Compressed air treatment
Introduction to compressed air drying:
Atmospheric air contains water vapor, the amount of which increases significantly at high temperatures. When we compress the air, this water concentration increases even more. For example, a compressor operating at 7 bar and a capacity of 200 l/s compressing air at 20˚C with a relative humidity of 80% will release approximately 10 liters of water per hour into the compressed air line. The presence of this excess moisture can cause problems such as corrosion, contamination of the end product and malfunction of pneumatic tools and equipment.
Why is it important to dry compressed air?
Compressed air drying is crucial to avoid problems related to water condensation in piping and equipment. The presence of moisture can lead to corrosion of piping, damage to pneumatic tools, and contamination of end products. In addition, condensed water can clog valves and regulators, causing malfunctions in the compressed air system. Therefore, proper compressed air drying is essential to ensure the efficiency and reliability of industrial processes.
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Compressed air drying methods:
1. Aftercooler: This technique involves cooling compressed air to condense and separate water. An aftercooler is a heat exchanger that cools the hot compressed air, precipitating the water that would otherwise condense in the pipes.
2. Refrigeration Dryer: In this method, the compressed air is cooled to condense the water, which is then separated. Subsequently, the air is heated again to prevent condensation in the pipes. Refrigeration dryers are effective for dew points between +2˚C and +10˚C and offer a cost-effective solution for compressed air applications.
3. Over compression: This method involves compressing the air to a higher pressure than the intended working pressure, followed by cooling to separate the water. However, this method is suitable only for small air flow rates due to its high energy consumption.
4. Absorption Drying: In this process, water vapor is attached to an absorbent material, such as sodium chloride and sulfuric acid. However, this method involves high consumption of absorbent material and only reduces the dew point to a limited extent.
5. Adsorption drying: This method uses a hygroscopic material to adsorb water vapor from compressed air. Adsorption dryers are ideal for applications requiring extremely dry air, with typical dew points as low as -40°C.
There are several types of adsorption dryers, each with specific characteristics and applications:
5.1. Purge Regenerated Adsorption Dryers: Also known as “cold regenerated dryers,” these dryers are suitable for small air flow rates. The regeneration is carried out with expanded (purged) compressed air, which requires about 15-20% of the dryer’s rated capacity at an operating pressure of 7 bar(e).
5.2. Hot Purge Regenerated Dryers: These dryers heat the expanded purge air by means of an electric heater, limiting the required purge flow to about 8%. This type consumes 25% less energy than cold regenerated dryers.
5.3. Blower Regenerated Dryers: In this type of dryer, ambient air is heated by an electric heater and used to regenerate the desiccant. As a result, no compressed air is needed for regeneration, which reduces energy consumption by up to 40% compared to cold regenerated dryers.
5.4. Heat of Compression (HOC) Reactivated Dryers: These dryers use the heat available from the compressor to regenerate the desiccant. This method can provide a typical dew point of -20°C without adding additional energy. HOC dryers are ideal for applications requiring extremely dry air and can provide optimum performance with minimal energy consumption.
6. Membrane dryers: These are devices that remove moisture from compressed air. They use a filtering technology that separates water vapor from the air. They work through thousands of tiny hollow fibers that allow dry air to pass through while trapping water vapor. These dryers are easy to use, quiet and require little maintenance. In addition to removing moisture, membrane dryers can separate the components of a gas, making them useful in the manufacture of nitrogen generators and other industrial applications. In summary, membrane dryers are an efficient and reliable option for keeping compressed air dry and clean in a variety of industrial environments.
In summary:
Compressed air drying is a critical part of any industrial compressed air system. By choosing the right drying method, companies can ensure operational efficiency, extend equipment life and reduce maintenance costs. It is important to understand the different drying techniques available and select the most appropriate one based on the specific needs of each industrial application.
Considerations when choosing a dryer
When selecting a compressed air dryer, it is essential to consider the dew point required for the specific application and the characteristics of the compressed air system, as well as the energy consumption and operating costs associated with each drying method.
Compressed air filters
The particles present in compressed air can be removed in a variety of ways. If they are larger than the pores of the filter material, they are separated mechanically (“sieve effect”), usually with particles larger than 1 mm. Particles smaller than 1 mm are collected by three physical mechanisms: inertial impact, interception and diffusion. The particle separation capacity of a filter is the result of these processes, although no filter is effective for the entire particle size range. Air quality, as defined in ISO 8573-1, is crucial to avoid contamination in critical processes, so it is recommended to use air classified as Class 0.
