Cycling vs. Non-Cycling vs. VSD Refrigerated Air Dryers: Which Do You Need?

If you are choosing a refrigerated air dryer, you’ve got options. Cycling vs. non-cycling dryers. VSD air dryers. Digital scroll vs. thermal mass. Picking the right air dryer for your application may seem overwhelming. We’re going to break it all down for you, so you understand the differences between the various types of refrigerated dryers, how they compare on cost and energy efficiency, and how to choose the right dryer for optimal performance of your compressed air system. Plus, find out how your energy company may help you pay for an upgrade to a cycling or VSD air dryer. 

Why Do You Need an Air Dryer for Compressed Air?

An air dryer is an essential component of an industrial compressed air system. Removing moisture in compressed air is important to prevent corrosion in the compressed air system and tools, ensure product quality, and prevent microbial growth in the system. Most applications require a consistent supply of clean, dry air. (Read about compressed air quality standards to determine exactly how clean and dry air needs to be for different applications.) An air dryer helps to maintain the compressed air quality by removing moisture, which can lead to various problems if left untreated. 

Read more: Moisture in Your Compressed Air Is a Bad Deal. 

Refrigerated Air Dryers: What They Are, How They Work, What They Can Do

Refrigerated air dryers are one of the two main types of dryers for compressed air. (Read more: Refrigerated vs. Desiccant Air Dryers.) There are many different types of refrigerated dryers, including cycling, non-cycling, and VSD air dryers. Cycling air dryers can be further broken down into digital scroll and thermal mass dryer types. But regardless of the specifics, refrigerated air dryers all work by cooling the compressed air down to allow moisture to fall out. 

Refrigerated air dryers are popular for their simplicity, cost-effectiveness, and ability to deliver a consistent dew point suitable for most industrial applications. However, they may not be suitable for applications that require extremely dry air (with dew points lower than 38°F); in such cases, desiccant or membrane air dryers might be more appropriate.

How Refrigerated Dryers Work

A refrigerated air dryer works by cooling the compressed air, causing moisture in the air to condense, and then separating and draining the condensed water from the air stream. When air is compressed, air exiting the compressor is very hot. Warm air can hold more moisture than cool air (this is why there is more humidity in the air in the summer vs. the winter). As the air cools down, its ability to hold moisture drops, and excess moisture falls out of the air as liquid water, or condensation — just like dew forms on the grass when temperatures drop at night. This liquid water is then drained off and disposed of. 

Here are the basic steps of operation for a refrigerated air dryer.

• Compressed air enters the air dryer: After being compressed, the hot, moist air enters the refrigerated air dryer for treatment.
• Air is cooled via refrigeration: Compressed air enters the refrigeration system, where it passes through a heat exchanger. A refrigerant flows through a separate circuit within the heat exchanger, absorbing heat from the compressed air and cooling it down. In some refrigerated air dryers, the incoming hot and moist compressed air is first pre-cooled by the outgoing cold and dry air in an air-to-air heat exchanger. This step helps reduce the load on the refrigeration system and enhances energy efficiency.
• Moisture separation: The cooling process causes the moisture in the air to condense into liquid droplets. The cool and wet compressed air then passes through a moisture separator, which separates the condensed water from the air stream. Separators use various mechanisms, such as centrifugal force or coalescing filters, to effectively remove the liquid droplets from the air.
• Draining the condensed water: The separated water is collected and drained away from the system through an automatic drain valve, ensuring that the compressed air is now dry and free of moisture.
• Reheating the dry compressed air: In systems with an air-to-air heat exchanger, the dry, cold compressed air is reheated by the incoming hot, moist air before leaving the dryer. This step helps prevent condensation from forming on the outside of the air pipes and reduces the risk of corrosion. It also can prevent icing or frost that can develop as compressed air expands and cools while flowing through the piping system or into tools. 
• Dry compressed air exits the dryer: The now-dry compressed air exits the refrigerated air dryer and continues through the compressed air system to be used in various applications.

Dew Point for Refrigerated Dryers

Refrigerated air dryers typically achieve a pressure dew point (PDP) in the range of 35-50°F (2-10°C). This dew point level is suitable for most general industrial applications where the removal of moisture is necessary to prevent corrosion, contamination, and equipment damage.

