Moisture in a compressed air system tends to be out of sight and often is out of mind too. But, left unchecked, even small amounts of condensation can cause damage.
A key part of compressed air moisture control centres around selecting the right compressed air dryer for your system and application.
In this article we look at why there is moisture in compressed air, the problems it can cause and how you can avoid compressed air moisture issues with the right air dryer for your application.
Why is there moisture in compressed air?
Ambient air contains moisture, this would be referred to as humidity on the weather forecast. When air is compressed, it is pressed into a more compact space. The contaminants (including water) can then be found at a higher volume in the compressed air.
There are a few factors affecting the amount of water produced by an air compressor, these include air temperature, humidity, compressor size and the required pressure.
An example: a 55 kW rotary screw air compressor running in an environment where the ambient temperature is 24 °C and the relative humidity is 75%, will generate up to 280 litres of water daily.
The threats of not controlling moisture levels:
- Corrosion and equipment wear
Moisture reacts with internal galvanised pipework, gradually degrading the surface. The corrosion not only weakens the infrastructure but also creates particulate contamination. Tiny particles can break loose and act as an abrasive inside valves, seals and actuators, leading to additional wear and premature mechanical failure. Over time, this shortens the lifespan of the entire compressed air system and raises maintenance costs.
In winter months or cold factory environments, condensed water can also freeze, risking pipe bursts or blockages.
- Tool malfunction
Pneumatic tools and air-driven machinery are designed to operate with clean, dry air. When moisture makes its way into these systems, the impact can be both immediate and long-term. Frequent tool failure doesn’t just mean replacement costs, it also leads to costly downtime, lost productivity, and, in some cases, safety hazards for operators handling malfunctioning equipment.
- Product quality and contamination
In many industries, especially food & beverage and pharmaceuticals, compressed air comes into direct or indirect contact with the end product. In these environments, moisture isn’t just a hassle, it’s a contaminant. Water in the air lines can ruin painted finishes, cause adhesive failure, or promote mould or bacteria growth. This poses a serious compliance risk and can lead to costly product recalls.
Understanding compressed air purity standards to help with moisture control
ISO 8573-1 is an international standard that defines air quality classes for compressed air systems. Many industries and applications have specific air quality requirements. For example, direct or in-direct contact compressed air in the food and beverage industry requires a purity designation ISO 8573-1:2010 [1:2:1].
Comparing the purity class you need with the allowed water content will help you specify the equipment you need to dry your air.
Prevention through proper compressed air moisture control
Available air dryers for compressors
- Refrigerant Dryers
Refrigerant dryers are the most common type, using heat exchangers to cool compressed air, condense moisture, and remove it through drainage. The air is then reheated to prevent condensation in the piping system and reduce cooling demand. They are available in air-cooled and water-cooled versions and work best when the relative humidity of the compressed air is kept below 50%.
Refrigerant dryers are also available with VSD technology, which adjusts the motor speed of the integrated compressor to match fluctuating air demand, cutting energy use and delivering the lowest operating costs.
- Desiccant Dryers
Desiccant dryers use a desiccant material to absorb moisture from compressed air, making them ideal for achieving very low pressure dew points. They are common in industries requiring extremely dry air, such as pharmaceuticals or electronics.
- Membrane Dryers
Membrane dryers are compact and low maintenance, they use semi-permeable membranes to remove moisture. They are suitable for smaller air flows or point-of-use applications where simple, silent drying is required.
How to select the right compressed air dryer
To determine which dryer is the best for your application, there are six key criteria that should be considered:
- Maximum air flow rate in litres per second
- Desired pressure dew point
- Air inlet pressure
- Air inlet temperature
- Ambient air temperature (and water temperature if the condenser is water-cooled)
- The installation environment of the dryer
Desiccant dryers can provide an ultra-low dew point, generally around -40°C. A reliable and predictable dew point can be important for demanding applications in sectors such as pharmaceuticals and food processing. Refrigerant dryers typically achieve a dew point of about 3°C, but they are less expensive to purchase, operate and maintain than desiccant dryers. They are mostly used for protecting the installation, rather than necessary for specific applications.
Question: I already have a dryer but still have issues with compressed air moisture control in my system, why is this?
If you already have a compressed air dryer but are struggling with moisture in your lines, there are a few things to check…
- The dryer could be faulty and so needs an engineer visit
- Your existing dryer could be undersized for your current production demand meaning it cannot keep up with demand
- If you are having moisture issues and you have a refrigerant dryer it might be that your compressed air is dry but not dry enough, in this case you would need a desiccant dryer.
IMPORTANT: Condensate Treatment
Trace amounts of oil can be found in the condensate from oil injected air compressors. Oil water separators are a legal requirement, not an option, in most instances.
Treatment is compulsory and regulated under the Water Resources Act (1991): law of discharge in the UK, Hazardous Waste regulations Act (2005) and European standard applicable since 23/10/2000. If you do not comply you are liable to imprisonment and potential fine of £20,000 or more.
In the UK, a site must have a consent to discharge from the local water authority. Generally, the limit is 20ppm, but you need to check with your local authority as regulations can differ between areas.
How can we help?
At PPS, we understand that no two compressed air systems are the same, so neither are their moisture management needs.
Whether you’re facing persistent water issues or simply want peace of mind, we can work with you to ensure your system delivers clean, dry and reliable air.
Contact our team for more support on your compressed air moisture control.