Editor's Note: This post was originally published in 2015 and has been completely updated for accuracy, comprehensiveness, and new information.
For compressed air systems to perform properly, the air must be dry; this means that the air must be free of liquid moisture and at a low relative humidity prior to entering the air network. Different methods can be used to remove the moisture content from compressed air, which will help it maintain a high level of air quality. One of the most popular pieces of equipment used to dry compressed air is a refrigerant air dryer, which can be classified as either non-cycling or as variable speed drive.
Like its name would suggest, a refrigerant dryer cools the compressed air. This allows the water present in the compressed air to condense and be separated from the air itself, and it is a crucial air system component for applications that require compressed air dried to a low dew point. In the first stage, compressed air enters the dryer and passes through an air-to-air heat exchanger, which pre-cools the incoming air. In the second stage, compressed air passes through an air to refrigerant heat exchanger to reduce the compressed air dew point, typically to 3°C / 37°F. Water vapor that was in the air condenses into liquid, which is trapped and removed. The dry air is then warmed up in the air-to-air heat exchanger to reduce the relative humidity in the air; this will prevent condensate from forming in downstream piping.
The moisture removal process in a refrigerated air dryer follows a straightforward refrigeration cycle. Here is a step-by-step look at what happens inside the dryer:
First, warm, moist compressed air enters the dryer and flows through an air-to-air heat exchanger, where it is pre-cooled by the outgoing dry air. This initial cooling stage reduces the energy required for the main refrigeration stage and improves overall efficiency.
Next, the pre-cooled air enters the air-to-refrigerant heat exchanger, where it is cooled further by the refrigerant circuit, typically down to around 3°C / 37°F. At this low temperature, the water vapor in the compressed air condenses into liquid water droplets.
The liquid water is then collected and removed from the system through a moisture separator and automatic drain. What remains is compressed air with a significantly reduced moisture content and a stable pressure dew point.
Finally, the dry air passes back through the air-to-air heat exchanger, where it is rewarmed by the incoming hot air. This rewarming step is important because it raises the relative humidity of the outgoing air, reducing the risk of condensation forming in downstream pipework and equipment.
This closed-loop process runs continuously, ensuring a steady supply of dry, clean compressed air to your system.
In a non-cycling (conventional) dryer, the refrigerant compressor runs continuously. Because ambient temperatures and the flow of compressed air vary, a bypass valve is often used to regulate the temperature of refrigerant while preventing freezing operating conditions within the system. Conventional refrigerant air dryers typically employ a piston compressor operating at one fixed speed. In terms of energy efficiency, this results in the same “on or off” shortcoming present in a fixed speed air compressor.
In contrast, a VSD refrigerant air dryer uses a scroll compressor with one spiral element orbiting around a second, fixed element. It’s compact and quiet, and the inverter that drives the scroll element can vary speed to match demand, saving energy. Because most compressed air systems do not operate under static flow and temperature conditions, variable speed refrigeration is used to control the refrigerant compressor during reduce loads and lower temperature conditions. The ability to match dryer energy consumption to air demand helps provide significant savings in operating costs.
For processes where compressed air demand is steady, a fixed speed compressor and fixed speed dryer can provide the required air cost-effectively. For processes where the demand for compressed air varies by factors such as process, workload or shift, a VSD compressor can match the production of compressed air to demand. While a fixed speed dryer can be sized to meet the maximum output of the VSD compressor, a VSD dryer’s ability to match dryer energy consumption to air demand can also provide significant savings in energy and operating costs.
While refrigerated air dryers are the most widely used drying solution for compressed air, they are not the only option available. Understanding how they compare to other drying methods can help you make the right choice for your application:
Refrigerated air dryers vs. desiccant dryers: Desiccant dryers use an adsorbent material to remove moisture from compressed air and can achieve significantly lower dew points than refrigerated air dryers, sometimes as low as -40°C / -40°F or below. This makes desiccant dryers the preferred choice for applications requiring ultra-dry air, such as certain pharmaceutical, electronics, or outdoor pipeline applications. However, desiccant dryers typically have higher operating costs than refrigerated air dryers due to the energy required for regeneration. For the majority of industrial applications where a dew point of 3°C / 37°F is sufficient, refrigerated air dryers offer a more cost-effective solution.
Refrigerated air dryers vs. membrane dryers: Membrane dryers use a semi-permeable membrane to separate moisture from compressed air and are valued for their compact size and simplicity. They require no electricity and have no moving parts, making them well suited for remote or hazardous locations. However, they are generally limited to smaller flow rates and may not achieve the consistent dew points that refrigerated air dryers can deliver across varying operating conditions.
For most general industrial, manufacturing, and commercial applications, refrigerated air dryers represent the best balance of performance, reliability, and cost, which is why they remain the most popular choice in compressed air systems worldwide.
What to consider when selecting a refrigerated air dryer
Choosing the right refrigerated air dryer for your compressed air system involves evaluating several key factors:
Flow rate: The dryer must be sized to handle the maximum flow rate of your compressor, measured in CFM. An undersized dryer will not be able to adequately dry the air, leading to moisture problems downstream.
Inlet air temperature: Higher inlet temperatures require more cooling capacity. If your compressor runs hot or operates in a warm environment, make sure your refrigerated air dryer is rated for those conditions.
Ambient temperature: Refrigerated air dryers have operating limits for ambient temperature. Exceeding these limits can reduce drying efficiency and damage the dryer's refrigeration components.
Required dew point: Most refrigerated air dryers achieve a pressure dew point of 3°C / 37°F. If your application requires a lower dew point, a desiccant dryer may be more appropriate.
Fixed speed vs. VSD: If your compressed air demand fluctuates, a VSD refrigerated air dryer will provide better energy efficiency by matching its refrigeration output to actual demand. For steady, consistent demand, a fixed speed dryer is a reliable and cost-effective choice.
Interested in learning more about refrigerated dryers and compressed air technology? You can download a free copy of An Overview of Compressed Air Drying whitepaper or contact an Atlas Copco expert for additional questions.
Frequently asked questions