It is important to note that the actual dew point achieved by a refrigerated air dryer depends on various factors, such as the design of the dryer, ambient temperature, inlet air temperature, and operating conditions. Proper sizing and maintenance of the air dryer are crucial to ensure that it consistently delivers the desired dew point.

For applications that require a lower dew point, desiccant air dryers are a more suitable choice, as they can achieve dew points as low as -40°F or even lower. 

Applications for Refrigerated Air Dryers 

Refrigerated air dryers are suitable for a wide range of industrial applications where the removal of moisture from compressed air is necessary to ensure process quality, prevent equipment damage, and avoid contamination. They are by far the most common type of air dryer used in industry and can be used for most industrial processes requiring air with a dew point of 38°F or higher. Some common industry applications for refrigerated air dryers include:

• Manufacturing: Compressed air is used to power pneumatic tools, equipment, conveying systems, and control systems in factories. Moisture-free air helps to maintain the efficiency and lifespan of these tools and prevent corrosion.
• Automotive: Compressed air is extensively used in automotive manufacturing and repair workshops for tasks such as painting, sandblasting, and operating air-powered tools. Dry air is crucial for achieving high-quality paint finishes and preventing moisture-related issues in the tools.
• Food and beverage: Refrigerated air dryers are used in food and beverage production and for packaging, filling, and conveying systems.
• Electronics: The electronics industry relies on clean, dry compressed air for processes such as component manufacturing, assembly, and testing. 
• Printing and paper: In printing and paper production, moisture-free air helps to maintain the quality of printed materials and prevents issues such as paper sticking or ink smudging.
• Plastics: In the plastics industry, dry compressed air is used for processes like injection molding, blow molding, and extrusion. 

What Is the Difference Between Cycling, Non-Cycling and VSD Air Dryers? 

Cycling, non-cycling, and VSD (Variable Speed Drive) air dryers are different types of refrigerated air dryers designed to remove moisture from compressed air. The primary differences between them lie in their operating principles, energy consumption, and adaptability to varying air demand.

Non-Cycling Air Dryers 

Non-cycling air dryers, also known as constant-speed or fixed-speed air dryers, maintain a continuous operation of the refrigeration system, regardless of the actual air demand. The compressor in the refrigeration system runs constantly, and the cooling capacity remains constant. These dryers are simpler in design and generally have lower upfront costs. However, they can be less energy-efficient, especially in applications with fluctuating air demand, as they consume the same amount of energy even when the demand for dry air is low.

Cycling Air Dryers

Cycling air dryers, also known as demand-based or load-responsive air dryers, adjust their cooling capacity based on the actual demand for dry air. They do this by cycling the refrigeration system on and off or by using other mechanisms such as thermal mass storage or digital scroll compressors. When the demand for dry air is low, the refrigeration system reduces its cooling capacity, resulting in energy savings. Cycling air dryers generally have higher upfront costs compared to non-cycling dryers but can offer improved energy efficiency and a more stable dew point in applications with fluctuating air demand.

VSD Air Dryers

Variable Speed Drive (VSD) air dryers are a type of refrigerated air dryer that uses variable speed technology to optimize energy consumption based on the actual demand for dry air. These dryers are also known as Variable Frequency Drive (VFD) air dryers, as they use a variable frequency drive to control the speed of the compressor. VSD air dryers can be considered a subtype of cycling refrigerated air dryers; they adjust their operation based on the demand for dry air, similar to digital scroll dryers. The main difference is that VSD air dryers use a different type of motor technology, which allows for more precise control of the cooling capacity. The speed of the refrigerant compressor is adjusted in real-time to match the compressed air flow, resulting in energy savings and improved performance.

Pros and Cons of Cycling vs. Non-Cycling Air Dryers

Types of Non-Cycling Refrigerated Air Dryers

There are two main types of non-cycling dryers: Direct Expansion (DX) Air Dryers and Plate Heat Exchanger Air Dryers. Both types of dryers can achieve similar dew points in the range of 35-50°F (2-10°C), suitable for most general industrial applications.

