In industry, compressed air is everywhere. Think of it like electricity – a power source that drives conveyers, packaging lines, spray-painting equipment, metal presses – the list goes on and on. But this comes at a cost, literally. Compressed air is one of the largest consumers of energy, which accounts for approximately 75% of the lifetime cost of a compressed air system.
Fortunately, energy saving opportunities are also everywhere, it just takes a little effort to recognize them. For compressed air systems, nothing offers a better return on investment than dew point measurement for ensuring energy efficiency and lowering the cost of ownership.
What follows is a review of dew point and methods for measuring it with compressed air systems.
Dew point measurement ensures energy efficiency and helps lower the cost of ownership of compressed air systems.
Quality Air Critical to Production and Products
Dew point is simply the temperature to which air must be cooled for the water vapor within to condense into dew or frost. At any temperature, there is a maximum amount of water vapor that the air can hold. This maximum amount is called the water vapor saturation pressure. If more water vapor is added beyond this point, it will result in condensation.
Condensation in pressurized air is a big problem because it causes blockages in pipes, machinery breakdowns, contamination and freezing. Modern facilities using compressed air can identify and avoid those problems simply by keeping an eye on dew point.
Dew point monitoring can be achieved by installing high-quality dew point sensors and monitors in the compressed air system. The use of a pressure dew point monitoring system allows you to be sure you are reliably maintaining the desired moisture level in a compressed air system.
The quality of the air you use is critically important to the end result of the process. Take spray painting as an example. Poor-quality air – i.e., air that contains dust, other particles, or water – will result in a poor-quality paint finish, and therefore a wasted product that can’t be sold. In semiconductor production, wet gases in compressed air systems can lead to problems such as low yield and poor reliability, while dust in the air can cause the products to short circuit.
On packing lines in the food and drinks industry, clean, dry air is an absolute must to maintain hygiene and preserve end-product quality. For example, if you’re using a filling gas to fill up packs of ham, you don’t want anything in there that isn’t supposed to be there. When air is pressurized, humid air condenses and forms water droplets, which can lead to rust – and this can end up in with the food. When you’re drying plastics to make soda bottles, excessive moisture can result in brittle bottles with a cloudy finish.
Air quality is critically important to the end result of the process.
Pressure Dew Point Defined
Dew point is focused primarily on temperature and relative humidity. However, that same point of saturation is affected by the pressure. So, before we continue, it is necessary to distinguish between “dew point” and “pressure dew point.”
Dew point is related to non-pressurized, atmospheric air (atmospheric dew point). The term “pressure dew point” is encountered when measuring the dew point temperature of gases at pressures higher than atmospheric pressure. It refers to the dew point temperature of a gas under pressure. This is important because changing the pressure of a gas changes the dew point temperature of the gas.
Because water vapor pressure and dew point are increased by air compression, it is critical to take this into consideration if you are bleeding the air to atmosphere before taking a measurement. The dew point at the measurement point will be different from the dew point in the process.
Dew point relates to non-pressurized, atmospheric air (atmospheric dew point). Pressure dew point is referred to when measuring the dew point temperature of gases at pressures higher than atmospheric pressure.
Dew point temperatures in compressed air range from ambient down to -80°C (-112°F) in special cases. Compressed air systems without air drying capabilities tend to produce compressed air that is saturated at ambient temperature. Systems with refrigerant dryers pass the compressed air through a cooled heat exchanger, causing water to condense out of the air stream. These systems typically produce air with a dew point no lower than 5°C (41°F). Desiccant drying systems absorb water vapor from the air stream and can produce air with a dew point of -40°C (-40°F) and drier if required.
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Supply and Demand Side Dew Point Measurement
Dew point is often measured on the supply side of a compressed air system. The measurement values can be shown directly on a display or on the dryer’s control panel. These values indicate a dryer’s performance and quality – and can also control desiccant tower regeneration to reduce energy consumption. Moisture contamination increases operating and maintenance tasks and costs in numerous ways. Whatever the source of the moisture, discovering the problem early on means that corrective actions can be implemented faster. This helps to avoid major problems that could lead to long and costly service breaks and lack of capacity.
The problem of excess humidity can be solved by using dryers, the two basic types of which are refrigerant dryers and desiccant dryers.
Refrigerant dryers use a refrigeration system and heat exchangers to drive down the temperature of compressed air to 2°C to 5°C (36°F to 41°F), which is also the dew point of the air. The excess water vapor condenses and is separated from the air, and the air is then warmed up.
Desiccant dryers operate on the basis of adsorption. In adsorption, material (adsorbate) travels from a gas or liquid phase and forms a superficial monomolecular layer on a solid or liquid substrate. Chemical beads, called desiccant, adsorb water vapor from compressed air. The most common desiccants are silica gel, activated alumina and molecular sieve. A desiccant dryer is considerably more effective than a refrigerant dryer. Although it typically provides a -40 °C (-40°F) dew point, even -100°C (-150°F) pressure dew points are possible.
