. However, they are capable of lowering the dew point of compressed air to -40°F or even -100°F. This is much, much drier air than a refrigerated dryer is capable of producing. If you need ultra-dry air for your processes (ISO Quality Classes 1, 2 & 3), a desiccant air dryer is the only way to go. Desiccant compressed air dryers are also essential if your applications run in freezing conditions; refrigerated dryers are not capable of reducing the dew point low enough to avoid condensation freezing when operating at very low temperatures.
A desiccant dryer uses energy and compressed air to regenerate spent desiccant materials. They may use between 5%-18% of your compressed air supply in the regeneration process, depending on the type of controls. There are three types of desiccant air dryers, which vary in their regeneration methods, energy costs, and compressed air use:
For the vast majority of manufacturers, a simple refrigerated compressed air dryer (cycling or non-cycling) is adequate. Unless your application requires ultra-dry air or is operating in below-freezing temperatures, it is hard to justify the added capital investment and operating costs for a desiccant dryer.
Questions to ask when deciding which type of air dryer to purchase include:
If you’re not sure which type of compressed air dryer is right for you, the specialists at Fluid-Aire Dynamics can help. We’ll help you evaluate your compressed air usage, dew point requirements, and potential energy savings to choose the right air dryer for your application.
Call 800-371-8380 or contact us to discuss your compressed air dryer options.
Every facility has differing application needs and usage demands, but selecting the right compressed air dryer for the situation will have a significant impact on energy savings and efficiencies.
Compressed air systems account for a significant portion of a facility’s overall operating costs. Choosing the right type of compressed air dryer for the application, and making sure that dryer is using energy in proportion to the demand when possible, can yield major savings over the life of the system.
Two categories of air dryers — refrigerated dryers and desiccant dryers — are widely used in industrial applications, and both have a place in the market. There isn’t a one-size-fits-all dryer solution for every facility. However, looking at the energy costs associated with the various options can help determine which solution will be most beneficial.
Typically, refrigerated dryers are the most economical to purchase and maintain, and they work well for most general manufacturing applications. These dryers yield air with a pressure dew point between 38 and 50 degrees F.
Refrigerated dryers reduce the temperature of compressed air through contact with a cold medium. Since cold air cannot hold as much moisture as hot air, saturated air condenses out moisture as the air temperature decreases, drying the air. The resultant moisture is removed using a moisture separator within the dryer and eliminated from the dryer through the drain system.
Refrigerated dryers generally fall into two categories: non-cycling and cycling, both of which use a refrigeration system to cool the compressed air. The two technologies differ in that once a non-cycling dryer is powered on, the refrigeration system runs continuously regardless of demand, while a cycling dryer can store cold energy within the unit until it is needed, which offers the ability to use energy in proportion to the demand. Most non-cycling dryers include a hot gas bypass valve to keep the dryer from freezing.
Because cycling dryers can store cold energy until it is needed, they help facilities conserve energy. Cycling dryers use the refrigeration system to cool a glycol-water mixture. This thermal mass exchanges heat with the warm air coming into the system, thereby cooling the air and warming the thermal mass. Once the thermal mass temperature rises above a set point, the refrigeration system is activated. The refrigeration system drives down the temperature of the thermal mass until it reaches the desired low temperature, at which point the refrigeration system turns off. This type of operation uses only the energy required to address the incoming air load on the dryer, another boost to energy efficiency.
Different air drying technologies yield different energy costs. The electrical costs for refrigerated dryers are essentially the refrigeration compressor, the controls and, in the case of an air-cooled unit, the condenser fans. Some units might have other components, such as a thermal mass pump, that make a minor contribution to the overall energy consumption.
Non-cycling refrigerated dryers are the least expensive models to purchase. However, cycling dryers provide the ability to use energy in proportion to demand, so they may ultimately be the least costly to own over the life of the dryer.
Instead of relying upon a refrigeration system that cools the air, desiccant dryers use porous desiccant beads to adsorb moisture from untreated air. Desiccant dryers, which provide air with a pressure dew point ranging from -40 to -100 degrees F, use two towers, each filled with desiccant material. While one tower adsorbs the moisture and dries the air, the second tower is regenerated. By alternating tower functions, desiccant dryers provide a constant stream of very dry air.
Desiccant dryers are good for applications where outdoor compressed air piping is subject to freezing. Critical applications, such as pharmaceutical and food applications, require the particularly dry air that is beyond what a refrigerated dryer is able to provide. There are three types of desiccant dryers used widely in the market: heatless, heated and blower purge. Energy costs vary by the type of desiccant dryer, with the energy use tied mostly to the manner of regeneration of the desiccant material.
