A white paper Engineering GREAT Solutions Compressed Air Dryers for the rail industry
02 A white paper A white paper 03 Executive Summary The use of compressed air technology in the rail industry has always presented a challenge to equipment manufacturers, as moisture and other contaminants make operating compressed air applications very problematic. To overcome the extreme conditions, air dryer packages, complete with multi-stage pre-filtration, can be utilised to clean and dry the compressed air before it reaches critical downstream applications, such as brakes and door systems, to ensure optimum performance, longevity and most importantly, reliability. There are two main products available for this purpose desiccant and membrane air dryers but currently both have limitations which significantly impact their effectiveness and reliability. However, things have changed thanks to the development of a new technology that inherits the benefits of both desiccant and membrane dryers, yet overcomes their collective weaknesses to offer an innovative, durable and effective compressed air solution for the rail industry. Brakes Horns Doors & Steps Pantographs Suspension Sanding Systems The need for air dryers In the rail industry, compressed air is used in some of the most sophisticated applications installed on rolling stock from braking systems and pneumatically operated doors, to self-levelling air suspension, pantographs and the train s horn. With so many critical functions now dependent on compressed air, the issue of reliability has been bought to the fore and the industry has responded with research and development to improve the lifecycle of compressed air driven applications. However, one of the best ways to make sure that such critical applications run efficiently is to ensure that the compressed air used to power them is free from moisture and contaminants. Compressors are generally mounted underneath the body of the train. The compressor draws in large volumes of air from the surrounding atmosphere, which contains water vapour and airborne contaminants. During the action of compression, the vapour and contaminants are heated up and compacted; and on subsequent cooling by an after-cooler, water vapour condenses within the air system to form liquid. With oil lubricated compressors the lubricating oil will degrade with the heat of compression and combine with the condensing liquid to form acid condensate of varying viscosities. If this wet and dirty liquid emulsion is not taken out of the system, it can cause erosion, corrosion, reduced performance, and result in costly maintenance and even safety implications. This liquid emulsion reaches the AMT drying stage to prevent over loading and oil contamination. Following inlet filtration stages the air will be free of most contaminants but 100% saturated with water vapour. If it enters the distribution system untreated and cools the water vapour will condense into bulk water. This is where the AMT technology acts to reduce the dew point to below 20% relative humidity so whatever the ambient conditions further condensate will be prevented from reaching downstream applications.
04 A white paper A white paper 05 Current system options and the issues with them There are two types of air dryers currently used in rail applications they are desiccant dryers and membrane dryers. Offering different approaches to the issue of removing moisture from the air, each technology has its strengths but it is acknowledged within the rail industry that neither option currently offers a wholly satisfactory solution. Membrane dryers use semi-permeable polymeric fibres (about the thickness of human hair) which allow fast gases such as water vapour to permeate through their walls directly to atmosphere. Lighter than a desiccant air dryer and able to be horizontally mounted, special membrane fibre bundles are loosely suspended between potted ends. Water vapour passes from the high concentration in the air line to the low concentration on the outlet. To maintain the lower concentration of moisture on the outside of the fibre bundle, a large percentage of the dry air produced must be employed to sweep away the collected water vapour. This air now laden with water vapour is swept into a small vent which releases it back into the atmosphere. This sweeping action is continual so the membrane self-regenerates with no cycling, pressure changes or the need for maintenance. There are distinct advantages to the membrane dryer including no maintenance, no dust, no external power requirement and their low mass. However there are also major disadvantages. The fibres in the membrane are very delicate, are susceptible to contamination and are easily broken often if one breaks, the others usually follow shortly after, causing a catastrophic failure. Shock and vibration, compressor stop starts and chemical contamination, all common in rail applications, lead to early failure. If air stops reaching the railway vehicle, the brakes are automatically applied, resulting in a breakdown, unscheduled maintenance and very unhappy customers. Membrane dryers also suffer badly from shock and vibration as well as chemical attack and temperature. Since the membrane fibres are subjected to full air pressure internally they have to support full line pressure and hence their vulnerability to the conditions that exist in rail applications is high. With a life cycle of anywhere between 4 and 24 months, membrane air dryers offer a less durable solution than desiccant dryers, and require constant monitoring and replacement as part of a regular maintenance schedule. Desiccant dryers are generally vertically installed, since horizontal orientation leads to by-pass. They adsorb moisture using two special canister towers or columns filled with adsorbent beads. The beads are typically made by mixing the adsorbent material with a clay