Turbocharger flow capacities continue to improve
Two-stroke turbochargers continue to evolve with the latest developments focusing on flow capacity, says Wendy Laursen.
Two of MAN Diesel & Turbo’s new turbo compound systems with power turbine and generator (TCS-PTG) will enter operation on two 4,700teu container ships in early 2014. MAN supplied a large number of earlier power turbines to AP Møller vessels back in the 80s and 90s, and now Reederei Horst Zeppenfeld has become a front-runner for the new generation of integrated power turbine systems.
The vessels are powered by individual MAN B&W 6S80ME-C9.2 low-speed engines, and the scope of supply includes two TCS-PTG20s and two TCA88 turbochargers. The new generation of TCS-PTG incorporates the latest versions of TCR turbocharger geometry and other new features such as electrical actuators for the regulation of exhaust gas flow over the turbine.
The thermal efficiency of these units can be significantly above 100%, says Andrea Bader, project manager for TCS-PTG at MAN. “Take for example an engine with 20MW. Running the power turbine will give a small consumption penalty of up to 2g/kWh on the main engine, which is equivalent to 400kW of thermal power. Fortunately this is overcompensated by our TCS-PTG producing 780kW of electrical power on such an engine. Divide the two figures and you achieve a thermal efficiency of about 190%. This is what makes our product so beneficial.
“There are also benefits for overhauling. We strongly believe that the maintenance of a TCS-PTG is cheaper than the maintenance and overhaul of a genset because you have fewer components and the ones you have to take care of are cheaper, smaller and lighter than, say, a cylinder liner,” says Mr Bader.
MAN sees a growing market for the technology and believes it is best suited to large cargo vessels including container ships, bulkers and tankers that have at least 20MW of installed power. The system automatically starts up between 40% and 50% MCR when the charge air pressure is sufficiently high. Below that, the system is not operational.
ABB’s first A200-L turbochargers were tested on engines at the beginning of May, and since that time, the customer response has, acording to ABB, been as "overwhelmingly positive" as when the A200-L was initially launched in autumn 2012. The turbochargers are ABB's latest generation of large turbochargers after the A100-L model, and as a result of an improved air flow of up to 30% with no reduction in pressure ratio, a smaller unit can be fitted. There are new components on the compressor side including the casing and the wall insert. The diffuser and silencer have also been optimised for the high volume flow.
The A200-L’s more compact frame means a smaller turbocharger can be used on a wide range of two-stroke engines. The benefits include less vibration, less installation space and better fuel consumption. “For shipowners and operators it also results in lower service costs. Smaller and lighter turbochargers translate into 25% lower costs for spare parts as well as easier maintenance, plus lower first cost,” says Arie Smits, senior general manager global turbocharging projects at ABB.
The A200-L will allow engine designers to further increase brake mean effective pressure, the basic measure of engine power, says Smits. At the same time it will make high pressure tuning (HPT) possible, a feature ABB launched in 2012. HPT involves using the full potential of a turbocharger by employing electronic engine valve control. The result is the same fuel consumption as with variable turbine tuning, but without the actual hardware of variable turbine geometry, or even an exhaust gas bypass. “No moving parts, no additional first costs, no exchange of a bypass with its inherent tendency to stick.” ABB Turbocharging announced earlier this year that its HPT had been added to MAN Diesel & Turbo’s two-stroke engine tuning options, and it is anticipated that the company will soon combine HPT with the new A200-L.
Wärtsilä continues to take-up new turbocharger technologies as they become available in the market. However, as the focus has changed from maximising turbocharger efficiency for engine loads of 80%-90% to the need to optimise for slow steaming or the derating of engines, the flexibility of the company’s cut-out solution has proved very successful. Increasing attention on dual-fuel engines is also driving research. Wärtsilä anticipates that a single-stage turbocharger will achieve good performance levels but continues to test two-stage options from a research point of view.
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