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In 1963, the USS Thresher, a US navy nuclear submarine sank to the bottom of the North Atlantic along with a crew of hundred and twenty nine members. It is known to be one of the greatest tragedies of the US Navy. Researchers found that the vessel sank owing to the presence of moisture in compressed air.



To comprehend why this seemingly minor fallacy led to a tragic accident, we must first grasp the physics behind the working of a submarine. Submarines are fascinating vessels that dive into the deep sea with utter finesse and resurface just as smoothly. Air compression technology helps the machines control their buoyancy by altering the level of water in the ballast tanks. So by either flooding the ballast tanks or venting it, the submarine can dive or rise.


Compressed air enters ballast tanks at a high pressure through a blowing valve. And since the air gets throttled in the process, its temperature drops substantially. If there is moisture in the air, there are chances of that moisture freezing in air passages or in the blowing valve itself.  Hence in the humid marine environment, maintaining air quality is crucial.


Following the USS Thresher tragedy, the usage of air dryers on all submarines has been made mandatory the world over. In modern day submarines, compressed air is therefore maintained at a minimum of -20°C at atmospheric dew point. However, the traditional absorption technology that is widely used in air dryers has its disadvantages. The desiccant granules in dryers turn into clay or become powdery over time and need to be replaced periodically.


ELGi Sauer took up this challenge and developed a high pressure dryer with a membrane that efficiently separates water vapour from an air stream. The membrane consists of thousands of hollow fibres bundled together. As moisture-laden air flows through the membrane, it travels around the hollow fibres. The fibre bundle is enclosed in a long tubular container that ensures efficient flow of air. Moisture is removed by selective permeation and dry air exits at the opposite end. The dryer is fitted before the final stage of the compressor and hence the name Inter-Stage Membrane Dehydrator (IMD). It improves the life of final stage valves and piston rings. This new technology is also cost effective.



Another big advantage of IMD is that it is maintenance free throughout its service life, except for periodic replacement of the pre­ filter element. These coalescing pre­ filters installed before the membrane dryer remove the bulk of the water vapour and any trapped compressor oil. Since the IMD does not need electricity, it is ideal for use in remote applications.


Moreover, since it has no moving parts, it is also ideal for heavy duty installations and work efficiently in harsh environments including shipboard systems and power plants. Its initial cost is higher than a desiccant dryer but its overall life cycle cost is far below other dryers that offer the same air quality.



IMD is one of the many examples of ELGi’s smart, cost effective solutions. With relentless R&D efforts, we constantly strive to take innovation to new heights, and depths in this case!



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