Applications for Industrial Dew Point Sensors

Industrial gases and compressed air

Moisture measurement using dew-point sensors or transmitters is one a number of parameters that are monitored in a wide range of industrial applications; in particular, the production of industrial and specialised gases, and the use of compressed air systems.

Monitoring the moisture content of process gases is key both to maintaining consistent product quality and protecting gas distribution and production equipment from damage. Increasingly, however, moisture measurement is also being used as a means of improving energy efficiency and reducing operating costs, especially where gas drying systems are employed.

Moisture measurement in compressed air systems

Commonly called ‘the fourth industrial utility’, compressed air provides the power that drives many different industrial processes and systems, from air tools and materials handling equipment, to food preparation and waste recycling units.

A typical system uses a compressor, which draws in and pressurises atmospheric air. This is then fed through a combination of air treatment filters and a dryer before being stored in a receiver, from where it is distributed on-demand through pipework to the point (or points) of use.

Atmospheric air typically contains contaminants such as particles, moisture and, depending on the environment, vaporised oil. Under normal conditions these are not generally perceived as hazardous, but as air is compressed and reduced in volume, so the concentration of contaminants – and the associated risk of damage to downstream equipment or manufactured products with which the air comes into contact – will increase. Moisture, for example, will cause corrosion on internal equipment and pipework surfaces and lead to the emulsification of lubricating oils, while in applications such as food production it can stimulate the growth of harmful microorganisms.

Under normal atmospheric and ambient conditions, a cubic metre of air at 35°C and at 60% relative humidity will contain around 23 grammes of water in the form of vapour. Raising the pressure to 7 bar will increase the concentration of vapour by a factor of eight; in this example, each cubic metre of compressed air will therefore contain 184 grammes of water vapour. Note that the temperature of air will increase as it passes through a compressor. As it cools, the ability of air to hold the same volume of water decreases, so that vapour condenses-out in liquid form.

A typical 90kW compressor running for eight hours a day will produce 970m³ of compressed air, plus 140l of water. he British Compressed Air Society (BCAS) has published a Best Practice Guide (BPG104) on the filtration and drying of compressed air. This explains that ‘after compression and subsequent cooling, the compressed air exits the after-cooler 100 per cent saturated with water vapour’.

Compressed air dryers

The role of dryers within each compressed air system is therefore crucial to minimise or eliminate the presence of moisture. Dryers use refrigerant, membrane or adsorption (desiccant) technologies and are generally specified to meet the anticipated maximum water vapour loading for each application

Calculating the optimum drying cycle has traditionally been based on the length of time required to condition a known volume of compressed air, with the choice of dryer often being made simply on the level of energy consumption quoted by each manufacturer. Although this approach may be acceptable when energy costs are low, it can be flawed when costs are escalating and operating margins under growing pressure, as it can result in dryers operating – and consuming energy – for far longer than is necessary.

Instead, what is required is precise control of the drying process, so that it stops as soon as the moisture content in the compressed air has reached the correct level. This is where advanced dew-point sensors and transmitters play a crucial role, as they can eliminate wasted energy while maintaining optimum air quality.

It is difficult to generalise about exactly how much energy can be saved by adding integrated moisture sensors, as every application is different. It is, however, fair to say that where compressed air is being generated for extended periods, such as in food production, there will be a significant reduction in energy consumption, compared with a dryer without effective moisture control.

Moisture measurement in industrial gas manufacture

A similar situation occurs in the manufacture of industrial gases, such as Oxygen and Nitrogen.

There has been a steady move away from the use of bottled gases and large gas storage tanks, due to factors such as manual handling and health and safety concerns, and transportation and storage costs. Instead, a growing number of companies are using on-site gas generators.

These systems typically use either pressure swing absorption (PSA) or membrane technologies to separate high purity streams of Oxygen and Nitrogen from compressed air sources. Although the adsorbent used in PSA systems and the hollow fibre, semi-permeable membranes will remove moisture during the gas separation process, it is still critical to demonstrate that each dryer is producing a supply of dry air to the correct standard.

Again, the integration of moisture measurement or dew-point sensors will ensure that drying systems operate to optimum levels of efficiency, ensuring optimum air quality and helping to reduce both energy consumption and operating costs.

Traceability, security and peace of mind

Regardless of the application, one of the key benefits of being able accurately to monitor the moisture content in compressed air or separated gas streams is the ability to demonstrate that the gas is safe and fit for purpose. This is especially true in critical manufacturing sectors such as food and pharmaceutical production, where the presence of moisture can have a damaging effect on the products being manufactured. The relevant ISO standard governing the quality of compressed air is ISO 8573.1; this specifies the maximum allowable concentration of particulate, moisture and oil contamination.

Caption: Compressed air quality standards

A wide choice of options

The use of the latest generation of advanced moisture sensors or dew-point transmitters, such as our SF range, will play a significant role in reducing energy consumption and enhancing product or process quality. These instruments are capable of measuring dew point, moisture content and trace moisture down to extremely low concentrations. They offer exceptional levels of repeatability, are available with a choice of options and are designed to be suitable for both OEM and retrofit installations.

To find out more about moisture control or energy reduction using moisture sensing please talk to one of our experts today.

Further information

The BCAS web site can be found here: Bcas.org.uk

More information on our dew-point sensors can be found here