Moisture Contamination in Compressed Air Supply

Compressed air is a vital part of the engineer’s tool kit but it has its challenges. It is an unavoidable truth that the presence of moisture in compressed air can limit its performance. Moisture is inherently contained in all air and, when air is compressed, the partial pressure of the water vapor rises. This has the knock-on effect of increasing the dew point. It is important to remember that compressed air is subject to the same properties as the air we breathe: for example, the humid air that is heavy with moisture on a hot summer's day. For more information on the effects of air pressure on moisture, Humidity Academy Theory 5 - Effect of Temperature and Pressure is a useful resource.

It is also worth keeping in mind that, unlike other stored gases such as nitrous oxide, compressed air is not highly regulated. This means the responsibility for quality control often falls squarely on the user, with no traceable line of accountability.

If the downstream process is contaminated with moisture, what is the impact?

Depending on the use case, moisture can cause an engineer a wide variety of issues that will lead to increased maintenance and downtime:

Internal corrosion of metal components and tubing

Freezing and expansion of moisture at low temperatures

Sensitive instruments can be affected by the presence of moisture

Potentially harmful bacteria can form and breed in moisture

Before we address how to mitigate the problems that can arise from the presence of contaminating moisture in compressed air, there are some factors that must be accounted for.

Which engineering moisture scale is used to express low moisture levels?

'Dew point’ refers to the temperature that air reaches, at which point water vapor takes on a liquid state. If the temperature is below this point, water forms solid droplets that attach to solid surfaces or saturate the gas stream. When air pressure increases, the dew-point temperature rises. By taking these factors into consideration, we can consider tools that control air pressure and temperature and measure the dew point. By using these tools, we can mitigate the negative effects of moisture in compressed air.

What international standards define compressed air quality levels?

ISO 8573-1 is the international standard for air quality. This ISO exists to regulate air quality for all industrial uses. The levels of moisture, pollutants, particulates and oils in compressed air are all subject to ISO 8573-1. Depending on the use case, it is vital that the level of moisture in our compressed air is found to be compliant with the respective quality class (see below). Once again, tools for moisture measurement can help us stay within acceptable parameters for water vapor levels.

How do we control moisture levels within the compressed air?

Moisture in compressed air has to be removed in order to ensure its safe use. An effective solution is to use a compressed air dryer.

A pressure swing absorption dryer works by binding the moisture within compressed air to a chemical substance. This removes it from the airflow but is considered an expensive option as it requires significant amounts of absorbent chemicals.

Adsorption drying is the more popular choice. This method involves passing compressed air over a hygroscopic material called a desiccant. This material will become saturated with water over time and require regeneration. This can be achieved by heating ambient air and using it to dry the hygroscopic material. Below is an example of how adsorption drying (also known as desiccant air drying) functions.

Filters are another great way to remove saturated moisture if a system is operating below the dew point. By passing the compressed air through a filter, droplets are collected and coalesced ready for removal from the system. Once heavy enough, these droplets will fall into a moisture trap. These filters are effective on low pressure setups, particularly as they can also remove aerosols meaning they align with ISO 8573-1.

Refrigerated air dryers work by cooling compressed air as it passes through the dryer. This causes the vapor to condense into moisture that can be drained out of the system. The air is then re-heated before leaving the dryer and returning to the air flow. These dryers are advantageous due to their low operational and maintenance cost. Non-cycling variants of these dryers continuously circulate refrigerant through the system, often using a hot gas bypass valve to regulate temperature. Variable speed dryers can control the compression of refrigerant to match the needs of the compressed air system as a whole.

Membrane air dryers utilize selective permeation to separate gas constituents within the air flow. By passing the compressed air through layers of hollow fibers, membrane dryers can separate moisture that is unable to pass through the material from the dry air that can. This moisture is then drained from the system whilst the dried air continues on with a small decrease in pressure. While these dryers are an effective solution that is easy to maintain, they have limited effectiveness when extremely low dew points are required.

How can we measure moisture levels in compressed air economically and reliably?

Moisture in compressed air can cause problems on a practical and regulatory level. It is therefore imperative that we account for and mitigate these problems whenever they arise. The best way to do this is to make sure that the level of moisture in our compressed air is closely and accurately monitored. We can then measure these readings against ISO 8573-1 parameters.

We have established that any application that utilizes compressed air also requires a method of dew-point measurement. But what options are available to modern engineers? Products such as the SF82 Dew-Point Transmitter are ideal for keeping the moisture levels of compressed air monitored in compressed air dryers. This transmitter is specially engineered to be temperature resilient in conditions typical of a compression room. Temperatures can reach +50 °C (+122 °F); the SF82 is still capable of accurate dew-point measurement down to -60 °C (-76 °F).

The Easidew PDP Dryer Portable has been designed with portability as a primary principle. It is engineered for spot-checking the outlet of gas dryers. The capacity to check moisture levels from a portable set-up is an invaluable tool for engineers working with compressed air. This device is fitted with a battery that lasts up to two working days, making it ideal for checking multiple compression setups on various projects.

The Michel Easidew EA2 is another great option for compressed air systems. This sensor is specially designed for use in hazardous area applications to measure both dew point and moisture content. It has a measurement range of -110 °Cdp up to +20 °Cdp, allowing measurement of moisture in varied conditions.

Moisture in compressed air poses significant challenges but addressing it doesn't have to be complicated. By using the right equipment, you can manage moisture levels successfully, ensuring ISO compliance while maintaining safe and efficient operations.

With 50 years’ experience in the development of moisture and dew-point technology, we are the application experts for all trace moisture measurement in compressed air and specialized gas applications. If you would like to discuss your requirements, please contact our team today. 