Direct Expansion (DX) Refrigerated Air Dryers

A direct expansion refrigerated air dryer is a type of refrigerated air dryer that uses a direct expansion cooling system to remove moisture from compressed air. The cooling process in a direct expansion air dryer is achieved by evaporating the refrigerant directly within the heat exchanger, which cools the compressed air and causes the moisture to condense. Direct expansion dryers typically have a shell-and-tube or tube-in-tube design, with the compressed air and refrigerant flowing through separate circuits within the heat exchanger. 

DX dryers are an older technology and are still widely used throughout the industry. Their relative simplicity makes them reliable, cost-effective, and easy to maintain, making them a popular choice for many applications. 

Plate Heat Exchanger (PHE) Refrigerated Air Dryers

These dryers use a plate heat exchanger to cool the compressed air indirectly. Plate heat exchanger dryers use a secondary fluid (such as glycol) in a closed loop between the refrigeration system and the plate heat exchanger. The refrigerant cools the secondary fluid, which then cools the compressed air as it passes through the plate heat exchanger. The plate heat exchanger consists of multiple corrugated metal plates stacked together. The compressed air and secondary fluid flow through alternating channels between the plates, allowing for efficient heat transfer. 

Plate heat exchanger dryers are less common compared to direct expansion dryers but offer certain advantages that make them suitable for specific applications. They tend to cost more up front, but usually have lower operating costs and better temperature control due to their high heat transfer efficiency. They also tend to be more compact than DX refrigerated air dryers. 

Types of Cycling Refrigerated Air Dryers

There are two main types of cycling dryers: Thermal Mass Air Dryers and Digital Scroll Air Dryers. Both types of cycling dryers offer advantages in terms of energy savings, but they operate very differently and have some different pros and cons of their own.

Thermal Mass Refrigerated Air Dryers

A thermal mass refrigerated air dryer is a type of cycling refrigerated air dryer that uses a thermal storage medium, often called a “thermal mass” or “thermal buffer” , to store and release thermal energy to optimize the dryer’s energy consumption based on the actual demand for dry air. The refrigerant cools the thermal mass, which helps stabilize the temperature and maintain a consistent dew point, even when the air demand fluctuates. The thermal mass can be either a solid or liquid material with high heat capacity and good thermal conductivity. 

Most modern thermal mass dryers use glycol or another heat transfer fluid as a thermal storage medium. In a glycol thermal mass dryer, the refrigerant cools the glycol, which then flows through a separate circuit within the heat exchanger, cooling the compressed air. Glycol has good thermal storage capacity and allows for efficient heat transfer. Other thermal mass dryers may use water, ice, or metal blocks, or plates made of materials with high thermal conductivity, such as aluminum or stainless steel. Some dryers use Phase Change Materials (PCMs), such as salt hydrates or low-melting-point metals like gallium; these materials store and release thermal energy when they change phase from solid to liquid or liquid to gas and back again. The thermal mass acts as a thermostat: when its temperature rises to a set level, the refrigeration cycle kicks in to cool it back down. 

The primary advantage of a thermal mass refrigerated air dryer is its ability to save energy during periods of low air demand. The refrigeration system cycles on and off based on the temperature of the thermal mass. When the demand for dry air is low, the refrigeration system turns off, and the thermal mass continues to cool the compressed air, providing energy savings and lower operating costs. 

However, these dryers tend to be larger and heavier than other cycling dryers due to the size and weight of the thermal mass itself. That means you need to consider whether you have the right space and supports for the dryer in your facility. 

Another issue to consider is dew point stability. Overall, thermal mass dryers do a great job of maintaining a stable dew point, especially if you are using air a little at a time. However, if you have a large influx of hot compressed air, this may cause the temperature of the thermal mass to spike, and it can take some time for the refrigerant to bring the thermal mass temperature down again. During this period, the dew point of your air may not be as low as expected — instead of being 38°F, for example, you may see a dew point of 50°F temporarily. This may not matter if your application is not particularly moisture sensitive, but a digital scroll or VSD compressor may be a better choice for applications with very tight moisture tolerances. 

Digital Scroll Refrigerated Air Dryers

A digital scroll air dryer is a type of refrigerated air dryer that uses a digital scroll compressor to control the refrigeration system’s capacity, optimizing energy consumption and providing a stable dew point. Digital scroll compressors have the unique ability to modulate their cooling capacity continuously, which allows them to respond efficiently to varying air demands. The digital scroll compressor’s capacity modulation reduces energy consumption during periods of low air demand and ensures that the refrigeration system only provides the necessary cooling capacity to maintain the desired dew point.

