The Use of Flow and Dew Point Sensors in Relation to Compressed Air Quality Classes

Compressed air systems are critical to many industries, ranging from manufacturing and food production to pharmaceuticals and electronics. To maintain reliable, efficient operations, compressed air quality must adhere to specific standards, most notably those outlined in?ISO 8573-1. This standard classifies compressed air quality into different "classes" based on the concentration of particles, water (humidity), and oil.

The use of?flow sensors?and?dew point sensors?plays a pivotal role in monitoring, controlling, and optimizing industrial compressed air systems to achieve desired quality classes. This article will explore how these sensors support compliance with ISO 8573-1 and examine how air quality impacts the?lifetime?and?service intervals?of system components, including the sensors themselves. This fact is often overlooked. Similar to the equipment and processes that are monitored, sensors also need proper care and maintenance. Here comes the credo “less is more”. When selecting and installing sensors, it is key to select the right number of sensors to provide just the data you need to generate the required KPI’s.

Flow Sensors in Compressed Air Systems

Flow sensors, such as?thermal mass flow meters?and?differential pressure (DP) flow meters, are essential for measuring the volume and rate of air delivered by the compressed air system. They provide critical data for assessing system performance and ensuring that air is delivered efficiently and cost-effectively. Flow meters with built-in pressure sensors provide additional value, as they can be used to identify pressure loss, in relation to high incidental demand, or due to undersized piping, the absence of storage or poor pressure control.

Thermal Mass Flow Meters

Thermal flow meters work by measuring the cooling effect of air as it passes over a heated sensor. These meters are highly accurate at measuring mass flow in compressed air systems and are unaffected by changes in temperature or pressure. The more advanced types can also measure reverse flow, which is crucial when mounted in ring networks.

• Applications:?Thermal flow meters are ideal for monitoring demand profiles after proper drying and filtering of the compressed air. Under these circumstances, they can be used to identify nominal consumption, detect leakage and size equipment such as filters, dryers. ?
• Relevance to Air Quality Classes:?Typically, thermal mass flow meters can work properly up to 4-5-4, or 4-4-3, if particles in the air flow are not causing any abrasive effect. When flow readings deviate from expected patterns, this could indicate blockages (by desiccant after a blow-out), fouling due to oil and aerosols. Application in high dew points is not a problem, as long as there is no condensation. Drops of condensate can cause peaks in the signal, and complete flooding situations should certainly be avoided as this can cause significant mis-readings.

Differential Pressure (DP) Flow Meters

Differential pressure flow meters measure the pressure drop across a restriction (e.g., an orifice plate or venturi) to calculate the flow rate.

• Application:?DP meters are widely used in systems requiring high durability under more extreme conditions.
• Relevance to Air Quality:?They can be used beyond the classification table, where air is condensing and oil mist/droplets are present. But it is not a magic technology for all circumstances: Care must be taken when installing the sensors as excessive water or flooding can impact their readout.

Recalibration advice for flow sensors

Typically, flow meters need a regular inspection and cleaning, especially in oil lubricated systems without proper downstream filtering. Some flow meters even require a bi-annual offset check, which may become very cumbersome and annoying for the user (“yet another maintenance task”). When investing in a flow meter, be aware of this feature when selecting a differential pressure flow meter.

Recalibration of flow sensors can be planned as described in your company’s Quality Management System or may be a strict requirement for compliance. Typically, an instrument should be calibrated once per year. So, what may affect the interval from an air quality perspective? The following points provide a guideline.

The dew point classification does not impact the recalibration period, if the dew point is kept between class 0 and 5. A water problem like flooding due to a dryer failure can have an impact and even break a thermal mass flow sensor. A DP sensor – when not properly installed- may also get flooded, resulting in false readings.

The oil classification can have an impact as the oil causes a fouling layer over the element or it may attract dirt, rust particles and so on. In most cases, the sensor can be cleaned off using water and a mild detergent. In a system with oil content class 2 or lower, this is less of an issue.

Finally, the particle class. Large particles may impact the sensor – literally. They can damage thermal mass flow sensors, especially those with ceramic elements. Sub-micrometer particles do not have a significant impact at low velocities, but when the velocity is high enough, they can “sandblast” the sensor surface. The only way to find this out is by regular inspection. And preventing desiccant blow out of course.

Dew Point Sensors in Compressed Air Systems

Dew point sensors measure the moisture content in compressed air by determining the temperature at which water vapor condenses into liquid. These sensors are critical in achieving the humidity requirements of ISO 8573-1. They are your watchdogs, to prevent collateral damage, for example failing valves in control equipment, or contamination of food products with water.