Generating compressed air is an expensive process, and heatless dryers use about 15 percent of the compressed air emerging from the dryer to remove moisture from the desiccant beads, in order to regenerate it. This means that even though heatless desiccant dryers are less elaborate and often have no added electrical components other than the controls on the dryer, they can actually be higher consumers of energy compared to the other desiccant technologies because the cost of diverting 15 percent of the compressed air must be factored into overall energy costs.
Heated desiccant dryers incorporate a heater in the regeneration circuit of the dryer. This type uses a combination of heat and airflow to regenerate the desiccant adsorption beads in the regenerating tower. So while heated dryers consume additional energy with the supplementary heater, they use about half the compressed air for regeneration than that of heatless dryers. Therefore, heated dryers often are less costly to operate than heatless desiccant dryers.
The third type of desiccant dryer, blower purged units, do not use compressed air to regenerate the desiccant. Instead, this model has a dedicated blower to draw air from the surrounding environment. Because the airflow is generated by the blower, the total air capacity of the air compressor is available at the dryer outlet. This means the expense of compressed air for regeneration is not a factor, but there is the added energy use from the electric motor used to drive the blower.
The bottom line with desiccant dryers regarding energy consumption: Dryers that rely on large quantities of compressed air for regeneration probably will be more expensive to operate than dryers requiring less compressed air. Of the different desiccant designs, the blower purge type has the greatest up-front cost but is often the most efficient to operate because it does not use expensive compressed air for regeneration.
Many manufacturers do make desiccant dryers with technology that can regulate the switching and the compressed air consumption based on the demands on the dryer, which helps to make them more energy efficient. Such energy management systems typically either sense if the moisture front in the tower has reached a certain level, or they measure the actual output dew point of the dryer. This technology can prolong the switch-over of the towers, so the dryer’s regeneration cycle is not starting on a fixed increment of time but instead being initiated based on demand. Alternatively, the energy management systems may hold the purge valves closed, preventing purge air from being used until it is needed for regeneration.
While energy use accounts for a significant portion of a compressed air system’s operating costs, reliability also should be factored in when considering the total cost of ownership.
Refrigerated dryers use hermetic refrigeration systems, meaning the refrigerant is not exposed to the atmosphere, so they typically require low maintenance and service to keep the system running.
Desiccant dryers involve frequent valve-switching to direct air to either the drying or regeneration tower, and these models often operate in high-heat, high-demand applications. Therefore, this equipment requires more service and valve maintenance. Downtime for that maintenance should be factored into the overall life cycle cost of the product.
First and foremost, user needs should dictate the choice of dryer technology. Refrigerated dryers have substantially lower up-front and energy operating costs than desiccant dryers, but they are not able to provide air that is as dry. For general manufacturing processes, the refrigerated dryer option probably will be sufficient. Desiccant dryers provide the driest air for critical applications, but have higher up-front and energy costs.
In making the selection, users should consider if the pipe work associated with the process is located in a conditioned or unconditioned space. Desiccant dryers are the best choice if the piping is exposed to harsh conditions, such as temperatures that are below 40 degrees F.
If a downstream process machine requires that the air be drier than what a typical refrigerated dryer can deliver, point-of-use equipment may be the right answer. Such a system may use refrigerated dryers for most of the applications and desiccant models only where they are needed for specific applications.
While energy savings can be achieved by selecting the proper dryer for specific applications, system optimization should also be considered for all air systems to maximize efficiency and minimize operating costs.
Dryers are one part of a full compressed air system, and inefficiency of the system as a whole can have as much impact on energy costs as the dryers themselves. Fixing and repairing piping and valve leaks, maximizing air use within a facility and maintaining all compressed air equipment can help recoup the up-front dryer costs by ensuring the system as a whole is operating more efficiently.
The value of a compressed air audit and consideration of the system as a whole to determine true energy costs should not be overlooked. Compressed air system optimization is key for organizations looking to reduce energy costs.
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About the Author
Christopher Ursillo is the product manager for air treatment products at Ingersoll Rand. He has a degree in mechanical engineering from Villanova University and diverse professional experience in engineered systems and product development. Ursillo has worked in the compressed air industry for 14 years with a focus on air treatment. His responsibilities have spanned from application engineering to managing air treatment marketing programs. He has authored several articles weighing compressed air treatment options and the practical application of each technology. For questions or comments for Ursillo, contact External Communications Leader Anne Wages.
To read more Air Treatment Technology articles, visit www.airbestpractices.com/technology/air-treatment.
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