The digital scroll compressor consists of two scroll-shaped components: a fixed scroll and an orbiting scroll. The motion of the orbiting scroll around the fixed scroll provides the compression for the refrigerant as it goes through the refrigeration cycle. The digital scroll compressor can adjust the effective cooling capacity by varying the amount of time spent in the compression and non-compression states.

Digital scroll is a newer technology for cycling air dryers, and it has some significant advantages compared to thermal mass dryers. Their design offers very precise temperature and dew point control, making them a good choice for applications with varying air demand and very tight moisture tolerances. They also offer better energy efficiency at partial load and faster response to changing air demand. And without the large thermal mass inside, these dryers are smaller and lighter.  

Digital scroll air dryers are not as commonly used as thermal mass, VSD or non-cycling air dryers. They tend to have a higher initial cost and, since they are more complex, higher maintenance costs. Since they are not widely available, finding parts and service for a digital scroll air dryer may be challenging. However, for some applications, especially those with very tight moisture tolerances, they may be advantageous. 

VSD Air Dryers

VSD air dryers are growing in popularity due to their energy efficiency, stable dew point, and ability to adjust to changing compressed air demand. While VSD air dryers typically have a higher upfront cost compared to non-cycling dryers, their energy savings and improved performance can make them a cost-effective choice in the long run, especially for applications with fluctuating air demand.

How Does a VSD Air Dryer Work?

In a VSD air dryer, the speed of the compressor is adjusted in real-time to match the compressed air flow, which results in energy savings and improved performance. When the demand for dry air is low, the compressor operates at a lower speed, reducing energy consumption. Conversely, when the demand is high, the compressor operates at a higher speed to provide the necessary cooling capacity. The variable-speed drive motor allows the dryer to respond to air demand in real time. 

Advantages of VSD Air Dryers 

VSD air dryers offer several advantages over traditional fixed-speed refrigerated air dryers, including:

• Energy efficiency: By adjusting the compressor speed based on demand, VSD air dryers can significantly reduce energy consumption compared to non-cycling dryers.
• Stable dew point: VSD air dryers maintain a more stable dew point due to their precise control of the cooling capacity, which results in better moisture removal and improved air quality.
• Reduced wear and tear: The variable speed operation can reduce the stress on the refrigerant compressor, potentially extending its lifespan and reducing maintenance costs.
• Scalability: VSD air dryers can adapt to changes in the compressed air system, making them suitable for applications with variable air demand.

Do You Need a Cycling Air Dryer?

Is cycling or VSD air dryer worth the additional up-front cost? Both cycling and VSD dryers cost a bit more up front than a standard non-cycling air dryer. Whether or not it is worth it depends on your application requirements and air demand. In general, the larger the dryer and the more variable your air demand, the more benefit you will see from cycling or VSD air dryer. 

When you think about how these dryers work, you will see why. If the rated capacity of your air dryer is 100 CFM, and you are always operating at or near 100 CFM, a cycling or VSD air dryer will have little benefit—it will always be operating at its maximum capacity anyway. But if your air demand fluctuates, a cycling or VSD air dryer may be a better choice. These dryers can adjust their cooling capacity to match the actual air demand, resulting in better energy efficiency and stable dew point control. The more fluctuation you see in your air demand, the greater the energy savings. 

You’ll also want to look realistically at how much energy savings you will actually achieve. A VSD air compressor is almost always a good deal for facilities with variable CFM requirements, as the air compressor uses a lot of energy. However, dryers are smaller and have much lower energy demands compared to the air compressor they are servicing. For a small 5 HP to 15 HP air dryer, the difference in energy draw and operating costs between a cycling and non-cycling air dryer may not be enough to justify the additional capital costs. 

How Much Energy Will You Save with a Cycling Air Dryer?

The energy savings from a cycling air dryer compared to a non-cycling air dryer depends on factors such as compressed air demand, operating hours, load factor, and the energy consumption of the air dryers. In general, cycling air dryers can save between 20% to 50% or more of energy costs compared to non-cycling air dryers. However, the actual energy savings will vary based on your specific application and operating conditions.

