Why is it Important to Dry Compressed Air?
13 November, 2022
Drying the output air is very important if you want to avoid problems in the compressed air system. Learn more about why drying is needed and how it is done.
Selecting the best compressed air dryer for your needs
Selecting the right compressed air dryer is essential for keeping your production efficient, reliable and cost-effective. Every compressed air system naturally produces moisture, which can harm equipment, contaminate products and increase maintenance costs. This guide explains how different dryer technologies work, what performance factors to consider and how to select the most suitable option for your application.
Moisture in compressed air is unavoidable. When air is compressed, water vapor condenses inside the system. Without proper air treatment, this moisture can cause corrosion in piping, damage pneumatic tools, clog valves and shorten the lifespan of critical components. The result is reduced performance, unexpected breakdowns and potential production interruptions.
Water vapor also creates contamination risks. In industries such as food, pharmaceuticals and manufacturing, wet compressed air can compromise product quality, lead to rejected batches and affect compliance. Even in general industrial applications, moisture can negatively impact processes such as painting, packaging and surface treatment. Inadequate air treatment also increases costs. Frequent maintenance, unplanned downtime and premature equipment replacement significantly raise the total cost of ownership.
The right dryer depends on your application. A refrigerated air dryer suits most industrial facilities, while a desiccant dryer is the better choice for laboratories, spray painting, or pneumatic tools. Read more on the importance of dryers in compressed air.
Every compressed air system needs a dryer because it
- Prevents damage caused by moisture
- Reduce the risk of product contamination
- Lower maintenance costs
- Improve overall system reliability
Refrigerated air dryers, desiccant dryers and membrane dryers are the three main types of compressed air dryers. Each suits different applications, dew point requirements and budgets.
Refrigerated Dryer
A refrigerated air dryer is the most common choice for industrial applications that require dry air but not a critical dew point. It cools the air to around +3°C PDP, causing moisture to condense and drain away.
Three options are available:
- Non-cycling: runs continuously regardless of demand. Simple, affordable and easy to install, but less energy efficient at low load. Works well with any rotary screw compressor.
- Cycling: turns on and off based on compressed air demand using thermal mass and frequency controllers. More energy efficient than non-cycling, with minimal noise and easy installation. The better choice if energy costs are a concern or demand fluctuates.
- VSD (Variable Speed Drive): automatically adjusts motor speed to match air demand, reducing energy consumption significantly. Delivers a stable, low dew point at all times and offers the lowest lifecycle cost of the three options.
Desiccant Dryer
When applications require a dew point below 0°C (such as cold environments, laboratories or sensitive manufacturing) a desiccant dryer is the right choice. Unlike refrigerated dryers, desiccant dryers use desiccant beads to adsorb moisture from the air without changing its composition. Once saturated, the desiccant is regenerated using purge air, heat, or both. Most designs use a dual-tower construction. One tower dries the air while the other regenerates.
Three types are available, each differing in how regeneration is handled:
- Heatless dryers: use 16–25% of compressed air as purge air for regeneration. The simplest and most affordable option, but the least energy efficient. Account for the extra purge air when sizing your compressor.
- Heated purge dryers: use internal or external heaters to assist regeneration, reducing purge air to less than 10%. Higher initial investment, but significantly more energy efficient over the lifecycle.
- Blower purge dryers: circulate heated air through the desiccant, resulting in minimal compressed air loss. The most expensive option upfront, but delivers the best return on investment through energy savings.
Membrane Dryer
A membrane air dryer uses selective permeation to remove moisture from compressed air. Air passes through a bundle of hollow fibres — water vapour permeates through the membrane walls and is swept away, leaving dry air to continue through the system. With no moving parts and no power supply required, membrane dryers are exceptionally reliable and virtually maintenance-free.
Three characteristics make membrane dryers distinct from other types:
- Point-of-use design: best suited for remote locations, outdoor installations or individual machines on a production line where a small, self-contained drying solution is needed
- Low energy demand: no power supply required, making them the most energy-efficient dryer type available
- Membrane air dryer vs desiccant: membrane dryers are the simpler, lower-maintenance choice for moderate dew point requirements; for critical processes requiring very dry air down to -70°C across a full system, a desiccant dryer remains the stronger option
Summary of air dryers types
The table below gives a quick overview before the sections go into more detail.
| Technology | PDP Range | Energy Use | Maintenance | Best Applications | Capital Cost |
|---|---|---|---|---|---|
| Non-cycling refrigerated | +5–7°C | Medium | Low | General industrial | Low |
| Cycling refrigerated | +3°C | Medium | Low | Variable demand | Medium |
| VSD refrigerated | +3°C | Low | Low | Variable demand | Medium–High |
| Desiccant (heatless) | -40°C / -70°C | Medium | Medium | Labs, pharma, food | Medium |
| Desiccant (heated) | -40°C / -70°C | Low–Medium | Medium | Cold environments | High |
| Membrane | -40°C | Very Low | Very Low | Point-of-use | Low–Medium |
Choosing the correct Pressure dew point (PDP) for your application and ambient conditions ensures reliable operation and prevents condensation-related issues.
