Inter-stage Membrane Dryer: Lower Life Cycle Costs, Zero Maintenance

How a submarine dives into the deep sea and re-surfaces is a question of simple physics. And aiding in this function is air compression technology. The vessel floats on the surface when the weight of water it displaces equals its own weight. This displacement of water creates an upward force called buoyancy that opposes the downward acting gravity. Since a submarine can control its buoyancy, it can submerge and re-surface.

A submarine controls its buoyancy with the help of its ballast tanks; these tanks are filled with water to increase the overall weight of the submarine, thus allowing it to submerge. And when that water is pumped out by displacing it with compressed air, its weight is reduced and the submarine re-surfaces. So by either flooding the ballast tanks or venting it, the submarine can dive or rise. But compressed air is required not just for charging ballast tanks to change buoyancy but also for life support. A constant supply of compressed air is therefore maintained on board in large steel pressure vessels called air flasks.

Clearly, the air compressor is a critical piece of equipment on board a submarine. However, storing a large amount of compressed air alone is not enough; that air also needs to be dry in order to ensure safe operation. Compressed air at a high pressure enters ballast tanks 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 not just for diving and rising but also for engine controls, weaponry and various other control systems.

Compressed air is therefore maintained at a minimum of -20o C at atmospheric dew point. Failure to do this resulted in the sinking of the USS Thresher, a US navy nuclear submarine in 1963. The vessel sank as the presence of moisture in the compressed air led to icing in the blowing valve. Following this disaster, the usage of air dryers on all submarines has been made mandatory the world over.

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 that need to be replaced periodically.

J.P. 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 through and 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.