Calculating Energy Savings for a VSD Air Dryer

To calculate energy savings for a VSD or cycling vs. non-cycling air dryer, follow these steps. 

• First, look at the kW rating for each dryer you are comparing. It is best to use actual manufacturer specifications for this calculation, as kW ratings may vary for different dryers of the same HP or CFM rating. 

• Multiply the power consumption in kW by the annual operating hours to calculate the annual energy consumption in kWh. 
• Then multiply your kWh by your energy rate (e.g., $0.10 per kWh) to calculate the annual energy cost for the dryer. 
• To estimate cost savings for the cycling dryer, you will need to estimate how much time it spends loaded vs. unloaded. For example, if it is operating at 50% capacity most of the time, you can estimate that it will use about 50% of the energy. 
• Look at the total estimated annual energy costs for the cycling vs. non-cycling dryer to determine whether the savings justify the additional up-front costs of the cycling or VSD dryer. 

Thermal Mass vs. Digital Scroll vs. VSD Air Dryers: Which to Choose 

So, you’ve made the decision to purchase a cycling air dryer: which is best, a thermal mass, digital scroll, or VSD air dryer? Here’s a quick comparison chart to help. 

Sizing Your Refrigerated Air Dryer 

Once you’ve decided what type of air dryer you need, you need to size your dryer for your application. Dryer size depends on several factors, including compressed air flow rate (CFM), operating pressures, inlet air temperatures, ambient air temperatures, and required dew point. As a rule of thumb, you should oversize your refrigerated dryer by about 50% — or even more if your air compressor is operating in very high ambient temperatures. Here’s why. 

Your compressed air dryer is rated for a particular CFM, at a particular operating pressure, and a particular inlet air temperature. If any of those factors change—particularly the temperature of air coming into the dryer — it will impact dryer performance. If the air coming into the dryer is warmer than its rated capacity, it will contain more moisture, and the dryer will have to work harder to cool the air down. As a result, the dryer may not be able to reach the dew point needed for your application. 

For example, a refrigerated dryer may be rated for 100 CFM at 100 PSI and 100°F inlet temperature. The inlet temperature for air coming out of the air compressor is directly related to ambient temperatures. In general, the air compressor aftercoolers will lower compressed air temperatures to within about 15°F of ambient conditions. So, if the compressor room is 100°F, compressed air coming into the dryer will be about 115°F. If ambient temperatures rise to 120°F, inlet temperatures will be about 135°F. This air is not only hotter, but also wetter. A dryer that can get 100°F air down to a dew point of 38°F may only be able to get 135°F air down to a dew point of 50°F. 

If you purchase an air dryer for the exact rated capacity of your application (e.g., 100 CFM at 100 PSI), it may not be able to keep up if airflow or pressure is increased or ambient air temperatures increase. In particular, dryers may struggle in the summer months when ambient temperatures and humidity rise. 

• For most applications, sizing the air dryer up by about 50% will provide enough wiggle room to account for variations in operating conditions. 
• If the compressor is running in a very hot environment, you may need to double air dryer capacity. 
• Increasing air dryer size doesn’t add much to the cost, so it doesn’t make sense to go cheap on air dryer capacity. If your air drying system has excess capacity built in, your dryers will have an easier time maintaining your required dew point. 

Understanding Correction Factors for Refrigerated Air Dryers

Dryer capacity depends not only on ambient and inlet air temperatures, but also airflow (CFM) and operating pressure (PSI). Of these, temperature makes the biggest difference, but changes in CFM and PSI can also impact air dryer performance. For this reason, air dryer manufacturers publish correction factor charts for refrigerated air dryers. These charts are used to adjust the dryer’s rated capacity based on actual operating conditions that differ from the standard conditions specified by the manufacturer.

Correction factor charts provide multipliers that can be applied to the rated capacity of the dryer to account for differences in operating conditions. These multipliers are typically presented as a table or graph, showing correction factors for different combinations of inlet air temperature, pressure, and flow rate. Correction factors will be specific to the make and model of the dryer; it is important to use the chart supplied by the dryer manufacturer. 