What is the dew point?
Not all dry air is equal. The key performance indicator of a compressed air dryer is the PDP. It is the temperature at which water vapor in compressed air starts to condense at operating pressure. In practical terms, it tells you how dry the air is inside your system. This differs from atmospheric dew point, which is measured at normal pressure. Because compressed air is pressurized, moisture condenses differently, so a compressor dryer dew point must always be specified at system pressure to reflect real drying performance.
In practice, a refrigerated dryer typically delivers a dew point of around +3–7°C PDP, which is sufficient for most industrial applications. More sensitive processes or cold environments may require a much lower PDP, such as -40°C, where a desiccant dryer is needed.
Choosing the wrong dryer size is a common and costly mistake in compressed air systems. An undersized dryer cannot remove all moisture, while an oversized dryer increases capital and energy costs. Correct sizing starts with your compressor’s actual airflow and adjusts for real operating conditions using correction factors.
Step by step dryer sizing
1. Determine compressor flow (FAD)
Find the compressor’s Free Air Delivery (FAD) in m³/min or CFM from the specification sheet or nameplate. Do not rely on motor horsepower, as it does not reflect actual airflow.
2. Identify operating conditions
Record the inlet air temperature, ambient temperature, and system pressure. Higher inlet temperatures significantly increase the moisture load on the dryer.
3. Apply correction factors
Manufacturers provide correction factors (CF) for conditions that differ from standard ratings. These factors adjust the dryer’s real performance at your site.
4. Select the correct dryer size
Choose a dryer whose corrected capacity exceeds your compressor FAD, and include a 10–15% safety margin for peak demand or future expansion.
5. Verify the pressure dew point (PDP)
Ensure the dryer achieves the required pressure dew point at the corrected flow rate, not just under standard conditions.
The purchase price of a compressed air dryer is rarely its biggest cost. Over its working life, energy consumption typically far exceeds the initial investment, making efficiency the most important factor when comparing options.
Refrigerated dryers Non-cycling models run at full power regardless of demand. Cycling and VSD designs adjust to actual load, reducing energy use significantly during quieter periods. Over a ten-year operating life, this difference can exceed the original purchase price.
Desiccant dryers Heatless designs use no electrical power for regeneration, but consume a portion of treated air as purge – a real ongoing cost that your compressor must cover. Blower purge designs eliminate most of this loss and typically recover their higher upfront cost within a few years.
Always evaluate dryers on total cost of ownership (purchase price, energy, consumables and maintenance) not capital cost alone.
Not sure which dryer to choose?
See the video for more info on what kind of air dryers you can get.
Let us help you to make the right choice
Choosing between a refrigerated or desiccant dryer depends on your air quality needs. The right system protects equipment, prevents downtime, and ensures clean, reliable production, which is saving you money over time. We are here to find you the best dryer for your application.
Do I need an air dryer for my compressor?
Yes, you do. Every compressor produces moisture as a natural byproduct of compression, and without a dryer that moisture will travel downstream, causing corrosion, equipment damage and potential product contamination. The only question is which type of dryer is right for your application.
What does a refrigerated dryer do?
A refrigerated dryer cools compressed air to around +3–7°C, causing water vapour to condense into liquid that is then drained away, leaving dry air to continue through the system. It is the most common dryer type for general industrial applications and suits the majority of installations where a critical dew point is not required.
What is an air dryer and how does it work?
An air dryer is a filtration device installed in a compressed air system to remove water vapour before it can cause damage downstream. Depending on the technology (refrigerated, desiccant or membrane) it works by either cooling the air until moisture condenses, passing the air through a moisture-absorbing desiccant material, or forcing water vapour through a selective membrane, in each case leaving drier air to continue through the system.
Which type of dryer is most energy efficient?
For most industrial facilities with variable demand, a VSD refrigerated dryer is the most energy-efficient choice, as it continuously adjusts its output to match actual airflow rather than running at full power regardless of load. For applications requiring a critical dew point, a blower purge desiccant dryer is the most efficient option, eliminating the purge air losses that make heatless desiccant designs costly to run.
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