ENGINEERING - Royal Australian Navy

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N A VY EN G I N E E R I N G B U LL ET I N F E B RU A RY 20 0 2 5 9 FIGURE 4 THE MECMAR EXHAUST SYSTEM INSTALLED IN THE SOUTH AFRICAN A200 FRIGATE However, being able to claim improved emissions will help the public relations aspect should the RAN equip any surface vessels with this system. South Africa has been the first to obtain the Blohm&Voss A200 class Frigate, a development of the MEKO 200, which in its 200ANZ version forms the RAN ANZAC class. Since the ships are still under construction, there has been little publicity regarding the details of its novel propulsion arrangements, discussed elsewhere in this report. However, from these drawings, it can be seen that the LM2500 gas turbine has been moved aft of the cruise diesel engines, since it drives a water jet directly rather than using the propeller shafts. The arra n gement is inte re sting in that a secondary ve rtical turbine e x h a u st is provided in the aft port qu a rter of the superst ru c t u re, next to the hanga r. This could be to a l l ow use of the turbine fo r m a n o e u v ring in harbour, where a transom exhaust might be re ga rded as unsuita b l e . The South African Blohm und Vo s s A200 fri ga te uses conve n t i o n a l s h a fts and controllable pitch p ro p e l l e rs, dri ven dire c t ly by th e c ruise diesels. While generally similar to the ANZAC, the Blohm & Voss MEKO A 200 has an entirely different means of getting the gas turbine “sprint” power into the water. Instead of using the shafts and controllable pitch propellers still used by the diesel engines for cruise, a separate water jet, mounted on the transom, is used. In particular, this allows the diesel and gas turbines to run at the same time, allowing a (slightly) higher top speed, estimated at 29 knots. . . . being able to claim imp rove d emissions will help the public re l a t i o n s aspect should the RAN equip any s u rface vessels with this syste m .
6 0 N A VY E N G I N E E R I N G B UL LE T I N FE B R U A RY 2 0 0 2 AN ALT E R NATIVE APPROACH An alternative approach to reducing the temperature of exhaust gases is shown in the Northrop Grumman/Rolls Royce WR21 gas turbine. This uses an approach vaguely analogous to the addition of a turbochar ger to a diesel engine, or a superheater in a marine boiler, in that otherwise waste energy is recovered for use in the engine. In this case the heat energy is re c ove red from the exhaust st ream as it is passed th rough a heat exch a n ger arra n ged above the power turbine, which is used to heat air already comp ressed by the gas ge n e ra tor immediate ly b e fo re the combustion ch a m b e r w h e re fuel is added and burn t . Since the gas turbine cycle is m a i n ly dependent on th e d i ffe rence in te mp e ra t u re betwe e n inlet and outlet, this re c ove re d e n e rgy can signifi c a n t ly incre a s e e ffi c i e n c y, part i c u l a rly at lowe r p owe rs where the specific fuel c o n s u mption of a turbine is wo rst . The manufacturers claim that the temperature reduction involved at full power (400 deg.F, about 200 deg C) compared to a simple cycle turbine is sufficient that no eductors, of the type illustrated earlier, would be required. The temperatures quoted are a significant reduction, but continuing improvement in IR technology would suggest that some form of exhaust cooling be maintained. T h e re is a considerable we i g h t i n c rease due to the re c u p e ra to r, about 50 tonnes, but this is fa i rly l ow in the ship. It is claimed th a t the fuel consumption imp rove m e n t would allow the elimination of c ruise diesel engines, which are even heav i e r, but this perfo rm a n c e is so far th e o re t i c a l . The Royal Navy Type 45 is the first vessel currently proposed to use the WR21, but the USN is interested in using them in the DD-X Prototype Destroyer. About the author Peter Clark works for the Directorate of Naval Platform Systems within Navy Systems Branch, and is the Project Liaison Officer for Amphibious and Afloat Support Projects He has previously worked with Naval Aviation Engineering, the RAAF Tactical Fighter Project and the Guided Missile Frigate Project. TOP THE COMBINED CONTROLLABLE PITCH PROPELLER AND WATER JET DRIVE OF THE A200. THE GAS TURBINE EXHAUST IS JUST ABOVE THE WATER JET. BOTTOM THIS SCALE MODEL OF THE WR21 CLEARLY SHOWS THE ADDITIONAL VOLUME REQUIRED BY THE “RECUPERATOR”, AND THE DUCTING TO TAKE THE COMPRESSED AIR FROM THE COMPRESSOR TO THE HEAT EXCHANGER AND BACK TO THE COMBUSTION CHAMBER. SHOULD A FAULT DEVELOP, THE RECUPERATOR CAN BE ISOLATED AND THE TURBINE WILL OPERATE AS A CONVENTIONAL OPEN CYCLE TURBINE.
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ENGINEERING - Royal Australian Navy

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