KBB has developed turbochargers that blur the boundary between single- and two-stage turbochargers
Kompressorenbau Bannewitz (KBB Turbo) has brought its 60+ years of experience of developing turbochargers to address current trends towards two-stage turbocharging. Its ‘Knowledge to Boost’ (K2B) developments are described below but it also sees the importance of closing the gap between single- and two-stage turbocharging products. With current demands for single-stage systems still predominating, KBB continues to push the boundaries of pressure ratio capabilities with its ST27-EP products.
The company began serial production of its ST27 single-stage turbocharger series in 2010, with the largest ST7 product being released in 2014. To date, approaching 5,000 units have been delivered with the range covering the needs of engines with power outputs from 500 kW to 5,000 kW and above per turbocharger. KBB’s more recent ST27-EP series products are now lifting pressure ratio capabilities from 5.5:1 up to 6:1 to meet increasing requirements of dual-fuel and strong Miller cycle engines.
To a great degree, this capability has been achieved by optimising KBB’s impeller cooling technology, which is applied in cases where pressure ratios of above 5.2:1 are required by engines. With no cooling, the temperature limit of the aluminium material is reached at 5.2:1 but a safe operational margin is achieved with standard cooling arrangements. The upgraded cooling now available further increases temperature margins allowing ST27-EP products to achieve 6:1 performance while still using less than 2% of the compressor airflow for cooling. Wide compressor maps are also maintained on the EP series to ensure good on-engine operability.
ST27-EP products retain turbine designs and they are also geometrically interchangeable with HPR series turbochargers. The products have been designed to extend service intervals and component lives, reducing through-life costs for operators. Other features include the ability for rapid service and quick and easy replacement of wear components. There are also options for supplying compressor and turbine washing equipment, speed measurement facilities, intake silencing and casing insulation. Water cooling of the bearing casing is also available on request, for specific high-temperature applications.
“We continue to see a strong demand for efficient, high pressure ratio, single-stage turbocharger products,” said KBB engineering director Klaus Buchmann. “By increasing the pressure ratios achievable from ST27-EP products, we can still provide highly cost-effective solutions for engines that do not yet need to make the move to two-stage systems.”
KBB had already anticipated market demands for even higher pressure ratios and, with its K2B products, already has two-stage turbocharger systems available to meet these. Based on a low-pressure stage using a radial or axial turbine turbocharger and coupled with a high-pressure stage radial product, the K2B system offers pressure ratios of up to 10:1 for engine powers of 500 kW to 5,000 kW.
The low-pressure stage uses an LPR series turbocharger for most flow-rate requirements. It is available in five frame sizes and is based on knowledge and experience gained from HPR and ST series turbochargers. For the highest volume flow requirements, the axial turbine HPA7000 turbocharger is available, capable of delivering approaching 10 m3/s of charge air at pressure ratios of over 3.5:1. To compliment the first stage, KBB has developed a new turbocharger to deliver the final boost pressure to the engine. The radial turbine HSR product also comes in five frame sizes and is based on HPR and ST turbocharger design technology.
To date, KBB has released two systems to the marketplace; the first for engine outputs of 1.5-2.5 MW and a larger system for 4-5 MW engines. These have been designed for compact installation and are suitable for use on both in-line and V engines, running on MDO, HFO or gas fuels. As previously reported, KBB has now gained considerable operating experience with the K2B, notably since 2014 on Yanmar six-cylinder main propulsion MDO/HFO engines. In addition to proving highly satisfactory in operation, significant reductions in fuel consumption have been confirmed.
“We see a slowly increasing demand for two-stage systems in the coming years, with drivers such as the introduction of IMO Tier III emissions limits now approaching” said KBB managing director Dr Roman Drozdowski. “Although capital costs are unavoidably higher than for single-stage turbocharging, operating costs will compensate for this making K2B a realistic and viable solution,” he said.
Meeting IMO Tier III
As engine manufacturers focus more closely on the solutions required for IMO Tier III there are clearly a number of different possibilities to consider. ST27-EP turbochargers are already capable of meeting of IMO Tier II’s engine emissions limits and IMO Tier III compliance can also be achieved when these are used in combination with exhaust gas after-treatment technologies such as selective catalytic reduction (SCR). The higher charge pressures available from two-stage turbocharging systems also present an opportunity but exhaust gas recirculation (EGR) can also be used to control NOx emissions.
With this in mind, KBB has developed its Exhaust Recirculation Turbocharger (ERT) product specifically for EGR duties; it is suitable for both single-stage and two-stage turbocharging applications. The EGR system takes a proportion of the engine exhaust gas and returns this to charge air system. A big advantage of this approach is its compact nature compared to large SCR installations.
For single-stage turbocharging, the ERT takes exhaust gas upstream of the main turbocharger turbine, dividing this into two streams. The ERT turbine is driven by one and the second is fed to the compressor, after primary cooling. The compressor discharge is then fed, via a further cooler, into the air feed, downstream of the main turbocharger aftercooler. The resultant intake mixture has the effect of reducing temperatures in the combustion process and, hence, NOx formation.
A similar arrangement is available for two-stage turbocharging but this also allows the possibility of a ‘semi-short’ system where high pressure exhaust gas leaving the engine can be fed, via a cooler, to the intake of the high pressure turbocharger, thus potentially avoiding the need for an ERT.
The KBB ERT20 is currently available for EGR applications and has been developed specifically for this purpose. It is suitable for engines in the power range from 500 kW to 2 MW and has a number of specialised features, including a titanium impeller for specific application requirements. The aggressive nature of the exhaust gas has also necessitated the optimisation of coatings and the use of alternative materials to cope with higher temperatures, prevent damage from corrosion and soot formation and to withstand impact damage which would otherwise be caused by particles in the gas flow. Turbocharger thermodynamics are also designed specific to the gas flows and bearings, shaft sealing and oil supply can cater for both operating conditions and circumstances where the EGR system is turned off.
The current KBB standard product portfolio comprises the ST27 and ST27-EP products, radial turbine HPR and axial turbine M40 series, and the K2B two-stage system. Other specialist products are available, such as the ERT20, and the company offers a full aftermarket support service for maintenance, overhaul and spare parts. A customisation service is also available for meeting specific application requirements or duty changes, including part-load re-optimisation.
A further initiative now being developed by KBB focuses on improving turbocharger instrumentation and condition monitoring. At present, the majority of turbochargers carry little instrumentation and, where this is fitted, it is likely to be limited to speeds, temperatures and pressures. KBB’s vision is to develop turbocharger monitoring to the level where condition based maintenance (CBM) can be effectively introduced. This principle is being increasingly used to achieve cost-effective and timely machinery maintenance on major capital equipment and KBB believes that there is an opportunity for CBM to be extended to turbocharging systems.