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.
Why Choosing the Right Air Dryer Is Important
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:
- How dry do you need your air to be?
- How cold are ambient temperatures?
- What is the CFM of your application?
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What Is the Purpose of a Compressed Air Dryer?
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:
- Liquid will tend to fall out of compressed air as it moves through the piping system and cools. Condensation in compressed air pipes and air-powered tools can lead to problems with corrosion and scale or wash away lubrication in production equipment. If piping is exposed to cold temperatures, water in control lines can freeze, causing blockages or damaging the lines.
- Water vapor remaining in compressed air is problematic for many manufacturing processes. For example, excess moisture in air-operated paint lines will have an adverse effect on color, adherence, drying times, and finish. In food processing or pharmaceutical applications, moisture in compressed air can lead to spoilage.
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.
What Are the Differences Between Refrigerated and Desiccant Air Dryers?
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|
Refrigerated Air Dryers
Refrigerated 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:
- Lower capital investment;
- Lower operating and maintenance costs;
- Higher dew points (more moisture left in the air).
There are two types of refrigerated compressed air dryers: non-cycling and cycling.
Non-Cycling Air Dryers
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.
Cycling Air Dryers
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:
- Thermal mass cycling refrigerated dryers use a thermal storage medium to store cooling capacity that can be used when the dryer is operating at less than full load. When the thermal mass reaches a predetermined temperature, the refrigeration compressor is switched off to save energy.
- Digital scroll cycling refrigerated dryers cycle the refrigeration compressor on and off in response to demand. Capacity is binary (digital); the system is either operating at full capacity or no capacity.
- Variable speed drive refrigerated air dryers (also called variable frequency drive, or VFD) adjust the motor speed for the refrigeration compressor and condenser fan in response to real-time demand.
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 Air Dryers
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:
- A heatless desiccant dryer simply uses up to 18% of the rated capacity of the dryer to purge back through the saturated tower for the regeneration process.
- A heated desiccant dryer heats a lower flow of purge air (approximately 5 – 7%) to regenerate the desiccant material.
- A blower purge dryer uses very little or no compressed air in the regeneration process but instead uses heat and a blower. It takes more energy to operate but preserves the compressed air supply.
Desiccant vs. Refrigerated Air Dryers: What Compressed Air Dryer Should I Choose?
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:
- What are the dew point requirements and ISO ratings for your application? Is ultra-dry air required?
- Will your application be operating below 38°F?
- Will your air dryer be running more or less consistently during each shift, or would you benefit from a cycling air dryer that can ramp up and down with demand?
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.