Importance of filters in air quality
Air quality in industrial processes is essential to avoid contamination and ensure the integrity of the final product. Compressed air filters, especially the coalescing filters, not only remove particles, but can also separate oil and water from the air. Air classification according to ISO 8573-1 is essential to ensure air purity in critical applications.
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Filtration mechanisms
Impingement, interception and diffusion are the three main mechanisms of particle filtration in compressed air.
Impaction occurs with large particles and high gas velocities, while interception occurs when a particle follows the flow path, but its radius is greater than the distance between the flow path and the fiber perimeter. Diffusion occurs with very small particles moving randomly due to Brownian motion.
Recommendations and regulations
It is crucial that filters are not only sized for the nominal flow rate, but also have an additional capacity threshold to cope with the pressure drop due to blockage. In addition, ISO 8573-1 sets standards for compressed air quality, and it is recommended to use only Class 0 rated air in critical applications to avoid air contamination.
Design of compressor installations
Compressor system design in general:
When designing a compressed air installation, it is crucial to consider user needs, operating economics, and the possibility of future expansion. Starting by detailing the applications and processes that will use compressed air is critical to guide the design. Calculating air requirements, reserve capacity and room for future expansion are critical. Working pressure, air quality and energy recovery capabilities are also key factors in the design.
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Calculation of the working pressure
The working pressure required in a compressed air installation is determined by the pneumatic equipment and the system design. The correct pressure depends not only on the compressor, but also on other elements such as piping, valves, dryers and filters. Compressor flow regulation and pressure drop in the system also influence the working pressure calculation. It may be convenient to perform the calculations using the following example:
It is mainly the consumption points and the pressure drop between these and the compressor that determine the pressure that the machine must produce. The working pressure can be determined by adding the pressure drop of the system, as shown in the example above.
Calculation of air consumption
The nominal compressed air consumption is calculated on the basis of the various consumers, including tools, machines and processes. The utilization factor of each piece of equipment must be considered, as well as leakage, wear and foreseeable changes in the future. The compressor capacity is essentially determined by the nominal compressed air consumption and the reserve capacity is calculated to cover possible supply failures.
Measurement and analysis of the required air
A detailed operational analysis provides crucial information on compressed air requirements, enabling the optimum amount to be produced to be assessed. Operational measurements, taken over a significant period of time, provide accurate data on system performance. This data serves as the basis for calculating the load factor, compressed air requirements and possible energy recovery. In addition, the analysis allows the control system to be adjusted to improve efficiency and to detect possible leaks in the system.
Key findings
The design of compressor installations is a complex process that requires detailed considerations and careful planning. At the end of the design process, it is important to highlight some key points:
- Comprehensive Consideration: The design of a compressed air installation must address present and future user needs, operational efficiency and expandability.
- Pressure and Consumption: The accurate calculation of working pressure and air consumption is essential to properly size compressors and ensure a reliable and efficient air supply.
- Operational Analysis: Performing operational measurements and analysis provides valuable information on actual system performance, allowing adjustments to improve efficiency and detect potential problems.
- Energy Recovery: The ability to recover energy in a compressed air installation can result in significant savings and should be considered as an integral part of the design.
By integrating these aspects into the design and planning of compressor installations, companies can optimize their performance, reduce operating costs and improve their environmental impact.
Centralized or decentralized installations
The choice between a large centralized compressor or several small decentralized compressors to meet specific compressed air needs depends on several factors, such as the cost of a production shutdown, availability of electrical power, load variations, compressed air system costs and available space.
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Centralized installations
In many cases, a centralized installation is the best solution. It is more economical to operate and maintain than several distributed compressors. Compressor rooms can be efficiently interconnected, reducing energy consumption and monitoring and maintenance costs. In addition, they offer better energy recovery opportunities. Centralized installation allows optimal sizing of filters, coolers and other auxiliary equipment, as well as noise reduction.
Decentralized installations
An installation with several decentralized compressors may be preferable for certain applications. It involves a smaller and simpler compressed air distribution system. Modern compressors, with integrated air conditioning equipment, can be installed on-site, reducing distribution costs and eliminating the need for separate buildings. Decentralized compressors can be used to maintain pressure in systems with large pressure drops, or installed near specific consumers to optimize consumption.
Key conclusion
In summary, the choice between a centralized or decentralized compressor installation depends on a number of factors including space availability, operating and maintenance costs, variability in compressed air demand and desired energy efficiency. Centralized installations offer advantages in terms of operational efficiency and maintenance, as well as energy management. On the other hand, decentralized installations may be preferable in specific applications where greater flexibility and more precise compressed air distribution is required. It is important to carefully evaluate each option and consider the specific needs of the application before making a final decision on compressor installation design.