Relevance to Air Quality Classes

Compressed air systems often aim to meet ISO 8573-1 humidity classes, ranging from?Class 0 or 1?(very low moisture content, -70°C pressure dew point or lower) to?Class 6?(higher allowable humidity, +10°C dew point). Dew point sensors ensure the system meets the target class by continuously monitoring the air's moisture level.

Importance for Compressed air System Health, Product Quality and Safety

Excess moisture in compressed air can lead to many issues:

• Corrosion?in piping and end-use equipment.
• Flooding when condensate drains are not working properly.
• Microbial growth?in systems serving sensitive industries like food and pharmaceuticals.
• Increased wear on pneumatic tools, and cylinders
• Reliability issues when control equipment is malfunctioning due to a stuck or broken valve.
• Safety may be compromised when equipment starts to show hiccups. This can also lead to increased stress and frustration for machine operators.

Maintaining a proper dew point through accurate monitoring ensures the longevity of system components and reduces the frequency of repairs.

ISO 8573-1 Air Quality Classes and Their Impact on System Lifetime

The ISO 8573-1 standard defines compressed air quality in terms of?particles,?water, and?oil. Each of these contaminants can have profound effects on the reliability, service life, and maintenance intervals of system components.

Particles

Particles include dust, dirt, rust, and other solid contaminants.

• Impact on System:?Particles can clog filters, wear down seals and valves, and erode piping.
• Sensor applications:?Abnormal pressure drops across filters (detected by DP flow meters) may indicate a high particle load.
• Effect on Lifetime:?Excessive particles lead to frequent filter replacements, reduced component life, and higher maintenance costs.

Oil (Aerosols and Vapor)

Oil contamination can come from lubricated compressors or ambient air.

• Impact on System:?Oil aerosols can damage sensitive pneumatic equipment, clog air preparation units, and contaminate processes in industries like food and pharmaceuticals.
• Sensor Applications: Flow sensors may detect irregularities caused by oil build-up in filters, while dew point sensors may indicate issues with desiccant degradation due to fouling.
• Effect on Lifetime:?Contaminated air shortens the life of filters, dryers,? pneumatic tools, necessitating frequent cleaning or replacement.

An on-line monitoring system for oil-content combined with the right filters and maintenance procedures can help to prevent such issues.

Humidity

Water vapor in compressed air can condense into liquid water, particularly if the air cools below its dew point. This water will generally build up at the lowest point of your compressed air system. This is caused by a thing called gravity.

• Impact on System:?Moisture causes rust in mild steel pipe material, reduces lubricant effectiveness, and damages pneumatic equipment. It also fosters microbial growth in sensitive applications.
• Sensor Role:?Dew point sensors help to ensure that the air humidity level stays within acceptable moisture limits for the desired quality class. They can also be used for active control of dryers, which can yield significant energy savings, extension of the lifetime of the dryer and carbon footprint reduction.
• Effect on Lifetime:?Excess moisture accelerates corrosion, increases maintenance costs, and reduces the longevity of tools and equipment. It can also cause severe damage to downstream components as previously mentioned.

Maximizing Service Intervals and uptime with Proper Monitoring

By integrating flow and dew point sensors into a compressed air system, operators can ensure that the air quality is compliant with the selected ?ISO 8573-1 class, ?while extending component lifespans. Here's how:

1. Proactive Maintenance:?Sensors provide real-time data, allowing for timely intervention before contaminants cause damage.
2. Leak Detection:?Flow meters help identify leaks that may lead to inefficiency and increased contamination exposure. Reverse flow measurement may also indicate failing dryers or drains, which can be the starting point of further problems with air quality. Last but not least, leaks cause water ingress in the pipe, which leads to a compromised dew point downstream.
3. Optimized Drying:?Dew point sensors ensure dryers operate only when necessary, reducing energy consumption and wear on desiccants or refrigerant systems. Flow sensors can also help to identify dryer capacity issues due to under- or oversizing. However, temperature at the inlet has the biggest impact on dryer performance: Flow sensors with integrated temperature measurement placed on the inlet can therefore be of great help to identify potential issues.
4. Filter Performance Monitoring:?By tracking pressure drops with 3-in-1 flow meters, operators can replace filters only when needed, avoiding premature or overdue maintenance.

Conclusion

The use of thermal and differential pressure flow sensors, combined with dew point sensors, and optional oil vapor/ particle sensors is essential for maintaining compressed air quality within ISO 8573-1 standards. These sensors help monitor and control contaminants (particles, oil, and humidity) that can significantly impact the lifetime and service intervals of compressed air systems. Properly maintained air quality not only extends the life of system components but also reduces energy consumption, minimizes downtime, and ensures compliance with industry standards.

Investing in sensor technology and implementing an effective monitoring strategy is a smart move for any organization seeking to improve efficiency and reduce total cost of ownership in compressed air systems, which eventually will have a significant impact on their bottom line.