To use the correction chart, determine what your actual operating conditions may be, including PSI, CFM, and inlet air temperatures (which will be based on the type of air compressor you are using and the ambient air conditions). For proper sizing, think about the “worst case” scenario: i.e., the operating conditions that would put the most stress on your dryer. Locate the corresponding correction factors on the chart for each variable. Multiply the rated capacity of the dryer by the correction factor to obtain the adjusted capacity. 

Considerations in Selecting a Refrigerated Air Dryer

When selecting a refrigerated air dryer for your compressed air system, there are several factors to consider to ensure optimal performance and efficiency. Here’s a list of important aspects to keep in mind.

Energy Use

The energy efficiency of the dryer can significantly impact long-term operating costs. The larger the dryer (in HP), the more energy it will use, and the more important it will be to consider energy efficiency. When deciding between a non-cycling and a cycling or VSD air dryer, look at your air use patterns: the more your air demand fluctuates, the more you could potentially save by selecting a cycling or VSD air compressor. 

Initial Costs vs. Operating Costs

In general, a cycling or VSD air compressor will cost more up front, but will have lower operating costs over time. Evaluate how much energy savings you may get with a cycling or VSD dryer and decide whether or not it is worth the increased up-front costs. But do remember that energy incentives from your power company (see below!) may reduce or eliminate the initial cost difference between cycling and non-cycling dryers. 

Size, Weight and Installation Requirements 

Different types of dryers have different footprints, weights, and installation requirements. Review the manufacturer’s specifications and make sure the dryer will work with your space. For example, thermal mass dryers tend to be much larger and heavier than other types and will require a larger space and heavier-duty supports. A VSD or digital scroll will have a smaller footprint per rated CFM and be lighter and easier to install. 

Maintenance Requirements 

Of all the dryer types, non-cycling dryers are the simplest and easiest to maintain, followed by thermal mass dryers. VSD dryers are a little more complex due to the addition of the VSD motor, but they are still fairly easy to maintain and operate. Digital scroll dryers, on the other hand, are the most complex in design, and because they are less common, it may be harder to get parts and service. 

Dew Point Stability 

Dew point stability is an important consideration when selecting a refrigerated dryer. In general, cycling air dryers will offer more temperature and dew point stability than non-cycling dryers. Among the cycling dryers, thermal mass dryers will have much more variability in dew point, especially if they are subjected to a large influx of very hot compressed air. VSD air dryers offer very good dew point control and are able to respond quickly to changes in CFM. Digital scroll dryers offer excellent dew point control, which gives them an advantage for applications with very tight moisture tolerances. 

Air Filtration

When selecting an air dryer, also consider air filtration options. Some dryers come with internal filtration, which can help in removing oil mists and aerosols from compressed air; oil is easier to remove while it is at its coolest point inside the dryer, before it warms up again in the distribution system. Make sure the internal filter is a coalescing filter to remove oil mists. Most compressed air systems will also use inline filtration after the dryers as part of a complete air treatment strategy. 

Pipe Size 

Many people believe that the outlet pipe for the air dryer has to match the pipe size for the distribution system. While that is ideal for maintaining efficient airflow, it is not absolutely necessary. It is possible to pipe a dryer with a 2” outlet to a 3” distribution pipe without losing too much pressure or airflow. 

Energy Incentives 

As mentioned above, energy incentives from your power company may make the difference in selecting a refrigerated air dryer for your compressed air system. These incentives may cover most or all of the cost difference between a non-cycling air dryer and a cycling or VSD air dryer. Check with your energy company to see what rebates are available in your area. Some companies differentiate between the types of air dryers based on the differences in energy efficiency they deliver. Check out available incentives from ComEd in Northern Illinois, Xcel in Minnesota, Consumers Energy and DTE in Michigan, and Focus on Energy in Wisconsin. 

Refrigerated Air Dryers from Fluid-Aire Dynamics 

Choosing an air dryer is an important part of compressed air system design. Not sure which type of air dryer is best for you? The Aire Experts at Fluid-Aire Dynamics can help! We can help you choose between cycling and non-cycling air dryers or decide whether a VSD dryer is right for you. We’ll even help you figure out what energy incentives are available in your area to help pay for it. 

Contact us for a consultation or quote.