binder, which is then formed into spheres of various diameters within a mesh range e.g. 2-5mm. The beads adsorb moisture and contaminants from the air that is pushed through them, allowing dry air to flow out to the reservoir and downstream equipment. Once the beads in the first column are saturated, the air dryer switches to the second column, and a percentage of the dry air is pushed back through the saturated unit (purge) to remove the moisture so it is ready to be used again. Adsorbent beads suffer from attrition, channelling and by-pass (particularly if mounted horizontally) leading to performance degradation and the generation of fine dust. One of the problems associated with rail applications is shock and vibration and this has a particularly detrimental effect on desiccant dryers. Although the adsorbing beads are tightly packed into the column their size varies and vibration from the train can cause them to rub together forming dust which can contaminate the air and damage downstream equipment. To counteract this, an additional downstream filter is usually installed to collect the dust near the output of the dryer, but this may not be 100% effective. And given that the dryer may fail due to degradation, misuse etc. this dust combined with water forms an abrasive sludge which travels throughout the entire distribution pipework system. As the beads abrade and erode, they become less tightly packed in the column, allowing more moisture laden air to pass through the gaps that have appeared in the desiccant bed, rather than across the adsorbent surfaces of the beads and flow into downstream equipment where condensation can lead to application breakdowns. Over time as the size of the desiccant bed reduces with vibration, it can become over saturated should this happen, the conventional beads will be destroyed by prolonged bulk water saturation and irreparable damage is done meaning they cannot be used again. Depending on where they are installed, desiccant air dryers have a life cycle of between 6 and 36 months. The frequent replacements needed means additional costs, extra maintenance (it is usually a two-person job, sometimes requiring special lifting equipment) and unnecessary downtime for a rail operator.
06 A white paper A white paper 07 Patented air dryer for rail offers best of both worlds Up to 7 years maintenance free life A revolutionary new air dryer has been brought to market to deliver a step change for compressed air drying for the railway industry. Combining the positives and learning from the negatives of desiccant and membrane technologies, the result is a more effective and reliable way of removing moisture from compressed air, ensuring a trouble-free life for rail operators between major overhaul schedules. The innovative new dryer uses Adsorbent Media Tube (AMT) technology. Best described as adsorbent tubes (typically 2mm in diameter and 0.9 mm bore) consisting of a high level of adsorbent ( 80%) and a non-adsorbing durable polymeric binder such as Polyether Sulphone (PES). They are not to be confused with semi permeable membranes. The AMT is extruded into a water bath where a phase inversion takes place and their structure is formed, hence they are totally resistant to bulk water. The structure of the AMT is such that it maximises the permeability and high surface area of the adsorbent crystals presenting the greatest number of active sites. The open structure of the AMT provides for low mass transfer resistance, enabling the flow of compressed air to permeate easily into the structure while offering very low pressure losses. Adsorption and desorption kinetics are faster than conventional materials leading to more efficient performance. The result is that moist air can get to the drying agent and be adsorbed quicker and during the regeneration process (purge) the moisture is removed just as quickly leading to a reduced purge figure. The adsorbent tubes are tightly packed into their housing, but as they are uniformly shaped, unlike traditional beads, they are unaffected by the vibration of the rolling stock. This also means there is a consistent air flow through the tubes, reducing pressure drop, and the unit s performance does not degrade over time. As there is no clay component, there is also no dust, which is a huge advantage over previous desiccant air dryer solutions. AMT is extruded into a water bath as part of the manufacturing process, so they are essentially born in water. This has significant benefits if the tube becomes saturated, there is no by-product or chemical reaction. The tubes are simply dried during the purge process, returned to their original state and reused as normal - no maintenance is required. Vibration tested to BS EN 61373:2010 and with an ambient operating temperature range of -50oC to +80oC, the new dryer provides a robust, durable yet compact solution for the rail industry. As a fit and forget product, it has a life span of up to six years or 21,000 operating hours in most applications, which changes the status of the air dryer from a regularly serviced item to a major refurbishment item. It has a considerably lower cost of ownership than its air drying predecessors and most importantly, it dries the air better to ensure a reliable flow of clean air to the downstream applications which keep the railway network moving. Conventional adsorbent bead technology Fail-safe -50ºC...+80ºC Vibration tested to EN 61373:2010 The way forward AMT technology The new AMT air dryer is a leap forward for the compressed air industry and to achieving levels of reliability never seen before for rail applications. By learning from the successes as well as the failures of the past, we can continue to create innovative technology for rail applications which goes beyond expectation and gives customers real engineering advantage.
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