Desiccant dryers usually have two desiccant-filled towers and switching valves that direct the compressed air flow first through one tower and then through the other. A desiccant dryer’s basic operation consists of one drying cycle and one regeneration cycle, which is continuously repeated. While one tower dries the air, the parallel drying tower is in the regeneration mode.
Dew point monitoring ensures the dryer is functioning according to its specifications. Concerning desiccant dryers, dew point measurement can also be used to control the desiccant regeneration interval. Regeneration is not started until the desiccant tower has been used to its full capacity as indicated by a rise in the outlet air dew point. Unlike conventional timer-based regeneration, this system takes into account the fact that when compressed air is dry, the regeneration interval may be much longer than for moist air. Because it avoids unnecessary regeneration, dew point dependent switching (DDS) provides the user with up to 80% savings in the energy costs of drying compressed air, typically making a realistic repayment period for the investment less than one year.
On the demand side of a system, dew point instruments are installed throughout the distribution network and before critical end-use applications to give operators and plant personnel a quick assessment of moisture conditions at specific points in the system. These instruments confirm that the compressed air produced has been kept dry enough throughout the entire facility.
The key goals for measuring dew point in a compressed air system are to ensure that energy is not being wasted and that capacity is not being lost.
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ISO8573.1 and Reliably Measuring Pressure Dew Point
ISO8573.1 is an international standard that specifies the quality of compressed air. The standard defines limits for three categories of air quality:
- Maximum particle size for any remaining particles.
- Maximum allowable dew point temperature.
- Maximum remaining oil content.
All remaining particles in the air will be 0.1 µm or smaller, and the maximum oil content will be 0.01 mg/m3. There are also several other ISO 8573 standards, as well as related topics such as measurement and test methods, to carefully research to avoid problems.
Users often ask how pressure dew point is reliably measured in compressed air. Some principles of measurement apply to all types of instruments:
- Select an instrument with the correct measuring range: Some instruments are suitable for measuring high pressure dew points, but not low pressure dew points. Similarly, some instruments are suitable for very low pressure dew points but are compromised when exposed to high pressure dew points.
- Understand the pressure characteristics of the dew point instrument: Some instruments are not suitable for use at process pressure. They can be installed to measure compressed air after it is expanded to atmospheric pressure, but the measured dew point value will have to be corrected if pressure dew point is the desired measurement parameter.
- Install the sensor correctly: Follow instructions from the manufacturer. Do not install dew point sensors at the end of stubs or other “dead end” pieces of pipe where there is no airflow.
Some technologies, such as the Vaisala DRYCAP® sensor, provide fast dew point measurements from ambient temperature down to -80°C (-112°F) with an accuracy of plus or minus 2°C ( plus or minus 3.6°F) over the entire range. In addition to the general principles given above, consider the following when selecting a dew point instrument:
- The best installation for a dew point sensor isolates the sensor from the compressed air line. This is accomplished by installing the sensor in a “sample cell” and connecting the cell to a “T” in the compressed air line at the point of interest. A small amount of compressed air is then bled past the sensor. The cell should be made of stainless steel and connected to the “T” with tubing (1/4 inch or 6 mm). It is useful to install an isolation valve between the cell and the air line. This enables easy installation and removal of the sensor.
- A flow-regulating device is necessary to control airflow past the sensor. The desired flow rate is only one slpm (two scfh). The regulating device can be a leak screw or a valve. To measure pressure dew point, the regulating device is installed downstream of the sensor, so that when the isolation valve is opened, the sensor is at the process pressure. To measure dew point at atmospheric pressure, the regulating device should be installed upstream of the dew point sensor.
- Do not exceed the recommended flow rate. When measuring pressure dew point, an excessive flow rate will create a local pressure drop at the sensor. Because dew point temperature is pressure sensitive, this will create an error in the measurement. The most common installation for a dew point sensor isolates the sensor from the compressed air line.
A word about calibration. We recommend a one- or two-year calibration interval, depending on the instrument. Sometimes a simple field check against a calibrated portable instrument is enough to verify correct operation of other instruments. Check detailed calibration information in the user’s guide shipped with the instrument. Any time you have doubts about the performance of your dew point instruments, it is wise to check their calibration.
Multiple Advantages of Dew Point Monitoring and Measurement
Since the importance of clean, dry compressed air – and the cost associated with it – is so high, carefully managing and monitoring of it becomes a crucial task for any industrial process or plant. By using stable dew point measurement devices, you can avoid over-drying, save energy and protect your equipment from corrosion.
Common Dew Point Monitoring Applications
Vaisala is a global leader in industrial and environmental measurement. Building on over 80 years of experience, Vaisala provides observations for a better world. We are a reliable partner for customers around the world, offering a comprehensive range of innovative observation and measurement products and services. Our newest offering – the Indigo 520 Transmitter – provides several industry innovations, including dual-probe and multi-parameter support. The transmitter is a universal host device for our Indigo-compatible, stand-alone smart probes – which include our dew point probes as well as many other probes. Headquartered in Finland, Vaisala employs approximately 1,850 professionals worldwide and is listed on the Nasdaq Helsinki stock exchange. For more information, visit www.vaisala.com/en or https://www.linkedin.com/company/vaisala/
All photos courtesy of Vaisala.
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