• MAN is pursuing a future-oriented concept for axial compressors

With support from the German Federal Ministry of Economics and Technology, development teams from MAN Diesel & Turbo and MTU Aero Engines have worked together intensively for three years to develop a new generation of axial compressors. This hybrid, which is based on proven and robust industrial design as well as on compressor technology derived from stationary gas turbines and aircraft engines, meets the requirements of future mega processing plants.

Air separation units (ASUs) represent a technically demanding and economically important application for turbocompressors. The engineering of air separation units is today increasingly influenced by the trends in the energy markets. Examples are synthesis gas-based processes for the production of high-quality fuels from gas (GTL), coal (CTL) or biomass (BTL), processes for CO2 capture and storage (CCS) – e.g. for CO2-free power plants (OxyFuel, IGCC) – or applications for enhanced oil recovery (EOR). ASU turbomachinery trains have a key role here as the air flow rates for downstream production of oxygen or nitrogen that can be achieved with current industrial compressor technology are barely sufficient for today's larger plants. In addition, the required discharge pressures are increasingly exceeding existing technological limits.

In the largest GTL plant in the world – the Pearl plant in Qatar – eight of these compressor trains will process 130 million m³ of air per day after commissioning in order to ensure the daily oxygen requirement of 28 000 tonnes for the GTL process. The necessary power input of 640 MW roughly corresponds to the average electricity requirements of a city of 700 000 inhabitants, including industrial, commercial and retail consumption.

Based on economies of scale, the medium-term planning for future GTL- and CTL-plants envisages oxygen requirements of 50 000 tonnes or more per day. These oxygen requirements will be met by compressor trains with corresponding air separation columns of 5 000 tonnes of oxygen per day in the next step, rising to 7 000 tonnes per day in a few years time. In the CCS sector similar or even larger compressor trains are expected. This is stimulating manufacturers of air compressor trains for large and very large ASUs to make efforts on the development front in order to deliver the required volumes flows and/or pressure ratios. This primarily affects the main air compressor (MAC), today normally realised as a combined axial/centrifugal compressor.

The MAC of an ASU machine train must meet a wide range of operating requirements. Firstly, operation is influenced by the process requirements, plant startup and overload operation. And secondly, consideration must be given to changes in intake temperature, air humidity and intake pressure in order to maintain process conditions. Ultimately, the customer wants to be able to operate this type of compressor for years without interruption, which makes it extremely important to allow some leeway for possible fouling of the compressor.

There are however limits to simple upscaling of the MAC. These include not only the rotor-dynamical feasibility of long rotors (at high pressures), but also the entire supply chain, starting with raw part procurement through production, assembly and transport to handling at the plant and therefore mark the technological limits of conventional industrial axial compressors.

Compressors of stationary gas turbines and aircraft engines have significantly higher power densities compared to conventional industrial axial compressors, i.e. greater stage pressure ratios, higher speeds and fewer stages.

However, gas turbine compressors are not suitable for industrial use for many reasons, e.g. they do not have the necessary operating range, the efficiency levels are too low and they do not have the necessary robustness and flexibility in terms of flow rates and/or pressure ratios.  In addition, gas turbine compressors have only a limited speed range of normally 95-105%, which does not meet the required API (American Petroleum Institute) standards.

At that point the R&D department at MAN Diesel & Turbo has taken a new approach. Without relinquishing the benefits of current industrial compressors, e.g. robustness, efficiency and operating range, the advantages of gas turbine compressor technology have also been implemented with a new type of blading – a hybrid based on proven industrial design and gas turbine design.

Three years ago MAN Diesel & Turbo managed to sign up with aircraft engine specialist MTU Aero Engines, for the joint realisation of this hybrid concept. The development of an industrial axial compressor in collaboration with a development partner from the aero engine sector was a new departure. During the concept phase all variables were investigated in order to ensure optimal compressor design. Comprehensive matrix studies were carried out in all areas of the design taking into account all aspects, e.g. aerodynamics, mechanics, rotor dynamics, production, costs etc., in order to ascertain the overall optimum for the new hybrid generation.

After three years of intensive collaboration on the development project, the prototype of this hybrid axial compressor, internally called the "MAX1", is ready for the testing phase at MTU's test stands in Munich.

"MAX1" is an original heavy-duty compressor, whose robustness even exceeds that of today's axial compressors. The speed is about 1/3 higher than in the existing series. This results not only in a considerable reduction of stage count (approx. 1/3 fewer) and considerable weight and cost savings in the axial compressor, but also in a rise in power density in the steam turbine drive.

Following the conclusion of the first test programmes at the MTU test stands towards the end of this year the new hybrid compressor will be released for sale.

 

MAN Diesel & Turbo SE, based in Augsburg, Germany, is the world’s leading provider of large-bore diesel engines and turbomachinery for marine and stationary applications. It designs two-stroke and four-stroke engines that are manufactured both by the company and by its licensees. The engines have power outputs ranging from 450 kW to 87 MW. MAN Diesel & Turbo also designs and manufactures gas turbines of up to 50 MW, steam turbines of up to 150 MW and compressors with volume flows of up to 1.5 million m³/h and pressures of up to 1,000 bar. The product range is rounded off by turbochargers, CP propellers, gas engines and chemical reactors. MAN Diesel & Turbo’s range of goods includes complete marine propulsion systems, turbomachinery units for the oil & gas as well as the process industries and turnkey power plants. Customers receive worldwide after-sales services marketed under the MAN PrimeServ brand. The company employs around 12,700 staff at more than 100 international sites, primarily in Germany, Denmark, France, Switzerland, the Czech Republic, Italy, India and China. MAN Diesel & Turbo is a company of the Power Engineering business area of MAN SE, which is listed on the DAX share index of the 30 leading companies in Germany.

 

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